Limestone Calcined Clay Cement (LC3): The Evolution of a Ternary Binder from Laboratory Innovation to Sustainable Industrial Application

Abstract

The urgent need to decarbonize the global cement industry is compounded by the declining availability of conventional supplementary cementitious materials (SCMs). Limestone-calcined clay cement (LC3) emerges as a highly sustainable alternative, enabling up to 50 percent clinker replacement and an approximate 40 percent reduction in carbon dioxide emissions. Unlike existing reviews that focus on basic material properties, this paper critically bridges the gap between fundamental hydration thermodynamics and next-generation sustainable engineering applications. Through a structured bibliographic analysis of 135 contemporary sources published between 2000 and 2026, it traces the evolution of LC3 from a laboratory innovation to a highly promising solution for large-scale industrial implementation and circular economy integration. The discussion highlights the synergistic alumina carbonate reaction. This reaction forms carboaluminate phases. These phases significantly densify the microstructure and enhance long term durability. Key engineering properties are examined, contrasting rheological challenges from high water demand and carbonation susceptibility against its exceptional chloride resistance in aggressive environments. The transition to field application is thoroughly assessed, emphasizing technological advances in flash calcination, environmental footprint reduction through life cycle assessment (LCA), and production scalability. Finally, rather than restating known challenges, this review exposes the limitations of current empirical mitigation strategies. It proposes a targeted research agenda focused on molecular-level green admixture design and field calibrated durability models to support the integration of LC3 into emerging sustainable technologies such as 3D concrete printing.

1. Introduction

1.1. Global Context: The Imperative for Low-Carbon Binders and SCM Scarcity

Modern construction is fundamentally dependent on Ordinary Portland Cement (OPC), which constitutes the primary binding agent in concrete and underpins global infrastructure development [1,2]. Yet this dependence carries a significant environmental burden: cement production is estimated to contribute between 5% and 8% of anthropogenic carbon dioxide (CO₂) emissions worldwide [1,3,4,5,6,7]. Of these emissions, roughly 60–70% arise from the chemical decarbonization of limestone (CaCO₃) during clinker manufacture, while the remaining fraction is attributable to the combustion of fossil fuels needed to sustain kiln temperatures of up to 1450 °C [8,9,10,11,12,13]. To put this environmental burden into precise quantitative perspective, the production of one ton of ordinary Portland clinker generates approximately 800 to 850 kg of carbon dioxide. Consequently, depending on the clinker factor, the manufacturing of one ton of OPC releases between 600 and 700 kg of carbon dioxide directly into the atmosphere.

The environmental footprint of traditional cement production extends far beyond greenhouse gases to include severe impacts on local air quality. The industry acts as a major source of particulate matter and secondary pollutant precursors. Transitioning to low-carbon binders like LC3 is therefore a critical component of broader health-oriented emission control frameworks [14].

With global cement demand expected to rise steadily through 2050, potentially reaching 6 billion tons annually, reducing the clinker content of cement has emerged as the most direct and pressing strategy for achieving carbon neutrality [15,16,17,18]. Historically, this has been accomplished through the use of Supplementary Cementitious Materials (SCMs) such as fly ash and ground granulated blast furnace slag, which partially replace clinker while contributing to hydration reactions [19,20,21,22,23].

However, the global availability of these conventional SCMs falls well short of current and projected demand. The progressive closure of coal-fired power plants is curtailing fly ash supplies, while changes in steel production technologies are reducing slag output [6,8,22,24,25,26,27,28]. Existing estimates suggest that fly ash and slag combined can meet no more than 15–20% of global cement requirements [29,30]. This widening supply and demand gap underscores the need for alternative binders. These binders must be derived from abundantly available raw materials with a low environmental footprint. Such materials are essential for ensuring both long-term sustainability and resource security [11,20,31,32,33].

1.2. The Rise of Calcined Clays: Abundance and Reactivity

In contrast to conventional SCMs, clays, particularly those of the kaolinitic type, rank among the most abundant raw materials in the Earth’s crust, with substantial deposits distributed across nearly all global regions, including many developing countries where cement demand is highest [20,24,27,34,35,36]. Raw clays undergo dehydroxylation when subjected to thermal treatment within the range of 600–850 °C. This process is referred to as calcination. During this phase, hydroxyl groups are eliminated from the crystal structure. This transformation yields an amorphous or semi-crystalline product known as metakaolin, which exhibits pronounced pozzolanic activity [11,27,37,38,39,40]. The fundamental pozzolanic potential of these clays is strictly governed by three critical mineralogical factors. The absolute kaolinite content determines the total available reactive alumina required for the subsequent synergistic reactions. The calcination temperature directly dictates the degree of structural disorder achieved during the dehydroxylation phase. A higher degree of amorphization creates a highly unstable thermodynamic state that drastically accelerates early hydration kinetics. In addition to these chemical parameters, the physical specific surface area of the calcined material plays a decisive role. Materials with high specific surface areas provide abundant nucleation sites for the rapid precipitation of calcium aluminosilicate hydrate gels, which directly dictates the early age mechanical performance of the final cementitious matrix. Although the pozzolanic use of calcined clays has been established for over half a century, its widespread adoption in the construction sector was historically limited by the high cost of pure kaolin. The recent paradigm shift, therefore, is not the discovery of calcined clay itself, but its synergistic combination with limestone and the utilization of abundantly available low-grade clay deposits.

Because the production of calcined clays requires substantially lower temperatures than clinker manufacture and does not involve the decarbonation of limestone, the associated CO₂ emissions and energy consumption are markedly lower than those of OPC [3,10,21,33,38,41]. The existing body of research indicates that kaolinitic clays, characterized by a 1:1 layer structure, exhibit the highest pozzolanic reactivity when compared to other clay types such as illite and montmorillonite, which possess a 2:1 clay structure [2,32,34,42,43,44]. Although high-purity kaolin remains the preferred material for the applications in the paper and ceramic industries, so-called “low-grade” clays containing 40–60% kaolinite have been shown to deliver adequate performance for cement substitution while offering greater cost effectiveness [18,27,44,45,46].

1.3. The LC3 Concept: Synergistic Hydration Mechanisms and Microstructure

The development of ternary systems incorporating both calcined clay and limestone, as opposed to binary systems relying on calcined clay alone, has marked a paradigm shift in cement technology [37,47,48,49]. This ternary blend is designated limestone-calcined clay cement (LC3). It derives its performance from a synergistic interaction. This interaction occurs between the reactive alumina in calcined clay and the carbonates supplied by limestone [6,37,44,47,50,51].

The standard formulation of LC3 technology, commonly referred to as LC3-50, typically comprises 50% Portland clinker, 30% calcined clay, 15% limestone, and 5% gypsum by mass [21,44,52,53,54,55]. Although this baseline ratio is widely adopted in the literature, it is crucial to recognize that these proportions are not strictly fixed. The optimum mix design is highly variable and must be dynamically adjusted depending on local material characteristics. Factors such as the specific kaolinite content of the raw clay, the grinding fineness of the individual components, and the targeted mechanical or durability performance dictate the final formulation. Tailoring the calcined clay to limestone ratio ensures maximum utilization of reactive alumina and prevents underperformance in real world applications. Throughout this review, to ensure consistency when comparing various studies, LC3 variants are uniformly denoted as LC3-XX, where XX strictly indicates the mass percentage of Portland clinker in the binder. This level of clinker substitution yields a 30–40% reduction in CO₂ emission relative to OPC, while mechanical performance equivalent to that of reference cement is achieved from 7 days onwards [16,56,57,58,59,60].

Hydration Kinetics and Phase Development: The hydration mechanism in LC3 systems is notably more complex than that of conventional Portland cement. Calcined clay participates in a pozzolanic reaction to generate additional calcium alumino silicate hydrate gel. Furthermore, the reactive alumina interacts with the calcium carbonate supplied by limestone to form space-filling carboaluminate phases. The precise thermodynamic stability of these hemi- and mono-carboaluminate phases is heavily supported by recent phase equilibrium models. Furthermore, advanced experimental characterization techniques including in situ X-ray diffraction and thermogravimetric analysis have explicitly validated this interpretation by tracking the continuous consumption of portlandite and the progressive crystallization of these space-filling products. This specific synergistic interaction contributes to massive microstructural densification. A detailed thermodynamic analysis of these complex phase developments and the resulting pore refinement is provided in Section 3.

Mechanical Performance and Durability: When evaluating these ternary systems, it is essential to distinguish clearly between early age behavior and long-term performance. During the initial curing phase of one to three days, LC3 frequently exhibits slightly lower compressive strengths due to the dilution effect associated with the reduced clinker content. However, this early strength deficit is rapidly compensated at later ages as the ongoing pozzolanic reactions and carboaluminate precipitation induce massive microstructural refinement. Studies have consistently shown that LC3 concretes attain compressive strengths comparable to those of OPC by 7 days and frequently exceed them at 28 days [44,47,61]. With respect to durability, LC3 systems demonstrate superior performance, most notably in terms of chloride impermeability [62,63]. The dense microstructure and refined pore network characteristic of these systems substantially reduce chloride ion diffusion relative to OPC. Enhanced resistance to alkali–silica reaction (ASR) and sulfate attack has also been documented [4,47]. However, with regard to carbonation resistance, LC3 tends to carbonate more rapidly than OPC, a consequence of its lower clinker content and the depletion of portlandite reserves, making adequate curing essential [49,62,64].

1.4. Rheology, Workability, and Challenges

A principal technical obstacle to the widespread adoption of LC3 technology lies in the increased water demand and reduced workability. Calcined clays possess high specific surface areas and layered structures that directly impair the effectiveness of conventional polycarboxylate ether superplasticizers (PCE). This interaction typically requires higher admixture dosages to maintain adequate flowability. The fundamental chemical mechanisms governing this rheological behavior and the limitations of current mitigation strategies are critically dissected in Section 4.1.

Furthermore, the thixotropic behavior and elevated yield stress exhibited by LC3 pastes represent critical considerations for pumpability and placement [55,65,66]. It must be emphasized that while issues such as elevated water demand, PCE incompatibility, and subsequent workability loss are heavily documented and well known within the scientific community, merely acknowledging these phenomena is no longer sufficient. The existing literature frequently treats these challenges as inherent material flaws rather than systemic formulation issues. Therefore, the subsequent sections of this review will not simply reiterate these recognized bottlenecks, but will critically dissect the fundamental limitations of the current empirical workarounds utilized in the industry.

1.5. Scope of the Review and Emerging Applications

The past two decades have witnessed an exponential increase in research devoted to binders incorporating calcined clay and limestone [67,68]. While several review articles have recently been published on LC3, the majority of these studies present a generalized overview of basic mechanical and durability properties. As the technology rapidly matures, there is a critical need for a comprehensive synthesis that bridges the fundamental atomistic mechanisms, such as aluminum uptake in C-A-S-H gels and carboaluminate thermodynamics, with cutting-edge structural applications and industrial scale-up challenges. This review addresses that specific gap. Whereas early investigations focused primarily on fundamental material characterization and mechanical properties, more recent investigations have shifted toward increasingly complex topics. Of particular note, the thixotropic nature and buildability advantages of LC3 have attracted considerable attention in the context of 3D concrete printing (3DCP) technology [38,47,65,66,67,69,70,71,72]. Furthermore, the application of LC3 in advanced material systems, including ultra-high-performance concrete (UHPC) [37,49,66,68,73,74] and engineered cementitious composites (ECC) [1,2,33,35,66,75,76], offers a pathway toward integrating environmental sustainability with high structural performance.

In these advanced structural applications, ensuring continuous durability under aggressive exposure conditions such as combined freeze thaw cycles and salt erosion remains a critical design parameter that benefits greatly from nano engineered microstructural modifications [77].

Recent reviews by various authors have primarily concentrated on either the fundamental chemistry of calcined clays or the basic mechanical properties of the binder. The critical research gap in the current literature is the absolute absence of a holistic framework that connects atomic-scale hydration thermodynamics directly to macroscopic operational bottlenecks. The novelty of this review lies in its targeted focus on the structural contradictions inherent to LC3 technology. Unlike previous literature reviews that merely summarize successful laboratory tests, this paper explicitly bridges the gap between microstructural potential and actual industrial scale-up challenges. It critically assesses the ongoing transition toward large-scale commercial deployment by emphasizing recent breakthroughs in flash calcination, color-control mechanisms, and updated environmental modeling. This study uniquely exposes the fundamental conflict between the rheological demands of highly reactive clays and the limitations of conventional superplasticizers to provide a realistic roadmap for emerging digital construction techniques. By conducting a structured bibliographic analysis of 135 contemporary sources, the study not only consolidates current knowledge but also identifies critical gaps, offering a strategic roadmap for the integration of LC3 into next-generation digital and high-performance construction. Table 1 summarizes key research findings on LC3 mixtures reported in the literature, while Table 2 presents a structured overview of research focus areas and corresponding mechanistic insights. Figure 1 illustrates the relationship between composition, performance characteristics, and environmental impacts of LC3 systems. Furthermore, Figure 2 establishes a clearly defined conceptual framework that explicitly maps the trajectory from fundamental hydration mechanisms to macroscopic engineering properties and ultimately to industrial implementation.

To ensure a highly coherent narrative and maintain focus, this review is structured into consecutive logical steps directly reflecting this conceptual framework. Section 2 and Section 3 establish the fundamental baseline by detailing binder optimization and microstructural hydration mechanisms. Building upon this baseline, Section 4 and Section 5 translate these microstructural characteristics into macro-scale engineering properties by evaluating rheological behavior, mechanical strength, and long-term durability. Finally, Section 6 links all these technical parameters to real-world feasibility by critically examining industrial production technologies, environmental assessments, and standardization processes.

1.6. Methodology and Literature Selection Process

The synthesis presented in this review is based on a structured evaluation of the literature published over the last two decades. The research strategy involved querying major academic databases including Scopus, Web of Science, and ScienceDirect for the period spanning from 2000 to early 2026. This search process utilized targeted keywords such as limestone-calcined clay cement, LC3, calcined clay mineralogy, synergistic alumina carbonate reaction, and industrial scale flash calcination. To capture the most recent paradigm shifts in the field, a specific weighting was given to high-impact articles published between 2021 and 2026, which account for a significant portion of the synthesized data.

The selection of the 135 sources was guided by three primary criteria. Initially, the focus was placed on mechanistic studies providing fundamental chemical insights into hydration kinetics through advanced characterization techniques. Furthermore, research documenting the transition from laboratory consistency to industrial performance was prioritized with a particular emphasis on workability retention and long-term durability metrics. Finally, the current literature regarding the integration of LC3 into innovative technologies such as 3D concrete printing and ultra-high-performance concrete was included. Data from these sources were critically compared to reveal the structural contradictions between laboratory innovation and industrial feasibility.

Table 1. Research findings on LC3 mixtures.

Binder Composition	SCM/Replacement Ratio (%)	Workability and Rheology Findings	Compressive Strength Outcomes	Durability and Porosity Observations	Microstructural Findings	References
OPC, Calcined Clay, Limestone, Gypsum	45–50%	High specific surface area of calcined clay necessitates higher HRWR/PCE dosages (up to 1.8%). Pronounced thixotropy and yield stress are attributed to clay clustering. Substitution levels exceeding 30% result in unsatisfactory slump retention.	Twenty-eight-day strength matches or surpasses OPC (approx. 45–55 MPa). Late-age strength (365 days) is superior to OPC. Early strength (3 day) is comparable to OPC and surpasses slag/fly ash blends.	Pore refinement reduces volume by 30–50%. Chloride diffusion coefficients reduced by 50–60%. Outstanding resistance to sodium sulfate expansion (0.03% vs. 0.12% for OPC).	Synergistic reaction between alumina and calcite forms carboaluminate phases (Hc and Mc). C-(A)-S-H gel formation is the primary binder matrix. ITZ width reduced to 10 μm.	[5,8,44,78,79].
OPC, Calcined Clay, Limestone, Gypsum (3D Printed)	45–50%	LC2/LC3 enhances printability, buildability, and thixotropy. Direct superplasticizer addition increases yield stress by a factor of 1.2–2.5 compared to delayed addition. VMA improves shape stability but reduces flow.	Printed samples show anisotropic behavior, 28-day strength (35–44 MPa); 3D-printed samples often exhibit 20–28% lower strength than cast samples. Higher grade metakaolin enhances early strength.	Total porosity of printed samples approx. 2.05%. Average compactness ratio increased by 28.2% compared to fly ash/silica fume controls.	C-(A)-S-H bridges identified as dominant in elastic behavior. Formation of extra C-A-S-H and carboaluminates enhances mechanical properties.	[67,69,70,72].
OPC, Calcined Clay, Limestone, Gypsum, Carbonated Waste Paste (CWP)	50%	Superplasticizer used for workability, CWP blends show earlier setting and shorter induction periods than standard LC3 due to silica-alumina gel reactivity.	CWP-LC3 reached 44.8 MPa at 28 days. Showed 15.7% higher 7-day strength than standard LC3 due to gel pozzolanic reactivity.	CWP refined early pore structure via rapid pozzolanic reaction. Chloride resistance improved compared to OPC.	Silica-alumina gel (from CWP) intermixed with calcite clusters. Synergistic hydration formed C-(A)-S-H and AFm phases (Hc/Mc).	[80]
OPC, Calcined Clay, Limestone, Gypsum, Recycled Aggregates	30–70%	Slump declines linearly with substitution (29.1% reduction at 70%). High water absorption of recycled fine aggregates (9.67%) impacts fluidity.	Strength for LC3 and OPC converges by 28 days. Replacement above 50% results in mechanical deterioration. 15% LC3 with 30% RFA is comparable to the control.	Water absorption approx. 11.9% (high due to recycled aggregates). LC3 improved pore size distribution and reduced total porosity (3.66% for LC3-30).	LC3 and RFA improve the performance of the Interfacial transition zone (ITZ). Formation of additional hydration products (Mc, Hc, and C-A-S-H).	[10,50,74]
OPC, Calcined Clay, Limestone, CSA Cement, Gypsum	45–50%	Flowability decreased with graphite content (256 mm to 191 mm), but still met self-leveling requirements (>170 mm).	Optimum binder reached 35.24 MPa (28 days); 12% graphite reduced UCS to 26.04 MPa.	CSA addition reduced LC3 shrinkage from −0.11% to −0.039%. 12% graphite reduced shrinkage to negligible −0.0091%.	TGA identified C-(A)-S-H, AFt (from CSA), Mc, and Hc. Graphite acted as a physical filler but inhibited hydration at high dosages.	[52]
GP/OPC, Calcined Clay, Limestone (LC3)	44–50%	Accelerated early age hydration compared to fly ash blends, higher chemical shrinkage until 20 days. High water demand in blends with >40% kaolinite.	Twenty-eight-day strength ranges from 31.0 to 38.4 MPa. Sodium silicate (SS) addition can increase 1-day strength from 9.4 to 22.1 MPa.	High autogenous shrinkage (1.5 times control) compensated by low drying shrinkage. SS addition reduced cumulative pore volume by 36% at 1 day.	Formation of carboaluminate phases (Hc and Mc) from 3 days. High Na2O (2.5%) leads to deleterious U-phase.	[3,6]
LC3 with Crystalline Admixture (CA)	50%	Superplasticizer used to achieve 200 ± 10 mm flow diameter.	LC3CA reached 61.5 MPa at 28 days compared to 54.8 MPa for plain LC3.	Superior chloride resistance (42% reduction vs. OPC). CA enhanced the crack healing ratio up to 85.9% in seawater.	Healing products in seawater are dominated by brucite and aragonite; the matrix contains carboaluminates, which bind chloride ions as Friedel’s salt.	[81]
LC3 with C-S-H Seeding/Slag/Fly Ash/Silica Fume	50–80%	Partial replacement of calcined clay with fly ash or GGBFS significantly improved workability and mortar flow. PCE demand decreases when clay is substituted with industrial by-products.	A 5% silica fume (SF) boosted 28-day strength by 36.1% over base LC3. Seeding allows parity with OPC even at a 25% clinker factor.	C-S-H seeding reduced the critical pore size from 0.26 μm to 0.10 μm at 1 day. 5% SF achieved the highest electrical resistivity (60 Ohm.m).	C-S-H seeding acts as a nucleus for hydration. Reactive metakaolin introduces Al into C-A-S-H. Mc transforms to Hc at high substitution levels.	[35,59,73,82]
OPC, Calcined Clay, GGBFS, Limestone	Up to 57%	GGBFS compensated for CC-induced workability reduction. Optimized mix achieved a slump of 65 mm with 1.2% SP.	Optimized mix achieved 42.8 MPa at 28 days. Retained 56 MPa residual strength after 400 °C exposure.	Porosity is reduced through the formation of amorphous phases and compact morphology.	FESEM identified C-A-S-H flakes in honeycomb structures and needle-like ettringite.	[2]
LC3 with varying w/b ratio	50%	High w/b (0.9) used for fabric impregnation, dewatering reduced final w/b to 0.5. At low w/b (0.25), yield strength was 7.5× higher than PC.	High-performance LC3 concrete achieved over 100 MPa at 28 days with w/b 0.25. 1-day strength was low (10 MPa) due to retardation.	LC3 had higher mesopore content (<100 nm), but strength improved through pore refinement.	Carboaluminate peaks (Hc and Mc) are less apparent at low w/b (0.25) due to space confinement and restricted crystal growth.	[23,50]
OPC, Calcined Clay, Limestone, Gypsum (High Strength)	30%	Flowable consistency achieved with a slump flow of 440–450 mm using 2% superplasticizer; slightly less flowable than plain OPC.	High compressive strength of approx. 70 MPa (7 days) and 90 MPa (28 days).	Very low chloride permeability (<1000 Coulombs). Water absorption approx. 2%. Drying shrinkage 203–213 microns (1 month).	Matrix contains a higher content of C-(A)-S-H gel and carboaluminate phases filling the pores.	[62]
OPC, Calcined Bentonite, Limestone, Plasterboard Waste/Gypsum	50%	Dissolution rate of sulphate source (PW vs. gypsum) influences early hydration. PW impacted reactions more than virgin gypsum.	CB-based blends achieved 95–102% of 28-day OPC strength (up to 62.8 MPa).	B-CB pastes had slightly coarser pores than metakaolin pastes, both coarser than the OPC control.	CB promoted C-S-H with higher Si/Ca ratios and strätlingite formation. PW improved early pozzolanic reactivity.	[32]
OPC, Limestone, Calcined Clay, Steel Slag	30–50%	SCMs decreased the slump, increased the SP dosage (0.8–1.7%) required due to the high fineness of CC and slag.	A 30% and 40% replacement gave higher 90-day strength than OPC. 50% replacement reached 54.56 MPa at 90 days.	Water absorption slightly increased in 50% replacement (up to 4.2% higher than control). UPV values indicated good quality.	Finer SCM particles fill interstices (filler effect). Pozzolanic reaction and hydraulic slag densify the matrix.	[16]
OPC, Limestone, Metakaolin (UHPSSC)	10–40%	Fluidity and setting time decreased with MK increase. LSMK30 flow spread was 67% of the control.	Optimum substitution at 20% MK + 10% LS reached 117.8 MPa at 90 days.	Chlorides in seawater promoted chemical binding by alumina to form Friedel’s salt, improving density.	MK consumed Portlandite to form C-(A)-S-H gels with higher Si/Ca and Al/Ca ratios.	[37]
OPC, Calcined Clay, Limestone, Soil	45%	Hydraulic conductivity between 4.69 × 10−6 and 1.65 × 10−5 cm/s.	Seven-day UCS increased from 150 to 500 kPa. Decline of 14.5–20% at 56 days due to micro-cracking and swelling.	Reduction in macro-pores, electrical resistivity increases with LC3 content, but decreases with curing time.	XRD/SEM confirms the formation of C-S-H and C-A-S-H gel; expansive soil causes micro-cracking.	[83]
OPC, Calcined Clay, Limestone, Nano-Silica	45%	Nano-silica increases SP demand (up to 0.85 wt%) and decreases flowability.	Adding 2% nano-silica increased the 1-day compressive strength by 55.8% compared to the reference LC3.	Improved carbonation resistance (7–11% reduction), resistivity, and ultrasonic pulse velocity increased significantly.	NS provides nucleation sites for C-(A)-S-H, resulting in a denser matrix with fewer pores.	[49]
LC3 with Waste Rockwool (RW) Bricks	60%	W/B ratios adjusted (0.5 to 0.59) to maintain workability, RW leads to higher water demand due to fineness.	Seven-day strength > 8.6 MPa, 10% RW replacement increased strength by 42.44% relative to reference LC3.	RW addition decreased porosity and water absorption (15.8% to 12.7%), and 90% residual strength after fire.	SEM showed improved packing with RW fibers and formation of three-dimensional staggered networks.	[4]
LC3 with Aloe Vera (AVM) Admixture	50%	AVM bio-admixture acts as a set retarder and reduces yield stress.	LC3 reached 31.67 MPa at 28 days, 2.5% AVM dosage enhanced compressive strength.	Bulk density approx. 2283 kg/m3, density reduced with higher AVM content.	Phenol groups in AVM adsorb on ions via steric hindrance to retain flow.	[84]
LC3 for HS-SHCC/LW-ECC	10–65%	LC3 required higher mixing energy and reduced flowability. LC3 weight fraction increase required more superplasticizer.	A 50% replacement reduced compressive strength but increased flexural strength/ductility due to C-A-S-H polymerization.	A 30% substitution densified pore structure, 50% replacement increased porosity. LC3 shifted the pore size distribution to the left.	Formation of highly polymerized C-A-S-H gel and needle-like ettringite acting as nano-reinforcement.	[33,85]
LC3: Effect of Clay Type/Calcination	45%	Metakaolinite (1:1 clay) reacts faster and requires more water/SP than 2:1 clays (illite/montmorillonite). Soak and flash calcination yield similar results.	Twenty-eight-day strength: 38.4 MPa (kaolinitic), 32.8 MPa (illite), 33.7 MPa (montmorillonite). Strength maximized at 850 °C calcination.	Porosity is higher with coarse clay. Illite pastes have larger threshold pore diameters than kaolinite.	Kaolinite produces denser microstructures. Hc formed in all blends; Mc depends on the clay type.	[40,42,86]
LC3 vs. Limestone-Calcined Laterite (LCLC)	45%	LC3 and LCLC have similar workability requirements. Limestone improved workability compared to calcined laterite alone.	No significant difference between laterite-based and clay-based concrete. LC3 28-day strength 85–90% of control.	Blended cements showed higher initial surface absorption (ISAT-10) than the control, normalizing with age.	Laterite is slightly more amorphous than clay. Both have flat, irregular shapes, limestone is rounder, aiding lubrication.	[64]
LC3 with Recycled Concrete Slurry Waste (CDCSW)/Powder (TARCP)	15–45%	CDCSW/TARCP decreased fluidity and significantly increased yield stress/viscosity due to porous morphology.	CDCSW mortar strength was higher than standard LC3 specimens at all ages. TARCP improved strength via matrix densification.	Refined pore structure with reduced critical pore diameter. Small effective chloride diffusion (1.08 × 10−11 m2/s).	Mc content at 28 days was 11.2% for LC3-CDCSW vs. 9.5% for standard LC3. TARCP facilitates greater Al incorporation into C-A-S-H.	[17,87,88]
Rubberized LC3 Self-compacting Concrete	50%	Slump flow (655–680 mm) satisfied EFNARC criteria, excellent flowability despite crumb rubber addition.	Twenty-eight-day strength decreased from 67.5 MPa (0% CR) to 25.2 MPa (40% CR).	Bulk electrical resistivity increased with CR, and chloride-ion penetrability remained very low.	Compactness and ITZ tightness improved, resulting in lower leaching of heavy metals due to improved microstructure.	[89]
Ultra-high Substitution LC3 (LCC)	75–95%	Yield stress increases (61.94% to 1383%) as the substitution ratio increases. The addition of Portlandite (CH) shortens the induction period.	Max strength 146 MPa at 55% substitution. Extra CH improved 28-day strength by up to 60% in ultra-high LCC systems.	Refined pore structure with a threshold size below 10 nm, total porosity remains below 7%.	Additional CH supports continuous pozzolanic reaction forming more C-A-S-H, gypsum encourages ettringite over mono-carboaluminate.	[24,30,73]
LC3 with Partially Calcined Limestone (PCL)	Up to 80%	PCL provides an alkaline environment and a core-shell structure (CaO shell, CaCO3 core), accelerating early hydration.	At a 52.3% calcination level, 28-day strength was 137% higher than standard LC3 with uncalcined limestone.	Total porosity reduced by half, pore sizes refined to the 5–10 nm range.	The core-shell structure of PCL promotes the formation of a dense layer of hydration products around particles.	[88]

Table 1. Research findings on LC3 mixtures.

Binder Composition	SCM/Replacement Ratio (%)	Workability and Rheology Findings	Compressive Strength Outcomes	Durability and Porosity Observations	Microstructural Findings	References
OPC, Calcined Clay, Limestone, Gypsum	45–50%	High specific surface area of calcined clay necessitates higher HRWR/PCE dosages (up to 1.8%). Pronounced thixotropy and yield stress are attributed to clay clustering. Substitution levels exceeding 30% result in unsatisfactory slump retention.	Twenty-eight-day strength matches or surpasses OPC (approx. 45–55 MPa). Late-age strength (365 days) is superior to OPC. Early strength (3 day) is comparable to OPC and surpasses slag/fly ash blends.	Pore refinement reduces volume by 30–50%. Chloride diffusion coefficients reduced by 50–60%. Outstanding resistance to sodium sulfate expansion (0.03% vs. 0.12% for OPC).	Synergistic reaction between alumina and calcite forms carboaluminate phases (Hc and Mc). C-(A)-S-H gel formation is the primary binder matrix. ITZ width reduced to 10 μm.	[5,8,44,78,79].
OPC, Calcined Clay, Limestone, Gypsum (3D Printed)	45–50%	LC2/LC3 enhances printability, buildability, and thixotropy. Direct superplasticizer addition increases yield stress by a factor of 1.2–2.5 compared to delayed addition. VMA improves shape stability but reduces flow.	Printed samples show anisotropic behavior, 28-day strength (35–44 MPa); 3D-printed samples often exhibit 20–28% lower strength than cast samples. Higher grade metakaolin enhances early strength.	Total porosity of printed samples approx. 2.05%. Average compactness ratio increased by 28.2% compared to fly ash/silica fume controls.	C-(A)-S-H bridges identified as dominant in elastic behavior. Formation of extra C-A-S-H and carboaluminates enhances mechanical properties.	[67,69,70,72].
OPC, Calcined Clay, Limestone, Gypsum, Carbonated Waste Paste (CWP)	50%	Superplasticizer used for workability, CWP blends show earlier setting and shorter induction periods than standard LC3 due to silica-alumina gel reactivity.	CWP-LC3 reached 44.8 MPa at 28 days. Showed 15.7% higher 7-day strength than standard LC3 due to gel pozzolanic reactivity.	CWP refined early pore structure via rapid pozzolanic reaction. Chloride resistance improved compared to OPC.	Silica-alumina gel (from CWP) intermixed with calcite clusters. Synergistic hydration formed C-(A)-S-H and AFm phases (Hc/Mc).	[80]
OPC, Calcined Clay, Limestone, Gypsum, Recycled Aggregates	30–70%	Slump declines linearly with substitution (29.1% reduction at 70%). High water absorption of recycled fine aggregates (9.67%) impacts fluidity.	Strength for LC3 and OPC converges by 28 days. Replacement above 50% results in mechanical deterioration. 15% LC3 with 30% RFA is comparable to the control.	Water absorption approx. 11.9% (high due to recycled aggregates). LC3 improved pore size distribution and reduced total porosity (3.66% for LC3-30).	LC3 and RFA improve the performance of the Interfacial transition zone (ITZ). Formation of additional hydration products (Mc, Hc, and C-A-S-H).	[10,50,74]
OPC, Calcined Clay, Limestone, CSA Cement, Gypsum	45–50%	Flowability decreased with graphite content (256 mm to 191 mm), but still met self-leveling requirements (>170 mm).	Optimum binder reached 35.24 MPa (28 days); 12% graphite reduced UCS to 26.04 MPa.	CSA addition reduced LC3 shrinkage from −0.11% to −0.039%. 12% graphite reduced shrinkage to negligible −0.0091%.	TGA identified C-(A)-S-H, AFt (from CSA), Mc, and Hc. Graphite acted as a physical filler but inhibited hydration at high dosages.	[52]
GP/OPC, Calcined Clay, Limestone (LC3)	44–50%	Accelerated early age hydration compared to fly ash blends, higher chemical shrinkage until 20 days. High water demand in blends with >40% kaolinite.	Twenty-eight-day strength ranges from 31.0 to 38.4 MPa. Sodium silicate (SS) addition can increase 1-day strength from 9.4 to 22.1 MPa.	High autogenous shrinkage (1.5 times control) compensated by low drying shrinkage. SS addition reduced cumulative pore volume by 36% at 1 day.	Formation of carboaluminate phases (Hc and Mc) from 3 days. High Na2O (2.5%) leads to deleterious U-phase.	[3,6]
LC3 with Crystalline Admixture (CA)	50%	Superplasticizer used to achieve 200 ± 10 mm flow diameter.	LC3CA reached 61.5 MPa at 28 days compared to 54.8 MPa for plain LC3.	Superior chloride resistance (42% reduction vs. OPC). CA enhanced the crack healing ratio up to 85.9% in seawater.	Healing products in seawater are dominated by brucite and aragonite; the matrix contains carboaluminates, which bind chloride ions as Friedel’s salt.	[81]
LC3 with C-S-H Seeding/Slag/Fly Ash/Silica Fume	50–80%	Partial replacement of calcined clay with fly ash or GGBFS significantly improved workability and mortar flow. PCE demand decreases when clay is substituted with industrial by-products.	A 5% silica fume (SF) boosted 28-day strength by 36.1% over base LC3. Seeding allows parity with OPC even at a 25% clinker factor.	C-S-H seeding reduced the critical pore size from 0.26 μm to 0.10 μm at 1 day. 5% SF achieved the highest electrical resistivity (60 Ohm.m).	C-S-H seeding acts as a nucleus for hydration. Reactive metakaolin introduces Al into C-A-S-H. Mc transforms to Hc at high substitution levels.	[35,59,73,82]
OPC, Calcined Clay, GGBFS, Limestone	Up to 57%	GGBFS compensated for CC-induced workability reduction. Optimized mix achieved a slump of 65 mm with 1.2% SP.	Optimized mix achieved 42.8 MPa at 28 days. Retained 56 MPa residual strength after 400 °C exposure.	Porosity is reduced through the formation of amorphous phases and compact morphology.	FESEM identified C-A-S-H flakes in honeycomb structures and needle-like ettringite.	[2]
LC3 with varying w/b ratio	50%	High w/b (0.9) used for fabric impregnation, dewatering reduced final w/b to 0.5. At low w/b (0.25), yield strength was 7.5× higher than PC.	High-performance LC3 concrete achieved over 100 MPa at 28 days with w/b 0.25. 1-day strength was low (10 MPa) due to retardation.	LC3 had higher mesopore content (<100 nm), but strength improved through pore refinement.	Carboaluminate peaks (Hc and Mc) are less apparent at low w/b (0.25) due to space confinement and restricted crystal growth.	[23,50]
OPC, Calcined Clay, Limestone, Gypsum (High Strength)	30%	Flowable consistency achieved with a slump flow of 440–450 mm using 2% superplasticizer; slightly less flowable than plain OPC.	High compressive strength of approx. 70 MPa (7 days) and 90 MPa (28 days).	Very low chloride permeability (<1000 Coulombs). Water absorption approx. 2%. Drying shrinkage 203–213 microns (1 month).	Matrix contains a higher content of C-(A)-S-H gel and carboaluminate phases filling the pores.	[62]
OPC, Calcined Bentonite, Limestone, Plasterboard Waste/Gypsum	50%	Dissolution rate of sulphate source (PW vs. gypsum) influences early hydration. PW impacted reactions more than virgin gypsum.	CB-based blends achieved 95–102% of 28-day OPC strength (up to 62.8 MPa).	B-CB pastes had slightly coarser pores than metakaolin pastes, both coarser than the OPC control.	CB promoted C-S-H with higher Si/Ca ratios and strätlingite formation. PW improved early pozzolanic reactivity.	[32]
OPC, Limestone, Calcined Clay, Steel Slag	30–50%	SCMs decreased the slump, increased the SP dosage (0.8–1.7%) required due to the high fineness of CC and slag.	A 30% and 40% replacement gave higher 90-day strength than OPC. 50% replacement reached 54.56 MPa at 90 days.	Water absorption slightly increased in 50% replacement (up to 4.2% higher than control). UPV values indicated good quality.	Finer SCM particles fill interstices (filler effect). Pozzolanic reaction and hydraulic slag densify the matrix.	[16]
OPC, Limestone, Metakaolin (UHPSSC)	10–40%	Fluidity and setting time decreased with MK increase. LSMK30 flow spread was 67% of the control.	Optimum substitution at 20% MK + 10% LS reached 117.8 MPa at 90 days.	Chlorides in seawater promoted chemical binding by alumina to form Friedel’s salt, improving density.	MK consumed Portlandite to form C-(A)-S-H gels with higher Si/Ca and Al/Ca ratios.	[37]
OPC, Calcined Clay, Limestone, Soil	45%	Hydraulic conductivity between 4.69 × 10−6 and 1.65 × 10−5 cm/s.	Seven-day UCS increased from 150 to 500 kPa. Decline of 14.5–20% at 56 days due to micro-cracking and swelling.	Reduction in macro-pores, electrical resistivity increases with LC3 content, but decreases with curing time.	XRD/SEM confirms the formation of C-S-H and C-A-S-H gel; expansive soil causes micro-cracking.	[83]
OPC, Calcined Clay, Limestone, Nano-Silica	45%	Nano-silica increases SP demand (up to 0.85 wt%) and decreases flowability.	Adding 2% nano-silica increased the 1-day compressive strength by 55.8% compared to the reference LC3.	Improved carbonation resistance (7–11% reduction), resistivity, and ultrasonic pulse velocity increased significantly.	NS provides nucleation sites for C-(A)-S-H, resulting in a denser matrix with fewer pores.	[49]
LC3 with Waste Rockwool (RW) Bricks	60%	W/B ratios adjusted (0.5 to 0.59) to maintain workability, RW leads to higher water demand due to fineness.	Seven-day strength > 8.6 MPa, 10% RW replacement increased strength by 42.44% relative to reference LC3.	RW addition decreased porosity and water absorption (15.8% to 12.7%), and 90% residual strength after fire.	SEM showed improved packing with RW fibers and formation of three-dimensional staggered networks.	[4]
LC3 with Aloe Vera (AVM) Admixture	50%	AVM bio-admixture acts as a set retarder and reduces yield stress.	LC3 reached 31.67 MPa at 28 days, 2.5% AVM dosage enhanced compressive strength.	Bulk density approx. 2283 kg/m3, density reduced with higher AVM content.	Phenol groups in AVM adsorb on ions via steric hindrance to retain flow.	[84]
LC3 for HS-SHCC/LW-ECC	10–65%	LC3 required higher mixing energy and reduced flowability. LC3 weight fraction increase required more superplasticizer.	A 50% replacement reduced compressive strength but increased flexural strength/ductility due to C-A-S-H polymerization.	A 30% substitution densified pore structure, 50% replacement increased porosity. LC3 shifted the pore size distribution to the left.	Formation of highly polymerized C-A-S-H gel and needle-like ettringite acting as nano-reinforcement.	[33,85]
LC3: Effect of Clay Type/Calcination	45%	Metakaolinite (1:1 clay) reacts faster and requires more water/SP than 2:1 clays (illite/montmorillonite). Soak and flash calcination yield similar results.	Twenty-eight-day strength: 38.4 MPa (kaolinitic), 32.8 MPa (illite), 33.7 MPa (montmorillonite). Strength maximized at 850 °C calcination.	Porosity is higher with coarse clay. Illite pastes have larger threshold pore diameters than kaolinite.	Kaolinite produces denser microstructures. Hc formed in all blends; Mc depends on the clay type.	[40,42,86]
LC3 vs. Limestone-Calcined Laterite (LCLC)	45%	LC3 and LCLC have similar workability requirements. Limestone improved workability compared to calcined laterite alone.	No significant difference between laterite-based and clay-based concrete. LC3 28-day strength 85–90% of control.	Blended cements showed higher initial surface absorption (ISAT-10) than the control, normalizing with age.	Laterite is slightly more amorphous than clay. Both have flat, irregular shapes, limestone is rounder, aiding lubrication.	[64]
LC3 with Recycled Concrete Slurry Waste (CDCSW)/Powder (TARCP)	15–45%	CDCSW/TARCP decreased fluidity and significantly increased yield stress/viscosity due to porous morphology.	CDCSW mortar strength was higher than standard LC3 specimens at all ages. TARCP improved strength via matrix densification.	Refined pore structure with reduced critical pore diameter. Small effective chloride diffusion (1.08 × 10−11 m2/s).	Mc content at 28 days was 11.2% for LC3-CDCSW vs. 9.5% for standard LC3. TARCP facilitates greater Al incorporation into C-A-S-H.	[17,87,88]
Rubberized LC3 Self-compacting Concrete	50%	Slump flow (655–680 mm) satisfied EFNARC criteria, excellent flowability despite crumb rubber addition.	Twenty-eight-day strength decreased from 67.5 MPa (0% CR) to 25.2 MPa (40% CR).	Bulk electrical resistivity increased with CR, and chloride-ion penetrability remained very low.	Compactness and ITZ tightness improved, resulting in lower leaching of heavy metals due to improved microstructure.	[89]
Ultra-high Substitution LC3 (LCC)	75–95%	Yield stress increases (61.94% to 1383%) as the substitution ratio increases. The addition of Portlandite (CH) shortens the induction period.	Max strength 146 MPa at 55% substitution. Extra CH improved 28-day strength by up to 60% in ultra-high LCC systems.	Refined pore structure with a threshold size below 10 nm, total porosity remains below 7%.	Additional CH supports continuous pozzolanic reaction forming more C-A-S-H, gypsum encourages ettringite over mono-carboaluminate.	[24,30,73]
LC3 with Partially Calcined Limestone (PCL)	Up to 80%	PCL provides an alkaline environment and a core-shell structure (CaO shell, CaCO3 core), accelerating early hydration.	At a 52.3% calcination level, 28-day strength was 137% higher than standard LC3 with uncalcined limestone.	Total porosity reduced by half, pore sizes refined to the 5–10 nm range.	The core-shell structure of PCL promotes the formation of a dense layer of hydration products around particles.	[88]

Table 2. Comprehensive literature summary and research focus areas for LC3.

Category	Research Variables and Methods	Key Findings, Mechanisms
Raw Material Characterization and Calcination	Variables: Clay mineralogy (kaolinite, Illite, montmorillonite), calcination temperature (600−900 °C), residence time, grinding fineness. Methods: XRD, TGA, R3 test (ASTM C1897 [90]), PSD, and BET surface area.	Kaolinitic Clays (more than 40% purity): Maximum pozzolanic reactivity is achieved at 800 °C. These clays yield 28-day compressive strengths of 45–60 MPa and reduce critical pore diameters strictly to the 5–10 nm range [63,79,91]. Flash Calcination (0.1–1.0 s): Increases specific surface area to 15–25 m2/g and enhances early age reactivity. This method increases superplasticizer demand (typically 1.5–2.0% dosage) while maintaining high late-age strength (40–55 MPa) [40,92]. Soak/Rotary Calcination (30–60 min): Produces more regular particle morphology and lower surface areas (10–15 m2/g). This results in lower admixture demand (0.8–1.2% dosage) with comparable 28-day strength ranges (40–50 MPa) [40,61,93]. Low-Grade Clays: Lower reactivity requires higher calcination temperatures (850–900 °C). They typically yield 28-day strengths of 30–40 MPa and slightly coarser critical pore diameters in the 10–15 nm range [42,86,94].
Hydration Mechanism and Phase Development	Variables: Clinker/clay/limestone ratios, sulfate balance (gypsum/anhydrite), curing temperature. Methods: Isothermal calorimetry, in situ XRD, SEM-EDS, and thermodynamic modeling (GEMS).	Synergistic Effect: Alumina from metakaolin reacts with carbonate from limestone to form carboaluminates (Hc and Mc), filling capillary pores and refining the microstructure [95]. Reaction Kinetics: This reaction often corresponds to a “third hydration peak” occurring after the main clinker hydration peaks [95]. Sulfate Balance: Proper sulfate optimization is critical to control the aluminate reaction, prevent flash set, and maximize strength [45,96]. Long-Term Evolution: Hydration continues significantly beyond 90 days, with Hc often converting to Mc and porosity continuing to decrease up to 3 years [18].
Rheology and Workability	Variables: w/b ratio, SP type (PCE, SNF), and dosage, addition time (delayed/direct). Methods: Rotational rheometry, slump flow, Zeta potential, TOC (adsorption).	Water Demand: Calcined clays have high BET surface area and layered structures, leading to increased water demand and yield stress compared to OPC [21,29]. PCE Compatibility: Clays can adsorb or intercalate PCE polymers, reducing their dispersing efficiency. Delayed addition of superplasticizers or using specific clay-mitigating admixtures improves workability [29,45]. Thixotropy: LC3 pastes exhibit higher thixotropy and structural build-up, which can be challenging for pumping but advantageous for 3D printing [65,97].
Mechanical Performance	Variables: Clinker replacement levels (35–80%), Accelerators (C-S-H seeding, nano-silica), and curing conditions. Methods: Compressive/flexural strength, elastic modulus.	Strength Development: LC3-50 (50% clinker) typically achieves comparable strength to OPC after 7 days and can surpass it at 28 days due to pore refinement [5]. Early-Age Strength: Early strength (1–3 days) is often lower due to the dilution effect but can be enhanced by alkali activation, nano-silica, or C-S-H seeding [47,59]. Clinker Factor: Reducing clinker below 50% (LC3-35) results in lower strength, but optimization of particle packing and chemical activation can mitigate this loss [57].
Durability Performance	Variables: Chloride exposure, carbonation, sulfate attack, ASR. Methods: RCPT, migration tests (RCMT), MIP (porosimetry), surface resistivity.	Chloride Resistance: LC3 exhibits superior chloride resistance compared to OPC (often by an order of magnitude) due to a highly refined pore network and tortuosity [63,78]. Carbonation: The carbonation rate is generally faster than OPC due to lower portlandite (CH) buffer capacity, making proper curing essential [98]. ASR and Sulfate: The system shows effective mitigation of alkali–silica reaction (ASR) expansion and improved resistance to sulfate attack [41].
Sustainability and Environmental Impact	Variables: CO2 emissions, embodied energy, cost analysis. Methods: Life cycle assessment (LCA), cost–benefit analysis.	Emission Reduction: LC3 technology can reduce CO2 emissions by 30–40% compared to OPC [26,99]. Energy: Calcination of clay requires lower temperatures (800 °C) than clinker production (1450 °C), resulting in energy savings [100]. Economics: The use of abundant low-grade clays and limestone overburden offers economic advantages, particularly in regions lacking high-quality fly ash or slag [44].
Advanced Applications (3D Printing, UHPC)	Variables: 3D concrete printing (3DCP), UHPC design, fiber reinforcement (ECC). Methods: Printability, extrudability, micromechanics.	3D Printing: High thixotropy and yield stress of LC3 are beneficial for shape retention (buildability) in 3D printing applications [43,66]. UHPC: LC3-based ultra-high-performance concrete demonstrates high packing density and sustainability, though it requires careful rheology management [73,74]. Composites: In ECC, LC3 contributes to high tensile strain capacity and controlled crack widths [33,71].

Table 2. Comprehensive literature summary and research focus areas for LC3.

Category	Research Variables and Methods	Key Findings, Mechanisms
Raw Material Characterization and Calcination	Variables: Clay mineralogy (kaolinite, Illite, montmorillonite), calcination temperature (600−900 °C), residence time, grinding fineness. Methods: XRD, TGA, R3 test (ASTM C1897 [90]), PSD, and BET surface area.	Kaolinitic Clays (more than 40% purity): Maximum pozzolanic reactivity is achieved at 800 °C. These clays yield 28-day compressive strengths of 45–60 MPa and reduce critical pore diameters strictly to the 5–10 nm range [63,79,91]. Flash Calcination (0.1–1.0 s): Increases specific surface area to 15–25 m2/g and enhances early age reactivity. This method increases superplasticizer demand (typically 1.5–2.0% dosage) while maintaining high late-age strength (40–55 MPa) [40,92]. Soak/Rotary Calcination (30–60 min): Produces more regular particle morphology and lower surface areas (10–15 m2/g). This results in lower admixture demand (0.8–1.2% dosage) with comparable 28-day strength ranges (40–50 MPa) [40,61,93]. Low-Grade Clays: Lower reactivity requires higher calcination temperatures (850–900 °C). They typically yield 28-day strengths of 30–40 MPa and slightly coarser critical pore diameters in the 10–15 nm range [42,86,94].
Hydration Mechanism and Phase Development	Variables: Clinker/clay/limestone ratios, sulfate balance (gypsum/anhydrite), curing temperature. Methods: Isothermal calorimetry, in situ XRD, SEM-EDS, and thermodynamic modeling (GEMS).	Synergistic Effect: Alumina from metakaolin reacts with carbonate from limestone to form carboaluminates (Hc and Mc), filling capillary pores and refining the microstructure [95]. Reaction Kinetics: This reaction often corresponds to a “third hydration peak” occurring after the main clinker hydration peaks [95]. Sulfate Balance: Proper sulfate optimization is critical to control the aluminate reaction, prevent flash set, and maximize strength [45,96]. Long-Term Evolution: Hydration continues significantly beyond 90 days, with Hc often converting to Mc and porosity continuing to decrease up to 3 years [18].
Rheology and Workability	Variables: w/b ratio, SP type (PCE, SNF), and dosage, addition time (delayed/direct). Methods: Rotational rheometry, slump flow, Zeta potential, TOC (adsorption).	Water Demand: Calcined clays have high BET surface area and layered structures, leading to increased water demand and yield stress compared to OPC [21,29]. PCE Compatibility: Clays can adsorb or intercalate PCE polymers, reducing their dispersing efficiency. Delayed addition of superplasticizers or using specific clay-mitigating admixtures improves workability [29,45]. Thixotropy: LC3 pastes exhibit higher thixotropy and structural build-up, which can be challenging for pumping but advantageous for 3D printing [65,97].
Mechanical Performance	Variables: Clinker replacement levels (35–80%), Accelerators (C-S-H seeding, nano-silica), and curing conditions. Methods: Compressive/flexural strength, elastic modulus.	Strength Development: LC3-50 (50% clinker) typically achieves comparable strength to OPC after 7 days and can surpass it at 28 days due to pore refinement [5]. Early-Age Strength: Early strength (1–3 days) is often lower due to the dilution effect but can be enhanced by alkali activation, nano-silica, or C-S-H seeding [47,59]. Clinker Factor: Reducing clinker below 50% (LC3-35) results in lower strength, but optimization of particle packing and chemical activation can mitigate this loss [57].
Durability Performance	Variables: Chloride exposure, carbonation, sulfate attack, ASR. Methods: RCPT, migration tests (RCMT), MIP (porosimetry), surface resistivity.	Chloride Resistance: LC3 exhibits superior chloride resistance compared to OPC (often by an order of magnitude) due to a highly refined pore network and tortuosity [63,78]. Carbonation: The carbonation rate is generally faster than OPC due to lower portlandite (CH) buffer capacity, making proper curing essential [98]. ASR and Sulfate: The system shows effective mitigation of alkali–silica reaction (ASR) expansion and improved resistance to sulfate attack [41].
Sustainability and Environmental Impact	Variables: CO2 emissions, embodied energy, cost analysis. Methods: Life cycle assessment (LCA), cost–benefit analysis.	Emission Reduction: LC3 technology can reduce CO2 emissions by 30–40% compared to OPC [26,99]. Energy: Calcination of clay requires lower temperatures (800 °C) than clinker production (1450 °C), resulting in energy savings [100]. Economics: The use of abundant low-grade clays and limestone overburden offers economic advantages, particularly in regions lacking high-quality fly ash or slag [44].
Advanced Applications (3D Printing, UHPC)	Variables: 3D concrete printing (3DCP), UHPC design, fiber reinforcement (ECC). Methods: Printability, extrudability, micromechanics.	3D Printing: High thixotropy and yield stress of LC3 are beneficial for shape retention (buildability) in 3D printing applications [43,66]. UHPC: LC3-based ultra-high-performance concrete demonstrates high packing density and sustainability, though it requires careful rheology management [73,74]. Composites: In ECC, LC3 contributes to high tensile strain capacity and controlled crack widths [33,71].

The synthesis of the data presented in Table 1 and Table 2 reveals a clear consensus regarding the superior chloride resistance and long-term mechanical stability of LC3 systems. Research confirms that formulations utilizing calcined clays with a kaolinite content exceeding 40 percent achieve strength parity or superiority compared to Portland cement by 28 days. This performance trend is driven by the synergistic reaction between alumina and calcite which forms carboaluminate phases that effectively fill capillary voids and refine the microstructure. Furthermore, the application of chemical activators such as calcium silicate hydrate seeding or nano-silica emerges as a consistent strategy to compensate for the early age dilution effect associated with high clinker replacement.

Despite these agreements, significant discrepancies remain in the literature concerning workability management and the long-term implications of carbonation. The high specific surface area and layered morphology of calcined clays lead to excessive adsorption of conventional superplasticizers which necessitates higher dosages or specialized addition sequences to maintain flowability. Regarding carbonation, while it is widely accepted that LC3 systems exhibit faster carbonation fronts due to lower portlandite reserves, there is an ongoing debate about the actual corrosion risk. Some studies suggest that the highly refined pore network and increased electrical resistivity significantly slow down the propagation of reinforcement corrosion even after the carbonation front reaches the steel. These findings highlight that the global adoption of LC3 requires a shift toward performance-based standards that account for specific exposure conditions and local material characteristics.

2. Evolution of Binder Design: From Random Blending to Scientific Optimization

One of the most consequential paradigm shifts in cement science over the last two decades has been the transition from a “random replacement” approach in the use of SCMs to a “scientific optimization” framework grounded in the chemical and physical synergy among components.

LC3 exemplifies this evolution more tangibly than any other contemporary binder system. The design philosophy underlying LC3 is not predicated on the simple substitution of clinker with an inert filler. Rather, it is founded upon the thermodynamic and kinetic balancing of a ternary interaction involving the reactive alumina contributed by calcined clay, the carbonates supplied by limestone, and the sulfate provided by gypsum or alternative sources [19,100]. This section examines the critical characteristics of raw materials, the methodologies employed for mix proportioning, and the strategies for optimizing sulfate balance, all considered in light of the most current literature.

2.1. Constituent Raw Materials and Characterization

The performance of LC3 systems is intrinsically governed by the mineralogical and physical attributes of the constituent raw materials. Reactivity, particle size distribution (PSD), and particle morphology constitute the primary parameters that dictate water demand, rheological behavior, and mechanical strength development of the final product.

2.1.1. Clays: Mineralogy, Calcination, and Reactivity

The pozzolanic activity of clays is imparted through the disruption of their crystalline structure and subsequent transformation into an amorphous phase, a process known as dehydroxylation, during thermal treatment (calcination).

Kaolinite Content and Dehydroxylation: The literature has consistently demonstrated that kaolinitic clays, characterized by a 1:1 layer structure, exhibit substantially higher pozzolanic activity than other clay minerals such as illite or montmorillonite, which possess a 2:1 layer structure [101]. In their investigation of Saudi Arabian clays, Abdulqader et al. [1] emphasized that the endothermic transformation of kaolinite to metakaolin (Equation (1)) enables precise quantification of kaolinite content through mass loss measurements in the 400–600 °C range using thermogravimetric analysis (TGA). Representative TGA and DTG curves, illustrating the characteristic dehydroxylation peaks employed for quantifying kaolinite content across different clay sources, are presented in Figure 3.

Al₂Si₂O₅(OH)₄ → Al₂Si₂O₇ + 2H₂O

(1)

Although high-purity metakaolin (exceeding 90% kaolinite) delivers superior performance, it has been established that “low-grade” clays containing 40–60% kaolinite can nonetheless achieve 28-day compressive strength equivalent to that of plain OPC, owing to their synergistic interaction with limestone [5,102].

Calcination Methods: The calcination regime exerts a direct influence on both the reactivity and particle morphology of the resulting clay. In a comparative study of traditional “soak” calcination in rotary kilns versus “flash” calcination methods, Koutsouradi et al. [40] reported that flash calcination mitigates particle agglomeration, while both techniques yield comparable chemical reactivity as assessed by the R³ test. Nevertheless, it should be noted that flash-calcined clays may occasionally induce higher water demand owing to their irregular particle shapes.

Low-Grade and Complex Clays: The utilization of mixed clays containing not only kaolinite but also montmorillonite and illite is gaining increasing attention. Ram et al. [46] demonstrated that even clays with kaolinite contents below 20% can be effectively incorporated into LC3 systems. Although durability properties, such as chloride resistance, may be somewhat compromised relative to systems employing high-kaolinite clays, they nevertheless exhibit performance superior to that of OPC.

2.1.2. Limestone

In LC3 systems, limestone functions not merely as a physical filler but as an active participant in hydration reactions.

Chemical Effect (Carboaluminate Formation): Reactive alumina (Al₂O₃) derived from calcined clay interacts with carbonate (CO₃²⁻) dissolved from limestone during hydration, leading to the formation of hemi-carboaluminate (Hc) and mono-carboaluminate (Mc) phases. Zunino and Scrivener [95] demonstrated that this reaction plays a critical role in pore clogging and microstructural densification, contributing to strength development through the filling of capillary voids.

Physical Effect (Nucleation): Finely ground limestone provides additional surface area for the facilitation of the nucleation and growth of C-S-H gels, a phenomenon commonly referred to as the “nucleation effect.” Berodier and Scrivener [103] reported that limestone particles accelerate clinker hydration by serving as preferential nucleation sites. However, beyond a certain fineness threshold (exceeding approximately 1000 m²/kg), the efficiency of this effect may diminish owing to the increased risk of particle agglomeration.

Fineness and Reactivity: Ferreiro et al. [86] observed that increasing the fineness of limestone enhances early-age strength development, particularly within the first 1–3 days, albeit at the cost of increased water demand. It is also noteworthy that limestone, being softer than clinker, tends to separate into finer fractions during inter-grinding with clinker, thereby contributing to a natural optimization of particle packing.

2.2. Mix Proportioning: From Empirical to D-Optimal Design

The optimization of the proportional relationship among clinker, calcined clay, and limestone represents the most critical stage in LC3 binder design. Whereas binary combinations of metakaolin and limestone were prevalent in the early 2000s, contemporary research increasingly emphasizes the stoichiometric balance inherent to ternary systems.

The 2:1 Clay/Limestone Rule: The canonical formulation of LC3 technology, commonly designated LC3-50, is predicated on a mass ratio of approximately 2:1 between calcined clay and limestone. This proportion approximates the ideal chemical stoichiometry required for the complete conversion of reactive alumina into carboaluminates phases. Nevertheless, Kanagaraj et al. [26] emphasized that this ratio should not be regarded as immutable across different clay sources or local materials availability, rather, it warrants optimization through statistical methodologies such as the Taguchi method or D-optimal design. A schematic representation summarizing the critical variables and the optimization steps employed in LC3 mixture design is presented in Figure 4.

Low Clinker Factor (Below 50%): Recent investigations have increasingly focused on systems in which the clinker content is reduced below 50%, typically to levels of 35–40%. X. Li et al. [59] and Sun et al. [57] examined the efficacy of strategies such as C-S-H seeding or nano-silica addition to compensate for the diminished early-age strength associated with such low clinker factors. Their findings indicate that as the clinker factor decreases, the alumina-to-sulfate balance becomes increasingly sensitive and requires more precise control.

Packing Density: For applications requiring UHPC, Luzu et al. [104] and X. Liu et al. [73] emphasized that LC3 mixture design must maximize not only chemical reactivity but also particle packing density. In mixtures optimized using the Andreasen and Andersen (A&A) packing model, exceptionally high compressive strengths can be achieved through concomitant reduction of the water-to-binder (w/b) ratio.

2.3. Sulfation Optimization

LC3 systems are characterized by a substantially higher alumina content relative to OPC. This compositional feature renders the adjustment of sulfate balance principally governed by gypsum content, the most critical parameter influencing both the rheological behavior and strength development of the binder.

Ettringite Stabilization and Shrinkage Control: Sulfate ions participate in early-age reactions with alumina to form ettringite. Insufficient sulfation (undersulfation) may precipitate rapid aluminate reactions and consequent loss of workability, whereas an appropriately calibrated sulfate balance enhances mechanical strength and mitigates shrinkage. Hay et al. [96] investigated the effect of different sulfate levels on the dimensional stability of LC3 systems and demonstrated that the optimum sulfate dosage should be determined on the basis of the combined contribution of C₃A from clinker and reactive alumina derived from calcined clay. Exceeding the optimum, a condition referred to as oversulfation, elevates the risk of dimensional instability.

Superplasticizer (PCE) Interaction: A competitive adsorption equilibrium exists between sulfate ions and PCE-based superplasticizers. Moghul et al. [29] and R. Li et al. [12] revealed that adjustments to sulfate content directly influence the adsorption of PCE onto clay surfaces and, consequently, the flowability of the concrete mixture. The presence of sufficient sulfate can enhance workability by promoting the retention of PCE molecules on clinker particles rather than their depletion through adsorption onto clay.

Determination via Calorimetry: Isothermal calorimetry constitutes the most reliable method for optimizing sulfate content in LC3 systems. The appearance of the aluminate peak subsequent to the silicate (alite) peak, separated by an appropriate time interval, serves as an indicator that the system is correctly sulfated. The influence of varying sulfate dosages on the hydration kinetics, specifically the shift in aluminate and silicate peaks as captured by isothermal calorimetry, is illustrated in Figure 5.

3. Hydration Mechanisms and Microstructure

Understanding the fundamental hydration mechanisms at the atomic level is the mandatory first step before addressing the macro-scale engineering challenges of LC3. The engineering performance of LC3 systems arises from a complex interplay among clinker hydration, pozzolanic reactions, and carbonate interaction. This section examines phase development, the atomic-scale structure of C-A-S-H gel, and the evolution of the pore network, all of which collectively govern the mechanical strength and durability of LC3, in light of the current literature.

3.1. The Synergistic Effect

The efficacy of LC3 systems derives from the chemical synergy between calcined clay (metakaolin) and limestone, rather than from the individual contributions of these components. When calcined clay is employed alone, as in binary LC2 systems, it generates an alumina-rich yet calcium-poor environment that favors the formation of stratlingite (C₂ASH₈). The introduction of limestone into the system, however, fundamentally alters this thermodynamic equilibrium.

Alumina–Carbonate Interaction: Reactive alumina (Al₂O₃) released from calcined clay and carbonate ions (CO₃²⁻) supplied by limestone react with calcium and water to form carboaluminate phases. As observed in heat flow curves, the synergistic interaction in calcined clay systems significantly alters hydration kinetics by accelerating the main silicate peak (Peak 1) and amplifying the subsequent aluminate peak (Peak 2). This intense reaction not only contributes a space-filling effect through the precipitation of carboaluminates but also ensures the stabilization of ettringite (AFt) by preventing the consumption of sulfate ions in AFm phases such as monosulfate [95,100]. The characteristic evolution of heat flow, demonstrating the accelerated hydration kinetics and the pronounced aluminate peak in calcined clay systems compared to general purpose Portland cement (GP), is illustrated in Figure 6. The intensification of the heat flow peaks captured in Figure 6 indicates a highly reactive environment where the filler effect of limestone and the pozzolanic activity of metakaolin operate concurrently. This thermodynamic shift facilitates the early precipitation of carboaluminate phases which bridges the intergranular spaces more effectively than the hydration products in general purpose cement. Furthermore, the stabilization of ettringite through this synergistic pathway prevents the formation of monosulfate, thereby ensuring a higher volume of stable hydrates within the first 48 h. Consequently, the kinetic acceleration observed in the calorimetry data serves as a direct indicator of the matrix densification required to sustain structural integrity at reduced clinker factors.

Reaction Kinetics: In a series of experiments conducted across a range of w/b ratios (0.25 to 0.60), Hay et al. [23] reported that the presence of limestone accelerates early-age hydration by providing nucleation surfaces for clinker phases, in addition to its role as a chemical reactant. This synergistic effect constitutes the fundamental mechanism by which LC3 systems maintain early-age strength despite the reduction of the clinker content to 50%.

To fully understand the aluminate reaction and the emergence of the third peak in calorimetry data, it is essential to rigorously separate thermodynamic driving forces from kinetic effects. Thermodynamically, the continuous dissolution of reactive alumina from calcined clay drives the precipitation of space-filling carboaluminate phases. This chemical equilibrium dictates the ultimate phase assemblage and is highly dependent on the total availability of reactive alumina. Conversely, kinetic factors are primarily governed by the physical characteristics of the particles. The high specific surface area of calcined clays provides abundant nucleation surfaces. This physical feature accelerates the initial precipitation rate regardless of the underlying chemical composition. It is critical to note that while the kinetic acceleration provided by this filler effect is robust and observed universally across various clay sources, the thermodynamic potential to form massive carboaluminate phases is not. The total volume of carboaluminate formed is strictly limited by the specific kaolinite content and the reactive alumina fraction of the local clay source.

3.2. Phase Assemblage Evolution

The hydration products formed in LC3 systems are not only more complex but also volumetrically more abundant than those generated in OPC. A combination of thermodynamic modeling (GEMS) and experimental data (XRD/TGA) reveals how phase assemblage evolves over time.

3.2.1. C-A-S-H Gel Structure and Al-Uptake

Unlike the C-S-H gel in Portland cement, the gel formed in LC3 systems contains high amounts of aluminum and is designated as C-A-S-H.

Aluminum Incorporation: Avet et al. [91] and Dhandapani et al. [78], stated that the high alumina concentration from calcined clay alters the gel structure by substituting for silicon at bridging positions in silicate chains. This situation increases the mean chain length (MCL) of the C-A-S-H gel and decreases the Ca/Si ratio.

Density and Morphology: Using machine learning-assisted image analysis, H. Sui et al. [106], determined that the C-A-S-H gel in LC3 exhibits a denser and more homogeneous distribution compared to OPC, which positively reflects on mechanical properties (microhardness). The presence of aluminum increases the density of the gel, forming a matrix more resistant to ion diffusion [85].

3.2.2. Carboaluminates

The most distinctive microstructural feature of the LC3 systems is the presence of hemi-carboaluminate (Hc, C₄Ac_0.5H₁₂) and mono-carboaluminate (Mc, C₄AcH₁₁) phases.

Formation Sequence: During the early stages of hydration, when the ratio of carbonate ions to aluminate is relatively low, the Hc phase forms preferentially. As hydration progresses and carbonate continues to dissolve, Hc may transform into the thermodynamically more stable Mc phase. Sun et al. [57] demonstrated that even at clinker content levels below 50%, as in LC3-35 systems, the formation of Hc and Mc plays a critical role in filling capillary porosity and compensating for the reduced clinker content.

Long-Term Stability: Zunino and Scrivener [18] confirmed through long-term studies extending up to 3 years that both Hc and Mc phases remain stable and porosity continues to decrease over time. These phases crystallize preferentially within capillary voids, creating a “space-filling” effect that progressively reduces permeability. The temporal evolution of these phases within the hydrated matrix is illustrated in Figure 7.

3.3. Pore Structure Refinement

The superior durability performance of LC3 systems is attributable not to a reduction in total porosity but rather to a fundamental shift in pore size distribution.

Critical Pore Diameter: Mercury intrusion porosimetry (MIP) measurements indicate that although LC3 systems may exhibit higher total porosity than OPC largely owing to an increased volume of gel pores, the critical pore entry radius is significantly smaller. Dhandapani and Santhanam [107] and Maraghechi et al. [63] reported that the critical pore diameter in uncarbonated LC3 samples typically decreases to below 10 nm, whereas for OPC, this value remains within the range of 20 to 50 nm.

However, MIP measurements also reveal how carbonation alters this highly refined microstructure across LC3 systems produced with different clay sources. In a comparative study of standard LC3 mixtures containing 50% clinker, 30% clay, 15% limestone, and 5% gypsum (designated as K0-0, K1-0, and K2-0 to represent varying clay types), the critical pore diameter significantly shifted rightward after carbonation. Specifically, for the uncarbonated K0-0, K1-0, and K2-0 pastes, the critical pore diameters were initially measured at 0.011 μm, 0.014 μm, and 0.151 μm, respectively. Following carbonation, this diameter coarsened to 0.069 μm for all pastes. Because portlandite content is limited in LC3 systems, C-S-H and AFt are the dominant phases controlling porosity. The coarsening of the pore structure is directly attributed to carbonation-induced volume changes. These changes include an 11.5% expansion for portlandite, a 2.4% contraction for C-S-H, and a massive 44.6% contraction for AFt. Despite this general coarsening, the total porosity of the K2-0 mixture actually decreased from 29.9% to 24.9% after carbonation, a densification that likely restricts further CO₂ ingress. In contrast, the K0-0 and K1-0 blends exhibited minimal total porosity changes. The distinctive log differential intrusion curves demonstrating these carbonation-induced microstructural shifts are clearly illustrated in Figure 8. The shifts in the log differential intrusion curves presented in Figure 8 demonstrate that carbonation induces a significant reorganization of the hydrated matrix. While the rightward shift of the peaks indicates a coarsening of the critical pore diameter, the relative stability of total porosity in kaolinitic blends suggests a protective densification mechanism. This behavior is linked to the decomposition of hydration products which partially fills the larger voids created during the carbonation process. The convergence of all pastes to a similar critical diameter after exposure underlines the importance of initial curing to establish a refined network before atmospheric interaction occurs.

Connectivity and Tortuosity: Ram et al. [46] reported that even in LC3 systems produced with low- to medium-grade kaolinitic clays, the tortuosity of the pore network increases while pore connectivity decreases. This so-called “ink-bottle” effect physically impedes the ingress of harmful agents such as chloride ions and water into the concrete matrix.

Effect of Water/Binder Ratio: Hay and Celik [23] emphasized that as the w/b ratio increases from 0.40 to 0.60, the pore structure of LC3 demonstrates enhanced resilience against deterioration compared to that of OPC. This enhanced resilience is directly attributed to the capacity of carboaluminate phases and pozzolanic reaction products to fill the expanding capillary voids. Microstructural investigations visually validate this densification mechanism. The hydration of LC3 generates a highly compact structure where the pore size is significantly reduced. As observed in the scanning electron microscopy (SEM) images, the matrix features distinct clusters of calcium aluminum silicate hydrate (C-A-S-H) gels alongside hexagonal platelet portlandite crystals. Furthermore, the continuous precipitation of plate-like carboaluminates combined with columnar ettringite crystals growing directly from the C-A-S-H effectively bridges the gaps between particles. This synergistic formation of space-filling hydration products physically confirms how LC3 systems mitigate porosity and ensure a dense microstructural morphology, as clearly illustrated in Figure 9. The morphological evidence in Figure 9 confirms that the durability of LC3 is not solely dependent on the presence of individual phases but on their spatial arrangement. The interlocking network of plate-like carboaluminates and needle like ettringite crystals acts as a structural reinforcement within the capillary voids. This intertwined growth pattern effectively reduces the mean free path for diffusing ions, thereby providing a physical explanation for the low permeability documented in ternary systems. The visual presence of these minerals growing directly from the gel phases supports the conclusion that chemical synergy leads to a more continuous and resilient microstructure compared to binary blends.

4. Fresh and Mechanical Properties

The industrial viability of LC3 systems hinges on achieving an appropriate balance between fresh-state workability governed by rheological behavior and hardened state mechanical performance particularly compressive strength. The physical and mineralogical characteristics of calcined clays render this balance considerably more complex than that encountered with OPC.

4.1. Rheology and Workability

The most distinctive characteristic of binder systems incorporating calcined clay is their increased water demand and reduced workability relative to OPC. This behavior originates from the high specific surface area (SSA) and layered particle morphology characteristic of calcined clays.

4.1.1. Water Demand and Yield Stress

Calcined clays, particularly metakaolin, exhibit a pronounced capacity for water absorption. In a study examining the rheological effects of different clay types, Sposito et al. [109] reported that calcined clays significantly increase both yield stress and plastic viscosity. The surface charge commonly quantified as zeta potential combined with the plate-like particle shape of the clays amplifies interparticle friction thereby reducing flowability. Hay and Celik [23] demonstrated that even when the water-to-binder ratio is increased from 0.25 to 0.60, LC3 systems consistently exhibit higher shear stress than OPC, resulting in an elevated demand for superplasticizer.

Beyond immediate water demand, the time-dependent structural build-up of the paste reveals further differences between OPC and LC3 systems. Experimental measurements of oscillatory stress amplitude conducted by Ez zaki et al. [60] illustrate the evolution of critical shear stress and elastic modulus over varying resting times. As detailed in

Table 3. Rheological properties showing the evolution of critical shear stress (τ_cr), elastic modulus (G′), and corresponding strain values for OPC (K) and LC3 pastes over varying resting times [60].

Binders	Rheological Properties	0 min	Resting Time 30 min	60 min
K	τcr (Pa)	0.17	7.91	54.83
	γcr (%)	3.10 × 10−3	3.11 × 10−3	5.63 × 10−3
	G′ (Pa)	5.27 × 103	2.52 × 105	9.50 × 105
	γco (Pa)	3.64 × 10−2	3.20	10.20
LC3	τcr (Pa)	0.12	11.83	32.61
	γcr (%)	3.30 × 10−3	5.60 × 10−3	9.82 × 10−3
	G′ (Pa)	5.94 × 103	2.08 × 105	9.87 × 105
	γco (Pa)	2.14 × 10−2	5.78	10.50

, both OPC (denoted as K) and LC3 initially exhibit comparable low critical shear stress values upon mixing. However, after 30 min of rest, LC3 demonstrates a more rapid initial structuration with a higher critical shear stress of 11.83 Pa compared to 7.91 Pa for OPC. This early structural build-up in LC3 is balanced by the combination of the highly reactive calcined clay and the dilution effect of limestone. Conversely, at 60 min, OPC surpasses LC3 in critical shear stress reaching 54.83 Pa. This later stage domination in OPC is attributed to the rigidification of C-S-H bridges and an earlier initial setting time. Despite these differences in yield stress evolution, the elastic modulus values of both binders increase significantly and reach comparable levels at 60 min. The comparable elasticity in LC3 is maintained by a complex interplay of chemical reactions from aluminates, the dilution effect of limestone, and the strong flocculation network established by the calcined clays.

4.1.2. Superplasticizer Compatibility

The effectiveness of conventional PCE-based superplasticizers is substantially diminished in LC3 systems owing to the presence of calcined clays.

Adsorption Mechanism: Rather than dispersing cement particles, PCE molecules tend to adsorb onto the extensive surface area of calcined clay. Akhlaghi et al. [110] and Ren et al. [13] reported that clays effectively “consume” PCE polymers through intercalation or excessive surface adsorption, thereby impairing the steric repulsion forces responsible for dispersion. This phenomenon necessitates the use of higher admixture dosages compared to OPC to achieve adequate workability.

Solution Strategies: To specifically mitigate these rheological bottlenecks, the industry has adopted targeted strategies focusing on both admixture design and addition timing. Modifying the timing of superplasticizer addition during the mixing process physically prevents the immediate consumption of the polymers by the highly reactive clay. This delayed addition method allows the molecules to selectively adsorb onto the clinker phases to effectively initiate steric repulsion. In addition to timing modifications, the development of specific clay-mitigating admixtures offers a permanent chemical solution. Utilizing sacrificial agents or engineering tailored polymers with extremely high side-chain densities neutralizes the electrostatic affinity of the lamellar clay particles. These customized admixtures restore the required workability without demanding excessive water content. Ren et al. [13] demonstrated that the sequence of addition of PCE and protein-based retarders (PAR) is critical for optimizing rheological performance. The “delayed addition” (DA) method minimizes flowability loss by promoting the adsorption of PCE onto clinker phases rather than onto clay surfaces. Furthermore, Moghul et al. [29] emphasized that sulfate balance significantly affects PCE adsorption and that maintaining an optimal sulfate level enhances admixture efficiency. While the current literature heavily focuses on delayed addition protocols to mitigate flow loss, a critical synthesis of these studies reveals significant operational limitations for large-scale ready-mix plants. Relying solely on addition sequencing is merely a temporary operational fix rather than a fundamental material solution. The actual bottleneck remains the molecular architecture of conventional PCEs which are designed for pure Portland cement. A meaningful advancement requires shifting the research focus from manipulating mixing procedures to developing entirely new clay-specific polymers with customized side-chain densities. Recent computational studies strongly support this approach, demonstrating that modifying the specific ionic interactions and molecular architecture of the gel phase fundamentally governs the adsorption dynamics of these chemical admixtures [111]. The impact of different addition sequences on polymer adsorption is illustrated in Figure 10.

4.2. Strength Development

The strength development of LC3 concretes is governed by the time-dependent contributions of clinker hydration, the filler effect, and the pozzolanic reaction.

4.2.1. Early-Age Strength (1–3 Days): Clinker and Limestone Effect

Reducing the clinker content to 50 percent in LC3 systems typically results in a strength reduction compared to OPC during the first 1 to 3 days, attributable to the dilution effect. However, this reduction is less pronounced than theoretically anticipated.

Nucleation: Finely ground limestone and calcined clay particles provide additional surface area for the nucleation and growth of hydration products, particularly C-S-H, thereby accelerating the reaction of clinker phases, most notably C₃S. Avet et al. [5] demonstrated that this effect partially compensates for the reduction in clinker factor.

Fineness Effect: Ferreiro et al. [22] reported that increasing the fineness of limestone significantly enhances early-age strength development, although this improvement requires careful optimization as it also elevates water demand. Furthermore, X. Li et al. [59] showed that strategies such as C-S-H seeding can elevate early strength levels comparable to OPC even in systems with very low clinker content (40 percent).

A macroscopic review of current mechanical optimization strategies exposes a counterproductive trend within the research community regarding early-age performance. The relentless pursuit of matching the 24 h compressive strength of pure OPC forces researchers to resort to extreme grinding of clays or the incorporation of costly nano additives. This approach fundamentally contradicts the low energy and cost-effective rationale of LC3 technology. A meaningful paradigm shift requires the construction sector to recalibrate its early-strength expectations and adopt structural design codes that accommodate the natural kinetic development of ternary binders.

To address the practical limitations of early-age strength targets, several performance-based acceptance alternatives are proposed for LC3 systems. Maturity-based monitoring utilizes the high temperature sensitivity of calcined clays to estimate real-field strength more accurately than standard laboratory cylinders [94]. Another critical indicator is the 3-to-7-day strength gain ratio. A high relative increase during this window confirms the successful triggering of the alumina–carbonate synergetic peaks and provides a more reliable structural milestone than the absolute 24 h value [100]. Furthermore, early-age durability markers such as surface electrical resistivity or ultrasonic pulse velocity (UPV) measurements offer superior insights. These non-destructive metrics capture the rapid pore refinement and matrix densification of LC3 as early as 3 days, serving as better proxies for long-term service life than early compressive strength [63,78,107]. Implementing these alternatives allows contractors to adjust formwork removal schedules based on microstructural maturity rather than rigid clinker-centric time windows [112].

4.2.2. Late-Age Strength (28+ Days): The Pozzolanic Contribution

From 7 days onwards, the pozzolanic reaction of calcined clay becomes a dominant contributor to strength gain. Alumina and silica released from metakaolin react with portlandite (CH) to form additional C-A-S-H gel. More importantly, the carboaluminate phases, Mc and Hc, generated by the reaction between alumina and limestone fill capillary voids, thereby densifying the matrix. As confirmed by Avet et al. [5], the mechanical strength achieved after 7 curing days is directly related to the amount of calcined kaolinite present in the blend. Systems incorporating high-quality calcined clay with metakaolin addition can demonstrate greater mechanical strength than pure OPC control mortars after just 7 days. However, for blends containing high amounts of calcined kaolinite exceeding 45 percent, the rate of strength gain tends to become less significant after this initial period. Avet et al. [5], and Dhandapani et al. [78] confirmed that typical LC3-50 mixtures achieve compressive strengths equivalent to or exceeding those of OPC (CEM I) at 28 days.

Long-Term Performance: Zunino and Scrivener [18] reported, based on microstructural analyses extending to 3 years, that both the pozzolanic reaction and carboaluminate formation continue over the long term, sustaining progressive strength gain. The ultimate performance is highly adaptable depending on the clay quality. Comparative tests reveal that even systems utilizing clays without metakaolin addition can perform adequately, achieving structural performance equivalent to 85 percent of the OPC control after 90 curing days. Meanwhile, optimally designed LC3 systems with appropriate kaolinite content easily match or exceed conventional cement performance in the long term. The comparative compressive strength development of these systems over extended curing periods is presented in Figure 11.

4.2.3. Comparative Performance with OPC

Studies have demonstrated that LC3 functions not merely as a filler cement but as a high-performance binder in its own right. LC3 concretes have been observed to exhibit a higher flexural-to-tensile strength ratio compared to OPC, an attribute attributed to the formation of a dense interfacial transition zone (ITZ) and improved microstructure [15]. This microstructural densification becomes especially critical when LC3 is utilized in advanced composites like ultra-high-performance concrete, where optimizing the physical bond at the fiber and matrix interface is essential for achieving exceptional mechanical toughness [114]. Although modulus of elasticity values broadly similar to those of OPC concretes are typically obtained, some variation may occur depending on the clay type and the water-to-binder ratio of the mixture [5].

The micropore structure of the cementitious matrix is one of the key factors influencing this macroscopic mechanical performance. Microstructural analyses reveal that with the incorporation of calcined clay and limestone, the pore size distribution peak shifts towards significantly smaller pore size regions compared to the reference binder systems. The remarkable pore refinement underlying these advanced mechanical properties is clearly illustrated in Figure 12. The significant reduction in the critical pore diameter shown in Figure 12 confirms that the synergistic interaction between calcined clay and limestone goes beyond a physical filler effect. This shift towards the left in the intrusion curves indicates that the formation of carboaluminate phases and additional calcium aluminum silicate hydrate gel effectively subdivides the capillary voids into smaller and less connected pores. This structural refinement is the primary driver for the high chloride resistance of LC3, as the reduced pore size significantly increases the tortuosity of the matrix. Furthermore, the stabilization of this refined network explains why LC3 concretes maintain structural integrity and exhibit higher flexural performance even at high levels of clinker substitution.

5. Durability Performance

The sustainability of LC3 systems is evaluated not only on the basis of reduced CO₂ emissions during production but also through the performance they exhibit over their service life. The literature indicates that the durability profile of LC3 is characterized by a dual nature: while it demonstrates superior performance compared to OPC in aspects such as chloride impermeability and ASR resistance, it tends to be more vulnerable to carbonation owing to its reduced clinker content.

5.1. Chloride Resistance

The most unequivocal advantage of LC3 technology lies in its exceptional resistance to chloride ion penetration. This resistance stems primarily from physical refinement of the pore structure rather than from chemical binding mechanisms.

Diffusion Coefficient and Impermeability: Comprehensive studies by Maraghechi et al. [63] and Scrivener et al. [79] reported that the apparent chloride diffusion coefficient of LC3 systems is up an order of magnitude lower than that of OPC. This finding confirms that despite a 50% reduction in clinker content, LC3 constitutes an ideal binder for marine environments. Ram et al. [46] further determined that even in LC3 concretes produced with low-grade clays containing as little as 18% kaolinite, chloride migration resistance is at least 50% higher than in equivalent OPC systems.

Chloride Binding Capacity: Examination of the mechanism underlying chloride resistance reveals that the formation of Friedel’s salt (C₃A⋅CaC_l2⋅10H₂O) resulting from the reaction between alumina and limestone in LC3 plays a contributing role. Recent experimental evaluations provide critical temporal nuances to this mechanism. As depicted in Figure 13, LC3 pastes exhibit a significantly higher initial chloride binding rate compared to OPC, reaching their maximum binding capacity within the first 14 days of hydration. This rapid early binding is highly advantageous in practical engineering applications as it quickly reduces the corrosion risk for internal steel reinforcement. However, during prolonged hydration up to 56 days, the ultimate binding capacity of LC3 stabilizes while OPC continues to rise and eventually surpasses LC3. This temporal behavior aligns perfectly with the conclusions of Avet and Scrivener [116] and Sui et al. [117], who emphasized that the long-term superiority of LC3 is not derived from an indefinitely greater chemical binding capacity. Rather, the principal defense mechanism lies in the physical impedance of ion transport due to the massive reduction in critical pore diameter and the increase in tortuosity of the pore network.

Seawater Effect and Self-Healing: G. Huang et al. [81] demonstrated that LC3 composites modified with crystalline admixtures exhibit self-healing capabilities when exposed to seawater through the formation of brucite and aragonite within cracks, thereby reducing chloride ingress by up to 42%. While the literature unanimously agrees on the superior chloride resistance of LC3, a critical evaluation of the prevailing testing methodologies reveals a widespread over reliance on rapid electrical methods such as RCPT (ASTM C1202 [118]). Because LC3 systems significantly alter the pore solution chemistry by binding alkalis, their intrinsic electrical conductivity drops drastically. Consequently, electrical-based tests may disproportionately exaggerate the physical impermeability of the matrix. To bring deeper insights into actual field performance, future research must pivot away from rapid electrical indicators and prioritize long-term natural bulk diffusion tests to establish accurate service life predictions.

5.2. Carbonation

Carbonation is widely regarded as the most vulnerable aspect of LC3 systems. This vulnerability arises from a reduction in pH buffering capacity. The lower clinker content and the continuous consumption of portlandite (Ca(OH)₂) through pozzolanic reactions both contribute to this issue.

Accelerated Carbonation: Jan et al. [119] reported that LC3-70 significantly enhances durability relative to CEM II, exhibiting a 40–60% reduction in chloride penetration depth and nearly 50% lower soluble chloride content. The binder also demonstrated superior resistance to both natural and accelerated carbonation. Although a slight reduction in performance was observed when recycled sand was employed, LC3-70 maintained overall superiority, confirming its effectiveness as a binder for extending the service life of sustainable recycled aggregate concrete. The comparative carbonation depths are illustrated in Figure 14.

Khan et al. [120] and Zunino and Scrivener [95] confirmed that LC3 concretes carbonate more rapidly than OPC when exposed to atmospheric CO₂ ingress, a consequence of their lower calcium reserve, which allows the carbonation front to advance more quickly. However, the implications of this behavior for reinforcement corrosion remain a subject of debate. Pillai et al. [121] argued that even if the carbonation front reaches the reinforcement in LC3 concretes, the combination of very high electrical resistivity and dense microstructure means that while the “initiation” time of corrosion may be relatively short, the “propagation” rate is negligibly slow. Consequently, in terms of total service life, LC3 can compete with, or surpass, OPC in many scenarios.

To explicitly reconcile the apparent contradiction between high chloride resistance and high carbonation susceptibility, it is necessary to distinguish between the physical pore structure and the electrochemical properties of the matrix. In chloride-rich marine environments, the service life prediction is strictly dominated by the initiation phase. Here, the highly refined pore network and increased tortuosity physically block the ingress of chloride ions. Conversely, in carbon dioxide-rich urban environments, the physical pore refinement cannot fully compensate for the reduced chemical buffering capacity caused by lower portlandite levels. This causes the carbonation front to advance rapidly and shortens the initiation phase. However, once the reinforcement is depassivated, the service life prediction becomes entirely dominated by the electrochemical propagation phase. At this stage, the extremely high electrical resistivity of the LC3 matrix restricts the flow of ionic currents between anodic and cathodic sites along the steel. This high electrical impedance physically stifles the corrosion reactions, ensuring structural integrity even after the carbonation front has reached the reinforcement.

Dhandapani et al. [78] further reported that carbonation resistance in LC3 systems is more sensitive to curing duration than in OPC and that inadequate curing, particularly less than 7 days, severely degrades performance. A critical evaluation of the existing durability literature uncovers a significant methodological inconsistency. Most researchers rely heavily on accelerated carbonation tests utilizing exceptionally high CO₂ concentrations. These extreme conditions disproportionately penalize LC3 systems by forcing rapid portlandite depletion and completely failing to capture the protective nature of its refined pore structure under natural exposure. Therefore, drawing direct service life conclusions based purely on these accelerated laboratory models leads to overly pessimistic predictions. A true comparative analysis dictates that future carbonation models must be calibrated specifically for the unique phase assemblage of ternary binders.

To establish a reliable testing framework, researchers must adopt methodologies that closely mimic natural exposure. It is highly recommended to limit carbon dioxide concentrations in accelerated tests to a maximum of 1 percent. Using higher concentrations fundamentally alters the chemical degradation mechanism by forcing the artificial decomposition of calcium aluminosilicate hydrate gels and ettringite. Furthermore, accelerated testing protocols must be coupled with field data from natural weathering sites to derive accurate time-dependent carbonation coefficients. Implementing a dual testing strategy combining low-concentration-accelerated chambers with long-term outdoor exposure provides the most scientifically robust approach for predicting the true service life of limestone-calcined clay cement structures.

To comprehensively address the long-term durability implications of carbonation in reinforced concrete, it is critical to evaluate the structural performance beyond the simple measurement of carbonation depth. Although the carbonation front in LC3 systems may reach the steel reinforcement earlier than in OPC concrete, the subsequent corrosion propagation is severely restricted. The highly dense and electrically resistive nature of the carbonated LC3 matrix physically chokes the corrosion current. As a result, the cross-sectional area of the steel reinforcement remains largely intact and the ultimate load bearing capacity of the structural element is preserved over its intended design life. To proactively mitigate the initial carbonation susceptibility, specific construction strategies must be implemented. Extending the initial moist curing period beyond the standard seven days ensures that the pozzolanic reactions achieve maximum pore refinement before atmospheric exposure. Additionally, adjusting the concrete cover depth based on predictive service life models rather than outdated prescriptive codes provides a simple yet effective physical barrier. The application of penetrating silane sealers or hydrophobic surface treatments during the construction phase also offers a highly reliable defense by repelling the moisture required for the carbonation reactions to proceed [78,120,121].

5.3. Alkali–Silica Reaction (ASR)

LC3 offers an effective solution for regions where aggregates susceptible to ASR are employed. Nguyen et al. [122] provided a detailed account of the mechanism by which calcined clay mitigates ASR. Alumina derived from calcined clay reduces the alkalinity of the pore solution by binding alkalis, primarily sodium and potassium, and incorporating them into the C-A-S-H structure. Furthermore, the presence of aluminum inhibits silica dissolution through the formation of a passivation layer on the surface of reactive silica particles [4].

Scrivener et al. [100] demonstrated, through accelerated mortar bar tests (AMBT) conducted with highly reactive aggregates, that LC3 mixtures maintained expansion below critical thresholds, whereas OPC mixtures exceeded these limits. This mitigating effect is maximized at a clinker substitution level of 50%, corresponding to the LC3-50 formulation.

5.4. Sulfate Resistance

The performance of LC3 systems under sulfate attack is governed by the interplay between pore structure refinement and phase transformations within the binder. C. Yu et al. [41] reported that LC3 mortars exhibited less expansion and lower mass than OPC mortars when immersed in sulfate solution. This improved behavior is primarily attributable to the dense, impermeable microstructure that impedes the ingress of sulfate ions. From a chemical perspective, although it has been hypothesized that carboaluminate phases (Mc) formed during LC3 hydration could transform into ettringite under sulfate attack, potentially inducing expansion, Sun et al. [57] demonstrated that such transformation can occur without generating significant crystallization pressure, thereby allowing the system to remain dimensionally stable. The sulfate penetration profiles illustrating this resistance are presented in Figure 15.

Thaumasite Risk: Skaropoulou et al. [123] noted that cements containing limestone may be susceptible to the thaumasite form of sulfate attack (TSA) when exposed to sulfates at low temperatures. However, the incorporation of calcined clay substantially mitigates the risk relative to binary systems containing limestone alone by lowering the Ca/Si ratio of the C-S-H gel and increasing the alumina content. Furthermore, the physical characteristics of the pore network play a decisive role in defining this resistance. Recent microstructural evaluations reveal a critical nuance regarding the use of low-grade clays in LC3 systems. MIP measurements demonstrate that LC3 mixtures can exhibit a higher total accessible porosity and a greater total mercury intrusion volume compared to reference Portland cement (CEM I) and CEM II systems. However, despite this higher overall pore volume, the threshold and critical pore entry diameters in LC3 are significantly finer. The skewness of the differential intrusion curves is visibly lower, confirming a superior level of pore refinement. This unique microstructural evolution ensures that even with elevated total porosity, the refined pore entry diameters effectively block the capillary ingress of harmful sulfate ions. The cumulative pore volume curves demonstrating this higher overall porosity yet fundamentally refined pore structure are shown in Figure 16.

6. From Lab to Field: Scale-Up and Implementation

The fundamental microstructural development and the resulting engineering properties discussed in the preceding sections ultimately serve a single objective: the large-scale industrial implementation of LC3. Following the validation of its superior laboratory performance, LC3 has demonstrated significant potential to catalyze a paradigm shift within the global cement industry. However, the transition from gram-scale laboratory specimens to multi-ton production entails a range of multifaceted challenges, including the adaptation of production technologies, color control, LCA, and international standardization. This section evaluates these critical phases in the industrial scale-up of LC3, drawing on historical developments and recent innovations.

To effectively bridge the gap between laboratory durability predictions and real-world service life, the industrial scale-up of LC3 must integrate advanced data-driven structural health monitoring systems. Implementing continuous condition management strategies similar to recent temperature and displacement warning methods developed for bridge cables or multi-rate data fusion techniques used for structural assessment under extreme environmental loads will provide the dynamic field data necessary to validate the long-term sustainability of LC3 infrastructure [124,125].

6.1. Production Technology

Although a primary advantage of LC3 production lies in the extensive utilization of existing cement manufacturing infrastructure, the calcination process of clay, specifically dehydroxylation, remains thermodynamically distinct from the clinkering process.

6.1.1. Adapting Existing Infrastructure

Initial industrial-scale trials focused on evaluating the feasibility of calcining clays in conventional rotary kilns. Bishnoi et al. [126] reported on pilot productions in India. They demonstrated that existing rotary kilns were successfully modified to calcine clays in the 700 to 850 °C range. This modification yielded a product matching laboratory quality. Similarly, Vizcaíno-Andrés et al. [93], confirmed through industrial trials in Cuba that LC3 produced via rotary kilns exhibited characteristics competitive with those of OPC. Nevertheless, rotary kilns may encounter operational challenges, including clay adhesion to kiln walls and particle agglomeration.

To further bridge the gap between laboratory research and commercial reality, recent years have witnessed the rapid transition of LC3 from pilot trials to full scale industrial production. Major cement manufacturers have successfully integrated LC3 into their commercial portfolios. For instance, large-scale facilities in Colombia are currently producing hundreds of thousands of tons of LC3 annually, which have already been deployed in major infrastructure projects including bridges and high-rise buildings. Similarly, commercial production lines in the Ivory Coast and Ghana are utilizing flash calcination to process local clays on a massive scale. Furthermore, in India, LC3 has been utilized in the construction of numerous real world structures including embassies and commercial complexes, demonstrating its field viability without requiring any modifications to standard construction practices [127].

6.1.2. Flash Calcination: Efficiency and Reactivity

In recent years, flash calcination technology has emerged as a preferred method for enhancing energy efficiency and enabling better control over reactivity. In this process, clay particles undergo dehydroxylation through rapid heating within seconds.

Reactivity and Morphology: Koutsouradi et al. [40] conducted a contemporary study examining soak (rotary kiln) and flash calcination methods. The findings indicated that flash calcination prevents the agglomeration of clay particles and generates amorphous structures with a higher specific surface area. The study revealed that flash-calcined clays exhibit pozzolanic reaction rates, assessed via the R³ test, compared to or exceeding those of rotary kiln products. A critical synthesis of production methodologies reveals a significant trade-off that is frequently overlooked in the literature. While flash calcination successfully prevents particle agglomeration and maximizes pozzolanic reactivity, the resulting irregular particle morphology directly exacerbates the rheological bottlenecks discussed in Section 4. Therefore, future industrial applications must treat calcination parameters and superplasticizer design not as isolated variables but as a coupled system where maximizing chemical reactivity may inversely compromise fresh-state workability. The distinctive microstructural differences arising from these two methods are evident in the SEM images presented in Figure 17.

To quantify this structural trade-off, comparative data demonstrate that flash-calcined clays typically exhibit specific surface areas between 15 and 25 m²/g along with highly irregular and crumpled particle morphologies. In contrast, traditional soak calcination produces more regular particles with lower surface areas in the range from 10 to 15 m²/g. This morphological difference translates directly into rheological penalties. Pastes incorporating flash-calcined clays can exhibit yield stress values up to 50 percent higher than those utilizing rotary calcined equivalents. To mitigate this severe workability loss without sacrificing the enhanced reactivity, several practical pathways are proposed. Utilizing tailored PCE polymers with higher side-chain densities effectively counteracts the adsorption affinity of the irregular clay surfaces. Adjusting the sulfate balance by optimizing the gypsum dissolution rate provides an early rheological buffer. Implementing delayed admixture addition sequences physically prevents the premature consumption of the polymer by the highly reactive flash-calcined particles.

Historical Context: Although the effects of flash calcination on clays were investigated as early as the 1990s by Salvador [128], research on the integration and energy optimization of this technology within LC3 systems has accelerated in recent years [92]. San Nicolas et al. [129] emphasized the industrial potential of this method at an early stage by examining the performance of flash-calcined metakaolin in concrete.

6.2. Color Control

A significant aesthetic constraint affecting the market acceptance of LC3 is the transformation of iron-bearing minerals, such as goethite and pyrite, present in kaolinitic clays into hematite (Fe₂O₃) during calcination. This phase change imparts a characteristic red or pink hue to the resulting cement. Scrivener et al. [79] noted that this issue can be effectively addressed by controlling the calcination atmosphere. When calcination is conducted under reducing conditions, or when oxygen ingress is prevented during the cooling phase, the red hematite is converted to black magnetite (Fe₃O₄). This modification enables the production of LC3 with a grey color comparable to that of conventional OPC. The fundamental mechanism governing these iron phase transformations during thermal treatment is illustrated in Figure 18. Importantly, this color-control strategy does not adversely affect the mechanical performance of the concrete, yet it remains essential for architectural applications and consumer perception.

6.3. Sustainability and LCA

The fundamental drivers underlying the development of LC3 technology are its environmental and economic advantages, which have been substantiated through quantitative LCA studies.

CO₂ Reduction Potential: Early research by Gartner [131] and Damtoft et al. [132] underscored the necessity of clinker substitution as a primary strategy for reducing carbon emissions in the cement industry. Drawing on more recent data, Kanagaraj et al. [26] confirmed that LC3 production results in 30 to 40% lower CO₂ emissions compared to OPC. This reduction is achieved by limiting the clinker content to 50% and conducting clay calcination at lower temperatures (approximately 800 °C) relative to clinker production (1450 °C). The significant reduction in both emissions and energy consumption is detailed in Figure 19. Furthermore, evaluating the binder based on its mechanical performance reveals an even more distinct environmental advantage. Recent comprehensive assessments by Haverkamp et al. [133] demonstrate the superior carbon efficiency of LC3 systems when environmental impacts, particularly climate change indicators, are evaluated relative to compressive strength. Their data confirm that increasing the OPC substitution rate up to 50% not only elevates the mechanical strength but drastically reduces the relative climate change impact per MPa. The comparative carbon efficiency normalized by compressive strength for different substitution rates is clearly illustrated in Figure 20.

A broader comparison of the recent literature reveals that normalized climate change outcomes often exhibit notable disparities across different studies. These variations can be attributed to several factors including specific mix designs, the use of low-kaolinite clays which yield lower compressive strength, the incorporation of recycled materials, and the types of admixtures used. Additionally, variations in energy consumption and fuel types across different geographical contexts significantly contribute to these disparities. Despite these differences, the primary environmental hotspots consistently remain the production of OPC and calcined clay. Furthermore, an analytical review of current LCA studies highlights major methodological limitations. Comparisons across various environmental impact categories are frequently not feasible due to the use of differing impact assessment methods. The vast majority of environmental evaluations are restricted to cradle-to-gate boundaries. By only accounting for production emissions, these studies fail to capture the cradle-to-grave implications of the unique durability profile of LC3. A truly comprehensive sustainability synthesis must mathematically offset the initial carbon savings against the potentially extended service life in chloride rich marine environments along with the risks of premature carbonation. Future research must bridge this gap by integrating time-dependent durability models directly into LCA calculations.

Three-dimensional Printing and Advanced Applications: Jin et al. [99] conducted an LCA specifically for 3D concrete printing applications, demonstrating that the global warming potential (GWP) of LC3-based inks is markedly lower than that of reference cements. In a complementary study, Arruda Junior et al. [134] provided regional data supporting the environmental benefits of utilizing waste clays from the Amazon region for LC3 production.

Cost Analysis: Sánchez Berriel et al. [135] estimated that, in the specific context of Cuba, LC3 production costs could be 15–25% lower than those of OPC. These savings are realized through reduced energy requirements and the ability to exploit widely available low-grade clays that are not suitable for other industrial applications.

Beyond these regional estimates, recent global financial models confirm that the industrial adoption of LC3 presents a highly compelling business case. Due to the significantly lower calcination temperature of clays compared to clinker and the complete elimination of limestone decarbonation energy, plants transitioning to LC3 can achieve operational cost savings of up to 33 percent. These cost reductions are heavily amplified in regions with high imported fuel costs. The combination of reduced energy expenses, lower clinker factors, and the utilization of widely available raw materials ensures rapid investment payback and makes LC3 a financially superior alternative to traditional Portland cement on a global scale [136].

6.4. Standardization

The integration of LC3 into existing regulatory frameworks is a fundamental prerequisite for its global commercial viability.

Antoni et al. [19] provided the scientific foundation for standardization by demonstrating the synergistic effect of combined calcined clay and limestone substitution. Although Sabir et al. [137] had investigated the use of metakaolin in cementitious systems as early as the 2000s, the formal inclusion of ternary blends into standards has been a gradual process.

Current Status: The European standard EN 197-5 (2021) [138] now permits ternary blends of the LC3 type by allowing clinker substitution up to 50% under the Portland-composite cement (CEM II/C-M) classification. In the United States, ASTM C595 accommodates such utilization under the Type IT (Ternary Blended Cement) designation. The strategic alignment between the geographical distribution of SCMs resources and regions of high cement demand is illustrated in Figure 21. While fresh fly ash supplies are declining, the potential for utilizing reclaimed resources such as landfilled coal ash is gaining attention as a secondary pathway for resource security. Such alternative materials must be integrated into future regulatory frameworks to ensure long term industrial viability.

Testing Methods: The R³ test (ASTM C1897), developed to provide a rapid and reliable assessment of the pozzolanic reactivity of clays, has become an indispensable tool for industrial quality control [139]. In a recent RILEM TC-282 CCL committee report, Kanavaris et al. [112] emphasized the need for standards to transition from prescriptive-based to performance-based approaches. This regulatory evolution is particularly supportive of emerging technologies such as 3DCP, as demonstrated by the applications shown in Figure 22. Furthermore, the current regulatory landscape reveals a critical disconnect. Existing concrete codes heavily rely on compressive strength as a universal proxy for durability. However, the microstructural synthesis of LC3 demonstrates that its durability stems from physical pore refinement rather than absolute matrix strength. Consequently, applying traditional strength-based durability specifications to LC3 systems is fundamentally flawed and severely underestimates their potential in aggressive exposure conditions.

7. Conclusions and Future Outlook

While the fundamental feasibility of LC3 as a low-carbon binder is now widely recognized, this review demonstrates that the transition to global industrialization requires a critical reevaluation of existing research paradigms. By synthesizing the ongoing contradictions between microstructural potential and macro-scale operational bottlenecks, this paper transcends conventional performance summaries. The following conclusions explicitly outline the systemic flaws in current formulation strategies and testing methodologies, thereby providing a fully justified scientific roadmap for the next generation of LC3 optimization and sustainable infrastructure development.

7.1. Summary of Current Knowledge

Rather than reiterating the established facts of clinker substitution and basic mechanical parity, this review has synthesized the underlying contradictions within current LC3 research to generate new scientific insights for sustainable implementation. First, the critical evaluation establishes that the overarching challenge in LC3 optimization is no longer achieving early-age strength. The actual technical bottleneck is resolving the inherent trade-off between the rheological demands of high-surface-area clays and the fundamental chemical limitations of traditional superplasticizers. Second, this review exposes a systemic bias in prevailing durability testing methodologies. The apparent vulnerability of LC3 to carbonation and its exceptional chloride impermeability are frequently distorted by accelerated laboratory protocols that fundamentally fail to capture the long-term protective nature of its refined pore network under natural exposure. Third, the synthesis of large-scale production data reveals an unaddressed structural conflict where maximizing clay pozzolanic reactivity through flash calcination inversely compromises the fresh-state workability of the concrete due to irregular particle morphology.

Ultimately, this review demonstrates that LC3 is rapidly advancing beyond a subject of laboratory curiosity, demonstrating strong potential as an industrially scalable solution for deep decarbonization, although widespread commercial viability remains subject to resolving local supply chain constraints. The critical synthesis of recent data confirms that overcoming the remaining rheological and standardization barriers requires an immediate shift from empirical mix designs to performance-based approaches. The novelty and significance of this review lie in its holistic synthesis of the competing mechanisms defining LC3 performance. By critically comparing microstructural data with industrial scale-up challenges, this paper demonstrates that the primary technical barrier is no longer early strength development or chloride impermeability. Instead, the ultimate challenge is the inherent conflict between rheological demand and long-term carbonation susceptibility. This review provides a new conceptual framework indicating that the next decade of LC3 research must pivot away from basic microstructural characterization and focus entirely on green chemical admixture innovation and the establishment of exposure-specific performance standards.

7.2. Critical Research Frontiers and Gaps

Despite the emerging large-scale applications of LC3, several fundamental areas require further investigation to reflect current uncertainties and ensure its global versatility and broader industrial adoption within circular economy frameworks. Although challenges such as high water demand, susceptibility to carbonation, and the variable reactivity of low-grade clays are widely acknowledged across the global research community, the prevailing mitigation strategies remain largely empirical and short sighted. Acknowledging a problem is not equivalent to solving it. Overcoming these well-documented bottlenecks requires a fundamental departure from conventional Portland cement logic. The future of LC3 research must transcend simple material characterization and address the following critical frontiers with entirely new methodologies.

While the necessity of utilizing low-grade clays and multi-mineralogical waste streams is a well-recognized imperative for global sustainability, current research stagnates at simply documenting their lower reactivity. The actual critical gap is the absence of predictive thermodynamic models capable of quantifying the activation potential of mixed clay systems. Future studies must abandon basic trial and error characterization and instead focus on targeted mechanochemical activation techniques to elevate the reactivity of these complex mineral assemblies. The high specific surface area and layered morphology of calcined clays lead to excessive adsorption of PCE-based superplasticizers. There is a clear need for molecular-level studies to develop clay-tolerant polymers or sacrificial agents capable of mitigating workability loss without increasing chemical admixture costs or environmental footprints. Furthermore, most durability assessments currently rely on accelerated laboratory protocols. Obtaining 10-to-20 year field data from LC3 structures exposed to diverse climatic conditions is essential to establish long-term industrial confidence and refine holistic life cycle assessment models.

7.3. Future Recommendations

The expansion of LC3 technology over the next decade is expected to align closely with the parallel imperatives of digital construction and circular economy frameworks. While initial studies have successfully demonstrated the basic extrudability of LC3, simply proposing its use in 3D concrete printing is no longer a forward-looking agenda. The true scientific frontier lies in mastering the dynamic rheo physical control of these complex ternary systems to maximize material efficiency. Future digital construction research must pivot towards active rheology manipulation, such as using targeted chemical accelerators at the printhead to instantly trigger the aluminate reaction for rapid structural build-up. The high water retention of calcined clays drives differential desiccation shrinkage at the layer interfaces. Resolving this shrinkage and mitigating microstructural anisotropy remain critical unsolved challenges for sustainable automated construction.

Similarly, while the incorporation of LC3 into UHPC and ECC is gaining traction, simply substituting clinker in these matrices is scientifically inadequate. The critical challenge lies in resolving the thermodynamic paradox where the extremely low water-to-binder ratios characteristic of UHPC physically restrict the dissolution of calcined clays and inhibit the full spatial development of carboaluminate phases. Future studies must explore targeted chemical activators to maximize clay reactivity under severe water deficiency. Furthermore, regarding circularity, hybridizing LC3 with functionalized carbon carriers like biochar or recycled aggregates requires moving beyond basic compressive strength testing. The research community must elucidate the complex ITZ chemistry and the competitive adsorption dynamics between these porous additives and the highly reactive LC3 matrix. Ultimately, to facilitate global adoption, standardizing reactivity assessments for diverse clay sources will enable the maximum utilization of local materials, minimize transportation emissions, and remove bureaucratic barriers to universally accessible low-carbon construction.

To ensure the successful global integration of LC3 technology, future research must prioritize three critical pillars. First, admixture design must evolve beyond empirical dosage adjustments to develop tailored polymers that mitigate the high water demand of calcined clays without compromising reactivity. Second, durability modeling needs to transition from accelerated laboratory tests to long-term field calibrated predictions, particularly concerning carbonation and chloride transport. Finally, regarding large-scale implementation, the industry must establish exposure-specific performance standards and optimize calcination protocols to balance pozzolanic reactivity with fresh-state workability. Addressing these specific gaps will transform LC3 from a promising alternative into the foundational binder of sustainable infrastructure.