Innovations and Applications in Lightweight Concrete: Review of Current Practices and Future Directions

Abstract

Lightweight concrete (LWC) has emerged as a transformative material in sustainable and high-performance construction, driven by innovations in engineered lightweight aggregates, supplementary cementitious materials (SCMs), fiber reinforcements, and geopolymer binders. These advancements have enabled LWC to achieve compressive strengths surpassing 100 MPa while reducing density by up to 30% compared to conventional concrete. Fiber incorporation enhances flexural strength and fracture toughness by 20–40%, concurrently mitigating brittleness and improving ductility. The synergistic interaction between SCMs and lightweight aggregates optimizes matrix densification and interfacial transition zones, curtailing shrinkage and bolstering durability against chemical and environmental aggressors. Integration of recycled and bio-based aggregates substantially diminishes the embodied carbon footprint by approximately 40%—aligning LWC with circular economy principles. Nanomaterials such as nano-silica and carbon nanotubes augment early-age strength development by 25% and refine microstructural integrity. Thermal performance is markedly enhanced through advanced lightweight fillers, including expanded polystyrene and aerogels, achieving up to a 50% reduction in thermal conductivity, thereby facilitating energy-efficient building envelopes. Although challenges persist in cost and workability, the convergence of hybrid fiber systems, optimized mix designs, and sophisticated multi-scale modeling is expanding the applicability of LWC across demanding structural, marine, and prefabricated contexts. In essence, LWC’s holistic development embodies a paradigm shift toward resilient, low-carbon infrastructure, cementing its role as a pivotal material in the evolution of next-generation sustainable construction.

1. Introduction

Lightweight concrete (LWC) has increasingly drawn attention due to its significant benefits, including reduced structural weight, enhanced thermal insulation, and sustainability. Traditionally limited to non-load-bearing structures, recent advancements in high-strength lightweight concrete (HSLWC) have significantly expanded its use in structural applications, demonstrating suitability in demanding environments and complex structures [1,2]. LWC has been valued since Roman engineers cast the 44-meter-span Pantheon dome with pumice-rich concrete, proving that strategic density reduction can cut weight without sacrificing strength [2]. LWC has an air-dry unit weight of 400–2000 kg/m³, with structural LWC typically ranging from 1400 to 2000 kg/m³—significantly lighter than normal-weight concrete (2400 kg/m³) [3]. Those mass savings translate into longer bridge spans, lower seismic demand, and lighter prefabricated elements, making LWC attractive for high-rise, bridge, and offshore applications [4,5].

Modern LWC is realized through three complementary strategies. The first—and still dominant—is the aggregate replacement: natural vesicular rocks (pumice, scoria) or manufactured lightweight aggregates (LWAs) such as expanded shale, expanded clay, sintered fly ash pellets, cold-bonded demolition fines, and bio-derived shells. Aggregate density, pore topology, and shape dictate the quality of the interfacial transition zone (ITZ). Carefully graded, low-absorption expanded shale can deliver 28-day strengths above 80 MPa [4], whereas highly porous aggregates drive the non-linear fall-off observed once density drops below about 1600 kg/m³ [6]. The second route is an air-void generation by either chemical gas release (e.g., Al powder, H₂O₂), surfactants, or mechanical pre-foaming. Densities below 800 kg/m³ are achievable, but brittleness and high permeability limit structural use unless fibers or nanomaterials are added [7,8]. The third emerging option is the hybrid filler concept, which disperses ultra-light hollow glass or fly ash microspheres within ultra-high-performance cementitious matrices; when combined with pre-wetted core-shell LWAs, they yield ultra-high-performance LWC with compressive strengths above 120 MPa at densities near 1900 kg/m³ and minimal shrinkage [9].

Lower density alone does not guarantee durability, so matrix optimization is essential. Replacing 5–10% of ordinary Portland cement with silica fume densifies the ITZ, halves chloride diffusivity, and improves freeze–thaw resistance [10], while ternary blends of fly ash, slag, and silica fume (≈10% each) balance slump retention with 60 MPa compressive strength at w/b ≈ 0.42 [11]. End-hooked steel fibers at 0.5–1.0 vol % raise flexural strength by up to 60% and increase impact toughness six-fold [12,13]; carbon fibers further boost splitting tensile strength to around 10 MPa in aggregate concretes [12]. Polypropylene or vinylon fibers mitigate explosive spalling and shrinkage in all-lightweight mixes [10]. Nano-silica refines the capillary network and restores transport-related properties that would otherwise deteriorate as density falls [3].

Sustainability imperatives now drive the incorporation of waste streams and alternative binders. Recycled construction-and-demolition fines sintered into LWAs can replace virgin materials while cutting embodied energy [14]. Bio-based aggregates and ashes—for example, oil palm shell [15] or rice-husk ash [16]—lower density, sequester carbon, and offer closed-loop valorization of agricultural waste if surface treatments restore a robust ITZ [17]. Calcium-sulfoaluminate, magnesium-phosphate, and alkali-activated binders reduce CO₂ emissions while delivering rapid early strength or chemical resistance [18].

Research trajectories are converging on three fronts. First, engineered core-shell LWAs combined with ultra-high-performance pastes promise stiffness-to-weight ratios that surpass NWC by 50% while maintaining permeability an order of magnitude lower than conventional LWC [19,20]. Second, multiscale models that couple pore architecture, moisture transport, and reinforcement corrosion are beginning to offer 100-year service-life predictions specifically calibrated for lightweight systems [18]. Third, fully circular supply chains, in which demolition fines are up-cycled into next-generation LWAs and binder carbon footprints are offset through carbonation curing, are moving LWC toward net-zero-carbon construction [14]. These advances reinforced lightweight concrete’s role as a cornerstone material for performance-driven, low-carbon infrastructure.

These rapid developments across materials science, durability engineering, and sustainability frameworks underscore the need for a comprehensive and up-to-date synthesis of lightweight concrete technologies. Although numerous studies have addressed specific components—such as aggregates, aeration techniques, or supplementary binders—few reviews have integrated recent advances in mechanical performance, environmental impact, and practical implementation across all LWC classes. This review, therefore, aims to systematically consolidate recent progress in the design, production, and performance of lightweight concrete, emphasizing materials innovations, structural capabilities, and durability under aggressive exposure. It identifies key knowledge gaps, evaluates sustainability metrics, and outlines future directions for optimizing lightweight concrete in structural and non-structural applications.

2. Materials and Fabrication Technologies

This chapter provides an in-depth overview of the materials and fabrication technologies essential to the design and performance of LWC. It explores how the selection of lightweight aggregates, the integration of supplementary cementitious materials, the use of fibers and fillers, and the adoption of advanced binders such as geopolymers influence the mechanical, thermal, and durability characteristics of LWC. These material strategies are critical for achieving structural efficiency and reduced density and enhancing sustainability, resilience, and constructability in diverse applications. To contextualize these innovations, Table 1 summarizes the fresh and hardened properties of LWC mixes incorporating various lightweight aggregates and SCMs, highlighting key mechanical outcomes and workability features that inform optimal mix design. A summary of LWC materials, fibers, and strategies is shown in Table 1.

2.1. Materials and Methods

Lightweight aggregates (LWA) effectively reduce concrete density while maintaining structural integrity. These aggregates include natural (pumice and scoria) and artificial (expanded shale, clay, slate, and industrial by-products like fly ash) types [14,21,22]. Sustainable aggregates such as oil palm kernel shells (OPKS) have shown comparable mechanical and structural capabilities to traditional aggregates [22]. Recycling construction and demolition waste (CDW) as lightweight aggregates is also gaining prominence, supporting sustainable construction [14]. Meng et al. [4] developed a lightweight ultra-high-strength concrete (L-UHSC) using a combination of ultra-high-performance cement paste, steel fibers, and pre-wetted lightweight aggregates, including fly ash ceramic (FAC) and shale pottery (SP), to achieve both reduced density (1995–2114 kg/m³) and high compressive strength (102.4–114.5 MPa). As shown in Figure 1a,b, the selected lightweight aggregates exhibit distinct morphologies—FAC having a denser internal structure while SP displays a more porous surface—significantly influencing the composite’s pore distribution and interfacial bonding. The study concluded that steel fiber content had the most pronounced effect on mechanical strength, and the integration of fine, pre-saturated LWA enhanced pore refinement and internal curing, leading to a specific strength above 50 MPa/(t/m³), surpassing that of conventional lightweight concrete (Figure 1c).

One of the key components influencing the performance of lightweight concrete (LWC) is the type of aggregate used. Expanded lightweight aggregates are commonly employed due to their low density, improved thermal insulation, and potential to reduce structural dead load. Figure 1d illustrates a lightweight expanded shale aggregate used as a fine aggregate in LWC, characterized by its angular shape and moderately porous internal texture [19]. These aggregates exhibit a precise gradation in pore morphology—from the relatively dense, closed-cell structure of Liaver to the highly porous and interconnected network observed in Ecoglas. Such microstructural differences critically impact water absorption, tortuosity, and overall permeability of the resulting concrete.

According to Chung et al. [5], Liaver demonstrated the most favorable durability due to its encapsulated pore system and low absorption. At the same time, Ecoglas showed the poorest performance because of its open, connected porosity. Liapor offered intermediate behavior. In a related study, Liao et al. [19] reported that incorporating engineered high-strength core-shell lightweight aggregates along with micro steel fibers significantly enhanced the mechanical properties of LWC, achieving compressive strengths up to 87 MPa and fracture toughness values of 1.86 MPa·m^1/2. Although the expanded shale shown in Figure 1d is not thermally expanded in the same manner as the other aggregates, it reflects the practical application of industrial by-products such as fly ash and perlite through energy-efficient cold bonding technology. While lightweight expanded glass aggregates offer the benefit of reduced density, their standalone use in geopolymer mortars often results in lower mechanical performance due to their porous nature. However, the overall behavior can be significantly improved with reinforcing fibers. Al Mohammad et al. [23] investigated the combined effect of lightweight expanded glass aggregates and polyvinyl alcohol (PVA) fibers on the mechanical performance of geopolymer mortars exposed to elevated temperatures. They found that while increasing EG content reduced density and compressive strength, incorporating 1% PVA fibers enhanced compressive, tensile, and flexural performance, particularly at 250 °C, by improving crack resistance and structural integrity. In contrast, Figure 1e–g depict thermally expanded coarse aggregates, including Liapor (expanded clay), Liaver (expanded glass), and Ecoglas (foamed glass), respectively [24].

The internal network of microscopic voids within lightweight aggregates significantly influences their performance characteristics, including strength, heat transfer properties, and water absorption. Within LWA structures, two distinct pore types exist: interconnected surface-reaching channels that allow fluid penetration and sealed internal cavities that prevent liquid movement through their walls. This dual-nature porosity system not only defines the fundamental properties of the aggregates themselves but also substantially impacts the resulting concrete’s structural performance metrics, with higher void content typically corresponding to reduced compressive capacity and elasticity [18,24,25].

2.2. Aerated Concrete

Aerated concrete is lightweight concrete composed of cement- or lime-based mortar in which air voids are uniformly distributed throughout the matrix using an appropriate aerating agent [7]. Aerated concrete can be broadly classified based on the pore-formation mechanism (air-entrainment using gas-forming agents such as aluminum powder or hydrogen peroxide; mechanical foaming via pre-formed or mixed foaming agents; or a hybrid of both methods), the type of binder used (cement-, lime-, or pozzolan-based), and the curing technique applied (non-autoclaved or autoclaved), each of which significantly influences the material’s pore structure, mechanical strength, shrinkage behavior, and overall performance [7]. Generally, depending on the technique used to generate internal porosity, aerated concrete can be categorized as air-entrained or foamed concrete [8,26].

Table 1. Summary of LWC materials, fibers, and strategies.

Material/Strategy	Density (kg/m3)	Compressive Strength (MPa)	Key Benefits	Notable Limitations	References
Natural LWA (pumice, scoria)	1600–1800	Moderate (~15–30)	Low cost, naturally available	Limited strength	[14,21,22]
Artificial LWA (FAC, SP)	1995–2114	102.4–114.5	High strength, microstructure control	Requires fiber for ductility	[4]
Recycled CDW aggregates	Varies (~1600–2000)	15–30	Sustainability, waste reuse	Variable quality	[14]
Engineered LWA (Liaver, Eco glass)	Lightweight (varies by type)	Low–High (depends on type)	Thermal/durability control	Porosity affects durability	[5,19,24]
Aerated Concrete (Al powder, foam)	300–1200	2–10 (non-structural)	Thermal insulation, low weight	Low mechanical performance	[7,8,26]
SCMs (Fly ash, SF, GGBFS, MK)	Depends on mix	Enhanced (up to 90 MPa with blends)	Improved durability and strength	May reduce workability	[6,11,27,28,29]
Fibers (Steel, PVA, PP, Carbon)	800–1800	10–87+	Crack resistance, ductility	Mix design sensitivity	[10,12,13,30,31]

Table 1. Summary of LWC materials, fibers, and strategies.

Material/Strategy	Density (kg/m3)	Compressive Strength (MPa)	Key Benefits	Notable Limitations	References
Natural LWA (pumice, scoria)	1600–1800	Moderate (~15–30)	Low cost, naturally available	Limited strength	[14,21,22]
Artificial LWA (FAC, SP)	1995–2114	102.4–114.5	High strength, microstructure control	Requires fiber for ductility	[4]
Recycled CDW aggregates	Varies (~1600–2000)	15–30	Sustainability, waste reuse	Variable quality	[14]
Engineered LWA (Liaver, Eco glass)	Lightweight (varies by type)	Low–High (depends on type)	Thermal/durability control	Porosity affects durability	[5,19,24]
Aerated Concrete (Al powder, foam)	300–1200	2–10 (non-structural)	Thermal insulation, low weight	Low mechanical performance	[7,8,26]
SCMs (Fly ash, SF, GGBFS, MK)	Depends on mix	Enhanced (up to 90 MPa with blends)	Improved durability and strength	May reduce workability	[6,11,27,28,29]
Fibers (Steel, PVA, PP, Carbon)	800–1800	10–87+	Crack resistance, ductility	Mix design sensitivity	[10,12,13,30,31]

2.3. Supplementary Cementitious Materials (SCMs)

Supplementary Cementitious Materials (SCMs) are increasingly used in lightweight concrete to enhance its properties while reducing the environmental impact of cement production. Common SCMs include fly ash, silica fume, ground granulated blast-furnace slag (GGBFS), and metakaolin. These materials improve lightweight concrete’s workability, durability, and strength and contribute to reducing CO₂ emissions by partially replacing Portland cement. For instance, silica fume is known to significantly enhance the compressive strength and reduce the permeability of lightweight concrete, making it suitable for high-performance applications. Table 1. represents the summary of LWC materials, fibers, and strategies.

SCMs have shown great potential in enhancing the performance of LAC. In their study, Cheng et al. [6] investigated the combined effect of 10% metakaolin (MK) and 10% ground granulated blast furnace slag (GGBFS) on LAC, revealing improved compressive strength, refined microstructure, and a significant reduction in chloride ion penetration. Similarly, Youm et al. [27] reported that incorporating 3.5% silica fume into LAC improved its mechanical strength and durability without adversely affecting workability. Both studies highlight the pozzolanic reaction and filler effect of SCMs as key mechanisms for densifying the cement matrix and interfacial transition zone. Overall, SCMs contribute effectively to improving the durability, strength, and long-term performance of LAC in aggressive environments. Chen et al. [11] investigated the effects of supplementary cementitious materials (SCMs) on the workability and strength of lightweight concrete (LWC). Individually, fly ash improved workability but reduced early strength, while silica fume enhanced strength significantly at the cost of rapid slump loss, and blast furnace slag offered the best balance of strength and rheology. The combination of SCMs—particularly the mix of 10% fly ash, 10% slag, and 10% silica fume—provided the highest compressive strength and a well-balanced performance in fresh and hardened states (Table 1). Xiong et al. [28] discuss the role of supplementary cementitious materials, particularly silica fume (SF), in enhancing concrete performance. Building on conventional high-strength lightweight aggregate concrete (HSLWAC) practices, the study incorporated 8% silica fume to partially replace ordinary Portland cement in all mixes. The authors found that under the vibration mixing process, silica fumes dispersed more effectively, creating additional nucleation sites, which increased the hydration degree by 2.70% at low w/b ratios. SF’s enhanced dispersion and pozzolanic activity contributed to a denser interfacial transition zone (ITZ), improved bond strength, and higher overall concrete performance.

Researchers have increasingly explored the synergy between lightweight aggregates and supplementary cementitious materials to enhance flowability and reduce density, especially in self-compacting applications. For instance, Li et al. [29] studied the design of lightweight aggregate self-compacting concrete (LWASCC) to achieve reduced density and high performance. Recognizing the limitations of relying solely on lightweight aggregates, they emphasized the importance of incorporating supplementary cementitious materials such as fly ash and silica fume to improve packing density, minimize water demand, and enhance long-term strength. Their results showed that an optimal ternary blend of 21% fly ash and 9% silica fume significantly improved the fresh and hardened properties of LWASCC. This combined approach supports the development of lightweight concrete mixes suitable for precast and heavily reinforced applications, where self-compaction, strength, and durability are critical.

2.4. Fibers

Incorporating fibers into lightweight concrete can improve its tensile strength, crack resistance, and overall durability [12,13,30]. Various types of fibers, such as steel, glass, polypropylene, and natural fibers (e.g., basalt, coconut, and sisal), are used for this purpose. Steel fibers are particularly effective in enhancing lightweight concrete’s flexural and impact strength, making it suitable for structural applications subjected to dynamic loads. On the other hand, natural fibers offer an eco-friendly alternative and can improve lightweight concrete’s thermal and acoustic insulation properties. Expanding on this, Wu et al. [12] investigated the effects of both steel and carbon fibers on lightweight aggregate concrete. They found that while compressive strength increased slightly, splitting tensile and flexural strength improved significantly—by over 60% at 0.9% carbon fiber volume. The study also revealed that carbon fibers contributed to a denser microstructure and better crack-bridging capacity, while steel fibers provided superior energy dissipation through pull-out mechanisms. These findings underscore the value of fiber type and dosage in tailoring the mechanical performance of lightweight concrete for specific structural needs. Recent studies have emphasized the role of fiber reinforcement under dynamic loading to address the brittle behavior of ultra-lightweight high-strength concrete (ULHSC). Zhang et al. [13] examined the effects of steel and polypropylene (PP) fibers on the static and dynamic compressive behavior of ULHSC-containing fly ash cenospheres. Their results showed that incorporating 1.0 vol% end-hooked steel fibers significantly improved dynamic compressive strength and impact toughness, achieving the highest specific strength (35.5 kPa/kg·m³) and best failure resistance.

Table 2. Benefits of different fibers in lightweight concrete.

Fiber Type	Key Benefits	Application Notes	References
Steel Fibers	Increases flexural and impact strength	Ideal for structural and dynamic load applications	[12,13,30,31]
	Enhances energy dissipation	Best performance with longer fibers (e.g., 60 mm)
	Improves compressive strength
Carbon Fibers	Improves splitting tensile and flexural strength (>60%)	Suitable for high-performance, crack-resistant concrete	[12]
	Densifies microstructure
	Enhances crack-bridging
Polypropylene (PP) Fibers	Boosts strain-rate sensitivity	Less effective in static strength due to weak bonding	[13]
	Increases energy absorption under impact	Effective for impact resistance
	Improves ductility
Natural Fibers (e.g., basalt, coconut, sisal)	Eco-friendly	Suitable for sustainable and non-structural applications	[12]
	Enhances thermal and acoustic insulation
Vinylon Fibers	Enhances flexural strength and fracture toughness	Effective for improving mechanical performance of all-lightweight concrete	[10]
Steel + Silica Fume (Sand-Wrapping Method)	Increases split tensile strength (25%)	Suitable for structural and impact-resistant applications with lightweight concretes of 800–1800 kg/m3	[31]
	Reduces 90-day drying shrinkage by half

. represents the benefits of different fibers in lightweight concrete.

Furthermore, while PP fibers exhibited lower static strength due to weak bonding, they led to the most significant increase in strain-rate sensitivity and energy absorption under impact, highlighting the influence of fiber type and content on mechanical and microstructural performance. As shown in Figure 2a,d, the plain specimens exhibit brittle failure with extensive cracking and fragmentation, indicating low energy absorption and weak bonding between the matrix and aggregates. According to Figure 2c, the ULHSC–1.0 PP (polypropylene) specimen under 0.4 MPa load shows partial crushing and fiber pull-out but not complete fragmentation like the plain specimen. In contrast, the fiber-reinforced specimens show more cohesive failure surfaces with restrained crack propagation, highlighting the enhanced ductility and crack-bridging ability provided by the fibers (Figure 2b,c,f).

Building on the growing interest in fiber-reinforced lightweight concretes, Aldikheeli et al. [30] investigated the influence of steel fiber addition on aerated concrete’s mechanical and durability properties. Their study showed that incorporating 1.5% hooked-end steel fibers significantly enhanced compressive strength by up to 37% and flexural strength by 64%, with the best performance observed using longer fibers (60 mm, aspect ratio 67). Additionally, the fiber-reinforced mixes demonstrated improved tensile stress–strain response and reduced water absorption, confirming that steel fibers improve strength and enhance durability in aerated concrete systems [30]. Choi et al. [10] demonstrated that incorporating fibers—especially 1.5% vinylon—markedly enhances the mechanical performance of all-lightweight concrete. Their results showed significant gains in flexural strength and fracture toughness, confirming that fiber reinforcement improves strength and ductility in lightweight concrete systems. Chen et al. [31] devised a premix “sand-wrapping” technique that blended expanded-polystyrene beads with silica fume and up to 0.7 vol % steel fibers, yielding lightweight concretes of 800–1800 kg m⁻³ that still reached compressive strengths of about 10–25 MPa. The silica fume improved bead–paste bonding, and the fibers raised split-tensile strength by roughly a quarter while halving 90-day drying shrinkage, positioning the material as a tough, energy-absorbing alternative for structural and impact-resistant applications.

2.5. Lightweight Fillers

Lightweight fillers like expanded polystyrene (EPS) beads and perlite are commonly incorporated into concrete mixtures to reduce density while preserving acceptable mechanical strength significantly [32,33,34,35]. EPS beads are especially favored for their ultra-low density and outstanding thermal insulation capabilities [34]. Perlite, a naturally occurring volcanic glass that expands when heated, is also an effective filler, enhancing thermal and acoustic insulation [33].

The strategic incorporation of lightweight fillers has been widely explored to fabricate LWC with enhanced mechanical performance, durability, and thermal insulation. Hanif et al. [35] demonstrated that functional fillers such as fly ash cenospheres and glass microspheres can significantly reduce the density of cementitious composites while maintaining high compressive, flexural, and tensile strengths when optimally proportioned. Complementing this, Silva et al. [32] highlighted the role of natural fillers such as vermiculite and perlite, showing that the selection and dosage of fillers critically influence mortar density, mechanical compatibility, and capillary water absorption, with vermiculite offering improved microstructural compactness. Further expanding on lightweight filler strategies, Dixit et al. [34] incorporated EPS beads into ultra-high-performance concrete matrices, successfully achieving low-density composites with adequate compressive strength and significantly enhanced thermal insulation, owing to the uniform dispersion of EPS and matrix densification. Recently, Gencel et al. [33] investigated the combined use of expanded perlite and fine-sized waste glass sand in foam concrete, revealing that expanded perlite improves insulation properties due to its high porosity, while glass sand addition substantially enhances physio-mechanical properties and reduces pore connectivity, leading to a more durable and sustainable lightweight system. These studies affirm that the judicious selection, optimization, and integration of lightweight fillers are critical for tailoring the structural efficiency, thermal performance, and environmental sustainability of LWC systems.

2.6. Geopolymers

Geopolymers are an alternative to traditional Portland cement, offering a more sustainable option for lightweight concrete [36]. These materials are produced by the chemical reaction of aluminosilicate materials (such as fly ash or slag) with an alkaline activator, resulting in a binder with excellent mechanical properties and durability [37]. Geopolymer-based lightweight concrete reduces the carbon footprint and offers superior fire and chemical attack resistance, making it suitable for various applications. The production of geopolymer concrete is shown in Figure 3 [37].

Lightweight geopolymer concretes (LW-GPCs) can reach structural-grade strength while cutting dead-load and clinker demand. A fly ash/slag binder combined with dune sand and expanded clay aggregate delivered 22 MPa after 24 h at 60 °C, confirming that moderate calcium accelerates geopolymer hardening [38]. Substituting ultra-porous expanded-glass granules and tuning particle packing lowered the dry density to 780 kg m⁻³ yet still produced 8–10 MPa—adequate for façade and panel elements [39]. Incorporating palm oil shell aggregate lightened the mix further, but replacing 40% of the binder with ground-granulated slag restored strength and halved 90-day sorptivity, refining the pore network [40]. Waste-expanded polystyrene supplied an additional route to mass and thermal-loss reduction: a hybrid inorganic–organic matrix reached >12 MPa at 950 kg m⁻³ with conductivity below 0.20 W m⁻¹ K⁻¹ [41]. Using bottom ash aggregate, fly ash geopolymers achieved 16–18 MPa and thermal conductivities of 0.43–0.47 W m⁻¹ K⁻¹, roughly 25% lower than comparable OPC mixes [42]. Fire-exposure studies show that pumice- or LECA-based LW-GPCs retain ≥70% of their original strength after 800 °C, far outperforming Portland counterparts [43]; cold-bonded geopolymer lightweight aggregates can even push compressive strength beyond 40 MPa while keeping CO₂ emissions below 124 kg t⁻¹ of concrete [44].

2.7. Recycled Materials

Integrating recycled materials into lightweight concrete is increasingly recognized as a sustainable approach to reducing construction waste. Studies have shown that crushed or powdered autoclaved aerated concrete (AAC) and recycled lightweight aggregates (RLA) can partially replace natural aggregates, improving thermal insulation and reducing density without compromising workability or durability [45,46]. For instance, AAC powder has been shown to enhance freeze–thaw resistance and compressive strength when used as a supplementary cementitious material [45].

2.8. Aerogels

Aerogels are ultra-lightweight materials with exceptional thermal insulation properties, making them an attractive addition to lightweight concrete. These materials are composed of a gel in which the liquid component has been replaced by a gas, resulting in a solid with extremely low density and high porosity. Aerogels can significantly reduce thermal conductivity when incorporated into concrete, making the material ideal for energy-efficient building applications. However, the high cost of aerogels currently limits their widespread use in construction.

Aerogels, whose nanoscale pore network forces heat to travel predominantly by gas-phase conduction (Knudsen effect) rather than through solid contacts, therefore act as ultra-efficient thermal “buffers” when dispersed in a cementitious matrix [47]. Jiang et al. [48] blended aerogel-reinforced epoxy microspheres and hollow glass microspheres into lightweight concrete; an optimum mix (90 vol % microspheres + 2 wt % 12 mm glass fibers) achieved 0.897 g cm⁻³ density yet still reached 11.5 MPa in compression—69% higher than the fiber-free analog—thanks to good matrix/particle bonding and fiber bridging. Kalkan and Gündüz [47] replaced quartz sand with up to 4.5 wt % micro-silica aerogel granules in cement mortars: thermal conductivity fell sharply to 0.155 W m⁻¹ K⁻¹ and heat-storage capacity rose, but higher aerogel dosages also increased porosity and lowered compressive strength, underscoring the insulation–strength trade-off. Tsioulou et al. [49] produced “high-strength aerogel concrete” by substituting 30% of sand with aerogel beads; the mix combined a 70 MPa compressive strength with a reduced thermal conductivity of about 1 W m⁻¹ K⁻¹, demonstrating that carefully graded beads can preserve structural capacity while imparting insulation.

2.9. Bio-Based Materials

Bio-based materials derived from renewable biological sources such as plants, agricultural waste, and natural fibers (e.g., hemp, straw, rice husk ash, and oil palm shell) are increasingly explored as sustainable alternatives in lightweight concrete. These materials offer several advantages, including low density, renewable sourcing, and good thermal and acoustic insulation properties. Singh et al. [21] reviewed the inherent characteristics of agro- and industrial by-products as alternative binders and aggregates in lightweight concrete. They systematically analyzed materials such as fly ash, rice husk ash, and sugarcane bagasse ash for their physical, chemical, and morphological properties and their influence on mechanical and thermal performance. Their findings revealed that these waste-based materials can significantly reduce concrete density while enhancing durability and insulation properties, making them viable for eco-efficient lightweight construction.

While bio-based materials are primarily used in non-structural applications, ongoing research aims to enhance their mechanical properties for broader use in construction. Bio-based lightweight aggregates, such as oil palm shells (OPS), have emerged as sustainable alternatives to conventional materials in lightweight concrete production. In this context, Yew et al. [17] investigated the effect of surface modification using grout coating at varying water–cement ratios on the performance of OPS in high-strength lightweight aggregate concrete. Their findings showed that the optimized treatment at a w/c ratio of 0.65 significantly enhanced compressive strength, elastic modulus, and interfacial bonding, confirming the potential of treated bio-based aggregates to achieve mechanical performance and environmental benefits. Expanding on using agricultural by-products in concrete, Shafigh et al. [15] investigated the potential of oil palm shell (OPS)—a solid waste from the palm oil industry—as a coarse bio-based lightweight aggregate. The study demonstrated that using old OPS, which has lower fiber content, improved the bond between the aggregate and cement paste, resulting in a 28-day compressive strength of up to 48.3 MPa and dry densities within the lightweight concrete range. Their findings confirm that when carefully selected and processed, untreated biomass waste like OPS can produce high-strength, lightweight concrete suitable for structural applications. Biomass-based ashes with high silica content, such as rice husk ash, are highlighted as promising materials for making lightweight concrete, offering reduced density and improved thermal insulation and environmental benefits through waste valorization [50].

3. Performance Characteristics of LWC

This chapter examines the key performance characteristics of lightweight concrete (LWC), focusing on its mechanical behavior, durability, and long-term structural reliability. Understanding these properties is essential for evaluating LWC’s suitability in structural, environmental, and energy-efficient applications. The chapter explores how aggregate type, mix design, and supplementary cementitious materials influence strength, impact resistance, durability in aggressive environments, carbonation behavior, and sustainability performance.

Table 3. Summary of LWC properties with various lightweight aggregates and supplementary cementitious materials [2].

Type of LWA	SCM	Density (kg/m3)	Slump (mm)	Compressive Strength (MPa)	W/C	Keynote	Refs.
Crushed slate (Stalite)	3.5% Silica Fume	1893	210	71.0	0.259	High compressive strength; good workability; superior chloride resistance and frost durability; and optimum silica fume dosage.	[27]
Artificial shale ceramsite (pre-soaked)	10% Metakaolin + 10% GGBFS	1881	210–220	52.6	0.3	Best overall performance: seawater + 20% SCM (MK + GGBFS) led to high strength, low chloride permeability, and refined microstructure.	[6]
Expanded clay (5–15 mm, 6–10% absorption)	30% Blast Furnace Slag (BS)	1479	238	58.6	0.42	Best single-admixture mix; BS improved strength and resisted bleeding; stable workability.	[11]
Lightweight expanded clay (30% Leca S + 70% Leca 4/12)	fly ash and limestone powder	1834	687	56.4	0.29	Highest compressive strength among tested LWSCCs; excellent flowability and aggregate distribution; adheres to SCC flow and passing criteria.	[51]
High-strength shale ceramic aggregates	8% Silica Fume + 92% OPC	1745	NM	36.2	0.28	Prepared with vibration mixing process; improved hydration degree by 2.70%; formed dense ITZ with C–S–H gel; impregnation effect enhanced LWA-matrix bonding.	[28]
Spherical shale ceramsite (5–16 mm)	21% Fly Ash + 9% Silica Fume	1811	755	54.2	0.35	Optimal performance among tested mixes; best balance between workability and mechanical strength; maximum specific strength and improved passing ability.	[29]
Coated Dura Shell (pre-treated oil palm shell)	fly ash, slag, silica fume	1927	120	51.2	0.35	Best-performing mix; 41% higher workability, 23% strength improvement over untreated; enhanced ITZ bonding from heat and grout coating; suitable for HSLWAC applications.	[52]
Pre-treated dura oil palm shell	5% densified silica fume	1981	145	56.5	0.3	Best bio-based LWAC performance; 21% higher compressive strength, 31% increase in elastic modulus, improved ITZ and water absorption under 10%.	[17]
Old oil palm shell (OPS), max size 9.5 mm	20% Limestone Powder (by weight of cement)	1903	230	48.33	0.42	The highest-strength mix uses bio-waste OPS and limestone powder; it has excellent early strength and low water absorption (3.12%) and is suitable for precast.	[15]
Crushed oil palm shell (OPS), max size 8 mm	NU	1922	205	53.05	0.305	Best-performing mix; 28% strength increase over CP1 due to reduced W/C ratio; crushed OPS improved ITZ bond and mechanical interlock.	[53]
Expanded shale ceramist	12% Fly Ash + 8% Silica Fume	1845	Acceptable	86	0.26	Carbon fiber-reinforced LWAC; 0.9% fiber volume yielded the highest tensile (10.4 MPa) and flexural strength (5.15 MPa), dense microstructure, and excellent ITZ bonding.	[12]
Expanded clay (5–15 mm, 6–10% absorption)	10% Fly Ash (FA) + 10% BS + 10% Silica Fume (SF)	1532	178	60.5	0.42	The combined use of FA, BS, and SF provides optimal workability and strength.	[11]
Fly ash cenospheres (hollow microspheres)	45% Silica Fume + 15% Fly Ash	1830	NM	105.8	0.25	End-hooked steel fiber (1.0%) provided the highest specific strength (35.5 kPa/kg·m3), dynamic toughness, and densified microstructure, best for dynamic loading resistance.	[13]
Aerated concrete (air voids via aluminum powder)	NU	1901	NM	36	0.5	Best-performing mix; 37% increase in compressive and 64% in flexural strength; improved tensile ductility and lowest water absorption (8.1%).	[30]
All-lightweight aggregates (fine + coarse) made from expanded shale	NU	1670	180 ± 20	46	0.45	Vinylon fibers (1.5%) yielded the highest flexural strength (9.53 MPa) and improved splitting tensile and fracture toughness (>550%), optimal for ductility and toughness in ALC.	[10]
Expanded slate (Stalite)	NU	1881	160	61.6	0.35	Crack widths >0.1 mm increased carbonation rate by over 80% and capillary absorption, with porous aggregates further amplifying durability loss in cracked structural lightweight aggregate concrete.	[54]
Expanded clay aggregates	Fly Ash (125 kg/m3)	1746	630	59.74	0.58	Highest flexural strength (7.62 MPa) and toughness (23 J); splitting tensile strength improved by 37%; concrete exhibited strain-hardening behavior at 1.25% steel fiber content.	[55]
Sintered fly ash aggregates (SFA)	10% Fly Ash + 0.6% Macro + 0.02% Micro Synthetic Fibers	1800	NM	40	0.3	Best combination of strength, fracture energy (1.78 × 10−3 kN/mm), and post-cracking resistance; flexural toughness index I50 = 21.04; suitable for precast structural use.	[20]
Only coarse lightweight aggregate (F6.5, 4.75–9.5 mm)	NU	1900	125	50.0	0.38	Best-performing LWAC mix; uses only coarse lightweight aggregate; good workability and strength with high durability.	[56]
Stalite (crushed slate aggregate)	Fly Ash (FA) + Undensified Silica Fume (SF) + 0.5% MPP fibers	1911.9	660	90.08	0.22	High-flowable lightweight concrete with 0.5% macro-polyresin fiber; improved ductility without compromising workability; strong ITZ and low water absorption.	[57]
Expanded shale + Hollow Glass Microspheres (HGM)	Silica Fume + HGM (Pozzolanic filler)	1929	210	123	0.22	Ultra-high-strength (123 MPa), low density (1929 kg/m3); excellent durability, acoustic absorption, low permeability, and minimal shrinkage; designed using CCD optimization.	[9]

Note: NU-Not used, NM-Not mentioned.

summarizes typical density, slump, compressive strength values, and key observations for LWC with various lightweight aggregates and SCMs to provide a foundational overview.

While lightweight concrete (LWC) offers numerous performance advantages such as reduced structural load, improved thermal insulation, and enhanced sustainability, it also presents notable limitations and trade-offs that must be carefully considered in both research and practical applications. One key challenge is workability, especially in fiber-reinforced LWC, where the addition of steel, polypropylene, or natural fibers can hinder flowability and uniform dispersion, requiring the use of superplasticizers or advanced mixing techniques [58]. Cost is another critical factor; although LWC can reduce overall structural costs through load reduction, the use of specialty materials such as carbon fibers, aerogels, or nano-silica can substantially increase initial material expenses [59]. For example, aerogels—used for thermal enhancement—offer superior insulation but are cost-prohibitive for large-scale use, limiting their application to niche or high-performance structures [60]. Similarly, aerated concretes, while lightweight and thermally efficient, often suffer from increased brittleness and lower mechanical strength, necessitating reinforcement or design compensation [61]. Acknowledging these constraints helps stakeholders make informed decisions regarding material selection, lifecycle costs, and structural performance, ultimately improving both the academic rigor and real-world applicability of LWC innovations.

Table 4. Limitations and trade-offs of lightweight concrete (LWC).

Aspect	Description	Implication	References
Workability	Fibers (steel, PP, natural) hinder flow and dispersion	Requires superplasticizers or special mixing techniques	[62]
Material Cost	Specialty additives (carbon fibers, nano-silica, etc.) increase unit cost	Higher initial investment despite long-term structural savings	[59]
Aerogels	High thermal performance but cost-prohibitive	Suitable only for high-end or niche applications	[60]
Brittleness	Aerated LWC has low mechanical strength	Requires reinforcement or altered design strategy	[61]
Lifecycle Trade-offs	Material selection impacts overall cost-efficiency and durability	Informed trade-off decisions needed for real-world application	[59,60,61]

represents the limitations and trade-offs of lightweight concrete (LWC).

3.1. Strength and Elastic Modulus

The mechanical properties of lightweight concrete (LWC), particularly compressive strength and elastic modulus, are highly influenced by the type and proportion of lightweight aggregates used. Compared to normal-weight concrete (NWC), LWC generally exhibits lower strength and stiffness due to its higher porosity. However, optimized mix designs and advanced material additions can significantly enhance these properties. LWC generally shows reduced compressive strength and elastic modulus compared to normal-weight concrete (NWC), primarily due to lightweight aggregates’ porous and less dense nature [54]. However, performance can be significantly enhanced through supplementary materials like silica fume, fly ash, or nanomaterials, which refine the microstructure and improve bonding [63]. Although the elastic modulus of LWC is typically 20–50% lower than NWC, fiber reinforcement using steel or polypropylene fibers boosts both stiffness and ductility, aiding structural performance [64].

For lightweight concrete (LWC), both the American Concrete Institute (ACI) and Eurocode 2 (EC2) provide empirical equations for estimating compressive strength and modulus of elasticity according to ACI 318 and ACI 213R-87 (Guide for Structural Lightweight Aggregate Concrete) [65]. ACI does not modify the compressive strength formula for LWC directly. The measured value of fc′ is used in design.

For LWC, ACI 318 (Building Code) recommends: Modulus of elasticity (E_c): [65,66]

$$E_c=0.043\ast {\rho }^{1.5}\sqrt{f_c^{\prime }}$$

E_c: Modulus of elasticity (MPa)

$\rho$: Density of concrete (kg/m³)

$f_c^{\prime }$: Compressive strength (MPa)

Eurocode 2 recommends a density-based reduction factor to calculate modulus of elasticity for lightweight aggregate concrete (LWAC): [67]

$$E_{cm}=\alpha E\ast E_{cm,0}$$

$$E_{cm,0}=22\ast {\left({\displaystyle \frac{f_{cm}}{10}}\right)}^{0.3}$$

$$f_{cm}=f_{ck}+8\mathrm{M}\mathrm{P}\mathrm{a}$$

$$\alpha E={\left({\displaystyle \frac{\rho }{2200}}\right)}^2$$

$$\rho =DensityofLWC\left(\mathrm{k}\mathrm{g}/{\mathrm{m}}^3\right)$$

The mechanical performance of lightweight concrete (LWC), particularly its compressive strength and Young’s modulus, is intricately tied to the type, morphology, and properties of the lightweight aggregates (LWAs) used. Structural lightweight concrete generally demonstrates lower compressive strength and stiffness than normal-weight concrete due to the high porosity and reduced density of its aggregates. Kockal and Ozturan [68] reported that the strength and elastic properties of LWC are significantly influenced by the aggregate’s density, water absorption capacity, and strength. Specifically, a higher porosity in LWAs often leads to reduced interfacial bonding and weaker stress transfer mechanisms within the composite, thereby diminishing both compressive strength and Young’s modulus. Kayali et al. [69] further emphasized that although the incorporation of fibers can partially offset these drawbacks by bridging microcracks, the fundamental mechanical limitations imposed by aggregate porosity persist.

Several analytical and empirical models have been proposed to predict the compressive strength and modulus of elasticity in LWC. Cui et al. [70] developed a model incorporating aggregate volume fraction, compressive strength of the matrix, and LWA properties, achieving good correlation with experimental results. The model demonstrated that the stiffness of LWC is particularly sensitive to the stiffness and content of the aggregate. Lu et al. [71] supported this finding in their experimental work on lightweight foam concrete, highlighting that expanded clay aggregates, while beneficial for density reduction, contribute to a notable reduction in Young’s modulus due to their highly porous and deformable nature. Furthermore, Inayath Basha et al. [72] observed that using recycled plastic LWAs significantly lowers both strength and modulus due to the soft, thermoplastic behavior of the aggregates, underscoring the importance of aggregate selection in engineering performance. Compressive strength vs. elastic modulus relationships in lightweight concretes are shown in Table 5.

Incorporating machine learning into mechanical property estimation has shown promise in enhancing predictive accuracy, especially when dealing with the complex interplay of multiple material parameters. Kazemi et al. [58] and Shafighfard et al. [73] utilized advanced machine learning models like ensemble learning and artificial neural networks to accurately estimate the compressive strength of LWC and alkali-activated concretes. These data-driven approaches allow for integrating a wide range of features, including mix proportions, curing conditions, and aggregate types, to predict mechanical performance more reliably than conventional empirical models. Behera et al. [74] also demonstrated that hybrid lightweight concretes incorporating sintered fly ash and palm oil shell aggregates could attain ultra-high-performance levels, but their strength and modulus depend heavily on optimized mix design and fiber reinforcement. Collectively, these studies suggest that while LWC inherently exhibits lower stiffness and strength compared to conventional concrete, strategic mix tailoring and computational modeling can significantly mitigate these drawbacks.

Table 5. Compressive strength vs. elastic modulus relationships in lightweight concretes.

Ref.	Authors (Year)	Concrete Type	Relationship (E vs. f′c)	Comments
[68]	Kockal and Ozturan (2010)	SLWC with expanded clay/slate	E = 4730 × √f’c	Empirical model; E in MPa
[70]	Cui et al. (2012)	Structural lightweight aggregate concrete	E = 0.043 × (f’c)^1.5	E in GPa; fits well for soaked aggregates
[69]	Kayali et al. (2003)	Fiber-reinforced LWC	E = 4400 × √f’c	LWC with fly ash-based LWA
[71]	Lu et al. (2020)	Foamed LWC with expanded clay	E = 0.053 × (f’c)^1.34	Quasi-brittle modeling applied
[72]	Basha et al. (2020)	Recycled plastic aggregate LWC	E = 0.038 × (f’c)^1.48	Low E due to plastic inclusions
[74]	Behera et al. (2022)	UHPC hybrid fiber LWC	E = 3900 × √f’c	With sintered fly ash and palm shell
[58]	Kazemi et al. (2025)	UHPC and advanced materials	ML-based; no fixed equation	GWO + ML ensemble model
[73]	Shafighfard et al. (2024)	High-performance alkali-activated concrete	ML-based estimation	ANN, RF, SVR, etc.

Table 5. Compressive strength vs. elastic modulus relationships in lightweight concretes.

Ref.	Authors (Year)	Concrete Type	Relationship (E vs. f′c)	Comments
[68]	Kockal and Ozturan (2010)	SLWC with expanded clay/slate	E = 4730 × √f’c	Empirical model; E in MPa
[70]	Cui et al. (2012)	Structural lightweight aggregate concrete	E = 0.043 × (f’c)^1.5	E in GPa; fits well for soaked aggregates
[69]	Kayali et al. (2003)	Fiber-reinforced LWC	E = 4400 × √f’c	LWC with fly ash-based LWA
[71]	Lu et al. (2020)	Foamed LWC with expanded clay	E = 0.053 × (f’c)^1.34	Quasi-brittle modeling applied
[72]	Basha et al. (2020)	Recycled plastic aggregate LWC	E = 0.038 × (f’c)^1.48	Low E due to plastic inclusions
[74]	Behera et al. (2022)	UHPC hybrid fiber LWC	E = 3900 × √f’c	With sintered fly ash and palm shell
[58]	Kazemi et al. (2025)	UHPC and advanced materials	ML-based; no fixed equation	GWO + ML ensemble model
[73]	Shafighfard et al. (2024)	High-performance alkali-activated concrete	ML-based estimation	ANN, RF, SVR, etc.

Effects on Carbon Nanotubes for Improving Compressive Strength in LWC

(a)

The Effects of Carbon Nanotubes on the Compressive Strength of Cementitious Composites:

Carbon nanotubes (CNTs), with their exceptional mechanical, thermal, and electrical properties, have emerged as one of the most promising nanomaterials for enhancing the performance of cementitious composites, particularly in terms of compressive strength. Over the past decade, increasing attention has been paid to their potential role in improving the structural integrity of both traditional and advanced cementitious systems, including ultra-high-performance concrete (UHPC) and lightweight foamed concrete (LFC). An overview of studies investigating carbon nanotubes’ (CNTs) effects on compressive strength is shown in Table 6.

(b)

Mechanisms of Compressive Strength Enhancement:

The integration of CNTs into cement-based matrices influences the microstructure and mechanical performance significantly. Their exceptionally high tensile strength (up to ~60 GPa) and modulus of elasticity (~1 TPa), combined with a nanoscale diameter, allow CNTs to bridge microcracks, refine pore structure, and improve matrix continuity. These mechanisms help delay crack propagation under compressive loads, leading to a more ductile fracture response and enhanced load-bearing capacity.

According to Li et al. [75], CNTs act as nanoscale fillers that densify the microstructure by occupying voids and bridging microcracks, which results in a significant reduction in porosity. Their study, which employed machine learning models to predict compressive strength, confirmed that CNT dosage, dispersion quality, and water-to-cement ratio are critical parameters governing the effectiveness of CNTs in improving mechanical properties. The incorporation of CNTs up to an optimal threshold (~0.1 wt %) led to increases in compressive strength by up to 30% compared to the control specimens.

(c)

Machine Learning Approaches to Predict CNT Influence:

Recent advances in computational modeling have facilitated more accurate predictions of CNT effects on concrete performance. Özyüksel Çiftçioğlu et al. [76] introduced a hybrid Grey Wolf Optimizer (GWO) integrated with boosting algorithms to model and predict the compressive strength of CNT-incorporated UHPC. The predictive framework accurately captured the complex, nonlinear relationships between CNT content, curing conditions, and mechanical outcomes. The results revealed that optimal CNT dispersion and integration significantly enhanced compressive strength, especially in UHPC matrices where high packing density and supplementary cementitious materials synergize with CNTs’ crack-bridging functions.

Similarly, Bagherzadeh and Shafighfard [77] applied ensemble machine learning (EML) techniques to evaluate the influence of CNTs on various mechanical and durability properties of cementitious composites. Their models showed strong predictive capability, reinforcing the notion that data-driven approaches are instrumental in optimizing CNT content and mix design parameters for maximizing compressive strength. The study emphasized that while low concentrations of CNTs improve performance, excessive dosages can lead to agglomeration, negatively affecting matrix uniformity and strength.

(d)

CNTs in Lightweight and Foamed Concrete

Beyond conventional and UHPC systems, CNTs have demonstrated notable efficacy in enhancing the compressive strength of lightweight foamed concrete (LFC), which typically suffers from low mechanical performance due to high porosity. Sldozian et al. [78] investigated the role of multi-walled CNTs (MWCNTs) in improving both compressive strength and water adsorption of LFC. Their findings indicated that the inclusion of CNTs led to a more compact and interconnected pore network, which increased strength by up to 25% and reduced water absorption significantly. The enhanced performance was attributed to improved pore structure refinement and the mechanical reinforcement provided by the CNTs within the weak matrix of the LFC.

(e)

Challenges and Optimization

Despite the promising benefits, the successful application of CNTs in cementitious materials is hindered by several practical challenges, notably dispersion and cost. Poor dispersion can lead to CNT agglomeration, acting as flaws rather than reinforcements. As highlighted in [75,77], achieving uniform CNT distribution through methods such as ultrasonication, use of surfactants, or functionalization is essential for realizing their full potential. Moreover, dosage optimization is key, as excessive amounts may lead to negative effects on workability and strength.

(f)

Summary:

Carbon nanotubes, when properly integrated into cementitious systems, offer significant enhancements in compressive strength due to their nano-bridging effect, densification of microstructure, and crack-arresting capabilities. As demonstrated across several studies [75,76,77,78], the use of machine learning provides a powerful tool for optimizing CNT incorporation strategies. Future advancements in dispersion techniques and cost-reduction strategies are expected to further accelerate the practical application of CNTs in high-performance and sustainable cement-based materials.

Table 6. Overview of studies investigating CNT effects on compressive strength.

Study	Cementitious Material	CNT Type	Key Findings	Modeling/Analysis Approach
[76] Özyüksel Çiftçioğlu et al. (2025)	Ultra-High-Performance Concrete (UHPC)		CNTs enhance compressive strength significantly; optimal dosage is critical	Grey Wolf Optimizer + Boosting algorithm
[75] Li et al. (2022)	Cement-based composites		CNTs increase strength up to 30%; dispersion and dosage are key factors	Machine learning regression models
[77] Bagherzadeh and Shafighfard (2022)	CNT-reinforced cementitious composites	Carbon nanotubes (general)	ML models show strength enhancement with optimal CNT content	Ensemble machine learning (EML)
[78] Sldozian et al. (2024)	Lightweight Foamed Concrete (LFC)	Multi-walled CNTs (MWCNTs)	Up to 25% improvement in compressive strength; water absorption reduced	Experimental analysis

Table 6. Overview of studies investigating CNT effects on compressive strength.

Study	Cementitious Material	CNT Type	Key Findings	Modeling/Analysis Approach
[76] Özyüksel Çiftçioğlu et al. (2025)	Ultra-High-Performance Concrete (UHPC)		CNTs enhance compressive strength significantly; optimal dosage is critical	Grey Wolf Optimizer + Boosting algorithm
[75] Li et al. (2022)	Cement-based composites		CNTs increase strength up to 30%; dispersion and dosage are key factors	Machine learning regression models
[77] Bagherzadeh and Shafighfard (2022)	CNT-reinforced cementitious composites	Carbon nanotubes (general)	ML models show strength enhancement with optimal CNT content	Ensemble machine learning (EML)
[78] Sldozian et al. (2024)	Lightweight Foamed Concrete (LFC)	Multi-walled CNTs (MWCNTs)	Up to 25% improvement in compressive strength; water absorption reduced	Experimental analysis

Furthermore, Kayali et al. [79] reported that LWC produced with expanded clay, shale, or slag aggregates can achieve compressive strengths in the 20–60 MPa range, making it suitable for structural applications. Incorporating supplementary cementitious materials further improves performance. For example, silica fume enhances compressive strength by refining the concrete’s microstructure and reducing pore size [80]. Similarly, nano-silica contributes to a 15–25% increase in strength by promoting a denser and more cohesive matrix [81]. Despite strength enhancements, LWC typically has a lower elastic modulus—about 10–20 GPa compared to 25–40 GPa for NWC—due to the inherent porosity of lightweight aggregates [82]. This limitation can be mitigated by incorporating steel fibers, which have been shown to increase the stiffness of LWC by 10–30% while also enhancing ductility and flexural performance [83]. Moreover, using two-phase composite models allows for accurate prediction of elastic modulus, aiding in the rational design of structural elements using LWC. Overall, with the right combination of materials and mix optimization, LWC can meet structural demands while maintaining its lightweight advantage. Jingjun Li et al. [29] reported that the compressive strength at 28 and 56 days was opposite to that observed at early ages (3 and 7 days), which was attributed to the stiffness differential between the aggregates and the mortar over time. Compressive strength tests were conducted at 3, 7, 28, and 56 days, along with splitting tensile strength at 28 days, using three 100 mm cubic specimens per mix. All specimens were cast in a single operation without vibration. As shown in Figure 4a, the mixture MT-1.8 achieved the highest compressive strengths at 28 and 56 days, recording 54.2 MPa and 61.1 MPa, respectively—values 11.5% and 21.2% higher than MT-1.4.

In contrast, MT-1.4 exhibited the lowest compressive strengths, measuring 48.6 MPa and 50.4 MPa at 28 and 56 days, respectively. These findings suggest that lightweight aggregates (LWAs) should be encapsulated in mortar with sufficient thickness to promote strength development at later ages. Furthermore, the proposed dry density prediction model proved effective in the lightweight self-compacting concrete mix design (LWASCC). The observed divergence in strength development between early and later ages was primarily attributed to the evolving stiffness relationship between the aggregates and the surrounding mortar.

Figure 4b illustrates that, at 3 and 7 days, cracks predominantly propagated along the aggregate–paste interface. This behavior is attributed to the limited strength development of the paste matrix at early ages, resulting from the low degree of cement hydration. The presence of intact lightweight aggregates (LWAs) and visible voids suggests that the coarse aggregates acted as a reinforcing phase relative to the weaker mortar matrix during the early curing period. This pattern of compressive failure closely resembles that observed in normal-weight aggregate concrete (NWAC), where the interfacial transition zone (ITZ) between aggregate and paste is typically the weakest region. Figure 4c presents a schematic representation of crack propagation within the concrete, further highlighting the critical role of the aggregate–paste interface in the overall fracture behavior.

Furthermore, Jian-Xin Lu et al. [84] developed a high-performance, lightweight aggregate concrete (HPLAC) by integrating ultra-high-performance cementitious composites (UHPC) with various types of aluminosilicate lightweight aggregates (LWAs). Figure 5a displays the physical appearance of the three LWAs used in the study: RS, CLWA, and SLWA. Jian-Xin Lu et al. [84] employed the nanoindentation technique to investigate the micromechanical properties within the interfacial transition zone (ITZ). The boundaries of the ITZ were identified using optical microscopy images combined with indentation grid mapping. Figure 5b–d presents the micro-hardness and modulus contour maps for the interfacial transition zones (ITZ) in samples 100R, 100C, and 100S, respectively. Among these, the RS aggregate exhibited significantly higher hardness and modulus values than the surrounding bulk paste, making the ITZ readily distinguishable. Additionally, unreacted clinker particles with elevated moduli were observed within the paste matrix. Notably, a distinct ITZ with reduced hardness and modulus was found at the RS–paste interface, which may be attributed to a localized increase in the water-to-binder (w/b) ratio caused by the wall effect of RS. Based on the indentation images and corresponding contour maps, both the LWAs and paste phases could be identified. The LWAs exhibited low hardness and modulus due to their porous nature; however, the surrounding ITZ demonstrated relatively higher mechanical properties.

However, Jian-Xin Lu et al. [84] employed the 100R (Figure 5b) sample, a pronounced drop in both hardness and elastic modulus was observed within the interfacial transition zone (ITZ), followed by a gradual recovery as the distance from the recycled sand (RS) increased. In contrast, specimens incorporating lightweight aggregates (LWAs) exhibited a different trend: the regions adjacent to the aggregates displayed higher hardness and modulus values than the aggregates themselves, with a subsequent decline that stabilized at further distances. Notably, the ITZ formed between the SLWA (synthetic lightweight aggregate) and the cement paste demonstrated superior micromechanical characteristics compared to that between the CLWA (conventional lightweight aggregate) and paste.

These findings indicate that the inclusion of LWAs contributes positively to enhancing the micromechanical behavior of the ITZ in ultra-high-performance concrete (UHPC). The improvement was particularly pronounced with SLWA, likely due to its higher pozzolanic reactivity. In this investigation, the UHPC matrix, characterized by an extremely low water-to-binder ratio, led to the formation of a compact ITZ, devoid of crystalline hydration products near the aggregates. The improved ITZ in both SLWA and CLWA mixtures is largely attributed to internal curing facilitated by the pre-saturated LWAs. In this context, water released from the LWAs promoted continued cement hydration, thereby reinforcing the ITZ. This internal curing mechanism appears to mitigate, at least partially, the negative effects associated with the inherent porosity of LWAs.

Specifically, the SLWA mixture benefitted from enhanced pozzolanic activity, which led to the generation of additional calcium-alumino-silicate-hydrate (C-A-S-H) within the ITZ. This densification process resulted in improved micromechanical performance in that region. Although both internal curing and pozzolanic action strengthened the ITZ, the compressive strength of the overall concrete remained largely constrained by the relatively low strength of LWA itself. Nevertheless, the more refined ITZ in the SLWA system may have contributed to its comparatively higher compressive strength relative to the CLWA system.

In lightweight aggregate concrete, the porous nature of the aggregates typically represents the weakest zone under mechanical loads or aggressive environmental conditions. Therefore, a robust ITZ is crucial in impeding crack propagation and blocking harmful ion ingress. It was also observed that the average modulus of C–S–H (calcium-silicate-hydrate) in this study exceeded 35 GPa, which is significantly higher than values reported in previous studies. This elevation is attributed to the emergence of an ultra-dense C–S–H phase, made possible by the low water-to-binder ratio, alongside the conventional low- and high-density C–S–H forms.

In the UHPC matrix, the high and ultra-high-density C–S–H phases dominated the microstructure, thereby enhancing the stiffness of the paste. Furthermore, the integration of LWAs promoted additional C–S–H gel formation within the confined ITZ space, further improving the structural packing and mechanical stiffness. Similar observations in traditional LWA concretes have shown that the ITZ surrounding LWAs often possesses greater microhardness than the adjacent matrix due to the densification of the interface by newly formed hydration products. These enhanced micromechanical properties of the ITZ are believed to play a significant role in improving the overall mechanical performance and durability of high-performance lightweight aggregate concretes (HPLACs).

3.2. Impact Resistance and Fracture Behavior

LWC exhibits lower impact resistance due to its heterogeneous and porous microstructure [85]. However, this porosity allows for better energy absorption under dynamic or impact loads [86]. Including hybrid fibers—such as a combination of steel and polypropylenes—substantially improves fracture toughness, making LWC suitable for more demanding structural applications [87].

The morphology of lightweight aggregates (LWAs) plays a pivotal role in influencing the pore structure and bonding characteristics of lightweight concrete (LWC). Porous and irregularly shaped LWAs, such as expanded shale, clay, or pumice, inherently contain internal voids that contribute to increased overall porosity in the concrete matrix. These pores extend into the interfacial transition zone (ITZ), which is already recognized as a weak zone in concrete microstructures. Wang et al. [88] found that the microstructural characteristics of LWC, including pore connectivity and aggregate surface texture, significantly influenced fracture parameters in notched beams. Irregular morphologies and open porosity in LWAs were shown to promote crack propagation and reduce the load-bearing capacity of the composite.

Furthermore, the surface roughness and absorptive nature of LWAs alter the water-to-cement ratio at ITZ, impacting hydration kinetics and bond development. Lo and Cui [89] demonstrated that porous LWAs absorb significant amounts of mixing water, affecting internal curing but also leading to weaker paste–aggregate bonding. This water absorption may create a denser paste in surrounding areas, but the interface itself often lacks the strength of the bulk matrix. Hu et al. [90], through finite element simulations, revealed that surface modifications, such as forming a dense shell around a porous core (cordierite–belite design), improved the strength and fracture behavior of LWC by minimizing interfacial porosity and improving load transfer efficiency.

Lu et al. [71,91] investigated lightweight foam concrete and reported that spherical and highly porous aggregates, like expanded clay, led to a discontinuous and heterogeneous pore structure that affected stress distribution during loading. Although these aggregates reduced the overall density of concrete, their morphology often created stress concentration zones due to poor bonding and inconsistent stiffness across the ITZ. This heterogeneity contributes to quasi-brittle fracture behavior and early microcracking under mechanical loading. The findings highlight the challenge of balancing weight reduction with structural integrity in LWC design. The influence of lightweight aggregate morphology on pore structure and bonding is shown in

Table 7. Influence of lightweight aggregate morphology on pore structure and bonding in LWC.

Reference	LWA Type/Feature	Morphological Characteristics	Observed Influence on Pore Structure	Impact on Bonding Behavior
[88] Wang et al. (2022)	Expanded shale and clay	Rough, porous surface, irregular shape	Increased interfacial transition zone (ITZ) porosity; visible microcracking	Weak ITZ bonding due to poor paste penetration in deep pores
[89] Lo and Cui (2003)	Porous lightweight aggregate	Highly porous internal structure	High overall porosity; larger total pore volume	Reduced mechanical interlock and paste adhesion at ITZ
[71] Lu et al. (2020)	Expanded clay	Spherical, porous granules	Induced foam-like pore distribution in matrix	Weaker paste–aggregate adhesion; stress concentration zones
[90] Hu et al. (2024)	Core-shell (cordierite–belite)	Smooth outer shell over porous core	Reduced open porosity in ITZ	Improved ITZ bonding due to shell barrier limiting water ingress
[92] Musalamah et al. (2024)	Polypropylene LWA	Coarse, irregular, with rough texture	Reduced pore continuity; heterogeneous distribution	Enhanced mechanical interlock but localized stress intensification
[91] Lu et al. (2019)	Foam concrete with LWA	Highly porous with cellular structure	High internal porosity; weak matrix continuity	Poor bonding due to mismatch in stiffness and microstructure
[93] Maglad et al. (2023)	Recycled aggregates with steel fibers	Irregular, cracked surfaces	Increased ITZ porosity; more voids around fibers	Disrupted bonding continuity, but fiber bridging compensates partially
[94] Sahoo et al. (2023)	Structural LWA with fiber reinforcement	Angular, fibrous LWA	Micro-porous zones; entrapped air at interfaces	Mixed behavior: weak paste bond, but fiber enhances crack bridging
[95] Sim et al. (2013)	Varying LWA sizes	Smooth vs. rough textures	Larger LWA led to higher porosity; smoother types reduced ITZ interaction	Smaller aggregates showed better bonding through finer particle interaction

.

Finally, efforts to enhance LWC performance have considered not only aggregate selection but also surface treatment and microstructural optimization. Musalamah et al. [92] used Digital Image Correlation (DIC) to assess fracture energy and found that coarser, rough-surfaced polypropylene-based LWAs showed improved mechanical interlock but suffered from localized stress intensification. In contrast, engineered aggregates with smoother and more uniform surface morphology were found to reduce internal defects but at the cost of lower frictional bonding. These studies collectively suggest that LWA morphology must be optimized not only for density reduction but also for compatibility with the cement matrix to ensure structural performance [58]. However, most existing research highlights qualitative relationships rather than providing robust quantitative models linking morphology, porosity, and fracture mechanics, pointing to a need for more comprehensive multi-scale modeling frameworks.

Furthermore, fracture behavior in lightweight aggregate concrete (LWC) is significantly influenced by its heterogeneous microstructure, particularly the porous nature of lightweight aggregates and the weak interfacial transition zones (ITZs) between the matrix and aggregates. Wang et al. [88] highlighted that fracture parameters such as fracture toughness and energy in LWC are closely linked to the microstructural arrangement and properties of the aggregates. The porous nature of lightweight aggregates can lead to stress concentrations and early crack initiation. Lo and Cui [89] emphasized that the incorporation of porous aggregates generally reduces the tensile strength of concrete, but the overall effect on fracture resistance depends on the interaction between matrix and aggregates, aggregate morphology, and distribution.

Advanced modeling approaches have been developed to simulate fracture behavior in LWC, including cohesive zone models (CZMs), finite element models (FEMs), and digital image correlation (DIC)-integrated systems. Lu et al. [71] applied a quasi-brittle fracture model for foam concrete using CZM, showing that LWC displays enhanced energy absorption before failure, but the crack path is highly influenced by aggregate distribution and matrix porosity. Hu et al. [90] extended this with a finite element simulation that accounted for the core–shell structure of lightweight aggregates, illustrating the challenges in accurately modeling crack propagation due to varying stiffness and toughness across phases. While these models provide valuable insights, they remain sensitive to input parameters such as aggregate geometry and ITZ strength. Modeling fracture behavior of lightweight aggregate concrete (LWC) is shown in

Table 8. Modeling fracture behavior of lightweight aggregate concrete (LWC).

Ref. No.	Authors	Focus Area	Model Type/Approach	Identified Limitations in Models
[88]	Wang et al. (2022)	Fracture parameters via three-point bending	Experimental + Linear Elastic Fracture Mechanics (LEFM)	Microstructure-fracture links are hard to generalize; limited to notched beams and lab conditions.
[89]	Lo and Cui (2003)	Strength of porous LWA concrete	Analytical	Oversimplifies porosity effects; lacks fracture zone modeling.
[71]	Lu et al. (2020)	Quasi-brittle fracture modeling of foam concrete	Cohesive Zone Model (CZM) + Experimental	Scale-sensitive; assumes uniform aggregate distribution and idealized bonding.
[90]	Hu et al. (2023)	Simulation of core–shell LWA concrete	Finite Element Modeling (FEM)	High computational cost; complex crack path difficult to model precisely.
[58]	Kazemi et al. (2025)	Machine learning prediction of mechanical properties	Ensemble ML (RAGN-R)	Non-physical predictions; lacks transparency and validation against fracture mechanics.
[92]	Musalamah et al. (2024)	DIC-based fracture energy estimation	Digital Image Correlation (DIC)	Resolution-sensitive; limited to surface cracks; integration with numerical models is lacking.
[91]	Lu et al. (2019)	Simulation of foam concrete fracture	FEM + Experimental	Fails to model interfacial transition zones (ITZ); geometric simplifications limit accuracy.
[93]	Maglad et al. (2023)	SF-reinforced RAC fracture	Experimental + Analytical	Incomplete fiber bridging models; insufficient parameter calibration for hybrid aggregates.
[94]	Sahoo et al. (2023)	Fracture of fiber-reinforced LWC	Experimental + LEFM	Does not fully capture energy dissipation from fiber pull-out; lacks multi-scale modeling.
[95]	Sim et al. (2014)	Size effects in LWC fracture energy	Fracture Mechanics (Size Effect Law)	Scaling laws are not tuned for lightweight systems; they need larger datasets for validation.

.

Emerging technologies such as machine learning (ML) and high-resolution experimental tools are being integrated to improve fracture prediction in LWC. Kazemi et al. [58] proposed an ensemble ML model (RAGN-R) that estimates mechanical and fracture properties with high accuracy by learning from multiple datasets. Musalamah et al. [92] employed DIC to measure strain localization and fracture energy in real time, offering detailed visualization of fracture processes. However, both ML and DIC approaches face limitations in generalization and integration with physics-based models. Studies by Sahoo et al. [94] and Sim et al. [95] also noted that existing fracture models inadequately capture the effects of fiber reinforcement, size scaling, and anisotropy in LWC, underscoring the need for hybrid frameworks that combine physical, empirical, and data-driven methods for more robust modeling.

Fiber-reinforced lightweight concrete (FRLWC) notably improves performance under impact loading, particularly using optimized combinations of polyvinyl alcohol (PVA) and steel fibers, significantly increasing energy absorption capacity and fracture toughness. To better understand the role of lightweight microspheres in governing fracture mechanisms, the microstructural failure patterns of the composites were examined. Figure 6a–d display BSE images revealing the fracture characteristics of glass-UHPC and HPLC containing various lightweight microspheres [96]. In the glass-UHPC, Figure 6a, fractures primarily occurred along the smooth surfaces of glass aggregates, indicating weak interfacial bonding due to the inert and brittle nature of the cult. In the HPLC with fly ash cenospheres, Figure 6b, the microspheres fractured through their hollow shells, attributed to non-uniform wall thickness and low structural integrity. In Figure 6c, cracks propagated along the particle-matrix interface rather than through the microspheres, reflecting the improved mechanical stability and resistance to stress-induced rupture provided by the denser, more robust particles.

In contrast, Figure 6d shows extensive rupture across the microsphere cross-sections, confirming lower structural integrity despite similar size and dispersion. These differences highlight the critical role of microsphere morphology and shell strength in governing the fractured behavior and impact resistance of lightweight cementitious composites. To further illustrate the fracture resistance mechanisms in fiber-reinforced lightweight concrete systems, the failure modes and microstructural interactions were examined using SEM and BSE imaging in the study of Liao et al. [19]. As shown in Figure 6f, incorporating micro steel fibers notably improved the compressive failure response of SF-LWC, reducing surface peeling and fragmentation compared to the brittle failure observed in fiber-free specimens and standard concrete. This macroscopic improvement is supported by microstructural evidence in Figure 6e, where SEM imaging reveals effective fiber bridging across microcracks and dense hydration product formation around the embedded fibers. The EDS mapping in Figure 6g further highlights the elemental composition at the fiber–matrix interface, indicating strong chemical bonding due to the high-strength cementitious matrix. These observations confirm that integrating micro steel fibers enhances the mechanical anchoring and energy dissipation mechanisms, resulting in improved fracture resistance and ductility of the lightweight concrete system.

The impact resistance and fracture performance of lightweight concrete (LWC) are critical for its use in structural and protective applications. Various materials and techniques have been studied to enhance these properties. Polypropylene fibers have proven effective in improving the impact resistance of concrete by 30–50%, primarily by controlling crack propagation and distributing stress under dynamic loading conditions [97]. Another innovative approach involves the incorporation of recycled rubber particles. Rubberized LWC exhibits enhanced energy absorption capabilities due to its elastic nature, though this comes at the cost of reduced compressive strength [98]. Such trade-offs make rubberized LWC suitable for non-structural or impact-absorbing applications where flexibility is prioritized over strength.

Steel fibers significantly enhance fracture energy by bridging microcracks and delaying crack propagation, improving ductility and toughness [99]. High-performance, lightweight concrete (HPLWC) exhibits superior fracture resistance, particularly when modified with nano-silica and fly ash. This improvement is attributed to a denser interfacial transition zone (ITZ), which helps minimize crack initiation and growth [100]. Collectively, these studies highlight that by integrating fibers, recycled materials, and nanotechnology, the impact and fracture resistance of LWC can be effectively tailored for specific engineering applications.

3.3. Durability in Aggressive Environments

While lightweight concrete (LWC) offers many advantages, its inherently porous structure can compromise durability in aggressive environments. However, its resistance to various forms of deterioration can be significantly improved through optimized mixed designs and supplementary materials. Table 9. represents the sustainability trade-offs and CO₂ sequestration across lightweight concrete (LWC).

Durability is a key challenge for LWC, especially in chloride-rich, sulfate-laden, or freeze–thaw environments. Its high porosity facilitates chloride ingress and sulfate attack, potentially compromising long-term performance [101]. However, air entrainment improves performance under freeze–thaw cycles by relieving internal stresses [102,103].

Table 9. Sustainability trade-offs and CO₂ sequestration across lightweight concrete (LWC).

LWC Type	CO2 Sequestration Potential	Sustainable Material Used	Cost Impact	Durability Trade-Offs	References
LWC with Recycled Aggregate	Moderate (~10–12% CO2 uptake)	Crushed concrete, C&D waste	Low to moderate (cost-saving in supply)	Variable quality, potential for increased shrinkage	[104,105]
CO2-Cured LWC (Carbonation Curing)	High (~15–20% CO2 uptake)	Accelerated carbonation of cement matrix	Higher (specialized curing needed)	Risk of carbonation-induced rebar corrosion if uncoated	[106,107]
Geopolymer LWC	Moderate (~8–12% CO2 reduction)	Fly ash, slag (no Portland cement)	Moderate (variable by material source)	Lower early strength, sensitive to curing conditions	[108,109]
GEGA-Cement-Grout Permeable LWC	Moderate (~8–12% CO2 reduction)	Granulated Expanded Glass Aggregate + cement (hydraulic binder)	Medium: energy for glass granulation and cement grout	High porosity & low bulk density (~1000 kg/m3)	[110]
Bio-based LWC (e.g., bacterial LWC)	Moderate (~10–15% uptake)	Bacteria-induced calcite precipitation	High (novel materials and systems)	Experimental; long-term durability under real conditions unknown	[111]

Table 9. Sustainability trade-offs and CO₂ sequestration across lightweight concrete (LWC).

LWC Type	CO2 Sequestration Potential	Sustainable Material Used	Cost Impact	Durability Trade-Offs	References
LWC with Recycled Aggregate	Moderate (~10–12% CO2 uptake)	Crushed concrete, C&D waste	Low to moderate (cost-saving in supply)	Variable quality, potential for increased shrinkage	[104,105]
CO2-Cured LWC (Carbonation Curing)	High (~15–20% CO2 uptake)	Accelerated carbonation of cement matrix	Higher (specialized curing needed)	Risk of carbonation-induced rebar corrosion if uncoated	[106,107]
Geopolymer LWC	Moderate (~8–12% CO2 reduction)	Fly ash, slag (no Portland cement)	Moderate (variable by material source)	Lower early strength, sensitive to curing conditions	[108,109]
GEGA-Cement-Grout Permeable LWC	Moderate (~8–12% CO2 reduction)	Granulated Expanded Glass Aggregate + cement (hydraulic binder)	Medium: energy for glass granulation and cement grout	High porosity & low bulk density (~1000 kg/m3)	[110]
Bio-based LWC (e.g., bacterial LWC)	Moderate (~10–15% uptake)	Bacteria-induced calcite precipitation	High (novel materials and systems)	Experimental; long-term durability under real conditions unknown	[111]

Lightweight concrete (LWC) exhibits unique durability challenges, particularly in environments involving chloride and sulfate exposure, as well as freeze–thaw cycles. To better understand these mechanisms, it is essential to categorize them into specific degradation processes. For instance, in chloride-rich environments such as marine or deicing salt-exposed structures, LWC typically displays higher chloride diffusion coefficients than normal-weight concrete due to increased porosity. However, studies show that incorporating pozzolanic materials such as fly ash and silica fume can significantly reduce chloride permeability. For example, M. Olivia [112] reported a reduction in the chloride diffusion coefficient from 3.5 × 10⁻¹² to 1.2 × 10⁻¹² m²/s when 25% fly ash was used in LWC mixes [112]. In sulfate environments, expansion and cracking due to ettringite formation remain concerns, but optimized binder composition and proper curing can mitigate these effects [108].

When addressing freeze–thaw durability, LWC often shows improved performance due to its internal curing potential, especially when made with pre-saturated lightweight aggregates (LWA). However, the inclusion of air-entraining agents remains critical. While air entrainment enhances freeze–thaw resistance by creating pressure-relieving voids, it also reduces the compressive strength of LWC by approximately 10–15%, as noted by D. P. Bentz [113]. This trade-off emphasizes the need to balance durability and mechanical performance based on exposure conditions. Organizing the content into such specific degradation categories—along with quantitative benchmarks—can help researchers and engineers optimize LWC for harsh environmental applications, ensuring both resilience and longevity [113].

Furthermore, blended binders such as ground granulated blast furnace slag (GGBS) or metakaolin significantly improve resistance to sulfate and other chemical attacks [114]. Incorporating silica fume into LWC can reduce chloride ion penetration by 40–60%, enhancing its suitability for marine environments [115]. Additionally, fly ash-based LWC shows strong resistance to sulfate attack, owing to the pozzolanic reactions that refine pore structure and reduce permeability [116]. Waste glass powder is another sustainable additive that helps lower water permeability and boost chloride resistance, making LWC more durable in coastal and chemically aggressive conditions. LWC with air entrainment demonstrates comparable freeze–thaw durability to normal-weight concrete (NWC), especially under moderate exposure conditions [117]. Several studies have confirmed the long-term durability of LWC in harsh environments. Fly ash–lightweight aggregate concrete maintains strength and resists degradation over time [118], while self-consolidating versions show comparable resistance to environmental stress [119]. Enhanced resistance to marine exposure was reported in lightweight concretes subjected to hot and saline climates [120].

3.4. Carbonation Performance

The carbonation resistance of lightweight concrete (LWC) has been widely studied to ensure its durability when alternative materials replace natural aggregates. The carbonation of concrete depends on its compressive strength and mixing proportions (e.g., the water-to-binder and aggregate-to-cement ratios). In contrast, the carbonation of LWAC is strongly affected by its internal pore structure and the moisture content of the lightweight aggregate [121]. However, supplementary cementitious materials (SCMs) like slag and metakaolin can significantly mitigate carbonation depth, enhancing long-term durability in atmospheric exposure [122]. Jung et al. [121] investigated the carbonation behavior of structural lightweight aggregate concrete (LWAC) incorporating expanded bottom ash and dredged soil aggregates. Their results demonstrated that carbonation depth is primarily influenced by compressive strength and density rather than lightweight aggregate content. Although lightweight aggregates have higher porosity, their inclusion slightly delayed CO₂ diffusion, resulting in lower carbonation depths than normal-weight concrete of similar strength. Empirical models were also proposed to predict carbonation depth and diffusion coefficients based on strength and density parameters [121]. Bogas and Real [123] reviewed the durability of structural lightweight aggregate concrete. They concluded that a well-structured cement matrix and a high-quality interfacial transition zone (ITZ) are critical for mitigating CO₂ ingress. They emphasized that despite the porous nature of lightweight aggregates, SLWAC can exhibit carbonation resistance comparable to regular concrete when designed with a low water-to-cement ratio. Furthermore, they introduced a biphasic carbonation model, explaining that carbonation initially progresses through the paste and only later involves the aggregates [123].

Lightweight concrete (LWC) is generally more porous than normal-weight concrete (NWC), which can make it more susceptible to carbonation—a process where CO₂ penetrates the concrete and reacts with calcium hydroxide, potentially reducing durability. However, this increased porosity does not straightforwardly translate into higher carbonation rates due to the unique behavior of lightweight aggregates (LWA). Research indicates that while the matrix of LWC may be more porous, the internal structure of LWA—especially pre-wetted types—plays a crucial role in mitigating carbonation effects through internal curing. This internal curing mechanism helps maintain internal humidity, promoting ongoing hydration and resulting in a denser interfacial transition zone (ITZ), which can reduce overall permeability and slow CO₂ ingress [108,124].

Moreover, some types of LWA, such as expanded clay or shale, have been shown to possess tortuous internal pore structures that delay the diffusion of CO₂ despite the material’s higher porosity [66]. Bentz et.al. also found that the ITZ in LWC, although typically weaker in conventional concrete, can be significantly refined due to prolonged hydration facilitated by internal curing, leading to improved resistance to carbonation [125]. Therefore, the slightly delayed CO₂ diffusion in LWC is primarily due to internal curing from moisture stored in the LWA and the formation of a more refined, less permeable microstructure in the cement matrix and ITZ.

Building upon the need for sustainable alternatives, recent investigations have focused on further using recycled and unconventional aggregates to enhance lightweight concretes’ environmental and durability performance. In an experimental study, Islam et al. [126] produced a novel sustainable lightweight concrete by replacing 100% coarse aggregates with waste tire rubber particles. Through compression casting, they achieved a significant reduction in carbonation depth, up to 46% lower than that of non-compressed samples. Improved microstructural packing and bonding at the rubber–matrix interface were identified as the main factors enhancing carbonation resistance, demonstrating the importance of optimized curing and compaction techniques [126]. Further advancing curing technologies, Zhao et al. [127] evaluated carbonation curing methods for lightweight concretes using alternative aggregates and alkali-activated binders. Their findings showed that early carbonation exposure accelerated strength development and refined pore structures, improving long-term durability and carbonation resistance.

3.5. Sustainability and CO₂ Absorption

Lightweight concrete (LWC) offers significant sustainability benefits by incorporating recycled and industrial by-products such as fly ash, expanded glass, and waste sludge as cement replacements. These strategies reduce resource consumption, lower emissions, and support circular economic principles and contribute positively to sustainability by reducing structures’ embodied energy and carbon footprint, which are compared to NWC [128]. Certain LWAs, like expanded clay, come from recycled sources and possess inherent CO₂ absorption capabilities, aiding in passive carbon capture [129]. Additionally, carbonation curing methods enhance LWC’s capacity to sequester CO₂, improving both mechanical properties and environmental performance [130,131].

Due to its porous microstructure, LWC enables 15% CO₂ uptake through natural carbonation over its service life [132]. This offsets part of the emissions from cement production and enhances surface densification. Using alkali-activated binders, such as geopolymers, further improves carbonation potential and durability in CO₂-rich environments [133]. Expanded glass aggregates enhance environmental performance while contributing to good mechanical and thermal properties [134]. LWC made with recycled aggregates such as furnace bottom ash [135], oil palm shell [136], and rubber particles [137] demonstrates not only mechanical adequacy but also better environmental profiles. Fly ash and silica fume additions improve the performance of rubberized concretes [137]. Other studies have shown effective shrinkage control and strength improvements using recycled polypropylene and expanded polystyrene particles [138,139]. Foamed LWC made with coal fly ash demonstrates a 20–30% reduction in CO₂ emissions compared to normal-weight concrete [110]. The lower density of LWC reduces the load on structural elements, particularly foundations, thereby saving on additional material use and energy-intensive construction processes. This directly contributes to decreased embodied energy across the lifecycle of buildings.

3.6. Circular Economy and Low-Impact Design

Reusing demolition aggregates [140], sludge-derived binders, and multi-blended cement systems [141] exemplify circular economy strategies. These sustainable designs align with lifecycle assessment (LCA) findings, highlighting the environmental benefits of using recycled and lightweight components [58]. In summary, LWC meets structural and performance standards and significantly contributes to environmental sustainability through CO₂ reduction, sequestration, and reuse of industrial and waste materials.

4. Cost Analysis Report

This report provides a comprehensive evaluation of the cost implications associated with the use of lightweight concrete (LWC) in construction. It examines both direct material costs and indirect savings related to structural efficiency, transportation, and labor, offering a balanced perspective on LWC’s economic viability.

Table 10. Material cost comparison [142,143].

Component	Lightweight Concrete (LWC)	Normal-Weight Concrete (NWC)
Cement (per m3)	Similar (~$100)	Similar (~$100)
Fine Aggregate (sand)	~$15	~$10
Coarse Aggregate	~$50 (expanded clay)	~$25 (crushed stone)
Water	~$2	~$2
Admixtures	~$10	~$5
Total Material Cost	~$177/m3	~$142/m3

Note: Prices are approximate and can vary based on location and market conditions.

,

Table 11. Material cost comparison [74].

C-1 100%Normal Concrete in NTD
Material	Unit Volume (m3)	Quantity (kg/m3)	Price/Ton	Total Cost (NTD)
Cement	0.19	600.00	5500.00	3300.00
Sand (FA)	0.25	651.96	450.00	293.38
NCA	0.38	1053.17	450.00	473.92
SFA	0.00	0.00	0.00	0.00
POS	0.00	0.00	0.00	0.00
Silica Fume	0.00	0.00	0.00	0.00
GGBS	0.00	0.00	0.00	0.00
Fly Ash	0.00	0.00	0.00	0.00
SP	0.01	6.00	0.00	0.00
Water	0.18	176.64	100.00	17.66
Total Quantity		2487.76		4084.97

,

Table 12. Material cost comparison [74].

E-3 Light Weight High Strength Concrete (80% SFA + 20% POS in NTD)
Material	Unit Volume (m3)	Quantity(kg/m3)	Price/Ton	Total Cost (NTD)
Cement	0.10	330.00	5500.00	1815.00
Sand (FA)	0.23	613.47	450.00	276.06
NCA	0.00	0.00	450.00	0.00
SFA	0.28	345.43	350.00	120.90
POS	0.07	82.82	5600.00	463.79
Silica Fume	0.02	60.00	30,000.00	1800.00
GGBS	0.09	210.00	1200.00	252.00
Fly Ash	0.00	0.00	800.00	0.00
SP	0.01	6.00	2000.00	12.00
Water	0.18	176.64	100.00	17.66
Total Quantity		1824.36		4757.42

NCA—Normal Coarse Aggregate; NTD—New Taiwan Dollar.

and

Table 13. Material cost comparison [74].

Cost Comparison Results—NTD (New Taiwan Dollar)
Mix	Unit Weight (kg/m3)	Cost (NTD)	fc’ (MPa)	Increment Cost	Increment Strength	Decrease Unit Weight
C1 (NC)	2487.76	4084.97	35.55	16.46%	55.98%	26.67%
E3 (LWC)	1824.36	4757.42	55.45

represent the material cost comparison of LWC and NWC.

Table 14. Labor and equipment costs [142,143].

Item	Lightweight Concrete (LWC)	Normal-Weight Concrete (NWC)
Labor	Slightly lower (easier handling)	Standard
Equipment	Standard	Standard
Transportation	Fewer trips due to lighter weight	More trips required
Total Labor and Equipment Cost	Potentially lower overall	Standard

Note: LWC’s reduced weight can lead to easier handling and fewer transportation trips, potentially lowering overall labor and equipment costs.

summarized the labor and equipment costs comparison of LWC and NWC.

4.1. Lightweight Concrete vs. Normal-Weight Concrete

Lightweight concrete (LWC) incorporates lightweight aggregates like expanded clay, shale, or slate, resulting in a lower density than normal-weight concrete (NWC). While LWC may have higher material costs per unit, it offers potential savings in structural design and construction logistics [142,143]. Table 10, Table 11, Table 12 and Table 13 represent the material cost comparison of LWC and NWC [8].

4.2. Structural and Design Implications

Lightweight concrete (LWC) offers significant advantages in reducing structural dead loads, which can lead to substantial cost savings in building construction [144]. Typically, LWC has a unit weight ranging from 90 to 115 pounds per cubic foot (pcf), compared to the 145 pcf of normal-weight concrete (NWC), resulting in a minimum weight reduction of approximately 20% [124,145]. This reduction translates into lighter structural demands, enabling the use of smaller and more cost-effective foundations and framing systems. For instance, a case study demonstrated that although LWC has a higher material cost than NWC, the total building cost was reduced by 9.2%. This was largely attributed to a 27% decrease in footing material costs and 10.5% savings in steel frame expenses [124,145]. These findings highlight how LWC can deliver not only structural efficiency but also economic benefits across a project’s lifecycle.

4.3. Logistical Advantages

Lightweight concrete (LWC) contributes to enhanced transportation and installation efficiency on construction sites. Due to its reduced density, LWC allows for a greater volume of material to be transported per truckload, thereby decreasing the number of required deliveries and reducing overall transportation costs [108,125,146]. Additionally, LWC components are generally lighter and easier to handle, which enables their installation using standard construction equipment. This eliminates the need for specialized heavy-duty cranes, often necessary for normal-weight concrete (NWC) elements, leading to further savings in equipment rental and labor costs [108,125,146]. These logistical advantages make LWC an attractive option for improving project efficiency and cost-effectiveness.

4.4. Summary

While lightweight concrete has a higher per-unit material cost compared to normal concrete, its benefits in reducing structural dead loads, foundation sizes, and transportation requirements can lead to overall cost savings in construction projects. It is essential to consider the complete project design and logistics to fully assess the economic advantages of using LWC.

5. Applications and Future Directions

Lightweight concrete (LWC) has emerged as a transformative material in modern construction, prized for its reduced density, superior thermal insulation, and structural efficiency. Its application in high-rise buildings and bridges is particularly notable, as the low self-weight of LWC significantly reduces seismic loads and foundation demands [147]. Beyond structural advantages, LWC’s excellent thermal and acoustic insulation properties make it highly suitable for energy-efficient buildings, especially when augmented with foaming agents or expanded aggregates [148]. Sustainability is further bolstered by incorporating industrial by-products such as fly ash, slag, and recycled polystyrene, effectively minimizing the material’s carbon footprint [149,150].

Recent advancements highlight the growing interest in cutting-edge manufacturing technologies like 3D printing, which facilitates the creation of complex geometries and optimizes material usage in LWC structures [147]. Parallel research on self-healing LWC, utilizing microencapsulated polymers or bacteria-based mechanisms, shows great potential in enhancing durability by autonomously repairing microcracks [151,152]. Innovations such as nano-engineered lightweight aggregates are also under exploration to improve mechanical performance while preserving low density [147,153]. Furthermore, machine learning is increasingly being integrated into mixed design processes, enabling precise optimization of strength, durability, and sustainability metrics [101,154].

Lightweight concrete (LWC) is being increasingly utilized in conjunction with 3D printing technologies to create sustainable and structurally efficient building components that minimize material consumption while offering greater architectural flexibility. For instance, researchers have successfully employed 3D-printed lightweight aggregate concrete to fabricate structural wall components with complex geometries and internal lattice structures, significantly reducing dead loads while maintaining mechanical performance [155]. Similarly, LWC mixes incorporating expanded perlite and pumice have been optimized for printability and low thermal conductivity, demonstrating potential for rapid, energy-efficient housing applications [156]. Artificial Intelligence (AI) is further transforming LWC design through machine learning (ML) models and surrogate-based optimization. Techniques such as artificial neural networks (ANNs) and support vector machines (SVMs) are being used to predict compressive strength and workability based on mix proportions and curing conditions, minimizing the need for trial-and-error experiments [157]. Additionally, genetic algorithms and Bayesian optimization are aiding in identifying optimal binder-to-aggregate ratios and admixture dosages, accelerating the development of LWC tailored for 3D printing and sustainability goals [158].

Looking forward, the trajectory of LWC research is steering toward carbon-neutral production methods, including CO₂-cured aggregates and geopolymer-based formulations, which promise to redefine the sustainability of concrete [153,159]. Developing smart LWC with embedded sensors for real-time structural health monitoring is another frontier with significant implications for infrastructure longevity and safety [160,161]. Despite these advancements, key challenges remain—particularly in balancing cost-efficiency, scalability, and long-term performance.

Future research should prioritize the broader adoption of recycled aggregates and geopolymer technologies, the advancement of fiber-reinforced and ultra-high-performance lightweight concretes, and the refining of predictive modeling techniques. Enhanced composite design, durability optimization, and interdisciplinary innovation will be instrumental in expanding the frontiers of lightweight concrete in critical structural applications. Figure 7 presents a flowchart summarizing the key takeaways from current lightweight concrete (LWC) advancements and outlining future research directions on sustainability, durability enhancement, and innovative material development.

In advanced applications, lightweight concrete is increasingly being engineered for use in modular construction, offshore structures, and aerospace-adjacent infrastructure where load reduction is paramount. LWC’s compatibility with prefabrication and automation technologies, including robotic placement and digitally controlled casting, offers substantial potential for reducing on-site labor and accelerating project timelines. Moreover, LWC’s adaptability in producing load-bearing panels, insulation boards, and sandwich composites is transforming façade engineering and energy-efficient retrofitting. With its reduced density, LWC is also being explored for use in floating structures and marine applications, where buoyancy and corrosion resistance are critical. In such cases, hybrid designs integrating fiber-reinforced LWC with corrosion-resistant reinforcement are demonstrating extended service life and improved structural integrity in chloride-laden environments [162,163].

From a material science perspective, the morphology of lightweight aggregates (LWAs)—including their shape, surface roughness, and internal porosity—has a direct and quantifiable effect on pore structure and interfacial bonding within the concrete matrix. Studies utilizing X-ray computed tomography (CT) and scanning electron microscopy (SEM) have shown that angular and rough-surfaced LWAs enhance mechanical interlock and reduce interfacial transition zone (ITZ) thickness by up to 25% compared to spherical aggregates, thereby improving compressive strength and modulus of elasticity [164]. Additionally, porous LWAs contribute to internal curing by gradually releasing absorbed water, which promotes continued hydration and reduces shrinkage-related microcracking. This behavior has been modeled through multiscale simulations and confirmed by mercury intrusion porosimeter (MIP) measurements, which correlate increased aggregate porosity with a 15–30% reduction in total shrinkage and improved long-term durability [165]. Future research should focus on optimizing LWA morphology through surface treatments or engineered coatings to further refine the pore distribution and microstructural stability of LWC under dynamic and environmental loads.

6. Conclusions

This review critically evaluated recent developments in material innovation, fabrication methods, and performance characteristics of LWC, with emphasis on optimizing mechanical properties, durability, and sustainability. Lightweight concrete (LWC) has undergone substantial development, transforming from a niche material to a high-performance, multifunctional composite tailored for modern structural and sustainability requirements. The integration of engineered lightweight aggregates, supplementary cementitious materials, fiber reinforcements, and innovative binders like geopolymers has significantly enhanced its mechanical, thermal, and durability properties. Advances in microstructural engineering and manufacturing technologies have broadened the applicability of LWC across diverse construction sectors. Despite some challenges related to cost, workability, and long-term durability, ongoing research and material innovations continue to unlock new potential, making LWC a key component of future low-carbon, resilient infrastructure. Based on the detailed analysis of Chapters 2,3,4, and 5, the following key conclusions are drawn.

Key Results:

(1)

High Mechanical Strength:

Through optimized mix designs and fiber reinforcements (steel, polypropylene, glass), LWC achieves compressive strengths surpassing 100 MPa and enhanced flexural and tensile properties.

(2)

Improved Durability:

Advanced lightweight aggregates and SCMs densify the interfacial transition zone, leading to reduced permeability and shrinkage, and improved resistance to freeze–thaw cycles and chemical attacks.

(3)

Thermal and Acoustic Insulation:

The inclusion of lightweight fillers like expanded polystyrene, aerogels, and glass microspheres substantially reduces thermal conductivity, enhancing energy efficiency in buildings.

(4)

Environmental Sustainability:

Use of recycled materials (recycled LWAs, construction waste), agricultural by-products, and low-carbon binders (geopolymers, calcium sulfoaluminate cement) reduces CO₂ emissions and support circular economy principles.

(5)

Microstructural Optimization:

Nanomaterials such as nano-silica and carbon nanotubes improve matrix density, crack resistance, and early-age strength development.

(6)

Enhanced Fracture and Impact Resistance:

Hybrid fiber systems and engineered aggregate morphology improve crack bridging, ductility, and energy absorption under dynamic and impact loading.

(7)

Adaptability to Advanced Construction:

LWC is increasingly compatible with 3D printing, prefabrication, and modular building techniques, facilitating rapid, customized construction.

(8)

Reduced Structural Loads:

Lightweight properties translate into lower dead loads, resulting in reduced foundation sizes, improved seismic resilience, and overall cost savings in structural design and transportation.

(9)

Long-Term Performance in Aggressive Environments:

Geopolymer-based LWCs and alkali-resistant fibers demonstrate superior durability in marine, chemical, and high-temperature environments.

(10)

Challenges and Research Needs:

Continued work is necessary to improve cost-efficiency, optimize workability, enhance multi-scale modeling of fracture and durability, and validate long-term field performance under diverse conditions.