Advancing 3D Printable Concrete with Nanoclays: Rheological and Mechanical Insights for Construction Applications

Abstract

Three-dimensional concrete printing (3DCP) is an emerging technology that improves design flexibility and material efficiency in construction. However, widespread adoption of 3DCP requires overcoming key material challenges. These include controlling rheology for pumpability and buildability and achieving sufficient mechanical strength. This paper provides a comprehensive review of the application of nanoclays (NCs) as a key admixture to address these challenges. The effects of three primary NCs (attapulgite (ATT), bentonite (BEN), and sepiolite (SEP)) on the fresh- and hardened-state properties of printable mortars are systematically analyzed. This review summarize findings on how NCs enhanced thixotropy, yield stress, and cohesion, which are critical for shape retention and the successful printing of multilayered structures. Quantitative analysis reveals that optimized dosages of NCs can increase compressive strength by up to 34% and flexural strength by up to 20%. For enhancing rheology and printability, a dosage of approximately 0.5% by binder weight is often suggested for ATT and SEP. In contrast, BEN can be used at higher replacement levels (up to 20%) to also function as a supplementary cementitious material (SCM), though this significantly impacts workability. This review consolidates the current knowledge to provide a clear framework for selecting appropriate NCs and dosages to develop high-performance, reliable, and sustainable materials for 3DCP applications.

1. Introduction

Three-dimensional concrete printing (3DCP) is a form of cementitious additive manufacturing used to fabricate buildings, houses, or construction components that involves the pumping, extruding, and precise placement of concrete or mortar to form these fabrications [1]. It is a new and exciting additive manufacturing technique that has the potential to revolutionize not just the concrete industry but the construction industry itself. Many advantages have been identified, yet further improvement in its material properties is necessary to ensure the durability and stability required for widespread adoption in the construction sector. The construction sector is a major industry as it contributes 38% of the global greenhouse gases (GHGs) [2]. Considering the magnitude of this pollution, it is crucial that the industry’s carbon footprints are minimized. The high cement content in 3DCP makes this technology a prime focus for emission reduction strategies, as cement production generates a significant amount of GHGs. This high carbon footprint is due to the extraction, transportation, and production of concrete constituents and a lack of cement production and handling efficiency. As 3DCP allows for precise concrete placement, material waste can be reduced greatly. Reducing human errors in constructing this material will greatly reduce emissions, a goal 3DCP can achieve.

To further increase the efficiency and performance of this emerging technology, researchers are exploring additives such as nanoclays and superplasticizers, particularly in light of growing supplementary cementitious materials (SCMs) shortages due to regulatory and industrial shifts [3]. These additives are known to enhance fresh and hardened concrete properties, including compressive strength [4,5], yield stress [6], cohesion, constructability [7], and printability [8]. However, the mechanisms behind these improvements are still not fully understood. Although the 3DCP sector is still in its early stages, significant progress has been made in both hardware development and material innovation. Additives such as fibers, recycled by-products, admixtures, and nanomaterials have contributed to performance improvements. However, several barriers continue to hinder widespread adoption, including a lack of standardized procedures, innovation resistance, and variability in material behavior [9,10,11]. Addressing these limitations is critical to unlocking the full potential of 3DCP as a transformative method in construction.

Over time, 3DCP has demonstrated significant potential for small-scale construction, and it continues to evolve with ongoing improvements. Grange Close, Dundalk, shown in Figure 1, is the first example of a small-scale application in Ireland. The entire structure was completed in 18 days, resulting in a 35% faster completion time than conventional methods [12]. Some benefits of this technology include increased design flexibility, immunity to labor shortages, low labor costs, and increased building safety and efficiency [13,14,15]. Additionally, this construction method can result in lower carbon emissions and address rapidly growing housing demands more effectively. This efficiency is attributed to the reduced error in material usage and considerably minimized formwork quantity. The latter is a significant advantage as it results in savings of both time and finances. It is estimated that roughly 35% of the total construction costs are attributable to the formwork [15]. The broader adoption of 3DCP stands to benefit various stakeholders, including prospective homeowners, on-site construction personnel through safer working conditions, and material suppliers, such as Roadstone, that support 3D printable concrete solutions.

While 3D concrete printing holds immense promise for transforming the construction industry through enhanced efficiency and sustainability, its broader implementation is currently limited by factors such as the absence of standardized protocols, resistance to innovation, and material property uncertainties [17]. Among these challenges, optimizing fresh-state and hardened-state material performance remains particularly critical, as inadequate rheology and mechanical properties directly impact printability and structural reliability. To address this, this review focuses on nanoclays (NCs), a promising class of additives known to enhance both rheological behavior and mechanical performance in cementitious systems. Although many experimental studies have explored the use of NCs to address these issues, there remains a significant lack of systematized analytical reviews that consolidate findings across studies, quantify performance impacts, and provide comparative insight into the role of NCs in 3D printable cementitious systems.

To ensure a comprehensive and unbiased evaluation of the effects of clay incorporation in cementitious materials, a structured literature review was conducted using multiple academic sources, including peer-reviewed journal articles, conference proceedings, and doctoral dissertations. The primary database employed was Web of Science, covering publications from 2010 to 2025, supplemented by other reputable platforms (Scopus, Google Scholar) to capture both recent advancements and foundational studies. The search was guided by targeted keywords, such as nanoclay, attapulgite, bentonite, sepiolite, rheology, printability, and mechanical performance. Only English-language publications were selected to maintain relevance and uniformity. While emphasis was placed on contemporary research to reflect the current state of the art, earlier influential studies were also included to provide a foundational background and continuity. This review is thematically divided into two main domains: fresh-state properties (Section 6.1), which include flowability, yield stress, viscosity, thixotropy, and rigidification, and hardened-state properties (Section 6.2), focusing on compressive strength and reinforcement-concrete bond behavior. Each section presents a focused analysis of the relevant literature to elucidate the impact of clay modification on printability and performance.

This review aims to quantify the effects of NCs on flowability, yield stress, buildability, and compressive and flexural strengths in 3D printable concrete. Ultimately, it identifies optimal NC dosages and develops a refined mix design tailored to 3DCP. By critically analyzing the current state of knowledge, this review intends to bridge existing research gaps, support future experimental work, and promote the development of high-performance, sustainable materials for digital construction.

2. NC Types

The focus of this study is on attapulgite, bentonite, and sepiolite. Attapulgite is the most commonly tested material in this field, and to build on this foundation, this area was explored. Two more NCs were included in order to produce a substantial analysis. According to the existing literature, all three of these NCs show promising potential for improving 3DCP mixtures. The chemical composition of these three types of NCs, as reported in the literature, is reproduced in

Table 1. Chemical composition of three NC types, reported by Hanratty, Khan [18].

Constituent	ATT (%)	BEN (%)	SEP (%)
CaO	3.51	2.49	0.5
SiO2	52.7	46.3	60.5
Al2O3	11.3	18.2	2.4
MgO	8.02	2.72	23.8
Fe2O3	3.96	5.72	0.9
K2O	0.894	1.5	0.5
Na2O	<0.0001	0.238	0.1
P2O5	0.476	0.0385	-
TiO2	0.616	0.485	-
MnO	0.0345	0.054	-
SO	-	-	-
S2−	-	-	-
Cl	-	-	-
L.O.I	-	-	11.3

.

2.1. Attapulgite (ATT)

Attapulgite (ATT) is a naturally occurring chain-structured magnesium silicate clay, characterized by a fibrous morphology, high surface area, and strong water absorption capacity. It is primarily composed of SiO₂ (approximately 62.7 wt.%), Al₂O₃ (approximately 12.1 wt.%), and MgO (approximately 11.0 wt.%) and is typically used in powder form as an additive in cementitious systems [19]. Also known as palygorskite, ATT exhibits a needle-like morphology with an average length of approximately 1.75 μm and a diameter of 3 nm [20]. Its fibrous structure and high specific surface area contribute to notable improvements in the rheological behavior of cementitious materials. As shown in Figure 2, scanning electron microscopy (SEM) reveals the presence of both fibrous and cubic morphologies, the latter associated with carbonate impurities [21]. These microstructural features promote stronger particle–particle interactions, thereby enhancing yield stress, thixotropy, and shape stability in fresh concrete mixtures. The increased contact surface area also facilitates greater physical bonding and chemical reactivity within the cement matrix, a behavior generally observed across various NCs used in concrete [22].

ATT is often used in a highly purified form to ensure consistent performance. According to Bohuchval et al. [23] and Sonebi et al. [7], the incorporation of ATT improves cohesiveness, buildability, and bleeding resistance, while simultaneously enhancing workability and thixotropic recovery. Bleeding refers to the upward migration of excess water to the surface of freshly placed concrete, which can compromise mechanical and durability performance. Moreover, the thermal activation of attapulgite at 650 to 750 °C has been shown to enhance its pozzolanic reactivity by increasing the availability of reactive SiO₂ and Al₂O₃. These active phases react with calcium hydroxide to form additional C–S–H and C–A–H products, promoting matrix densification and strength development. Notably, Hanratty and Khan [18] reported that ATT can increase compressive strength by at least 34%, highlighting its multifunctional role as both a rheology modifier and a strength-enhancing additive in cement-based materials.

2.2. Bentonite (BEN)

Bentonite (BEN) is a naturally occurring smectite-rich clay formed through the in situ alteration of volcanic ash or glass and is primarily composed of montmorillonite. Its chemical composition typically includes SiO₂ (approximately 49.6–52.1 wt.%), Al₂O₃ (13.4–21.1 wt.%), and MgO (2.6–12.6 wt.%), meeting ASTM requirements for pozzolanic materials, as the combined content of SiO₂, Al₂O₃, and Fe₂O₃ exceeds 70 wt.% [24]. Figure 3 presents the morphology of BEN particles, where SEM images reveal irregularly shaped aggregates with a broad size distribution arranged in stacked, sheet-like configurations, characteristic of its layered clay mineral structure [25]. A bright crystallite, potentially corresponding to free silica (quartz), can also be observed on the surface. Moeini et al. [26] further described BEN as having a sheet-like layered structure with a thickness of approximately 1 nm and a specific surface area (SSA) of 1.22 m²/g. This high SSA, typical of NCs, plays a critical role in modifying the rheological behavior of cementitious materials.

In practical applications, BEN is typically incorporated in powder form as an SCM, often replacing 15–20 wt.% of Portland cement. Its flaky morphology and layered microstructure increase internal friction within the mix, leading to reduced flowability and fresh density. Nevertheless, BEN contributes significantly to durability improvements, including resistance to acid attack, sulfate exposure, and drying shrinkage [27]. In terms of rheology, BEN demonstrates a strong effect on the early structural buildup of cementitious pastes. Under identical conditions, BEN produces a higher initial static yield stress than other NCs, such as halloysite [25]. Despite potential drawbacks, such as reduced workability, which can be mitigated through the use of superplasticizers, BEN contributes to long-term strength gain due to progressive pozzolanic reactivity. Some studies [28] report an increase of up to 2.7% in 56-day compressive strength at optimal replacement levels, though slight reductions have also been observed depending on bentonite origin and processing. Overall, BEN functions both as a sustainability-promoting SCM and as a rheology modifier, enhancing green strength and structural robustness in cement-based 3D-printed materials.

2.3. Sepiolite (SEP)

Sepiolite (SEP) is a mineralogical fibrous magnesium silicate clay with a porous microstructure, high water absorption capacity, and strong thixotropic characteristics. Its general chemical formula is [(OH₂)₄Mg₈(OH)₄Si₁₂O₃₀]·8H₂O, comprising mainly SiO₂, MgO, and minor quantities of Al or Fe. Structurally, SEP belongs to the palygorskite family and shares similarities with attapulgite (ATT), but it exhibits larger needle-shaped particles and a higher Brunauer–Emmett–Teller (BET) specific surface area. As reported by Hanratty et al. [29], its median particle size is approximately 57 μm, and the elevated BET value enhances its entanglement and flocculation capacity in cement suspensions. Figure 4 shows the characteristic needle-like morphology and fibrous architecture of SEP [30], which contribute to increased viscosity and cohesion by promoting particle interlocking and network formation.

When incorporated into cementitious systems, typically at a dosage of 0.5% to 2.0% by the mass of binder, SEP acts as an effective rheology modifier, significantly enhancing static yield stress, dynamic yield stress, thixotropy, and the rate of structural build-up and recovery over time [31,32]. Unlike conventional chemical viscosity-modifying agents (VMAs), SEP displays a delayed progressive influence on rheological properties, thereby maintaining long-term stability during extrusion-based 3D printing processes [33]. Its impact on shear thickening behavior is also notable, as SEP was found to be more effective than other NCs, such as ATT and BEN, in mitigating excessive viscosity increases under high shear strain [32]. Moreover, a compressive strength increase of at least 25% was observed in SEP-modified systems, underscoring its multifunctional role in improving both fresh-state printability and hardened mechanical performance [18].

3. Admixtures

It is important to note that cement, fly ash, silica fume, and ground granulated blast furnace slag (GGBS) are commonly used as binders. Zhang et al. [34] reported that most 3DCP mixes are characterized by a high binder content and relatively small aggregate size compared to concrete mixtures of traditional construction methods, such as cast concrete. This high binder content increases the environmental impact. Therefore, using SCMs like fly ash and GGBS is crucial [8].

3.1. Superplasticizer (SP)

Superplasticizers (SPs), or high-range water reducers (HRWRs), are water-soluble polymer dispersants added to concrete mixes to improve flowability and ensure uniform distribution of solid components [35]. Lignosulfonate (LS), polynaphthalene sulfonate (PNS), and polycarboxylate ether (PCE) are some commonly used SPs in 3DCP mixes [9]. These additives are essential in 3DCP as the concrete must be sufficiently fluid to be pumped and extruded through the printer. From the constant shear rate test, an SP was shown to have the ability to increase the thixotropic coefficient, A_thix. This represents the structuration of 3DCP. Compared to all the other mixtures, the mixture containing the NCs and the SP exhibited the highest static yield stress. This is because the SP more evenly distributes the NC particles, and they can contribute more to the static yield stress. The ability of SPs to make materials such as concrete behave more fluidly is crucial in 3DCP. For example, as reported by Sonebi et al. [7], it was discovered that a decrease in SP content resulted in a corresponding reduction in slump spread diameter. Marchon et al. [9] suggested that using an SP resulted in a more uniform matrix, improved workability, and facilitated the development of mixtures with reduced water-to-cement ratios for better durability. Kanagasuntharam et al. [4] stated that an SP enhanced the workability and increased the open time of concrete. Open time is when the material can be adequately extruded [23]. The range of contents of superplasticizers considered in the previous literature is given in Table 2.

Table 2. Superplasticizer content in the past literature.

Reference	Content of SP (% of Binder Weight)
Abdalqader et al., 2022 [36]	0.2–0.4
Kilic et al., 2023 [37]	1.2
Nikravanet al., 2025 [38]	1–1.5
Bodur et al., 2025 [39]	3
Sadeghzadeh et al., 2025 [40]	0.5
Yan et al., 2023 [41]	1
Saka et al., 2023 [42]	1
Gou et al., 2024 [43]	1
Hanratty et al., 2024 [18]	1.2
Kanagasuntharam et al., 2024 [4]	0.471
Li et al., 2024 [3]	0.1
Moeini et al., 2022 [44]	0.2, 0.25, 0.35, 0.4, 0.5
Varela et al., 2023 [29]	0.38–5
Varela et al., 2024 [45]	0.4–1.8

Table 2. Superplasticizer content in the past literature.

Reference	Content of SP (% of Binder Weight)
Abdalqader et al., 2022 [36]	0.2–0.4
Kilic et al., 2023 [37]	1.2
Nikravanet al., 2025 [38]	1–1.5
Bodur et al., 2025 [39]	3
Sadeghzadeh et al., 2025 [40]	0.5
Yan et al., 2023 [41]	1
Saka et al., 2023 [42]	1
Gou et al., 2024 [43]	1
Hanratty et al., 2024 [18]	1.2
Kanagasuntharam et al., 2024 [4]	0.471
Li et al., 2024 [3]	0.1
Moeini et al., 2022 [44]	0.2, 0.25, 0.35, 0.4, 0.5
Varela et al., 2023 [29]	0.38–5
Varela et al., 2024 [45]	0.4–1.8

3.2. Viscosity-Modifying Admixture (VMA)

Figure 5 presents the effect of increasing NC content on the fresh properties of cementitious mixtures [46]. As the NC dosage increases, both slump flow and penetration depth decrease, indicating a reduction in workability. An increase in static yield stress is observed, which reflects an enhanced ability to maintain structure and resist deformation under minimal shear. These trends confirm that NCs improve the rheological structure by increasing cohesiveness and internal friction, although accompanied by reduced flowability. In the context of 3DCP, such rheological modifications are particularly relevant. Ma and Kawashima [47] emphasized that the ability to uniformly distribute water throughout the paste matrix is critical for minimizing bleeding and evaporation losses. To this end, viscosity-modifying admixtures (VMAs) are employed to adjust water retention, surface moisture dynamics, and yield behavior. Common VMAs include NCs, colloidal silica, cellulose-based additives, and modified starches [44].

Bohuchval et al. [23] conducted a systematic investigation using an NC-based VMA in concrete mixes. Their results showed that VMA addition significantly reduced both flow diameter and penetration depth while increasing the estimated yield stress. These effects were attributed to the VMA’s fine particle size and SSA, which allowed for uniform dispersion and stronger interaction with the cementitious matrix. This led to increased shear resistance, higher cohesiveness, and a greater demand for mixing water. The same study also noted improvements in buildability and mix stability following VMA incorporation. Leemann and Winnefeld [48] further supported these findings by demonstrating that VMAs can effectively suppress bleeding, improving the stability of fresh mixtures. In a separate study [7], purified palygorskite (a type of ATT NC) was applied as a VMA at 0.8%, 1.1%, and 1.6% of binder weight. This treatment induced pronounced shear-thinning behavior and substantially reduced extrusion time, attributed to accelerated internal structuring and a shortened setting time. The addition of NC-based VMAs also proved effective at enhancing cohesion and viscosity even under elevated water-to-binder ratios, thereby facilitating improved extrusion and shape retention in 3DCP applications.

4. Mixing Procedures

The mixing procedure is foundational to achieving the desired rheological behavior, structural performance, and reliability of 3DCP. Moreover, standardized and optimized mixing procedures are essential for scaling up 3D printing processes and integrating them into automated production systems. Due to the time-dependent nature of the contents of concrete mixtures, a strict protocol must be adhered to so that reliable comparisons between each mixture can be made, i.e., the mixing time of each concrete mix must be equal [7,23]. Mostly, concrete mixtures are mixed in a rotational mixer or blender [18]. However, Kazemian et al. [49] and Ma and Kawashima [47] used a drum and a hand mixer, respectively. Generally, the dry components, i.e., binder (cement, fly ash, GGBS, silica fume, etc.), and sand are mixed first for 30 s to 3 min at a low to medium speed to achieve optimal component dispersion. Often, in a separate container, admixtures, such as NC, SP, VMA, retarders, etc., are mixed with water for 1–2 min during or directly after the dry mixing to be added to the mixture as a suspension. Then, the wet mix is slowly combined with the dry mix and mixed for 30 s to 3 min at a low speed [4,7,8,23,49,50]. It was noted by Kanagasuntharam et al. [4] that the mixing of the wet and dry mixes should be paused momentarily to allow scraping of the side of the bowl to ensure a homogenous mix. In [26], the mixing method followed ASTM C 305 specifications. Hanratty et al. [18] employed a combination of low-speed and high-speed mixing, as shown in Figure 6, to balance the dispersion energy and structural integrity of the mix. The initial low-speed dry mix prevented particle segregation, while the high-speed wet blend ensured homogeneity and effective SP action. The return to low-speed final blending is strategic, minimizing damage to fragile flocculated networks vital for thixotropy and buildability in 3D printable concrete. During high-speed mixing, excessive shear can prematurely degrade structured flocs formed by clays (notably in ATT and BEN), reducing their effect on early yield stress and buildability, but it is also required to disperse SP particles and agglomerated clay clusters to achieve a homogeneous microstructure.

5. Pumping and Extruding

Zhang et al. [34] and Sonebi et al. [7] identified several critical process-related material properties that are essential for the successful implementation of 3D concrete printing, particularly those associated with the fresh state of the mix, such as pumpability. Pumpability or flowability refers to the mix’s capacity to flow effectively through a pressurized pumping system while maintaining its integrity and consistency. Extrudability describes the ability of the mixture to be continuously and uniformly extruded through the nozzle without interruption or segregation. Buildability is defined as the ability of the extruded material to retain its intended geometry, shape, and size under sustained and progressively increasing loads. Printability, a broader term, encompasses extrudability and buildability, reflecting the overall feasibility of producing stable and accurate prints. Finally, constructability refers to the maximum period a mix can remain in the nozzle before blockage occurs, directly affecting printing efficiency and continuity.

The traits mentioned above must be optimized to achieve a satisfactory product. Some challenges arise from attempting to satisfy all the criteria. Zhang et al. [34] mentioned how pumpability and buildability contradict each other. The material must be fluid enough to be pumped but stiff enough to retain the layer’s shape, mainly when more material is deposited on top. This results in pumpability being dependent on printing machine pump parameters. The composition of the mix and the printer’s nozzle have been highlighted as essential factors for adequate extrusion. The composition directly affects extrudability and the mixture’s thixotropic properties. A high plastic viscosity and yield stress are desired to achieve proper buildability. These properties are critical, as deformations in the extruded layers will result in misalignments and instabilities in the composition of the final product, drastically reducing its durability and mechanical properties. Sonebi et al. [7] used a ‘printability box’ to visualize the threshold of adequate 3DCP. If the properties of a given mixture are inside this box, he rheological characteristics are adequate to obtain the correct printed layers. This idea is applied to the graphs of the penetrometer, flow table spread results, and the estimated yield stress.

6. Effects of NCs

6.1. Fresh Properties

The fresh-state behavior of cement, mortar, and concrete is critical for the successful implementation of 3DCP. This section examines the influence of NC additions on key rheological parameters, including workability, yield stress, plastic viscosity, thixotropy, and early-age rigidification. Special attention is given to the roles of ATT, BEN, and SEP, as summarized in Table 3, Table 4 and Table 5, with emphasis on their potential to enhance printability-related functions, such as extrudability, buildability, and shape stability.

Table 3. The effect of attapulgite clay on rheological properties.

Ref. (Year)	Clay Type	Mix Design	Rheology Parameter	Change Trend	3D Printability
[51] 2019	Attapulgite	0.1–0.5 wt.% in 70% fly ash	Static yield stress Viscosity Thixotropy	At 0.5% attapulgite, static yield stress and thixotropy increase without significantly increasing viscosity.	At 0.5% attapulgite, there is higher viscosity recovery and enhanced structural build-up over time, indicating improved printability.
[52] 2021	Attapulgite	1 wt.% in alkali-activated material (AAM)	Flowability Yield stress Viscosity	At 1.0% attapulgite, thixotropy and static yield stress increase.	Attapulgite functions as both a rheology modifier and reinforcement, consistent with [44] (0.25–0.5 wt.% combined with VMA), enhancing printability, mechanical performance, extrusion and buildability.
[53] 2022	Attapulgite	0.5–3.0 wt.% in 90%OPC and 10% sulphoaluminate cement (SAC)	Dynamic yield stress Plastic viscosity Static yield stress Structural build-up rate Flowability	Dynamic yield stress: +192%. Plastic viscosity: +129%. Static yield stress: +400%. Structural build-up rate: +717%. Flowability: increases the rate of fluidity loss over time (optimal dosage is 3.0 wt.%).	Excessive attapulgite content (>1%) leads to a significant reduction in interlayer bonding (slant shear) strength due to increased interfacial porosity and reduced actual contact area between layers.
[54] 2014	Attapulgite	0.2 and 0.5 wt.%	Peak normal force (cohesion) Storage modulus (G′)	Peak normal force increases significantly with attapulgite content at all pulling velocities. Higher G’ observed with attapulgite at all times, indicating a stiffer and more cohesive microstructure.	——
[55] 2018	Attapulgite	0.25 and 0.5 wt.% with changed PCE dosage	Dynamic yield stress Thixotropic index Characteristic time for de-structuration	At 0.5 wt.% attapulgite, the dynamic yield stress and thixotropic index increase, and the characteristic time for de-structuration decreases, indicating faster structural breakdown due to a denser initial microstructure.	Enhanced shape stability and buildability, with increased resistance to deformation after deposition.
[37] 2023	Attapulgite	0.2, 0.4, and 0.6 wt.% in OPC and CSA cement, combined with silica fume, class F fly ash, and changed VMA dosage	Dynamic yield stress Static yield stress Plastic viscosity Storage modulus (G′) Loss factor	Dynamic and static yield stresses increase significantly with attapulgite dosage, storage modulus rises consistently, loss factor decreases, and plastic viscosity shows no clear trend unless combined with high VMA. Attapulgite shows stronger statistical influence than VMA.	Moderate attapulgite dosages (0.4–0.6%) enhance buildability and stability by increasing G′ and printable layers without collapse, enabling robust prints, though excessive attapulgite can reduce extrudability.
[56] 2016	Attapulgite	0.1, 0.3, and 0.5 wt.%	Static yield stress Thixotropic rebuilding Critical strain	Static yield stress increases significantly with attapulgite content (0.5% attapulgite nearly doubles it); thixotropic rebuilding is faster, while critical strain slightly decreases, indicating a stiffer structure.	Attapulgite enhances buildability by increasing static yield stress and accelerating thixotropic recovery; the creep recovery method proves effective for evaluating highly thixotropic systems where conventional tests fall short.

Table 3. The effect of attapulgite clay on rheological properties.

Ref. (Year)	Clay Type	Mix Design	Rheology Parameter	Change Trend	3D Printability
[51] 2019	Attapulgite	0.1–0.5 wt.% in 70% fly ash	Static yield stress Viscosity Thixotropy	At 0.5% attapulgite, static yield stress and thixotropy increase without significantly increasing viscosity.	At 0.5% attapulgite, there is higher viscosity recovery and enhanced structural build-up over time, indicating improved printability.
[52] 2021	Attapulgite	1 wt.% in alkali-activated material (AAM)	Flowability Yield stress Viscosity	At 1.0% attapulgite, thixotropy and static yield stress increase.	Attapulgite functions as both a rheology modifier and reinforcement, consistent with [44] (0.25–0.5 wt.% combined with VMA), enhancing printability, mechanical performance, extrusion and buildability.
[53] 2022	Attapulgite	0.5–3.0 wt.% in 90%OPC and 10% sulphoaluminate cement (SAC)	Dynamic yield stress Plastic viscosity Static yield stress Structural build-up rate Flowability	Dynamic yield stress: +192%. Plastic viscosity: +129%. Static yield stress: +400%. Structural build-up rate: +717%. Flowability: increases the rate of fluidity loss over time (optimal dosage is 3.0 wt.%).	Excessive attapulgite content (>1%) leads to a significant reduction in interlayer bonding (slant shear) strength due to increased interfacial porosity and reduced actual contact area between layers.
[54] 2014	Attapulgite	0.2 and 0.5 wt.%	Peak normal force (cohesion) Storage modulus (G′)	Peak normal force increases significantly with attapulgite content at all pulling velocities. Higher G’ observed with attapulgite at all times, indicating a stiffer and more cohesive microstructure.	——
[55] 2018	Attapulgite	0.25 and 0.5 wt.% with changed PCE dosage	Dynamic yield stress Thixotropic index Characteristic time for de-structuration	At 0.5 wt.% attapulgite, the dynamic yield stress and thixotropic index increase, and the characteristic time for de-structuration decreases, indicating faster structural breakdown due to a denser initial microstructure.	Enhanced shape stability and buildability, with increased resistance to deformation after deposition.
[37] 2023	Attapulgite	0.2, 0.4, and 0.6 wt.% in OPC and CSA cement, combined with silica fume, class F fly ash, and changed VMA dosage	Dynamic yield stress Static yield stress Plastic viscosity Storage modulus (G′) Loss factor	Dynamic and static yield stresses increase significantly with attapulgite dosage, storage modulus rises consistently, loss factor decreases, and plastic viscosity shows no clear trend unless combined with high VMA. Attapulgite shows stronger statistical influence than VMA.	Moderate attapulgite dosages (0.4–0.6%) enhance buildability and stability by increasing G′ and printable layers without collapse, enabling robust prints, though excessive attapulgite can reduce extrudability.
[56] 2016	Attapulgite	0.1, 0.3, and 0.5 wt.%	Static yield stress Thixotropic rebuilding Critical strain	Static yield stress increases significantly with attapulgite content (0.5% attapulgite nearly doubles it); thixotropic rebuilding is faster, while critical strain slightly decreases, indicating a stiffer structure.	Attapulgite enhances buildability by increasing static yield stress and accelerating thixotropic recovery; the creep recovery method proves effective for evaluating highly thixotropic systems where conventional tests fall short.

Table 4. The effect of bentonite clay on rheological properties.

Ref. (Year)	Clay Type	Mix Design	Rheology Parameter	Change Trend	3D Printability
[57] 2018	Bentonite	8, 10, 12, 14, 16, and 18 wt.%	Mini-slump cone test Density measurement Bleeding test	Mini-slump decreases significantly with higher bentonite dosage (8–18%), fresh density declines consistently, bleeding is markedly reduced and eliminated at 16% dosage, and water absorption in the hardened state decreases, indicating a denser matrix.	——
[58] 2022	Bentonite	2.5, 5, 7.5, 10, 12.5, and 15 wt.% combined with silica fume, limestone powder, and steel fiber	Flowability Static yield stress Dynamic yield stress Plastic viscosity Thixotropy	Flowability decreases by 55.7% as bentonite increases to 15%; static and dynamic yield stress increase by ~17× and ~5.6×, respectively, plastic viscosity rises moderately (~1.2×), and thixotropy shows slight improvement (~0.04×).	——
[59] 2020	Bentonite	1, 2, and 3 wt.%	Static yield stress Thixotropy Dynamic yield stress Plastic viscosity	At 2% bentonite, static yield stress and thixotropy increase significantly, with dynamic yield stress < 645.54 Pa and plastic viscosity < 2.50 Pa·s; thixotropy correlates closely with yield stress.	Bentonite significantly enhances thixotropy and static yield stress, improving shape stability and creep resistance for deformation-free 3D structures.
[60] 2025	Bentonite	1, 3, and 5 wt.% with varied grouting pressures and water/cement ratios	Plastic viscosity Yield stress Time dependence Thixotropy	Plastic viscosity and yield stress both increase with bentonite; viscosity shows time dependent behavior, with exponential growth at 5% due to strong thixotropy and water retention, while all mixes exhibit shear thinning Bingham behavior that becomes more pronounced at higher dosages.	——
[61] 2021	Bentonite	1, 2, and 3 wt.% combined with magnesia, KH2PO4, K2HPO4, fly ash, and ground granulated blast furnace slag	Fluidity Static yield stress Dynamic yield stress Thixotropy Storage and loss modulus Creep resistance	Fluidity decreases significantly with bentonite addition; static and dynamic yield stresses increase strongly, while plastic viscosity changes slightly. Thixotropy improves modestly, with thixotropic area increasing to 1.05× the reference at 3% bentonite. Storage and loss modulus rises rapidly, indicating earlier solid-like behavior; creep resistance improves as maximum shear stress increases from approximately 50 Pa at 1% to 200 Pa at 3%.	Adding 3% bentonite reduces the deformation rate from 18.13% to 1.11%, significantly enhancing shape stability and enabling more printable layers.
[62] 2020	800 °C calcined Na-bentonite and Ca-bentonite	5, 10, 15, 20, 25, and 30 wt.% cement replacement	Slump flow T500 flow time V-funnel flow time L-box blocking ratio Sieve stability test	Slump flow decreases and T500/V-funnel times increase with higher bentonite content, indicating reduced flowability and increased viscosity; the segregation index improves, showing enhanced stability.	——
[63] 2024	Bentonite	1, 2, 3, 4, and 5 wt.% cement replacement	Static yield stress Thixotropy Fluidity	Static yield stress and thixotropy increase markedly with bentonite content (up to 203.8% and 98.5%, respectively, at 5%), while fluidity decreases consistently.	Buildability improves significantly with bentonite addition, enabling stable multilayer printing (up to 52 layers), though excessive dosage may hinder extrusion due to reduced fluidity.
[64] 2014	Bentonite	2, 4, 6, 8, and 10 wt.% cement replacement	Viscosity Yield stress Thixotropy	Viscosity and yield stress increase with bentonite, especially at low shear; thixotropy improves, while superplasticizer mitigates yield stress and enhances fluidity.	——
[65] 2020	Bentonite	2~8 wt.% cement replacement	Yield stress Plastic viscosity Apparent viscosity	Yield stress increases from 2 Pa at 2% bentonite to 31 Pa at 8% bentonite; both plastic and apparent viscosities also rise with a higher bentonite content.	——
[66] 2024	Bentonite	1.25, 1.8, and 2.4 wt.% combined with fly ash-based aqueous nano-silica and alkali-activated binders	Yield stress Viscosity	Yield stress increases from ~10 Pa (no additive) to 66.6 Pa (1.8% bentonite) and 263.7 Pa (2.4% bentonite); viscosity also increases with higher bentonite content.	Buildability and shape retention significantly improve with optimal mix (1.8% bentonite); excess clay (>2.4%) reduces extrudability and print quality.
[67] 2011	Na-bentonite Ca-bentonite	0.5~10 wt.% with and without added electrolytes	Apparent viscosity Yield stress Thixotropy	Apparent viscosity and yield stress increase with bentonite content; Na-bentonite shows ~100× higher thixotropy than Ca-bentonite; yield stress becomes significant above 4~6% bentonite content,	——

Table 4. The effect of bentonite clay on rheological properties.

Ref. (Year)	Clay Type	Mix Design	Rheology Parameter	Change Trend	3D Printability
[57] 2018	Bentonite	8, 10, 12, 14, 16, and 18 wt.%	Mini-slump cone test Density measurement Bleeding test	Mini-slump decreases significantly with higher bentonite dosage (8–18%), fresh density declines consistently, bleeding is markedly reduced and eliminated at 16% dosage, and water absorption in the hardened state decreases, indicating a denser matrix.	——
[58] 2022	Bentonite	2.5, 5, 7.5, 10, 12.5, and 15 wt.% combined with silica fume, limestone powder, and steel fiber	Flowability Static yield stress Dynamic yield stress Plastic viscosity Thixotropy	Flowability decreases by 55.7% as bentonite increases to 15%; static and dynamic yield stress increase by ~17× and ~5.6×, respectively, plastic viscosity rises moderately (~1.2×), and thixotropy shows slight improvement (~0.04×).	——
[59] 2020	Bentonite	1, 2, and 3 wt.%	Static yield stress Thixotropy Dynamic yield stress Plastic viscosity	At 2% bentonite, static yield stress and thixotropy increase significantly, with dynamic yield stress < 645.54 Pa and plastic viscosity < 2.50 Pa·s; thixotropy correlates closely with yield stress.	Bentonite significantly enhances thixotropy and static yield stress, improving shape stability and creep resistance for deformation-free 3D structures.
[60] 2025	Bentonite	1, 3, and 5 wt.% with varied grouting pressures and water/cement ratios	Plastic viscosity Yield stress Time dependence Thixotropy	Plastic viscosity and yield stress both increase with bentonite; viscosity shows time dependent behavior, with exponential growth at 5% due to strong thixotropy and water retention, while all mixes exhibit shear thinning Bingham behavior that becomes more pronounced at higher dosages.	——
[61] 2021	Bentonite	1, 2, and 3 wt.% combined with magnesia, KH2PO4, K2HPO4, fly ash, and ground granulated blast furnace slag	Fluidity Static yield stress Dynamic yield stress Thixotropy Storage and loss modulus Creep resistance	Fluidity decreases significantly with bentonite addition; static and dynamic yield stresses increase strongly, while plastic viscosity changes slightly. Thixotropy improves modestly, with thixotropic area increasing to 1.05× the reference at 3% bentonite. Storage and loss modulus rises rapidly, indicating earlier solid-like behavior; creep resistance improves as maximum shear stress increases from approximately 50 Pa at 1% to 200 Pa at 3%.	Adding 3% bentonite reduces the deformation rate from 18.13% to 1.11%, significantly enhancing shape stability and enabling more printable layers.
[62] 2020	800 °C calcined Na-bentonite and Ca-bentonite	5, 10, 15, 20, 25, and 30 wt.% cement replacement	Slump flow T500 flow time V-funnel flow time L-box blocking ratio Sieve stability test	Slump flow decreases and T500/V-funnel times increase with higher bentonite content, indicating reduced flowability and increased viscosity; the segregation index improves, showing enhanced stability.	——
[63] 2024	Bentonite	1, 2, 3, 4, and 5 wt.% cement replacement	Static yield stress Thixotropy Fluidity	Static yield stress and thixotropy increase markedly with bentonite content (up to 203.8% and 98.5%, respectively, at 5%), while fluidity decreases consistently.	Buildability improves significantly with bentonite addition, enabling stable multilayer printing (up to 52 layers), though excessive dosage may hinder extrusion due to reduced fluidity.
[64] 2014	Bentonite	2, 4, 6, 8, and 10 wt.% cement replacement	Viscosity Yield stress Thixotropy	Viscosity and yield stress increase with bentonite, especially at low shear; thixotropy improves, while superplasticizer mitigates yield stress and enhances fluidity.	——
[65] 2020	Bentonite	2~8 wt.% cement replacement	Yield stress Plastic viscosity Apparent viscosity	Yield stress increases from 2 Pa at 2% bentonite to 31 Pa at 8% bentonite; both plastic and apparent viscosities also rise with a higher bentonite content.	——
[66] 2024	Bentonite	1.25, 1.8, and 2.4 wt.% combined with fly ash-based aqueous nano-silica and alkali-activated binders	Yield stress Viscosity	Yield stress increases from ~10 Pa (no additive) to 66.6 Pa (1.8% bentonite) and 263.7 Pa (2.4% bentonite); viscosity also increases with higher bentonite content.	Buildability and shape retention significantly improve with optimal mix (1.8% bentonite); excess clay (>2.4%) reduces extrudability and print quality.
[67] 2011	Na-bentonite Ca-bentonite	0.5~10 wt.% with and without added electrolytes	Apparent viscosity Yield stress Thixotropy	Apparent viscosity and yield stress increase with bentonite content; Na-bentonite shows ~100× higher thixotropy than Ca-bentonite; yield stress becomes significant above 4~6% bentonite content,	——

Table 5. The effect of sepiolite clay on rheological properties.

Ref. (Year)	Clay Type	Mix Design	Rheology Parameter	Change Trend	3D Printability
[68] 2019	Ca-Sepiolite	2.5, 5, 7.5, 10, and 15 wt.% cement replacement	Yield stress plastic viscosity Workability Water absorption	Cement paste and mortar with 15% Ca-sepiolite show increased yield stress (+88.8% and +116.8%) and plastic viscosity (+8.1% and +52.1%); workability decreases and water absorption rises due to Ca-sepiolite’s high surface area and porosity.	——
[69] 2021	400, 600, 700, 800, 900, and 1000 °C calcinated sepiolite	20 wt.% cement replacement	Yield stress Plastic viscosity	Sepiolite increases yield stress, peaking at 700 °C calcination (407.3 Pa) and then declining at higher temperatures; all sepiolite types raise plastic viscosity above the control, though viscosity decreases with higher calcination temperature and is lowest at 1000 °C (2.0 Pa·s).	——
[70] 2020	Unmodified sepiolite and modified (acid and silane treatment) sepiolite	0.5, 1.0, 1.5, and 2.0 wt.% combined with carbon fiber in oil well cement	Apparent viscosity Thixotropy Workability Water loss Dispersion	At 2% dosage, unmodified sepiolite increases apparent viscosity by 328%, while modified sepiolite raises it by 83%; modified sepiolite enhances viscosity and thixotropy less than unmodified sepiolite, maintains better fluidity (>21 cm), reduces water loss more effectively, and exhibits superior dispersion stability.	——
[38] 2025	Sepiolite	1 wt.% combined with 20 wt.%FA and (or) 0.15% polyamide microfiber	Flow table test	Sepiolite acts as a rheology modifier that enhances thixotropy by improving water retention and microstructural locking, leading to a 20.9% reduction in deformation within 2 h.	——
[71] 2017	Sepiolite	1 and 2.5 wt.% lime replacement in white Portland cement (WPC) 1 and 2 wt.% cement replacement in OPC	Yield stress Expansion speed Water retention Build-up thickness	Sepiolite addition increases paste yield stress (e.g., >307 Pa at 2.5% WPC after 10 min), significantly enhances water retention, promotes thicker buildup (>2 cm before slump), and accelerates expansion while reducing bulk density when combined with fly ash or metakaolin.	Sepiolite-containing mortars (with or without metakaolin) demonstrate excellent shootability and build-up thickness (>2 cm), enabling vertical spray application with no slumping, strong adhesion to ceramic substrates, and stable, low-density hardened layers suitable for facade insulation.
[72] 2022	Sepiolite	0.5 and 1 wt.% combined with 20% FA	Static yield stress Dynamic yield stress thixotropy	Sepiolite at 0.5–1% markedly increases static and dynamic yield stress and enhances thixotropy, especially in FA blends, though excessive dosage may hinder printability due to overly high yield stress; it exhibits superior nano-montmorillonite in rheological improvement.	At a 0.5% dosage, sepiolite enables successful extrusion with improved buildability and shape retention, while a 1% dosage leads to excessive stiffness and poor extrudability.
[73] 2023	Sepiolite	0, 5, 10, 15, and 20 wt.%	Water demand Viscosity Setting time	Water demand and viscosity increase linearly with sepiolite dosage, while a higher sepiolite content also prolongs setting time.	——
[40] 2025	Sepiolite	0.3 and 0.5 wt.% in 70% fly ash and 30% cement	Static yield stress Dynamic yield stress Structure build-up	Sepiolite addition has minimal impact on initial static yield stress, but significantly increases both static and dynamic yield stress over 60 min due to water absorption and structural development, outperforming VMA in long-term build-up.	——

Table 5. The effect of sepiolite clay on rheological properties.

Ref. (Year)	Clay Type	Mix Design	Rheology Parameter	Change Trend	3D Printability
[68] 2019	Ca-Sepiolite	2.5, 5, 7.5, 10, and 15 wt.% cement replacement	Yield stress plastic viscosity Workability Water absorption	Cement paste and mortar with 15% Ca-sepiolite show increased yield stress (+88.8% and +116.8%) and plastic viscosity (+8.1% and +52.1%); workability decreases and water absorption rises due to Ca-sepiolite’s high surface area and porosity.	——
[69] 2021	400, 600, 700, 800, 900, and 1000 °C calcinated sepiolite	20 wt.% cement replacement	Yield stress Plastic viscosity	Sepiolite increases yield stress, peaking at 700 °C calcination (407.3 Pa) and then declining at higher temperatures; all sepiolite types raise plastic viscosity above the control, though viscosity decreases with higher calcination temperature and is lowest at 1000 °C (2.0 Pa·s).	——
[70] 2020	Unmodified sepiolite and modified (acid and silane treatment) sepiolite	0.5, 1.0, 1.5, and 2.0 wt.% combined with carbon fiber in oil well cement	Apparent viscosity Thixotropy Workability Water loss Dispersion	At 2% dosage, unmodified sepiolite increases apparent viscosity by 328%, while modified sepiolite raises it by 83%; modified sepiolite enhances viscosity and thixotropy less than unmodified sepiolite, maintains better fluidity (>21 cm), reduces water loss more effectively, and exhibits superior dispersion stability.	——
[38] 2025	Sepiolite	1 wt.% combined with 20 wt.%FA and (or) 0.15% polyamide microfiber	Flow table test	Sepiolite acts as a rheology modifier that enhances thixotropy by improving water retention and microstructural locking, leading to a 20.9% reduction in deformation within 2 h.	——
[71] 2017	Sepiolite	1 and 2.5 wt.% lime replacement in white Portland cement (WPC) 1 and 2 wt.% cement replacement in OPC	Yield stress Expansion speed Water retention Build-up thickness	Sepiolite addition increases paste yield stress (e.g., >307 Pa at 2.5% WPC after 10 min), significantly enhances water retention, promotes thicker buildup (>2 cm before slump), and accelerates expansion while reducing bulk density when combined with fly ash or metakaolin.	Sepiolite-containing mortars (with or without metakaolin) demonstrate excellent shootability and build-up thickness (>2 cm), enabling vertical spray application with no slumping, strong adhesion to ceramic substrates, and stable, low-density hardened layers suitable for facade insulation.
[72] 2022	Sepiolite	0.5 and 1 wt.% combined with 20% FA	Static yield stress Dynamic yield stress thixotropy	Sepiolite at 0.5–1% markedly increases static and dynamic yield stress and enhances thixotropy, especially in FA blends, though excessive dosage may hinder printability due to overly high yield stress; it exhibits superior nano-montmorillonite in rheological improvement.	At a 0.5% dosage, sepiolite enables successful extrusion with improved buildability and shape retention, while a 1% dosage leads to excessive stiffness and poor extrudability.
[73] 2023	Sepiolite	0, 5, 10, 15, and 20 wt.%	Water demand Viscosity Setting time	Water demand and viscosity increase linearly with sepiolite dosage, while a higher sepiolite content also prolongs setting time.	——
[40] 2025	Sepiolite	0.3 and 0.5 wt.% in 70% fly ash and 30% cement	Static yield stress Dynamic yield stress Structure build-up	Sepiolite addition has minimal impact on initial static yield stress, but significantly increases both static and dynamic yield stress over 60 min due to water absorption and structural development, outperforming VMA in long-term build-up.	——

6.1.1. Workability and Flowability

Workability is a critical parameter in both conventional and 3D-printed concrete. While often used interchangeably with flowability, the two are not strictly synonymous. Traditionally, workability refers to the ease with which fresh concrete can be placed and compacted [74], whereas in the background of 3DCP, it encompasses a broader range of attributes, specifically extrudability, buildability, and constructability [7,75,76]. As such, workability directly governs open time, layer stability, and shape retention, all of which are essential for successful 3D printing. Several studies have highlighted the negative impact of NC additions on flowability. Natanzi and McNally [77] reported that incorporating NCs reduced the flowability of fresh mixtures, while Mousavi et al. [78] demonstrated that clay-based concretes generally exhibit lower flowability than conventional mixes due to the strong flocculation behavior of clay particles. However, this reduction can be partially mitigated through the inclusion of SPs, which improve dispersion and reduce internal friction.

Experimental observations further confirm that different NC types uniquely influence rheology. As shown in Figure 7, Hanratty et al. [18] reported that ATT, BEN, and SEP clays reduced slump flow to varying degrees. At just a 0.5% dosage, SEP caused the largest reduction (~125 mm), outperforming BEN (~30 mm) and ATT (~45 mm). This pronounced thickening effect is attributed to SEP’s fibrous morphology, higher surface area, and stronger water retention. Interestingly, a nonlinear response emerged at 1% SEP, possibly indicating overdosing effects, such as stiffening or particle clumping. These findings highlight the need to optimize the NC content to avoid compromising extrudability while still achieving adequate buildability.

Among the NCs, ATT is primarily used as a rheology modifier. It enhances cohesion and structural integrity in cementitious systems, particularly those incorporating fly ash or sulfoaluminate cement (SAC) [51,53]. Moderate dosages (≤0.5 wt.%) improve viscosity recovery and resistance to bleeding without significantly reducing flow. The fibrous structure of ATT promotes particle entanglement, forming a denser and more cohesive matrix. However, excessive ATT (>1 wt.%) may lead to interfacial porosity and reduced interlayer bonding, underscoring the importance of dosage control in 3DCP applications.

BEN is widely studied for workability adjustment in OPC-, silica fume-, and limestone-based systems. At 1–5 wt.% replacement levels, BEN consistently reduces slump flow and increases flow time (e.g., T500, V-funnel) due to its layered structure and high water absorption capacity [57,62]. These characteristics increase interparticle friction and reduce free water, which improves shape stability but can hinder extrudability if overdosed. Moderate levels (~3 wt.%) offer a favorable balance, enhancing dimensional integrity and reducing deformation in printed layers [63,66].

SEP serves a dual function as a workability regulator and internal curing agent in systems with OPC, white Portland cement, and fly ash. At 0.5–2 wt.% dosages, SEP significantly decreases paste fluidity and increases interparticle resistance due to fiber entanglement and strong water absorption [68,70,71]. While this reduces workability, modified forms of SEP with acid or silane treatment can alleviate excessive stiffening, improving dispersion and maintaining water retention. When appropriately incorporated, SEP contributes to higher buildability and shape fidelity, crucial for consistent, high-resolution 3D printing. Although NCs tend to reduce flowability due to their microstructural effects, they also enhance the structural cohesion and shape stability necessary for 3DCP.

6.1.2. Yield Stress

Yield stress is a fundamental rheological parameter that governs the onset of flow in cementitious materials. It is commonly divided into static yield stress (SYS), which represents the stress required to initiate flow after rest, and dynamic yield stress (DYS), which reflects the stress needed to sustain flow once movement has begun. Both play a crucial role in 3DCP, as they influence the extrusion, buildability, and structural integrity of printed elements. SYS is typically measured by applying increasing shear until flow initiates; the peak stress before flow indicates the material’s inherent resistance due to structuration. In contrast, DYS is determined during continuous flow and is sensitive to interparticle friction and dispersion state [79].

Qian et al. [80] examined the impact of ATT on SYS using creep recovery and tack tests. Their results (Figure 8) revealed a linear increase in SYS with increasing ATT content, highlighting its role in enhancing cohesion and microstructural build-up. The directional consistency across different test methods suggests that ATT induces rigid, isotropic network formation. This is attributed to ATT’s fibrous morphology (length ~ 1.75 µm, diameter ~ 30 nm), which enables particle bridging and interstitial pore filling, contributing to a denser and more cohesive matrix.

This trend is reinforced by other studies [7,50], which report that NCs increase SYS in 3DCP pastes, promoting shape stability and resistance to deformation. Natanzi and McNally [77] emphasized the need for higher SYS in printed layers to prevent slumping. Ramakrishnan et al. [50] further demonstrated enhanced structuration via a rapid increase in yield strength over 30 min in NC-modified pastes (Figure 9), driven by accelerated flocculation and early hydration. However, they also noted that excessive NCs (beyond 0.2 wt.%) adversely affect extrudability due to nozzle blockage, indicating the need for dosage optimization. Dejaeghere et al. [81] quantified the relationship between NC content and SYS across various mix designs (

Table 6. Mixes and corresponding SYS values [81].

Mix/Ref.	SP (%)	FA (%)	Nanoclay (%)	SYS (Pa)
1	0.60	5.0	0.50	11.7
2	0.60	5.0	2.50	1749.0
3	3.00	5.0	0.50	5.6
4	3.00	5.0	2.50	39.7
5	0.60	20.0	0.50	25.2
6	0.60	20.0	2.50	116.2
7	3.00	20.0	0.50	8.6
8	3.00	20.0	2.50	171.4
9	1.80	12.5	1.50	40.8
10	1.80	12.5	1.50	51.5
11	1.80	12.5	1.50	43.7

), confirming that an increased NC content consistently elevated SYS, even when other parameters remained constant.

Beyond static behavior, ATT also enhances DYS, especially in OPC, calcium sulfoaluminate (CSA), and alkali-activated systems at 0.2–0.6 wt.% dosages [37,52,53,56]. It improves particle flocculation and network rigidity, resulting in up to 400% increase in SYS and nearly 192% in DYS [53]. These improvements directly contribute to print layer stability and resistance to deformation under self-weight.

BEN has also shown substantial influence on both SYS and DYS across OPC, fly ash, and alkali-activated binders. At dosages of 1–5 wt.%, studies report increases in SYS by up to 17 times and DYS by 5.6 times [58,59,61,63,66]. This is attributed to BEN’s high surface area, layered morphology, and capacity to form a gel-like structure, promoting particle interlocking and flocculation. Na-bentonite consistently outperforms Ca-bentonite in yield stress enhancement [67]. Improved yield stress contributes to enhanced buildability and deformation resistance, although excessive dosage may reduce extrudability.

SEP is another effective yield stress modifier, which is characterized by its high aspect ratio and water retention. At dosages between 0.5 and 2.5 wt.%, it has been shown to significantly elevate SYS and DYS, especially in fly ash-blended or WPC matrices [40,68,69,71,72]. For example, calcination at 700 °C resulted in a maximum SYS of ~407 Pa [69]. The fibrous structure facilitates interparticle bridging and floc formation, enhancing structural rigidity over time. However, excessive inclusion (≥1 wt.%) can result in excessive stiffening, undermining smooth extrusion and print fidelity [72].

In summary, the inclusion of NCs, such as ATT, bentonite, and sepiolite, significantly enhances both static and dynamic yield stresses, with implications for flow initiation, shape retention, and interlayer stability in 3D concrete printing.

6.1.3. Viscosity

Viscosity describes the internal resistance of a material to shear-induced deformation and is a critical parameter for evaluating the extrudability and shape stability of 3D printable cementitious systems [82]. In extrusion-based 3DCP, apparent viscosity is commonly assessed because of the non-Newtonian and time-dependent behavior of fresh mixtures [26]. It is generally accepted that low viscosity facilitates smooth flow through nozzles, while moderate viscosity is necessary to prevent slump and maintain buildability [83].

A rheometer is the standard tool for characterizing viscosity in 3DCP studies, often using the modified Bingham plastic model (Equation (1)) to express the nonlinear relationship between shear stress ($\tau$) and shear rate ($\dot{\gamma }$), with plastic viscosity (${\eta }_p$) and dynamic yield stress (${\tau }_0$) as fitting parameters [84]:

$$\tau ={\tau }_0+{\eta }_p\cdot {\dot{\gamma }}^2$$

(1)

Alternatively, the Herschel–Bulkley model (Equation (2)) is employed for more complex shear-thickening or shear-thinning behaviors, introducing a consistency coefficient (k) and a flow index (n) [85]:

$$\tau ={\tau }_0+k\dot{{\gamma }^n}$$

(2)

Studies consistently report that NCs increase apparent viscosity due to enhanced flocculation and interparticle interactions [6,7,49,86]. Hanratty et al. [18] observed enhanced shear-thickening behavior in clay-containing mixtures compared to a reference mix (Figure 10). Specifically, 0.5% BEN improved both yield strength and shear-thickening characteristics, whereas higher dosages (1%) indicated over-saturation and reduced flocculation effectiveness. Conversely, ATT at 0.5% achieved an optimal balance between flow and resistance, essential for maintaining printability. Viscosity recovery tests by Ramakrishnan et al. [50] and Shahmirzadi et al. [6] highlighted NCs’ ability to enhance viscosity, attributed to increased flocculation. Aydin et al. [72] noted that SEP addition significantly raised dynamic yield stress and improved the time-dependent build-up of static yield stress, demonstrating its role as an effective rheology modifier. Panda et al. [51] reported that incorporating 0.5% ATT markedly increased the static yield stress in high-volume fly ash mortars due to enhanced electrostatic interactions and rapid structural re-flocculation (Figure 11). Importantly, despite the increased yield stress, apparent viscosity under shear remained below 10 Pa·s, signifying favorable conditions for extrusion while maintaining adequate structural stability.

ATT notably raises plastic viscosity by up to 129% within moderate dosages (0.2–0.5 wt.%), particularly when combined with low-VMAs [53]. This increase originates from its high water retention and network-forming capability, restricting free particle movement and promoting stable rheological behavior. Optimal dosing of ATT ensures a beneficial balance between viscosity and smooth extrusion, critical for achieving desirable printability without increasing pumping difficulty excessively.

An increase in BEN dosage also leads to elevated plastic and apparent viscosity. In OPC and alkali-activated systems, 1–8 wt.% addition increases viscosity by approximately 1.2 times, particularly at low shear rates [58,60,65]. Its layered structure increases internal friction and promotes shear-thinning behavior, which benefits extrusion and layer deposition. However, high viscosity at elevated dosages may compromise extrudability, making dosage control essential [64].

SEP significantly affects viscosity within cementitious mixes, typically employed at dosages of 0.5–2 wt.% [68,69,70,73]. Its high surface area and water absorption elevate viscosity proportionally to dosage. For instance, 15 wt.% Ca-SEP increased plastic viscosity by approximately 52.1% relative to control mixes [68]. Additionally, calcination of SEP modulates viscosity with optimal control observed at intermediate temperatures (700 °C), whereas excessively high temperatures (1000 °C) reduce effectiveness [69]. The controlled use of SEP ensures stable filament deposition, although overly high dosages can impair extrudability and thus require precise optimization for successful application in 3DCP [72].

In summary, while NC additions uniformly increase viscosity and contribute to structural stability, achieving optimal printability depends on balancing viscosity with yield stress and thixotropy. Controlled dosage and consideration of binder systems and admixture interactions are essential to maintain extrusion efficiency in 3DCP.

6.1.4. Thixotropic Properties

Thixotropy was defined by Ramakrishnan et al. [50] as the capacity of fresh concrete to reduce viscosity under shear stress and subsequently regain its structure after shear removal. In contrast, Kazemian et al. [49] described thixotropy as the cyclical build-up and breakdown of the internal structure within cementitious paste due to shear. These definitions collectively underscore thixotropy’s crucial role in 3DCP, particularly in ensuring rapid structural recovery necessary for successive layer deposition.

Thixotropic properties generally correlate with the degree of particle flocculation and internal chemical interactions. As indicated by Moeini et al. [44], incorporating NCs intensifies the flocculation process, thereby enhancing the thixotropic index of cementitious pastes. Hanratty et al. [18] suggested measuring thixotropy via the area between the ascending and descending curves (hysteresis loop) of stress–shear rate flow curves, as shown in Figure 12. Ramakrishnan et al. [50] validated this approach, demonstrating that incorporating 0.2 wt.% NCs increased this hysteresis loop area by 255% compared to a control mixture, thus confirming enhanced thixotropy and improved layer stability through faster structural recovery.

ATT significantly enhances thixotropic behavior, particularly in OPC and CSA cement systems. Effective ATT dosages typically range from 0.25 to 0.6 wt.% [55,56]. At around 0.5 wt.%, ATT nearly doubles the structural recovery rate, reducing the characteristic breakdown time and yielding a denser and more responsive microstructure [55]. Such rapid rebuilding is especially advantageous for extrusion-based 3DCP, facilitating quick rigidity recovery between layers and thus supporting enhanced buildability and higher printing speeds without compromising geometric accuracy.

Similarly, BEN significantly promotes thixotropic rebuilding and structural recovery after shear deformation, with optimal dosages ranging from about 2 to 5 wt.% in OPC-based, fly ash-blended, and alkali-activated mixtures [61,63,67,87]. BEN’s layered structure and high water absorption capacity promote particle flocculation and interparticle bonding, considerably elevating both static yield stress (up to 203.8% increase) and thixotropy (up to 98.5% increase) at a 5 wt.% dosage [63]. Notably, Na-BEN exhibits more substantial thixotropic behavior than Ca-BEN due to stronger ionic interactions [67]. This improved structural recovery is essential in 3D printing to rapidly stabilize each printed layer, enhancing overall buildability and dimensional accuracy [59,63].

SEP also notably enhances thixotropic characteristics when incorporated into cementitious systems, such as those containing OPC, fly ash, or modified cement blends. Typical dosages range from 0.5 to 1.0 wt.% [38,70,72]. SEP’s fibrous structure promotes the rapid rebuilding of the microstructural network following shear-induced deformation. At dosages of 2 wt.%, unmodified SEP increases apparent viscosity and thixotropic recovery by as much as 328% [70]. Acid- or silane-modified SEP achieves better particle dispersion and maintains improved fluidity, slightly lowering thixotropy compared to its unmodified counterpart while still significantly enhancing structural recovery [70]. Such improved thixotropic behavior supports the rapid stabilization required between successive layers, thereby maintaining shape accuracy and structural integrity during 3D printing operations [38,72].

6.1.5. Rigidification and Storage Modulus

Rigidification refers to the progressive increase in stiffness and elastic character of fresh cementitious mixtures, characterized primarily by the storage modulus (G′). The storage modulus quantifies the elastic response, indicating the solid-like behavior of the material, critical for ensuring structural stability immediately after deposition in 3D printing applications [88,89]. Moeini et al. [44] employed a highly purified magnesium alumino-silicate NC (ATT) in concrete mixtures to examine its influence on rigidification, finding that ATT significantly increased the rigidification rate compared to the control mixtures, as shown in Figure 13. The rigidification rate, defined as the rate at which G′ surpasses viscous properties, illustrates the rapid structural development induced by ATT, essential for enhanced dimensional stability and shape retention. In cementitious systems incorporating OPC, CSA, silica fume, and fly ash, ATT notably elevates the storage modulus while reducing the loss factor, particularly effective at dosages of 0.4–0.6 wt.% [37,54]. The enhanced mechanical rigidity arises from ATT’s fibrous morphology and high surface area, fostering robust internal networks and rapid structural stiffening. This rheological improvement supports immediate layer stability and minimizes deformation post-extrusion. Nonetheless, ATT dosages must be carefully optimized to avoid excessive rigidity that could impede smooth extrusion, balancing structural accuracy with printability.

BEN has been widely investigated for its influence on rigidification and viscoelastic properties, particularly within mixtures containing magnesia, fly ash, and GGBS. Effective dosages around 3 wt.% significantly enhance both the storage and loss moduli, indicating the rapid formation of a rigid and solid-like structure in fresh mixes [61]. Due to its lamellar arrangement and enhanced interparticle bonding, EBN induces the formation of structurally dense and durable matrices. The increased storage modulus directly correlates with improved shape stability, creep resistance, and dimensional precision during the layered deposition process in 3D printing. Therefore, the optimal incorporation of BEN ensures the superior structural rigidity and dimensional accuracy necessary for high-quality printed structures.

SEP similarly demonstrates substantial improvements in rigidification and the long-term storage modulus in OPC and fly ash-blended systems at moderate dosages ranging from 0.3 to 0.5 wt.% [40]. Unlike conventional viscosity-modifying agents, SEP induces a delayed but prolonged increase in both static and dynamic yield stress, which is attributed to its ongoing water absorption and the gradual evolution of the microstructure. This sustained rigidification facilitates the formation of a dense and interconnected particle framework, which, in turn, strengthens interlayer adhesion, reduces gravitational deformation, and ensures consistent accuracy and stability throughout prolonged printing durations. Consequently, controlled addition of SEP markedly improves structural performance and reliability for advanced cement-based 3D printing applications.

6.2. Mechanical Properties

This section systematically reviews the effect of different NCs on the mechanical properties of cementitious materials. The discussion specifically focuses on the influence of ATT, BEN, and SEP clays, analyzing their impact on compressive properties, flexural properties, and other relevant characteristics such as density and porosity. The key findings derived from various experimental studies and literature reports are organized and summarized in Table 7 (ATT), Table 8 (BEN), and Table 9 (SEP). This offers a systematic synthesis that enables clear comparison of the distinct effects introduced by each type of NC.

Table 7. The effect of attapulgite clay on mechanical properties.

Ref. (Year)	Clay Type	Mix Design	Mechanical Test Method	Sample Size	Strength Increase
[41] 2023	750 °C calcined attapulgite	6 wt.% cement replacement	Compressive strength	50 mm cube	85 MPa with +13.3%
[90] 2022	500 °C calcined attapulgite	10 wt.% cement replacement	Flexural strength Compressive strength Splitting tensile	40 mm × 40 mm × 160 mm for flexural and compressive; 100 mm × 100 mm × 100 mm for splitting tensile	Compressive strength: +102.1% Flexural strength: +55.6% Splitting tensile strength: +59.7%
[91] 2022	500 °C calcined attapulgite	2, 4, 6, and 8 wt.% cement replacement with recycled aggregate	Compressive strength	100 mm × 100 mm × 100 mm	+13.2~21.25% with recycled aggregate replacement rates from 0% and 100% (optimal dosage is 8 wt.%)
[39] 2025	Attapulgite	10, 20, and 30 wt.% cement replacement, combined with basalt fiber (0.5%, 1%, 2%) in foam concrete	Compressive strength	100 mm × 100 mm × 100 mm	28 days: +129.3% 90 days: +85.3% (optimal 30% attapulgite + 0.5% basalt fiber)
[92] 2023	750 °C calcined attapulgite	2, 4, 6, 8, and 10 wt.% cement replacement	Compressive strength	40 mm × 40 mm × 40 mm	3 days: +11.6% 7 days: +17.5% 28 days: 82 MPa with +9.5% (optimal dosage is 6 wt.%)
[51] 2019	Attapulgite	0.1–0.5 wt.% in 70% fly ash	Compressive strength Tensile bond strength	50 mm × 50 mm × 50 mm	53.37 MPa with +6.4% (optimal dosage is 0.5 wt.%)
[18] 2024	Attapulgite	0.5% and 1% in OPC and GGBS (50% cement replacement)	Compressive strength	100 mm × 100 mm × 100 mm	The value increased and then decreased with an optimal dosage of 0.5%, showing a maximum improvement of +16.67%
[52] 2021	Attapulgite	1 wt.% in alkali-activated material (AAM)	Flexural strength Compressive strength	3D-printed samples	Compressive strength: +20% Flexural strength: +43%
[53] 2022	Attapulgite	0.5~3.0 wt.% in 90% OPC and 10% sulphoaluminate cement (SAC)	Compressive strength	70 mm × 70 mm × 70 mm	Compressive strength: +17.6% Slant shear (interlayer bond) strength: −51% (optimal dosage is 3.0 wt.%)

Table 7. The effect of attapulgite clay on mechanical properties.

Ref. (Year)	Clay Type	Mix Design	Mechanical Test Method	Sample Size	Strength Increase
[41] 2023	750 °C calcined attapulgite	6 wt.% cement replacement	Compressive strength	50 mm cube	85 MPa with +13.3%
[90] 2022	500 °C calcined attapulgite	10 wt.% cement replacement	Flexural strength Compressive strength Splitting tensile	40 mm × 40 mm × 160 mm for flexural and compressive; 100 mm × 100 mm × 100 mm for splitting tensile	Compressive strength: +102.1% Flexural strength: +55.6% Splitting tensile strength: +59.7%
[91] 2022	500 °C calcined attapulgite	2, 4, 6, and 8 wt.% cement replacement with recycled aggregate	Compressive strength	100 mm × 100 mm × 100 mm	+13.2~21.25% with recycled aggregate replacement rates from 0% and 100% (optimal dosage is 8 wt.%)
[39] 2025	Attapulgite	10, 20, and 30 wt.% cement replacement, combined with basalt fiber (0.5%, 1%, 2%) in foam concrete	Compressive strength	100 mm × 100 mm × 100 mm	28 days: +129.3% 90 days: +85.3% (optimal 30% attapulgite + 0.5% basalt fiber)
[92] 2023	750 °C calcined attapulgite	2, 4, 6, 8, and 10 wt.% cement replacement	Compressive strength	40 mm × 40 mm × 40 mm	3 days: +11.6% 7 days: +17.5% 28 days: 82 MPa with +9.5% (optimal dosage is 6 wt.%)
[51] 2019	Attapulgite	0.1–0.5 wt.% in 70% fly ash	Compressive strength Tensile bond strength	50 mm × 50 mm × 50 mm	53.37 MPa with +6.4% (optimal dosage is 0.5 wt.%)
[18] 2024	Attapulgite	0.5% and 1% in OPC and GGBS (50% cement replacement)	Compressive strength	100 mm × 100 mm × 100 mm	The value increased and then decreased with an optimal dosage of 0.5%, showing a maximum improvement of +16.67%
[52] 2021	Attapulgite	1 wt.% in alkali-activated material (AAM)	Flexural strength Compressive strength	3D-printed samples	Compressive strength: +20% Flexural strength: +43%
[53] 2022	Attapulgite	0.5~3.0 wt.% in 90% OPC and 10% sulphoaluminate cement (SAC)	Compressive strength	70 mm × 70 mm × 70 mm	Compressive strength: +17.6% Slant shear (interlayer bond) strength: −51% (optimal dosage is 3.0 wt.%)

Table 8. The effect of bentonite clay on mechanical properties.

Ref. (Year)	Clay Type	Mix Design	Mechanical Test Method	Sample Size	Strength Increase
[93] 2020	Na-bentonite Ca-bentonite Mg-bentonite	2, 4, 6, 8, and 10 wt.% cement replacement	Flexural strength Compressive strength	40 mm × 40 mm × 160 mm for flexural and compressive	10 wt.% Na-bentonite: Compressive strength: +77.5% Flexural strength: +54.5% 10 wt.% Mg-bentonite: Compressive strength: +71.6% Flexural strength: +52.2% 10 wt.% Ca-bentonite: Compressive strength: +62.2% Flexural strength: +47.9% Na-bentonite shows the greatest improvement, followed by Mg-bentonite and Ca-bentonite
[28] 2012	Bentonite	3, 6, 9, 12, 15, 18, and 21 wt.% cement replacement	Compressive strength	150 mm × 300 mm	3 days: −3.1~−16.5% 28 days: +0.75~+2.9% 56 days: +0.9~+2.7%
[94] 2020	Bentonite	5, 10, 15, and 20 wt.% cement replacement in natural aggregate concrete and recycled aggregate concrete	Compressive strength Splitting tensile	150 mm × 150 mm × 150 mm for compressive; 150 mm × 300 mm for splitting tensile	Compressive strength: +25% (optimal dosage is 15 wt.%)
[95] 2011	500 °C calcined bentonite	20, 30, 40, and 50 wt.% cement replacement	Compressive strength	Mortar: 50 mm × 50 mm × 50 mm Concrete: Φ150 mm × 300 mm	Mortar: Compressive strength at 20 °C: −30% Compressive strength at 500 °C: −21% Concrete: Compressive strength at 500 °C: −21% (optimal dosage is 30 wt.%)
[96] 2019	Bentonite	4 and 8 wt.% cement replacement	Flexural strength Compressive strength Impermeability pressure	40 mm × 40 mm × 160 mm for flexural and compressive 80 mm × 70 mm × 30 mm for impermeability pressure	Compressive strength: +61.5% Flexural strength: +42.1% Impermeability pressure: +76.5%
[57] 2018	Bentonite	8, 10, 12, 14, 16, and 18 wt.% cement replacement	Compressive strength	40 mm × 40 mm × 40 mm	−0.07~−6.14%
[58] 2022	Bentonite	2.5, 5, 7.5, 10, 12.5, and 15 wt.% combined with silica fume, limestone powder, and steel fiber	Flexural strength Compressive strength	40 mm × 40 mm × 160 mm for flexural and compressive	Compressive strength: −20.8% Flexural strength: −20%
[59] 2020	Bentonite	1, 2, and 3 wt.%	Compressive strength	3D-printed	Structure deformation: −42.1% Compressive strength: −5.4%
[60] 2025	Bentonite	1, 3, and 5 wt.% with varied grouting pressures and water/cement ratios	Compressive strength Deformation modulus	Φ50 mm × 110 mm	Deformation modulus: +26.0~−10.1% Compressive strength: +16.6~−13.9% (optimal dosage is 2 wt.%)
[61] 2021	Bentonite	1, 2, and 3 wt.% combined with Magnesia, KH2PO4, K2HPO4, fly ash, and ground granulated blast furnace slag	Compressive strength	20 mm × 20 mm × 20 mm	29.68 MPa with +27.9%
[97] 2009	Bentonite and 150 °C calcined bentonite	20, 25, 30, 40, 50%, and 100 wt.% cement replacement in mortar and concrete	Compressive strength Modulus of rupture	Compressive strength: Mortar: 50 mm × 50 mm × 50 mm Concrete: Φ150 mm × 300 mm Modulus of rupture: Concrete: 150 mm × 150 mm × 750 mm	Compressive strength: Mortar at 21 °C: 7 days: −85% 14 days: −91% 28 days: −93% Mortar at 150 °C: 7 days: −81% 28 days: −80% Concrete at 21 °C: 7 days: −57% 14 days: −44% 28 days: −50% 56 days: −40% Concrete at 150 °C: 7 days: −66% 14 days: −62% 28 days: −64% 56 days: −60% Modulus of rupture: Concrete at 150 °C: 28 days: −25%
[62] 2020	800 °C calcined Na-bentonite and Ca-bentonite	5, 10, 15, 20, 25, and 30 wt.% cement replacement	Compressive strength	70 mm × 70 mm × 280 mm	90 days: 15% Na-bentonite: ~74 MPa with +21% 10% Ca-bentonite: ~70 MPa with +16% Higher than 20 wt.%, strength decreases. (optimum dosage is 10~15 wt.%)

Table 8. The effect of bentonite clay on mechanical properties.

Ref. (Year)	Clay Type	Mix Design	Mechanical Test Method	Sample Size	Strength Increase
[93] 2020	Na-bentonite Ca-bentonite Mg-bentonite	2, 4, 6, 8, and 10 wt.% cement replacement	Flexural strength Compressive strength	40 mm × 40 mm × 160 mm for flexural and compressive	10 wt.% Na-bentonite: Compressive strength: +77.5% Flexural strength: +54.5% 10 wt.% Mg-bentonite: Compressive strength: +71.6% Flexural strength: +52.2% 10 wt.% Ca-bentonite: Compressive strength: +62.2% Flexural strength: +47.9% Na-bentonite shows the greatest improvement, followed by Mg-bentonite and Ca-bentonite
[28] 2012	Bentonite	3, 6, 9, 12, 15, 18, and 21 wt.% cement replacement	Compressive strength	150 mm × 300 mm	3 days: −3.1~−16.5% 28 days: +0.75~+2.9% 56 days: +0.9~+2.7%
[94] 2020	Bentonite	5, 10, 15, and 20 wt.% cement replacement in natural aggregate concrete and recycled aggregate concrete	Compressive strength Splitting tensile	150 mm × 150 mm × 150 mm for compressive; 150 mm × 300 mm for splitting tensile	Compressive strength: +25% (optimal dosage is 15 wt.%)
[95] 2011	500 °C calcined bentonite	20, 30, 40, and 50 wt.% cement replacement	Compressive strength	Mortar: 50 mm × 50 mm × 50 mm Concrete: Φ150 mm × 300 mm	Mortar: Compressive strength at 20 °C: −30% Compressive strength at 500 °C: −21% Concrete: Compressive strength at 500 °C: −21% (optimal dosage is 30 wt.%)
[96] 2019	Bentonite	4 and 8 wt.% cement replacement	Flexural strength Compressive strength Impermeability pressure	40 mm × 40 mm × 160 mm for flexural and compressive 80 mm × 70 mm × 30 mm for impermeability pressure	Compressive strength: +61.5% Flexural strength: +42.1% Impermeability pressure: +76.5%
[57] 2018	Bentonite	8, 10, 12, 14, 16, and 18 wt.% cement replacement	Compressive strength	40 mm × 40 mm × 40 mm	−0.07~−6.14%
[58] 2022	Bentonite	2.5, 5, 7.5, 10, 12.5, and 15 wt.% combined with silica fume, limestone powder, and steel fiber	Flexural strength Compressive strength	40 mm × 40 mm × 160 mm for flexural and compressive	Compressive strength: −20.8% Flexural strength: −20%
[59] 2020	Bentonite	1, 2, and 3 wt.%	Compressive strength	3D-printed	Structure deformation: −42.1% Compressive strength: −5.4%
[60] 2025	Bentonite	1, 3, and 5 wt.% with varied grouting pressures and water/cement ratios	Compressive strength Deformation modulus	Φ50 mm × 110 mm	Deformation modulus: +26.0~−10.1% Compressive strength: +16.6~−13.9% (optimal dosage is 2 wt.%)
[61] 2021	Bentonite	1, 2, and 3 wt.% combined with Magnesia, KH2PO4, K2HPO4, fly ash, and ground granulated blast furnace slag	Compressive strength	20 mm × 20 mm × 20 mm	29.68 MPa with +27.9%
[97] 2009	Bentonite and 150 °C calcined bentonite	20, 25, 30, 40, 50%, and 100 wt.% cement replacement in mortar and concrete	Compressive strength Modulus of rupture	Compressive strength: Mortar: 50 mm × 50 mm × 50 mm Concrete: Φ150 mm × 300 mm Modulus of rupture: Concrete: 150 mm × 150 mm × 750 mm	Compressive strength: Mortar at 21 °C: 7 days: −85% 14 days: −91% 28 days: −93% Mortar at 150 °C: 7 days: −81% 28 days: −80% Concrete at 21 °C: 7 days: −57% 14 days: −44% 28 days: −50% 56 days: −40% Concrete at 150 °C: 7 days: −66% 14 days: −62% 28 days: −64% 56 days: −60% Modulus of rupture: Concrete at 150 °C: 28 days: −25%
[62] 2020	800 °C calcined Na-bentonite and Ca-bentonite	5, 10, 15, 20, 25, and 30 wt.% cement replacement	Compressive strength	70 mm × 70 mm × 280 mm	90 days: 15% Na-bentonite: ~74 MPa with +21% 10% Ca-bentonite: ~70 MPa with +16% Higher than 20 wt.%, strength decreases. (optimum dosage is 10~15 wt.%)

Table 9. The effect of sepiolite clay on mechanical properties.

Ref. (Year)	Clay Type	Mix Design	Mechanical Test Method	Sample Size	Strength Increase
[98] 2019	High anionic charge (HAC) and low anionic charge (LAC) sepiolite	1 and 2 wt.% in MgO-SiO2-limestone filler combined with 8 wt.% cellulose fiber	Dynamic elastic modulus (DEM) Modulus of rupture (MOR)	80 mm × 30 mm × 8 mm for DEM 160 mm × 40 mm × 6 mm for MOR	Dynamic elastic modulus: +27% Modulus of rupture: LAC: 9.78 MPa with +4.5% HAC: 9.00 MPa with −3.8% (optimum dosage is 1 wt.%)
[99] 2021	Sepiolite	5, 10, 15, 20, and 25 wt.% in sandy clay soil and clayey sand soil	Uniaxial compressive strength (UCS) Direct shear	UCS: Φ38 mm × 76 mm Direct shear: 120 mm × 120 mm	Uniaxial compressive strength: Clayey sand soil: +6.1× Sandy clay soil: +3× Shear strength (cohesion): Cohesion: +2.37× Internal friction angle: +1.75×
[100] 2025	750 °C calcined sepiolite	1, 3, and 5 wt.% kaolin soil replacement	Unconfined compressive strength (UCS)	Φ 35 mm × 76 mm	Unconfined compressive strength: +3.5× Modulus of elasticity: +6.82×
[42] 2023	Crude sepiolite and 900 °C calcined sepiolite	5, 10, 15, and 20 wt.% cement replacement in glass fiber-reinforced concrete	Compressive Strength Modulus of rupture Impact strength Abrasion resistance	50 mm × 50 mm × 50 mm for compressive strength 10 mm × 600 mm × 600 mm for flexural strength 40 mm × 40 mm × 160 mm for impact strength	Compressive strength: Crude sepiolite: −63.7% Calcined sepiolite: −27.3% Modulus of rupture: Crude sepiolite: −19.1% Calcined sepiolite: −44.8% Impact strength: Crude sepiolite: −21.1% Calcined sepiolite: −15.8% Abrasion resistance: Crude sepiolite: −20.8% Calcined sepiolite: −11.8%
[101] 2024	Crude sepiolite and 500, 700, and 900 °C calcined sepiolite	5, 10, 15, and 20 wt.% cement replacement	Compressive strength	50 mm × 50 mm × 50 mm	Water demand: Crude sepiolite: +59% Calcined sepiolite: +28% Compressive strength: Crude sepiolite: −69.6% Calcined sepiolite: −39.0%
[102] 2015	Sepiolite	Sepiolite/hydraulic lime (S/L = 1/3, 1 and 3) in hydraulic lime mortar	Flexural strength Compressive strength	40 mm × 40 mm × 160 mm for flexural and compressive	Flexural and compressive strengths both decrease with increasing sepiolite content.
[103] 2021	Sepiolite	5, 10, 15, 20, 25, and 30 wt.% cement replacement combined with 20 wt.% fly ash	Flexural strength Compressive strength	40 mm × 40 mm × 160 mm for flexural and compressive	Flexural and compressive strengths both decrease with increasing sepiolite content and optimal dosage is 25 wt.%.
[68] 2019	Ca-Sepiolite	2.5, 5, 7.5, 10, and 15 wt.% cement replacement	Flexural strength Compressive strength	40 mm × 40 mm × 160 mm for flexural and compressive	Compressive strength: 3 days: +7.6% 7 days: +11.1% 28 days: +14.8% 56 days: +22.2% Flexural strength: 3 days: +10.0% 7 days: +11.7% 28 days: +14.7% 56 days: +18.0% Water absorption: Paste: +61.5% Mortar: +12.9% (optimum dosage is 7.5 wt.%)
[69] 2021	400, 600, 700, 800, 900, and 1000 °C calcinated sepiolite	20 wt.% cement replacement	Compressive strength	30 mm × 30 mm × 30 mm	Calcinated sepiolite improves compressive strength over uncalcined sepiolite; strength peaks at 800 °C (45.1 MPa at 28 days) and then declines at higher temperatures.
[70] 2020	Unmodified sepiolite and modified (acid and silane treatment) sepiolite	0.5, 1.0, 1.5, and 2.0 wt.% combined with carbon fiber in oil well cement	Compressive strength Flexural strength Impact strength	50.8 mm × 50.8 mm × 50.8 mm for compressive strength 40 mm × 40 mm × 160 mm for flexural strength 10 mm × 15 mm × 120 mm for impact strength	Compressive strength: Unmodified sepiolite: −24.1% Modified sepiolite: −10% Impact strength: Unmodified sepiolite: +6.8% Modified sepiolite: +17.7%, Flexural strength: Unmodified sepiolite: +5.4% Modified sepiolite: +25.8%
[38] 2025	Sepiolite	1 wt.% combined with 20 wt.%FA and (or) 0.15% polyamide microfiber	Compressive strength Flexural strength Interlayer bonding	40 mm × 40 mm × 40 mm printed sample	Compressive strength: Casted: −20% Printed: −28.1% Flexural strength: Printed: +19.6% Interlayer bonding: Casted: +32% Printed: +13%

Table 9. The effect of sepiolite clay on mechanical properties.

Ref. (Year)	Clay Type	Mix Design	Mechanical Test Method	Sample Size	Strength Increase
[98] 2019	High anionic charge (HAC) and low anionic charge (LAC) sepiolite	1 and 2 wt.% in MgO-SiO2-limestone filler combined with 8 wt.% cellulose fiber	Dynamic elastic modulus (DEM) Modulus of rupture (MOR)	80 mm × 30 mm × 8 mm for DEM 160 mm × 40 mm × 6 mm for MOR	Dynamic elastic modulus: +27% Modulus of rupture: LAC: 9.78 MPa with +4.5% HAC: 9.00 MPa with −3.8% (optimum dosage is 1 wt.%)
[99] 2021	Sepiolite	5, 10, 15, 20, and 25 wt.% in sandy clay soil and clayey sand soil	Uniaxial compressive strength (UCS) Direct shear	UCS: Φ38 mm × 76 mm Direct shear: 120 mm × 120 mm	Uniaxial compressive strength: Clayey sand soil: +6.1× Sandy clay soil: +3× Shear strength (cohesion): Cohesion: +2.37× Internal friction angle: +1.75×
[100] 2025	750 °C calcined sepiolite	1, 3, and 5 wt.% kaolin soil replacement	Unconfined compressive strength (UCS)	Φ 35 mm × 76 mm	Unconfined compressive strength: +3.5× Modulus of elasticity: +6.82×
[42] 2023	Crude sepiolite and 900 °C calcined sepiolite	5, 10, 15, and 20 wt.% cement replacement in glass fiber-reinforced concrete	Compressive Strength Modulus of rupture Impact strength Abrasion resistance	50 mm × 50 mm × 50 mm for compressive strength 10 mm × 600 mm × 600 mm for flexural strength 40 mm × 40 mm × 160 mm for impact strength	Compressive strength: Crude sepiolite: −63.7% Calcined sepiolite: −27.3% Modulus of rupture: Crude sepiolite: −19.1% Calcined sepiolite: −44.8% Impact strength: Crude sepiolite: −21.1% Calcined sepiolite: −15.8% Abrasion resistance: Crude sepiolite: −20.8% Calcined sepiolite: −11.8%
[101] 2024	Crude sepiolite and 500, 700, and 900 °C calcined sepiolite	5, 10, 15, and 20 wt.% cement replacement	Compressive strength	50 mm × 50 mm × 50 mm	Water demand: Crude sepiolite: +59% Calcined sepiolite: +28% Compressive strength: Crude sepiolite: −69.6% Calcined sepiolite: −39.0%
[102] 2015	Sepiolite	Sepiolite/hydraulic lime (S/L = 1/3, 1 and 3) in hydraulic lime mortar	Flexural strength Compressive strength	40 mm × 40 mm × 160 mm for flexural and compressive	Flexural and compressive strengths both decrease with increasing sepiolite content.
[103] 2021	Sepiolite	5, 10, 15, 20, 25, and 30 wt.% cement replacement combined with 20 wt.% fly ash	Flexural strength Compressive strength	40 mm × 40 mm × 160 mm for flexural and compressive	Flexural and compressive strengths both decrease with increasing sepiolite content and optimal dosage is 25 wt.%.
[68] 2019	Ca-Sepiolite	2.5, 5, 7.5, 10, and 15 wt.% cement replacement	Flexural strength Compressive strength	40 mm × 40 mm × 160 mm for flexural and compressive	Compressive strength: 3 days: +7.6% 7 days: +11.1% 28 days: +14.8% 56 days: +22.2% Flexural strength: 3 days: +10.0% 7 days: +11.7% 28 days: +14.7% 56 days: +18.0% Water absorption: Paste: +61.5% Mortar: +12.9% (optimum dosage is 7.5 wt.%)
[69] 2021	400, 600, 700, 800, 900, and 1000 °C calcinated sepiolite	20 wt.% cement replacement	Compressive strength	30 mm × 30 mm × 30 mm	Calcinated sepiolite improves compressive strength over uncalcined sepiolite; strength peaks at 800 °C (45.1 MPa at 28 days) and then declines at higher temperatures.
[70] 2020	Unmodified sepiolite and modified (acid and silane treatment) sepiolite	0.5, 1.0, 1.5, and 2.0 wt.% combined with carbon fiber in oil well cement	Compressive strength Flexural strength Impact strength	50.8 mm × 50.8 mm × 50.8 mm for compressive strength 40 mm × 40 mm × 160 mm for flexural strength 10 mm × 15 mm × 120 mm for impact strength	Compressive strength: Unmodified sepiolite: −24.1% Modified sepiolite: −10% Impact strength: Unmodified sepiolite: +6.8% Modified sepiolite: +17.7%, Flexural strength: Unmodified sepiolite: +5.4% Modified sepiolite: +25.8%
[38] 2025	Sepiolite	1 wt.% combined with 20 wt.%FA and (or) 0.15% polyamide microfiber	Compressive strength Flexural strength Interlayer bonding	40 mm × 40 mm × 40 mm printed sample	Compressive strength: Casted: −20% Printed: −28.1% Flexural strength: Printed: +19.6% Interlayer bonding: Casted: +32% Printed: +13%

6.2.1. Compressive Strength

The compressive strength of cementitious materials is a critical mechanical property, typically evaluated in the hardened state using a uniaxial compression test. For this test, cubic specimens (commonly 50 mm) are cast in molds and cured for standard intervals, such as 7 and 28 days, with the latter generally representing the material’s full design strength [104,105]. A constant loading rate is applied to the specimen until failure, allowing for the generation of stress–strain data.

The incorporation of NCs into cementitious composites can significantly alter their compressive strength. However, the literature presents conflicting findings on its effects. Some studies report that NCs are crucial for improving compressive strength [78,106], with enhancements observed even at low dosages of 0.5–2.0% [104,107,108,109]. Conversely, other research indicates that the addition of NCs can lead to a reduction in strength [36,77]. This variability highlights that the impact of NCs is not straightforward and is highly dependent on several interacting factors, including the dosage of other admixtures like SPs, where a higher SP content has been linked to lower compressive strength [77,110].

ATT generally demonstrates a more consistent ability to enhance compressive strength across various binder systems. Optimal dosages typically range from 0.5 to 10% by weight. Calcined ATT is particularly effective. For instance, a 6 wt.% replacement with ATT calcined at 750 °C improved 28-day strength by 13.3% [100]. The mechanisms for this enhancement include ATT’s high surface area, which promotes pozzolanic reactions, refines the pore structure, and densifies the hardened matrix [6]. However, the curing environment is critical. Under sealed curing, ATT provides strength benefits, but under open-air curing, these gains can be reversed due to rapid water evaporation that hinders proper hydration and geopolymerization, as shown in Figure 14 [6].

BEN’s influence is particularly complex and varies with its type (e.g., Na-BEN, Ca-BEN), processing, and dosage. Raw BEN often reduces early-age strength due to its high water demand and inherently weak structure [78]. In contrast, calcined BEN treated at temperatures around 800–900 °C can significantly enhance performance. Optimal dosages, typically between 5 and 15% by cement weight, can improve 28-day strength by up to 77.5% [93,94]. This improvement is attributed to its pozzolanic reactivity and ability to refine the pore structure, promoting the formation of secondary calcium silicate hydrate (C-S-H) [62]. However, excessive bentonite content (≥20%) invariably deteriorates mechanical properties by creating a looser, more porous microstructure and introducing air voids due to the high viscosity of the slurry [59,111].

SEP’s impact also depends heavily on its form (crude vs. calcined) and dosage. At optimal dosages of 1–7.5 wt.%, calcined or modified SEP can yield significant strength gains with improvements as high as 22.2% at 56 days [68]. Calcination at 800 °C has been shown to be particularly beneficial [69]. Conversely, crude SEP or excessive dosages (≥10%) typically reduce compressive strength. This negative effect is primarily due to SEP’s high water absorption, which increases porosity and impedes hydration [42,101]. Therefore, careful control of dosage and processing is essential to leverage SEP’s potential benefits.

In summary, the conflicting results reported in the literature arise because the effect of NCs on compressive strength is a delicate balance. While optimal dosages of processed NCs can enhance strength through pozzolanic activity and microstructure densification, several factors can lead to a negative result. The primary reasons for strength reduction include poor dispersion, which leads to the formation of weak agglomerates and interference with cement hydration due to excessive water absorption [112,113,114,115].

6.2.2. Flexural Strength/Modulus of Rupture

Flexural strength testing typically employs rectangular prisms (40 mm × 40 mm × 160 mm), following standardized procedures involving two-point supports and central loading until failure. While numerous studies demonstrate the positive effects of NCs on flexural strength, the results from the literature are not entirely consistent [36,116,117]. Natanzi and McNally [77] reported an inverse relationship between NC content and flexural strength, indicating that optimal dosages exist, as also supported by Abdalqader et al. [36]. For example, NC dosages of 0.25% enhanced flexural strength, but further increases reduced strength, suggesting dosage optimization is essential to achieving beneficial results.

ATT also effectively improves flexural strength in cement-based systems, especially when utilized in alkali-activated or calcined ATT formulations. Typical dosages for flexural strength enhancement range between 1 and 10 wt.% cement replacement. For instance, the addition of 1 wt.% ATT to alkali-activated materials significantly enhanced flexural strength by approximately 43%, demonstrating its effectiveness in strengthening the brittle matrix through improved cohesion and crack bridging [52]. Moreover, incorporating 500 °C calcined ATT at 10 wt.% into OPC systems increased flexural strength by 55.6. These improvements result mainly from ATT’s fibrous morphology, enhancing toughness, crack resistance, and stress-transfer capabilities within the cementitious composites [98].

BEN incorporation also enhances flexural strength and the modulus of rupture in OPC systems. The effective dosage typically ranges from 4 to 10 wt.%, with Na-BEN exhibiting superior performance among clay types (Na, Mg, Ca). At a 10 wt.% replacement, flexural strength improved significantly, reaching up to 54.5% (Na-BEN) [93]. Another study using 4–8 wt.% bentonite showed flexural strength enhancements of about 42.1% [96]. The enhancement mechanism primarily involves BEN’s layered microstructure and improved particle bonding capacity, increasing crack resistance and stress redistribution within the cement matrix. Conversely, excessive BEN may negatively affect flexural performance due to increased matrix porosity and reduced bond integrity [58].

SEP enhances flexural strength and the modulus of rupture significantly in certain conditions, especially when chemically modified or combined with fibers. The optimal dosage typically ranges from 1 to 2 wt.% or lower (<10 wt.%). Chemically modified SEP (acid/silane-treated, combined with carbon fiber) at 2 wt.% substantially increased flexural strength by 25.8% due to improved particle–matrix adhesion and microstructural densification [70]. Ca-SEP at an optimal 7.5 wt.% enhanced flexural strength by approximately 18% under 56 days curing age [68]. In contrast, excessive SEP content (≥20 wt.%) or crude forms negatively impacted the modulus of rupture, reducing performance by approximately 45% due to increased porosity and reduced cohesive matrix integrity [42].

6.2.3. Others

In addition to enhancing compressive and flexural properties, NCs also have a positive impact on impact strength, abrasion resistance, deformation modulus, interlayer bonding, and porosity. However, these effects may vary depending on the type of NC and its effective chemical composition. ATT significantly influences mechanical properties, including splitting tensile strength, interlayer bond strength, and porosity characteristics. Optimal dosages typically lie within the range of 0.5–10 wt.%. For splitting tensile strength, incorporating 10 wt.% of calcined ATT at 500 °C resulted in a notable improvement of approximately 59.7% using a standard sample size of 100 mm × 100 mm × 100 mm. This improvement arises from enhanced particle–matrix bonding and crack arrestment capabilities imparted by ATT’s unique microstructure [98]. Conversely, interlayer bond strength assessed through slant shear tests revealed that excessive ATT dosage (e.g., 3.0 wt.%) could reduce bonding by about 51% due to increased interfacial porosity and weakened interlayer adhesion [53]. Therefore, precise dosage control is critical to optimizing interfacial performance. ATT’s role in refining porosity was also observed in blends containing recycled aggregates at an optimal 8 wt.% dosage, significantly enhancing strength by reducing microstructural defects [99].

Beyond compressive and flexural strengths, BEN significantly affects splitting tensile strength, impermeability, and the deformation modulus in various cementitious composites. A 15 wt.% BEN content optimally improved splitting tensile strength, tested using cylindrical samples (150 mm × 300 mm) in natural aggregate systems [94]. Impermeability was notably enhanced (by 76.5%) with an optimal bentonite content (4–8 wt.%) due to pore refinement and reduced permeability, which was tested with 80 mm × 70 mm × 30 mm specimens [96]. Furthermore, the deformation modulus changed positively within a 1–3 wt.% dosage, increasing by up to 26% due to enhanced matrix stiffness and refined pore structure [60]. These improvements result from BEN’s lamellar structure, which enhances densification and reduces microstructural defects, thereby positively affecting durability and mechanical integrity.

SEP significantly influences various mechanical properties depending on its physical form, dosage, and modifications. Modified SEP at 2 wt.% substantially improved impact strength by 17.7% (10 mm × 15 mm × 120 mm samples) due to superior dispersion and enhanced ductility, whereas crude SEP demonstrated only a modest improvement (+6.8%) [70]. Both crude and calcined sepiolite at higher dosages (≥15 wt.%) negatively impacted abrasion resistance (~20% decrease) due to increased matrix softness and porosity [42]. For the deformation modulus, small dosages (1 wt.%) effectively reduced structural deformation by 42.1% (3D-printed samples) and enhanced dimensional stability [59]. Additionally, SEP significantly improved interlayer bonding strength, particularly in casted (32%) and printed samples (13%) when combined with FA and polyamide fibers, reflecting improved adhesion and reduced interface porosity [38]. However, higher dosages of crude SEP significantly increased water absorption by approximately 61.5% (cement paste) and 12.9% (mortar), which negatively affected mechanical durability [68].

7. Discussion and Future Work

The preceding review demonstrates that NCs are a critical rheology-modifying admixture for advanced construction techniques, like 3DCP, due to their significant effects on material properties at various stages. Typically, these NCs are introduced in powder form during the dry mixing stage, alongside primary binder components such as cement, GGBS, and aggregates, before the introduction of water and chemical admixtures, like SPs. The principal function of NCs in these cement-based materials is to refine the fresh-state properties essential for successful 3D printing, namely, extrudability and buildability [51,118]. Owing to their high specific surface area and unique particle morphologies, NCs enhance the cohesion of the mixture, which mitigates segregation and water drainage, leading to a more stable extrusion and superior filament quality [119]. Furthermore, they significantly augment the thixotropy and yield stress of the mortar [50,80]. This is achieved by promoting the formation of a flocculated particle network that breaks down under the shear stress of pumping and extrusion but rapidly rebuilds at rest, providing the immediate structural integrity required to support subsequent printed layers. While this process improves cohesiveness, the inherent water-absorbing nature of NCs also increases the overall water demand of the mix. The concluded effects of NCs on 3DCP are shown in Figure 15.

The specific impact on mortar properties varies considerably between different types of NCs, such as ATT, BEN, and SEP, due to their distinct structural and chemical characteristics. ATT with needle-like morphology effectively improves cohesiveness and buildability while enhancing both yield stress and thixotropy for better shape stability. BEN, as a swelling clay, markedly increases viscosity and yield stress even at low concentrations, though it tends to increase water demand more significantly than the other NCs. SEP, which shares a fibrous structure with ATT, has proven particularly effective in improving static and dynamic yield stress and the rate of structural recovery. And some studies indicate that it provides the best fresh mechanical performance. The combination of SEP with VMAs, like methylcellulose, can further enhance cohesion and produce a properly extrudable paste. Regarding hardened properties, the addition of ATT and SEP at a 0.5% dosage by binder mass has been shown to increase 28-day compressive strength by over 30% in some cases while also improving flexural strength. The influence on cement hydration is complex, as the increased demand for SP to maintain workability can delay setting times, but thermally activated attapulgite may exhibit pozzolanic reactivity, forming additional C–S–H products that densify the matrix and improve long-term strength [29,45].

The existing literature occasionally presents conflicting findings on the effects of NCs, which can be attributed to several influencing factors. Dosage is a primary consideration, as there appears to be an optimal concentration for achieving desired properties. For instance, the significant compressive strength gains observed with a 0.5% dosage of ATT and SEP were not replicated at a 1% dosage, suggesting that excessive concentrations can become detrimental. The effectiveness of NCs is also heavily dependent on their dispersion within the cementitious matrix. Poor dispersion can result in agglomerates that act as weak points, negatively impacting mechanical performance. Finally, the overall mix design, including the interaction of NCs with SCMs and chemical admixtures, plays a crucial role. The type and amount of SP are particularly critical for balancing the workability and stability of the mixture.

Based on the reviewed research, an optimal dosage for NCs appears to be approximately 0.5% by binder weight, at which SEP particularly demonstrates superior performance in enhancing the static and dynamic yield stress required for high-quality 3D printing. For 3DCP applications, SEP is a promising candidate, and its performance can be further augmented through synergistic combinations with specific VMAs to improve cohesion and extrudability. However, the use of NCs necessitates careful control over the SP dosage to maintain sufficient open time without compromising the post-deposition stability of the printed structure.

Despite significant advancements, several research gaps remain in the application of NCs in 3D printable concrete. The majority of studies focus on fresh-state properties and early-age mechanical strength, leaving a need for more extensive investigation into long-term durability, including resistance to shrinkage, creep, and chemical attack. Further research is also required to understand the interaction between NCs and a broader range of sustainable binders beyond conventional Portland cement systems. The lack of standardized testing procedures for 3D printable materials makes direct comparison between studies difficult and highlights the need for developing reliable and repeatable characterization methods. Lastly, a comprehensive cost–benefit analysis is necessary to evaluate the economic feasibility of employing NCs in large-scale construction projects, ensuring their viability beyond the laboratory.

8. Conclusions

This comprehensive review critically examined the role of NCs in enhancing the fresh- and hardened-state performance of 3D printable concrete (3DPC), with a particular focus on attapulgite, bentonite, and sepiolite. The findings from recent studies revealed that incorporating NCs notably enhances critical rheological properties, such as thixotropy, static yield stress, viscosity, and buildability, which are vital for effective 3D concrete printing. The literature also revealed that NCs significantly improve mechanical properties such as compressive strength and flexural strength. The following key conclusions are drawn from this systematic and unbiased literature review:

• NCs, such as attapulgite, demonstrate significant potential in enhancing the performance of 3D printing concrete mixes, particularly in terms of rheological behavior, strength, and stability. Its unique needle-like structure and high surface area contribute to improved thixotropy, yield stress, and compressive strength.
• Studies show that NCs like attapulgite and sepiolite can enhance static yield stress and thixotropic recovery without significantly increasing apparent viscosity, thus achieving a rare but ideal rheological balance for 3DCP.
• The inclusion of NCs significantly enhances thixotropy by promoting flocculation and internal structural rebuilding. Studies demonstrate that even small additions of NCs markedly increase thixotropic indices and hysteresis loop areas, indicating stronger structural recovery.
• The addition of NCs tends to reduce flowability due to increased flocculation, as evidenced by reduced slump flow. However, this reduction can enhance shape retention and buildability if optimized. The findings suggest that a 0.5% dosage of sepiolite may offer a balanced compromise between sufficient flow and structural stability.
• NCs generally improve early-age and 28-day compressive strength due to their pore-filling and microstructure-densifying effects. However, these benefits are highly dependent on curing conditions and NC dosage, with excessive content or improper curing possibly reducing strength. Similar trends are observed in flexural strength testing, where optimal NC levels yield improvements, while overdose may lead to reduced flexural strength.
• Experimental evidence indicates that the optimal dosage of NCs enhances compressive strength and flexural strength, although excessive NCs may reduce workability.
• The incorporation of NCs, particularly attapulgite, enhances SYS by strengthening interparticle cohesion and encouraging microstructural densification. Multiple studies confirm a proportional relationship between NC content and SYS, with optimal dosages significantly improving structuration without compromising extrudability.