Superabsorbent polymers (SAPs) are effective internal curing agents for mitigating early-age autogenous shrinkage in low water-to-binder cementitious materials. However, the rapid water uptake of SAP during mixing usually compromises fluidity, and the pores formed by SAP water release lead to strength loss. In this study, two chemistry-driven modification routes are proposed to decouple “internal curing efficiency” from “workability/strength penalties”: (i) coupling-agent modification using vinyltrimethoxysilane (VTMS) (V-series), and (ii) a sodium metasilicate–assisted VTMS strategy (SiV-series) that forms in situ SiO2 particles within the SAP network, providing abundant grafting sites to construct a denser and more stable siloxane-crosslinked structure. Both modified SAPs exhibit progressive water absorption (low initial absorption, followed by gradual uptake during hydration), thereby suppressing early-stage free-water competition and significantly improving fresh-state performance. The fluidity reductions of V5 and SiV5, representative samples of the V- and SiV-series, were only 3.2% and 2.9%, respectively, compared with 13.8% for unmodified SAP. Mechanistically, fluorescence impregnation-based quantification, MIP, TG, and 1H LF-NMR reveal that VTMS hydrolysis and cleavage regulates swelling–release kinetics, while SiO2-assisted grafting effectively reduces SAP-induced porosity and refines pore formation governed by the pore-forming effective water-to-cement ratio at final setting. As a result, the compressive strength retention increased markedly, reaching 97.2% of the reference value for the cement paste incorporating SiV5 and 91.7% for that containing the optimal V-series sample, compared with 80.1% for the cement paste incorporating unmodified SAP. In addition, the autogenous shrinkage was significantly reduced. This work provides a feasible design strategy for internal curing SAPs compatible with low water-to-binder systems, achieving high shrinkage mitigation with near-original fluidity and strength.

Reinforcement implantation in concrete 3D printing is always a challenging task. Herein, a saddle stitching reinforcement (SSR) was developed to reinforce 3D printed concrete (3DPC). Compression and splitting tests were conducted to evaluate the mechanical properties of SSR-3DPC with anisotropy assessment; digital image correlation (DIC) analysis was integrated to track the failure process and mode; and X-ray computed tomography (XCT) was employed to characterize the interfacial microstructure of stitched 3DPC specimens.Results demonstrate that the proposed saddle stitching technique effectively mitigates the splitting strength anisotropy, and enhances the slipping strength, ductility, and absorption energy through the combined action of stitch legs and crotch reinforcement. However, a weaker stitch-matrix interface forms due to the inadequate self-recovery of 3DPC slurries after implantation. These findings deepen the understanding of reinforcement implantation in viscous concrete slurries, and provide valuable insights for optimizing reinforcement strategies in concrete 3D printing.

Abstract The hydrophobic groups in the main chain of polycarboxylate superplasticizers play an important role in the dispersion of cement particles. In this study, a series of macromonomers with different molar ratios of ethylene oxide to propylene oxide (EO/PO) were introduced to modify the molecular conformation of polycarboxylate superplasticizer. Subsequently, the influence of macromonomers with different hydrophilic-lipophilic balance (HLB) values on the solution conformation, surface tension, and absorption behavior of synthesized polycarboxylate superplasticizers (PCE) in a cementitious environment was investigated. Furthermore, the rheological behavior of cement paste with different PCE dosages was investigated. The HLB value of macromonomers showed a good correlation with the viscosity of cement paste. By adjusting the HLB values of the macromonomers, the dispersion ability and viscosity-reducing performance of PCEs can be greatly enhanced.

To date, the production of all types of traditional prestressed concrete has been limited by the use of steel tendons. Inspired by the working principles of tempered glass and prestressed ceramics, a new type of prestressed concrete without reinforcement (PCWR) was designed and prepared for the first time. A theoretical model was established to seek the critical factors affecting prestress and to guide the experimental design. Flexural tests and strain monitoring were conducted in order to ascertain the material's properties. The cross-sectional height ratio (RH), the expansion rate of the surface layer, and interfacial bonding all exerted a significant effect on the flexural strength of the PCWR. The highest flexural strength of PCWR was 7.11 MPa, representing a 41% improvement. A 7.35 MPa of residual compressive stress in the surface layer of PCWR was monitored. The present study demonstrated that PCWR exhibited superior flexural to compressive strength ratios (FCR) and flexural toughness before first cracking (Tfc), with respective improvements of 76% and 78%. As an innovation in materials reinforcement, PCWR offers a novel strategy for the strengthening of concrete materials, making it the first prestressed reinforced concrete material comparable to tempered glass and prestressed ceramics. The introduction of prestress without reinforcement has been demonstrated to significantly improve crack resistance, and may help reduce the ingress of external substances into the surface layer.

Ground granulated blast furnace slag (GGBS) is the main by-product of steel industry and has been widely utilized as mineral addition in concrete. Due to the difference of chemical composition and hydration reactivity of slag and cement particles, the ionic environment of cement paste changed a lot and influenced the dispersion performance of PCEs. For clarifying the ionic environment change and their effects on the dispersing ability of PCEs. The ICP-OES was used for clarifying ionic environment of cement paste, and their effects on dispersing performance of PCEs were determined by flowability and rheological experiments. Test results showed that ionic environment influenced the surface charge of slag, affected adsorption behavior and conformation of PCEs inducing the different dispersing properties. PCE with longer side chain length or smaller acid-ether ratio exhibited better dispersion performance.

3D printed concrete (3DPC) technology，while contributing to a profound revolution in the construction industry，still faces huge challenges in large-scale applications due to the presence of weak interfacial structures and properties. Herein, an innovative solution for interfacial improvement based on Micro-Region Vibration (MRV) of magnetorheological fluid (MRF) in 3DPC interfaces, has been proposed. Results indicate that cross printed specimens subjected to magnetic field treatment demonstrate 114% increase of interfacial tensile strength, reaching as high as 2.27MPa, and 36.2% decrease in interfacial porosity as opposed to the control group. Higher MRV frequencies reduced the strengthening effect irrespective of the magnetic particle size. The optimal activation duration, however, was longer for the finer particles than for the coarser ones. The interfacial fusion of cement slurry by MRV mitigates the interfacial defects in 3DPC. This technology presents a novel perspective to address the interfacial issue in 3DPC and holds great potential to surmount the technical bottlenecks and propel the progress of the industry.

CO2-curing, CO2-mixing and single-component carbonation offer promising strategies for reducing the carbon footprint of concrete; their current sequestration efficiencies remain insufficient to achieve carbon neutrality. This study proposes an innovative pathway towards carbon-neutral concrete by developing Total Component CO2-sequestration Concrete (TC3). The key innovation involves that total components are incorporated into the overall decarbonization framework, either through direct pre-carbonation treatment of carbonatable raw materials, inherent carbon sequestration potential during the in-service stage and indirect carbon benefits from cement usage reduction. CO2 reduction evaluation shows that TC3 achieves a CO2 reduction around 160 kg/m3, equating to an approximately 42% reduction per unit volume compared to ordinary concrete. This reduction is almost 2 times that of CO2-curing and 8 times that of CO2-mixing compared to typical literature-reported values. The synergistic hydration effect of carbonated supplementary cementitious materials and CO2-rich mixing water reached a 28-d compressive strength of over 60 MPa. Furthermore, the compressive strength carbon emission index of TC3 is over 40% and 30% lower than that of CO2-mixing and CO2-curing, respectively. By overcoming the limitations of diffusion (CO2-curing) and dissolution (CO2-mixing), TC3 establishes a better and more comprehensive carbon management strategy. This paradigm transforms concrete from a carbon source into a CO2-storing unit, providing a robust pathway to reconcile growing global concrete demand with stringent carbon neutrality commitments.

To quantify the correlation between dynamic drying shrinkage and pore-scale water removal kinetics, the pore-scale water allocation and dynamic shrinkage of white cement pastes upon drying at 75%, 43%, and 11% RHs were monitored and analyzed. Experimental results show a bilinear dependence of dynamic shrinkage on the removals of interlayer and gel water within CSH gel irrespective of RHs. CSH gel behaves like flexible hydrous sponges skewered by a stiff skeleton. Although CSH sponges lose water and contract remarkably upon drying, the spatial constraint of skeleton limits the deformation of pastes. Consequently, only 0.72% to 4.23% of interlayer and gel water losses are translated into measurable shrinkage. The removal of gel water contributes to shrinkage more than that of interlayer water due to the larger size of gel pores, though both their contributions decrease with declining RH and become similar. Mitigating shrinkage necessitates reducing CSH contraction and enhancing skeleton stiffness.

The extensive application of recycled aggregate concrete (RAC) in the construction industry has gained significant attention currently due to its potential environmental and economic benefits. However, numerous time-consuming experiments are required to evaluate the properties of RAC. Furthermore, multiple conflicting objectives need to be considered for RAC mix design, such as high mechanical strength, high durability, low carbon footprint, and cost-efficiency. Thus, this study employs nine machine learning models to predict the compressive strength (CS), carbonation depth (CD), and passing electric charge (PEC) of RAC after hyper-parameter optimization, utilizing datasets of 965, 698, and 231 samples, respectively. LightGBM achieved the highest performance for CS (R2=0.9523, RMSE=2.7163) and CD (R2=0.9286, RMSE=1.8723), while XGBoost excelled in predicting PEC (R2=0.8985, RMSE=514.3255). Generally, the predicted values of selected models and the actual values were well fitted, with an overall control at around 20%. The selected machine learning models are superior to conventional quantitative models. By combining the objectives of carbon footprint and cost, the Adaptive Geometry Estimation based MOEA II (AGE-MOEA-II) algorithm was used for multi-objective optimization. A hybrid combination of Analytic Hierarchy Process (AHP) and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) methods was employed to obtain the optimal solution in two design scenarios. This proposed framework can be applied to various engineering applications, which could improve the efficiency in the mix design optimization and promote the use of RAC in engineering.

Despite extensive advancements in Engineered Cementitious Composites (ECCs), mixture design remains predominantly empirical, due to the absence of a quantitative parameter directly linking fiber-matrix interfacial mechanics to strain-hardening performance. This study identifies fiber-matrix interfacial friction as a quantifiable parameter and establishes a micromechanics-guided interfacial regulation framework to enhance the toughness of ECC by regulating fiber failure modes. First, a critical fiber-matrix interfacial frictional stress, (τ0)crit, corresponding to the transition between fiber pull-out and fracture, was theoretically derived based on energy dissipation maximization during crack propagation. A back-calculation approach was further developed to determine interfacial frictional stress (τ0) directly from tensile stress-crack opening responses under single-crack tension, eliminating reliance on single-fiber pull-out testing. Then, τ0 was tuned toward (τ0)crit through interfacial regulation using fly ash. Experimental results demonstrate that the toughness of ECC is maximized when τ0 approaches (τ0)crit, confirming the validity of the proposed toughness enhancement mechanism. The study establishes an explicit mechanistic linkage between interfacial micromechanics and macroscopic strain-hardening performance, providing a predictive and quantitative design pathway that transcends empirical mixture adjustment.

This paper discusses the flexural and tensile strength properties of 3D printed concrete, based on the results of a RILEM TC 304-ADC interlaboratory study on mechanical properties. These properties are determined using different testing techniques, including 3- and 4-point flexural tests, splitting tests, and uniaxial tension tests, on specimens extracted from large 3D printed elements in accordance with a prescribed study plan. The relationship between compressive and flexural or tensile strengths, cast or printed samples, different types of tests, and different loading orientations, are analysed to understand the influence of 3D printing. As expected, the strength can reduce significantly when the main tensile stress is acting perpendicular to the interface between layers. The role of deviations from the standard study procedure, in terms of the time interval between the placing of subsequent layers, or the adoption of a different curing strategy, are also assessed. While the increased time interval significantly impacts the strength in the critical direction, the use of variable curing conditions does not seem to have a clear-cut effect on the strength ratios of the printed to cast specimens. Additionally, the paper looks at the variability in the results for the printed specimens, in order to emphasize the need for multiple replicates for obtaining a proper result. An extensive insight into the aspects affecting the variability is presented in the paper. Finally, with the limited dataset available for specimens tested at a larger scale, it is difficult to arrive at a clear understanding of the role of specimen size (i.e., greater number of layers).

The two major themes in the current construction industry are digital construction and low environmental impact. As a prominent digital construction technology, concrete 3D printing has attracted increasing attention. However, the current understanding of the durability of 3D printed cement-based materials (3DPCM) remains limited, which hinders its wider application, especially as load-bearing, reinforced concrete structures. This work shares the knowledge acquired during a broad interlaboratory study regarding the durability of 3DPCM with 15 laboratories from 13 countries participating, under the framework of TC 304-ADC ‘Assessment of Additively Manufactured Concrete Materials and Structures’. Anisotropy in water absorption capacity, carbonation and chloride ingress resistance of 3DPCM were evaluated by 15 institutes with their own printable materials and printing equipment. Additionally, the impacts of cold joints on these properties were investigated and a comparison between printed and cast samples was carried out. The outcome of this study indicates that the water absorption test provides information on the bulk porosity of the samples, while the carbonation and chloride ingress tests are more effective and visually reflect the local defects, especially the layer interfaces and cold joints. The water ingress depth of cast samples prepared with printable mixtures is an order of magnitude higher compared to conventional concrete, while their carbonation and chloride ingress resistance are comparable. The sorptivity and estimated water ingress height of printed samples measured in the direction parallel to the filaments is generally higher than that measured in the perpendicular direction and in cast samples. Similarly, the carbonation and chloride ingress depth and rate of printed samples measured in the direction parallel to the filaments is generally higher than that measured in the perpendicular direction or in cast samples. The overall durability of 3DPCM is weakened by anisotropy, these effects can be addressed with targeted mixture design and processing strategies. Due to the variations in printers, printing parameters and materials, three types of cross-section geometries were observed in printed samples with cold joints. The carbonation depth that measured from the maximum carbonation ingress point near the cold joint to the sample edge effectively captures the effect of cold joints in all these three types of cross-section geometries of printed samples. Finally, the participants identified areas of improvement in the methodology and suggestions were made to refine the procedure for adoption in future research.

Understanding the porosity characteristics and mechanisms of superabsorbent polymers (SAPs) in cement-based materials is crucial for evaluating their effects on autogenous shrinkage and compressive strength in cementbased materials. In this study, SAPs with different chemical compositions were synthesized by copolymerization of acrylic acid (AA) and acrylamide (AM). Absorption and release behaviors were analyzed, while pore formation parameters were assessed using ultraviolet fluorescence impregnation and digital image processing. Combined with FTIR, TG/DTA, and performance tests, water release characteristics, pore formation mechanisms, and regulatory effects on cement paste were evaluated. Key findings include in terms of absorption behavior, the equilibrium absorption capacity in cement filtrate decreases as AA content in SAP increases. Pore-forming absorption capacity in actual cement paste is higher than that obtained in cement filtrate. The ability of SAP to reduce autogenous shrinkage depends on its water storage and release properties. The optimal composition for minimizing shrinkage is mAA:mAM = 25 %:75 %. For compressive strength, SAP-induced porosity (eta pf) is the primary factor. Similar eta pf values for certain compositions lead to lower strengths. When SAP contains only -CONH groups, the compressive strength is highest. This study provides a theoretical basis for optimizing SAP composition to reduce autogenous shrinkage and enhance concrete strength.

Belite-rich cement (BRC), as a low-carbon footprint and low early heat-release cement, is garnering increasing attention. However, its early strength development is not competitive compared to ordinary Portland cement (OPC). Enhancing the early strength of BRC is a crucial area of research. This study used 20 degrees C as a reference temperature to investigate the long-term performance development of cement-based materials after early temperature-rise curing (55 degrees C, 90 degrees C), including mechanical strength, phase composition, microstructure, and gel chemical structure. The results show that raising the early curing temperature significantly enhanced the mechanical performance of BRC compared to OPC. Phase composition characterization revealed that the higher early temperature greatly accelerated the hydration of belite in BRC, resulting in more amorphous hydration products compared to OPC. The Q1 content in C-(A)-S-H increased significantly, the Q2 proportion grew slowly, and the average chain length difference narrowed, approaching that of OPC. The volume of coarse pores decreased rapidly, with pores of 30-200 nm becoming predominant. During subsequent long-term standard curing, an early curing temperature of 55 degrees C resulted in continuous strength gains, surpassing OPC at 28 days, making it the optimal early curing temperature for BRC. BRC cured at 90 degrees C initially showed strength deterioration during subsequent standard curing, similar to OPC, but recovered between 28 and 300 days, eventually exceeding OPC. Pore structure differentiation, delayed ettringite formation, and non-uniform distribution of hydration products explained this deterioration mechanism. BRC exhibits excellent early strength and sustained long-term development under moderate elevated curing temperatures, and at higher early temperatures, it demonstrates superior mechanical performance and potential for self-recovery, offering broad application prospects.

This study investigates the hydration behavior and microstructural evolution of Belite-rich cement (BRC) following early accelerated carbonation and subsequent water curing. Comparative experiments with ordinary Portland cement (OPC) under identical curing regimes were conducted to evaluate compressive strength development, reaction kinetics, and phase assemblage. Results show that early carbonation forms a spatial CaCO3 shell-hydration core structure in BRC, which facilitates continued hydration by providing abundant nucleation sites and ion pathways. This leads to further precipitation of hydration products that embed the carbonate skeleton, resulting in a compact composite structure. After 90d, BRC samples subjected to 3 h and 15 h of early carbonation achieved compressive strengths of 70.2 MPa and 68.1 MPa, respectively-9.9 % and 12.1 % higher than the non-carbonated reference (62.6 MPa), and significantly outperforming OPC (58.7 MPa). Based on these findings, a three-stage synergistic mechanism is proposed: initial hydration, surface carbonation, and reactivated hydration. This mechanism elucidates the strength enhancement and structural densification of BRC under coupled carbonation-hydration conditions. The results provide new insights into the performance evolution of BRC systems and offer practical guidance for carbon utilization strategies in cement-based materials.

This study investigates the surface damage evolution of low-heat cement concrete (LHCC) and PI cement concrete (PICC) under the combined effect of bending stress and freeze-thaw (FT) cycles, using a self-invented stress-FT coupling test system. The line laser scanning technique (LLST), an innovative non-contact and non-destructive testing method, is employed to quantify multiple surface damage parameters with considerable precision. The surface flatness and average vertical displacement obtained by LLST show strong consistency with traditional scaling weight measurements, confirming the accuracy and effectiveness of LLST. The experimental results demonstrate the exceptional superiority of LLST, and also reveal that the bending stress-FT synergy accelerates damage rate by 200% compared to FT cycles alone, with stress contributing over 50% of total damage. Moreover, LLST demonstrates exceptional capability in detecting the localised extreme damage (LED). The variation coefficient is introduced to quantify the discrepancy between global damage and LED. A difference exceeding 500% highlights that LED can escalate to unpredictable structural consequences, such as sudden brittle fracture without any macroscopic warning signs. These advancements establish LLST as an effective tool for real-time structural health monitoring, particularly for critical infrastructure in cold regions where coupled stress and FT threats exist.

This study investigates an alkali-resistant superabsorbent polymer (SAP-I) for cement-based materials, featuring a specialized "claw" distribution in concrete structures after pre-absorption water. To evaluate the performance changes in cement-based materials with SAPs, we compared laboratory-prepared alkali-resistant SAP (SAP-I) with commercially available ones (SAP-S). Firstly, the alkali-resistant SAP's adsorption capacity in tap water (185 g/g) is similar to commercial SAPs. However, its adsorption capacity in cement pores (21.63 g/g) far exceeds that of commercial SAPs (2.67 g/g), indicating a more significant internal curing effect. Secondly, the strength increase in cement-based materials with alkali-resistant SAP is significantly greater than with commercial SAPs. The alkali-resistant SAP maintains higher internal relative humidity in cement-based materials, promoting hydration and compensating for pore structure degradation. Thirdly, adding alkali-resistant SAP significantly improves the shrinkage performance of cementbased materials, with SAP-I having a mitigate shrinkage 2.11 times that of commercial SAPs. The combination of SAP particles with the cement forms an organic-inorganic interface, ensuring more uniform water release and significantly reducing autogenous shrinkage effects. In summary, compared to commercial SAPs, alkali-resistant SAPs show better compatibility with cement-based materials and greater potential for large-scale production, making them suitable for specific engineering applications.

This study aims at clarifying the effect of the functional group's grafting positions in PCE molecule on its dispersing performance, adsorption behavior, molecular conformation and cement pastes' setting times. Two kinds of PCEs modified by (3-cyclodextrin ((3-CD) with different grafting positions were synthesized. Test results showed that the grafting position of (3-CD in PCE molecule significantly influences its adsorption behavior and molecular conformation. ((3-M)n-AA-HPEG with (3-CD grafted on the main chain effectively increases adsorption amount on cement and decreases surface adsorption on clay. While PAA-g-((3-P)nHPEG with (3-CD grafted on the side chain slightly decreases adsorption amount on cement and hinders both surface and interlayer adsorption by clay. Moreover, DLS test results showed that the side chain as the grafting position would increase PCE's steric hindrance more effectively than grafting at the main chain. Surprisingly, through adjusting the grafting position, the setting times of cement pastes have exhibited the opposite trend.

Internal curing in high-performance cementitious materials (HPCM) is effectively achieved using superabsorbent polymers (SAP), which significantly mitigate autogenous shrinkage. However, the efficiency of internal curing is determined by factors such as the particle size, absorption capacity, and dosage of the SAP. Currently, the scientific community has not reached a consensus on the optimal SAP system for HPCM. This study examines the compatibility of SAP's absorption capacity, dosage, and particle size with HPCM through orthogonal experiments, focusing on their effects on mechanical properties and shrinkage performance. Additionally, the compatibility of SAPs with various chemical structures and the optimal SAP system was evaluated. Range and variance analysis results show that the optimal SAP system includes a dosage of 0.20 %, a particle size of 100-50 mu m, and an absorption capacity of 19.7 g/g. Compared to the control group (without SAP), the 28-day compressive strength decreased by only 0.77 %, while autogenous shrinkage reduced by 40.32 %. The optimal SAP system facilitated the hydration process of HPCM, reduced porosity, and increased the proportion of transition pore volume, significantly improving the pore structure. Furthermore, autogenous shrinkage decreased significantly with higher SAP dosage, smaller particle size, and greater absorption capacity. However, this also caused a substantial reduction in material strength and a marked increase in porosity, adversely affecting durability. The proposed optimal SAP system is compatible with superabsorbent polymers of various chemical structures. These findings offer valuable insights into SAP applications in HPCM, facilitating its broader adoption in practical engineering projects.