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Currently, a novel shrinkage‐reducing polycarboxylate superplasticizer (SR‐PCA) is used to control cementitious shrinkage. To clarify its mechanism when applied in cementitious materials, the influence of SR‐PCA on the composition, morphology, and structure of synthetic calcium–silicate–hydrate (C–S–H), together with the interaction between SR‐PCA and C–S–H at the atomic level, is investigated. For comparison, a commercial polycarboxylate superplasticizer (PCA) is also employed. The results show PCA and SR‐PCA can adsorb on the C–S–H surface rather than intercalate into the layers. Compared with PCA, SR‐PCA has a milder impact on C–S–H crystallinity. SR‐PCA refines the pore structure of C–S–H drastically, whereas PCA loosens the structure by increasing the mesopore volume. In addition, the adsorption effect of SR‐PCA on the C–S–H surface is less significant than that of PCA. At the atomic level, this less adsorption of SR‐PCA is attributed to the lower adhesion energy of the C–S–H/SR‐PCA interface due to the weaker Ca–O bond strength. [ABSTRACT FROM AUTHOR]

The deterioration of cement-basted materials serviced in leaching environments is strongly dependent on the decalcification behavior of calcium silicate hydrates after complete dissolution of calcium hydroxide. This paper examines the deteriorating condition of calcium silicate hydrates (hereinafter referred to C–S–H(I)) samples with the decalcification degree which is controlled by immersing samples into leaching solutions with different NH4Cl concentrations. The composition, chemical structure, micromorphology and pore structure of un-leached and leached C–S–H(I) samples were determined using XRD, TG, FTIR, 29Si NMR, SEM, TEM and N2 adsorption–desorption measurements. Results reveal that the decalcification degree of C–S–H(I) samples increases with the increasing NH4Cl concentration with the Ca/Si atomic ratio decreasing from 0.91 to 0.02. The area of flocculent micromorphology of C–S–H(I) samples increases with the decalcification degree due to the continuous generation of cross-linking C–S–H(I) and amorphous silica gel. In addition, the decalcification behavior damages the crystallinity, coarsens the pore structure and decreases the chemically-bound water content of C–S–H(I) samples. Molecular dynamics simulations prove that increasing NH4Cl content results in weaker connectivity stability and more breakage of Ca–O bond in C–S–H(I) molecules. [ABSTRACT FROM AUTHOR]

Belite-rich cement (BRC) has gained increased attention in recent years due to its low energy consumption and high long-term performance. However, the carbonation behavior of the binder formed by its combination with fly ash (FA) has been scarcely revealed. This study investigated the influence of carbonation curing and FA replacement level on the carbon capture and mechanical properties of BRC. The carbon uptake, phase composition, microstructure, and micromorphology of BRC-FA composites after carbonation curing were characterized through quantitative XRD, TG, FT-IR, MIP, and SEM-EDS. The results indicated that carbonation curing significantly increased the compressive strength by up to 81.6% of BRC compared to standard curing due to the high reactivity of belite phase with CO 2 , whilst the hydration products of BRC gradually disappeared, and calcite and amorphous silica became the main phase accompanied with pore refinement behavior. Moderate addition of FA increased the degree of carbonation and compressive strength of BRC-FA composites, however excessive addition (45%) of FA results in adverse effect. Additionally, FA accelerated the phase recombination but coarsened the pore structure. The rapidly formed calcium carbonate crystals stacked and squeezed together, its blend with amorphous silica gel, and the pore structure rapidly densified explain the carbonation strength growth of BRC-FA composites. [ABSTRACT FROM AUTHOR]

Fly ash and slag are two commonly used supplementary cementitious materials (SCMs) in modern concrete, which can significantly enhance the long-term durability of concrete. However, their application is somewhat limited due to the negative impact on early strength development and shrinkage cracking. This study proposed using a recently developed shrinkage-reducing polycarboxylate admixture (SR-PCA) to alleviate the above problems. The effect of SR-PCA on compressive strength, drying shrinkage, autogenous shrinkage, mass loss, pore structure and hydration process were also investigated. The results showed that SR-PCA enhanced the 28d compressive strength due to the refinement of pore structure and improved hydration degree. With the addition of SR-PCA, the drying shrinkage and autogenous shrinkage were significantly reduced, particularly in the mixture blended with 50% fly ash, while showing a slight impact in the sample with 50% slag. SR-PCA decreased the drying shrinkage by lowering the hydrous compressibility factor (CF) value and mesopore volumetric percentage. The incorporation of SR-PCA retarded cement hydration at an early age, thereby reducing the early-age autogenous shrinkage, while the long-term autogenous shrinkage was mitigated due to the enhancement effect on the hydration degree at 28 days. Therefore, SR-PCA is a practical approach for enhancing shrinkage resistance and compressive strength in cement-fly ash-slag blends, thereby contributing to the broader application of these industrial by-products. [ABSTRACT FROM AUTHOR]

Terminate hydration and drying procedures were necessary before characterizing the cement-based materials. This study investigated the influence of drying method (-including freeze-drying and vacuum-drying) on the composition-structure of main hydration product (C-S-H) in cement-based materials. In addition, the effect of pH value was considered. The in-situ atomic scale change of C-S-H under different drying conditions was described using molecular dynamics simulations (MDS). The results reveal that freeze-drying has a stronger drying effect than vacuum-drying. Freeze-dried C-S-H samples have lower bound water content, smaller interlayer distance, longer mean chain lengths (MCL), denser pore structure and lower crystallinity than vacuum-dried samples. As the pH values increase, the bound water content, interlayer distance, MCL and cumulative pore volume decrease. In addition, although the micromorphology of freeze-dried samples is similar to vacuum-dried ones, the crystallinity of freeze-dried samples is lower than that of vacuum-dried samples. MDS provides evidence for the influencing mechanism of drying method on interlayer distance and chain length of C-S-H from the atomic scale. It also proves that the complexation between water molecules and interlayer Ca2+ is more accessible to be destroyed in freeze-drying than vacuum-drying, resulting in a higher reduction of interlayer distance after freeze-drying. Furthermore, water loss due to drying action breaks the Ca-OH bond, enhances Ca2+ movement in the interlayer of C-S-H and facilitates the generation of Ca-O structure in the interlayer, leading to the increase in chain length of C-S-H. • Examined C-S-H gel performance under vacuum and freeze drying through a systematic study. • Identified stronger impact of freeze-drying on C-S-H: less bound water, longer MCL, denser pores. • Utilizes molecular dynamics simulations to show atomic scale changes in C-S-H under drying conditions. • Uncovered mechanisms of drying methods on C-S-H interlayer distance, chain length, and water-Ca2+ disruption. [ABSTRACT FROM AUTHOR]

This paper examines the deteriorating behavior of C-(A)-S-H samples with three Al/Si ratios from atomic to mesopore scales under the decalcification action. The composition, chemical structure, pore structure, micro/nanomorphology of samples were studied using XRD, FTIR, 29Si and 27Al NMR, TEM, SEM and N 2 adsorption. Results indicate bridging Al(IV) in alumino-silicate chain and most of Al-incorporated phases (e.g., Doyleite and Zeolite) dissolve in NH 4 Cl solution. Furthermore, the decalcification action improves polymerization and elongates chain length with simultaneous transformation of C-(A)-S-H into amorphous Al-Si gel. Therefore, the crystallinity of C-(A)-S-H is damaged and the nanomorphology becomes flocculent from foil-like, with large reduction on Ca/Si ratio but slight reduction on Al/Si ratio. The dissolution of C-(A)-S-H coarsens but the generation of amorphous Al-Si gel refines pore structure. Samples with moderate Al/Si ratio exhibit stronger resistance to structural evolutions than samples with other Al/Si ratios. Our work paves the way for understanding Ca-leaching induced multiscale deteriorating mechanism of blended cement-based/alkali-activated materials and the improvement on structural stability by moderate Al addition. [ABSTRACT FROM AUTHOR]

• Clay slightly alleviated the autogenous shrinkage while significantly aggravated the drying shrinkage. • The initial swelling of clay is responsible for reducing autogenous shrinkage. • Incorporation of clay limitedly affects the relationship between IRH and autogenous shrinkage. • The coarsened pore structure accelerates the drying shrinkage and the self-contraction of clay aggravates the drying shrinkage. • Pronounced volume shrinkage of hydrated clay occurs as RH drops, but the conversion of the self-contraction of clay to the shrinkage of mortar is low. Clay-like substances in manufactured sand significantly impact the shrinkage of cementitious materials. This study investigated the effect of clay content on autogenous shrinkage, drying shrinkage, hydration rate, internal relative humidity (IRH), elastic modulus and pore structure of cementitious materials. The water content and self-contraction of clay during desorption were also studied to clarify the underlying mechanism of its influence on shrinkage. The results indicate that the clay slightly alleviated the autogenous shrinkage while significantly aggravated the drying shrinkage. Pronounced self-contraction of clay occurred as RH dropped, but the conversion ratio of the self-contraction of clay to the shrinkage of mortar was very low. It is concluded that the self-contraction of clay obviously influences the pore structure and the evolution of the various performance of mortars. Furthermore, the initial swelling of clay and the gentle decline of IRH are account for the reduction in autogenous shrinkage. Meanwhile, the coarsened pore structure accelerates the drying shrinkage and the self-contraction of clay intensifies the drying shrinkage. [ABSTRACT FROM AUTHOR]

• A novel SR-PCA is prepared via free-radical copolymerization. • The retardation effect of SR-PCA on cement hydration is studied. • The adsorption and dispersion behaviors of SR-PCA in cement paste are studied. • The shrinkage-reducing mechanism of SR-PCA in cement paste is studied. The incorporation of polycarboxylate superplasticizers may increase the risk of shrinkage cracking of cement-based materials. Co-adding shrinkage-reducing admixtures (SRAs) may decrease the compressive strength or arise the incompatibility problem. Designing a novel superplasticizer with excellent shrinkage-reducing function by modifying molecular structures of polycarboxylate superplasticizers can be a promising approach. This study aims to synthesize a novel shrinkage-reducing polycarboxylate superplasticizer (SR-PCA) possessing shrinkage-reducing and water-reducing functions and to investigate its performance and working mechanisms in cementitious systems. The results indicate that SR-PCA improves the fluidity of cement paste and has a distinguished fluidity retention capability. SR-PCA prolongs the cement hydration induction period, hinders the nucleation and growth of C-S-H in the acceleration period, while enhances hydration degree and refines the pore structure at later ages. Moreover, it can also be deduced that most PCA adsorbs on the cement surfaces, however, SR-PCA is mainly in the pore solution. The shrinkage-reducing mechanism of SR-PCA in cement-based materials is attributed to lowering the surface tension of the pore solution, due to the presence of the unreacted esterification macromonomers (AA-C4-PE-600) and the unadsorbed SR-PCA molecular. [ABSTRACT FROM AUTHOR]

Nowadays, the efficient recycling of construction waste and the sustainability of recycled concrete have faced challenges due to the reduced performance of recycled aggregate (RA). One significant factor impacting the performance of RA is solar irradiation, which occurs throughout the entire service life of a construction project. However, the analysis of the effect of solar irradiation on hardened cement paste has not been fully addressed. This research presents a comprehensive investigation of the multi-scale deterioration of RA caused by solar irradiation. The dynamic evolution of the atomic and chemical structure of synthetic Calcium Silicate Hydrate (C-S-H) gel, the morphological changes in tricalcium silicate (C 3 S) hydration products, and the alteration of pore structure in the cement paste are unveiled through the combination of reactive molecular simulation and experimental approaches. The results indicate that solar irradiation disrupts the structure of C-S-H, leading to the migration of water molecules from the interlayer spaces of C-S-H to the pore spaces. This prompts the formation of a new, denser gel. However, as a consequence, more capillary pores are formed within the cement paste. The research outcomes provide a bottom-up, multi-scale insight into the reduced performance of RA under solar irradiation. Furthermore, the transmission mechanism of deterioration information at various scales is established, facilitating comprehension of the correlation between gel characteristics and macro-performance. This research paves the way for strategies to ameliorate the sustainability of recycled concrete, addressing the challenges associated with solar irradiation and enhancing the overall performance of recycled aggregate in concrete applications. • Radiation instigates the dissociation of water molecules. • Dissociated water fosters internal crosslinking within the gel network. • The shrink of the gel network concurrently bolsters the porosity of paste. [ABSTRACT FROM AUTHOR]

As a promising and environmentally friendly cementitious material for mass concrete applications, the long-term performance evolution of low-heat cement (LHC) and moderate-heat cement (MHC) under the alternating environmental temperature remains unclear. To address this gap, a comprehensive comparative study based on laboratory acceleration test was conducted by evaluating the physicochemical properties and cracking behavior of LHC and MHC in comparison to ordinary Portland cement (OPC) following 1200 thermal cycles (TC) at temperatures ranging from 5 to 45 °C. Cracking behavior is revealed by a quantitative identification analysis. The results indicate a negative linear relationship between porosity, crack density, and compressive strength, highlighting their competing mechanisms in determining the degree of damage under TC. TC sharply diminished the gel pores (<10 nm) and promoted the formation of capillary pores (50–1000 nm), which were the primary forms of pore coarsening observed. Notably, the different proportions of hydrated products such as HD-CSH, LD-CSH, and CH influence the toughness properties of the matrix and contribute to different resistances to cracking and the formation of cracks with varying fracture morphology, although they also impact the stability of the pore structure. Further analysis revealed different degrees of rehydration occurring within the first 200 cycles of both LHC and MHC, which contributed to the mitigation of performance degradation. Moreover, an increase in the water-to-cement (w/c) ratio had a tendency to coarsen the matrix and facilitate the formation of long and narrow cracks. • The competing mechanism between porosity and crack density in determining the degree of damage was proposed. • Different proportions of hydrated products influenced the toughness properties. • Theevolution mechanism of LHC under thermal fatigue was proposed with comparison with MHC and OPC. [ABSTRACT FROM AUTHOR]

• Thermal fatigue of cement-based materials in a mild temperature range. • Quantification of microcracks based on image processing and grayscale algorithm. • The double-edged effect of the w/c ratio on the deterioration. • Evolution of macro-properties and micro-structure under thermal cycling fatigue. • The establishment of the relationship between microcracks and Vickers hardness. Concrete structures in the atmosphere are continuously subjected to thermal cycle (TC) induced by varying environmental temperature. However, the influencing mechanism of thermal cycling fatigue on the mechanical properties and microstructure of cement-based materials has not been thoroughly documented. To address this issue, the relationship between the macro-properties and microstructure of cement paste under TC was analyzed using micro-properties measurement and quantitative microcrack evaluation. In this work, ordinary Poland cement pastes with three different water-to-cement (w/c) ratios (0.4, 0.5, and 0.6) were exposed to TC in the range of 5–45 °C. The mechanical properties, pore structures, and micromorphology of cement pastes after different cycle times were measured. The result indicated that as the number of cycles increased, TC negatively affected the macro-mechanical characteristics, pore structure, and micromorphology of cement pastes. Cement pastes with greater w/c ratios were more susceptible to TC-induced deterioration. In addition, thermal cycling fatigue caused the propagation of microcracks including crack width, length, and density. Thermal cycling fatigue also coarsened the pore structure of cement pastes, especially the large pores (i.e., diameter > 100 nm). The underlying damage mechanism caused by TC is the coupled actions of fluid flow, thermal deformation, and thermal fatigue. Moreover, a negative linear relationship was established between microcrack density and Vickers hardness of cement pastes. [ABSTRACT FROM AUTHOR]

Calcium silicate aluminate hydrate (C(-A)-S-H) is the main hydration product used in cement-based materials. It plays a specific role in both the mechanical properties and volumetric deformation of the cement matrix. This study aims to clarify the effect of porosity and chemical composition of C(-A)-S-H on its micromechanical properties using nanoindentation. The results showed that porosity significantly affected the indentation, elastic, and creep moduli of the porous C(-A)-S-H compacts. Micromechanical properties of the "pure" C(-A)-S-H were improved by decreasing the Ca/Si ratio. The incorporation of Al slightly altered the creep modulus by affecting the volume of the large gel pores, basal spacing, and mean chain length (if no cross-linking occurred in C(-A)-S-H). Finally, the nanoindentation results of the mixed C(-A)-S-H and Al(OH) 3 confirmed the distinct contribution of the individual component characteristics to the overall properties of porous mixtures. [ABSTRACT FROM AUTHOR]