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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]

• The influence law of clay content on properties related to plastic shrinkage cracking is revealed. • The addition of clay reduces the permeability of mortar. • The inclusion of clay decreases the plastic tension strength. • The interaction between clay and PCE keeps the surface tension of the pore solution the same at consistent fluidity. • The action mechanism of clay in exacerbating plastic shrinkage cracking is elucidated. Clay minerals in manufactured sand exacerbate plastic shrinkage cracking of cementitious materials. The impact of clay content on plastic shrinkage cracking and the underlying mechanism were comprehensively investigated. The results indicate that clay considerably mitigates the plastic settlement while significantly aggravates the plastic shrinkage, triggering more severe cracking. Water absorption of clay substantially lowers the permeability of mortars, which accounts for the reduction in bleeding, evaporation and plastic settlement. The intensified plastic shrinkage can be attributed to the accelerated build-up of capillary pressure and the self-contraction of clay. Furthermore, the critical cause of the more severe cracking with higher clay content is the rapid increase of plastic shrinkage and the reduction in plastic tensile strength. [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]

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]