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Steam curing is a crucial stage for the strength development of steam-cured concrete. This study comprehensively investigated the hydration kinetics , mechanical properties , and autogenous shrinkage (AS) of cement pastes during steam curing. A hydration-dependent analytical method was proposed to analyze the mechanisms governing the development of mechanical properties and AS of steam-cured cement pastes (SCPs). Results revealed that in this investigated curing regimes, the peak hydration rate of SCPs was approximately 7 times higher than that of standard-cured pastes (NCPs). SCPs exhibited faster time-dependent mechanical property development but slower hydration-dependent mechanical property development than NCPs. This difference was primarily attributed to variations in initial structure establishment and free water content. Moreover, an elevated curing temperature reduced the growth rate of hydration-dependent AS due to large capillary pores and low surface tension , which suggested that AS should not be considered as a cause of heat damage to SCPs. Introduction The hydration and strength development of concrete at initial ages are crucial for its subsequent evolution [1]. This is particularly true for accelerated curing concrete, wherein the development of hydration and strength are considerably promoted, resulting in the rapid attainment of high mechanical strength and hydration degree in a short time. For example, the pre-tensioned prefabricated concrete track slabs used in China's high-speed railway can achieve 75 % of their designed strength through a mere dozen hours of steam curing. Steam curing remains an essential method for expediting the production of prefabricated concrete components due to its superior effectiveness and susceptibility to precise control compared with the use of chemical accelerating agents [2,3]. The past years have seen a considerable amount of research findings related to steam curing [4]. However, steam curing, which is a crucial stage for the strength gain of steam-cured concrete, has received little attention in previous studies [[5], [6], [7]]. Steam curing promotes the strength formation of concrete but typically results in poor performance at later ages [[8], [9], [10], [11], [12], [13]]. Several feasible explanations for the strength degradation of late-stage steam-cured concrete exist as follows: (1) Hydration inhibition. At elevated curing temperatures, the surfaces of cement particles develop dense hydrate shells that impede subsequent hydration [14]. However, some controversy remains given that the final hydration degree does not seem to decrease with increasing hydration temperature [15,16]. (2) Increase in high-density C-S-H content. At high hydration temperatures, hydrates have substantially higher precipitation rate than diffusion rates, resulting in the accumulation of hydrates in the vicinity of cement particles, thus increasing the high-density C-S-H content [[17], [18], [19]]. Previous research [16] discovered that increasing the hydration temperature from 20 °C to 60 °C increased the apparent density of C-S-H by 12 % from 2.03 g/cm3 to 2.27 g/cm3. This effect consequently led to a reduction in the generated C-S-H volume and an elevation in capillary pore content. (3) Irreversible expansion. During the heating period of steam curing, the concrete remarkably swells due to its high free water content and increased thermal expansion coefficient. As the concrete hardens, its thermal expansion coefficient decreases, leading to thermal shrinkage during the cooling period and insufficient compensation for the expansion that occurred earlier. This phenomenon results in residual expansion at the end of steam curing [8]. The considerable expansion that occurs during the heating period can compromise the establishment of the initial structure of concrete, whereas residual expansion leads to an increase in capillary pore content. (4) Degradation of the interfacial transition zone (ITZ). The varying coefficients of thermal expansion amongst the ingredients within concrete will result in thermal stress under the conditions of dramatic temperature changes [20]. Accelerated hydration leads to increased self-induced stress [21]. Therefore, the degradation of the ITZ can be exacerbated by elevated treatment temperatures during steam curing [22]. (5) Surface degradation. The strength and anti-permeability of the surface layer considerably reduce due to heat and mass transfer between the exposed concrete surfaces and the steam chamber [13,23]. In consideration of the aforementioned concerns, attention is currently focused on optimizing steam curing regimes and cementitious material compositions to mitigate the degradation of steam-cured concrete. Shi et al. [24] suggested that a step heating curing regime was advantageous for initial structure development and resulted in improved mechanical properties and impermeability at later stages. Zou et al. [25,26] proved that subsequent curing methods were effective in alleviating the damage caused by steam curing to concrete. Chen et al. [27] proposed incorporating metakaolin and limestone into concrete to mitigate the adverse effects of steam curing by reducing its treatment temperature. Moreover, lightweight aggregates were added to alleviate some adverse effects of steam curing by homogenizing temperature distribution and enhancing the performance of ITZ, yielding favorable results for concrete [20,28,29]. However, these studies primarily focused on concrete performance after steam curing and gave little attention to hydration behavior and property evolution during steam curing. Liu et al. [[30], [31], [32], [33]] conducted a preliminary study on the hydration and pore distribution of steam-cured cement pastes (SCPs). A comprehensive investigation is required to investigate the changes in SCPs to gain further insight into their degradation mechanism. High autogenous shrinkage (AS) remarkably contributes to reductions of mortar strength [34]. An elevated curing temperature can lead to rapid AS and self-induced stress development, which may increase cracking risk [21,35]. Nevertheless, this aspect has not been thoroughly studied by current research. In addition, the disparity between the early-stage performance of steam curing and that of normal curing is primarily attributed to differences in hydration levels. Therefore, aside from time-dependent factors, the hydration-dependent aspect should be considered when evaluating the early-stage performance of SCPs. In this study, the hydration kinetics, mechanical properties, and AS of SCPs were investigated by using the isothermal calorimetry, the equivalent age function, the resonance fundamental frequency method and the corrugated tube method. Furthermore, on the basis of the results of these methods, the hydration-dependent mechanical properties and AS were established to evaluate its developmental characteristics and mechanisms comprehensively. The findings of this work will provide new insights into understanding the degradation characteristics of SCPs. Section snippets Materials Type P·I 42.5 Portland cement was utilized. It contained clinkers and gypsum with the compositions shown in Table 1. Deionized water with an electrical resistivity exceeding 18.2 MΩ·cm was used. Monoclinic C3S synthesized in the laboratory was employed to analyze the micromorphology of primary hydrates. Sample preparation Cement pastes with varying water-to-cement ratios (w/c), including those with the w/c of 0.3 and 0.4, were employed in this study. The 40 mm cubic specimens were used to determine the compressive Hydration behaviors during steam curing The hydration heat curves acquired during steam curing are compared with those obtained during standard curing, as depicted in Fig. 2. Fig. 2(a) shows that the rate of cement hydration rapidly increases during the heating period of steam curing and peaks at approximately 5 h, which is close to the end of the heating period. The peak hydration rates of the 0.3 and 0.4 w/c SCP are 24.5 and 21.6 mW/g, respectively, whereas those of the corresponding 0.3 and 0.4 w/c NCPs are only 3.3 and 3.1 mW/g, Feasibility of simulating exothermic hydration behaviors through the equivalent method Fig. 7(a) and (b) present the curves of heat flow and accumulative hydration heat of the 0.4 w/c pastes. The pastes cured at 20 °C, 40 °C and 60 °C have the main peak heat flow values of 3.07, 9.14 and 20.34 mW/g, respectively, and the cumulative hydration heat within 72 h values of 253.80, 292.39 and 325.57 J/g, respectively. Hydration may be confirmed to be obviously promoted by elevated temperature. According to previous studies, the reaction degree α( t ) can be determined by the accumulative Conclusion and outlook Steam curing is a crucial stage for the strength formation of SCPs. In this study, the characteristics of time-dependent and hydration-dependent mechanical property and AS were investigated to gain further insight into the degradation mechanisms of SCPs. Based on the findings, the following conclusions are listed below: (1) Under the steam curing regime adopted in this study, the main exothermic peak of hydration in SCPs occurs predominantly between 3 and 8 h and reaches its maximum at approximately CRediT authorship contribution statement Jionghuang He: Conceptualization, Methodology, Investigation, Data collection, Writing-Original draft, review & editing; Guangcheng Long: Conceptualization, Project administration, Writing-review & editing; Kunlin Ma: Analysis, Writing-review & editing; Youjun Xie: Funding acquisition, Writing-review & editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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