硅酸盐学报

低真空环境会加速水泥基材料的水分损失，这会改变其微观结构从而严重威胁其长期力学性能。为此，对砂浆及不同水胶比、养护龄期的净浆与混凝土试件开展了为期18个月的模拟暴露试验，系统研究了长期低真空暴露对水泥基材料力学性能、质量损失、物相组成、水化程度以及孔结构的影响。结果表明：长期低真空环境下水泥基材料的强度演变呈现显著的非线性时变特征，即短期暴露会促进强度的提升，而长期暴露会导致强度的下降与损失。低真空环境下混凝土抗压强度与水胶比呈负相关，而与养护龄期呈正相关。此外，与普通大气环境相比，长期低真空暴露虽然不会改变水化产物类型，但会加速基体的失水从而抑制水化并导致总孔隙率和有害孔径(>100 nm)的增大。同时，低真空环境下基体的水化程度随水胶比的增大或养护龄期的延长而提高，这与基体孔结构的变化规律相对应。此外，低真空环境下混凝土的抗压强度遵循Abrams定律与成熟度理论，拟合曲线与测试数据高度吻合，其决定系数(R²)为0.94，并且基于增强数据集构建的GWO-LSTM模型可精准预测不同水胶比、养护龄期、暴露时间下的混凝土抗压强度(R²>0.96)。研究成果揭示了低真空环境下水胶比、养护龄期与暴露时间对水泥基材料力学性能的影响机制，并据此构建了多因素耦合的强度预测模型，为低真空环境下水泥基材料的力学性能评估与预测提供了理论依据。

Introduction With the expansion of human activities, the application of cementitious materials has gradually extended from normal air condition to low vacuum environment. For instance, in high-altitude regions, concrete structures are affected by low pressure due to increased elevation. Moreover, for recently proposed the construction of low-vacuum pipeline transportation systems and space bases, concrete infrastructure will also face a challenge of operating under a low vacuum condition. Compared to normal air condition, low vacuum condition can accelerate the evaporation of moisture. This rapid moisture loss can affect hydration and alter pore structure of matrix, thereby posing a serious threat to its long-term mechanical properties. However, the existing studies generally use relatively high air pressure values(i.e., above 50 kPa) and short exposure time when characterizing early-age cement-based materials. Moreover, there is still a lack of consistent conclusions regarding the time-varying laws of the mechanical properties of mature cement-based materials under long-term low vacuum condition, requiring a further research. This study was to investigate the effects of water-cement ratio, curing age, and exposure time on the mechanical properties, mass loss, phase composition, hydration degree, and pore structure of cement-based materials under a low vacuum condition. A multi-factor coupling prediction model considering water-cement ratio and curing age was proposed. This research could hold a great significance of the evaluation and prediction of the mechanical properties of cement-based materials under a low vacuum condition. Methods Portland cement P·O 42.5 was used. Fine aggregates were medium river sand with a fineness module of 2.96, and coarse aggregates were continuous grading gravels with the diameters of 5–25 mm. In addition, tap water in laboratory was used for blending, and polycarboxylate superplasticizer(SP) was adopted to reduce water. Besides, cement paste, mortar, and concrete specimens were employed at different water-cement ratios(i.e., 0.43, 0.53, and 0.63), respectively. To investigate the effect of low vacuum condition on the concrete performance at different curing ages, cement paste and concrete specimens with a water-cement ratio of 0.53 were divided into three groups, and cured for 3, 28 d, and 60 d, respectively. All the other specimens were cured for 28 d. For the respective curing ages, the specimens in each group were further split into two groups, i.e., one group was placed in a self-designed low vacuum chamber, while another was stored in a constant temperature and humidity chamber(i.e., temperature:(20±2) ℃, relative humidity: 60%±5%, air pressure: 99.85 kPa). For the low vacuum chamber, the temperature was set to(20±2) ℃, and an automated evacuation process was performed every 6 h at 9–10 kPa. The exposure time was 0, 6, 12, and 18 months, respectively. The effect of low vacuum condition on the mechanical properties and moisture loss of mortar and concrete after reaching the corresponding exposure ages was determined via testing the dynamic elastic modulus, compressive strength, flexural strength, and mass loss. Meanwhile, the evolution of phase composition, hydration degree, and pore structure of matrix under long term low vacuum condition were characterized by X-ray diffraction, thermogravimetric analysis, and mercury intrusion porosimetry. Finally, a multi-factor coupling prediction model considering water-cement ratio and curing age was proposed to achieve an accurate prediction of concrete compressive strength under a long-term low vacuum condition. Results and discussion The compressive strength of concrete under low vacuum condition is negatively correlated to water-cement ratio, while positively correlated to curing age. Meanwhile, reducing the water-cement ratio or extending the curing age can improve the compactness of matrix, thereby mitigating the degradation degree of concrete mechanical performance after a long-term low vacuum exposure. Although long-term low-vacuum exposure does not change the type of hydration products, it accelerates water loss, thus inhibiting a further hydration of matrix. In addition, the hydration degree of matrix increases with the increase of water-cement ratio or the extension of curing age, which corresponds to the variation law of the total amount of main hydration products in the matrix. Compared with normal air condition, a long-term low-vacuum exposure leads to an increase in total porosity and most probable pore size. Moreover, the total porosity and most probable pore size of matrix under a low vacuum basically decrease with the reduction of water-cement ratio or the extension of curing age. The compressive strength of concrete under a low vacuum condition conforms to Abrams' Law and maturity theory, with the fitting curve showing a high degree of agreement with the test data(R² = 0.94). In addition, the enhanced database constructed based on this equation shows that the GWO-LSTM model can effectively predict the compressive strength of concrete at different water-cement ratios and curing ages after a low vacuum exposure(i.e., R² > 0.96). Conclusions The strength evolution of cement-based materials could exhibit a nonlinear time-dependent characteristic under a low vacuum condition. A short-term exposure boosts strength, while a long-term exposure causes degradation. Besides, the compressive strength of concrete under a low vacuum condition conformed to Abrams' Law and maturity theory. Moreover, compared with normal air condition, a long-term low vacuum exposure could not alter hydration product types but could accelerate water loss of matrix, inhibiting hydration and increases total porosity and harmful pores(i.e., >100 nm). Meanwhile, under a low vacuum condition, the hydration degree of matrix increases at a higher water-cement ratio or a longer curing age, corresponding to the evolution law of the pore structure. Furthermore, the GWO-LSTM model based on enhanced dataset could accurately predict compressive strength of concrete under a low vacuum condition at different water-binder ratios, curing ages, and exposure time(i.e., R² > 0.96).