Micromechanical modeling of drying shrinkage of cement based on pore size distribution and on capillary forces

由于相对湿度的变化，混凝土的干燥、收缩和膨胀对于混凝土构件的耐久性、回弹性和结构设计是非常重要的。即使在低相对湿度下，液体水也已经存在于微孔中并且作为C-S-H相中的层间水存在。在几种不同组成的硬化水泥浆体中，经常观察到低湿度下固体结构的水蒸气吸附和体积变化的明显滞后。通过比较普通波特兰水泥和铝酸钙水泥的不同处理和制备的硬化水泥浆体的行为，我们能够确认在低湿度下的滞后，这与微孔的存在有关。我们开发了一个理论模型，可以解释水蒸气吸附滞后和它的连接的基础上在微孔和层间水的吸水膨胀和收缩滞后。该模型能够再现重要的拓扑特征的吸附等温线的微孔的量，以及不同类型的等温线的水蒸气和氮吸附低水蒸气压。实验吸附图的建模和解释，以及在整个湿度范围内的硬化水泥浆体的膨胀和收缩需要考虑到微观和介孔，以及改进的建模的化学细节的狭缝孔壁在不同的水泥浆体通过计算机模拟。在该项目的延续，理论基础的一个部分开发的模型计算吸附，膨胀，收缩和滞后将完成列入相应的滞后效应的基础上测得的孔径分布。将在不同的养护条件下，用高炉矿渣水泥生产具有几种水灰比的其他类型的水泥浆体。它们的水和氮吸附性能和相关的溶胀/收缩将作为相对湿度的函数进行测量。相应的孔径分布将通过高压压汞孔隙率测定法和SAXS测定。直接的计算机模拟的水蒸气吸附在现实的层结构的改进水泥模型的计划。特别是二价离子如钙离子对水泥浆体中毛细力的影响进行了分析。在这里，要确定冷凝物和多孔壁之间的力（壁应变）以及封闭孔壁的粘附能。模拟结果为水蒸气吸附模型和水泥浆体积变化模型提供了额外的一致性测试。

Drying, shrinkage, and swelling of concrete due to variations of the relative humidity are very important for durability, resilience, and the structural design of concrete members. Even at a low relative humidity, liquid water is already present in micropores and as interlayer water within the C-S-H phases. In several types of hardened cement pastes with different compositions, pronounced hysteresis of water vapor sorption and volume change of the solid structure at low humidity is frequently observed. By comparing the behavior of differently treated and prepared hardened cement pastes from Ordinary Portland- and Calcium Alumina Cement, we were able to confirm a hysteresis at low humidity which is linked to the presence of micropores. We developed a theoretical model that can explain water-vapor sorption hysteresis and its connection to hysteresis of swelling and shrinkage based on water sorption in micropores and interlayer water. The model is capable of reproducing important topological features of sorption isotherms in relation to the amount of micropores as well as the different types of isotherms of water vapor and nitrogen sorption for low water vapor pressures. Modelling and interpretation of experimental sorption diagrams as well as the swelling and shrinkage of hardened cement paste in the whole humidity range needs to take into account both micro- and mesopores as well as an improved modeling of chemical details of slit-pore walls in different cement pastes by computer simulations. In the project continuation, the theoretical foundations of a partially developed model for calculating sorption, swelling, shrinkage, and hysteresis are to be completed by inclusion of the corresponding hysteresis effects based on measured pore size distributions. Additional types of cement pastes from Blast-furnace slag cement with several water to cement ratios will be produced at different curing conditions. Their water- and nitrogen sorption properties and the associated swelling/shrinkage will be measured as a function of relative humidity. The corresponding pore size distribution will be determined by high-pressure mercury intrusion porosimetry and SAXS. Direct computer simulations of water vapor sorption on realistic layer structures of improved cement models are planned. In particular, the effect of divalent ions such as calcium ions on the capillary forces in cement pastes are to be analyzed. Here, the forces (wall strain) between the condensate and the micropore walls as well as the adhesion energy for the walls of closed pores are to be determined. The simulation results provide additional consistency tests for the models of water vapor sorption and the resulting volume changes in cement paste.