Project Details
Projekt
Micromechanical modeling of drying shrinkage of cement based on pore size distribution and on capillary forces
Subject Area
Construction Material Sciences, Chemistry, Building Physics
Term
from 2014 to 2021
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 262927646
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.
DFG Programme
Research Grants
