A numerical model for the densification process of C-S-H in microstructural development of Portland cement paste

The main hydration product of cement, calcium silicate hydrate (C-S-H), is characterised by a high specific surface area (SSA) and a complex multi-scale pore structure, which leads to difficulties and controversies regarding the quantitative evaluation of the microstructural properties of C-S-H. This study addresses some controversies by considering the combined effect of densification and volume expansion of C-S-H. Based on this concept, a constitutive law is proposed and incorporated into a numerical model. The model predicted well the various microstructural properties of cement paste (e.g. time-dependent bulk density of C-S-H, nitrogen-accessible surface area, capillary pore volume and quantities of various hydrates). Nitrogen adsorption tests (NATs) were performed to validate the numerical results. The SSA was found to have no clear relationship with the total pore volume measured by the NAT. However, the SSA showed an almost perfect linear relationship with porosity of finer pores (less than ∼8 nm). Therefore, it was concluded that only this portion of finer pores detected by nitrogen are ‘gel pores’ within low-density C-S-H, which truly reflects its growth. This redefinition and the findings of this work provide better understanding of the microstructural development of cementitious materials and more appropriate interpretation on the widely used NAT.