Evolution of tricalcium silicate (C3S) hydration based on image analysis of microstructural observations obtained via Field's metal intrusion

The morphological organisation of cementitious materials greatly affects their mechanical and engineering properties, as well as overall durability. However, detailed image-based characterisation and analysis are challenging due to the complex and multi-scale morphological characteristic of this class of materials. In this study, C3S served as a simplified model of cementitious material. Samples with different hydration times were investigated using Field's metal intrusion to obtain back-scattered electron (BSE) images for detailed microstructural analysis. The degree of hydration was estimated and a metastable barrier surrounding a pack of unhydrated particles was found in the BSE images of C3S microstructure. The length of this barrier correlated well with the hydration rate of C3S. The hydration process was further found to induce a notable change in the pore size and pore profiles of C3S pastes. Generally, the critical pore size limiting C3S hydration was found to be 4 mu m, and the hydration process caused both a reduction in the number of large pores and the formation of small pores. The C3S samples were found to reach 80% of hydration at 7 days of hydration and the pore structure developed continuously after 7 days even though the porosity and degree of hydration stayed similar.