Estimation of C–S-H and calcium hydroxide for cement pastes containing slag and silica fume

The estimation of Calcium–Silicate–Hydrate (C–S–H) in pure Portland cement paste is difficult due to the uncertainty in obtaining the reasonably accurate Ca/Si ratio. The replacement of pure Portland cement with mineral admixtures further complicates the estimation process due to the formation of a secondary C–S–H with different Ca/Si ratios. This study considered a novel approach of using combined microanalysis by Energy Dispersive X-ray Spectroscopy (EDS) and nonlinear optimization to accurately define the Ca/Si ratio for Portland cement pastes containing slag and silica fume at four different ages. Based on the Ca/Si ratios, the volumes of primary and secondary C–S–H, and calcium hydroxide were estimated. The volume fractions of the primary C–S–H predicted by the optimization method were found to be in close agreement with the experimentally obtained values from EDS. The estimated mass fractions of calcium hydroxide from EDS and those obtained by thermo gravimetric analysis (TGA) also agreed well with R2 of 0.97.