Density and water content of nanoscale solid C–S–H formed in alkali-activated slag (AAS) paste and implications for chemical shrinkage

Alkali-activated slag (AAS) paste was analyzed using small-angle neutron scattering (SANS). The scattering response indicates that the microstructure consists of a uniform matrix of hydration product with a high surface area studded with unhydrated cores of slag particles. In contrast with portland cement paste, no surface fractal scattering regime was detected, and elevated temperature curing (at 60 °C) had no detectable effect on the microstructure at any length scale studied. The specific surface area of the AAS pastes is about 25% higher than that of a portland cement paste cured under the same conditions. The composition and mass density of the nanoscale solid C–S–H phase formed in the AAS paste was determined using a previously developed neutron scattering method, in conjunction with a hydration model. The result ((CaO)0.99–SiO2–(Al2O3)0.06–(H2O)0.97, d = (2.73 ± 0.02) g/cm3) is significantly lower in calcium and in water as compared to portland cement or pure tricalcium silicate paste. These values were used to calculate the chemical shrinkage that would result from complete hydration of the AAS paste. The result, (12.2 ± 1.5) cm3 of volumetric shrinkage per 100 g of unhydrated cement, is about twice the amount of chemical shrinkage exhibited by normal cement pastes.