Effect of Slag Reactivity Influenced by Alumina Content on Hydration of Composite Cements

Abstract

This work investigates the role of alumina content in the slag and presence of limestone in composite cement on the hydration and compressive strength evolution of composite cement. A coupled experimental and modeling approach was applied.

Increasing Al₂O₃ content of slag from 8 % to 12 % has pronounced impact on slag kinetics while further increase from 12 % to 16 % has a limited effect. However, this increase affects the hydrates assemblage across the whole compositional range investigated; the composition of the C-S-H phase and hydrotalcite is modified. Additionally the AFm and AFt phases content changes. Presence of limestone has a pronounced impact on the hydration, being more pronounced for the slags of high alumina content. This is related to the formation of hemi- and mono-carbonate and stabilization of ettringite. Despite of these changes, the pore size distribution is similar among the investigated cements. Thermodynamic modelling matches favorably the experimental data and enables calculating the evolution of the pore volume over the time. A relationship between calculated porosity and compressive strength is proposed and verified. This relationship shows that all hydrated phases have a similar impact on compressive strength and the strength is mainly related to the pore volume.