Rheology, early-age hydration and microstructure of alkali-activated GGBFS-Fly ash-limestone mixtures

In this study, the effects of limestone powder on the rheological behavior, pore solution chemistry, mechanical properties and microstructure of alkali-activated cements have been investigated. The results exhibit that, with the increasing content of limestone powder in the ternary alkali-activated system, the structural build-up of the mixture increases earlier. It was observed that flow curves of pastes fit the Bingham model well. With the increasing content of fly ash in the ternary mixtures, the plastic viscosity decreased as expected by the particle packing effect and the increased water film thickness as well as the spherical shape of fly ash particles. As a result of the higher specific surface and improved nucleation provided by the limestone powder, the reaction process was enhanced and accelerated for the mixtures with higher limestone powder contents. The calcium and alumina concentrations in the pore solution rapidly evolved at first for a certain time, but decreased afterwards. The significant influence of the Ms value of the activator was observed on the evolution of the elemental concentrations. Microstructure analysis revealed that the early age reaction product is C-A-S-H for the slag mixtures incorporating limestone or fly ash. The compressive strength of the ternary mixtures decreased with the incorporation of limestone powder due to the inert character of the limestone powder.