Mechanisms of alkali-activation of limestone: Reaction kinetics and influence of drying parameters

Abstract

This paper focuses on the study of the reaction between limestone and sodium hydroxide solution. This reaction is used to produce alkali-activated mortars after curing and in the presence of unreactive siliceous sand. The kinetics of this reaction is investigated by semi-adiabatic calorimetry experiments. The formulation parameters are fixed but the curing parameters are tested: temperature (38 and 45°C) and time of drying (14, 25, 27 and 28 days). Their impact on reaction products (identified by FTIR and thermal analysis), on microstructure (SEM observations and porosity measurements) and on mechanical strength (flexural and compressive tests) are determined. The reaction was fast and a temperature drying was required to obtain stabilized hydration products in mortars. The products of the alkali-activation of limestone are double sodium/calcium carbonate (mainly pirssonite and with specific conditions of curing gaylussite) and calcium hydroxide. The microstructure and the proportions of reaction products, impacted by drying conditions, influence the mechanical strength of mortars. With an appropriate time (25 days) and temperature of drying (45°C), the compressive strength reached, after 28 days, almost 25 MPa.