Early-age cracking tendency of Alkali-activated slag binders compared to Ordinary Portland Cement
Résumé
Ground granulated blast furnace slag (GGBFS) is a by-product of steel manufacturing, increasingly used as an alkali-activated hydraulic binder. The substitution of Ordinary Portland Cement (OPC) based binders by this type of materials represents essentially an ecological advantage due to the reduction in CO2 emissions. Additionally, these materials constitute relevant alternative of OPC binders in some specific industrial applications [1] [2]. Hydration advancement and microstructural properties' evolution of alkali-activated slag materials have been extensively studied in the literature. However, a large dependency both on the chemical and physical properties of the activated GGBFS, and the type and concentration of the alkaline activator used, can be observed. Additionally, few studies have addressed the evolution of the mechanical properties of these binders and the early-age creep and shrinkage characteristics. Thus cracking tendency of this type of materials, especially at early ages when the volumetric changes of the hydraulic binders are the most important, needs to be investigated in order to study the durability of any structural application. In this context, the current paper describes, first, an experimental campaign comparing the early-age behavior of an alkali-activated slag mortar to that of an OPC mortar, then thermo-chemo-mechanical simulations allowing comparing the cracking tendency of both materials.The developed experimental campaign covers the delayed strains (autogenous shrinkage and creep) and the mechanical properties (mechanical strengths and Young's modulus) evolutions of the mortars. Results show that the alkali-activated mortar undergoes autogenous shrinkage strains higher then OPC mortar and showing an increase even at long term. However, its basic creep strains are more important than OPC mortar ones tested in the same conditions. This implies a higher capacity of stress relaxation for the alkali-activated slag mortar. Regarding the evolution of mechanical properties, Young modulus, compressive and tensile strengths of the alkali-activated mortar are lower of those of OPC based mortar at all ages. A simplified cracking index comparison applied at this stage of study shows comparable cracking risks of both materials at 7 and 28 days.The performed numerical simulations are performed by the means of a thermo-chemo-mechanical model developed based on the experimental study. The equivalent time approach model is adopted in order to describe the early-age evolution of the binders. Basic creep strains are expressed using a visco-elastic model combining a Kelvin-Voigt chain and dashpot [3]. This model is revisited to add a time-equivalent evolution for the ageing parameters. Finally, finite element calculations are performed on a bar fixed at both ends in order to compare the cracking tendency of the studied mortars.