Abstract
The effect of two precursors (slag and fly ash), different particle size distribution, and three types of aggregate (siliceous sand, limestone, and recycled concrete) on alkali-activated material (AAM) mortar rheology were studied and compared to their effect on an ordinary Portland Cement (OPC) mortar reference. Stress growth and flow curve tests were conducted to determine plastic viscosity and static and dynamic yield stress of the AAM and OPC mortars. In both OPC and AAM mortars, a reduction of the aggregate size induces a rise of the liquid demand to preserve the plastic consistency of the mortar. In general terms, an increase of the particle size of the siliceous aggregates leads to a decrease of the measured rheological parameters. The AAM mortars require higher liquid/solid ratios than OPC mortars to attain plastic consistency. AAM mortars proved to be more sensitive than OPC mortars to changes in aggregate nature. The partial replacement of the siliceous aggregates with up to 20% of recycled concrete aggregates induced no change in mixing liquid uptake, in either AAM or OPC mortars. All the AAM and OPC mortars studied fitted to the Bingham model.
Highlights
Alkali-activated materials (AAMs) are considered eco-efficient and sustainable alternatives to Portland cement (OPC)
Plastic viscosity as well as static and dynamic yield stress increase with the decrease in maximum particle size. This effect is due to the increase of the interparticle friction and the decrease of the packing density of the mortars due to the narrower size distribution of the aggregate with the smaller size (Dmax = 0.5); The AAM mortars, the activated fly ash (AAFA)-10M materials, required higher liquid/solid ratios than ordinary Portland Cement (OPC) mortars to attain plastic consistency, whereas the partial replacement of the siliceous aggregate with up to 20% of recycled aggregate induced no change in in mixing liquid uptake in either AAM or OPC mortars; Aggregate nature affected mortar packing density and voids content slightly more intensely in waterglass-activated slag (AAS) mortars than in NaOH-AAFA and OPC mortars
The most visible effect was found for mortars prepared with limestone aggregate, which, under all the conditions studied, exhibited lower packing density and a higher voids content than siliceous aggregate-containing mortars, due to the uneven shape of limestone aggregate particles
Summary
Alkali-activated materials (AAMs) are considered eco-efficient and sustainable alternatives to Portland cement (OPC). This effect has been observed in mortars [13] and concretes [14], and in both cases, the use of longer mixing times led to a better workability This described phenomenon has not been observed in alkali activated slag pastes with NaOH, KOH, or alkaline carbonates (or even mixtures of all of them). A smaller size of aggregates leads to a higher demands of mixing liquid in the preparation of their mortars and concretes while preserving their rheological properties These effects are well known in OPC systems, but much less information is available for AAM systems. The research described here aimed to determine the effect of the nature of the sand (siliceous, limestone, or recycle aggregates) on the rheological properties of alkali-activated slag (AAS) and alkali-activated fly ash (AAFA) mortars. The impact of the granulometry of the siliceous sand and its total or partial replacement by limestone and recycle aggregates on the AAS and AAFA mortars rheology has been investigated comparing the findings with the results for OPC mortars
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