Influence of gradation of aggregates using particle packing methods on strength and microstructure of blended cement mortars

Abstract

Blended cements having partial replacement of fly ash (FA) and granulated blast furnace slag (GBFS) with Ordinary Portland Cement (OPC) have proven to be more sustainable cements with several ecological benefits, increased performance and durability aspects compared to plain OPC. In this study, three blended cements, namely Portland Pozzolana Cement (OPC + FA), Portland Slag Cement (OPC + GBFS) and Composite Cement (OPC + FA + GBFS) are examined. Three Particle Packing Models (PPM) viz. Modified Toufar Model (MTM), J D Dewar model (JDD) and Compressible Packing Model (CPM) are considered to evaluate the gradation of fine aggregates used in designing the mortars and are compared with the mortars designed as per IS 650:2008. The theoretical maximum packing density values obtained based on MTM, JDD and CPM models are 0.681, 0.674 and 0.668 respectively, while the experimental packing density values are 0.617, 0.626 and 0.642 respectively. The compressive strength of Composite Cement (CC), Portland Slag Cement (PSC) and Portland Pozzolana Cement (PPC) mortars exhibited a delay in gain of strength compared to OPC mortars. The mortar samples prepared using CPM model, showed an average increase in compressive strength of 20.71% at 7 days, 12.26% at 28 days and 10.29% at 90 days compared to samples designed using IS 650:2008. Similar trend was observed from MTM and JDD mortar samples also with strength of mortars varying as CPM > MTM > JDD > IS 650. This shows that particle packing models are useful in overcoming the delay in early strength of blended cements due to slow pozzolanic reaction. The hydrated phases of cement and microstructure is examined using Scanning Electron Microscopy and X-Ray Diffraction.