PHYSICO-CHEMICAL AND MECHANICAL PROPERTIES OF BLENDED CEMENT PASTES CONTAINING RICE HUSK ASH AND METAKAOLIN

metakaolin (MK) on the physico-chemical and mechanical properties of the hardened OPC-RHA-MK blended cement pastes. OPC was partially replaced by different ratios of MK (10, 15 and 20%) and a constant ratio of RHA (5%) and the resulted cement blends were hydrated in the paste form by using the water/cement ratios required for the standard water of consistency; the pastes thus obtained, were hydrated for 1, 3, 7, 28, and 90 days. At the end of hydration period, the cement pastes were tested for compressive strength, total porosity and hydration kinetic via determination of free lime contents.The phase composition of the formed hydration products was investigated using X-ray diffraction (XRD) and differential thermal analysis (DTA) techniques. It was found that, the substitution of ordinary Portland cement (OPC) by rice husk ash (5% RHA) enhances the physico-chemical and mechanical properties of the hardened blended cement pastes as compared with the neat OPC. The results of compressive strength indicated slightly higher values for the pastes made of OPC–RHA-MK blends containing 5% RHA blended with 10, 15 % MK. However, the blended cement paste derived from OPC-RHA-MK blend containing 5% RHA blended with 20% MK, for economic reasons, was taken as the most suitable mix containing both RHA and MK. The partial substitution of OPC by RHA and MK leads to higher porosity values with a consequent decrease in the compressive strength values especially during the early ages of hydration. It was found that, the increase of MK content in OPC-RHA-MK blended cement pastes resulted in an increase in water consistency and setting times. Lower values of free lime contents were obtained for OPC-RHA-MK blended cement pastes, with the formation of further additional amounts of CSH, as a result of the pozzolanic reaction.