Hydration behaviour and properties of binary blended rice husk ash cement paste

Abstract

For the hydration process of Rice Husk Ash (RHA) – cement blended mixtures to be fully explored and understood, the degrees of Cement, RHA-Cement reactions, non-evaporated water and free-lime contents as well as fresh properties and compressive strengths were determined at varying ages. Mixes of PC and RHA-PC blended cement pastes at replacement levels of 0, 5, 10, 15, 20, 25, 30, 35, 40 45 and 50 % respectively, by mass of PC labelled as PC, 5RHA-PC, 10RHA-PC, 15RHA-PC, 20RHA-PC, 25RHA-PC, 30HA-PC, 35RHA-PC, 40RHA-PC, 45RHA-PC and 50RHA-PC at a constant W/b ratio of 0.50 were used for this study. From the aforementioned mixes, fresh properties such as water of consistency and setting times were determined. Mortar cubes (50 x 50 x 50 mm) were cast and cured at 7, 14, 21, 28, 56 and 90 days and compressive strength determined at each aforementioned test age, then, there was stop hydration of the crushed specimens by immersion in a mixture of methanol to acetone (1:1) for one hour, filtered and dried for two hours. The sample was crushed and sieved through a 75μm sieve. Non- evaporable water content was determined by the loss on ignition method as well as free- lime content by the glycerol/ethanol method for each mix at same curing ages. PC and PC – RHA mixes were compared. Test results showed that water of consistency and setting times increased gradually with the addition of RHA as it replaced PC. Furthermore, compressive strength increased steadily up to 28 days and then gradually up to 90 days for the PC, but for the blended (RHA-PC) mixes, equating and surpassing control mix values at ages after 28 days for pastes with RHA less than 30 % replacement of PC. This is attributed to pozzolanic reactions with Ca (OH)2 liberated from cement hydration at later ages in addition to the cement hydration at early ages. For pastes containing RHA more than 30 % of PC, compressive strength was less than control values attributed to dilution effect. The trend in behavior for the non- evaporable water content is same with compressive strength but the free – lime content steadily increased up to 28 days and gradually with increase in RHA content up to 90 days for the PC paste, but for RHA-PC Pastes, they increased up to 56 days and then decreased up to 90 days. These results could enhance modelling of the behavior of RHA-PC blended cement paste.