Pozzolanic activity of rice husk and its application in the production of lightweight mortar

Abstract

The properties of lightweight mortar using rice husk (RH) can benefit from pozzolanic reaction and from organic fibre reinforcement. The use of RH as a partial replacement to cement is economical, besides creating a better environment. The study investigates the pozzolanic activity of rice husk and its effect on the strength and physicochemical properties of lightweight mortar (LWM) containing RH. The pozzolanic activity of RH was determined via conductivity measurement. In this study, RH was used as a cement replacement at 10% (w/w) and 20% (w/w). The strength development of LWM was monitored at 3, 7, 14, 28, 56 and 90 curing age. Physicochemical composition of RH and LWM were studied by field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray (EDX), X-ray diffraction (XRD), single point BET and thermogravimetry analysis (TGA). The studies showed that RH contains 96% silica (SiO2). XRD analysis confirmed the presence of amorphous silica. Conductivity studies indicated that the RH exhibit pozzolanic activity as reflected by a decrease in conductivity. The decrease is attributed to the interaction between Ca(OH)2 and the SiO2 in RH. The amorphous silica contained in RH can react with Ca(OH)2 to form a type of C-S-H gel. FESEM analysis showed a smooth internal surface and irregular morphology at external surface of the RH. The morphology of the LWM samples showed the development of portlandite, C-S-H gel and enttrigite indicating the occurence of cement hydration. EDX microanalysis gave higher silica content at the external surface than the internal surface which may promote a pozzolanic action. XRD and TGA showed that Ca(OH)2 increased with time for LWM without RH, indicating cement hydration. The strength development of the LWM showed increasing strength at 3, 7, 14 and 28 days, but the strength of conventional LWM is almost constant after 28 days. Eventhough the mortar containing RH showed lower strength than the conventional LWM, but it showed an increasing strength after 28 days due to cement hydration, pozzolanic reaction and possibly fibre reinforcement.