Experiment study on the mechanical properties and alkali silica reaction (ASR) of mortar blended rice husk ash (RHA)

To ensure the serviceable popularization of RHA and avoid the alkali silica reaction of mortar blended RHA, the impact of RHA particle size and dosage on the mechanical strength and alkali silica reaction expansion of the mortar prepared with nonreactive silica sand and river sand has been evolutionarily investigated. The hydrates products of the mortar containing RHA were conducted by thermogravimetric (TG) and X-ray diffraction analysis (XRD). The microstructure of the mortar containing RHA was also explored by scanning electron microscope with an energy-dispersive X-ray spectroscopy analysis (SEM-EDX). The experiment results demonstrate that the compressive strength of mortar could be maximum enhanced by 21.1 % when RHA8.8 substituted 20 % of cement due to the pozzolanic effect and physical filling effect of RHA. Moreover, RHA with massive amorphous silica could react with Na ions in pore solution to form alkali-silica gel, thereafter resulting in expanding and destroying the microstructure of the mortar. The real ASR expansion value fabricated by RHA increased as the RHA particle size increased, which was mostly up to 0.0177 % when 20 % cement was replaced by RHA with a mean particle of 27.6 µm. Cracks noticeably emerged on the surface and edge of the mortar containing RHA with the particle size from 8.8 µm to 27.6 µm when RHA substituted 20 % of cement (by mass). Overall, the RHA particle size should be lower than 8.8 µm, while the substitution of cement by RHA also be below 10 % of cement, which can safely avoid the destructive expansion of RHA in an alkali environment.