Microstructure of cement pastes with residual rice husk ash of low amorphous silica content

Residual rice husk ash (RHA) has been frequently suggested as possible mineral addition in cements. However, the characteristics of residual RHAs produced by different manufacturers and the effect of residual RHA with low amorphous silica content in addition in cement pastes are poorly understood. This paper aims at characterizing the different varieties of residual RHA produced in Brazil and at investigating the microstructure of cement pastes with 20% replacement of ordinary cement by RHAs which presents high and low amorphous silica content. A broad array of techniques was employed in the characterization, such as X-ray diffraction (XRD), thermogravimetric analysis (TG), scanning electron microscopy (SEM), X-ray microanalysis, and porosimetry by mercury intrusion (PMI). The results demonstrate that RHA with low amorphous silica content, finely divided, reduces the calcium hydroxide content and the Ca/Si ratio of calcium silicate hydrate (C-S-H), and also refines the porous structure of the pastes. Residual RHAs with low amorphous silica content thus proved to be suitable mineral addition for cements.