Abstract
The present work aims to introduce a novel and eco-friendly method, i.e., a water-leaching pretreatment for extracting highly reactive biomass silica from rice husk (RH), for viability as a pozzolanic additive in cement. For comparison, the traditional acid pretreatment method was also employed throughout the experimental study. The silica from RH was extracted using boiled deionized water and acid solution as leaching agents to remove the alkali metal impurities, and then dried and submitted to pyrolysis treatment. The results indicated that potassium was found to be the major contaminant metal inducing the formation of undesirable black carbon particles and the decrease in crystallization temperature of amorphous RHA silica. The boiling-water-leaching pretreatment and acid-leaching pretreatment on RHs significantly removed the metallic impurities and reduced the crystallization sensitivity of RHA silica to calcination temperature. A highly reactive amorphous silica with purity of 96% was obtained from RH via 1 N hydrochloric acid leaching followed by controlled calcination at 600 °C for 2 h. The acid treatments increased the crystallization temperature of silica to 1200 °C and retained the amorphous state of silica for 2.5 h. In the case of water-leaching pretreatment, leaching duration for 2.5 h could yield an amorphous silica with purity of 94% and render the silica amorphous at 900 °C for 7 h. The RHA silica yielded by water-leaching pretreatment presented a comparable enhancing effect to that of acid leaching on hydration and improved the strength of cement. Furthermore, compared with the acid-leaching method, the water-leaching pretreatment method is more environmentally friendly and easier to operate, and hence more widely available.
Highlights
Rice husk (RH) is the outer shell of the rice grain, which is a by-product of the rice milling process.It is an agricultural waste in all rice-producing countries
In view of the fact that the biomass metallic salts are mostly soluble, this study explores using boiling water as a leaching argent for rinsing off alkali metal impurities in rice husk ash (RHA) silica and obtaining a purified and highly reactive RHA silica
Chemical compositions and physical properties of RHAs without and with pretreatment are summarized in Tables 1 and 2, respectively
Summary
Rice husk (RH) is the outer shell of the rice grain, which is a by-product of the rice milling process. It is an agricultural waste in all rice-producing countries. The major components of RH are organic materials such as hemicellulose, cellulose, and lignin, totaling about 85%, and the remaining ash content is 15–20% [4]. Of all the residues of edible plants, the ash obtained from the calcined RH has the highest silica content [5]. RH becomes rice husk ash (RHA), which normally contains 85–95% of silica (SiO2 ), 5–8% of alkali metal oxides, and some carbonaceous materials by mass [6]
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