Thermochemical Pretreatments for Agricultural Residue Ash Production for Concrete

Abstract

Agricultural residue ash is known to be a very reactive source of supplementary cementitious material (SCM) for use in concrete. The influence of thermochemical pretreatments on the reactivity of agricultural residue ash (ARA) was studied for its use as an SCM. It was shown that pretreatments are effective in the partial removal of alkali metals and other impurities out of both wheat straw and rice straw, leading to ARA with lower loss on ignition (LOI), higher internal surface area, and higher amorphous silica content than unpretreated ARA. It was shown that the alkali content correlated with the LOI and amorphous silica content of the ash. When used at a cement replacement rate of 20% by mass, pretreated ARA accelerated the hydration of cement paste samples, whereas unpretreated ARA retarded the cement hydration. Pretreatments were found to increase ARA reactivity, as measured by calcium hydroxide content reduction with time. ARA increased the compressive strength of mortar samples by 25% when used as 20% replacement of cement in the samples. The calcium hydroxide content of paste samples and mortar compressive strength was found to correlate to the amorphous silica content of the ash.