Effects of sodium oxide content on the durability of alkali-activated mortar utilizing botswana copper mine tailings and fly ash

¹ Botswana International University of Science and Technology, Department of Civil and Environmental Engineering, Botswana
² University of the Witwatersrand Johannesburg, School of Civil and Environmental Engineering, Johannesburg, South Africa

^* Corresponding author: abraham.sannoh@studentmail.biust.ac.bw

Abstract

This investigation aims to provide experimental data on the performance of alkali-activated mortar made of copper mine tailings (CMT) and fly ash (FA) exposed to acid-sulphate attacks and elevated temperature environments as a measure of durability. FA was used as a replacement material, substituting 20 to 40% of the CMT by mass, and sodium hydroxide was the alkaline activator, which was added in terms of Na₂O content by mass of the total binder at 5%, 10%, and 15%. The durability performance of alkali-activated mortar was evaluated against 5 % and 10% concentrations of sulphuric and hydrochloric acids, and magnesium and sodium sulphates up to 180 days of exposure, as well as elevated temperature environment. The specimens were first visually examined, and weight change was measured before being exposed to an elevated temperature environment and the residual compressive strength was measured. It was observed that Na₂O content and elevated temperature environment influence the residual compressive strength of alkali-activated mortar. Increasing the Na₂O content for all CMT-FA-based mortar samples performed well in sulphates and acids mediums in terms of durability, but with a slight reduction in its durability performance in terms of residual compressive strength. However, these effects were more profound in samples exposed to acids, particularly those with lower FA replacement levels and sodium oxide content. For the high-temperature exposure, the residual compressive strength of all CMT-FA samples was much higher than the initial values. The findings also revealed that the partial replacement of CMT by FA significantly improved the residual compressive strength in terms of the durability performance of the alkali-activated mortar