Properties of ‘green’ concrete engineered from industrial waste materials: paper mill boiler ash & waste foundry sand.

Abstract

The environmental impacts of cement manufacturing, coupled with the largescale consumption of natural resources, have placed concrete production techniques under scrutiny. In response, supplementary materials for green concrete production are being investigated. The South African pulp and paper industry generates large quantities of waste paper mill boiler ash (PMBA), whilst foundries are inundated with waste foundry sand (WFS) from metals production. Despite their high generation rates and the national landfill crisis, landfilling is the dominant disposal method. However, due to their inherent qualities and respective production processes, these materials may have potential for meaningful reuse in concrete. This study investigated the properties of PMBA-integrated concrete and WFS-integrated concrete. PMBA partially replaced cement whilst WFS partially replaced fine aggregate. Replacements occurred in 5, 10, 15, and 20 percent by mass. Conventional concrete served as the control sample. Previous work of a similar nature left substantial gaps as testing was predominantly limited to mechanical strengths and occurred in international contexts. This study employed local waste materials to assess concrete workability, density, durability, mechanical strengths, and batch leaching tests to determine the pH value, ion conductivity and nitrates content in filtered concrete leachate. The following conclusions were drawn: A 10% replacement of cement, with PMBA, was identified as the overall optimum concrete mix as it achieved the highest density, the highest compressive and flexural strengths, a ‘medium’ degree of workability, a ‘good’ degree of durability in terms of oxygen permeability, ‘excellent’ degrees of durability against water sorptivity and chloride conductivity and a sufficient pH value to preserve the concrete passivation layer for steel protection. Out of all WFS samples, the 5%WFS mix was found to be superior as it displayed a ‘high’ degree of workability, a suitable pH for protecting steel, ‘excellent’ degrees of durability against oxygen permeability, water sorptivity and chloride conductivity, and when compared to the control, exhibited a higher density and a lower compressive, flexural and tensile-splitting strengths. In terms of leachate quality, ion conductivity assessments indicated that conventional concrete leachate exhibited a higher ion conductivity and leached more ionic species than all ‘green’ concrete samples. The leachates of all samples, except the 7-day WFS sample, exhibited nitrate contents which were low enough to meet the requirement for drinking water and the maximum contaminant level. Viability assessments, based on overall concrete performance, indicated all PMBA samples performed better than all other samples whilst the only 5%WFS sample performed better than the control sample.