Utilization of andesite processing wastewater treatment sludge as admixture in concrete mix

The dried physicochemical sludge from the coagulation–flocculation treatment of an andesite processing plant wastewater was used as concrete admixture in powder form. Various physicochemical sludges obtained with various coagulant/flocculants (alum, FeCl3, and sepiolite) were experienced dry particulate (powder) form of the sludge was replaced at 0.5–1.5% w/w of the cement for 250 kg/m3 cement dosage. The admixture doses were complied with local standard. Slump tests were conducted on fresh concrete, and compressive strengths were measured for 7 and 28 days hardened concrete samples. Physical properties, namely freeze/thaw resistance, water absorption, capillary suction, void ratio and porosity were determined at the end of 28 days for the hardened concrete samples. The concrete consistency (slump) was improved up to 16–18 cm from 14.5 cm by the admixture. The best compressive strength was achieved with 0.5% w/w for admixtures. The admixture sludges obtained with organic flocculants were worse than the other admixtures. The structural changes after freeze/thaw tests were very close to the reference concrete (concrete without admixture); only the capilary suction had increased. The results confirmed that up to 0.5% of physicochemical sludge can be replaced with cement in concrete for use in certain non-load bearing structures that require medium strength concrete.