Influence of sludge from residual concrete recycling systems on fresh and hardened cement paste

Abstract

Samples of two different compositions were prepared. Cement mixtures were made from cement, superplasticizer, liquid sludge, dried concrete sludge and clean water. In the compositions with dried concrete sludge, the cement was replaced with different amounts of dried concrete sludge added at 0%, 5%, 10%, 15%, 20%, 25%, 30%. In the compositions with liquid sludge, the cement content was kept constant, but clean water was partially replaced with liquid sludge, thus reducing clean water content by 0%, 10%, 20%, 30%, 40%, 50%, and 60%. The slump and setting time of the cement paste with different dried concrete sludge and liquid sludge content were tested. Density, compressive and flexural strengths and water absorption of hardened cement paste were determined. It was found that both technological and mechanical properties of the samples deteriorate with lower cement content in the samples that contain a lover level of dried concrete sludge; however, the replacement of cement with dried concrete sludge up to 5% does not have a very significant effect on the final performance of hardened cement paste. In the mixtures with liquid sludge, an increase in strength and density was observed.