Synergic effects of the substitution of Portland cement for water treatment plant sludge ash and ground limestone: Technical and economic evaluation

Abstract

The coupled use of calcined clay and ground limestone has been a promising alternative to partially replace Portland cement due to the synergic effects between these two additions. Therefore, substituting calcined clay with a material of similar chemical composition, such as water treatment plant sludge ash (WTPSA), yields several environmental advantages. In this study, the pozzolanic activity of WTPSA was evaluated by determining the strength activity index (SAI) and using the modified Chapelle method. Statistical analyses were used to define the best conditions for WTPSA calcination and grinding. Response Surface Method (RSM) was used to find the Portland cement substitution content by WTPSA and limestone that result in higher concrete compressive strength values. The results of the evaluation of pozzolan activity of WTPSA showed that this material has the potential of being used as pozzolan. The joint analysis of the results confirms that calcination at 700 °C, residence time of 1 h, and grinding time of 1 h were the best conditions for treating WTP sludge. The optimal WTPSA and limestone content found by the Response Surface Method (RSM) were validated experimentally. Ternary mixtures containing WTPSA and limestone resulted in higher compressive strength than binary mixtures and plain cement, which shows the synergistic effect between the two materials. For compressive strength levels between 45 and 60 MPa, the ternary mixture composed of 15% WTPSA +7.5% ground limestone reduced Portland cement consumption from 34.7 to 38.4%.