Degradation process of alkali-activated slag/fly ash and Portland cement-based pastes exposed to phosphoric acid

High costs are currently involved in the repair and replacement of sewage concrete pipes due to the severe damage caused by the acid-rich aggressive environments of sewage and drainage wastewater. Although it is known that, a high concentration of phosphoric acid exists in some sewer pipes, very few studies have been focused on its impact on various cementitious materials. The purpose of this study is to experimentally investigate the resistance of alkali-activated slag/fly ash-based pastes and compare them to ordinary Portland cement-based pastes exposed to phosphoric acid with different pH values (pH = 2, 3 and 4) for a period of 150 days. The experimental results revealed that alkali-activated slag/fly ash-based pastes are more resistant to phosphoric acid attacks compared to ordinary Portland cement-based peers. Furthermore, it is also observed that the kinetics of degradation of the pastes are vastly different, depending on their binders: the degradation process of ordinary Portland cement-based pastes can be described by using Power law, while the degradation process of alkali-activated slag/fly ash-based pastes is divided into an early stage and a later stage which can be described by using the Hill function and the Power law, respectively. The theoretical model developed in this study predicts that, under strong phosphoric acid condition for a period of 50 years, alkali-activated slag/fly ash-based pastes could potentially reduce the degradation depth by around 70%–80% in comparison to ordinary Portland cement-based pastes.