Performance of Low-Carbon Mortar Prepared with CO₂-Carbonated Carbide Slag as a Partial Cement Substitute

Abstract

To explore the feasibility of using CO₂-carbonated carbide slag in low-carbon cementitious materials, this study investigated the basic properties of raw carbide slag and CO₂-carbonated carbide slag, and further used CO₂-carbonated carbide slag as a partial cement substitute to prepare low-carbon mortar. Five replacement ratios of CO₂-carbonated carbide slag, namely 0%, 5%, 10%, 15% and 20%, were designed to evaluate their effects on mortar flowability, compressive strength, water absorption, bulk density, leaching pH and Ca²⁺ release behavior. The results showed that CO₂ carbonation increased the CaCO₃ content of carbide slag from 4.0 wt.% to 41.6 wt.%, decreased the Ca(OH)₂ content from 92.0 wt.% to 48.2 wt.%, and reduced the pH from 12.60 to 10.55, indicating that carbonation can reduce alkalinity and form carbonate-rich products. When the replacement ratios of CO₂-carbonated carbide slag were 5% and 10%, the 28-day compressive strengths of mortar were 50.2 MPa and 49.6 MPa, respectively, slightly higher than that of the control group (48.5 MPa). When the replacement ratio increased to 20%, the 28-day compressive strength decreased to 41.3 MPa. The water absorption and bulk density results indicated that 5%-10% replacement was beneficial to improving mortar compactness, whereas excessive replacement weakened the dense structure. Leaching tests showed that the leachate pH and Ca²⁺ concentration slightly decreased with increasing replacement ratio, suggesting that CO₂-carbonated carbide slag did not increase the risk of alkaline release. Overall, CO₂-carbonated carbide slag can be used as a partial cement substitute in low-carbon mortar, and a replacement ratio of 5%-10% is considered suitable.