Mechanical properties and durability of low-carbon cement containing limestone powder and clay residue under mild alkali activation

The cement industry is a major source of global CO 2 emissions, highlighting the need for sustainable binders with reduced clinker consumption. This study evaluated the setting time, relative slump, compressive strength, retained compressive strength after exposure to 5% MgSO 4 and 5% H 2 SO 4 solutions, weight variation, and environmental impact of low-carbon cement (LCC) pastes produced by replacing 50 wt% of commercial hydraulic cement (CHC) with clay residue (CR) and/or limestone powder (LP). The performance of mixtures activated with a mild alkaline solution containing 0.5 M sodium hydroxide (SH) was compared with that of tap water (TW)-activated systems. Test results indicated that the incorporation of CR and LP generally prolonged setting times while enhancing workability, particularly under alkaline activation. Although both CR and LP replacement reduced compressive strength because of the dilution effect, all mixtures showed continuous strength development up to 28 days. LP-containing mixtures consistently achieved higher strengths than CR-rich systems, with 50CHC50LP (50 wt% LP) demonstrating the best mechanical performance among the blended mixtures. CR-rich LCC mixtures exhibited lower weight loss and less surface deterioration under H₂SO₄ exposure, whereas LP-rich mixtures were more susceptible to H₂SO₄ attack due to their higher calcium content. Replacing 50 wt% of CHC with CR and/or LP reduced embodied CO 2 emissions by approximately 50%. Overall, replacing 50 wt% of CHC with LP provided the most favorable balance between mechanical performance and carbon reduction, whereas CR-rich mixtures were more suitable for acidic environments.