Hybrid alkali activated cements (HAACs) system: A state-of-the-art review on fresh, mechanical, and durability behaviour

Hybrid alkali activated cements (HAACs) are getting worldwide attention as a potential alternative binder in construction industry. The HAACs are composed of blending low proportions of Portland cement and high proportions of supplementary cementitious materials with an alkali activator. This paper reviews fresh, mechanical, and durability properties in terms of carbonation, sulfate attack, alkali silica reaction (ASR), and chloride penetration of HAACs. The different factors affecting fresh and mechanical properties of HAACs were comprehensively discussed. The workability and setting time of HAACs are lesser due to rapid chemical reaction of OPC in presence of alkali activators. Further, paste, mortar and concrete prepared with HAACs show higher early age strength gain than that prepared with OPC or alkali activated materials (AAMs) due to coexistence of C-S-H gel along with geopolymer gels i.e., N-A-S-H and C-A-S-H in the system. As reported in limited research studies, HAACs are more susceptible to carbonation than OPC even though HAACs have denser microstructure as compared to OPC system. On the other hand, HAACs have shown higher resistance to sulfate attack, ASR, and chloride penetration than OPC. However, intermediate calcium and alkali content of HAACs as compared to OPC and AAMs need further study on ASR resistance. Further, lack of long-term durability studies on HAACs acts as barrier for its suitability as an alternative binder for practical applications. Based on literature review, this paper identifies the research gap to carry out further investigation on development of sustainable HAACs.