Performance evaluation of limestone-calcined clay (LC2) combination as a cement substitute in concrete systems subjected to short-term heat curing

Abstract This paper evaluates the suitability of limestone-calcined clay (LC2) combination as a cement substitute material in concrete systems subjected to steam curing. Concrete with blends of portland cement and LC2 were subjected to short-term heat curing and compared against concrete with plain portland cement and fly ash-based binder (at 30% replacement) for two different concrete systems (a) 400 kg/m3 binder content and 0.40 w/b, and (b) 360 kg/m3 binder content and 0.45 w/b. Several parameters were considered to evaluate the potential of LC2 in such applications, which include - improvements in the 1-day compressive strength, strength gain at later ages, and resistance to chloride-ion penetration. Furthermore, the pore structure, phase assemblage and SEM micrographs were used to assess the alteration in the microstructure at 1-day to elucidate the microstructural development due to steam curing regime. The results show that concrete prepared with LC2 could achieve 1-day compressive strength equal to OPC concrete, which was not achieved with fly ash mixes. The durability performance was similar in both normal and steam-cured concretes across all the binder systems considered in the study. Concrete prepared with LC2 showed significant improvement in 1-day strength due to short-term heat curing. Correspondingly, there was a significant reduction in the porosity and critical pore diameter at 1-day for LC2 systems subjected to steam curing, while fly ash mixes showed almost negligible change in pore structure. LC2 systems could produce a dense microstructure at 1-day, despite the absence of ettringite and carboaluminate phases at elevated curing temperature. The study put forward the beneficial use of LC2 in precast fabrication units with steam-curing requirement for improved early-age strength and durability performance.