Mechanical properties and hydration of ultra-high-performance seawater sea-sand concrete (UHPSSC) with limestone calcined clay cement (LC3)

Abstract

Ultra-high-performance seawater sea-sand concrete (UHPSSC) has attracted increasing attentions over the past few years as it exhibits exceptional mechanical and durability properties similar to conventional ultra-high-performance concrete (UHPC). Comparing to conventional UHPC, UHPSSC has great advantages when applied in marine engineering, particularly in offshore projects where construction materials are not easily accessible, and the transportation is costly. Nonetheless, current production of UHPSSC still necessitates large amounts of cement due to its low water/binder ratio. The larger demands of cement in manufacturing UHPSSC/UHPC than ordinary concrete can lead to substantial increase in the carbon emission. Therefore, an eco-friendly UHPSSC is developed in this paper by incorporating a promising ternary binder system (LC3) that consists of limestone (LS), calcined clay (metakaolin) and cement. The influence of the metakaolin (MK) dosage on the mechanical, hydration and microstructure properties of UHPSSC is comprehensively studied by carrying out a variety of experiments including mechanical test, isothermal conduction calorimetry test, TGA, XRD, SEM etc. The results showed that the UHPSSC mixed with LC3 exhibited lower early-age compressive strength than the reference mix, but its long-term strength was better than the reference group owing to the formation of more hydrates at the late age by the pozzolanic reaction of MK and the synergistic reaction between LS, MK and cement. Based on this study, the coupled replacement rate of cement by LS and MK could reach up to 40% without strength compromise. Nonetheless, 20% MK and 10% LS appeared to be the optimum substitution rate (total substitution rate of 30%) where the highest long-term strength was achieved. The main exothermic peak and total heat of hydration normalized per binder weight decreased with the increase of MK content level, but the main peak occurred earlier. Moreover, the addition of seawater can promote the hydration, leading to a greater main exothermic peak.