Effects of limestone calcined clay, fly ash and seawater on early hydrating behavior, mechanical properties, microscopic performance and sustainability of eco-friendly cement-based pastes

In some remote areas or islands, the availability of freshwater for infrastructures has encountered a huge shortage, which has prompted more researchers to explore the possibility of applying seawater to design green concrete or composites. This study investigated the mechanical and microstructural properties of cement-based pastes (CBPs) mixed with fly ash (FA) and limestone calcined clay (LC2) and seawater at different curing ages. The early-age hydration, exothermic evolution, hydration phase, unconfined compressive strength (UCS), pore distribution, microstructure and sustainability of CBPs were investigated. It was observed that seawater has an accelerating effect on OPC hydration and can significantly enhance the early UCS. Combining with seawater, the pozzolanic activity of LC2/FA can result in generating more C–S–H gels. Therefore, the combination of LC2 or FA with seawater can obtain better the mechanical properties of CBPs. Besides, the micro characteristics of CBPs are observed by X-ray diffraction, thermogravimetric analysis, mercury-pressure porosimetry and scanning electron microscopy technologies, and it is revealed that seawater and SCMs play significant role in the micro morphology of CBPs due to silicate, semicarbonate, calcium aluminate, C–S–H gels. This also leads to a denser and more complex microstructure, which lowers the total porosity, obviously leads to transferring macropores into gel pores and enhances the UCS. Additionally, CBPs with FA or LC2 or seawater show about 1/6–1/5 lower embodied energy (EE), 1/6–1/5 lower embodied carbon (EC) and 1/6–1/4 lower cost than the typical CBPs with freshwater, CBPs with SCMs and seawater are even more perfect in terms of EE, EC and cost per unit UCS. Overall, this work assesses the feasibility of adding LC2/FA and seawater to CBPs, and also can provide a better insight into the applications of seawater cementitious composites for marine infrastructures.