Upcycling waste seashells with cement: Rheology and early-age properties of Portland cement paste

To reduce solid waste discharges into environment and virgin resource consumptions in construction, waste seashells from seafood and aquaculture industry were recycled into powder form and reused in cementitious construction materials. This work aimed to use this biologically renewable waste material as a possible alternative to the nonrenewable limestone mineral used conventionally with Portland cement. The study explored roles of seashell powder as both a partial replacement and an additional additive to cement through evaluating the rheological and physico-mechanical properties of early-age cement paste. The cyclic rheological measurement including model analysis (Bingham, Casson, and Herschel-Bulkley models) was conducted to reveal the time- and shear-dependent workability of fresh cement paste during the first 2 h. Connecting to rheology, physico-mechanical measurements were carried out to understand hydration mechanisms of cement paste with seashell powder. Seashell powder was found to improve the rheological and early hydration behaviors of Portland cement paste. A parallel comparison between limestone and seashell powders was conducted to understand different roles of the two calcium carbonate minerals in cement system. Results from this study indicated the unique physical features and chemical reactivities of seashell powder in cement system, and thus suggested a promising feasibility to recycle waste seashells with cementitious construction materials.