Supplementary cementitious material from food industry: Closing the loop using mollusk seashell waste

Cementitious materials are the most common and effective building materials. While guaranteeing excellent performance, their production has an adverse effect on the environment in terms of greenhouse gas emissions and resources depletion. Thus, this research investigated the use of seashells, a waste product from the food industry, as a cement substitute and explored the optimal pre-treatment and mix design for preparing cementitious mortar. Seashell waste was characterized in terms of physical and chemical constitution and ground for use as a filler for the substitution of cement in mortar at 10, 20, and 30 wt%, with a 0.5 water-to-binder (w/b) ratio. The results were also compared to the reference mortar (0 % substitution) with the same w/b ratio. The fresh-state and hardened properties were evaluated. The compressive strength (Rc) showed that the performance decreased by up to 50 % when 30 % seashell waste filler was used. To mitigate this loss of performance, three different approaches were adopted: (i) the pre-treatment of seashell waste, (ii) a reduction in the w/b ratio, and (iii) the preparation of ternary mixes with blast-furnace slag as pozzolanic material and seashell waste as a partial substitute for cement. When substitution was performed with seashell waste from which the organic part was recovered, a 40 % reduction in Rc was recorded. With the reduction in the w/b ratio from 0.5 to 0.4, a superplasticizer was added to maintain the same workability, and the decrease in Rc was 30 % when 30 % binder was used for substitution. However, when 10 % seashell waste filler was used for substitution, the reduction in Rc was negligible for both approaches. The addition of slag to the sand–cement–seashell–slag mixture at 3:0.7:0.1:0.2 wt % also permitted the recovery of Rc when compared to 30 % seashell waste filler substitution, with a reduction in Rc of about 25 % when compared to the reference mortar