Understanding the Influence of Morphology and Chemical Compositions of Algae on the Hydration Reactions of Portland Cement

Cement accounts for up to 10% of global carbon dioxide emissions and has become one of the targets of recent decarbonization efforts. Algae is an abundant and easy-to-cultivate biomass, which can sequester carbon during growth and can be used to replace cement. Recently, we showed that certain microalgae hinder the primary hydration reaction of ordinary Portland cement at high concentrations, significantly hampering mechanical performance. To optimize the structural performance of algae-cement composites, here we focus on the effects of morphology and chemical compositions of algal biomatters on cement hydration. Applying a bottom-up approach, we first study the effects of algae-related biopolymer building blocks, including proteins, carbohydrates, and lipids, on the resulting hydration products through thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM), and their relationships with the composites' compressive strength. To further characterize the influence of particle size, bonding structure, and chemical compositions of the algae on the composites, we then pretreat the microalgae and macroalgae through grinding, hot-pressing, and biopolymer extraction. SEM is used to investigate the effects of particle morphology on the binding interfaces and distribution in the cement matrix. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) are used to characterize the bonding environment and chemical compositions of the biomass, which are correlated with the hydration products in the algae-cement composites and their contribution to mechanical properties. We show that using specific pretreatment for both algal biomatters, the strength of composites can be improved by up to twofold compared to the unprocessed algal biomatter. The proposed processing methods allow for higher algae replacement content, effectively reducing the environmental impact of the cementitious binder.