Chlorella-Laden Cements for Green Construction Materials—Effects of Algae Incorporation on the Structure and Mechanical Properties of Cement

Concrete industry, which accounts for 5-8% of global CO2 emissions, raises increasing concerns regarding its environmental impacts. To improve the environmental footprint of cementitious materials, a wide variety of biobased fillers across macro-to-nano scales, including hemp fibers, nanocellulose, and cellulose microfibrils, has been studied. Promising findings on the effects of incorporating such biobased fillers to cementitious materials include improvements on the mechanical properties of the produced composites, as well as enhancement of thermal and acoustic insulation properties. However, limitations involving the use of costly food crops as a source for biobased materials, and the energy-intense processing methods to extract and pretreat such materials prior to introducing them to concrete, have impeded their further applications. Algae-based materials, owing to their high capacity of carbon fixation, rapid growth rate, and capability of growing on non-arable spaces, represent a promising biomass resource for construction materials.<br/><br/>In this work, we investigated the effects of adding a photosynthetic algae, Chlorella, in forms of dry powder into Type I/II ordinary Portland cement (OPC) at concentrations ranging from 0.5-15 wt.%. We find that the mixing process, which is crucial to disperse chlorella powder in cement mixture, together with the water absorption capacity and particle sizes of chlorella increase the viscosity and affect the workability of the fresh paste. The compressive strength evolution across 3-91 days is studied and associated with the development of microstructure by scanning electron microscopy. We observed a drastic decrease in strength throughout the entire curing duration with only 5 wt.% addition of chlorella, which suggests a hindrance of the conventional hydration reactions. We further examine the interactions between chlorella and OPC at different concentrations by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and energy-dispersive X-ray spectroscopy (EDS). Our results show that the presence of chlorella significantly alters the hydration reactions of OPC with the formation of different compounds, which subsequently leads to micro morphological changes and notable difference in macroscopic mechanical properties.