Influence of Thermomechanical Processing of Biomass Ash on Mineralogical and Microstructural Evolution of Alkali-Activated Biomass Ash Binder

The production of portland cement is CO2-intensive (contributing 7%–8% of global CO2 emissions), and large volumes of biomass ash (BA) from agricultural and bioenergy industries are landfilled yearly; both scenarios represent environmental risks. This research tackles both issues by developing a one-part alkali-activated binder composed of 71%–77% by weight of BA. The BA was activated using a dry mix of BA, CaO, and Na2CO3. The mortars developed compressive strength of 10 MPa and were stable underwater. The optimal pretreatment (by ANOVA) was sieving the BA through a 75-μm mesh, then ball-milling and calcinating at 650°C for 2 h. Sieving BA through a 600-μm mesh achieved a useful strength of 6 MPa. X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) showed the high chemical/mineral variability of the BA that after alkali activation formed calcite and gaylussite intermixed with amorphous SiO2 and calcium silicate hydrates (C-S-H). The microstructure studies showed a heterogeneous matrix with agglomerations of silica gel poorly interacting with other phases.