Sustainable Cement Production Using Forest Biomass Ash: Insights into Alkaline Cements, Blended Cements and Hybrid Alkaline Cements

The search for and utilization of alternative precursors for the development of low-carbon binders has become a major research priority for both the scientific community and the construction industry. The future availability of the traditional aluminosilicate sources commonly used in cement production, such as coal fly ash and ground granulated blast-furnace slag, is expected to decline due to the progressive phase-out of coal-fired power plants and ongoing changes in the industrial landscape. This scenario highlights the importance of exploring new waste streams with suitable chemical and mineralogical characteristics for use as cementitious precursors. This study evaluates the potential of forest biomass ashes, namely fly ash and bottom ash, as reactive components in the production of alkaline cements, hybrid alkaline cements, and blended cements. Cement pastes were prepared using these precursors, with ash contents ranging from 30 to 100 wt.%, and cured for 28 days. Their mechanical performance, mineral phase evolution, and microstructural development were subsequently assessed using a range of characterization techniques, including X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), backscattered electron microscopy coupled with energy-dispersive X-ray spectroscopy (BSEM/EDX), and mercury intrusion porosimetry (MIP). The results highlight the critical role of the chemical and mineralogical composition of the precursor, as well as the chemical composition of the amorphous phase, in determining the mechanical performance of the different cement systems developed. Several formulations exhibited satisfactory mechanical strengths, comparable to those of the Portland cement (PC) reference, especially in the hybrid and blended systems. Moreover, the reaction products identified were similar to those typically found in PC systems, including C-(A)-S-H gels, portlandite, and ettringite, together with (N,C)-A-S-H gels in formulations containing low-CaO content ashes.