Effect of calcium hydroxide on biomass wood ash-based alkali activated mortar

With the worldwide promotion of renewable clean energy, biofuel energy based on the combustion of wood sawdust and chips has witnessed significant growth in recent years, especially in countries with abundant forestry resources. This brings forth the challenges of waste management of biomass wood ashes (BWAs), which are a by-product of wood burning. Except for a small quantity of them consumed in the manufacture of agricultural fertilizers, most of them are disposed of in landfills, arousing a great concern for the environment. One more sustainable possibility is the valorization of these ashes as raw materials to produce building and construction materials. There is already some research regarding the BWA utilization in cement-based materials as a partial replacement for Portland cement or aggregates. Also, some investigations focused on its application in alkali-activated materials as a binary or ternary precursor. These studies primarily utilized BWA at a small ratio due to the consideration of mechanical property evolution; however, regarding the possibility of developing alkali-activated materials based on 100% BWA, the investigations are still limited. Under this background, an alkali-activated mortar (AAM) was developed, using two categories of BWAs (biomass wood fly ash and biomass wood bottom ash) as precursors. Recycled sand from biofuel plants replaced natural river sand due to the consideration to conserve non-renewable natural resources. Besides sodium hydroxide (SH) and sodium silicate (SS), in order to improve the eco-efficiency, a more sustainable activator, calcium hydroxide (CH), was added as a ternary activator at the ratio of 0, 5, 10, 15, and 20% by precursor weight. Its effects on AAM were assessed via mechanical strength, water absorption, and microstructural analysis which was conducted through SEM-EDS, XRF, XRD, and FTIR to also reveal the reaction mechanisms. The carbon footprint and energy consumption of the mortar production were evaluated via a lifecycle assessment. According to the obtained results, CH, as a ternary activator, served as an active source of calcium and can improve the alkalinity of the ambient. This importantly contributed to the enhancement of strength in a range between 16.64% and 24.26%. Its incorporation enhanced the densification of the AAM microstructure and was related to a reduction of water absorption. This was confirmed by the SEM graphs in Fig.1, the reference sample with 0% CH exhibited a mal-bond microstructure. With a 10% CH addition, a denser morphology was observed, with a decreased amount of micropores and gaps. The following increment of CH content to 20%, however, led to less densification of the microstructure and the propagation of voids. This corresponded well with the strength evolution. The shifts of bands aligning with the Si-O-Si, O-H, and C-O vibrations, compared to the raw materials, were observed in the FTIR spectra in mortar samples, indicating the formation of hydrates and carbonates. This was testified to in the XRD analysis, where crystalline hydrates were identified, together with an amorphous hydrate region. The intensity of the peaks belonging to calcium silicate hydrates showed an enhancement after adding CH, revealing its positive effects on the facilitation of the activation process. In the XRF analysis, the introduction of CH increased the Ca/Si ratio of the samples, which significantly influenced the mechanical properties of the mortar. In the lifecycle assessment, it is noted that BWA had very few greenhouse gas emissions and energy consumption, while most of the environmental impacts came fromalkali activators, especially SH and SS, which accounted for nearly half of the impacts. In contrast with them, CH contributed to much less impacts, with an energy demand of five times less than that of SH. In conclusion, the valorization of biofuel energy wastes to be applied as precursors for alkali-activated materials greatly improved sustainability from the perspective of both waste management and building material production. The usage of CH as a ternary activator was sustainable and improved the mechanical properties of AAM due to its positive effects on facilitating the production of hydrates, improving the microstructure, and contributing to a more homogeneous pore structure.