Valorization of Wood Biomass Fly Ash for the Development of Sustainable Low-carbon Cementitious Materials

Abstract

With growing concern for global warming, the electricity industry is actively promoting the transition from coal to renewable energy sources. Due to the carbon neutrality of wood biomass energy, it has become one of the most popular options of renewable energy sources. However, the by-products resulting from biomass combustion, particularly wood biomass fly ash (WBFA), have not received sufficient attention. Direct disposal of WBFA poses environmental threats and causes pollution. From the perspective of the construction industry, the energy transition has led to a scarcity of coal fly ash (CFA) that is intensively used as supplementary cementitious materials (SCMs) in construction industry. With the increasing demand for raw materials in construction industry, it is of great interest to explore whether WBFA can be integrated as a new material in construction industry. This motivates the initiative of this research, driven by significant industry demand.

This thesis aims to enlarge the utilization efficiency of WBFA by recirculating WBFA as a valuable mineral to develop sustainable low-carbon cementitious materials. Prior to the experiment, a literature review on the properties of WBFA and current WBFA utilization methodologies in cementitious materials are summarized. An application-oriented WBFA classification was proposed, providing a guideline for the utilization of WBFA in preparing cementitious materials.

For the experimental research, three types of WBFA were initially characterized and screened based on their physicochemical properties, i.e., the content of unfavoured metallic aluminum and reactive components. One type of WBFA with representative properties was selected as the most suitable candidate for further investigation. To remove the metallic aluminum in WBFA, a two-step pretreatment method is proposed. The feasibility of WBFA for binder formulation was then evaluated through dissolution tests.

Based on the characteristics of WBFA, two types of binders were proposed. In chapter 4, considering the high alkalinity of WBFA, WBFA was used to enhance the reaction of aluminosilicates. An innovative clinker-free binary binder with WBFA and blast furnace slag (BFS) was developed. The effects of different WBFA to BFS ratios on reaction kinetics, hydration products, and microstructure evolution were studied. Following this, WBFA was further used as a mineral additive for BFS replacement in BFS blended cement. The effects of WBFA on both BFS and cement reactions were comprehensively studied using different characterization techniques. These two chapters together provide new solutions for the valorization of WBFA in novel binder formulations.

The carbonation of WBFA-containing binders investigated in chapter 4 and 5 was studied in chapter 6. The carbonation kinetics of pastes were calculated based on the carbonation depth development. It was found that in WBFA-BFS binary pastes, mixture with 50% of WBFA and 50% of BFS showed the best carbonation resistance, although the carbonation coefficient was much larger than cement pastes. When WBFA was introduced in BFS blended cement, it was observed that there was a decreased carbonation resistance in pastes with more WBFA. By analysing the microstructure evolution, it was concluded that pore connectivity played the key role in governing the carbonation process of pastes. Further grinding of WBFA to reduce its porous structure was recommended to reduce the pore connectivity and improve the carbonation resistance of pastes with WBFA.
The environmental impact evaluation is of great significance for waste utilization in the construction industry, as it can, to a certain extent, help quantify the sustainability of specific products. In chapter 7, bio-ash bricks and bio-ash composite cement were developed based on the mixtures studied in Chapters 4 and 5, respectively. A cradle-to-gate life cycle analysis (LCA) was conducted to evaluate the contribution of integrating WBFA for the production of these construction products. Possible improvements regarding further reducing the environmental impact of these products are discussed.

In summary, this thesis offers novel options for the utilization of WBFA in the construction sector as a binder component. Binary paste containing up to 70% WBFA demonstrates satisfactory mechanical properties for low-strength applications. In BFS-blended cement, substituting up to 30% of BFS with WBFA enhances early compressive strength, only a 7.67% reduction in compressive strength after 90 days. These findings indicate high utilization efficiency for WBFA. The investigations in the reaction kinetics and microstructure evolution of pastes with different WBFA to BFS ratios in the binders yield valuable insights into the function of WBFA in binder reaction. This knowledge can serve as a valuable reference for engineering practitioners seeking to customize the properties of these binders. The development of building products such as bio-ash bricks and bio-ash cement exemplifies the conversion of WBFA into construction materials. This emphasizes the notable ecological advantages of WBFA as a resource for the construction sector, highlighting the academic and industrial interests in utilizing WBFA for the development of sustainable low-carbon cementitious materials.