Concrete is one of the primary contributors to global carbon emissions, mainly due to the widespread use of Portland cement. Transitioning to more sustainable binder alternatives is therefore very crucial. The potential advantages of using wood biomass fly ash (BFA) as a sustainable precursor in the manufacturing of alkali-activated concrete is investigated in this study. Several biomass fly ashes (BFA1, BFA2, BFA3, and WA4) were characterised to identify the most suitable candidate for CO₂ sequestration. Focusing on the existence of reactive Ca- and Mg-bearing phases, the selection was based on both chemical composition and phase analysis using XRF and XRD, followed by a free lime content analysis. Among these investigated ashes, BFA1 showed the most interesting characteristics, particularly in regard to its possible CO₂ absorption capacity.
The selected biomass fly ash (BFA1) underwent a two-stage pretreatment process intended to improve its environmental performance as well as to improve its use in alkali-activated concrete. The first step involved a water-interaction treatment. This treatment was primarily meant to prevent the generation of hydrogen gas, an issue that typically originates from the reaction of metallic aluminium in the ash with the alkaline environment. In addition to that, this treatment also enables the immobilisation of heavy metals. The second pretreatment was an accelerated carbonation pretreatment. This is mainly focusing on an increase in the CO₂ sequestration by transforming reactive CaO and Ca(OH)₂ into stable carbonates such as calcite. In addition to carbon absorption, this carbonation process contributed further to the immobilisation of heavy metals.
The effectiveness of each of these methods was assessed. The main assessment criteria were the reduction in heavy metal concentration, removal of metallic aluminium, and overall carbonation efficiency. Both treatments effectively dropped heavy metal levels to below the Dutch Soil Quality Limit (SQL). Furthermore, the combination of both treatments proved most successful for totally mitigating metallic aluminium content. Hereby resolving major challenges for the safety of including BFA into concrete. The carbonation of the ash was carried out through two routes, a dry route and a wet route (L/S = 0.3). The effectiveness of the wet carbonation method exceeded the gas-solid method by achieving complete carbonation in just eight hours. In contrast, the gas-solid approach was significantly slower, and after two months, total carbonation was nevertheless not achieved. The persistent presence of free lime confirmed the limited efficiency of the gas-solid route. The CO₂ absorption capacity of BFA1 was eventually determined to be 6.59% by weight, highlighting the effectiveness of the wet carbonation method in facilitating carbon sequestration.
Combining QXRD, ICP-OES dissolution, FTIR and isothermal calorimetry allowed for the assessment of the reactivity of the pretreated BFA1 samples. Raw BFA showed the most reactivity among the various treatments, based on its largest cumulative heat release from the isothermal calorimetry test. Its finer particle size distribution and lower degree of particle agglomeration were mostly responsible for this increased reactivity. Raw BFA also showed the highest dissolution levels of alumina and silica in the ICP-OES test, further confirming its superior chemical reactivity in alkaline environments. On the other hand, the samples that underwent carbonation treatment showed a reduction in reactivity. This was clear from the lower total heat generated in the isothermal calorimetry tests and the lower dissolution of reactive elements in the ICP analysis. In addition to that, FTIR spectra showed the presence of gel-like structures in both water-treated and carbonated BFA1 samples, demonstrating the initial formation of reaction products. These early reactions reduce the reactivity of the material by consuming some of its available reactive content, thereby influencing the reactivity during subsequent alkaline activation.
Following the pretreatment process of the ash, the water-carbonation-treated BFA was incorporated into alkali-activated concrete mixtures by partially replacing slag at varying replacement levels. These concrete mixtures were developed and tested in order to meet the requirements for the production of sidewalk pavement blocks. Mechanical testing showed that a 25% replacement level of slag with treated BFA was sufficient to satisfy the compressive strength class of C30/37, as defined in the regulation. The reference mix (AAC-REF) reached a compressive strength of 49 MPa, while the mix including 25% pretreated BFA showed a slightly lower strength of 46 MPa. Durability testing showed that the treated BFA significantly lowers freeze-thaw resistance. The mass loss resulting from freeze-thaw after 28 cycles increased from 3.63 kg/m² in the AAC-REF sample to 5.66 kg/m² in the BFA-containing mix at the 25% replacement level, a significant reduction in the long-term durability under freeze-thaw conditions.
Paste samples were prepared to analyse the impact of BFA addition on the microstructure and phase composition. FTIR analysis of the paste determined the degree of polymerisation, showing a slightly increased polymerisation degree in the BFA-containing paste. QXRD and TGA were applied to determine the amount of reaction products formed after alkali activation. Both pastes contained similar types of reaction products, but the reference paste contained a greater amount of amorphous phases. Lastly, SEM analysis was performed to examine morphological and compositional changes due to the incorporation of BFA. BFA incorporation resulted in the development of micro-cracks between the BFA grain and the surrounding matrix and within the BFA particle itself. SEM-EDX point analysis revealed that the reaction gel consisted mainly of C–A–S–H, although the BFA-containing specimens had a larger Ca/Si ratio, likely due to the high calcium level in the BFA and the existence of calcite.
A life cycle assessment (LCA) was conducted to measure the environmental benefits of including water-carbonation-treated BFA, with a focus on lowering its carbon footprint. The results showed that replacing 25% of slag with pretreated BFA reduced CO₂ emissions by 21.28% relative to the reference mix, which consisted of 100% slag. This shows the potential of using BFA as a sustainable alternative precursor in an alkali-activated system.
Finally, this thesis demonstrates that pretreated biomass fly ash can be utilised in alkali-activated binder systems, especially under combined water and carbonation treatments. Although these treatments might slightly reduce the reactivity of the ash, their environmental benefits, including CO₂ sequestration, metallic aluminium elimination, and heavy metal immobilisation, provide strong justification for their incorporation in sustainable building materials.