Utilisation of Wood Ash in Cement-Based Materials

Wood ashes originating from combustion of wood and woody biomass were investigated with the aim of utilising wood ash as a partial cement replacement.
The physicochemical properties of wood ashes varied significantly depending on the combustion technique and temperature, the type of woody biomass used etc. Multivariate data analysis of chemical compositions was performed to determine which production parameters yielded the wood ash most suitable for utilisation as a partial cement replacement in cement-based materials. The standards EN 450 [1] and EN 197 [2] were used for the assessment of possible pozzolanic and hydraulic activity, respectively, in the multivariate data analysis. The multivariate data analysis revealed that the combustion technique had the greatest influence on the physicochemical characteristics of the wood ash. It was determined that wood ashes originating from grate combustion had the highest probability of possessing hydraulic properties, while, wood ashes originating from fluidised bed combustion had the highest probability of possessing pozzolanic properties. Based on this analysis, two wood ashes were selected for further investigations: one as the wood ash most likely to possess hydraulic properties and the other as the wood ash most likely to possess pozzolanic properties.
Pozzolanic activity was tested on mixtures with 20% cement replacement with wood ash, and hydraulic activity was assessed based on the phase and strength development of 100% wood ash and water mixtures. Contrary to the results obtained by multivariate data analysis, both of the selected wood ashes displayed hydraulic properties (they set, hardened and developed strength) and neither of them showed pozzolanic activity. Despite both possessing hydraulic properties, a large difference in the strength development of the two wood ash and water mixtures was observed. Subsequent analysis of the phase development determined the formation of ettringite as a major factor in strength development. Formation of ettringite requires aluminium, and upon lack of aluminium formation of gypsum was observed instead, leading to a significantly lower compressive strength development. The difference in strength development of the two wood ash water mixtures was therefore attributed to the difference in aluminium content.
Low-levels of cement replacement and pre-treatment of wood ashes have generally been shown in literature to enhance compressive strength and reduce the risk of durability issues in cement-based materials. In this project, an in-depth analysis of phase and strength development of both untreated and washed samples with wood ashes utilised as low-level cement replacements (10%) has been performed, expanding the knowledge on the influence of such pre-treatments on the performance of wood ashes as low-level cement replacements. A decrease in compressive strength was observed for all cement-wood ash mortars when compared to a reference cement mortar, however, an increase is seen compared to a mortar with a 10% cement replacement with an inert filler. This increase in compressive strength is attributed to the formation of ettringite in all cement-wood ash specimens, due to the aluminium provided by the cement clinker and the increased sulphate content from the wood ash. Washing of wood ashes prior to use was found to remove SO₃ and thus enable regulation of ettringite formation by adjusting the SO₃/C₃A ratio. Based on differences in compressive strength an optimum SO₃/C₃A ratio in the binder of 0.4 ≤ optimum < 0.5 was proposed. This ratio ensures the maximum contribution to compressive strength from ettringite formation, without excessive ettringite formation leading to microcracking. Several potential durability issues resulting from the presence of oxides in the wood ashes were reduced by a washing pre-treatment (risk of sulphate attack, chloride corrosion and alkali-silica reactions). However, potential durability issues related to the content of CaO_free, MgO and LoI were still a risk, thus, other types of treatments could be required. An assessment of the leaching behaviour of heavy metals concluded that none of the wood ashes, untreated or washed, posed any significant environmental risk when used as low-level cement replacements in cement-based materials. In summary, ettringite was determined to be the most important phase when wood ash was used as a partial cement replacement, due to the contribution to compressive strength. However, even at low-level cement replacement ratios, partial cement replacement with wood ash led to a decrease in the compressive strength of cement-based materials. The hydraulic properties of wood ash could facilitate usage in other types of cement-based materials with lower strength requirements. A washing pre-treatment greatly decreased the risk of several potential durability issues, therefore, further optimisation of wood ash characteristics by pretreatment may be a viable option to explore in future work.