Properties of biomass fly ash concrete
Authors:   , Elbuaishi Eman Elhadi 
Published: April 2021

The environmental concerns of carbon emissions by the energy industry have led to a change in the way energy is generated as the UK moves to a low carbon future. While biomass combustion is gaining attraction as the most available renewable energy source, the resulting ash is most often landfilled and is still not accepted in the concrete industry as in the case of coal fly ash. This is mainly because of the limited knowledge of the in-service life of concrete made with this fly ash. This research investigates the use of two types of wood biomass fly ash, obtained from two power plants in the UK, in cement and concrete production to provide a performance-based database for evaluating its utilization in the concrete industry. The study comprises of three parts, the first part deals with determining the chemical, mineralogical and physical properties of these two fly ashes enhanced biomass ash (EBA) and virgin wood biomass ash (WBA). The results show that EBA has a chemical composition more similar to coal fly ash (CFA) than WBA and EBA satisfies the BS EN 450-1 requirements for the main oxides and other chemical components. The mineralogical structure of both ashes is mainly amorphous; EBA particles are mainly spherical whereas the morphology of WBA particles is fibrous irregular in shape and size. WBA has a higher surface area than both EBA and CFA while its pozzolanic reactivity is less. The mechanical and durability properties investigated in parts 2 and 3 are related to these characteristics (e.g., chemical compositions, pozzolanic reactivity and particle size) and also to pore properties investigated in part 2. Part 2 of this study is concerned with the effect of both ashes on the fresh and hardened properties of concrete compared to coal fly ash. Blended fly ash pastes and mortars substituting the cement at 10, 20 and 30% were produced and numerous tests were performed. The results show that the incorporation of EBA reduces the water demand and improves the workability similar to the effect of coal fly ash while the behavior of WBA is the opposite. The coarse and high surface area of WBA particles contributes to its higher water demand. The early age hydration behavior of EBA is quite similar to CFA. The CFA and EBA mixes release considerably higher heat than WBA mixes, indicating a higher rate of hydration. The compressive and flexural strength decreases gradually as the percentage of both EBA and WBA in the mix increases. The compressive strength of CFA mixes is higher than EBA mixes while WBA mixes give the lowest strength. The incorporation of EBA and WBA increases the total porosity of cement pastes. Part 3 investigates the durability properties of enhanced biomass fly ash concrete by exposing it to long-term sulphate, chloride and carbon dioxide environments which are substances that cause deterioration and damage to concrete structures. Durability properties were tested under laboratory conditions over a period of one year and control samples of ordinary OPC concrete and coal fly ash concrete were produced for comparison. Generally, enhanced biomass fly ash concrete shows better durability properties than OPC concrete except for the carbonation resistance. The depth of carbonation of enhanced biomass fly ash concrete is higher than OPC concrete but less than coal fly ash concrete which shows the highest carbonation depth. The results also show that the incorporation of enhanced biomass fly ash improves the sulphate resistance compared to control OPC, however, it is still less effective than coal fly ash in resisting sulphate attack. The chemically and physically bound chloride of enhanced biomass fly ash concrete is lower than OPC concrete but it is higher than coal fly ash concrete. The efficiency of both enhanced biomass fly ash and virgin wood biomass ash in mitigating alkalisilica reaction was also examined based on the accelerated mortar bar test. The results show that enhanced biomass fly ash reduced the expansion caused by ASR to the low-risk level of deterioration according to ASTM C1260/1576 standards whereas the reduction of expansion in the case of virgin wood biomass ash was not sufficient to reduce the risk from potentially deleterious level to low risk.