Sustainable transformation of fly ash industrial waste into a construction cement blend via CO2 carbonation

This study investigates the potential approach of utilising CO2 to produce carbonated fly ash (CFA) in a cement blend for new 'green' construction materials. The aqueous fly ash (FA) carbonation experiments are performed in a batch bench-scale glass reactor to study the effect of different CO2 flow rates (of 2 and 4 ml/min) and water-to-FA ratios (15 and 7.5 ml/g) on the carbonation process. A longer carbonation time is obtained for the water-to-solid ratio of 7.5 ml/g compared with 15 ml/g at the same CO2 flowrate while a faster carbonation process is achieved at higher CO2 flowrate at the same solid dosage. The maximum carbonation efficiency of 16.9% is calculated for water-to-solid ratio of 15 ml/g based on the variation in the Ca2+ ions concentration in the liquid phase. However, using the carbonate content of the solid samples results in a much higher carbonation efficiency of 83.5% corresponding to a final CO2 uptake of around 3.2%, i.e. 32 g CO2/kg FA. The CFA is then used to prepare cement paste blends in which Portland cement (PC) is replaced with CFA at different replacement levels of 10 and 30% (w/w) using the water-to-cementitious material ratio of 0.4. Mechanical strengths of CFA-PC mortars, cured for 14 days under ambient conditions, are compared with those prepared from pure PC in order to investigate the effect of FA pre-carbonation on the performance of green cement with low-carbon footprint. The results show that the CFA-PC mortars containing 10% CFA has slightly lower compressive strength than that of the pure cement, however with increasing the CFA replacement level to 30%, significant reduction in the compressive strength of mortars is observed.