Development of greener alkali-activated cement: utilisation of sodium carbonate for activating slag and fly ash mixtures

Alkali activated fly ash/slag (AAFS), a newly evolved type of alkali-activated cements (AACs), is here studied with the aim of developing a more sustainable alternative to Portland cement (PC), known for its adverse environmental impact. In this study, sodium carbonate (Na2CO3) was used as the alkali activator for the fly ash (FA)/slag blends. The effects of different factors on the strength, reaction rate, hydration products and microstructure were examined; these factors include the activator dosage, FA/slag ratio, and curing regime. It was found that increasing the Na2CO3 dosage significantly increased the compressive strength. The inclusion of up to 25 wt% fly ash marginally decreased the compressive strength up to 28 days while the inclusion beyond 25 wt% can lead to a remarkable reduction in strength, particularly for water-cured specimens. Sealed curing in general was found to be beneficial to the strength development of AAFS paste especially at a 50/50 ratio of FA/slag. Both the activator dosage and FA/slag ratio were found to have notable influences on reaction rate and reaction products and microstructure. Increasing the activator dosage accelerates the kinetics of the reaction while increasing the FA/slag ratio slows the reaction rates. The main binding phase is C-(N)-A-S-H with varying Ca/Si ratios ranging between 0.6 and 1.0 depending mainly on FA/slag ratio. The results indicated the possibility of production of greener cementing materials by utilizing appropriate ratios of FA/slag, dosage of sodium carbonate, and curing regimes.