Use of sodium/potassium citrate to enhance strength development in carbonate-activated hybrid cement

Hybrid alkaline cement (HAC) typically has a high amount of fly ash (70–80 wt%) and a low amount of clinker (20–30 wt%). A small amount of activator is commonly incorporated in HAC to promote its strength development. Compared with those highly corrosive and hygroscopic alkaline activators (e.g. sodium hydroxide), sodium and potassium carbonate are much preferred because of their lower cost and easier handling and storage. Carbonate-activated HAC, however, usually has low early strength and slow strength development (usually <40 MPa at 28 days) because of the slow reaction of fly ash. This paper investigates the strength development of HAC activated by sodium or potassium carbonate, where a small amount of sodium or potassium citrate is added to promote strength development. It was found that the 28-day compressive strength of the HAC could be improved from 38.5 MPa up to 72.4 MPa by adding 1 wt% sodium or potassium citrate to the mixture. Microstructure characterisation revealed that the inclusion of sodium or potassium citrate led to the formation of denser C-S-H structure in the paste. The environmental impact and cost of the developed HACs were further analysed. Compared with ordinary Portland cement, the HACs’ greenhouse gas emissions can be reduced by up to 73 %, and their costs can be reduced by up to 28 %.