Influence of Admixtures on the Workability and Strength of Sodium Carbonate-Activated Slag/fly Ash Cement

A study of the fresh state properties and mechanical strengths of different formulations of alkaline cement activated by sodium carbonate was conducted to determine the effect of slag-fly ash proportion and admixture incorporation. The utilization of sodium carbonate as an alkaline activator considerably reduces the environmental impact of cement, but its use has not spread due to certain limitations, as low early-age strength or long setting time. To overcome these drawbacks two approaches are evaluated in this work: to adjust the ratio of slag-ash, so blends of 100-0, 85-15 and 70-30% slag-ash were studied, and to add some admixtures to optimize the workability of these cements. Results show that an increase in the ash content delays the reaction and improves workability but reduces compressive strength. Relating to admixtures, these improve fluidity at the first moments after mixing but their plasticizing function is not well preserved over time. In addition, they are associated with a decrease in strengths at 28 days. The highest impact on workability is observed for polycarboxylate-based admixtures with very high chain density.