Optimizing the concentration of Na2O in alkaline activators to improve mechanical properties and reduce costs and CO2 emissions in alkali-activated mixtures

•In the most of cases, the optimum dosage for compressive strength was 6% Na2O.•Microcracking clearly affected the mechanical properties, especially flexural strength.•The production of C-S-H achieved maximum values for the concentration of 6.0% Na2O.•The blends with 4.5 and 6.0% Na2O present the higher efficiency indexes for CO2 emissions and costs.