Geopolymers are novel binding materials produced by the alkaline activation of rich aluminosilicate materials. Geopolymer binders are considered as green building materials that have increasing potential to replace the ordinary Portland cement in the concrete industry. This study investigated the alkali solution effects on the physical and mechanical properties of the high calcium fly ash based geopolymers. The parameters involved in this study were the NaOH solution molarity and the Na2SiO3/NaOH ratio. Three NaOH concentrations (8, 10, and 12 molars) were investigated. The Na2SiO3/NaOH ratio was varied between 1 and 2.5. The results showed that the studied parameters significantly affected the properties of the produced geopolymer mortars. The workability and setting time were found to decrease by increasing the NaOH concentration or by increasing the Na2SiO3/NaOH ratio. The influence of the NaOH concentration was higher on the workability and setting time while the Na2SiO3/NaOH ratio highly affected the compressive strength. The initial setting time was in the range of 45-105 minutes while the final setting occurred quickly after that within 70-115 minutes. Oven curing method resulted in high strength at early ages where all of the mixes were able to achieve more than 75% of the 28 days strength within the first 3 days. The 28-days compressive strength ranged between 60-85 MPa, which promotes the use of the high calcium fly ash for the production of early high strength geopolymer concrete. (C) 2016 The Authors. Published by Elsevier Ltd.