The compressive strength achieved in alkali-activated low-calcium fly ash depends on the total reactive oxide ratios in the activated system, the reactive alumina content in fly ash and the initial molarity of NaOH. The total reactive silica in the system is the reactive silica contributed by the fly ash and soluble silica from the alkaline solution. A minimum molarity of NaOH is required to ensure complete dissolution of the glassy phase present in fly ash. The maximum compressive strength depends on the amorphous reaction product content formed, which is determined by the reactive alumina content in the fly ash. The highest compressive strength is achieved for the ratio of the total reactive silica to sodium ratio in the activated system equal to 4.72 and lower. More Na is incorporated in the reaction product on increasing the Na 2 O content relative to the reactive oxides. © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.