The effects of various parameters on the strength development of alkali-activated binders produced using pulverised fuel ash (pfa) have been investigated.Combinations of sodium hydroxide and sodium silicate gave the highest compressive strengths.The proportions of these two alkalis and their concentrations were defined through two parameters: the alkali dosage, which is a proxy for the concentration of the alkali activator solution and was varied from 7.5% to 13.5%, and the alkali modulus, which is a proxy for the amount of added silica in the activator solution and was varied from 0.5 (corresponding to all sodium silicate) to ∞ (all sodium hydroxide).The water/solids ratio, i.e.the ratio between the total mass of water and the total mass of pfa and alkali solids,value was kept constant at 0.37.The total mass of water included the water content of the sodium silicate solution.The sand/pfa ratio was fixed at 2.75.It was found that an increase in the alkali dosage resulted in an increase in strength but only up to the value of 12.5%.Beyond this value the strength decreased, which has been attributed to a saturation of the gel with alkali ions, such that less free water is available for speciation of silica and alumina oligomers from the dissolution ofpfa.This shows that there is an ideal range for the alkali modulus,above and below which strengths drop.Alkali modulus between 0.75 and 1.00 gave the best strengths across the alkali dosages studied.The decrease in strength with increasing modulus is likely to be a result of the reduced amount of available silica that needs to be involved in the 'reorganization-gelation-polymerization' for the geopolymer formation, ultimately leading to a denser and more complete and long polymer chain.The 'sweet spot' of the optimum alkali dosage (≈ 12.5 %) and modulus (≈ 0.90) resulted in strengths of around 70 N/mm2.Advanced microstructural techniques were used to study the reaction products.These indicate that there is a change in the reaction products which affects strength, i.e.it is not only the percentage ofpfa that has reacted.