The Impact of Mixed NaOH and KOH Mole Fraction on Mechanical Performance of Metakaolin Based Geopolymer Material

Geopolymer is one of the newly promising materials with potential comprehensive implementation. Its parameters can affect its synthesis and mechanical performance, and accordingly influence the area of geopolymer applications. The type and concentration of alkaline activators is a powerful parameter that activate geopolymerization. In the following study, metakaolin was used as a source for aluminum and silicate. The aim of this study is to optimize the mole fraction effect of NaOH to KOH as mixed alkaline activators on the hardness and compressive strength of the geopolymer paste. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) were used as a mixed alkaline activator in 9 mole ratios from 0.1up to 0.9, while keeping the ratio of NaOH and KOH to aluminum content is 1:1of the final geopolymer formula. The other parameters were fixed like the silica/alumina ratio is equal to 3.8, the alkaline hydroxide to alkaline silicate is 3.02. The results showed that the best mole ratio was 0.3NaOH + 0.7KOH which gave the highest compressive strength and optimum hardness around 62.85MPs and 78.21VHN, respectively. It can be inferred that the metakaolin based geopolymer compressive strength can be modified by the type of used alkaline elements. The strength of the geopolymer can increase by increasing the amount of KOH related to NaOH up to 0.7 KOH + 0.3 NaOH. After this ratio, the compressive strength decreased. The effect of the little amount of Na element has vanished from the effect of the other K element.