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This research aimed to develop a scale to predict the effect of alkali-activated solution on compressive strength and microstructure of geopolymer based fly ash. The starting material for geopolymer synthesis was fly ash obtained from Mea Moh electricity thermal power plant. Sodium hydroxide 10M, sodium silicate solution and calcium hydroxide solution were used as alkali-activated solutions for geopolymer. The proportions of alkali -activated in the geopolymer mixture were calculated by simplex lattice design. The compressive strength of geopolymer was investigated at the age of 7, 14 and 28 days. The surface area of the geopolymer samples was characterized by OM technic. The results of compressive strength were input into a Minitab program to construct the scale for predicting and plotting the mixture contour. The results showed that the compressive strength of geopolymer had a range between 0 and 200 ksc. Using sodium hydroxide and sodium silicate solution to be alkali activated solution for geopolymer provided the highest compressive strength. It is worth mentioning that sodium hydroxide is a strong alkali hydroxide which dissolved Si and Al from fly ash; additionally, geopolymer could also be formed from free silica in the sodium silicate solution. On the other hand, geopolymer synthesized with calcium hydroxide solution presented the lowest compressive strength due to the solution being a less strong alkaline to dissolve Si and Al from fly ash. The equation for predicting the compressive strength in ksc of geopolymer with three alkali-activated solutions is Y = 202.49NH-3.63NS+43.20CH+461.24(NH*NS)-61.81(NH*CH)+559.15(NS*CH)+45.98(NH*NS*CH) where Y is compressive strength of geopolymer in ksc.
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