The main aim of the current study is to search the impact of variable matrix phase features on fly ash based lightweight geopolymer mortars (LWGM). Another scope of the study is to obtain performance oriented optimum mixture proportions through response surface method (RSM). In order to have low unit weight for LWGMs, pumice aggregate was utilized as a part of the aggregate. The investigated engineering properties are water absorption, drying shrinkage and thermal conductivity. By performing optimization analysis, it was aimed to obtain the best numerical models representing the experimental results depending on the input variables. The decrease of liquid (alkali activators) to powder (fly ash) ratio, Na2SiO3 solution to NaOH solution ratio and increase of sodium hydroxide molarity led to improvement of compressive strength. Dry thermal conductivity values in dry state were observed to be less than those of saturated ones. Moreover, the higher sodium hydroxide molarity and lower Na2SiO3 solution to NaOH solution ratios, and liquid to powder ratios resulted in further shrinkage reduction. Depending on the goals of maximum compressive strength, minimum water absorption, and drying shrinkage, optimum values for molarity, SS/SH, and l/p factors were determined as 14 M, 1.586, and 0.45, respectively.