This paper describes the research developed to evaluate the performance of stabilised residual granitic soil from the north region of Portugal. It is part of a research project designed to investigate the possibility of using this soil for rammed earth construction. Since its reduced plasticity makes the use of lime less effective, and the increasing environmental concerns with cement production can no longer be ignored, the study was focused on binders made from alkaline activation of low calcium fly ash. Therefore, the aim of this work was to determine, through a parametric analysis using laboratory tests, the optimum fly ash – based alkaline activated binder for improving granitic soil, in terms of mechanical strength. Identification of the key parameters allowed the characterisation of the most effective mixtures, which will be later used in the construction of large rammed earth wall panels that will be tested according to proper specifications.