In this paper, alkali activated slag-fly ash mortars were prepared by applying the modified Andreasen & Andersen particle packing model in order to maximize the packing of the granular solid materials. The effects of two key manufacturing factors: slag/fly ash ratio and activator modulus, on the fresh behaviors, gel structure development, compressive strength, and drying shrinkage were investigated. The results show that a large variation of slump flow can be found when different combinations of slag/fly ash ratio and activator modulus were used. The microstructure analysis carried out by FTIR and TG shows that the gel structure remains stable after 1 d of curing, mixes with higher slag content and lower activator modulus show a slightly higher bound water content. The main reaction product is a chain structured C-A-S-H type gel regardless of the slag/fly ash ratio and activator modulus, but a slightly higher main absorption band is shown in samples with high fly ash content. A higher content of slag in the slag-fly ash mix leads to a higher strength in general. An optimum activator modulus of 1.4 in terms of strength appears in all cases. Both slag content and activator modulus strongly affect the drying shrinkage, and using high amount of fly ash and low activator modulus can effectively reduce the drying shrinkage.