•Slag/fly ash ratio and activator modulus show synergetic effects on reaction.•Activator modulus has a more significant influence on early age reaction.•Gel structures remain stable regardless of activator modulus and slag/fly ash ratio.•Slag content shows a dominating effect on compressive strength. Room temperature cured alkali activated slag/fly ash blends have shown their advantages in field applications. Given that alkali activated materials are extraordinarily sensitive to the composition of the starting materials, identifying their influences is essential for their application. This paper focuses on the effects of two compositional factors: activator modulus (SiO2/Na2O from 1.0 to 1.8) and slag/fly ash mass ratios (between 90/10 and 50/50) on reaction kinetics, gel characters and compressive strength. The results show that when lowering the activator modulus, the early age reaction is significantly accelerated with a higher reaction intensity, and increasing the slag content also leads to an increased reaction rate, especially at low activator modulus. Regardless of the two influential factors, the main reaction products are chain structured C-A-S-H gels with similar water contents and thermal properties, and no typical N-A-S-H type gels are formed in the system. Slight differences in terminal SiO bonds and crystallization temperature are caused by the activator modulus and slag/fly ash mass ratios, respectively. The compressive strength results show that the optimum activator modulus changes with the slag/fly ash mass ratio, and higher slag/fly ash mass ratios prefer higher activator moduli in general, while either too high or too low activator modulus has detrimental effect on strength. Understanding the reaction, gel structure and strength changes are fundamental for determining key manufacturing parameters and tailoring the properties.