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 NA-S-H type gels are formed in the system. Slight differences in terminal SiAO 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.