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A wide use of fly ash (FA) as a supplementary cementitious material (SCM) can result in an enhancement in its durability performance in our civil engineering applications. Although FA is either agricultural or industrial by‑product and is abundant for use in cement and concrete works for many decades now, the utilization of FA still has a practical challenge. The challenge is due to variability and their heterogeneity. In Southeast Asia, fly ash is used in concrete production replacing cement as a pozzolanic material. However, there are few standard guidelines for using fly ash across the region. This study evaluates the durability of cementitious materials after the including of five types of fly ash in different mix proportions from three countries, Myanmar, Thailand, and Indonesia. For the mathematical model, each type is used to replace ordinary Portland cement in 15 % with a water-to-binder ratio of 0.54. Moreover, this work is to perform both experimental and numerical studies to have a better insight for controlling the different FA, reflecting its durability. FA from five different sources were assessed to investigate the impacts on its chemical composition and physical properties. The blended FA-cement systems were also prepared and evaluated on its flow, compressive strength of mortar, hardened porosity, ability to resist chloride penetration, apparent chloride diffusivity coefficients (Da), and carbonation resistance after 7, 28, and 91 days, and degree of reaction. Because the chemical composition of each country’s fly ash is slightly different, their chemical reaction with cement is marginally different. Thus, the durability performances of the mortar are better for finer fly ash types at an early age. The service life model using Life365 was performed to predict the service life of blended FA-cement system. The experimental results were statistically analyzed using analysis of variance (ANOVA) and sensitivity analysis with linear regression. The analyses indicate that chemo-physical mechanisms of multi-scale structures are the main factors, and both degree of reaction of blended FA-cement systems and its hardened porosity are among the most positive factors influencing its durability.