Microstructural verification of the strength performance of ternary blended cement systems with high volumes of fly ash and GGBFS

•Ternary cement blends with high volumes of GGBFS and fly ash were investigated.•Microstructural investigations were performed using XRD, TGA, MIP, and SEM/EDS.•The 3:5:2 mixture possesses equivalent characteristics to 4:4:2 for microstructure.•Strengths of ternary pastes were more vulnerable to pore volume and size than OPC. Ternary combinations of Portland cement, GGBFS, and fly ash with a typical weight ratio of 4:4:2, respectively, have been widely used for concrete production in South Korea. Recently, to achieve lower heat generation and material costs, the construction society of Korea is investigating the use of a 3:5:2 mixing ratio for construction; however, despite earlier laboratory testing results exhibiting satisfactory mechanical properties, the industry remains reluctant to apply the 3:5:2 mixture because there has been little verification of the microstructural characteristics of the mixture proportions. Therefore, this microstructural study examines the influences of the higher ratio of GGBFS to cement on the microstructure and hydration products to explain the comparable mechanical performances of the ternary mixtures of 3:5:2 and 4:4:2 based on the testing results of powder X-ray diffraction analyses, thermogravimetry, mercury intrusion porosimetry, and scanning electron microscopy with energy dispersive spectroscopy.