Nanoscale pore structure analysis of cementitious materials subjected to delayed ettringite formation

• Microstructural pore size responsible for DEF of up to 4000 days was estimated via SAXS. • The peak pore size of DEF samples deceased due to the initial ettringite formation filling up the largest pores. • The critical pore size threshold necessary to induce cracking stress was approximately 20 to 25 nm. The current study investigates the nanoscale pore structure of cementitious materials subjected to delayed ettringite formation (DEF) under different heat curing conditions up to 4000 days via small angle x-ray scattering (SAXS). Four types of commercially available cements were used and a heat curing temperature of up to 100 °C was applied. Results indicated that the peak pore size deceased due to the initial ettringite formation filling up the largest pores. Over time, ettringite continues to form in the smallest pores during supersaturation, leading to an apparent increase in average pore size in later age. Once ettringite crystalline pressure exceed the tensile strength of the mortar, nano-cracking initiates. Results revealed that the critical pore size threshold necessary to induce cracking stress due to crystalline pressure in the microstructure was approximately 20 to 25 nm based on the SAXS analysis. The main outcome of this study was to recognize the pore size responsible for the mass expansions of certain mortars subjected to DEF under different heat curing conditions in the long-term of up to 4000 days.