Time-dependent behavior of cementless concrete-like material based on oil shale fly ash binders in ambient air

The time-dependent behavior of cementless concrete-like materials in ambient air was assessed by checking long-term strength of laboratory specimens on the basis of high calcium oil shale (HCOSFA) binders as well as X-Ray Diffraction and Scanning Electron Microscopy, studying composition and microstructure of cured binder material. It was established that there are two distinct stages in the strength development of specimens containing only HCOSFA as a binder: An increase of compressive strength for the first month and the continuous drop until the 60th month of exposure. In specimens prepared with using HCOSFA and low calcium coal fly ash (LCCFA) binder, three stages of strength change were identified: An increase of compressive strength for the first month, a sharp drop thereafter until the third to sixth month followed by a gradual loss or stabilization over a period of 60 months. It was resolved that binder hydration results in the formation of ettringite and gypsum together with other compounds. It was found that ettringite is not stable in ambient air and over time the amount of ettringite was diminished. This disintegration resulted in a continuous loss of strength in specimens containing only HCOSFA. The addition of LCCFA to HCOSFA generated an additional amount of high-strength, stable secondary CSH in the hardened system. The secondary CSH helped to neutralize consequences of ettringite transformation and improved the performance of cementless concrete-like materials in ambient air.