Time-Dependent Behavior of Cementless Concrete-Like Material Based on Oil Shale Fly Ash Binders in Ambient Air

Abstract

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