The occurrence of duplex films on flyash surfaces

Abstract

This note reports observations made on several flyash-bearing Portland cement pastes in which duplex films apparently are found to occur around all visible grains, including the flyash particles. The films consist of a thin 0.5 ..mu..m uniform continuous layer of calcium hydroxide deposited quickly on the exposed surface, plus a thin single layer of parallel, more-or-less widely spaced C-S-H gel particles. The pastes were 0.5 pastes prepared from 50 wt percent flyash-50 wt percent Portland cement mixtures hydrated for 1 day at room temperature. Examination suggests that the coating that develops around these flyash grains, which should be inert at the short period of hydration involved, is in fact the same kind of calcium hydroxide -C-S-H gel duplex film previously shown to form on any inert surface in contact with solutions developing in hydrating cement paste. The long term cementitious reaction, associated with the late strength gain of such flyash-containing concretes must take place through this skin, presumably by diffusion of material in or out of the grain through it. It appears that the same hydration shells may represent the effects of deposition from solution onto surfaces of grains that are chemically inactive. 3 figures. (DP)