Water, acid, and calcium carbonate pretreatment of fly ash: The effect on setting of cement-fly ash mixtures

The treatment of class C, I, and F fly ash (FA) with water, HNO3, and aqueous CaCO3 has been investigated to develop a simple chemical route to change the morphology and surface chemistry of fly ash particles to enhance the setting properties of a cement/fly ash (C/FA) composite. The treatment of C-FA with an aqueous CaCO3 solution results in a dramatic improvement in the setting time and the setting profile on C-class FA; in contrast, the treatment has no effect on the set time for F-FA and reduces the set time and appears to result in an even more nonideal induction setting curve as compared to the untreated C/I-FA. The enhancement observed for the treatment of C-FA with aqueous CaCO3 solution is not a consequence of the water solution since simply washing with water (i.e., C-FA(H2O)) results in the extraction of Na and Ca with a concomitant increase in surface area and a performance similar to those observed for untreated I-FA and F-FA despite a much higher surface area. The acid (HNO3) treatment of I-FA and F-FA results in the formation of an inert filler-like material, while acid treatment of C-FA results in a material with completely undesirable setting properties. Clearly, the enhancements observed for the aqueous CaCO3 treatment are not as a result of simply either the aqueous or acidic nature of the HCO3 Containing CaCO3 solution. Based upon the forgoing, we propose that the efficacy of the aqueous CaCO3 treatment on C-FA is associated with the availability of "reactive calcium". Exposure of C-FA to dry CO2 does not affect the set time or set profile for C/C-FA mixture, but the retarding effect of the aqueous CaCO3 treatment on C-FA can be replicated by the exposure of the C-FA to a stepwise reaction with water and CO2. Exposure of C-FA to wet CO2 results in the improvement of the setting induction profile without significantly affecting the set time. We propose that this process should allow for the addition of fly ash to cement samples while the induction profile remains flat, providing predictability in pumping for down-hole applications.