Factors Affecting the Ability of a Fly Ash to Contribute to the Sulfate Resistance of Fly Ash Concrete
- PDF / 173,176 Bytes
- 2 Pages / 420.48 x 639 pts Page_size
- 83 Downloads / 232 Views
FACTORS AFFECTING THE ABILITY OF A FLY ASH TO CONTRIBUTE TO THE SULFATE RESISTANCE OF FLY ASH CONCRETE G.J. McCARTHY,a P.J. TIKALSKY,b R.L. CARRASQUILLOb O.E. MANZC and A. THEDCHANAMOORTHYa a. Department of Chemistry, North Dakota State University, Fargo, ND 58105 b. Ferguson Structural Engineering Laboratory, University of Texas, Austin, Texas 78758 c. Energy and Mineral Research Center, University of North Dakota, Grand Forks, ND 58202 Received 28 March, 1989; communicated by R.T. Hemmings SUMMARY The objective of this summary is to report on work in progress that is examining parameters, measurable through chemical and XRD analyses, that could indicate whether a fly ash will enhance, degrade or have no effect on the sulfate resistance of fly ash concrete. Mehta [1-4] has discussed the factors that contribute to attack of sulfates on fly ash concrete. As noted in his review paper on this subject in the preceding volume in this series [1], the agents responsible for concrete expansion and cracking are alumina-bearing hydrates, such as calcium monosulfoaluminate and calcium aluminate hydrate, that are attacked by the sulfate ion to form ettringite, calcium trisulfoaluminate. Acidic type interactions between sulfate ions and calcium hydroxide also lead to strength and mass loss. When trying to decide whether a particular fly ash will be suitable for making concrete in a sulfate susceptible environment, one would like to be able to predict how the fly ash and cement will interact to form alumina-bearing hydrates. Furthermore, it would be clearly desirable if, for each of the types of Portland cement (whose sulfate resistance behavior is well known), this prediction could be based on readily measurable parameters of the fly ash itself. Dunstan [5] was the first to attempt this prediction. He defined an R-Factor, which considered only the analytical CaO and Fe 2 0 3 determined by chemical analysis to predict sulfate resistance. Although Dunstan [5] recognized that certain chemical compounds were the likely source of the problem (he suggested C 3 A, C 4 A3 S, and gehlenite-rich glass that forms strdtlingite on hydration), his R-Factor formula does not call for measuring any of these compounds. In fact, missing altogether from this R-Factor is the alumina concentration of the fly ash. As noted by Mehta [1], Manz et al. [6] have argued that the fact that some ashes contain substantial amounts (10-20 wt%) of nonreactive crystalline aluminates such as mullite and melilite, and that much of the iron oxide in a fly ash is present as nonreactive hematite and magnetite, any R-Factor must consider the mineralogy (chemical compounds) as well as the chemical composition of the ash. Manz et al. [6] also point out that the amount of sulfate in fly ash must also play a role, as this is available to react with alumina-bearing hydrates before the concrete sets. Mehta [4] has also made the case for the necessity of considering mineralogy of fly ash in predicting sulfate resistance, but he believes that it is the cement-fly ash interacti
Data Loading...
