Date of Award
Doctor of Philosophy
Civil, Construction, and Environmental Engineering
The available alkalis of four Class C and two Class F fly ashes were evaluated according to ASTM C 311. The effects of alkalis in three Class C fly ashes on expansion of mortar bars with high and low-alkali cements were studied using the procedures described in ASTM C 441. The purpose of this research was to study: (1) the significance of the standard available alkali test for analyzing different fly ashes, (2) the possibility of reducing the time required to complete the test, (3) the significance of the standard mortar bar test in determining the degree to which high and low-alkali cements could be replaced by Class C fly ashes, (4) effect of fly ash alkali contents on alkali reactivity, and (5) the ability of Class C fly ash to improve concrete's resistance to external alkali attack;The results indicated that the available alkali test described in ASTM C 311 tends to underestimate the ultimate available alkali content in Class C fly ash. An empirical equation, %Eq. Na(,2)O = -0.1486 + 28.6153 (N/S), was developed to estimate the ultimate concentration of available alkalis in fly ash or of active alkalis in cement fly ash paste from the known total equivalent Na(,2)O/SiO(,2) (N/S) mole ratio. More than 85% of total equivalent alkalis is mobilized into the pore solution when the CaO/SiO(,2) mole ratio is greater than 1.75. It was also found that increasing curing temperature to 55(DEGREES)C is not a suitable way for decreasing curing time needed for an available alkali test. The results also indicated that replacements of small amounts of high-alkali cement with Class C fly ash resulted in increased expansion as compared to plain cement mortars. Replacements of low-alkali cement with Class C fly ash up to 40% also increased expansion. A critical N/S mole ratio was identified and found to characterize alkali reactivity. The replacement of the cement with fly ash was found to provide a degree of protection from external alkali attack. Expansions caused by external alkali attack were found to be inversely related to the N/S mole ratio of the cementitious materials.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Lee, Chau, "Available alkalis in fly ash and their effects on alkali-aggregate reaction " (1986). Retrospective Theses and Dissertations. 8264.