The present invention is directed to a fly ash composition, and more particularly, to a fly ash composition including lithium carbonate which has a reduced susceptibility to alkali-aggregate reactivity based when used in cementitious compositions.
Fly ash comprises finely divided inorganic products produced by the combustion of coal. Enormous amounts of fly ash are produced annually nationwide, principally from burning coal in electric power plants. Disposal of fly ash poses an increasingly difficult problem because the volume, the particulate nature and the varied chemical composition of fly ash limit the number of acceptable disposal sites. Such sites require sophisticated and expensive engineering, design, construction and operational controls to be in place to manage and dispose of the fly ash. Furthermore, the capacity of these disposal sites is not unlimited.
Fly ash of particular composition has been used as an additive in portland cement but such cements have not been hydraulic, i.e., self-setting. Rather, the fly ash has been added to strengthen the cement, evidently, by reacting with excess lime and by reacting with sulfate compounds which would otherwise attack the tricalcium aluminate and tetracalcium aluminoferrite compounds of portland cement.
U.S. Pat. No. 4,382,649 to Heitzmann et al. describes a cement composition which contains fly ash. The composition contains from 50 parts to about 80 parts portland cement; from 13 parts to about 35 parts fly ash; and from 1 part to about 5 parts potassium carbonate. The composition may additionally include up to about 10 parts metakaolin; up to about 6 parts slag; and up to 4 parts of an admixture. However, such a composition may still be subject to alkali silica reactivity.
Chemical reactions between alkalies and aggregates in concrete mixtures result in a shorter useful life and deterioration of structures formed from the concrete mixture. The alkali compounds in the mixture react with certain aggregates in the concrete resulting in an increased pH of the pore solution in the concrete. For such reactions to take place, the concrete must be subjected to a certain amount of moisture during service.
Concrete is essentially a two component system comprising a paste portion and an aggregate portion. The paste portion is a product of the hydration reaction between the cementitious materials and water. The paste portion is formed from a matrix of solid material with an internal network of interconnected pores. When water enters these pores, it moves through the pores solubilizing salts of calcium and alkali metals. As these salts are dissolved, the pH of the pore solution increases as a result of an increase of hydroxide ions in the pores. As the hydroxide ions react with reactive silica in the aggregate to solubilize the silica, a gel is produced which may cause the concrete to crack. Researchers believe that this cracking is a result of the gel absorbing water.
In order for alkali-aggregate reactivity to become problematic, three factors are necessary. First, reactive forms of silica must be present in the aggregate. Amorphous, or noncrystalline and poorly crystalline silica, such as volcanic glasses and opal, are the most reactive forms. Other rapidly reacting forms include tridymite, cristobalite and chalcedony. Second, the concrete pore solution must have a high alkali content. This is because the hydroxide ion concentration increases as the alkali content increases. Third, sufficient moisture must exist to initiate the reactivity. Concrete which contacts the ground or water or is periodically wetted most likely will contain sufficient moisture to cause reactivity.
Thus, a need has developed in the art for a hydraulic fly ash composition which can be used for a variety of purposes and which is less susceptible to alkali silica reactivity.