THE INFLUENCE OF CEMENT PORE SOLUTION ON ALKALI-SILICA REACTION
Abstract
The alkali-silica reaction, which may cause expansion and sometimes cracking of concrete, takes place between alkali (sodium and potassium), usually derived from the cement, and certain forms of silica that may occur in the aggregate. The goal of the study described here, which concerns the influence of the composition of the aqueous solution preseent in the pores of hydrating mortar, is to improve our understanding of certain details of alkali-silica reaction mechanisms. Compositions of the pore solutions were varied through the selection of cements with various alkali contents and distributions. Expansion studies were carried out using these cements and certain reactive aggregates. The expansion studies indicated an influence of the cement alkali distribution on expansion due to alkali-silica reaction. Effects of the chemical composition of pore solutions on expansion were studied directly. Pore solutions were expressed from limestone (control) mortars prepared from the same cements, and concentration levels of various ionic species were measured. The solutions were shown to consist primarily of hydroxyl and alkali ions, with pH levels ranging from 13.4 to 14.0. When composition results were compared with the expansion results of mortars containing opal, the levels of hydroxyl and alkali ions were shown to influence the expansion. A threshold in pH was observed, between 13.65 and 13.83, below which there was little or no expansion, and above which there was significant expansion. In addition, a direct correlation was observed between expansion and the level of either sodium or sodium plus potassium. Reactions of the aggregates in model pore solutions were studied and compared with expansion levels produced by the aggregates in mortars. The reactions were shown to produce various levels of dissolved silica and reduction in pH, and the levels of silica correlate directly with the mortar-bar expansion levels produced by each aggregate and proportion. Thus expansion appears to be a function of the extent of the reaction of aggregate in pore solution, which depends on the material, its proportion, and the solution pH level, and is further influenced by the amount of sodium or sodium plus potassium in the pore solution.
Degree
Ph.D.
Subject Area
Civil engineering
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