Similarity between alkali-aggregate reaction and the natural alteration of rocks

This article aims to demonstrate that the mechanisms governing the natural alteration of rock also lead to the development of alkali-aggregate reaction. We have used the alteration of granite to pinpoint two processes that are often involved. In the first, existing minerals are transformed into minerals with expansive properties (such as the transformation of biotite into chlorite and expansive smectite). In this case, the expansion occurs in the structural layers of the mineral and may cause relatively localized deterioration in the original material. Ln the second process, the mineral constituents are initially hydrolyzed in the surrounding water, which gradually becomes saturated with silica, alumina, and various cations. New phases evolve from this water, yielding gels or well-crystallized products. In the case of the granite investigated, the product is laumontite (Al2O3. 4SiO(2). CaO . 4H(2)O), which is formed from the water in the cracks. The crystallization of this zeolite generated significant cracking in the samples examined. We have demonstrated experimentally that similar type of zeolite (Al2O3. 2SiO(2). CaO . 5H(2)O) forms readily when a mixture of lime and metakaolin is reacted with water in the presence of sodium hydroxide. This mixture contributes the silica, alumina, and cations required to initiate the reactions. (C) 2001 Elsevier Science Ltd. All rights reserved.