MECHANISM OF ALKALI-AGGREGATE REACTION

AN INTERFEROMETER PROCEDURE HAS BEEN DEVELOPED TO DETERMINE THE ATTACK ON CHARACTERISTIC ACTIVE AND NONREACTIVE AGGREGATES OVER AN EXTENDED PH RANGE, IN SOLUTIONS OF SODIUM HYDROXIDE, CALCIUM HYDROXIDE, SODIUM HYDROXIDE AND CALCIUM HYDROXIDE, AND AQUEOUS EXTRACTS OF HIGH AND LOW ALKALI CEMENTS. MICROSCOPIC STUDIES OF REACTIVE AGGREGATES IN HIGH ALKALI CEMENTS AND PHOTOGRAPHS SHOW HOW MAJOR REACTION TAKES PLACE INSIDE THE OPAL GRAIN RATHER THAN ON THE OUTSIDE. IT IS CONCLUDED THAT THE ALKALI-AGGREGATE DISINTEGRATION IS INITIATED BY THE ION EXCHANGE REACTION BETWEEN FREE WATER, MIGRATING SODA AND THE REACTIVE AGGREGATE. THIS REACTION BRINGS TOGETHER THE COMPOUNDS REASSURING FOR THE FORMATION OF A HYDRATED SODIUM SILICATE COMPLEX OF MUCH GREATER VOLUME THAN THE ORIGINAL AGGREGATE. CONTINUED SWELLING MAY THEN BE CAUSED BY THE OSMOTIC ACTION OF THE CEMENT PASTE AND BY INHIBITION OF WATER BY THE HYDRATED SODIUM SILICATE COMPLEX. /AUTHOR/