THE ALKALI-SILICA REACTION .1. USE OF THE DOUBLE-LAYER THEORY TO EXPLAIN THE BEHAVIOR OF REACTION-PRODUCT GELS

An understanding of the expansion mechanisms resulting from the alkali -silica reaction is essential to the assessment of the susceptibility of a concrete structure to deterioration by these processes and to the planning and implementation of preventive measures As a result of the alkali-silica reaction between certain reactive aggregates and the hi ghly alkaline pore fluids in a cement paste, a reaction-product gel de velops that in the presence of water expands and may cause cracking of mortar or concrete. To explain the volume change behavior of mortar b ars containing a reactive aggregate, a theoretical model is proposed i n this paper The expansion of the alkali-silica reaction-product gels is attributed to swelling caused by electrical double-layer repulsive forces. For a given colloidal system, double-layer theory indicates th at the larger the valence of the counterions in the double layer; or t he larger the concentration of these ions, the smaller the double-laye r thickness and the repulsive forces that may be generated in the pres ence of water (aqueous solution). Experimental results available in th e literature support the double-layer model. According to these result s, the expansion of mortar bars in the ASTM C 1260 test is related to the composition of the reaction-product gels. The reaction-product gel s containing larger amounts of equivalent sodium oxide (Na2Oe) and sma ller CaO/Na2Oe cause larger expansions in the mortar bars.