Chemical stability of cementitious materials based on metakaolin

The alkali activation of metakaolin is a way of producing high strength cem entitious materials. The processing of these materials has been the subject of numerous investigations. The present paper describes the results of a r esearch project initiated to study the stability of these materials when ex posed to aggressive solutions. Prisms of mortar made of sand and alkali-act ivated metakaolin were immersed in deionized water, ASTM sea water, sodium sulfate solution (4.4% wt), and sulfuric acid solution (0.001 M). The prism s were removed from the solutions at 7, 28, 56, 90, 180, and 270 days. Thei r microstructure was characterized and their physical, mechanical, and micr ostructural properties were measured. It was observed that the nature of th e aggressive solution had little negative effect on the evolution of micros tructure and the strength of these materials. It was also found that the 90 -day and older samples experienced a slight increase in their flexural stre ngths with time. This tendency was most pronounced in those samples cured i n sodium sulfate solutions. This behavior may be related to the change in m icrostructure of the cementitious matrix of the mortars cured longer than 9 0 days. Some of the amorphous material present had crystallized to a zeolit e-like material belonging to the faujasite family of zeolites. (C) 1999 Els evier Science Ltd. All rights reserved.