Weakening of Alkali-Activated Metakaolin During Aging Investigated by the Molybdate Method and Infrared Absorption Spectroscopy

Aluminosilicate gels were synthesized by mixing metakaolin with potassium water glass solution. Mechanical strength values showed a strong increase between 25 and 100 h but decreased again above 200 h for aging at 25 degrees C. A similar curve was observed during aging at 50 degrees C but the maximum strength shifted to shorter time. Aging at 90 degrees and 130 degrees C revealed only the decreasing wing in mechanical strength. The strengthening and weakening during aging at 25 degrees C were further investigated by the molybdate method and infrared absorption spectroscopy. The results indicate basically that different time-dependent formations of two structural units dominate the mechanical properties: (i) A fast formation of longer (polymeric) silicate chains is enforced by the consumption of hydroxide in the solution of metakaolin up to about 25 h of aging. (ii) An aluminosilicate network grows slowly and encloses the chain-like units giving rise to an initial increase in strength within 100 h. During further aging the destruction of the polysilicate chains weakens the geopolymer. The destruction of the chain-like units is explained by the increasing amount of hydroxide produced during network condensation. The results imply that the same reactions occur for aging at a higher temperature but on faster time scales.