Composition of alkali silica gel and ettringite in concrete railroad ties: SEM-EDX and X-ray diffraction analyses

Abstract

Alkali silica gel and ettringite observed in concrete railroad ties were analysed by SEM-EDX and X-ray diffraction. The compositions, represented in the Si CaNa + K system, of clear gel were in the range of 53 to 63 atomic% Si and 20 to 30 atomic% Ca. The K + Na contents were constant and about 15% (excluding oxygen, water and trace elements). Analyses of crystallized gel showed a constant ratio between Si, Ca and Na + K of 70: 15: 15. Analyses of the Si and Ca content, of crystallized gel in cracks inside the aggregate particles, and clear gel at the mouth of the cracks and in air voids revealed a compositional difference with the crystallized gel as higher Si and lower in Ca. The content of Si decreases and Ca increases in gel away from the aggregate particles. The gel compositions are in the range published in the literature. The composition of ettringite is close to stoichiometrical values for ettringite, however, slightly lower in sulfur. The X-ray diffraction analyses identified the ettringite as crystalline.