Changes in the chemical composition of silica fume in the concrete composite system

Abstract

The paper demonstrates a spontaneous change in the micro-chemical composition of silica fume micropellets in a composite system of concretes with pozzolans admixtures. For 18 years, concrete was modified in laboratory conditions only with the use of drinking water. Using quantitative techniques of scanning electron microscopy (SEM) and microanalysis (EDX), the paper presents results of a hydration analysis of concrete prepared according to three different recipes from Portland cement mixed with silica fume and slag. Hydration and changes in the chemical composition of the silica fume micropellets were evaluated over a time-frame of 0, 5, 15, 18 years. The paper describes “spontaneous pure hydration”, during which compaction and change of the micro-chemical composition, density and chemical composition occur in two different morphology types of silica fume micropellets. The experiment confirmed the preventive effect of silica fume micropellets on the course of expansion reactions in concrete structures. Even after 18 years, no expansive alkaline silica gels were identified in the concrete. Silica fume has a positive effect on the speed of the expanding alkali-silica reaction, which, even at very low content (2 %), it slows down.

Keywords

• Silica fume
• SEM
• EDX
• ASR
• Concrete