Role of an amorphous silica in portland cement concrete

An amorphous silica (AS) replacing up to 10% of the portland cement by mass in concrete behaved partly as a traditional pozzolan, partly crystallized to quartz and the rest underwent some (crystal) structural reorganization without morphological changes. The pozzolanic behavior was evident from a reduction in the calcium hydroxide content of AS-cement paste with time. A new crystalline phase was observed in the scanning electron microscope only when AS was present, and energy dispersive X-ray microanalysis coupled with X-ray absorption near edge structure spectroscopy confirmed that it was the most stable form of silica, quartz. The presence of AS changed the concrete's microstructure, aggregate-paste interface, and pore size distribution.