Chloride binding capacity of metakaolin and nanosilica supplementary pozzolanic cementitious materials in aqueous phase

Abstract

This work reports the differences in chloride binding capacity as well as formation of hydrated calcium silicate and hydrated calcium aluminium silicate gels in aqueous phase of calcium hydroxide and sodium chloride with different pozzolanic materials such as metakaolin and nanosilica or blends of both. The resulting solid materials, are characterized by differential thermal analysis and thermogravimetry, X-ray diffraction, Fourier transform infrared spectroscopy and Confocal Raman Spectroscopy. Metakaolin favours the chemical chloride binding capacity through the formation of Friedeĺs salt and it has higher trend to form C-A-S-H gel than C-S-H one. With simultaneous presence of both pozzolanic additions, nanosilica, due to its very high surface, prevents the access of metakaolin to chlorides, avoiding the chemical chloride binding, and therefore, the consequent formation of Friedel's salt. Moreover, the presence of high reactive nanosilica could change metakaolin structure due to a coordination change of Al forming bounds Al-O-Al similar to polymerized alumina leading a decrease in metakaolin reactivity, lowering its chloride binding capacity. C-S-H gel formed in blend only with nanosilica shows very high tendency to carbonation.