Pozzolanic reactivity of nanoscale pyrogene oxides and their strength contribution in cement-based systems

This paper deals with the pozzolanic reactivity of pyrogene oxides (represented by Aerosil) that belong to the colloidal silicas. Pyrogene oxides are highly reactive pozzolanic nanoscale additives. They react much faster pozzolanically than other commonly used pozzolans and in this way they consume the available calcium hydroxide in a much shorter time. It is shown by using in situ X-ray transmission microscopy, pH-change measurements and the strength property results that Aerosil begins to react as early as the first hour of contact with both the added calcium hydroxide in solution and with the products of the tricalcium silicate (C₃S) hydration reaction. In comparison with the other commonly used pozzolans, that is, silica fume, pyrogene oxides contribute by developing a better early and final strength. The concrete specimens that were cast by combining silica fume with small quantities of Aerosil were considered to belong to the ultra high performance class and were additionally characterised by finer pores and reduced total porosity.