Effect of ammonium nitrate solution on the structure and strength of cement pastes containing silica fume

Ordinary Portland cement pastes containing 0, 5, 10 and 15% of condensed silica fume (csf) were investigated after different times of hydration up to 250 days and after immersion in ammonium nitrate solution. Changes in compressive strength and pore size distribution were monitored and the oligomeric-polymeric fraction in the silica anion size dispersion was estimated.

The nitrate solution attacks the structure of the pastes, temporarily increasing the proportion of AF_t and AF_m phases, and increases the polymerization of the silica anions. X-ray diffraction of the pastes immersed for a long time showed that amorphous calcium silicate hydrates were formed and peaks were identified as belonging to the ferrite phase. In immersed OPC/csf pastes, some minor crystalline components were found which were derived from the added silica fume.