Comparison study on the sulfate attack resistivity of cement-based materials modified with nanoSiO2 and normal SCMs: Pore structure and phase composition

The effects of nanoSiO(2) (NS) on the sulfate attack resistivity of cement paste, including the pore structure and the phase assemblage, were studied in this work, by using which the macro-property development of the previous work was explained. Comparison studies were made between silica fume (SF), fly ash (FA) and ground granulated blast-furnace slag (GGBS). Paste samples were initially cured for 7- and 28-day before being immersed in 5 wt% Na2SO4 solution for another 180 days. For the 7-day initially cured samples, NS-added samples showed the largest porosity (31.59% of 1 wt% NS, 25.94% of 3 wt% NS) and critical pore diameter (59 nm of 1 wt% NS, 46 nm of 3 wt% NS), while the control samples showed 21.22% and 40 nm after sulfate attack for 180 days. The best performance shown by the 7-day initially cured NS added cement mortar in previous work could be due to the filling of the pores by the expansive constituents as well as the pozzolanic hydrates of NS. For the 28-day cured samples, smallest porosity (15.48% of 1 wt% NS, 13.99% of 3 wt% NS) and critical pore diameter (8 nm of 1 wt% NS, 9 nm of 3 wt% NS) after sulfate attack were shown by the NS-added samples even though a slight reduction of expansive products was shown when compared with SF-, FA- and GGBS-added samples due to the smaller dosage. A higher crystallization pressure resistance of NS-added sample could be accounted for the better sulfate attack resistivity, which can also be highlighted by the mechanical strength development and the volume stability study of the previous work. This study shows the feasibility of improving the sulfate attack resistivity of cement-based materials by improving the pozzolanic reactivity of the SCMs. (C) 2019 Elsevier Ltd. All rights reserved.