Influence of fly-ash, silica fume, and slag on sulfate resistance of concrete

The paper presents results of an investigation on the sulfate resistance of concrete containing different levels of fly ash, silica fume, or ground-granulated blast furnace slag to partially replace ordinary portland cement. The total cementitious content was 350 and 450 kg/m(3) and the water/cementitious materials ratio was 0.45. The porosity and pare structure of representative pastes of the matrix were measured using mercury intrusion porosimetry, and the extent of carbonation of these pastes was also determined by spraying them with phenolphthalein solution. Specimens were cured initially for 14 days after casting under different temperatures (20 and 45 C) and humidity (25, 55, and approximately 100 percent) before immersion in a sulfate solution. The results show that cement replacement by 22 and 32 percent weight of fly ash produced maximum sulfate resistance; the sulfate resistance being superior in initially air-cared specimens, compared with initially wet/air-cured specimens. The inclusion of 5 to 15 percent silica fume also resulted in a great improvement in sulfate resistance, even though the intruded pore volume and proportion of large pores of diameter > 0.1 mu m were found to increase with the use of silica fume. An 80 percent replacement of cement by ground-granulated blast furnace slag increases the sulfate resistance of concrete, whereas 40 percent replacement has a contrary effect. In a 40 percent ground-granulated blast furnace slag blended mix, a lower pore volume and finer pore structure, under initial wet/air-curing at 45 C, 25 percent relative humidity, does not result in higher sulfate resistance, campared with the control mix. Similarly, a higher extent of carbonation under initial air-curing does not lead to higher sulfate resistance.