Determination of free silicon content in silica fume and its effect on volume of gas released from mortars incorporating silica fume

This paper presents the results of a laboratory study to determine the free silicon content of different silica fume samples, and its effect on the vo lume of gas released from mortars incorporating silica fume. The effect of the water-cementitious materials ratio (w/cm) of the mortars was also inves tigated. The free silicon content of the silica fumes nas determined by measuring th e volume of hydrogen gas released from the reaction of silica fume in sodiu m hydroxide solution. For the 25 silica fume samples tested, the gas genera ted from the reaction of the silica fume with the sodium hydroxide solution at 70 C ranged from 4.1 to 10.6 mt per gram of silica fume after approxima tely 24 h. The free silicon content of the silica fume was calculated and r anged from 0.23 to 0.60% by mass of the silica fume. The calculation was ma de assuming that the gas generated was 100% hydrogen, and all the hydrogen gas teas generated by the reaction of free silicon with the sodium hydroxid e solution. For mortars incorporating 10% silica fume and I to 5% free sili con addition by mass of the silica fume (w/(cm+SF) = 0.30 to 0.50), no gas release was detected from mast mortar mixtures for approximately 6 h after casting at 22 C; the gas evolution ceased after approximately 4 to 5 days;f or mortars with w/cm of 0.35, the volume of the gas released increased gene rally with an increase in the silicon content in the mortars. For mortars containing 10% silica fume and a 3% silicon addition fly mass o f the silica fume, the amount of hydrogen gas released increased with an in crease in the w/cm. At a low w/cm of 0.30, the hydrogen gas released was ne gligible. The hydrogen gas released in the mortars kept at 22 C far approxi mately, a month was 3.1% bf the maximum amount generated in the NaOH soluti on.