Title:
Determination of Free Silicon Content in Silica Fume and
Its Effect on Volume of Gas Released from Mortars
Incorporating Silica Fume
Author(s):
Min-Hong Zhang, V. Mohan Malhotra, and John Wolsiefer, Sr.
Publication:
Materials Journal
Volume:
97
Issue:
5
Appears on pages(s):
576-586
Keywords:
mortar; silica fume; water-cementitious materials ratio
DOI:
10.14359/9290
Date:
9/1/2000
Abstract:
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 volume of gas released from mortars incorporating silica fume. The effect of the water-cementitious materials ratio (w/cm) of the mortars was also investigated. The free silicon content of the silica fumes was determined by measuring the volume of hydrogen gas released from the reaction of silica fume in sodium hydroxide solution. For the 25 silica fume samples tested, the gas generated from the reaction of the silica fume with the sodium hydroxide solution at 70 C ranged from 4.1 to 10.6 mL per gram of silica fume after approximately 24 h. The free silicon content of the silica fume was calculated and ranged from 0.23 to 0.60% by mass of the silica fume. The calculation was made assuming that the gas generated was 100% hydrogen, and all the hydrogen gas was generated by the reaction of free silicon with the sodium hydroxide solution. For mortars incorporating 10% silica fume and 1 to 5% free silicon addition by mass of the silica fume (w/(cm+SF) = 0.30 to 0.50), no gas release was detected from most mortar mixtures for approximately 6 h after casting at 22 C; the gas evolution ceased after approximately 4 to 5 days; for mortars with w/cm of 0.35, the volume of the gas released increased generally with an increase in the silicon content in the mortars. For mortars containing 10% silica fume and a 3% silicon addition by mass of the silica fume, the amount of hydrogen gas released increased with an increase in the w/cm. At a low w/cm of 0.30, the hydrogen gas released was negligible. The hydrogen gas released in the mortars kept at 22 C for approximately a month was 3.1% of the maximum amount generated in the NaOH solution.