ASTM C 1240 Standards

ASTM C 1240

The version discussed here is the 2004 edition of the specification. A copy of the cover page is presented in Figure 4.2. Each of the mandatory, optional, and report only items is described below.

Silicon dioxide (SiO₂) content (mandatory). This requirement calls for a minimum SiO₂ content of 85%. Because SiO₂ is the reactive ingredient of silica fume, a limit on the content is deemed appropriate. Other standard specifications (for example, Canada) allow the use of silica fume with a SiO₂ content less than 85% after appropriate testing in concrete.

Moisture content (mandatory). This requirement limits maximum moisture content to 3%. The intent here is simply to minimize the amount of moisture that is brought along with the silica fume.

Loss on ignition (mandatory). This requirement limits maximum loss on ignition to 6%. Fly ashes have had this requirement for many years because of the potential for partially combusted coal particles being included in the fly ash. This coal can be of a form with a very high surface area, which significantly increases the demand for air entraining admixture in air-entrained concrete. It is not clear whether any coal of this same nature is present in silica fume, so the LOI requirement is more of a control on any unburned coal or other material from the electric-arc furnace.

Oversize material (mandatory). This requirement limits the amount of oversize material retained on a 45-μm (No. 325) sieve to a maximum of 10%. There is a further requirement that the maximum variation from average be no more than 5 percentage points. As noted in Chapter 2, silica fume is an extremely fine material and a sieve analysis will not provide any significant information on particle size or surface area. This requirement is aimed at minimizing the amount of foreign material in the silica fume. Such material could include uncombusted materials from the furnace or rust particles from the silica fume collection system.