Chloride-Ion Attack on Low Water-Cement Ratio Pastes Containing Silica Fume


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Title: Chloride-Ion Attack on Low Water-Cement Ratio Pastes Containing Silica Fume

Author(s): R. Gagne, M. Pigeon, E. Revertegat, and P. C. Aitcin

Publication: Special Publication

Volume: 132


Appears on pages(s): 1471-1490

Keywords: cements; chloride ions; durability; microstructure; pastes; porosity; silica fume; water-cement ratio; Materials Research

Date: 5/1/1992

The chloride-ion attack on low water-cement ratio pastes containing silica fume was studied by soaking small paste disks in four different pH-controlled sodium chloride solutions for periods of up to 12 months. The pastes were made using water-cementitious material ratios of 0.30 and 0.25. Three types of cementitious materials were used: an ASTM Type III cement (Canadian CSA Type 30), the same Type III cement with 6 percent silicafume, and a French CPA-HPR cement with 6 percent silica fume. The four solutions in which the paste disks were soaked were the following: 3 percent NaCl (by weight) at a pH of 13.0, 3 percent at 11.5, 0 percent at 13.0, and 0 percent at 11.5. The curing period was fixed at 28 days for all mixtures. Mercury intrusion porosimetry, x-ray diffraction, scanning electron microscopy, and electron microprobe measurements were the techniques used to study the various samples after removal from the solutions. The chloride-ion attack on these low water-cementitious material ratio pastes was always very small, even after 12 months of exposure to 3 percent NaCl solutions at pH values of 13.0 and 11.5. After several months of exposure at a pH of 13, only very small amounts of chloride ions ( 1 percent) were detected and only minor changes to the microstructure were noted. At a pH of 13.0, the penetration of chloride ions was not found to be a function of the paste characteristics [w/(c + sf), type of cement, silica fume content]. The major parameter controlling chloride-ion penetration in low water-cementitious material ratio pastes is the pH of the NaCl solutions. When the pH is 11.5, the penetration of chloride ions into the pastes is easier because of the leaching of calcium ions creating a very fine microporosity. For this relatively low pH, it was found that the use of lower water-cementitious material ratios and silica fume can reduce the amount of chloride ions that can penetrate the cement paste.