Title: Effects of Silica Fume on Expansion Characteristics of Expansive Cement Pastes
Author(s): Colin Lobo and MenashiI D. Cohen
Publication: Materials Journal
Appears on pages(s): 481-490
Keywords: cement pastes; clinker; evaporation; expansion; porosity; pore-size distribution; silica fume; sulfate resistance; water; Materials Research
Effects of silica fume on the expansion characteristics of Type K expansive cement paste are presented. An accelerated expansion was observed at an early age (3 days), and the expansion period was shortened considerably when silica fume was added to the expansive cement pastes. A result of the shortened expansion time was that the paste microstructure-repairing mechanism of the damage associated with the expansion process was more efficient. Mercury intrusion porosimetry analysis of pastes indicated that the addition of silica fume does not decrease the cumulative porosity of the paste, measured at 60,000-psi (414-MPa) intrusion pressure, but the threshold pore diameter is shifted to a finer size. The observation suggested that a more impermeable concrete could result due to silica fume addition. Another advantage of adding silica fume was that the delayed expansion typically seen with coarser expansive clinkers was prevented. Besides the reduction in permeability and, therefore, improved durability of concrete, the shortening of expansion time due to silica fume addition can result in reducing the required curing period (i.e., from 7 to 4 days). The high sensitivity of expansion characteristics of pastes on the fineness of the expansive clinkers is also reduced. The sulfate resistance of unrestrained expansive cement pastes with silica fume was also studied and found to be much better than expansive cement pastes without silica fume.