The Chemical Shrinkage of Pozzolanic Reaction Products

The total chemical shrinkage of silica fume and Class F fly ash, both as pozzolanic materials reacting with lime and as mineral additives replacing portland cement, was studied. By increasing pH, the rate related to the pozzolanic reaction decreased for silica fume and increased for fly ash. Although the presence of alkalis are catalytically necessary for a rapid pozzolanic reaction of silica fume, the pH increase reduces the solubility of calcium hydroxide (CH) due to the common ion effect. This may explain why the reaction rate decreases if dissolution of CH, followed by precipitation of CSH, is the rate-limiting step. The increased reactivity of fly ash, caused by a pH increase, indicates that the dissolution of the glassy aluminosilicate phase by alkalis was determining the overall rate of the process. The total chemical shrinkage was crudely estimated to be 8.8 ml/100 g of reacted silica fume and 10.0 ml/100 g of reacted fly ash, as compared with 6.3 ml/100 g of portland cement. The measured shrinkage for silica fume could be higher than the above value since minor amounts of silicon metal in the silica fume could produce an expansion due to evolved hydrogen gas.