The beneficial effect of pozzolanas in suppressing the alkali-silica reaction

An investigation has been made of the effect of replacement of part of the ordinary Portland cement (OPC) in an OPC/silica glass (SG) mortar by pulverized fuel ash (PFA), precipitated silica (PS) and ground granulated blastfurnace slag (ggbs). The expansion of the mortar due to alkali-silica reaction is reduced, and it is found in all cases that alkali dilution due to the replacement can play only a minor role in the reduction of expansion. It is hypothesized that a major mechanism of this reduction is an increase in the effective SiO₂/Na₂O ratio to values in excess of the pessimum value; it is found that the greater is the deviation of this ratio from the pessimum value, the less is the expansion, the order of effectiveness of the replacements being PS > PFA > ggbs. The replacements are more effective in reducing the expansion due to alkali-silica reaction than they are in reducing the loss in bending strength of mortar bars. Observations by transmission electron microscopy of the microstructure of PFA/OPC/SG mortars show that the glass particle surfaces develop a layer of CSH reaction product of lower thickness than that of equivalent mortars without replacement, and that the PFA particles also develop a surface layer of CSH reaction product, but of different morphology to that of glass particle reaction product layers.