Effect of Slag on Expansion Due to Alkali Aggregate Reaction in Concrete

Abstract

The efficacy of ground granulated blast furnace slag in controlling expansion caused by alkali-aggregate reaction was evaluated using both concrete prism and accelerated mortar bar testing. Six aggregates were included in the program; four of these were classified as alkali-silica reactive (siliceous limestone, sandstone, greywacke, and granite), one as alkali-carbonate reactive (dolomitic limestone), and one as nonreactive (dolostone). The partial replacement portland cement with slag in the range of 25-65% was effective in retarding the rate of expansion and limiting the ultimate expansion at 2 years in concrete prisms cast with all four alkali-silica reactive aggregates. However, the minimum level of slag required to control expansion to an acceptable level, (for example, 0.04% at 2 years) was found to vary depending on the nature of the aggregate and the amount of alkali present in the concrete. There appears to be a reliable correlation between the expansion of mortar bars after 14 days storage in 1M NaOH at 80 deg C and the expansion of concrete prisms after 2 years storage over water at 38 deg C. The accelerated mortar bar test appears to be an appropriate tool for determining the minimum safe level of slag required for a particular reactive aggregate source. The use of slag also reduced the initial rate of expansion of concrete prisms containing the alkali-carbonate reactive (ACR) aggregate. However, the expansion at 2 years exceeded 0.20% for all slag levels tested (25-65%), and this final expansion was actually greater for concretes with 25-50% slag than for the control concrete. The expansion of mortar bars was less than 0.10% at 14 days for all the mixes tested, which confirms the unsuitability of this test for detecting ACR aggregates.