Alkali – aggregate reaction and internal release of alkalis

Portland cement concrete may deteriorate because of expansive reactions between certain types of aggregates and alkalis. The alkalis are mainly derived from the cement, but it has also been postulated that some aggregates contain minerals that contribute alkalis by internal reactions and so cause increases in expansion due to alkali–aggregate reaction, Dawsonite, a rare mineral found in volcanic silicocarbonatite dykes and sills in the Montreal area, has been found to react with Ca(OH)₂. Expansion of concrete due to alkali–carbonate reaction and alkali–silica reaction increased when the alkali–reactive coarse aggregates were partially replaced by a dawsonite-containing silicocarbonatite. Chemical analysis showed increases in the alkali content of the concrete test-specimens. NaOH is a product of the proposed dawsonite reaction, which also indicates an increase in solid volume that probably accounts for the early age expansion of normally non-expansive control prisms. Comparison also showed that some prisms subjected to accelerated steam curing had higher early-age expansions than fog-room cured prisms, implying that a short exposure to elevated temperature accelerated the dawsonite and alkali–carbonate reaction. Delayed ettringite formation is probably a factor in late-age expansions of samples subjected to accelerated curing.