The behavior of silicocarbonatite aggregates from the Montreal area

In a previous paper, it was concluded that silicocarbonatite aggregates from the Francon quarry, Montreal contributed to durability problems in Portland cement concrete. Results show that, at 2 days after casting, concrete made with silicocarbonatite aggregates contained over 1.5% more Na₂O than similar bars made with Exshaw limestone aggregates. A reaction involving the rare mineral dawsonite in the silicocarbonatite is thought responsible for the higher Na₂O content. In turn, this caused increased expansion of concrete bars made with alkali expansive aggregates. Also, concrete made with alkali-carbonate reactive Pittsburg aggregate showed more expansion when cured at 80 °C than bars cured at 23 °C. Concrete bars made with Exshaw limestone aggregates cured for 4 h at 85 °C showed late-stage expansion, which is attributed to delayed ettringite formation (DEF). However, no expansion was shown by heat-cured concrete prisms or mortar bars made with silicocarbonatite aggregates. Release of alkalis, aluminates and carbonates by the dawsonite reaction may have inhibited DEF. Concrete bars made with nonreactive Nelson dolostone and 10% silicocarbonatite cured at 80 °C for 4 h showed up to 0.15% expansion after several years at 23 °C and 100% relative humidity (R.H.), indicating that a deleterious reaction did occur.