Sulphate Resistance of Slag Cements

Abstract

<p>The vulnerability of concrete to sulphate attack has been recognized and studied for at least two centuries. The basic needs that prompted that study were: (a) there are no available Canadian Standards on the sulphate resistance of slag cements; and (b) slag cements in general are not well known in Canada. There were three main phases in the study: Phase One - involved 76.2 by 152.4 mm (3 by 6 inch) concrete cylinders made from actual field mixes and subjected to various sulphate solutions representative of field conditions; Phase Two - involved an accelerated test program employing 25.4 and 25.4 by 285.75 mm (1 by 1 by 11½ inch) mortar prisms and 50.8 mm (2 inch) mortar cubes stored in more concentrated sulphate solutions; and Phase Three - followed along the same lines as Phase Two, but encompassed a wider range of samples, solutions and tests. The slag cement studied in detail was cementitious hydraulic slag meeting CSA A363. Experimental results show that: the sulphate attack is generally intensified with higher sulphate concentrations but the degree of attack is not proportional to concentration; Na₂SO₄ and K₂SO₄ solutions produced more rapid deterioration than MgSO₄ solutions; sulphate attack increases in rate and magnitude as the C₃A content of the portland cement increases and as the alumina (Al₂O₃) content of the slag cement (i.e. pelletized blastfurnace slag) increases; the higher the slag cement content of the blended cement, the greater its resistance; and a 50 per cent slag cement (cementitious hydraulic slag)/50 per cent Type 10 portland cement blend appears to be equivalent in sulphate resistance to a Type 50 portland cement. No abnormal, or significant, amount of ettringite was found in any of the specimens examined, which tends to substantiate one hypothesis that ettringite formation has little to do with sulphate expansion. The 76.2 by 152.4 mm concrete cylinder program did not yield any significant results. The accelerated mortar test procedure was found to be the most reliable test for evaluating a cements' sulphate resistance performance in a relatively short time, and it is recommended that this test be adopted as a standard.</p>