Wednesday, 11 September 2019
Austroads has released a report which concludes that it would be inadvisable to entirely replace the currently used Australian general-purpose cement which contains up to 7.5% limestone, with cement containing 12% limestone.
Australian Standard AS3972:2010 specifies that general-purpose cement may contain up to 7.5% limestone as a mineral addition. The cement industry has proposed that general-purpose cement contain up to 12% limestone without changing the cement designation from general-purpose or providing alternative cements in the market with lower limestone content.
Austroads commissioned research to investigate whether adoption of general-purpose cement containing 12% limestone could be justified with respect to performance and durability.
The project compared the properties of concrete made with a general-purpose cement containing 2.9% limestone with concrete incorporating cement with 12% limestone. Physical, mechanical, chemical and microstructural aspects of both plain cement concrete, and concrete incorporating cement blended with fly ash and slag, were tested.
Generally, there was no significant difference in compressive strength of corresponding pairs of concrete. However, the concrete containing cement mixed with 12% limestone was inferior regarding drying shrinkage, modulus of elasticity and creep behaviour.
Resistance to carbonation and chloride ingress was significantly lower in the plain cement with higher limestone content, particularly in concrete steam-cured at 80 °C. This indicated that concrete made with cement containing 12% limestone was less durable and unsuitable for aggressive exposure conditions such as those found in coastal environments.
The differences in durability were reduced when cements were blended with 25% fly ash or 65% slag.
Resistance to sulfate attack was similar and adequate for moist cured concretes made with the two plain cements at a water/cement ratio of 0.40. However, steam-cured concrete showed much lower resistance to both sulfate and delayed ettringite formation when made with cement containing 12% limestone compared with cement containing 2.9% limestone.
The results of the study indicate that it would be highly inadvisable to entirely replace the currently used Australian GP cement, which may contain up to 7.5% limestone and complies with AS 3972:2010, with the cement proposed by the industry that would contain 12% limestone, without the option of having cements of up to 7.5% limestone addition readily available in the market to all cement and concrete users.