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For more than 30 years the Cement and Concrete Association publication Winter concreting has provided practical guidance on whether or not concreting operations should proceed when freezing weather is forecast. However, the basic guidance curves do not adequately cover contemporary strength grades of concrete, or the use of composite cements, and modern Portland cements tend to differ from those produced when the curves were derived. To remedy this deficiency, the original Bergström model describing the thermal behaviour of newly cast concrete has been adapted for computerization and applied to mix designs in current use. Strength development and heat of hydration measurements were also carried out in order to produce new guidance curves covering 30, 40 and 50 N/m2 strength grades, and concretes made with: OPC; 70:30 OPC:PFA; and 40:60 OPC:GGBS. Discrepancies between the new and old curves are attributed mainly to the different maturity functions employed in the calculations.

It has been shown that the reaction between aluminium powder and the aqueous phase of cement pastes is chiefly characterised by a definite inhibition period when no hydrogen evolves, followed by a slow evolution of gas. The utility of aluminium powder in reducing the early shrinkage of cement pastes depends upon this gradual reaction. The inhibition is specifically associated with the presence of ettringite on the surface of aluminium particles. It is suggested that the ettringite is formed by reaction of calcium sulphate with the hydrous oxide film on aluminium particles: termination of a period where no hydrogen is evolved, probably results from a more or less complete conversion of aluminium oxide to ettringite.

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