| The addition of calcium sulphate to Portland cement clinker during milling prevents flash set of the cement on addition of water. However, in addition to controlling setting time, sulphate has also long been known to improve compressive strength and reduce drying shrinkage. Unfortunately, too much sulphate can result in detrimental expansive effects. Thus, the optimum sulphate content maximizes strength while minimizing expansion and shrinkage.;This investigation studies the effects of incremental additions of sulphate on four cements of varying tricalcium aluminate and alkali contents. Optimum sulphate contents are determined based on maximum compressive strength, minimum expansion in water and minimum drying shrinkage. A relationship between porosity and compressive strength is found, but results from pore-size distribution and pore solution analyses are inconclusive. The current standard test method for optimum sulphate, ASTM C563, and the current CSA and ASTM limits on sulphate additions are evaluated. Finally, the use of empirical equations is examined. |
