Sammendrag
Portland cement is Portland clinker interground with calcium sulphate mainly as set retarder, even though it is also known that sulphates accelerates the hydration rate of alite and partly counteracts early shrinkage by forming ettringite in reaction with C3A etc. Calcium sulphates may be in the form of anhydrite, hemihydrate or dihydrate (i.e. gypsum). In the present study, ground Portland clinker was blended with 3.5% gypsum as the regular dosage serving a reference and compared to the same clinker where added 2% calcium nitrate, formate or acetate dissolved in the mixing water to make a paste of w/c 0.40. The flow curve as a function of time until 1.5 hours was measured, as well as the rate of hydration heat under isothermal conditions.The resistance to flow for the pastes with other calcium salts than gypsum was lower than for the paste with gypsum as reflected in lower Bingham viscosity, and was constant or decreased with time in opposition to the increase with time for the paste with gypsum. This effect was attributed to growth of ettringite needles on surface of the cement grain in the case of gypsum that will have a negative impact on the flow (e.g. slump loss) in addition to the high water binding by ettringite compared to the assumed formation of AFm like phases by the other calcium salts being responsible for the set regulation of C3A.The isothermal calorimetry showed that the characteristic double peak of AFt to AFm conversion was absent in the case of other calcium salts than gypsum, but also several other different features. For instance was the cumulative hydration heat the first 24 h equal for 2% calcium formate and 3.5% gypsum, but significant lower for 2% calcium nitrate and acetate. The latter effect might imply that a low energy Portland cement can be made by ground clinker and calcium nitrate or acetate as set regulators, without changing clinker properties. Another implication is that cement with other calcium salts than gypsum may be allowed to