Properties of Gypsum-free Portland Cement

【Author】

Harald Justnes;

【Abstract】

<正>Calcium sulphate(e.g.gypsum) is the common set regulator in Portland cement.It has been shown that ground clinker can be set regulated by other calcium salts than gypsum that has sufficient solubility,in particular calcium acetate,formate,nitrate,nitrite and propionate.Finely divided calcium carbonate is not sufficiently soluble to act as a replacement for gypsum. Rheology of paste is improved when gypsum is replaced by the other salts,probably due to less water binding and less needle shaped surface hydration products than ettringite (i.e.AFm type products). For in particular calcium nitrate the cumulative hydration temperature is reduced, which opens up for low-energy cement without changing clinker chemistry.The effect has been documented by curing of mortar under semi-adiubatic conditions and comparing the temperature profile for 3.5%gypsum and 2.0%calcium nitrate, respectively,as set retarders for the ground clinker. Removing calcium sulphate all together may also allow heat curing or evolution of hydration heat in massive structures to exceed 70℃without risking delayed ettringite formation(DEF).Mortar based on clinker with 3.5%gypsum and 2.0%calcium nitrate, respectively,was heat cured at 80℃for 3 days.Thereafter linear expansion was monitored as a function of time.The expansion was larger for the mortar with gypsum than for the mortar with calcium nitrate,probably caused by delayed ettringite formation(DEF) in the former. Originality The originality of this research is the finding that gypsum can simply be be replaced by nearly any other souluble calcium salt to regulate setting time.Replacing calcium sulphate with other calcium salts as setting regulator in Portland cement may imply; 1.Less water demand for equal rheology due to no ettringite formation in the fresh state. 2.Calcium nitrate replacing gypsum leading to less heat of hydration first 24 h means that low-energy cement can be produced without changing clinker chemistry. 3.No gypsum in the system can open up for heat curing above 70℃improving productivity in element production,or allowing temperature increase above 70℃in massive structures removing need for cooling,without risking delayed ettringite formation(DEF) leading to later expansion and cracking.

【Keywords】

chemical admixtures;;gypsum;;low energy cement;;delayed ettringite formation;;rheology

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