Role Of Alkali In The Hydration And Hardening Process Of Low Alkali Cement

Gypsum and alkali have significant influence on the early hydration and hardening process of Portland cement.Present studies generally believe that gypsum can delay cement hydration and prolong the setting time,while alkali accelerates the early hydration process and deteriorates the workability and rheological properties of cement.Although the alkali content in cement is increasing in the overall trend,Chinese vast territory leads difference.Some areas' cement clinkers contain unusually low alkali content.The setting time of these low alkali clinkers cannot be prolonged by gypsum or worse,its even shortened sharply,leaving the setting time fail to meet the minimum requirements of national standards.In this paper,the effects of gypsum and alkali on the hydration and hardening process of low alkali cement were explored,the cause of the abnormal setting were studied.Practical measures to control the setting and rheological properties of low alkali cement were proposed as well.The effects of different kinds and addition of gypsum on the setting time of low alkali clinker(Na2Oe content: 0.10%)were analyzed by mixing of gypsum and low alkali clinker and comparing with high alkali clinker(Na2Oe content: 0.64%,1.08%)whose setting time can effectively adjusted by gypsum.Isothermal calorimeter,X-ray diffractometer and viscometer were used in the experiments.The results show that gypsum,natural anhydrite,soluble anhydrite and Ca-langbeinite all lead to abnormal setting.Gypsum will advance the second exothermic peak of cement paste.Besides,gypsum added to low alkali clinker can not effectively participate in the hydration reaction of cement as unable to improve the rheological properties and early-age compressive strength of low alkali cements.The effects of thenardite and arcanite on the hydration and hardening process of low alkali cement were studied by adding gypsum and alkali sulfate into the low alkali clinker.The results indicate that an appropriate amount of alkali sulphate can greatly promote the reaction of gypsum with cement compounds,postpone the exothermic peak of cement paste and reduce the heat release,prolong the setting time,increase the fluidity and improve the rheological properties of the cement paste.Moreover,the addition of thenardite and arcanite both enhance the strength of low alkali cement.The threshold value of the alkali to sulfate molar ratio in the cement(ie,the molar ratio of Na2 Oe / SO3)is discussed based on the effect of thenardite and arcanite on the early-age performances of low alkali cement.When the alkali to sulfate molar ratio of the cement is less than 0.30,gypsum alone can not retard the setting time,the fluidity and viscosity of the slurry almost have no improvement.While adding an appropriate addition of alkali to improve the ratio can significantly promote the gypsum to play the retarding role;when the alkali to sulfate molar ratio of the cement is higher than 0.60,further increase of alkali equivalent will lead to abnormal settting and considerable deterioration of the rheological properties of the cement paste.The effects of thenardite,sodium hydroxide and sodium carbonate on the early hydration process of low alkali cement were compared by adding those alkalis to the cement made with low alkali clinker and gypsum respectively.Incorporation of sodium hydroxide to the cement with the alkali to sulfate molar ratio kept below 0.60 can prolong the setting time of the cement paste,while sodium carbonate shortens the setting time.Different forms of alkali all can improve the early-age rheological properties of low alkali cement,increase the pH value of cement paste,decrease the concentration of K+ in the liquid phase and the concentration of Ca2+ in the early stage of hydration.The AFt and C-S-H gel amount at 2 h were also promoted.Through the above research,it is clear that an appropriate amout of alkali added to increase the alkali content of low alkali cement can significantly improve its setting and hardening process as well as rheological properties.Specifically,Na2Oe/SO3 molar ratio is meaningful in optimizing the performance of fresh concrete and the application of admixtures.