Effect Of Alkali Sulfates On Early Hydration And Hardening Properties Of Portland Cement

Sulfate is an important minor component of Portland cement,including protogenetic sulfate such as alkali sulfate,sulfate in silicates and aluminates and anhydrite,and exogenetic sulfate such as added gypsum,which has important impact on setting time,development of compressive strength and volume stability.In the past,the production quality of cement was usually controlled by the total SO3 content,so as to give the content of sulfate content with a positive role,and to avoid the quick set,degradation of volume stability and durability of concrete induced by the introduction of excessive sulfur content in the cement production.However,the different types and contents of sulfate in cement and their different influences on cement hydration and hardening properties did not be taken into account.With the innovation of cement production technology and the wide use of high sulfur coal,the content of protogenetic sulfate which maily occurs as alkali sulfate increased significantly.Due to the difficulty in detection of the type and content of protogenetic sulfate,there is a lack of understanding the phases,content and function of protogenetic alkali sulfates especially alkali sulfate in cement.The adverse impact of alkali sulfate on the properties of cement is studied by using chemical reagents,which is difficult to reflect the practical impact of protogenetic alkali sulfate in cement.It is the prerequisite for the control of alkali sulfate content in cement production,the adjust of properties of fresh concrete and the function of admixtures,to accurately analyse the quantitative distribution of alkali sulfate in cement and their impact on cement early hydration and hardening properties.The ethanol method that inhibits the reacting of dissolved SO42-and dissolving of gypsum in a short time,can be used to measure the content of alkali sulfate in clinker,cement and mineral admixtures,which is more effective than 2-min water-dissolving method.Saturated soda method that promotes the dissolving of gypsum and inhibits its reacting,can be used to measure the content of gypsum in cement and mineral admixtures.The quantitative distribution of sulfates in 60 of Portland cements,26 clinkers and more than 40 of mineral admixtures which were collected from cement plants were analysed through optimized measuring method of sulfate,ethanol method,saturated soda method and Taylor empirical formula method.The result shows that the protogenetic sulfate in cement and clinker is mainly alkali sulfate which is high in clinker than cement.There is a low content of alkali sulfate in fly ash and CFBC ash.The effects of alkali sulfates which were prepared in the laboratory and added into low SO3 clinker on cement hydration and hardening properties were studied.It is found that arcanite(K2SO4),thenardite(Na2SO4)and aphthitalite(K3NS4)do not significantly shorten the setting time of cement,and promote 1-day compressive strength of mortar,but significantly reduce the compressive strength at 28 and 90 days,and increase drying shrinkage of mortar.Compared to gypsum,Ca-langbeinite(KC2S3)will reduce the compressive strength of mortar,and increase drying shrinkage and water expansion.But,Ca-langbeinite can still replace gypsum to adjust the setting time,to promote the development of compressive strength,and to compensate drying shrinkage.It can be concluded that the adverse effects of arcanite,thenardite and aphthitalite are obviously,which cannot replace gypsum to play a good role even though including in total SO3 of cement.Hydration product of AFm is formed in cement added with Ca-langbeinite compared with the AFt induced by gypsum.The function of alkali sulfate in cement would be affected by cement composition,cement fineness and curing conditions.Considering the C3 A content,cement fineness and curing conditions,the functions of alkali sulfates in cement were studied,then the optimum content of alkali sulfate in cement was proposed.The results show that mortars have high compressive strength at early and later ages,lower(less than 0.1%)drying shrinkage and water expansion ratios when the molar ratio of [Na2Oeq][C3A]/[SO3]2 should be less than 0.5.It is found that [Na2Oeq][C3A]/[SO3]2 molar ratio can provide guidance for the improvement of cement performance by analysing strength of cement samples.With the increased fineness of cement,the increase of alkali sulfate or Na2 Oeq would delay the initial setting time of cement,but decrease compressive strength at ages of 3,28 and 90 days,and increase the drying shrinkage.Thus,the content of alkali sulfate should be lower with increasing of cement fineness.Under the condition of heat curing at high temperatures of 55 oC and 85 oC,28-day compressive strength of mortar decreases with the increase of alkali sulfate,and may even be lower than that cured at room temperature.With heat curing at 20 oC and 55 oC,water expansion ratio of mortar is low,but higher with curing at 85 oC.The content of alkali sulfate should be as lower as possible for steamed concrete products and mass concrete.With the widely use of minerals such as fly ash and slag in cement,the effects of alkali sulfates on the hydration and hardening properties of cement with fly ash and slag were studied to guide the practical application of cement.The result shows that compressive strength of mortar added with fly ash and slag at the ages of 28 d and 90 d decreases,and the drying shrinkage of mortar increases with the increase of alkali sulfate in cement.The adverse effects on strength and drying shrinkage would decrease with the use of fly ash and slag.That can be attributed to the dilution of the alkali sulphate with the use of mineral admixtures and the activating effect of alkali sulphate on fly ash and slag.The effect of Ca-langbeinite on the early hydration and hardening properties of cement with fly ash and slag is not obvious.It should be noticed that the adverse effect of alkali sulfate on early hydration and hardening properties of cement is obvious.In practical,the impact of alkali sulfate should be noticed,especially for the adjustment of properties of fresh concrete and the function of admixtures,when [Na2Oeq][C3A]/[SO3]2 mole ratio > 0.5,even though the adverse effect of alkali sulfate can be reduced by the use of fly ash and slag.