The Roles And Regulation Of Gypsum In Portland Cement With Large Amounts Of Supplementary Cementitious Materials

Gypsum is an important component of Portland cement,which has positive effect on hydration and hardening performance of Portland cement.Usually the positive role of gypsum in Portland cement is mainly reflected in adjusting early setting,improving cement strength and compensating drying shrinkage.The supplementary cementitious materials(SCMs)are also important components of cement and concrete,which could improve the performance of Portland cement and concrete,and reduce the cost during production.There are many types of SCMs used in cement concrete,most of which are industrial by-products with high amounts of reactive Al₂O₃ and Si O₂.In practical applications,the amount of SCMs in cement could get high level,and part of SCMs was even introduced in concrete production,so the amount of SCMs in concrete would be further increased.When high amounts of SCMs were introduced into cement and concrete,the amounts of the aluminate phase in cement concrete increased,the roles and impact of gypsum would change.Although gypsum plays a positive role in promoting the strength development of Portland cement with SCMs,the SO₃ content is still strictly limited.The understanding of the interactions between gypsum and the SCMs was mostly qualitative and lacked of quantitative results.When high amounts of SCMs are introduced into cement,the amounts of reactive aluminate phases will increase,and the positive roles of gypsum have a great relationship with the amount of aluminate phases,so it is necessary to further understand the role of gypsum in cement with consideration of the relationship between gypsum and the aluminate phase in SCMs.In this paper,two representative SCMs,blast furnace slag and circulating fluidized bed combustion fly ash(CFBC ash),were selected to study the positive effect of gypsum when large amounts of SCMs were introduced in blended cement.The roles of gypsum in pure Portland cement were studied with consideration of alkali sulfates and C₃A in Portland clinkers.The SO₃ content of Portland cement was adjusted by adding gypsum into practical clinkers,and alkali sulfates were also added.The results showed that when the alkali sulfate in clinker was less than a certain content(Na₂O_e<1.0),it would not influence the positive effect of gypsum on adjustment of early setting,strength enhancement and the maintenance of volume stability.Considering that the roles of gypsum was closely related to C₃A,the interaction between C₃A and gypsum was studied.The results indicated that gypsum content could appropriately increase with the increase of C₃A content in Portland cement.Contour maps were designed to determine the optimal proportion of slag-gypsum-clinker system by comprehensive considerations of strength,expansion,hydration,microstructure and crystallization stress.The results indicated that that there were two independent regions with relatively high strength in the contour map of compressive strength,and the blended cements in the two corresponding regions exhibited low and controllable expansion values.In the first region,the blended cement was composed of60%?75%slag,0?10%gypsum and 20%?40%clinker.The blended cement in another region with higher compressive strength consisted of 70%?85%slag,15%?25%gypsum and<5%clinker.The XRD?SEM and MIP results indicated that an increase of slag and gypsum in a certain range promoted the formation of ettringite and C-A-S-H,which could improve the strength development and maintain the volume stability.By comparing the amount and crystallization stress of ettringite,microstructure of hardened cement paste of slag-gypsum-clinker system,the results indicated that the expansion deformation was mainly caused by the ettringite crystallization stress,and there presented critical gypsum content which would rapidly increase the expansion deformation.It could be concluded that when high amounts of slag were introduced into cement,gypsum could correspondingly increase.The fly ash and CFBC ash were selected,and the ternary CFBC ash(fly ash)-anhydrite-clinker system was designed to determine the optimal proportions of blended cement by comprehensive considerations of strength,expansion,and hydration products.The strength contour map indicated that there were also two separate regions with higher cement strength in the strength contour.The first region corresponded to the CFBC ash blended cement,which mainly composed 50%?60%CFBC ash and 40%?50%clinker.The blended cement in another region with higher compressive strength consisted of60%?75%CFBC ash,5%?12%anhydrite and 20%?25%clinker.The XRD and SEM results revealed more ettringite formed and mainly deposited in pores,which was beneficial to the strength enhancement and volume stability.The fly ash-anhydrite-clinker system was also designed,and the hydration products were determined by XRD.The phase assemblage revealed that the anhydrite content could correspondingly increase with the increase of fly ash in Portland cement.Finally,the synergistic effect of gypsum and aluminum phase was studied,and the gypsum content in blended cement with large amounts of SCMs was optimized.By comprehensively analyzing the relations among SO₃/Al₂O₃ molar ratio,the hydration products,strength development and expansion deformation,the SO₃/Al₂O₃ molar ratio could be introduced as the parameter for gypsum optimization.Generally,the SO₃/Al₂O₃should be no less than 0.6 for enhancement of high early strength;when SO₃/Al₂O₃ was around 1.0,the formation and stability of ettringite in later ages could be ensured,as well as the long-term strength development;and the SO₃/Al₂O₃ molar ratio of 1.0?1.2 was recommended to ensure the coexistence ettringite and monosulfate,which could further benefit for strength enhancement and volume stability.Although the type and amount of aluminum phases were different in different SCMs,it was believed that the roles and amount of gypsum in blended cement could be optimized by controlling the molar ratio of SO₃/Al₂O₃,and the SO₃/Al₂O₃ molar ratio of 0.8?1.2 was recommended for safely and effectively increase of SCMs and gypsum in cement and concrete.