Effect Of Anhydrous Gypsum On Hydration And Microstructure Evolution Of Cement At Low Water-Binder Ratio

Nowadays,with the rapid development of infrastructure industry,the requirements for the strength and durability of concrete are getting higher and higher,and high-performance concrete with low water-binder ratio and ultra-high-performance concrete are developing rapidly.In cement production,gypsum dihydrate is usually added to adjust setting.However,with the natural gypsum dihydrate resources being exhausted in China,natural anhydrite and a large number of industrial by-products gypsum are also used to produce cement.Due to the gradual application of coal-fired ash with high anhydrous gypsum content in cement,the content of anhydrous gypsum in cement is increasing year by year.In high-performance concrete with low water-binder ratio,the dissolution of gypsum in cement will be affected,which may lead to changes in the hydration process and hydration products of cement,and it is very likely that there will be unhydrated gypsum in cement stone.If these residual gypsum is hydrated in the later stage of hardened concrete,it may induce the expansion and cracking of cement concrete materials and affect its long-term performance.Therefore,it is of great theoretical guiding significance and practical application value to explore the hydration process of anhydrous gypsum in cement and its influence on the microstructure evolution of cement paste at low water-binder ratio for the safe application of high strength and high performance concrete.In this thesis,the influence of anhydrous gypsum on the macro-properties,hydration process and microstructure evolution of cement at low water-binder ratio is studied.By comparing the effects of anhydrous gypsum on cement properties such as fluidity,setting time,compressive strength and volume stability under different water-binder ratios and different gypsum contents,the influence law of anhydrous gypsum on cement properties under low water-binder ratio is analyzed.By studying the influence of anhydrous gypsum on the hydration degree and hydration products of cement at low water-binder ratio,the mechanism of the influence of anhydrous gypsum on cement at low water-binder ratio is revealed.By studying the effect of anhydrous gypsum on the microstructure,pore structure and micro-mechanical properties of cement paste at low water-binder ratio,the potential harm of anhydrous gypsum at low water-binder ratio is explored.The main conclusions are as follows:（1）The effect of anhydrous gypsum on the macro-properties of cement is not different from that of dihydrate gypsum at a large water-binder ratio.At a low water-binder ratio of 0.2,the retarding effect of anhydrous gypsum on cement is obviously lower than that of dihydrate gypsum.Increasing the content of anhydrous gypsum in cement can improve the retarding effect of gypsum at a low water-binder ratio,but the compressive strength of cement may shrink in the later stage of hydration,and the expansion of cement will increase in the later stage.This shows that the cement containing anhydrous gypsum is difficult to be used in high performance concrete with low water-binder ratio.（2）Different from gypsum dihydrate,the water-binder ratio has a more significant effect on the hydration process of anhydrous gypsum in cement.At the low water-binder ratio of 0.2,the solubility of anhydrous gypsum decreases,which leads to the acceleration of cement hydration and the formation of more hydration products such as C-S-H gel and calcium hydroxide,but there will be anhydrous gypsum left in cement.The more gypsum content in cement,the greater the residue of anhydrous gypsum,and the delayed ettringite will be generated in the later stage of hydration,thereby increasing the cracking risk of high performance concrete with low water-binder radio.（3）The influence of anhydrous gypsum on the microstructure of cement paste is different under different water-binder ratios.The lower the water-binder ratio is at the same gypsum content,the larger the most accessible pore size is in the early stage,and the microhardness of cement paste decreases in the later stage of hydration.At the low water-binder ratio of 0.2,the higher the content of anhydrous gypsum in cement,the coarsening of the most accessible pore size of cement paste in the later curing period will be gradually realized,and the microhardness of cement paste will be reversed,which will lead to the deterioration of the structure,therefore,the application of anhydrous gypsum cement in high performance concrete with low water-binder radio is limited.（4）Based on the influence of anhydrous gypsum on the macro-properties,hydration process and microstructure of cement,it is found that at a high water-binder ratio（0.3 and 0.4）,when the total SO₃ content of anhydrous gypsum in cement is2.5%～3.5%,it can give full play to the beneficial effect of retarding and strengthening of anhydrous gypsum.At low water-binder ratio（below 0.2）,the content of anhydrous gypsum in cement should be strictly controlled due to the possible risks caused by the residue of anhydrous gypsum.