Study On The Volumetric Stability Of Portland Cement Based On The Control Of Crystal-to-gel Ratio

In this paper,anhydrous calcium sulfoaluminate and high temperature gypsum are used as control materials to systematically study the relationship between the crystal-to-gel ratio of hydration products of different compositions in Portland cement body-based cementitious materials and their mechanical properties,volume stability,and stress development.Laws and principles.It has certain guiding significance and practical value for preparing Portland cement-based cementitious materials with stable volume.Based on the adjustment of the crystal-to-gel ratio of hydration products,20groups of cementitious materials with silicate clinker as the base material,high-temperature gypsum and anhydrous calcium sulfoaluminate as the control materials were designed to cure different contents of Portland cement.The materials（95%,90%,85%,80%,75%）and control materials with different CS/C₄A₃S molar ratios（2,5,8,11）are orthogonally designed.The mechanical properties,free expansion rate,restricted expansion rate,stress,etc.of the formulated cementitious material composition were tested,and combined with X-ray diffractometer（XRD）,differential scanning calorimeter（TG-DTG）,scanning electron microscope（SEM）and mercury intrusion（MIP）and other test methods,to study the correlation law between the crystal-to-gel ratio of hydration products of different cementing materials and their properties,and use the microscopic features（hydration products,microscopic morphology,pore size distribution）to further analyze The corresponding mechanism is discussed.The results show that:（1）The crystal-gel ratio increased with the increase of high temperature gypsum and anhydrous calcium sulphoaluminate in the system.In the same age,the effect of the smaller hydration product gel ratio on the strength of cement is not obvious.The strength of cement can be improved by appropriately increasing the hydration product gel ratio,and the strength is the highest when the ratio is 0.151.However,when it exceeds a certain range,the strength will drop sharply.For the sample with a large crystal gel ratio,cracks will appear on the surface of the specimen after 7d,skin will rise and the specimen will be completely crushed in the later period.When Portland cement clinker is 90%and CS/C₄A₃S molar ratio is 11,the strength of cement specimen can reach 80.4MPa.There was no correlation between the crystal-gel ratio and strength of hydration products at different ages.（2）The free expansion rate is positively related to the limit expansion rate in water and the crystal-to-gel ratio.For the samples with CS/C₄A₃S molar ratios of 8,11,the limit swelling rate drop of the specimens after 7 days of water-cultivated to dry air has a good match with the cement strength.For the sample with a total content of10%of high-temperature calcined gypsum and anhydrous calcium sulfoaluminate,the limit expansion rate of the specimen is the closest to 0,the volume is the most stable,and the strength is the highest.The ettringite generated under this crystal-to-gel ratio condition is used to fill the pores,so that the microstructure is relatively dense and the porosity is the lowest.When converted to dry air,the capillary and gel pores lose less water,resulting in shrinkage.The smaller it is,the more stable the volume and the higher the strength.（3）During the same age,the crystal-to-gel ratio is positively correlated with ettringite crystal expansion stress and free expansion.The compressive strength first increases and then decreases as the crystal-to-gel ratio increases.When the crystal gel is relatively low,with the extension of the hydration age,the crystal gel ratio becomes lower,the expansion stress decreases,the free expansion rate increases,and the strength increases.Mainly due to the lack of sulfur in the environment,ettringite will be converted into AFm,The content of ettringite is reduced,and the expansion stress is reduced.At this time,the main effect on expansion is Ca（OH）₂.When the crystal-to-gel ratio gradually increases,the ettringite gradually increases,the expansion stress increases,and the free expansion rate gradually increases.The ettringite that plays a major role in the expansion becomes the ettringite.When the expansion stress is greater than the tensile strength of the material,new pores will appear,the structure will be loose,and the strength will decrease.