Experimental Study On The Performance Regulation Of High Belite Sulphoaluminate Cement

With the deepening of China’s sustainable strategy,the development of new green cementitious materials,high-performance concrete and other building materials has attracted more and more attention.As a low-carbon cementitious material,high belite sulphoaluminate cement（HBSAC）has the properties of high strength,rapid setting,low hydration heat,low shrinkage and excellent durability.It is widely used in rush repair and construction,anti-corrosion and impermeability projects.However,HBSAC has some problems,such as too short setting time,low alkalinity,and further improvement of later strength,which has become the bottleneck hindering for its wide application.In order to effectively control the hydration properties of HBSAC and expand its application range,this paper has conducted experimental research on the modification of HBSAC.Three new composite cementitious material systems,i.e.HBSAC-RA system,HBSAC-CN system and HBSAC-CH system,were constructed by adding three modified components of new composite retarder（RA）,calcium nitrite（CN）and calcium hydroxide（CH）to HBSAC respectively.The physical and mechanical properties,hydration characteristics and durability of these composite cementitious material systems were systematically studied by the combination of macro performance test and microstructure analysis.The main research contents and conclusions are as follows:（1）The influence mechanism of the composite retarder on the hydration of HBSAC is to slow down the dissolution rate of gypsum and inhibit its early hydration with C₄A₃S,so as to retard the setting of cementitious materials.With the addition of composite retarder,in terms of physical and mechanical properties,the water consumption of the standard consistency of the cementitious material is reduced,the setting time is prolonged,and the early strength decreases,but later strength can be significantly improved by the composite retarder with the appropriate amount of composite retarder In addition,in terms of durability,the dry shrinkage of cementitious materials increases,but the sulfate attack resistance is enhanced.Considering the above properties,the modification effect of HBSAC is the best when the composite retarder content is 1.0%,the initial setting time and final setting time are extended to 75.9 min and 88.5 min respectively,and the 28 days’flexural strength and compressive strength are 1.16 times and 1.48 times of HBSAC respectively.（2）The influence mechanism of calcium nitrite on the hydration of HBSAC is to improve the dissolution rate of early minerals,and increase the content of early ettringite,so as to accelerate the setting and hardening of cementitious materials.With the addition of calcium nitrite,in terms of physical and mechanical properties,the water consumption of standard consistency of cementitious materials changes little,the setting time is shortened,and the improvement effect of early strength is very little,but the development of middle strength and late strength have obvious advantages.In addition,in terms of durability,the dry shrinkage of cementitious materials is reduced,and the sulfate attack resistance is enhanced.Considering the above properties,the modification effect of HBSAC is the best when the content of calcium nitrite is 1.0%～1.5%,the maximum 28 days strength increases to 1.43 times of HBSAC,and the dry shrinkage rate is about 50%of HBSAC.（3）The influence mechanism of calcium hydroxide on the hydration of HBSAC is to delay the early hydration process by inhibiting the early dissolution of C₄A₃S and changing the crystal morphology of AFt,which is conducive to the later hydration of cementitious materials.With the addition of calcium hydroxide,in terms of physical and mechanical properties,the water consumption of standard consistency of cementitious materials increases and the setting time is prolonged,but the change range is small;the early strength decreases,while the middle and late strength is better improved,and the 7days strength is up to 1.29 times of HBSAC.In addition,in terms of durability,the dry shrinkage of cementitious material increases,and its sulfate attack resistance is obviously weakened,when the content of calcium hydroxide is 4.0%,the strength loss of mortar specimen is significant.