Research On Properties Of Cenospheres Lightweight Cement-based Composites

With the development of super-large engineering and the improvement of seismic requirements,higher requirements are put forward for the lightweight,high strength and toughness of cement-based composites.Although reactive powder concrete has better strength,durability and flexural toughness than ordinary concrete,its bulk density is too large to meet the requirements of light weight and high performance of materials under specific circumstances,which restricts the universality of application.However,the strength and performance of ordinary light materials are limited,so it is difficult to prepare high-strength and high-performance concrete.In order to solve this problem,in this paper,fly ash cenospheres were used to replace quartz sand.Based on the study of the influence of mixing process on the performance of the cement-based composites,a better process was obtained to prepare the cement-based composites.The effects of material composition on the mechanical properties,drying shrinkage properties and hydration products of composites were studied,and the flexural toughness of the composites was evaluated by ASTM C1018 method and JCI SF4 method.Research shows that:Different mixing processes had little effect on the fiber dispersion coefficient,but had a greater impact on the mixture fluidity,the bulk density,the flexural strength and compressive strength of the cement-based composites.There were some differences in the better mixing process of different mesh cenospheres with different hybrid fibers content.Mainly considered the influence of the mixing process on the bulk density,taking into account its influence on the fibers dispersion,the fluidity of the mixture and the strength of cement-based composites.Combined with different mesh cenospheres materials,it is better to use the process of the final addition of steel fibers for mixing.Defoamer could reduce the air content of mixture and improve the compactness,strength and flexural performance of cement-based composites.Based on different hybrid fibers content systems of 1.0% steel fibers +0.5% PVA fibers and 1.5% steel fibers+1.25% PVA fibers,lightweight cement-based composites were prepared with the bulk density of 1440kg/m3?1850kg/m3,compressive strength of 50.4MPa?96.0 MPa and flexural strength of 8.3MPa?15.9MPa,which were superior to quartz sand cement-based materials in specific strength and good flexural performance.With the increased of waterbinder ratio,the bulk density,flexural strength,compressive strength and flexural toughness coefficient of the composites decreased,but the specific strength of quartz sand cement-based composites decreased faster than that of lightweight cement-based composites.With the increased of the content of cenospheres,the bulk density and flexural toughness coefficient of lightweight cement-based composites with different grading decreased gradually,while the flexural strength,compressive strength,specific strength and toughness index were different.PVA fibers and steel fibers formed a fibers network structure,which could promote the steel fibers to play the role of strengthening and toughening the lightweight cement-based composites.The strength and flexural toughness of the composites were greatly improved with the increase of the content of hybrid fibers.However,when the amount of hybrid fibers was higher,the cenospheres were broken seriously and the bulk density increased greatly,especially the composites with large particle size cenospheres.The drying shrinkage of lightweight cement-based composites developed rapidly in the early stage,and the shrinkage value in the 7th day was about 2/3 of that in the 28 th day.The drying shrinkage increased slightly with the increase of water binder ratio,but firstly decreased and then increased with the increase of the content of cenospheres.In the early stage of drying,the shrinkage of lightweight cement-based composites was less than that of quartz sand cement-based composites,while in the 28 th day drying period,the shrinkage of the two composites were close.Among the different cenosphere grading,the drying shrinkage rate of composites with the grading of fine particle size were slightly higher in the 28 th day.Based on the thermal analysis of hydration products,quartz sand contained a certain amount of Ca CO3,which led to a sharp increase in the total weight loss of composites.Compared with quartz sand material,fly ash cenospheres material had a certain pozzolanic effect,and the smaller the particle size was,the higher the corresponding pozzolanic effect was,the longer the curing period,the more significant the pozzolanic effect,and the broken cenospheres could increase the corresponding activity.The specific strength value of composites under different cenospheres grading are mainly considered,taking into account the degree of cenospheres crushing,mechanical strength,flexural performance and drying shrinkage property of microbeads.Based on different cenospheres grading,it is better to give priority to the cenospheres of fine particle size with high activity.