Mechanical Properties Of Composite Modified Calcareous Sand Of South China Sea Islands And Microscopic Mechanism

At present,the construction of island reef projects is underway in the South China Sea,and the calcareous sand obtained by excavating lagoons and waterways is used as filling material.Calcareous sand itself is a kind of particle porous,easy to break and low strength,which is difficult to meet the construction needs of the island reef project.Cement,as a conventional cementing material,can effectively improve the strength of calcareous sand by mixing it into calcareous sand,but the improvement of strength is limited.And nano zirconium dioxide and basalt fiber as new nano materials and fiber materials can be applied in the modification of mechanical properties of cement materials.In this paper,nano-zirconium dioxide and basalt fibers were compounded with cement into calcareous sand,and specimens of nano-zirconium dioxide and cement composite modified calcareous sand(referred to as nano-cement)and basalt fibers and cement composite modified calcareous sand(referred to as fiber-cement)were obtained,and the nano-zirconium dioxide and basalt fibers and cement composite modified calcareous sand(referred to as fiber-cement)specimens were studied and analyzed by using unconfined compressive test and direct shear test as well as microscopic test.The mechanical properties and micromechanisms of the modified calcareous sand with zirconium dioxide and basalt fibers and cement were investigated and analyzed using lateral limit compressive tests and direct shear tests as well as microscopic tests.The main findings of this paper are summarized as follows:(1)In terms of the basic physical properties of calcareous sand,the basic geotechnical tests yielded an inhomogeneity coefficient Cu=2.17,a curvature coefficient Cc=0.9,a moisture content of 0.92 wt%,and a specific gravity of 2.64.In terms of engineering properties,the particle shape is angular and porous compared with conventional terrestrial sands,the cohesion between the skeletons is poor,there is no directional confinement distribution,and there are large differences in elemental composition.(2)The compressive strength stress-strain curves of calcium-cement sand,nano-cement and fiber-cement calcium sand showed softened hump-like convex curves at different dosing and curing ages.And in order to obtain the accuracy of the test data,five sets of repeated independent test data with discrete nature were fitted to make them fit into a typical representative stress-strain curve.When nano-cement and fiber-cement were blended in different ratios,the compressive strength of the calcareous cement sand was substantially increased.In this paper,it is concluded that nano-cement composite with 8wt% is the optimum dosing,and the compressive strength of the sand increases by49%-101%,30%-105%,and 38%-92%,respectively,with the increase of the curing age compared with that of the calcium cement sand.At the same time,it was concluded that the optimum dose was obtained at 0.6 wt% of fiber-cement composite,and the compressive strength increased by 66%-94%,55%-65%,and 44%-63%,respectively,with the increase of curing age compared with that of calcium cement sand.Comparing the results of compressive performance of nano-cement and fiber-cement modified calcareous sand,the effect of nano-cement composite modification was better than that of fiber-cement composite modification,and the compressive strength of nano-cement was higher than that of fiber-cement as affected by the age of curing.In addition,the effect of fiber-cement on peak strain is better than that of nano-cement,and it can improve the deformation resistance and delay the damage time of specimens.(3)Direct shear of calcareous sand was performed when no material was incorporated,and with the increasing normal stress,the shear stress also increased,and both showed a linear growth trend.The cohesive force was also measured to be 0,and the angle of internal friction was 34.41 degrees.Both the cohesive force and the internal friction angle were enhanced after the incorporation of cement.When nano-cement was compounded,the optimum admixture was selected when nano-cement was blended with 8 wt%.The shear strength was improved by 46%-98% and 35%-73% compared with that of calcareous cement sand at the curing age of 7d and 28 d,respectively.When the fiber-cement composite was used,the optimal amount was selected when the fiber-cement was mixed with 0.6wt%.The shear strength was increased by 18%-59% and 15%-67% compared with that of calcium cement sand at the curing age of 7d and 28 d,respectively.And the shear performance effect of the composite modified calcareous sand is mainly achieved by enhancing the cohesion,which has some effect on the internal friction angle,but the effect of modification is not as obvious as the cohesion.Comparing the results of shear performance of nano-cement and fiber-cement modified calcareous sand,the shear performance of nano-cement composite modified calcareous sand is better than that of fiber-cement.(4)Microscopic experiments yielded:When cement is mixed into calcareous sand,a cementitious compound C-S-H is generated through the hydration reaction,which has a certain bonding property and can be glued together with calcareous sand to play the role of bonding and reinforcement.When the nano-cement compound is mixed into the calcareous sand,nano-zirconium dioxide plays two roles,one is to fill the particle pores of the calcareous sand,making the calcareous sand skeleton more dense to each other,thus improving its compressive strength.Secondly,it induces catalytic effect to increase the rate of cement hydration reaction and produce a large amount of cementing compounds to gel with nano-zirconium dioxide to achieve the purpose of modifying and improving the strength of calcareous sand.When fiber-cement is mixed into calcareous sand,basalt fiber plays two roles,one is to fill the pores between the bonding slurry by penetration,so that the free agglomerated bonding slurry is adsorbed on the surface and connects it to form a block structure to achieve modification to improve the compressive strength of calcareous sand.The second is the anchoring and reinforcing effect,basalt fibers interspersed in the cement mortar,forming a dense mesh structure,wrapping and binding the cement mortar,when the external load,play the role of anchoring and reinforcing.