The Mechanical Properties Of Cemented Calcareous Sand

Calcareous sand widely covered in South China Sea and nearby islands.There are large amount of Construction Projects occurred in this areas as the result of nature resource development and national defense.Therefore,the systematic research of cementing characteristics of calcareous sand has important practical significance and theoretical value.In addition,we have been done a larger amount of research on calcareous sand,and obtained many valuable results.However,we found that there are hardly understand well on mechanical properties of cemented calcareous sand.In this study,the calcareous sand can be used as test material which comes from coral reef in Nansha islands.Experiments with loose calcareous sand,Portland cement cemented calcareous sand and gypsum cemented calcareous sand were conducted by laboratory tests and microscopic tests.The characteristics of shear dilatancy and contraction,particle breakage,cementing characteristics of microstructure,mechanism of cementing,and elastic-plastic model were studied in detail.The main conclusions as follows:(1)The NMR tests found that the pore volume decreased with curing period and cement content.Defines the values as the current total pores divided the initial total pores of curing period and cement content,respectively.They were used to calculate the relative pore ratio.The conclusions of curing period is the key factor for increasing strength and there have an optimal amount of cement content are observed.The relationship between pore volume and strength was established.(2)With the increase of curing period and cement content,the pore volume of specimens is filled with hydration products.With the curing period increased,the semi-permeable gels of hydration product harden into mesh structure space(structure locking strength),and the pore volume decreased.When the hydration reaction completed,a column-mesh structure cementing products(cementing strength)are formed.(3)The cementing characteristics of calcareous sand are influenced by the cement content,curing period,confining pressure,and dry density.The peak strain of cementing calcareous sand is that in contrast with the maximum dilatancy strain.The lower cement content of Gypsum is the better strength improved have,and it is greater than that of Portland cement.The resistance of strength is mainly comes from the material itself at larger confining pressure.(4)The loading capacity of Portland cement cemented calcareous sand increased with the curing period,cement content,and dry density,which represented at the value of secant modulus increased by 0.5-90 times.The strength envelope of shear strength peak point is described by the curvilinear equation.The friction angle is unchanged,and the cohesion is increased to 3.23 times.The brittleness indexes increased with the increase of cement content,and the changing from plastic destruction to brittle destruction is obtained.(5)The value was introduced in the tangent modulus,which calculated from stress ratio divided peak stress ratio,and the equation modified to implicitly consider the effect of particle breakage.The equation of tangent volume ratio was derived and obtained the modified southern water model.The model calculation is well with the test data.(6)A method was obtained to find the volume yield surface equation,and the curve-shaped failure criterion of Portland cement cemented calcareous sand was summarized.The equations of tangent modulus and tangent volume ratio are considered the effect of particle breakage,and they are combined to establish a double-yielding surface plastic model by using the rule of orthogonal flow.It was found that the computational value can describe the test data well before the peak stress point.(7)The strength prediction model was obtained which consider the effects of curing period and cement content.Besides,the model calculation is consistent with the test data.The relationship between relative pore ratio and curing period was substituted into the model,and the model can clarify the relationship between pore volume and strength enhancement.Comparing with other models,we found that this model is more precision.