Experimental Study On Design,Preparation Andproperties Of Reef Sandaggregate Concrete

The reef sand aggregate is a kind of natural lightweight aggregate according to the concrete aggregate academic classification because of its lightweight,water absorption,porous and porosity,which is a mixture materialfrom dredging harbor,digging channel and mineralized coral reef.Without destroying the natural conditions and save the land resources,using the reef on the construction process of sand rock as a source of concrete aggregate,designing and preparing for the reef sand aggregate concrete.It is very important significance that reduces land resources in the process of transportation facilities construction,makes full use of the construction of tailings,so as to decreas the construction cost,reduce the environment load.In this paper,according to the material composition performance requirements of gelled material composition,combining with physical and chemical properties of reef sand aggregate,preparation of reef sand aggregate concrete.According to the characteristics of the coarse porous reef sand aggregate,in order to improve the workability of concrete as a focus for design concrete,study the physical and mechanical properties and microstructuresystematically,explore the active mineral admixture to improve its mechanical properties and durability of principle and key technology.Utilizing process of paste cladding,developed high-performance reef sand concrete.Pore structure,the composition of hydration products and the interface transition zone of hardened cement and aggregate were tested by XRD,TG-DSC,SEM and MIP etc.The main results of the paper are as follows:?1?Through the analysis of the design orthogonal test of water to cement ratio,cement dosage and sand ratio on the influence of reef sand concrete,the results show that the optimal combination of main materials level is the water-to-cement of 0.4,cement dosage 420kg/m³ and sand ratio of 38%.Mixed with metakaolin and slag can increase the strength 13.8-34.7%.?2?Compared with the nature river sand concrete,the reef sand concrete early strength develop rapidly,however,the later strength develop slowly lead to the ultimate strength is lower than the nature sand river.The strength of sand concrete mixed with fly ash?15wt%?,slag?15wt%?and metakaolin?5wt%?reach 49.3MPa,while thereef sand concrete strength is 47.8MPa and strength increased by 21%.?3?Comparing with the reef sand concrete hydration products and thermogravimetric analysis under the standard curing shows that the reef sand concrete and river sand concrete with the same hydration products,the 28d hydration products of ettringite,calcium hydroxide,hydrated calcium silicate,and hydrated calcium aluminate.Mineral admixtures compactmicrostructure and increase the intensity of the reef sand concrete,mixing 5wt%metakaolin is the best and compound additionis 15wt%slag and 5wt%metakaolin.For pore structure analysis,the mineral admixtures can optimize the reef sand concrete pore structure and make reef sand concrete pore diametersmaller,pore distribution more reasonable and density increased.?4?Chloride ion penetration resistance of reef sand concrete is superior to the nature sand concrete at the early age,thechloride ion diffusion coefficient of 28daysis 3.6×10^-12m²/s,according to evaluation index of concrete chloride ion diffusion coefficient,reef sand concrete chloride ion permeability is“medium”.Compared to thecontrol group,thechloride ion diffusion coefficient mixed with 5wt%metakaolin and compound addition 15wt%slag and 5wt%metakakaolin decline 66.7%,79.2%respectively,chloride ion permeability is“medium”and“low”.?5?The drying shrinkage of reef sand concrete is greater than the nature sand concrete.Mixed mineral admixtures reduce thereef sand drying shrinkage,compared to the control group,the addition15wt%fly ash,15wt%slag and 5wt%metakaolin reduce more than 50%drying shrinkage.Reef sand concrete has superior ability to resist carbonation than nature sand concrete at early ages,however,the carbonation depth is higher than nature sand concrete at the late age.With addition fly ash,the carbonation depth of reef sand concrete increase,but the slag and metakaolin can reduce the carbonation depth.?6?Compared to the control group,the chloride ion diffusion coefficient and carbonation depth of reef concrete grately decline with utilizingprocess of paste cladding of different admixtures,which mixed with 5%metakaolin composite 20%fly ash and 10%slag is the most significant and reduce 69.1%of chloride ion diffusion coefficient.The utilization process of paste cladding and inner curing make formation a high porosity and low permeability structure of reef concrete.