Experimental Study On Mechanical Properties Of Seawater Sea-sand Concrete With Sea-sands From Different Regions

In this study,undisturbed sea sand from Shandong,Fujian,and Guangxi were selected as the research object,and the influence mechanism of sea sand from different regions on the mechanical properties of concrete was analyzed.Optical microscope,element titration,XRD,TGA,SEM and other methods were used to determine the physical and chemical properties of sea sand from different regions,and compared those with local river sand from Changsha.Then the seawater sea-sand concretes（SWSSCs）with three different strength gradients（C30,C40 and C50）were prepared with the sea sands from diffferent regions and the artificial seawater.The mechanical tests were conducted to obtain the key mechanical properties,including compressive strength,elastic modulus and splitting tensile strength,etc…Afterwards the related mechanism was analyzed with the help of XRD,TGA,SEM and NMR and the results were also compared with those of the ordinary concrete（OC）.Finally,combined with the workability and compressive strength of concrete,the optimal replacement ratio of sea sand partially replacing river sand was studied.The research results could provide a significant basis for the mix proportion optimization design and engineering application of sea sand concrete.The main research content s and related results of this paper were as followed:（1）The physical properties of sea sand from three different regions were tested and compared with ordinary river sand according to JGJ52-2019.Besides,the mineral composition,ion concentration,organic matter content and chemical element composition of sea sand were analyzed by XRD,titration test,SEM and EDS,respectively.It was showed that there was no significant difference in density and hardness between sea sand and river sand.Sea sand and river sand were both continuously graded,belonging to the medium sand in Zone II.Both of it were suitable fine aggregates for concrete preparation.Sea sand had finer particle size,rounder and glossier surface,while river sand had a rougher one.Sea sand contained more marine substance such as shell and organic matter.Moreover,the mud content of sea sand was higher than that of river sand.There was no obvious difference in mineral composition between sea sand and river sand,and the main components were quartz（Si O₂）,followed by feldspar（（K,Ca,Na）（Al Si）₄O₈）.Compared with river sand,the chemical element composition of sea sand was more complex,and the contents of Cl^-and SO₄^2-were higher,which will affect the properties of concrete.（2）The physical properties,mechanical properties and the working performance of different types of concrete were tested.The drying shrinkage value,cubic compressive strength and splitting tensile strength of concrete at different ages,as well as the flexural strength and elastic modulus under the standard age（28 d）were studied.The results showed that due to the influence of marine impurities,such as shells in sea sand,the slump of fresh concrete was diminished.However,we could prepare concrete that met the engineering requirements by adjusting the amount of water reducing agent either river sand or sea sand.Because of the influence of salt ions such as Cl^-and SO₄^2-,the early shrinkage and compressive strength of seawater sea-sand concrete were higher than those of ordinary concrete.With the development of curing age,there was no distinct difference in long-term performance between seawater sea-sand concrete and ordinary concrete.The splitting tensile strength of SWSSC at different ages was higher than that of ordinary concrete,but the gap between these two gradually narrowed with the development of curing age.There was no apparent difference between sea-sand concrete and ordinary concrete in elastic modulus and flexural strength.（3）In order to study the evolution mechanism of concrete mechanical properties,the microstructure of concrete at different ages was observed by XRD,SEM,NMR.It was found that the salt substances containing Cl^-and SO₄^2-could accelerate the hydration of concrete,generate more hydration products,which could make the concrete denser,and this behavior played a key role in early curing age.This was also the reason for the higher compressive strength of early-age seawater sea sand concrete.The compressive strength of seawater and sea sand concrete was lower than that of ordinary concrete after 28 d curing age.The reason was that the compactness of SWSSC was decreased because of the decomposition of Friedels’salt,which was generated and filled in pores earlier.Another reason was that the crystallization of salt ions led to crystallization pressure inside the concrete,causing damages to the microstructure of seawater and sea-sand concrete,which hindered the strength of concrete.The porosity of sea sand concrete was lower than that of ordinary concrete at 28 d curing age,while it was higher than that of ordinary concrete at 90 d and 360 d curing age.The total porosity of concrete decreased with the increase of age.The content of macropores（>1000 nm）in ordinary concrete decrease d with the increase of age while the content of macropores in seawater and sea sand concret e increased.This law was consistent with the slow development of its intensity in the later stage.（4）Determined the optimal replacement ratio of sea sand part ially replacing river sand by testing the physical and mechanical properties of concrete with different sea sand contents（15%-50%）.When the replacement ratio of sea sand was 25%-35%,the compressive strength,tensile strength and elastic modulus of mi xed sand concrete could reach the maximum.Compared with pure sea sand or river sand concrete,it has improved.All three kinds of sea sand from different region have reached similar conclusions.In summary,the optimal replacement ratio of sea sand was 25%-35%.