| On islands far from the mainland,raw materials for concrete production are often more difficult to obtain,so converting coral waste generated during island construction into raw materials for concrete preparation is not a bad strategy for environmental protection.In this experiment,two sets of water-cement ratios of 0.45 and 0.30 were selected,and coral powder（CP）was used to replace cement in the net slurry by equal masses（10%,15%,20%）,and the optimal replacement rate of CP was obtained by compressive strength test.On the basis of this,coral sand（CS）of equal mass（10%,20%,30%,40%）was used to replace standard sand in the mortar to prepare coral mortar,and its mechanical properties and shrinkage properties were tested.In addition,various micro-scale and nano-scale tests such as X-ray diffractometer（XRD）,thermogravimetric analyzer（TG）,nitrogen adsorption,scanning electron microscopy（SEM）and nanoindentation techniques were used to analyze the micro-and nano-scale structures of coral cement-based materials.The composition and content of mineral phases in cementitious materials were determined by XRD-TG.the pore structure and pore distribution of cementitious materials were determined by nitrogen adsorption.the microstructure of matrix and interface transition zone（ITZ）of cementitious materials were characterized by SEM.the nano-mechanical properties of matrix and ITZ of CS-containing cementitious materials were analyzed by nanoindentation technique.The volume fraction of each hydration product was analyzed and calculated to further explain the macroscopic properties of CP and CS cementitious materials.The main studies and conclusions are as follows:（1）At 0.30 and 0.45 water-cement ratio,the coral net slurry has better mechanical properties at 10%CP substitution.0.45 water-cement ratio,the compressive strength of coral net slurry increases with the increase of CP and then decreases;at 0.30 water-cement ratio,the compressive strength decreases with the increase of CP.The reason for this is that under high water-cement ratio,the water absorption of CP reduces the effective water-cement ratio in the slurry,and the high fineness of CP has better microaggregate filling effect and crystal nucleation effect,which optimizes the pore structure to a certain extent,thus reducing the porosity of the slurry and improving the compactness of the slurry.While under low water-cement ratio,the benchmark group itself has high compactness,and the admixture of CP absorbs a certain amount of water,which makes a large number of unhydrated cement particles exist in the system and the cement slurry is not compact inside,thus leading to the decrease of slurry strength.（2）At 0.30 and 0.45 water-cement ratio,the mechanical properties of coral mortar increased and then decreased with the increase of CS mixing.28 days of age,at 0.45 water-cement ratio,when the CS substitution was 30%,the compressive strength at 28 d increased by 28%compared with the reference group;at 0.30 water-cement ratio,the compressive strength of coral mortar specimens with 30%CS substitution increased by 30%compared with the reference group specimens.The reason is that the optimized effect of CS on coral mortar at 0.45 water-cement ratio is mainly reflected in its surface roughness and irregularity,and the cement paste adheres to the aggregate more tightly;while the optimized effect of CS on coral mortar at 0.30 water-cement ratio is mainly reflected in the internal maintenance effect,which promotes the secondary hydration of unhydrated cement particles and thus increases its hydration product content.（3）The self-shrinkage strain of coral cement-based materials decreased with the increase of CP and CS dosing at 0.30 and 0.45 water-cement ratio.And the drying shrinkage strain tends to decrease first and then increase with the increase of CP and CS.The shrinkage strain of coral cement-based materials grows faster at the early age and slows down and levels off at the later age.The main reason is that CP plays the role of water absorption at high water-cement ratio,while it plays the filling effect and nucleation effect at low water-cement ratio,filling the internal pores of cementitious materials.CS mainly plays the role of internal maintenance,releasing water at the late stage of hydration,promoting cement hydration,improving the density of the matrix,slowing down the water dissipation inside the cement paste,and thus reducing the shrinkage strain value of cementitious materials.（4）The 10%CP doping had a good improvement on the microstructure of the net cement paste.For the hydration products,the addition of CP did not lead to the formation of new mineral phases in the hydration products,Ca（OH）2,C2S,C3S and Si O2 were the main crystalline phases.For the pore structure,at 0.45 water-cement ratio,CP can improve the pore structure of cement slurry better,and the slurry has better pore structure at 10%CP dosing;while at 0.30 water-cement ratio,the improvement effect of CP for harmful pores is more obvious,mainly playing the filling effect.For microscopic morphology,at 28d,10%CP replaces cement and can fill the pores in cement slurry and promote hydration,while the cement slurry has certain pore structure and microcracks inside at 20%CP admixture.（5）On the basis of 10%CP admixture,30%CS admixture has a better improvement effect on the micro and nano structure of cement mortar.It was concluded from the nitrogen adsorption test that the improvement of CS on the pore structure of cement mortar at 0.45water-cement ratio was not obvious,but it reduced the average pore size of cement mortar.at0.30 water-cement ratio,the improvement of CS on the pore structure of cement mortar was mainly reflected in the gel pores,and the mortar with 30%CS admixture had the optimal pore structure.It is concluded from SEM and nanoindentation tests that there is an obvious ITZ between standard sand and cement matrix,while CS is more tightly connected with cement matrix without obvious ITZ.the internal curing effect of porous CS can play an improving role on the weak ITZ.In addition,the nanoindentation test yielded that the internal curing effect of CS caused secondary hydration of the unhydrated particles and a 22%increase in C-S-H gel phase content compared to the baseline group,with a significant increase of 18%in HD C-S-H content and a small increase in LD C-S-H.In addition,the volume fraction of the unhydrated phase was reduced compared to the baseline group.This indicates that CS can promote cement hydration,allowing more cement to be converted to C-S-H. |
