Effect Of Coral Sand On The Mechanical And Hydration Properties Of Magnesium Potassium Phosphate Cement Mortar

Damaged structures on coral islands have been spalling and cracking due to the dual corrosion of tides and waves.Magnesium potassium phosphate cement is widely used in repairing damaged structures by its fast setting,high early strength,and good bonding performance.To ensure repair effectively and easy access to materials,coral sand and magnesium potassium phosphate cement are mixed to repair damaged structures on coral islands.However,coral sand is a specific geotechnical material with a porosity of up to 50%.Its mineral composition is mainly aragonite and calcite（≥96%）and has characteristics with water absorption-release and low particle strength,which will inevitably have an influence on the hydration process and performance of magnesium potassium phosphate cement mortar.To elucidate the mechanical properties,corrosion resistance,and hydration characteristics of magnesium potassium phosphate cement-coral sand mortar,a series of macroscopic and microscopic experimental studies were conducted in this paper.We analyzed the primary factors affecting the compressive strength of magnesium potassium phosphate cement-coral sand mortar by controlled variable and orthogonal method.The macroscopic and microscopic mechanical properties of coral sand mortar were studied by compressive strength,flexural strength,bonding strength,and micro-hardness tests.The changing patterns of mechanical properties of coral sand mortar after exposure to water or corrosion solution were investigated.The relative contents and the morphological evolution of the hydration product phases were revealed by X-ray diffraction（XRD）and scanning electron microscope-energy dispersive spectrometry（SEM-EDS）.The main studies and findings are as follows.（1）The effects of the main factors on the compressive strength of magnesium potassium phosphate cement-coral sand mortar were elucidated.The compressive strength regression model and the optimal water-cement ratio prediction model were constructed.The results showed that w/b and s/b were the main factors affecting the compressive strength of coral sand mortar.Based on the hydrolysis of borax,water absorption by coral sand,and water consumption by the reaction between Mg O and KH₂PO₄ in the coral sand mortar system,the theoretical water-cement ratio of the mortar was predicted as 0.18.The optimal doping amounts M/P:2.0,w/b:0.18,s/b:0.5,and borax:0.11 were obtained by combining the regression model and the predictive theoretical model of the optimal water-cement ratio.（2）The macroscopic and microscopic mechanical properties of magnesium potassium phosphate cement-coral sand mortar have been investigated.The results demonstrated that compared with magnesium potassium phosphate cement-river sand mortar,the coral sand mortar interface had higher microhardness values and was linearly correlated with compressive strength（y=-36.30+0.70x,R²=0.99）.The interfacial bonding strength of coral sand mortar and concrete was higher than that of river sand mortar,and its shear damage surface was irregular,which exhibited shear expansion characteristics.Interfacial contact area and loading conditions were the main factors affecting the interfacial bonding performance.The larger the interface area,the higher the bond strength.In addition,under different loading conditions,the adhesion strength was as follows:slant shearing>direct shearing>flexural bending>tensile bonding.（3）The corrosion resistances of magnesium potassium phosphate cement-coral sand mortar were clarified after exposure to clear water,15%Na₂SO₄ solution,and 15%Na Cl solution under long-term immersion and wet/dry cycling conditions.The results illustrated that the coral sand mortar exhibited better resistance to Na₂SO₄ and Na Cl solutions,whose resistance coefficient to salt solution erosion was greater than that of the river mortar.The dry and wet cycling conditions promoted the resistance of coral sand mortar under the corrosive attack of water and SO₄^2-,while the long-term immersion conditions enhanced the resistance of coral sand mortar under the corrosion of Cl^-.（4）The hydration characteristics of magnesium potassium phosphate cement-coral sand mortar were revealed.The results indicated that the morphology of coral sand mortar crystalline phases appeared flocculent,agglomerate,pillar,and plate sequentially with increased curing time.The coral sand mortar generated the flocculent product 6KPO₂·8H₂O at the stage of 1 h～12 h.Then,at the stage of 12 h～28 d6KPO₂·8H₂O gradually dissolved and participated in the reaction to form the pillar shape products Mg KPO₄·6H₂O.With the concentration of Mg KPO₄·6H₂O increased from 0.6%to 8.6%,the crystals continuously overlapped and formed dense plate-like structures,which guaranteed the excellent mechanical properties and corrosion resistance of the coral sand mortar.