Preparation And Properties Of Nickel-iron Slag-based Magnesium Phosphate Cement

Magnesium phosphate cement is a new kind of cementitious material which is fast hard and early strong.Compared with traditional Portland cement,it has the advantages of fast setting and hardening,high early strength,good volume stability,good wear resistance and frost resistance,and is widely used in the rapid repair of some civil structure engineering.But the traditional magnesium phosphate cement has high cost,which limits its large-scale use in engineering.As a kind of metallurgical solid waste,ferronickel slag has a high magnesium content,and most of it is stored in the open air or landfill,which not only poses a potential threat to the surrounding ecological environment,but also causes a waste of magnesium resources.Therefore,this paper proposed the preparation of ferronickel slag-based magnesium phosphate cement material using ferronickel slag with high magnesium contentFirstly,the basic physical and chemical properties of ferronickel slag were studied,and the feasibility of preparing magnesium phosphate cement was analyzed Then the effects of ferronickel slag on the mechanical properties and working performance of the materials were investigated by single factor optimization experiments,such as ferronickel slag-to-ammonium dihydrogen phosphate mass ratio m(FS/ADP),water-to-binder mass ratio m(W/B),admixture(MgO)content m(M/FS)and retarder(boric acid)content m(BA/FS).The optimal preparation conditions of magnesium phosphate cement was determined,and the phase composition and microstructure of magnesium phosphate cement at different ratio were analyzed to reveal the hydration mechanism of magnesium phosphate cement.In addition,the water-resistance and alkali-resistance of the samples under the optimal preparation scheme were investigated,and the changes in phase composition and microstructure of the materials under different wet curing methods,different water immersion environment and different mass fraction of alkali solution immersion conditions were analyzed.The results and conclusions are as followed(1)The main chemical compositions of ferronickel slag is SiO2,MgO and Fe2O3,in which the MgO content is as high as 32.88%in the form of forsterite(Mg2SiO4),Moreover,ferronickel slag itself meet the requirements for natural radionuclides as the main building material specified in GB6566-2010(2)The optimal preparation conditions for ferronickel slag-based magnesium phosphate cement is as follows:m(FS/ADP)=4:1,m(M/FS)=4%,m(W/B)=0.17,m(BA/FS)=0.3%after reacting 8 h at 50? constant temperature water bath.Under these conditions,the compressive strength of samples at 1 d,3 d,7 d and 28 d can reach 32.6 MPa,39.9 MPa,48.7 MPa and 54.9 MPa,and the setting time was 12 min(3)Charaterization analyses show that the hydration products of ferronickel slag-based magnesium phosphate cement is mainly struvite,and some content of schertelite.The prepared material is a high strength hardened body which was bonded and wrapped by unreacted ferronickel slag particles with hydration gel and crystallization as binder.pH value is an important factor throughout the hydration process of ferronickel slag-based magnesium phosphate cement,which affectes the formation of schertelite and struvite(4)The samples were leached by different pH leaching solutions,and the results shows that the leaching concentrations of total Cr in the leaching liquid do not exceed the limit of 15 mg/L stipulated by the national standard(GB5085.3-2007),and the leaching concentrations of Cr in the ferronickel slag-based magnesium phosphate material is significantly lower than that of the raw ferronickel slag,indicating that the ferronickel slag-based magnesium phosphate material has a certain sealing effect on the heavy metal in ferronickel slag and improve itself environmental safety(5)Durability study of ferronickel slag-based magnesium phosphate cement shows that curing in high temperature and too humid environment will affect the development of mechanical properties of the material.The retention rates of sample strength for 28 d of fresh water immersion and seawater immersion are 87.4%and 80.2%,respectively.The compressive strength loss rate and mass loss rate of the sample increase gradually with the increase of concentration when the material was immersed in different concentrations of NaOH.This results may be due to the reaction of the hydration product guanfossil in the material with NaOH,resulting the formation of gel-like Mg(OH)2.