Study On The Preparation Of Magnesium Phosphate Cement With Calcined Dolomite

Magnesium phosphate cement(MPC),as a chemically bonded ceramic,is formed mainly through the acid-base reaction between magnesia(MgO)and phosphates.MPC has a series of vital properties compared with ordinary Portland cement,however,the high cost of raw materials limits the practical application of MPC.Although partial substitution of MgO or MPC with some admixtures can reduce the cost of MPC and improve some performances,excessive substitutions will reduce the early strength due to the low reactivity of the admixtures.To radically reduce the cost of raw materials,it is necessary to find a breakthrough from the source or the preparation technology of raw materials.Dead burned MgO is preferred in MPC production and generally produced by magnesite(Mg CO₃)at calcination temperatures above 1500°C.MgO accounts for a large proportion(50?80%in mass ratio)of raw materials in MPC.Dolomite[Ca Mg(CO₃)₂]ores as another abundant magnesium resource have larger deposits that are more widely distributed than that of magnesite.However,calcined dolomite can not be directly used as alkaline components of MPC because it contains free CaO,which will cause either a more rapid setting or volume instability in MPC.In this paper,other substances were introduced during the calcination process of dolomite to chemically combine with CaO to form minerals with a certain reactivity,while a large amount of MgO was still reserved.The calcination products were expected to completely replace dead burned MgO as alkali components to prepare MPC.So the cost of MPC would be reduced and the minerals containing calcium would play a positive role in MPC.In this paper,suitable minerals containing calcium were selected as calcium-fixing products.Moreover,the CaO-MgO-SiO₂ and CaO-MgO-Al₂O₃-SO₃-SiO₂ systems were designed and their feasibility was evaluated by calculating the thermodynamic data.Finally,the effect of calcium-fixing products on the hydration of MPC was studied by experiments on suspensions prepared with synthetic minerals.The results showed that dicalcium silicate and calcium sulphoaluminate minerals were possible to be used as calcium-fixing products separately or together for their suitable formation temperature,solubility and reactivity.The thermodynamic calculations showed that CaO was easy to react with Si or with S and Al,while MgO was very difficult to react with those chemical elements.The values of Gibbs free energy changes of the chemical reactions to generate dicalcium silicate and calcium sulfoaluminate were also very low.These two calcium-fixing pathways were possible in thermodynamics.In the C₂S-MgO suspensions,when the replacement amount of C₂S was less than 40%,the p H value of the prepared MPC suspension did not increase significantly and struvite could still be observed at 28 d in the XRD results.In the MgO-CSA-C₂S ternary system,the effects of CSA and C₂S on the p H value of the MPC hydration system could cancel each other out.When the p H value of the system was controlled between 10.50 and 11.00,the content of MgO,CSA and C₂S in the calcination products should be controlled at 20%–40%,40%–60%and 15%–35%,respectively.Based on the CaO-MgO-SiO₂ system,the quartz was used to fix free CaO and the calcination products of dolomite-quartz were used as alkali components to prepare MPC.Through the calcination of dolomite-quartz powders with the same fineness,the calcination of the coarse quartz and fine dolomite was put forward.Based on the analysis of the performance of the calcination products,the suitable mix design and calcination regime were selected.The performance,hydration products and microstructures of the MPC pastes prepared by the calcination products were investigated.Finally,the mechanism of the new hydration system,especially the effect of C₂S,was studied.The results showed that when the mass ratio of the fine dolomite to coarse quartz powders was 1.5 and the calcination temperatures were 1250°C,the final calcined products contained 17%MgO,26%C₂S and 41%SiO₂.The calcination products were used to replace dead burned MgO to prepare MPC with a setting time of 16 min,3-h compressive strength of 22 MPa and 7-d strength of 63 MPa.The main crystal hydration product was still struvite and a small amount of Ca(OH)₂was also observed.If there is a lot of free CaO in the calcination products,it would have a negative effect on strength development and volume stability at later ages.However,by further optimizing the calcination temperature and the particle sizes of dolomite and quartz,the free CaO in the calcination products could be reduced or eliminated.C₂S minerals had certain reactivity in MPC and could react with phosphate to generate some amorphous phosphosilicate phases,which were beneficial to the strength development of MPC.Based on the CaO-MgO-Al₂O₃-SO₃-SiO₂ system,bauxite-gypsum was used to fix free CaO and the calcination products of dolomite-bauxite-gypsum were used as alkali components to prepare MPC.The mix-design theory and calcination regime were investigated firstly,and then the properties of the calcination products were investigated.The calcination products were used to prepare MPC and the properties of the cement were studied.The hydration mechanism,especially the effect of C₄A₃(?),was also investigated.The results showed that the products containing 26%MgO,17%C₂S and 51%C₄A₃(?)could be prepared by the calcination of dolomite-bauxite-gypsum mixtures at 1200°C for30 min.The calcination products were used to replace dead burned MgO to prepare an MPC with a setting time of 24 min,3-h compressive strength of 46 MPa and 360-d strength of 80 MPa.Compared with MPC with the dead burned MgO,the MPC pastes prepared by the calcined products had a longer setting time,lower hydration heat release and similar mechanical properties.Struvite was still the main crystal hydration product.With increasing curing ages,the content of C₄A₃(?)decreased gradually.Part of C₂S participated in the hydration reactions and generated some amorphous phases containing polysilicon chains.More C₄A₃(?)participated in the chemical reactions and formed some amorphous phases containing Al-P.Comparing the above two calcium-fixing ways,the system of dolomite-bauxite-gypsum is better.The calcination products of dolomite-bauxite-gypsum were used as alkali components to prepare MPC.The influence of the proportion of the cement mix design on the performance of MPC,its stability in water curing,performances in elevated temperature conditions and the performances of the mortar were studied.The results showed that the compressive strength development of MPC pastes containing dicalcium silicate and calcium sulfoaluminate was closely related to the phosphate types,alkali-component/ADP,borax content and W/B.At environment temperatures from 200–950°C,the cement pastes containing dicalcium silicate and calcium sulfoaluminate were more stable than the MPC pastes prepared by dead burned MgO.The compressive strength of the MPC pates was significantly improved before 90 d in water curing,and the strengths remained stable in the later ages.Besides,the volume changes of the pastes in water curing were small and the length change at 360 d was only about 0.08%.The water environment would promote the hydration of C₄A₃(?)in the early ages,and some amorphous phases containing Al-P phase with flocculent structure would form.The amorphous phases filled in the pores,which was the main reason for the strength increase in water curing.The MPC mortar prepared by calcined dolomite-bauxite-gypsum at1200°C had a setting time of 27 min,free fluidity of 150 mm,3-h compressive strength of 41.0 MPa,7-d strength of 57.5 MPa and 180-d strength of 74.4 MPa.In addition,the mechanical properties of the mortar in water curing were also improved.