Thermodynamic Modeling Of Modifiers,Temperature And Aggressive Media On Phase Assemblages In Magnesia-Based Cements

There are abundant magnesium reserves in China,and one third of the world’s magnesite reserves are in China.Meanwhile,China is rich in seawater,brine,dolomite,brucite and other magnesium resources,and there are many salt lakes containing Mg Cl₂and Mg SO₄ in China.These abundant magnesium resources are favorable for the application of magnesia-based cements in China.Magnesium phosphate cement（MPC）and magnesium oxychloride cement（MOC）are two types of commonly used magnesia-based cements.Magnesium phosphate cement sets very fast,and it can set at-20℃.It has strong chemical bonding and high early mechanical strength,good durability and volume stability etc.MPC is usually used for rehabilitations of infrastructure and waste stabilization/solidifications at present.MOC also sets very fast,and due to the high compressive and tensile strength,high fire resistance and abrasion resistance and low thermal conductivity,MOC is widely used in grinding wheels,industrial flooring,lightweight insulation wall panel,interior plaster and decorative panels.The current investigations,which are labor-and time-intensive,are primarily concerned with identifying the optimal mix proportions of MPC and MOC based on the setting time,hydration heat evolution,macro-mechanical characteristics,durability,and microstructure.There are also few studies focusing on the mechanisms of hydration,modification,and durability degradation.Since the performance of MPC and MOC mainly depends on the phase assemblage,and thermodynamic simulation can efficiently and accurately study the equilibrium phase composition of the system under given conditions.Therefore,this thesis explores the equilibrium phase composition of MPC and MOC under given conditions through thermodynamic simulation,and establish the relationship among the composition of raw materials,the influence of external environment and the composition of hydration products.Revealing hydration mechanism,modification mechanism and durability degradation mechanism will be the focus of this paper.Firstly,this thesis constructs the thermodynamic database for MPC（magnesium ammonium phosphate cement,MAPC and magnesium potassium phosphate cement,MKPC）and MOC.The reliability of the constructed database verified by ion concentration,p H and phase assemblage.Hydration products which belong to MOC may form in MPC under chloride attack.Hydration products generated in MPC may for in MOC modified with phosphate.Therefore,this thesis studies the effect of mix proportions,modifiers,temperature,sulfate attack,chloride attack and carbonation on the phase assemblages of MPC and MOC through thermodynamic simulation.The effects of mix proportions,modifiers and temperature on the hydrate phase assemblages are as follows:（1）For MAPC:（a）Thermodynamic database of MAPC is constructed;（b）The hydration products of MAPC at standard state are（NH₄）₂Mg（HPO₄）₂?4H₂O,Mg HPO₄?3H₂O and Mg NH₄PO₄·6H₂O,and the first two are metastable phases which can further react with Mg O and H₂O to form Mg NH₄PO₄·6H₂O as M/P（n（Mg O）/n（NH₄H₂PO₄））increases to 1.（c）The ternary phase diagram of Mg O-NH₄H₂PO₄-H₂O at standard state is constructed,and the effects of mix proportions are that the main hydration products are（NH₄）₂Mg（HPO₄）₂?4H₂O,Mg HPO₄?3H₂O when M/P is less than1,and（NH₄）₂Mg（HPO₄）₂?4H₂O,Mg HPO₄?3H₂O will transform to Mg NH₄PO₄·6H₂O when M/P is greater than 1.（d）Borax and boric acid have no influence on the type of the hydration products,and they only affect the formation amount of each phase.Borax and boric acid promote the dissolution of Mg O and the formation of Mg NH₄PO₄·6H₂O in MAPC.（e）The formed Mg NH₄PO₄·6H₂O in MAPC with M/P less than 1 will convert into（NH₄）₂Mg（HPO₄）₂?4H₂O with the increase of temperature,leading to the volume increase of the solid phases.While for MAPC matrix,when M/P is greater than 1,the formed Mg NH₄PO₄·6H₂O will dehydrate to form Mg NH₄PO₄·H₂O with the increase of temperature,resulting in a rise in solid phases.（2）For MKPC:（a）The main hydration products of MKPC at standard state are Mg HPO₄?3H₂O,Mg₂KH（PO₄）₂·15H₂O and Mg KPO₄·6H₂O,and there is also a small amount of Mg（OH）₂ formed.Mg HPO₄?3H₂O and Mg₂KH（PO₄）₂·15H₂O are intermediate phases which will transform into Mg KPO₄?6H₂O when M/P(n（Mg O）/n（KH₂PO₄）is greater than 1.（b）Borax and boric acid have no influence on the type of the hydration products,and they only affect the formation amount of each phase;（c）As for MKPC,when M/P is less than 1,the formed Mg₂KH（PO₄）₂·15H₂O will convert into Mg KPO₄?H₂O and Mg HPO₄?3H₂O.When the M/P is greater than 1,all the Mg KPO₄?6H₂O transforms into Mg KPO₄?H₂O and Mg HPO₄?3H₂O.The volume of solid phases of MKPC decreases with the increase of temperature.（3）For MOC,this thesis improves the thermodynamic database at standard state to study the modification mechanism of KH₂PO₄and Fe SO₄·7H₂O,as commonly modifier of magnesium oxychloride cement（MOC）.KH₂PO₄ and Fe SO₄·7H₂O only effect the relative amount of formed hydration products in MOC at standard state.The addition of KH₂PO₄inhibits the formation of phase 5,leading to the increase of the concentration of Mg²⁺and Cl^-in the solution.According to the law of mass action,the increase in the concentration of relevant ions is good for the stability of phase 5 in MOC.Fe SO₄·7H₂O can promote the formation of phase 5 and increase the concentration of Mg²⁺,improving the stability of formed phase 5 in the system and the water resistance of MOC.The effect of mix proportions on the hydrate phase assemblage is greater than that of temperature when the curing temperature is lower（T is lower than 42.7℃）,that is to say,the phase assemblage under low temperature is almost the same with that cured under standard state.Phase 3 becomes to be the stable phase as temperature rises（T is greater than 42.7℃）,and the formed phase 5 will convert into phase 3.Volume expansion of MOC happens when the content of H₂O and Mg Cl₂ in the mixture is higher,and the volume contraction occurs in other cases.After verifying the reliability of the constructed thermodynamic database of MAPC,MKPC and MOC,phase assemblages of MPC under chloride attack and sulfate attack and MOC soaked in sulphate solutions are investigated.The phase evolution of MPC and MOC under carbonation is also studied through thermodynamic simulation.The results are as following:（a）Phase 3 or phase 5 cannot form in MAPC and MKPC soaked in 20%Na Cl solution.Unidentified phase is formed in MAPC immersed in 20%Na Cl solution for 6months,while for MKPC,hazenite（KNa Mg₂（PO₄）₂·14H₂O）is formed.（b）3-1-8（3Mg（OH）₂·Mg SO₄·8H₂O）and 5-1-7（5Mg（OH）₂·Mg SO₄·7H₂O）will form in MPC immersed in 20%Mg SO₄ solution.MKPC experiences a volume expansion under the sulfate attack.MOC decomposes in low concentration Mg SO₄solution,while in high concentration Mg SO₄ solution,5-1-7 will form in MOC.（c）The unreacted Mg O in MPC will react with CO₂and H₂O to form Mg CO₃·3H₂O.However,this transformation process is very slow due to that the first reaction occurs during MPC carbonation is the reaction of Mg O with H₂O to form Mg（OH）₂ which further react with CO₂ to form hydrate Mg-carbonates.The Mg O used in MPC is dead-burnt Mg O whose reactivity is very low,making it hard to react with H₂O to form Mg（OH）₂.In addition,Mg（OH）₂ is only stable under alkaline conditions,while MPC is low-alkali cement.（d）Mg₂（CO₃）Cl（OH）·3H₂O is not the only carbonation products of MOC,and there are other Mg-carbonates can be formed.The evolution of carbonation products in MOC is as follows:Mg₂（CO₃）Cl（OH）·3H₂O→Mg CO₃·Mg Cl₂·7H₂O→Mg CO₃·3H₂O→Mg₅（CO₃）₄（OH）₂·4H₂O.The addition of modifier does not affect the types of carbonation products.However,modifiers influence the process of the carbonation.Fe SO₄·7H₂O does not affect the carbonation process of MOC:phase 5 is the first to react with CO₂ and H₂O to form Mg₂（CO₃）Cl（OH）·3H₂O.While in KH₂PO₄ modified MOC,the first phase that react under carbonation is Mg（OH）₂,which reacts with CO₂to form Mg CO₃·3H₂O.Mg₂（CO₃）Cl（OH）·3H₂O is predicted to form through the reaction of phase 5,CO₂and H₂O after Mg（OH）₂ is consumed.Modifiers in MOC enhance the carbonation resistance of MOC,however,they do not affect the final carbonation phase assemblages of MOC.This thesis reveals the thermodynamic mechanism of hydration,modification and the performance degradation of MAPC,MKPC and MOC,which can provide guidance on engineering design and application of magnesia-based cements.