Water Corrosion Behavior And Modification Mechanism Of Magnesium Potassium Phosphate Cement-based Materials

Magnesium potassium phosphate cement?MKPC?are highly colloid mixed with crystal materials produced by the acid–based chemical reactions between dead-burned magnesium oxide?MgO?and potassium dihydrogen phosphate?KH ₂PO₄?in the presence of a proper proportion of water,small amount of borax or other composite retarder compounds was used as setting retarders to meet its early working performance.The final reaction product is magnesium potassium phosphate hexahydrate or k-struvite?MgKPO₄·6H₂O,MKP?.As compared to other inorganic cementing materials,the paste has many superior properties such as fast setting,high early-strength,low shrinkage and good bonding and high adhesive.These superior properties make MPC attractive for applications such as fast repair of damaged structures and stabilization of toxic waste.However,magnesium phosphate cement is sensitive to water,the mechanical properties of MPC paste can be significantly weakened when immersed in,or even in contact with water.This can limit its applications in moist environments.There have been many studies on the short-term water corrosion behaviors of MKPC paste,but previous studies primarily concentrated on the macroscopic properties of MKPC paste after water corrosion,and most of them are focusing on the improvement effects mechanism in water stability of MKPC paste modified by different mineral admixtures.However,the mechanisms of water corrosion behavior of MKPC paste are still not yet clear.Understanding of the water corrosion mechanism of the system is helpful for developing new additives to improve the water stability of the hardened MKPC paste,to enhance its application in long term water contact engineering such as marine architecture.Firstly,the hydration performance and mechanical properties,phase composition and microstructures of MKPC paste under different curing temperatures and humidity was studied to determine the appropriate curing condition,then the effects of curing humidity,initial air curing age,water-solid ratio and state of water on the corrosion behavior of MKPC paste subjected to long-term water attack?0-240d?were investigated.The pH value and precipitated ions(K⁺,Mg²⁺,total P)of soaking liquid were innovative employed to evaluate the water corrosion behavior.The microstructure and hydration product changes of the corrosion paste were also analyzed.Through the research to above content,the deterioration mechanism of MKPC paste was proposed.Then pure hydration product k-struvite was synthesized and its chemical stability in water environment was also studied to further clarify the deterioration mechanism of MKPC paste and the corrosion mode was established.Finally,the influence of limestone powders and nickel slag powders on the water stability of MKPC paste was studied and the addition of nickel slag powders can significant improved the water stability of MKPC paste.The main conclusions are as follows:?1?Curing condition significantly effect on the hydration temperature and strength of MKPC paste.Too much curing humidity and too high water bath curing temperature will reduce the hydration reaction rate,improve early strength,but later strength and volume stability decreased.For improved durability dry air curing and30?water-bath curing with temperature below the critical peak hydration temperature is the best,and 30?water-bath curing is preferable to moist air curing,if rapid strength development is required.?2?The strength and volume stability of MKPC paste will deteriorate or even lose its durability completely in long-term wet or water environment.The strength retention ratio of the samples under moist air curing was 87.61%at 240 days,while the strength retention ratio of the samples under water curing was only 64.26%.When the curing humidity increased,the inner structure of MKPC paste was looser and the content of hydration products decreased,the crystal structures were all destroyed and the damage degree increased.Prolong initial air curing time bring a favorable effect on the water resistance of MKPC paste,as it reduce the corrosion degree of hydration products in MKPC paste,decrease the cumulative leaching amounts,and the crystal grown was healthier with bigger size.Prolong initial air curing time bring a favorable effect on the water resistance of MKPC paste,which is attributed to the fact that long initial air curing time can facilitate better formation,crystallization and growth o f hydration products,reduce the cumulative leaching amounts and corrosion degree of hydration products.Increase of the water-solid ratio reduced the content of the crystallized hydrate product and worsed the crystal morphology,resulting in poor water stability.The compressive strength and volume stability of MKPC paste under flowing water curing are significantly lower than those under static water curing,the strength residual rate of flowing water curing at 180 days is only 46.83%,and the volume expansion strain was 1.48×10^-4,which was almost 2 times higher than that under static water curing.?3?K-struvite dissolution and phase transformation was the ultimate causation of poor water stability of MKPC paste.The K-struvite corrosion was peeling off between layers from the outside to the inside and began from the end of adhesion with the matrix,the bonding between MKPC and MgO reduce and the structure bec ame loose.Besides,K-struvite can be replaced by new crystals such as Mg₃?PO₄?₂·22H₂O,both leading to the evident macroscopic properties decrease.?4?When the samples immersed in water,the unreacted phosphates dissolved rapidly and reacted with the residual MgO form more K-struvite until the potassium dihydrogen phosphate used up.The pH value of the medium increased,the hydration products such as K-struvite gradually dissolve and release Mg²⁺,K⁺and P ions in water.After a certain degree of dissolution,the covered residual magnesia is exposed and could react with the leaching ions on a later stage and lead to new needle-like hydration products formation that could generate expansion and cracking.Besides,the new hydration product has small crystal size and poor appearance,it is easier to dissolve under continuous water curing,further decrease the water stability.?5?The dissolution character of synthetic products of K-struvite under pure water and different pH solutions was studied.The surface area of K-struvite began to dissolve when contact with the soaking liquid to release Mg²⁺?K⁺and PO₄^3-ions into solutions,the ion concentrations increased fast at first,then slow down after 1d.The dissolutions reach a balanced state with solution pH value steady at around 10.6.During the dissolution process,the ions are saturated and recombined to form new hydration products such as Mg₃?PO₄?₂·22H₂O,Mg₂?PO₄??OH?·3H₂O,TMP8 and Mg₃?PO₄?₂·10H₂O.These reactions are accompanied by the volume expansion of the crystal structure.Therefore,the expansion stress caused by the phase transition of k-struvite is another reason for the decline of the performance.?6?The addition of limestone powder degraded both the compressive strength and water stability of MKPC.For MKPC paste with 5%limestone powder,the residual strength rate achieved a maximum value of 0.86,which was slightly higher than that of control MKPC samples.The residual strength rate MKPC paste decreased with the increasing amount of limestone powder.The residual strength rate of samples prepared with more than 15%limestone powder was less than 0.80.When limestone powder added,the total porosity of MKPC paste increase and the structure was much loose,the formation,crystallization and growth of hydration products w ere decreased.Hence,limestone powder should not excess 10%in magnesiu m phosphate cement to keep original water stability.The addition of nickel slag powders improved both the compressive strength and water stability of MKPC.Amorphous hydrates were generated in the MKPC blended with nickel slag powders paste bound together with MgO and K-struvite particles,forming a higher density.Besides,the dissolution rate of hydration products decreased.Increasing the amount of nickel slag powders improved the water stability,and the optimum water stability was obtained at 40 wt%nickel slag powders content with the compressive strengths were 82 MPa at the strength retention ratios of 122%.