Magnesium Potassium Phosphate Cement Paste Research Articles

Leaching of magnesium potassium phosphate cement pastes under alkaline conditions

Some legacy radioactive waste containing aluminum (Al) metal need to be stabilized and solidified before their final disposal. Currently, Portland cement (PC) is extensively used for conditioning low- or intermediate-level radioactive waste. However, the high alkalinity of PC leads to strong corrosion of Al metal, which is associated with significant dihydrogen release. Therefore, it is important to investigate alternative binders that show better chemical compatibility with Al metal. Magnesium potassium phosphate cements (MKPCs), comprising equimolar amounts of MgO and KH2PO4, are interesting candidates since their pore solution pH may fall within the passivation domain of Al metal. The understanding of their long-term durability, especially under alkaline conditions, is however incomplete. Hence, MKPC paste samples (with fly ash as a filler) were submitted to semi-dynamic leaching tests using an alkaline solution under well-controlled conditions. The leachates were analyzed over time using ICP-AES, and the leached solids were characterized by XRD, SEM/EDS, and 31P MAS-NMR spectroscopy. Leaching induced a decrease in the content of crystalline K-struvite (MgKPO4·6H2O), the main hydrate of the paste samples, as well as the precipitation of calcium-deficient hydroxyapatite (CDHA), brucite (Mg(OH)2) and possibly magnesium silicate hydrates, OH-LDH or PO4-LDH phases. The experimental data were then used as an input for numerical simulations using the reactive transport code HYTEC.

Chemical degradation of magnesium potassium phosphate cement pastes during leaching by demineralized water: Experimental investigation and modeling

The long-term durability of magnesium potassium phosphate cement (MKPC) pastes was investigated by examining their leaching behavior. MKPC comprised magnesium oxide (MgO) and potassium dihydrogen phosphate (KH2PO4) in equimolar amounts and yielded K-struvite (MgKPO4·6H2O) and a nearly neutral pore solution pH upon hydration. Semi-dynamic leaching tests were performed on MKPC paste samples using demineralized water with a pH set at 7, and the leached solids were analyzed using XRD, SEM/EDS, 11B and 31P MAS-NMR spectroscopy. Leaching was mainly governed by diffusion of dissolved species through the pore network of the paste. Three main zones were observed in the leached solids: (i) a poorly cohesive residual layer where K-struvite was fully depleted, (ii) an intermediate zone where K-struvite coexisted with cattiite (Mg3(PO4)2·22H2O), and (iii) a third zone without any cattiite. Reactive transport modeling made it possible to predict the extent of degradation and the phase evolution in the MKPC paste samples.

Investigation of corrosion mechanism of ribbed mild steel bars coated with magnesium potassium phosphate cement paste

This study investigated the anti-corrosion performance of magnesium potassium phosphate cement (MKPC) paste applied to the surface of ribbed mild steel bars – which was exposed to simulated accelerated corrosive environment. Four electrochemical approaches were used including open-circuit potential (OCP), electrochemical impedance spectroscope (EIS), polarization resistance (PR) and potentiodynamic polarization (PDP) over a period of 5376 h (224 days). Moreover, visual inspection, optical microscope, and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) were used to assess the extent of corrosion on the samples. To understand the mechanism of corrosion protection of the coating system, X-ray photoelectron spectroscopy (XPS) was employed to characterize the chemical groups on the surface of mild steel, and the chemical changes in the coating layer were characterized using thermogravimetric/differential thermal analysis (TG/DTA) and X-ray diffraction (XRD). The MKPC paste coated bars were compared with not only uncoated bars, but also bars coated with ordinary Portland cement (OPC) that is known to passivate steel due to its high alkalinity. Results indicated that MKPC paste coating layer could effectively protect the ribbed mild steel bars, and its protectiveness significantly surpassed that of OPC. Both the de-passivation effects of chloride ions and carbonation of the OPC resulted in relatively severe corrosion of the OPC coated bars during the long exposure duration; while the anti-corrosion merit of the MKPC paste coating layer could be attribute to a double-protection system– the dense microstructure of MKPC and the formation of an iron (III) phosphate passivation layer between the substrate steel and the MKPC paste coating layer.

Investigation of Corrosion Mechanism of Ribbed Mild Steel Bars Coated with Magnesium Potassium Phosphate Cement Paste

Investigation of Corrosion Mechanism of Ribbed Mild Steel Bars Coated with Magnesium Potassium Phosphate Cement Paste

Effect of water content on microstructure and properties of magnesium potassium phosphate cement pastes with different magnesia‐to‐phosphate ratios

AbstractMagnesium potassium phosphate cement (MKPC) is a class of chemically bonded ceramic for special engineering applications. The selection of mass ratios of MgO‐to‐KH2PO4 (M/P) and water‐to‐cement (W/C) is an important aspect for formulating MKPC paste. This paper analyzed the stoichiometry of the primary reaction of MKPC and investigated the effect of W/C (0.14–0.40) on paste microstructure and properties at different M/P (1.5–4). At high M/P, low W/C was sufficient to complete the hydration of KH2PO4 and higher W/C yielded looser microstructure, dictating that properties of high M/P pastes were monotonic functions of W/C. However, a clear W/C threshold effect was found in medium M/P pastes at both micro‐ and macro‐levels. Low W/C below stoichiometric value suppressed the formation of K‐struvite and favored the presence of an amorphous phase, leading to intense early shrinkage, low early compressive strength, and poor long‐term water resistance. The increase in W/C above stoichiometric value resulted in the segregation of excess water and thus to porous structure, with detrimental effects on dimensional stability and strength. The micro–macro analysis highlighted the importance of stoichiometric water amount in the optimal design of MKPC.

Influence of hybrid graphene oxide/carbon nanotubes on the mechanical properties and microstructure of magnesium potassium phosphate cement paste

This research reports the effect of hybrid graphene oxide (GO)/carbon nanotubes (CNTs) on the mechanical and microstructural properties of the magnesium potassium phosphate cement (MKPC) paste. The experimental results confirmed that the hybrid GO/CNTs exhibited better dispersion performance than the CNTs and GO individuals by improving the dispersion stability and reducing the diameters of dispersed nanomaterials, making the hybrid GO/CNTs more suitable for nano-modification of cement composites. Furthermore, the addition of GO/CNTs shortened the final setting time and reduced the fluidity of the MKPC paste. The compressive and flexural strength of the MKPC paste was improved by 13.77% and 17.50% respectively by the addition of 0.05% GO/CNTs (by weight of cement). The incorporation of the hybrid GO/CNTs also refined the pore size distribution and improved the crystallization of the hydration products. A higher content of hybrid GO/CNTs and the utilization of polycarboxylate superplasticizer further enhanced the mechanical properties of the MKPC paste. The X-ray diffraction and Fourier transform infrared spectrometer results confirmed the absence of new phases or any chemical bonding between the hybrid GO/CNTs and the pastes. It could be concluded that the hybrid GO/CNTs were relatively promising when compared with the GO or CNTs individuals, possessing great potential for modifying the MKPC paste.

Behaviour of magnesium phosphate cement-based materials under gamma and alpha irradiation

Stabilization and solidification of low- and intermediate-level radioactive waste using Portland cement, possibly blended with fly ash or blastfurnace slag, is a well-established practice. However, when the waste contains high amounts of alpha emitters, this solution can be restricted by the strong release of radiolytic gases, wherein H2 is the most abundant. This work investigates the interest of using magnesium potassium phosphate cement (MPC), a binder with a high chemical water demand, as a possible substitute to Portland cement (PC). The radiolytic gas production of PC and MPC pastes and mortars is determined under external gamma and internal alpha irradiation. The H2 radiolytic yield of MPC materials is found to be 2 to 3 times smaller than that of PC references, provided that the main part of the mixing water is consumed by K-struvite formation. Moreover, gamma irradiation of a MPC mortar up to an integrated dose of 10 MGy has no significant influence on its mechanical strength (flexural, compressive) nor on its mineralogy. MPC materials are thus potential candidates for the conditioning of high amounts of radioactivity with limited H2 release. The H2 production of MPC materials can be reduced further by adding radical scavengers or H2 getters within the matrix. However, other radiolytic gases such as O2 are often produced, making these solutions potentially less attractive considering the concern of pressure build-up within the cemented waste package.

Effect of fly ash on the corrosion resistance of magnesium potassium phosphate cement paste in sulfate solution

In order to explore the effect of fly ash on the corrosion resistance of magnesium potassium phosphate cement (MKPC) paste in sulfate solution, pure MKPC pastes (M0) and MKPC pastes containing 20% fly ash (M1) were prepared and soaked in water and 5% Na2SO4 solution for long-term immersion corrosion test. The strength, volume deformation and water absorption rate of MKPC paste specimens at different soaking ages were measured. The pore structure, phase composition and microstructure of MKPC paste samples were also analyzed. The results show that adding a proper amount of fly ash can improve the strength and the pore structure of MKPC paste hardened for 28 under natural conditions. Compared with those of M0, the residual strength rates of M1 soaked in water and 5% Na2SO4 solution for 360 days both increased by more than 5%. At 360-day soaking age, the volume expansion rates of M1 soaked in water and 5% Na2SO4 solution were two orders of magnitude smaller than those of M0 under the same condition. Therefore, adding a proper amount of fly ash can improve the resistance of hardened MKPC paste to long-term water and sulfate corrosion. The morphological, filling and pozzolanic effects of fly ash enable MKPC paste to have a dense initial structure, which make corrosive media not easy to penetrate, newly generated phases containing sulfate filled the pores of hardened MKPC paste, further making the structure denser and reduce the structural deterioration of hardened MKPC paste under the condition of long-term sulfate erosion.

Feasibility study on sustainable magnesium potassium phosphate cement paste for 3D printing

3D printing of cementitious materials is an innovative and promising approach in the construction sector, attracting much attention over the past few years. Use of waste cementitious materials in the production of 3D printable components increases the sustainability and cost-effectiveness of this process. This work proposes an environmentally friendly 3D printable cementitious material involving the use of magnesium potassium phosphate cement (MKPC) with various ratios of fly ash replacement ranging from 0 to 60 wt% to increase the working time of the binder. Silica fume was used at up to 10 wt% to adjust rheological and mechanical properties. The performance of the developed MKPC binders with different formulations in the context of 3D printing was assessed via a detailed investigation of the workability, extrudability, buildability, compressive strength, porosity and microstructural analysis. Amongst the mixtures studied, the optimum MKPC formulation involving 60 wt% fly ash and 10 wt% silica fume with a borax-to-magnesia ratio of 1:4 was selected for a small-scale printing demonstration in line with its rheological and mechanical properties. Finally, a 20-layer component with a height of 180 mm was printed in 5 min to demonstrate the feasibility of the adopted mixture in 3D printing.

Experimental study on the deformation characteristics of magnesium potassium phosphate cement paste at early hydration ages

This paper aims to study the deformation characteristics of magnesium potassium phosphate cement (MKPC) pastes at early hydration ages. A jar unit was designed to measure the volume deformation of MKPC samples in balloon, and the volume deformation of MKPC prism specimens was tested with micrometer. Additionally, the hydration temperature of fresh MKPC pastes was measured. The phase compositions and micro-structure of MKPC paste were also analyzed. The results showed that there was a good nonlinear correlation between the volume shrinkage rates of MKPC sample in balloon and MKPC prism specimen from 5-h to 48-h hydration ages. The correlation coefficient (R2) was 0.991 and the correlation can be mathematically described as y = 2.7645 × 2 - 0.1672x + 0.0027. For calculating the linear volume deformation rate of MKPC pastes with controllable early hydration reaction rate, the length of MKPC prism specimens at 24-h hydration age (L24h) is suggested as the reference length (L0) in order to eliminate the early fluctuation of volume deformation. Adding some fly ash in MKPC led to higher peak values of the volume expansion rate and residual volume expansion rate of sample in balloon due to the morphology and activity of fly ash. The volume shrinkage rate of MKPC prism specimen containing 10% fly ash was only 2/3 of that of reference specimen M0 at 28-day hydration age, which was also due to the activity and micro-aggregate effects of fly ash.

Microstructural evolution/durability of magnesium phosphate cement paste over time in neutral and basic environments

This study investigates the evolutions of the mineralogy, pore solution and porosity of magnesium potassium phosphate cement pastes (MKPC) over time in neutral and basic environments, media that MKPC pastes might encounter when used as patch repair materials or as stabilization materials for hazardous wastes. Mineralogy evolutions were characterized by X-ray diffraction/Rietveld (XRD) and thermogravimetric analyses (TGA) and completed with scanning electron microscopy (SEM/EDX) microanalyses. Pore solution compositions (ICP-OES, IC) and the porosity of the materials were also analyzed. The results showed an amorphization of MKPC pastes over time that might be related to structural changes for the K-struvite phase. Achieving chemical equilibrium between the cementitious waters and the MKPC pore solution can result in porosity evolutions within the materials. Transport and mechanical changes might be expected, especially for MKPC pastes kept in basic media, since K-struvite [MgKPO4·6H2O], could be partially dissolved, leading to a higher total pore volume.

Properties of magnesium potassium phosphate cement pastes exposed to water curing: A comparison study on the influences of fly ash and metakaolin

Under moist condition the magnesium phosphate cement (MPC) exhibits poor water resistance. In this study, the effects of metakaolin (MK) and fly ash (FA) on the properties of magnesium potassium phosphate cement (MKPC) pastes exposed to water curing were investigated. The compressive strengths, water resistance and microstructures of the MKPC pastes containing 30 wt% and 50 wt% of MK or FA as replacements of MKPC under various water curing regimes for up to 180 d were examined. Results showed that the water curing yielded much slower compressive strength development and lower ultimate compressive strengths of all the MKPC pastes during the curing age in comparison to air curing. Incorporation of MK or FA improved the water resistance of MKPC pastes and the MK contributed more considerably than the FA. This may be attributed to the reaction of MK in the MKPC system, which led to the formation of an intermixed phase of struvite-K with Al and Si and/or other amorphous aluminosilicate-bearing phase and thus was beneficial for the microstructure densification, compressive strength increase, and water stability improvement of the reaction products in the MKPC pastes. In comparison, fewer contents of Al and Si were incorporated in the hydration product of MKPC pastes containing FA probably owing to the lower reactivity of FA in the MKPC pastes.

Mechanical properties of micro-steel fibre reinforced magnesium potassium phosphate cement composite

This paper presents the mechanical properties of micro-steel fibre (MSF) reinforced magnesium potassium phosphate cement (MPPC) composites with multi-composite retarder. The compressive strength, flexural strength, flexural toughness and flexural ductility of the MSF reinforced MPPC composite (MSFRMC) were experimentally explored. The variables of the experiment included sand-cement mass ratio, water-cement mass ratio, curing time and fibre volume fraction. In addition, the effect of different types of cement on the mechanical properties of MSF reinforced composites was investigated. It was found that with the increase of water content in the MPPC paste, the average compressive strength, flexural strength and flexural toughness of MSFRMC decreased significantly and the ductility of MSFRMC increased slightly. With the increase of MPPC content in the paste, the flexural toughness of MSFRMC increased significantly and the flexural ductility of MSFRMC increased moderately. With the increase of the addition of MSF, the compressive strength, flexural strength, flexural toughness and flexural ductility of MSFRMC improved significantly. The compressive strength, flexural strength, flexural toughness and flexural ductility of MSFRMC were high at the early stage of curing, especially during the first 3 days. The addition of MSF in MPPC composite improved the compressive strength, flexural toughness and flexural ductility significantly more than the addition of MSF in sulphoaluminate cement and ordinary Portland cement composites.

Experimental research on seawater erosion resistance of magnesium potassium phosphate cement pastes

This paper aims to study the resistance of magnesium potassium phosphate cement (MKPC) paste to seawater erosion. MKPC paste specimens were prepared and their compressive strength, mass change, hydration temperature and water absorption were measured. The phase composition and microstructure were also analyzed. The results show that the initial hydration age of the reference sample M1 under natural curing condition has a significant effect on its compressive strength development in seawater. The residual ratios of the compressive strengths of M1(3d) and M1(28d), hydrated for 3 and 28 days, respectively, under natural curing condition, are 76.6% and 91.1% at 360-day seawater immersion age, respectively. This also indicates that early-age M1 has poor resistance to seawater erosion. Adding some limestone (LS) powders and silica fume (SF) can significantly improve the strength of early-age MKPC paste. The 3-day compressive strength of M2 with LS and SF is about 50% higher than that of M1. After immersion for 360 days in seawater, the residual ratios of the compressive strength of M2(3d) and M2(28d) are both close to 93%. This implies that initial hydration age of M2 under natural curing condition has almost no effect on its compressive strength development in seawater. Compared with M1, M2 has higher resistance to seawater erosion, which is attributed to the higher hydration degree and more compact structure of M2. This paper might be useful for preparing MKPC-based materials that can meet the requirements of marine concrete.

Influence of fly ash and metakaolin on the microstructure and compressive strength of magnesium potassium phosphate cement paste

The influences of fly ash and metakaolin added as substitutions (by up to 50 wt%) of magnesium potassium phosphate cement (MKPC) on the microstructures and compressive strengths of the MKPC pastes were investigated. The results indicate that the aluminosilicate fractions of both fly ash and metakaolin are involved in the acid-base reaction of MKPC system, leading to a preferential formation of irregular crystalline struvite-K incorporated with Al and Si elements and/or amorphous aluminosilicate phosphate products. Metakaolin is more reactive than fly ash in the MKPC system. For the same addition dosage, the MKPC pastes containing metakaolin exhibit higher compressive strengths than the pastes containing fly ash. This is attributed to the formation of more highly reinforced microstructures and denser interfaces between the metakaolin particle and hydration products (e.g. struvite-K) in the MKPC paste containing metakaolin. Addition of 30 wt% metakaolin increases the compressive strengths of MKPC pastes at all test ages.

On the influence of Mg/P ratio on the properties and durability of magnesium potassium phosphate cement pastes

This paper investigates the influence of magnesium to phosphate molar ratio (Mg/P) on the properties of magnesium phosphate cement pastes (MPPC). The aim of the present study is to investigate the durability of these materials. The results show that excess potassium dihydrogen phosphate (KH2PO4) has a negative impact on overall properties. High quantities of KH2PO4 lead to poor water resistance, as the anhydrous phosphate remained unreacted in the microstructure is highly soluble and generates important values of open porosity when samples are immersed in water. Another drawback to excess KH2PO4 is the appearance of an efflorescence phenomenon, confirmed by XRD analysis. Finally, samples having a high Mg/P molar ratio show important swelling, especially when water is present in the system (autogenous conditions).

Potential to design magnesium potassium phosphate cement paste based on an optimal magnesia-to-phosphate ratio

Magnesium potassium phosphate cement (MKPC) is an important engineering material, and it has been attracting increasing attention. Properties of MKPC based materials depend largely on the magnesia-to-phosphate molar ratio (M/P) and the water-to-cement mass ratio (W/C). While the effect of W/C has been clear, experimental results of the effect of M/P in the literature seem to be inconsistent. In this paper, a model is developed to unify these experimental findings. The model hypothesizes that the strength of MKPC based materials is dominated by the volume ratio of magnesium potassium phosphate hexahydrate and space (MKP/space ratio), and the ultimate MKP/space ratio can be limited by the availabilities of the three reactants, i.e. magnesia, phosphate and water. It proves that, if the W/C of MKPC-based material is kept constant, there exists an optimal M/P that results in the highest compressive strength. It appears that the higher the W/C is, the smaller this optimal M/P will be.

Effects of graphene oxide on the properties and microstructures of the magnesium potassium phosphate cement paste

This paper presents the effect of graphene oxide (GO) on the properties and microstructures of the magnesium potassium phosphate cement (MKPC) paste with a lower water to solid ratio (w/s) of 0.15. The influence of GO on the workability, hydration degree, mechanical behavior and microstructures of the MKPC paste is systematically investigated. The experimental results showed that the addition of GO shorted the final setting time and decreased the workability of the MKPC paste, but the compressive and flexural strength of the MKPC paste were improved by a moderate addition of GO. It was clearly indicated that compared with the fresh MKPC paste, the addition of 0.05wt.% GO can improve the compressive and flexural strength of the MKPC paste by 6.8% and 8.3%, respectively. The improved mechanical strength is not only attributed to the excellent mechanical properties of GO itself, but also the higher hydration degree and lower porosity of the MKPC paste by the GO addition. The microstructures of the MKPC paste with GO addition of 0.05wt.% become much denser and better crystallization of the hydration products can be achieved, compared with the fresh MKPC paste. However, more GO addition 0.10wt.% has a negative effect on the crystallization of the hydration products and mechanical behavior of the MKPC paste. Finally, the FTIR spectra indicated that there was no chemical bonding between the hydration products of the MKPC paste and GO. It is concluded that GO, as a promising nanofiller, has a great potential for reinforcing the MKPC paste.

Influence of magnesia-to-phosphate molar ratio on microstructures, mechanical properties and thermal conductivity of magnesium potassium phosphate cement paste with large water-to-solid ratio

This paper describes the influence of the magnesia-to-phosphate (M/P) molar ratios ranging from 4 to 12, on the properties and microstructures of magnesium potassium phosphate cement (MKPC) pastes with a large water-to-solid ratio (w/s) of 0.50. The setting behavior, compressive strength, tensile bonding strength and thermal conductivity of the MKPC pastes, were investigated. The results show that an increase in the M/P ratio can slow down the setting reaction, and clearly degrade the mechanical strengths, but clearly improve the thermal conductivity of MKPC pastes. Furthermore, micro-characterizations including X-ray diffraction, scanning electron microscopy and thermogravimetric analysis, on the MKPC pastes reveal that a lower M/P ratio can facilitate better crystallization of the resultant magnesium potassium phosphate hexahydrate (MKP) and a denser microstructure. Moreover, strong linear correlations are found between the mechanical strengths and the MKP-to-space ratio, and between thermal conductivity and the volume ratio of the unreacted magnesia to the MKP.

Effects of water content, magnesia-to-phosphate molar ratio and age on pore structure, strength and permeability of magnesium potassium phosphate cement paste

In this study, the pore structure of magnesium potassium phosphate cement paste is investigated using mercury intrusion porosimetry. Several mix proportions, obtained by changing the magnesia-to-phosphate molar ratio (M/P) and the water-to-cement mass ratio (W/C) of the material, are involved. It is found that lower W/C and longer material age make the porosity lower and the pore structure finer. When the W/C is kept constant, both porosity and critical pore diameter are not monotonic functions of M/P, but the M/P of 6 gives the lowest porosity and the smallest critical pore diameter. Also, the M/P of 6 shows the highest compressive strength and the lowest intrinsic permeability. Based on the experimental results, empirical models describing the relations between the properties and pore structure parameters (porosity ϕ and critical pore diameter dc) of MKPC paste are developed. The compressive strength is inversely proportional to ϕ, and the intrinsic permeability is directly proportional to dc2ϕ.