Potassium Magnesium Research Articles

High-temperature properties of fly ash and silica fume composite magnesium potassium phosphate cement

Magnesium potassium phosphate cement (MKPC) is extensively employed in patching materials and fireproof coatings owing to its outstanding properties. This study employs a blend of fly ash and silica fume to substitute a portion of the cementitious materials, aiming to enhance the performance of MKPC under high-temperature conditions. The relative proportions of fly ash and silica fume were considered. MKPC was subjected to cubic compression and three-point flexural tests. Additionally, the phase and microstructure of MKPC were also characterized using XRD and SEM. The compressive strength, flexural strength, visual appearance, and mass loss of MKPC specimens were evaluated before and after exposure to various high temperatures. Following exposure to elevated temperatures, MKPC specimens without fly ash and silica fume doping showed a significant decrease in strength. Conversely, specimens doped with 15 % or 10 % silica fume exhibited the most remarkable strength enhancement. The dehydration of K-struvite at elevated temperatures was the primary reason for the mass loss and strength reduction of MKPC. Silica fume and fly ash can facilitate the sintering of phases and amorphous particles derived from K-struvite decomposition, leading to the formation of ceramic-like structures. Notably, the contribution of silica fume to the sintering effect exceeds that of fly ash. Therefore, blending fly ash and silica fume proves to be an effective approach to enhancing the high-temperature resistance of MKPC.

Long-term performance of magnesium phosphate cement concrete prepared in the winter

Previous studies have established the strength development of magnesium ammonium phosphate cement (MAPC) concrete and magnesium potassium phosphate cement (MKPC) concrete at sub-zero temperatures. However, the long-term performance of these magnesium phosphate cement (MPC) concrete, prepared at low temperatures and exposed to complex environmental effects of cold regions, remains underexplored. This study monitored the mechanical properties and durability of MAPC concrete at different ages for 520 d, which fully demonstrated the feasibility of MAPC concrete for winter construction in cold regions. At the age of 520 d, MAPC concrete had a compressive strength of over 65 MPa and excellent resistance to chloride ion penetration with a measured charge of 129.1 C, and could endure more than 250 rapid freeze-thaw cycles. In contrast, MKPC concrete exhibited significant disadvantages in terms of freeze-thaw resistance during natural freeze-thaw cycles. No expansive substances or damage were found in MKPC concrete that was prepared at low temperatures and subsequently experienced temperature increases along with accelerated hydration. The relatively poor freeze-thaw resistance of MKPC concrete could be attributed to a combination of factors including larger capillary size and volume, reduced air content, greater air-void spacing factor, wider and weaker interface zone between cement mortar and coarse aggregate.

In-depth insight into the driving factors of the compressive strength development of MKPC based on interpretable machine learning methods

Magnesium potassium phosphate cement (MKPC) is a kind of Mg-chemically bonded phosphate ceramic commonly used for rapidly repairing dilapidated structures. In this study, a compressive strength dataset of MKPC was constructed, and four advanced machine learning (ML) algorithms (XGBoost, RF, GBDT and ANN) were selected to establish a high-precision compressive strength prediction model of MKPC. The SHAP and PDP methods are also used for interpretability analysis of ML-MKPC models. The XGBoost model has good generalizability and reliability while achieving high prediction accuracy. The RF and GBDT models performed similarly to the XGBoost model on the training set but performed poorly on the testing set. The ANN model is poorly trained on both the training and testing sets, with a risk of underfitting. The R2 of the XGBoost model at the different compressive strength stages still reaches above 0.80, indicating that it not only captures the complex relationships of the overall dataset well but also effectively predicts the staged strength dataset. Feature importance analysis revealed that the curing age (T), water-to-binder ratio (W/B), mineral admixtures-to-binder ratio (MA/B) and phosphate-to-magnesium ratio (P/M) are the principal variables affecting the compressive strength of MKPC. The partial interpretation shows that the optimum value range is determined when W/B is 0.10–0.18, MA/B is 0–0.20, P/M is 0.40–1.0, and R/M is 0–0.12. The composition of mineral admixtures with high-Ca, high-Si and low-Al systems seems to be more conducive to participating in the hydration reaction of MKPC. The ML-MKPC compressive strength prediction model developed in this study can provide theoretical support for the subsequent composition design and performance optimization of MKPC.

Magnesium Potassium Phosphate Cement for Encapsulation of Ash from Burning Radioactively Contaminated Wood

When wood from Chornobyl radioactively contaminated forests is burned in the Incinerator created in the Chornobyl zone under support of the European Commission, ash with a high content of Cs radionuclides is formed. Recently, magnesium potassium phosphate cements (MKPC) have been proposed as an effective alternative to Portland cement-based protective matrices. In this work, MKPC compositions were optimized for laboratory-scale encapsulation of inactive model ash, used as surrogate of the radioactive ash. The influence of the MgO/KH2PO4 ratio and the amount of added model ash on the temperature and setting time of the MKPC paste were determined. The compressive strength after 28 days of MKPC curing without ash was 22.7 MPa. After adding 20 wt.% model ash, the compressive strength of the MKPC matrix was 34.5 MPa, 40 % model ash was 23.4 MPa and 60 % model ash was 8.3 MPa. Thus, the accepted amount of the added ash that does not initiate the essential strength decrease of the MKPC matrix is 40 %. X-ray diffraction analysis of the MKPC matrix with 40 % model ash shows the presence of the main phase – K-struvite MgKPO4·6H2O. In addition to K-struvite, there are peaks of unreacted MgO and compounds such as calcite CaCO3, quartz SiO2 and potassium chloride KCl, which are parts of the model ash. It has been shown that ash components such as hydroxyapatite Ca5(PO4)3(OH) and calcium oxyphosphate Ca4(PO4)2O are involved in the formation reaction of phosphate cement and it contributes to increasing of the permissible amount of ash encapsulated into the MKPC matrix. SEM images of MKPC samples demonstrate a dense structure with well-connected ash particles. The leaching behavior of the MKPC matrices and their thermal stability were also assessed using immersion tests and thermogravimetric analysis, respectively. The Cs leaching rate of 1.2×10-4 g/cm2·day from the MKPC matrix containing 40 % MA is lower than the rates typical for cement waste forms. The obtained results demonstrate the possibility of using new magnesium-potassium phosphate cement as a matrix for the encapsulation of ash from the combustion of radioactively contaminated wood in the Chornobyl zone.

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.

Effect of seawater on hydration of magnesium potassium phosphate cements

Magnesium potassium phosphate (MKP) cement-based materials could be suitable for concrete repairs in marine environments. In this study, the effect of seawater on the hydration of MKP cements at different magnesium-to-phosphate molar ratios was investigated using mainly diluted suspensions. Compared to deionized water, seawater can slow down MKP cement hydration, which is attributed to the combined effect of Na+, K+, Mg2+and Ca2+ ions. The Cl− and SO42− in seawater can reduce the pH in MKP cements. The amount of seawater influences the hydrate assemblages, resulting in mainly K-struvite at low liquid-to-solid (l/s) ratio of 0.25, to hazenite at l/s ratio of 5 and to bobierrite at high l/s ratio of 50, consistent with the thermodynamic calculations. The modelling and experiments showed the destabilization of K-struvite to hazenite by Na+ and the formation of bobierrite and possibly amorphous hydroxyapatite by Mg2+ and Ca2+.

Molybdenum tailings calcination for phase composition of Si-Ca-K-Mg fertilizers: Modification of phase transformation and its effects on plant growth

The high value and comprehensive utilization of molybdenum tailings for the production of fertilizer of calcium silicon magnesium potassium (Si-Ca-K-Mg fertilizer) was investigated, considering the abundant potassium feldspar (K-feldspar) content in the tailings. The effects of calcination temperatures, times, and additives (CaCO3 and CaSO4) on reaction degree, final product type, and phase transition process were studied. The results indicated that at 1200 °C, calcination time of 120 min and m(molybdenum tailings):m(CaCO3):m(CaSO4) = 1:0.7:0.2, the final product met Type I product requirements according to Chinese national standards GB/T36207-2018 as well as extremely low heavy metal contents. In the calcination process, CaO resulted from CaCO3 absorbs the liquid phase produced during K-feldspar decomposition to form CaSiO3, facilitating the K-feldspar decomposes into KAlSiO4. Simultaneously, the Ca2+ in CaSO4 replaces undecomposed K-feldspar’s K+. Additionally, CaO reacts with Al2O3, MgO, and SiO2 to produce gehlenite (Ca2Al2SiO7) and akermanite (Ca2MgSi2O7), providing plants with Ca, Mg, and Si elements. The nutrients of the fertilizer were readily assimilated by plants, improving acidic soil without causing any harm to the soil or vegetation. This work offers new ideas for large-scale and high-value utilization of molybdenum tailings as well as addressing the shortage of soluble potassium resource in China.

Influence of Ultrafine Fly Ash and Slag Powder on Microstructure and Properties of Magnesium Potassium Phosphate Cement Paste.

This study investigated the influences of ultrafine fly ash (UFA) and ultrafine slag powder (USL) on the compressive strengths, autogenous shrinkage, phase assemblage, and microstructure of magnesium potassium phosphate cement (MKPC). The findings indicate that the aluminosilicate fractions present in both ultrafine fly ash and ultrafine slag participate in the acid-base reaction of the MKPC system, resulting in a preferential formation of irregularly crystalline struvite-K incorporating Al and Si elements or amorphous aluminosilicate phosphate products. UFA addition mitigates early age autogenous shrinkage in MKPC-based materials, whereas USL exacerbates this shrinkage. In terms of the sustained mechanical strength development of the MKPC system, ultrafine fly ash is preferred over ultrafine slag powder. MKPC pastes with ultrafine fly ash show greater compressive strength compared to those with ultrafine slag powder at 180 days due to denser interfaces between the ultrafine fly ash particles and hydration products like struvite-K. The incorporation of 30 wt% ultrafine fly ash enhances compressive strengths across all testing ages.

Impact of Zinc Oxide on the Structure and Surface Properties of Magnesium–Potassium Glass–Crystalline Glazes

Impact of Zinc Oxide on the Structure and Surface Properties of Magnesium–Potassium Glass–Crystalline Glazes

A study on particle size of slag on properties of magnesium potassium phosphate cement

AbstractMagnesium potassium phosphate cement (MKPC) presents a broad application prospect in engineering fast repairing, reinforcement, hazardous material, and national defense construction, due to its good properties such as fast hardening and early strength, high bonding strength, and wide environmental adaptability. In this work, the effect of slag with different particle size on the workability, mechanical properties, and hydration products of MKPC was investigated. The results showed that the compositions of slag in the particle size range 75–300 µm are quite different. A decrease in the particle size of slag resulted in reduction of the fluidity of MKPC from 175 to 108 mm and shortening the setting time from 1300 to 850 s. The compressive strength of MKPC (curing for 3 h) does no change much as the particle size of slag varies, and slag with the particle size in the range of 115–300 µm slightly increase the 3‐h‐compressive strength of MKPC, since the compressive strength values are in the vicinity of 13 ± 2 MPa. As the curing age increases, physical effect and hydration activity of slag concurrently dominate the mechanical properties of MKPC, and when the hydration activity of the slag exceeds the adverse effect caused by fine particles, the compressive strength increases. Slag particle size can significantly improve the long‐term compressive strength of MKPC, which can effectively reduce the cost and can be used as a repair material with better durability.

Novel insights into the amorphous feature of granulated blast furnace slag-supplemented magnesium potassium phosphate cement and its effect on bulk properties

This paper reports the effects of MgO:PO4 (3, 9, and 15) ratio and granulated blast furnace slag dosage (0 wt%, 40 wt%, and 80 wt%) on the phase formation, microstructure, and bulk properties of magnesium potassium phosphate cement (MKPC). Results show that the supplementary of slag and the increment in MgO:PO4 inhibit the formation of K-struvite via X-ray diffraction. While the detections of 29Si magic-angle spinning nuclear magnetic resonance spectroscopy and scanning electron microscope-energy dispersion spectrum further unveil the dissolution of slag and the formation of Q4 amorphous structure in slag supplemented MKPCs. Although the Q4 structure increases the interplanar spacing of K-struvite even leads to its amorphization in MKPCs with high MgO:PO4 (>9), the optimum coexisting of K-struvite and Q4 structure minimizes the porosity and maximizes the compressive strength of MKPCs with MgO:PO4=3 and 40 wt% slag. These results are crucial to developing optimal mixture formulations of MKPC with superior mechanical performance and lower costs associated with their production.

Medical waste incineration fly ash-based magnesium potassium phosphate cement: Calcium-reinforced chlorine solidification/stabilization mechanism and optimized carbon reduction process strategy

The traditional solidification/stabilization (S/S) technology, Ordinary Portland Cement (OPC), has been widely criticized due to its poor resistance to chloride and significant carbon emissions. Herein, a S/S strategy based on magnesium potassium phosphate cement (MKPC) was developed for the medical waste incineration fly ash (MFA) disposal, which harmonized the chlorine stabilization rate and potential carbon emissions. The in-situ XRD results indicated that the Cl− was efficiently immobilized in the MKPC system with coexisting Ca2+ by the formation of stable Ca5(PO4)3Cl through direct precipitation or intermediate transformation (the Cl− immobilization rate was up to 77.29%). Additionally, the MFA-based MKPC also demonstrated a compressive strength of up to 39.6 MPa, along with an immobilization rate exceeding 90% for heavy metals. Notably, despite the deterioration of the aforementioned S/S performances with increasing MFA incorporation, the potential carbon emissions associated with the entire S/S process were significantly reduced. According to the Life Cycle Assessment, the potential carbon emissions decreased to 8.35 × 102 kg CO2-eq when the MFA reached the blending equilibrium point (17.68 wt.%), while the Cl− immobilization rate still remained above 65%, achieving an acceptable equilibrium. This work proposes a low-carbon preparation strategy for MKPC that realizes chlorine stabilization, which is instructive for the design of S/S materials.

Preparation and properties of phosphogypsum based plastering mortar

In order to reduce the negative impact of phosphogypsum accumulation on the environment, this paper uses phosphogypsum, magnesium slag, potassium dihydrogen phosphate, fly ash and calcium carbide slag to prepare gypsum based plaster mortar as a cementing material, and explores the optimal ratio of the composite cementing material through orthogonal experimental design. The mechanical properties of the test block were analyzed by the flexural and compressive strength of the test block, and the slurry properties were evaluated by the initial and final setting time, stability measurement, fluidity test and water resistance test. The experimental results show that when phosphogypsum: potassium magnesium phosphate: fly ash: When titanium gypsum =50:25:20:20, the strength of the composite system can reach 2.98Mpa in 60 days, and the initial setting time is 118min and the final setting time is 223min, which meets the requirements of GB/T25181-2019 building gypsum plaster mortar that the initial and final setting time is greater than 60min and less than 480min. Its fluidity is 237mm, which meets the requirement that the fluidity of GB/T25181-2019 building gypsum plaster mortar is greater than 100mm.

A preliminary study on preparation and properties of magnesia based clinker with zeolite and magnesite

Dead burnt magnesia is the main raw material of magnesium phosphate cement, which is widely used in road repair works. However, due to the high energy consumption and high cost in the production and preparation process of dead burnt magnesia, its application is limited to a certain extent. In this work, a magnesia based clinker containing a considerable number of magnesium oxide and magnesium silicates phase was prepared by co-firing of zeolite powder and magnesite at low temperature (1200–1300 ℃). The results show that high temperature than 1200℃ results in less free MgO. Intermediate phases 2CaO·MgO·2SiO2 formed in clinker at 1250℃ and 1300℃. The workability of the pastes made from calcined cement clinker was proved to be better than that of traditional magnesium potassium phosphate cement (MKPC). The compressive strength can reach 30 MPa after curing for 7 days. Compared with the traditional MKPC, the hydration heat of the new magnesia based cement is greatly reduced. Lower calcination temperature on the cement clinker resulted in fewer cracks and pores on the surface of magnesia based cement and denser structure of the hydration products.

MgO/KH2PO4 and Curing Moisture Content in MKPC Matrices to Optimize the Immobilization of Pure Al and Al-Mg Alloys.

Magnesium Potassium Phosphate Cements (MKPCs) are considered a good alternative for the immobilization of aluminium radioactive waste. MKPC composition and moisture curing conditions are relevant issues to be evaluated. The corrosion of pure aluminium (A1050) and AlMg alloys (AA5754) with 3.5% of Mg is studied in MKPC systems prepared with different MgO/KH2PO4 (M/P) molar ratios (1, 2, and 3M) and moisture curing conditions (100% Relative Humidity (RH) and isolated in plastic containers (endogenous curing)). The Al corrosion potential (Ecorr) and corrosion kinetic (icorr and Vcorr) are evaluated over 90 days. Additionally, the pore ion evolution, the matrix electrical resistance, the pore structure, and compressive strength are analysed. The corrosion process of Al alloy is affected by the pH and ion content in the pore solution. The pore pH increases from near neutral for the 1M M/P ratio to 9 and 10 for the 2 and 3M M/P ratio, increasing in the same way the corrosion of pure Al (AA1050) and AlMg alloys (AA5754). The effect of Mg content in the alloy (AA5754) becomes more relevant with the increase in the M/P ratio. The presence of phosphate ions in the pore solution inhibits the corrosion process in both Al alloys. The MKPC physicochemical stability improved with the increase in the M/P ratio, higher mechanical strength, and more refined pore structure.

Effect of K+ Diffusion on Hydration of Magnesium Potassium Phosphate Cement with Different Mg/P Ratios: Experiments and Molecular Dynamics Simulation Calculations.

Magnesium potassium phosphate cement (MKPC) is formed on the basis of acid-base reaction between dead burnt MgO and KH2PO4 in aqueous solution with K-struvite as the main cementitious phase. Due to the unique characteristics of these cements, they are suitable for special applications, especially the immobilization of radioactive metal cations and road repair projects at low temperature. However, there are few articles about the hydration mechanism of MKPC. In this study, the types, proportions and formation mechanism of MKPC crystalline phases under different magnesium to phosphorus (Mg/P) ratios were studied by means of AAS, ICP-OES, SEM, EDS and XRD refinement methods. Corresponding MD simulation works were used to explain the hydration mechanism. This study highlights the fact that crystalline phases distribution of MKPC could be adjusted and controlled by different Mg/P ratios for the design of the MKPC, and the key factor is the kinetic of K+.

Effect of surface treatment by mineral admixture-magnesium potassium phosphate cement on the mechanical properties, durability, and microstructure of recycled aggregate concrete

Magnesium potassium phosphate cement (MKPC), a clinker-free cement, was used as the recycled coarse aggregate (RCA) surface strengthening material to enhance the mechanical properties and durability of recycled aggregate concrete (RAC) under two surface treatment methods. The effect of strengthening slurry properties on RAC reinforcement was investigated by adding different types and amounts of mineral admixtures (MA). An interface model was prepared to detect the microscopic characteristics of interfaces. The results indicated that all MA-MKPC slurries improved the RAC macroscopic performances, attributed to the fact that MA-MKPC slurries effectively improved the RCA surface pore structure and promoted a secure connection with RCA or new mortar. MA-MKPC slurry properties significantly affected the performances of modified RAC, and the MA optimal dosage of 10 % was found. Furthermore, the pre-coating without pre-curing method outperformed the pre-coating plus pre-curing method, forming an enhanced new interfacial transition zone within the RAC.

Phase and microstructure evolution of the hydration products of magnesium phosphate cements under sulfuric acid environments

Magnesium phosphate cement is a good canditate of repairing material served under harsh environments and its acid resistivity has drawn a lot of interest, but rare detailed research on the evolution of its physiochemical features has been reported. In this work, the impacts of sulfuric acid solution (pH = 2 – 7) on the hydration products of magnesium potassium phosphate cement and magnesium ammonium phosphate cement. Results showed that the main hydration products, struvite/struvite–K, experienced partial corrosion in the absence of the sulfuric acid solution replacement. The phase transition of struvite–K is clearly visible, leading to the formation of the new crystalline hydration product Mg3(PO4)2•22 H2O. The initial blossoms cluster microstructure transformed into a diffused dispersion that resembled a plate. When the solution was replaced, struvite/struvite–K underwent significant corrosion and even complete decomposition in more acidic (pH = 2, 3) solutions.

Cs+ Promoting the Diffusion of K+ and Inhibiting the Generation of Newberyite in Struvite-K Cements: Experiments and Molecular Dynamics Simulation Calculations.

Struvite-K cements, also called magnesium potassium phosphate cements (MKPCs), are applicable for particular applications, especially the immobilization of radioactive Cs+ in the nuclear industry. This work focuses on how Cs+ affects the hydration mechanism of struvite-K cements because newberyite and brucite in the hydration products are deemed to be risky products that result in cracking. Experiments and molecular dynamics simulations showed that Cs+ promoted the diffusion of K+ to the surface of MgO, which greatly facilitates the formation of more K-struvite crystals, inhibiting the formation of newberyite and brucite. A total of 0.02 M Cs+ resulted in a 40.44%, 13.93%, 60.81%, and 32.18% reduction in the amount of newberyite and brucite, and the Cs immobilization rates were 99.07%, 99.84%, 99.87%, and 99.83% when the ratios of Mg/P were 1, 3, 5, and 7, respectively. This provides new evidence of stability for struvite-K cements on radioactive Cs+ immobilization. Surprisingly, another new crystal, [CsPO3·H2O]4, was found to be a dominating Cs-containing phase in Cs-immobilizing struvite-K cements, in addition to Cs-struvite.

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.