Magnesium Potassium Phosphate Cement Research Articles

Thermodynamic study on the phase assemblage of MPC exposed to natural and accelerated carbonation conditions

AbstractMagnesium phosphate cement (MPC), which belongs to chemically bonded phosphate ceramics, is commonly applied as a repair material and waste stabilization/solidifications. However, studies on the influence of CO2 on the phase assemblages in MPC are limited. A thermodynamic simulation approach is employed to explore the influence of CO2 on the equilibrium phase assemblage of MPC. The mechanisms of natural and enforced carbonation of MPC have been investigated in this work. The results disclose that Mg carbonates are less likely to precipitate in magnesium ammonium phosphate cement (MAPC) cured under CO2, while MgCO3·3H2O is the only carbonation product of magnesium potassium phosphate cement (MKPC). The carbonation resistance of MAPC is better than that of MKPC. The increase of activity of magnesia employed in MPC obviously enhances the formation of brucite and slightly promotes the carbonation of MPC.

Tailoring magnesium potassium phosphate cement-based adhesive for the improved bonding performance of CFRP/concrete interface

Magnesium potassium phosphate cement (MKPC) with rapid hardening and early high strength can be used as inorganic adhesive for externally bonded carbon fiber reinforced polymer (CFRP). This study aims to tailor MKPC as inorganic adhesive with good infiltration and bonding strength for CFRP-reinforced concrete. The influences of water-to-binder (w/b) ratios and particle sizes of potassium dihydrogen phosphate (PDP) on the fresh properties, hydration kinetics, pore structure, and strength development of MKPC were studied. The bonding strength of CFRP/concrete with MKPC-based adhesive was evaluated by double-shear tests. Results showed that a lower w/b ratio and a finer PDP enhanced the strength of MKPC due to a refined microstructure. The bonding strength of CFRP/concrete was dependent on the rheological characteristics and strength of MKPC. The use of a coarse PDP in MKPC was beneficial to enhance the bonding strength of CFRP/concrete. MKPC with coarse PDP and a w/b ratio of 0.2 resulted in the greatest bonding strength, corresponding to a 20% enhancement compared to epoxy resin. The superior bonding performance of MKPC was ascribed to its higher strength, a better infiltration for CFRP, and a larger MPC/concrete interfacial transition zone (ITZ).

Preparation of magnesium potassium phosphate cement from magnesite tailings under lower calcination temperature

The identification of a more economical source of magnesium, coupled with reduced preparation costs, is essential for enhancing the competitiveness of magnesium potassium phosphate cement (MKPC) production. This study explores the utilization of magnesite tailings, waste materials produced during the mining and ore dressing process of magnesite, as raw materials for the preparation of low-cost dead-burnt magnesium oxide (MgO-T) at comparatively low calcination temperatures. The research demonstrated that impurities, specifically silicon and calcium, formed compounds with MgO during calcination, leading to the encapsulation of certain MgO particles. This encapsulation reduced both the specific surface area and hydration activity of the MgO. Despite the calcination temperature of 1100 °C being significantly lower than that of commercial dead-burnt MgO (MgO-C) at 1700 °C, MgO-T exhibited lower activity compared to MgO-C, which rendered it more suitable for the production of MKPC. Under optimal conditions, characterized by a molar ratio of magnesium to phosphorus (M/P) of 3, a borax retarder content of 15%, and a water-cement ratio of 0.15, the MKPC prepared using MgO-T demonstrated an initial setting time exceeding 60min and achieved a compressive strength of up to 79.6MPa. XRD and microstructural analyses revealed that struvite-K was the primary strength phase; however, the porosity of the specimens had a more pronounced effect on strength. By producing lower activity MgO at reduced calcination temperatures, this method obviates the need for complex purification processes when utilizing magnesite tailings, thereby decreasing the costs associated with MKPC preparation and presenting promising applications for the future.

Anti-Corrosion Performance of Magnesium Potassium Phosphate Cement Coating on Steel Reinforcement: The Effect of Boric Acid.

It has recently been found that magnesium potassium phosphate cement (MKPC) paste coating applied on the surface of steel reinforcement can effectively retard the onset of corrosion and suppress corrosion reactions. However, the fast-setting nature of MKPC-which is a merit in repair-can be problematic in a practical engineering process of coating the steel reinforcement with MKPC paste. To address this problem, boric acid (H3BO3) was added as a retarder in an MKPC formulation to prolong the setting time. This work investigated the impact of boric acid (at 5% by weight of MgO) on the anti-corrosion performance of MKPC paste coating through a series of electrochemical (EC) tests. The results showed that the anti-corrosion performance of MKPC paste coating for a mild steel bar could be interfered with by the presence of boric acid. In the same testing situation (immersed in 3.5 wt.% NaCl corrosion solution), the polarization resistance and corrosion current density of the group including boric acid were inferior and exceeded the corrosion thresholds prior to the control group without boric acid. Meanwhile, the time constant phase in the frequency range from 1 Hz to 10 kHz was rarely observed, implying that the presence of boric acid probably impaired the formation of the passivation layer. This decrease in anti-corrosion performance of MKPC paste coating could be related to the larger volume fraction of pores in the range from 0.1 to 10 µm that are formed during the initial stage of coating formation.

Effect of zinc oxide on synthesis of magnesium potassium phosphate cement

Magnesium potassium phosphate cements (MKPCs) have been developed for the rapid repair of civil engineering structures and the solidification of hazardous waste. However, the rapid curing and high heat of hydration of MKPCs pose challenges in bulk production. To address this problem, the use of zinc oxide as a novel retarder for MKPC synthesis was investigated. Compared with conventional retarders like boric acid, zinc oxide was found to delay the initial hydration of MKPCs significantly, although its effect diminishes over time. The addition of zinc oxide accelerated the dehydration of MgHPO4.7H2O to MgHPO4.3H2O. It is likely that Zn2+ ions inhibit the dissolution of magnesium oxide by surface adsorption or dehydration of Mg(H2O)62+. Using zinc oxide as a retarder provides sufficient mixing time, making it advantageous for practical applications where a controlled setting time is crucial.

Sustainable Utilization Method of Zn-Bearing Iron Dust in Magnesium Potassium Phosphate Cement: Mechanical Properties and Stabilization Behavior of Zn

Sustainable Utilization Method of Zn-Bearing Iron Dust in Magnesium Potassium Phosphate Cement: Mechanical Properties and Stabilization Behavior of Zn

Study on the high temperature performance of magnesium and potassium phosphate cement mixed with sucrose and its mechanism of action

Magnesium potassium phosphate cement (MKPC) is susceptible to high-temperature damage when used as a fireproofing coating or structural repair material and as a nuclear waste encapsulation shell, and the strength loss and dimensional shrinkage at about 100 °C severely limit the application of MKPC. Therefore, in this paper, the macroscopic properties, physical phase composition and microscopic morphology of sucrose-modified MKPC at room and high temperatures (20°C, 50°C, 120°C) are investigated from the perspective of modified retarder, and the mechanism of sucrose's influence on MKPC at high temperatures and the optimal dosage are speculated, and the Pearson correlation analysis is performed on the various macroscopic and microscopic indexes. The results showed that 2.5 % sucrose doping could reduce dimensional shrinkage by 59.6 % and increase late strength by 21.8 % at high temperatures, significantly optimizing the macroscopic properties of MKPC. The inhibition of the high-temperature decomposition product MgKPO4·H2O by sucrose and the reduction of pores formed by water evaporation are among the main reasons for the optimization of the properties. Additionally, the adsorption of amorphous products to form new polymers that fill the pores is also an important reason for the optimization of performance. However, Pearson's correlation analysis showed that too much sucrose contributed to the reduction of pulp strength, and its optimal dosage should be between 2 % and 3 %.

Durability of Magnesium Potassium Phosphate Cements (MKPCs) under Chemical Attack.

Magnesium phosphate cements (MPCs), also known as chemically bonded ceramics, represent a class of inorganic cements that have garnered considerable interest in recent years for their exceptional properties and diverse applications in the construction and engineering sectors. However, the development of these cements is relatively recent (they emerged at the beginning of the 20th century), so there are still certain aspects relating to their durability that need to be evaluated. The present work analyses the chemical durability of magnesium potassium phosphate cements (MKPCs) during 1 year of immersion in three leaching media: seawater, a Na2SO4 solution (4% by mass) and deionized water. For this, pastes of prismatic specimens of MKPC, prepared with different M/P ratio (2 and 3), were submitted to the different chemical attacks. At different ages, the changes on the mechanical strengths, microstructure (BSEM, MIP) and mineralogy (XRD, FTIR, TG/DTG) were evaluated. The results obtained indicate that, in general terms, MKPC systems show good behavior in the three media, with the more resistant system being the one prepared with a M/P molar ratio of 3.

Rheology and buildability of sustainable 3D printed magnesium potassium phosphate cement composites incorporating MgO-SiO2-K2HPO4

MKPCs with unique advantage of superior strength, excellent durability and high thixotropy provides alternative binder for 3D printing. The acid-base reaction between dead burnt MgO and K2HPO4 is intense and the retarding effect is limited in practical cases. This paper presents the investigation of MKPCs incorporating MgO-SiO2-K2HPO4 binders prepared with dead burnt MgO, soluble phosphate (K2HPO4•3 H2O) and silica fume (SF). The effects of magnesium-to-phosphate (M/P) mass ratio, SF and slag ratios on rheological properties and the printability were tailored and setting time reached to 50 min by increasing the alkaline environment of the solution. In addition, the pozzolanic activity of SF and slag were synergistically motivated and thus promoted the precipitation of K-struvite, MgSiO3 and C-(A)-S-H hydrates. Two-stage Athix values were calculated to describe the structuration rate. It is confirmed excellent thixotropy was dominated by early-age physical re-flocculation and later-age cement hydration. Although the structure deformation increased significantly as SF content increased, the beneficial effect of SF content on the increase of strength is significantly important than that of adverse effect on the buildability. Slag powder and fine aggregate were incorporated to make the binders more “printable”. A cylinder with 16 layers on optimum mixture was printed without deformation.

New magnesium cement optimized in MgO-K2HPO4-SiO2 system and its hardening performance

Silica fume (SF) is commonly used in producing magnesium potassium phosphate cement (MPC) and magnesium silicate hydrate cement (MSHC). However, its roles in magnesium cement would differ from each other when mono hydrogen phosphate was involved. This paper aims to find optimized compositions in MgO-K2HPO4 DKP- SF system with comprehensive considerations of the micro and macro performances. Ternary contour maps were designed to determine the optimized proportions of MgO, DKP and SF. The hydration reaction mechanism was analyzed with a variety of techniques including XRD, SEM, TG-DTG and NMR. Magnesium cement containing 60–75 % dead-burnt MgO, 10–25 % DKP and 12.5–25 % SF exhibited excellent compressive strength and volume stability. K-struvite and M-S-H were confirmed to promote the hydration and hardening performance of the blended MgO-DKP-SF system with optimized proportions. An excess of soluble K+ from DKP may precipitate as KOH and then dissolute in solution, increasing pH and release of Si from SF.

Investigation of self-desiccating cement-based materials for dihydrogen sequestration: Interactions between γ-MnO2/Ag2CO3 getter and the cement matrix

Mitigating the release of dihydrogen resulting from metal corrosion or water radiolysis is an important issue for the disposal of certain types of cemented radwaste packages. The approach investigated in this work consists in adding an oxide getter (γ-MnO2/Ag2CO3) to the cement matrix. Since the efficiency of the getter decreases under wet environment, two self-desiccating binders (calcium sulfo-aluminate and magnesium potassium phosphate cements), are used to obtain significant desaturation of the pore network by the sole hydration reactions. The getter slightly influences the rate of cement hydration at early age, but has no effect afterwards. Sorption of ions released by dissolution of cement phases onto γ-MnO2 is evidenced, as well as partial or total destabilization of silver carbonate. Nevertheless, the getter still enables to reduce strongly the outgassing of dihydrogen from mortars encapsulating Al-metal, which opens new perspectives to improve the conditioning of waste producing H2 in a cement matrix.

Effect of coal fly ash and CO2 curing on performance of magnesium potassium phosphate cement

This study explores the combined effects of coal fly ash (FA) and CO2 curing on the flexural strength, compressive strength, and water resistance of magnesium potassium phosphate cement (MKPC). Additionally, the hydration products and microstructure of MKPC and MKPC-FA blends are examined using X-ray diffraction (XRD), thermogravimetric analysis (TGA), mercury injection porosity (MIP), and scanning electron microscopy (SEM). The results demonstrate that carbonation curing effectively improves the mechanical strength and water resistance of MKPC-FA blends by refining the pore structure and reducing porosity. Incorporating fly ash into magnesium phosphate cement leads to a longer setting time and appropriate enhancement in the water resistance of MKPC-FA blends. It should be mentioned that the mechanical strength of MKPC-FA blends declines with increasing fly ash content, and carbonation curing can partially ameliorate these negative effects. Therefore, both incorporating fly ash and storing carbon dioxide have positive effects on the durability and environmental sustainability aspects associated with MKPC preparation.

Interaction of aluminum alloys with MKPC and Portland-based cements on the metal-matrix interface

Cementitious matrices are considered for the immobilisation of radioactive aluminium. The processes occurring at the interface between both materials are relevant for the long-term stability of the repository. The present study compares Ordinary Portland cement (OPC), CEM I-type, with alternative Portland blended cement (CEM IV and CEM I + 50% silica fume) and magnesium potassium phosphate cement (MKPC). Al A1050 and Al AA5754, with 3.5% Mg, have been used as reactive metal alloys. Two exposure conditions were employed: (1) water immersion and (2) isolated in sealed plastic films. Long-term corrosion monitoring (Ecorr, icorr and Vcorr) was evaluated to understand the effect of Al reactivity in the matrix. The associated H2 release was quantified to understand the changes at the metal/matrix interface. Furthermore, pore ion concentrations and the pore microstructure were evaluated. The metal/matrix interface alterations were analysed at the end of the tests. The study revealed that after 300 days of water immersion, the CEM I + 50% of silica fume matrix had reduced the pore solution pH down to 10.5 compared to CEM I, which remained high alkaline (pH 12.9), and CEM IV with no significant decreases (pH 12.3). In contrast, MKPC showed the lowest pH (7–9.8). Low Al corrosion rates were found with MKPC, followed by CEM I + 50% SF according to their lower pH. A corrosion product layer of 90-50 μm thickness was observed at the Al/CEM I matrix interface constituted of aluminium and oxygen. Furthermore, enrichment in Al was detected in the matrix (around 1 mm depth), causing the formation of ettringite nodules. Voids were detected at the interface level associated with the high volume of H2 released. The MKPC matrix showed no alteration at the metal/matrix interface due to the lower reactivity of the matrix and lower H2 release. A homogeneous and dense region (30 μm) rich in phosphorous, potassium and magnesium was observed near the metal surface.

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+.

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.