Magnesium Phosphate Cement Mortar Research Articles

Role of Fly Ash in the Repair Interface between Magnesium Phosphate Cement and Cement Concrete

This paper examines a rapid repair material for cement concrete pavement based on magnesium phosphate cement (MPC) blended with fly ash (FA). Three kinds of stress forms for rapid repair of Portland cement concrete pavement (PCCP) were evaluated and the role of FA in the repair interface was studied. The optimal mixture ratio of FA-MPC mortar was determined, and the compressive and flexural strength tested. The composite specimens were then designed to test the interfacial bonding, tensile, and plain shear strength. Finally, the multi-scale mechanism of FA in the interface was analyzed by a pull-off adhesion test, nanoindentation test, scanning electron microscope (SEM)/energy dispersive spectrometer (EDS), Raman spectrum, and molecular dynamics simulation. The optimal content of FA in MPC mortar was 20%. The laminated beam structure had the maximum strength, and the interface load bearing capacity was the lowest. For further multi-scale analysis, the results of the pull-off adhesion test showed the adhesion strength of interface was increased by the curing age and loading rate. Adhesion capacity of basalt aggregate and FA-MPC was stronger than that of Portland cement mortar and FA-MPC. The elastic modulus of MPC-aggregate interface was more than that of MPC-OPC mortar. SEM/EDS and Raman spectrum results showed that the MPC blended with FA had cracks and defects at the interface formed with cement mortar. Less FA formed near the interface. The good adhesion ability of FA and basalt aggregate was verified by molecular dynamics simulation. By contrast, poor adhesion strength with cement mortar was confirmed.

Bond strength and interface characteristics between magnesium phosphate cement mortar and ordinary Portland cement concrete in a frigid environment

ABSTRACT The rapid repair of airport runways and concrete pavements using magnesium phosphate cement mortar (MPCM) in frigid environments requires the MPCM to have high bond strength with ordinary Portland cement (OPC). In this study, the effects of the pouring temperature of MPCM, surface roughness, water content, and strength grade of concrete on the interfacial bond strength and characteristics between MPCM and OPC were investigated in a frigid environment (−20°C ± 2°C). The results indicated that the 3-h compressive strength of MPCM reached 30 MPa, whereas the 3-h flexural bond strength between MPCM and OPC (BS MPCM-OPC) exceeded 2.5 MPa when the pouring temperature for the MPCM mixture is 0-5°C in a frigid environment. The moisture content of concrete before freezing had a more significant impact on BS MPCM-OPC than the influence of concrete roughness in a frigid environment; however, the influence of the concrete strength grade can be disregarded. The hydration heat of MPCM can quickly release with an increase in the pouring temperature, which accelerates phosphate dissolution and promotes the generation of struvite and the development of the BS MPCM-OPC. Furthermore, the interface bonding between MPCM and OPC involves physical bonding, mechanical anchoring, and weak chemical bonding in a frigid environment.

Strength and Microstructural Evolution of Magnesium Phosphate Cement Mortar in Plateau Environment

Climatic conditions in plateau areas can enormously affect the properties and microstructure of cement-based materials. This research investigates the strength development and microstructural changes in magnesium phosphate cement (MPC) mortars in a plateau environment. Experiments were conducted in parallel in a plateau area (Lhasa) and a plain area (Chengdu) to evaluate the effects of the water-to-binder ratio (w/b = 0.12, 0.14 and 0.16) and sand-to-binder ratio (s/b = 0.5, 0.75 and 1) on the compressive and flexural strength of MPC mortars. At the same time, hydration products were characterized via XRD, TGA, and SEM/EDX micro-analyses, and the porosity of the materials was also analyzed via MIP. The results demonstrated that curing in a plateau environment resulted in a decrease in workability and yielded higher strength at an early age (before 1 day) but degraded the long-term (180-day) strength of MPC mortars when compared with curing in a plain environment, irrespective of w/b and s/b ratios. Unlike the plain group, the plateau group revealed the deterioration of microstructures over time, including the decrease in struvite content, the morphology change in struvite crystals, and the increase in porosity, which resulted in the degradation of mechanical properties between 1 and 180 days. The strength loss can be effectively alleviated at lower w/b and s/b ratios.

Study on Bonding Performance of Magnesium Phosphate Cement Mortar in Chloride Corrosion Environment

The effects of nano‐SiO2 and hydroxypropyl methyl cellulose (HPMC) on the physical and mechanical properties of magnesium phosphate cement (MPC)‐based mortar under salt corrosion environment were studied. The variation of strength under salt corrosion environment was compared and studied. The chloride ion permeability of modified MPC mortar was evaluated by electric flux method. X‐ray diffraction (XRD) and scanning electron microscopy (SEM) were used to observe the composition and morphology of the samples under salt corrosion environment. Results showed that appropriate amount of nano‐SiO2 can significantly improve the compressive strength, flexural strength, and chloride ion penetration resistance of MPC‐based mortar under 3% NaCl solution corrosion. There existed an optimal dosage. The incorporation of HPMC would significantly reduce the compressive and flexural strength and the resistance to chloride ion penetration of MPC‐based mortar. Appropriate amount of nano‐silica and HPMC could improve the bonding performance of modified MPC‐based mortar under salt corrosion environment.

Investigation of Interface Fracture between BFPMPC and Cement Concrete

Recently, there has been wide concern about the mechanical properties of the repair interface for the magnesium phosphate cement mortar as a repair material with Portland cement concrete pavement. In this paper, based on the previous research results about basalt fiber reinforced and polymer modified magnesium phosphate cement (BFPMPC) mortar, the fracture behavior of the interfacial transition zone (ITZ) for BFPMPC and Portland cement concrete (PCC) was further pursued and studied. Firstly, a nanoindentation test was carried out on the repair interface with creep characteristics. Results showed that a synergistic effect and elastic moduli of multiple ITZs in repair interface were verified and determined. Then, the creep characteristic of ITZ was described by proposed fractional rheology characteristics in BFPMPC-PCC ITZ with further validation by finite element analysis. Finally, the interface fracture model combined with dislocation theory was proposed and analyzed. The results showed that satisfactory agreements had been obtained between the calculated results of interface fracture model and experiments. It was indicated that the fracture essence for BFPMPC-PCC ITZ was revealed by the interface fracture model. This finding, as a novel aspect and insight for interface fracture of bi-cementitious-based materials, was provided and the pursued direction of materials design for interface enhancement in pavement rapid repair was expected.

Properties of red mud blended magnesium phosphate cements: Workability and microstructure evolution

Alumina and iron oxide which constitute significant components of red mud can participate in the hydration reaction with phosphate. This paper partially replaced magnesia with red mud in the preparation of magnesium phosphate cement (MPC) and investigated the effects of red mud substitution on the performance of fresh and hardened properties of the MPC. The fresh properties of MPC were analyzed by measuring the setting time, fluidity, early hydration heat release and rheological properties, while the hardened mechanical properties were characterized by compressive strength measurement. The phase composition and micro-morphology were examined by XRD, SEM, EDS, and TG-DTA. The experimental results demonstrated that the incorporation of red mud prolonged the setting time, and postponed the occurrence of the peak temperature, and reduced the hydration reaction intensity. Regarding to the rheological properties, an increase in the dynamic yield stress was followed by a decline with gradually addition of red mud. Conversely, the plastic viscosity increased consistently. Concerning hardened properties, adding appropriate amount of red mud powders to MPC can significantly improve the compressive strength of hardened MPC mortars. The samples containing 15 % red mud yield the highest compressive strength, especially in the early strength, it was even more pronouncedly improved by 14.8 % comparing to the controlled samples at 3 h. Furthermore, detailed information about the modification effects of red mud on the MPC microstructure has been studied. The density of the MPC matrix was improved, microcracks and gaps appeared less, and some new hydrates could be determined, which is responsible for the improvement of the MPC’s mechanical properties.

Effect of Metakaolin on the Water Resistance of Magnesium Phosphate Cement Mortar

In order to study the effect of metakaolin on the water resistance of magnesium phosphate cement mortar, we took four factors as research objects: the setting time, fluidity, compressive strength, and water resistance of magnesium phosphate cement mortar. These were studied after adding equal gradations of metakaolin, and we then carried out SEM microcosmic experiments on the best group. The results showed that with the increase in metakaolin content, the setting time of magnesium phosphate cement decreased gradually, and the fluidity of the mortar decreased as a whole. The effect of metakaolin on the fluidity of the magnesium phosphate cement mortar was not significant when the content of metakaolin was less than 8%. Metakaolin was able to improve the pore structure of magnesium phosphate cement mortar; for example, it improved its compressive strength and water resistance due to both chemical and physical interactions. The water resistance of mortar samples in the same timeframe increased first and then decreased with the increase in metakaolin content. When the content of metakaolin was 12%, the water resistance of magnesium phosphate cement mortar achieved the optimal results. A 50.32% improvement of the 56 d strength retention rate and a 54.89% improvement of the 90 d strength retention rate proves its effective long-term water resistance.

Recycling of waste magnesia refractory brick powder in preparing magnesium phosphate cement mortar: Hydration activity, mechanical properties and long-term performance

Waste magnesia refractory bricks (MRBs) that remain from industrial production are landfilled in large quantities or recovered to prepare recycled bricks with low added value. In consideration of the significant level of magnesia in waste bricks, this work proposes to extract and recover magnesia from spent bricks for manufacturing magnesium phosphate cement mortar (MPCM). The essential attributes of recycled magnesia (RM) and its influence on the workability, hydration temperature rise, mechanical properties, phase evolution, long-term performance and microstructure of MPCM were analyzed. The results reveal that RM significantly prolongs the setting time and reduces the overall hydration heat release, which is beneficial to the application in large-scale engineering. The compressive strength of MPCM prepared by RM (MPCMRM) initially grew and then decrease with the increase of magnesia to phosphate ratio (M/P ratio). The optimal performance was observed at M/P = 4, and the compressive strength cured for 3 h and 28 d were 39.1 MPa and 68.5 MPa respectively. While the mechanical properties of MPCMRM were marginally inferior to those of dead-burned magnesia (DM), it was found to satisfy the early and long-term strength requirements of engineering repair materials. In addition, MPCMRM did not generate new mineral phases, and the crystal morphology of hydration products (struvite) was worse than that of mortar prepared by DM. MPCMRM also exhibits excellent water resistance and volume stability due to the crystal characteristics of low-activity RM. It seems that RM can completely replace DM to prepare MPCM and achieve satisfactory performance, which not only reduces the cost of repair materials, but also provides a fresh approach for the recycling of waste MRBs.

Evaluation of feasibility and performance of foamed Fire-Resistant coating materials

A preliminary study found high-performance cement mortar, geopolymer mortar, and magnesium phosphate cement mortar (MPCM) have the potential as new fire-resistant materials. In this study, foam was added to these three fire-resistant materials to further improve their rheological, mechanical, and fire-resistant performance and reduce costs. Systematic design and experimental programs were conducted. The results showed the addition of foam enhanced workability, adhesiveness, and fire resistance, allowing the materials to withstand higher temperatures and further delay heat transfer. A mixture of 70% MPCM and 30% foam was identified as the optimum design, which could withstand 1000 °C with low heat transfer rates.

The influence of calcium sulphoaluminate on the properties of low-cost magnesium phosphate cement mortars

Magnesium phosphate cement (MPC) exhibited many excellent performances, however, the high cost of raw materials of MPC limited its application in construction areas. Calcined dolomite powders containing CaO could not be applied in MPC production. The dolomite-bauxite-gypsum powders after calcination were used to replace dead burned MgO and the free-CaO in dolomite was transferred into calcium sulphoaluminate (ye'elemite, C4A3S‾). This study intends to investigate the performance of the low-cost MPC mortars prepared with calcined dolomite-bauxite-gypsum powders containing C4A3S‾. The setting times, fluidities, hydration temperature developments, compressive strength developments and volume changes of MPC mortars with C4A3S‾ were investigated. Furthermore, the differences in compressive strength, flexural strength and bond strength development of MPC mortars with C4A3S‾ in air and water curing were compared. Phase developments, microstructures and pore diameter distribution of MPC mortars with ye'elemite were also investigated. MPC mortars prepared with the calcined dolomite powders containing 51% C4A3S‾ and 26% MgO showed a setting time of 27 min, free-fluidity of 150 mm, 3-h compressive strength of 41 MPa, 7-d compressive strength of 58 MPa and 180-d compressive strength of 74 MPa. C4A3S‾ in MPC mortars could form some amorphous hydration products and its hydration may be promoted in water curing. The new amorphous phases may have filling and cementitious effects, leading to denser structures and higher strengths.

A novel insight into the interface fracture between magnesium phosphate cement mortar and cement concrete

ABSTRACT This paper focused on fracture problem about interfacial transition zone (ITZ) formed after repairing between magnesium phosphate cement (MPC) mortar and old Portland cement concrete (PCC) pavements. Firstly, the mode Ⅱ cracks on the ITZ were hypothesized and the interface fracture model with dislocation theory was proposed. The stress field at the crack tip was represented by Airy stress potential function and Fourier Transform, and the structural characteristics of the kernel solution of Cauchy were retained. Then, combined with the oscillation term, the Dirac function was replaced by the normal distribution function to obtain stable fracture parameters. Besides, the ϵ about 8 ‰ of the crack length in model was analyzed. Secondly, the elastic moduli of ITZs were obtained by nanoindentation experiments. Results showed the elastic modulus of ITZ between MPC mortar and ITZ-1 (the interface in concrete) was lowest. Such weakest position in repair interface was further verified by finite element analysis (FEA). Finally, a twodimensional model was developed to validate the fracture model. Simulation results were found consistent with the theoretical solution of interface fracture model. The developed interface fracture model can provide insights for evaluating the shear behavior of the interface between MPC mortar and concrete.

Effect of Biomass Ash on the Properties and Microstructure of Magnesium Phosphate Cement-Based Materials

The disposal of biomass ash (BA) will be of great importance for environmental protection and sustainability, and the aim of this study is to analyze the feasibility of the resourceful use of biomass ash in civil engineering materials. The effects of the content and type of biomass ash on the flowability, setting time, compressive strength, flexural strength, bonding strength, and drying shrinkage of magnesium phosphate cement (MPC) mortar were investigated. In addition, the effects of BA on the hydration and microstructure of MPC were investigated by X-ray diffraction (XRD), thermogravimetry, mercury intrusion porosimetry (MIP), and scanning electron microscope (SEM). The results showed that BA significantly affects the flowability and setting time of MPC mortar. The compressive and flexural strength of MPC mortars decreases with increasing amounts of BA. The drying shrinkage of MPC mortar specimens increases exponentially with the increase of BA content. The incorporation of BA will reduce the bonding strength of the MPC mortar, which is associated with increased drying shrinkage. The incorporation of BA into MPC results in low hydration product generation and poor pore structure. The incorporation of BA into MPC has a significant effect on the microstructure morphology and the hollow columnar-like hydration product may be formed by the reaction of BA with MgO in the paste.

High-Temperature Resistance of Modified Potassium Magnesium Phosphate Cement.

To study the high-temperature mechanical properties of potassium magnesium phosphate cement mortar and the high-temperature resistance of its laminates. Potassium magnesium phosphate cement (MKPC) was prepared by using heavy-burning magnesium oxide and potassium dihydrogen phosphate as the main raw materials, borax as the retarder, and compounded with a certain amount of fly ash and silica fume. The effect of the mass ratio of magnesium to phosphorus (M:P), compounded fly ash and silica fume on the setting time and mechanical properties of MKPC was investigated. Furthermore, based on the better M:P, the compressive strength of MKPC mortar was studied after 3 h of constant temperature at 400 °C, 600 °C, and 800 °C, and the effect of fly ash and silica fume on the high-temperature resistance of MKPC was analyzed. The high-temperature resistance of MKPC was further evaluated by analyzing the temperature variation of potassium magnesium phosphate cement laminate during a constant temperature of 650 °C for 3 h. The results showed that the mechanical properties of potassium magnesium phosphate cement were influenced by different raw material ratios, and the mechanical properties of potassium magnesium phosphate cement were optimal when M:P was 2:1, fly ash was 5% and silica fume was 15%. The internal temperature of MKPC laminate increased slowly with time, and its high-temperature resistance was better.

The mechanical and conductive properties of intelligent magnesium phosphate cement mortar

•Mechanical performance of MPM with conductive fibers is presented. •Conductive properties of MPM with conductive fibers is determined. •Self-sensing performance of MPM and the corresponding properties is investigated.

Water-resisting performances and mechanisms of magnesium phosphate cement mortars comprising with fly-ash and silica fume

Design of hydraulic structures using magnesium phosphate cement (MPC) composites is a still challenging issue because of the volume instability of MPC matrices in water environment. In this research, an attempt was taken to progress the water-resistance performance of MPC blended with fly ash (FA) and silica fume (SF). The FA was introduced as one third of magnesia in the mortar specimens, where SF dosages were 5%, 10% and 15% as the substitution of magnesia. Physico-mechanical and microstructural properties of mortar samples were examined considering the air and water curing systems. The analytical results bared that compressive strength (CS) and flexural strength (FS) of specimens composed with MPC + FA+5%SF were observed about 25% higher as compared to control. Both strength properties having the optimized content of SF (i.e. 5%) was reduced close by 3–4% in water system, whereas the pure MPC matrix exhibited around 20% less at 28 d. In addition, mass loss (ML) was found nearby 1.56% at 28 d. Moreover, SEM, EDS and XRD tests were detected that intermediate minerals namely Berlinite, Mullite, Lizardite and Enstatite were formed in the microstructures, and micro-cracks and micro-pores were significantly reduced. Consequently, compact microstructure was obtained that resulted the well water-resistant of proposed MPC mortar. The outcomes of this research might be a potential information for constructing the MPC-based water structures.

Investigating the Hybrid Effect of Micro-steel Fibres and Polypropylene Fibre-Reinforced Magnesium Phosphate Cement Mortar

To overcome the drawbacks caused by the intrinsic brittleness of cementitious materials, various types of fibres were incorporated as reinforcements. Extensive research on Ordinary Portland cement indicated that compared with the use of a single type of fibres, the mixed-use of multiple fibres can significantly improve both strength and toughness of the cementitious composites, which is referred to as the hybrid effect. However, such hybrid effect in multiple fibre-reinforced magnesium phosphate cement-based composite (HFRMC) still lack quantitative understanding. Therefore, this study conducted a series of experiments, including slump flow tests, compression tests, four-point bending tests and microstructure analysis, to investigate the hybrid effect of micro-steel fibres (MSF) and polypropylene (PP) fibres in HFRMC. Two types of mixed designs of HFRMC were conducted: 1. total fibres fraction (including both PP fibres and MSF) was fixed to be 1.6%; 2. PP fibres fraction was fixed to be 1.6% with different addition of MSF. Our results indicated that the slump flow of magnesium phosphate cement mortar varied around 7.6–8.8% with the hybrid use of MSF and PP fibres, while the flexural strength and toughness increased around 13.7–23.1% and 1.6–45.9%, respectively.

Preparation and Properties of Magnesium Phosphate Cement-Based Fire Retardant Coating for Steel.

Magnesium phosphate cement (MPC) is a potential inorganic binder for steel coating due to setting and hardening rapidly, and bonding tightly with steel. NH4H2PO4-based MPC as a fire-retardant coating for steel was investigated in this work. MPC coatings were prepared from MPC paste and MPC mortar with expanded vermiculite (EV). The physical-mechanical properties and fireproof performance of MPC coatings were investigated in detail. An infrared thermal imager was employed to collect the temperature distribution and temperature rise with time on the coating samples automatically. The X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analyses were carried out on the MPC coating after the fireproof test. Re-fire test and corrosion resistance were performed preliminarily on the MPC coating. The results showed that the fireproof performance of MPC coating met the fire protection requirement for steel as long as the thickness of the MPC paste coating was up to 10 mm, while the thickness of MPC mortar coating decreased to 4 mm when adding 40% EV (by mass). Dehydration and decomposition of reacted products in the hardened MPC coating were, to some extent, contributed to the excellent fireproof performance during the fire test. The slight ceramic formation and integration of MPC coating during the fire test would compensate for the decreasing of strength due to the dehydration and decomposition, so that the MPC coating would keep certain fireproof performance when undergoing fire again. MPC is suitable for a fire-retardant coating, while higher tensile bonding strength with steel and potential corrosion resistance on steel, as well as rapid surface drying and hardening can be achieved.

Fresh properties and compressive strength of MPC-based materials with blended mineral admixtures

To explore the feasibility of using blended mineral admixtures to improve the performance of magnesium phosphate cement (MPC) composites, fly ash (FA) + ground granulated blast furnace slag (GGBS) and fly ash (FA) + silica fume (SF) were used to partially replace the MPC, respectively, in this study. Their effects on the setting time and flowability of MPC paste and the compressive strength of MPC mortar (MPCM) were investigated through setting test, flowability test, and compressive test. The test results showed that the blended FA and GGBS adversely affected the setting time of MPC paste but slightly improved the flowability of MPC paste and the compressive strength of MPCM if an appropriate content was used. And the blended FA and SF adversely affected both the setting time and flowability of MPC paste. However, compared to the MPCM without any mineral admixture or with SF (FA) alone, the 25 % FA+ 5 % SF, 32.5 % FA+ 7.5 % SF, and 35 % FA+ 5 % SF could further enhance the compressive strength of MPCM. In addition, the XRD and SEM tests showed that the blended FA and GGBS mostly resulted in loose microstructure of MPCM. However, an appropriate content of blended FA and SF could lead to the formation of a dense microstructure, and a new magnesia-silica gel substance (MgSiO3) was generated, which improved the compactness and compressive strength of MPCM.

Evaluating the mechanical strength prediction performances of fly ash-based MPC mortar with artificial intelligence approaches

Introduction of Fly ash (FA) in the magnesium phosphate cement (MPC) mortars is considered as sustainable way to advance the microstructural characteristics and reduce the manufacturing cost of MPC products. However, artificial intelligence (AI) approaches are still need to forecast the strength properties of MPC compositions blended with FA and estimate the governing input elements for appropriate mix design with suitable contents. For this aims, the current research elected five AI models based on deep neural network (DNN), optimizable gaussian process regressor (OGPR) and gene expression programming (GEP) to judge the prediction accuracy of mechanical strength values of the MPC-FA compounds, where the literature data was collected for training the models. In addition, laboratory tests were conducted in this study for producing the data and validating the recommended AI methods. As is observed, DNN2 having 3 hidden layer and Bayesian optimization based Gaussian process regressor techniques presented prediction skills above 95% with errors below 5% at the training and validation phases. Moreover, sensitivity analysis of each input variable revealed that FA content has the prime impact on strength achievement of MPC-FA mixtures, which was corroborated by the correlation analysis between inputs and outputs of whole data points. Finally, forecasting the mechanical strength properties of FA-based MPC mortars using the DNN2 and OGPR methods might be applied in the practical field for reducing the workload, labor and material ingesting through optimizing the mix combinations.

Experimental Study on a Novel Modified Magnesium Phosphate Cement Mortar Used for Rapid Repair of Portland Cement Concrete Pavement in Seasonally Frozen Areas

Experimental Study on a Novel Modified Magnesium Phosphate Cement Mortar Used for Rapid Repair of Portland Cement Concrete Pavement in Seasonally Frozen Areas