Dead-burned Magnesia Research Articles

Additive manufacturing of alumina refractories by binder jetting

In this work, refractory components based on alumina were produced by binder jetting using a large-scale 3D printer. The formulation contained several particle fractions up to a grain size of 3 mm, equal to the printer resolution. The binder system contained fine dead burnt magnesia, milled citric acid and reactive alumina, which were added to the aggregate mixture to create the powder bed. Deionized water was deposited from the printer's nozzles and triggered the binding reaction between the magnesia and citric acid. After 24 h, the printed samples were removed from the powder bed, dried and sintered at 1600 °C for 5 h. Reactive alumina contributed to the in situ creation of magnesium aluminate spinel at high temperature. The samples were characterized in terms of Young's modulus of elasticity, bending and compressive strength in 2 directions (parallel and perpendicular to the printing direction). The broken parts were used to investigate physical properties such as the open porosity and bulk density. The microstructure was studied by means of computed tomography. Finally, powder samples were used to determine the phase composition at different stages of production by means of XRD.

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.

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.

Waste glass recycling in magnesium phosphate coatings for the fire protection of steel structures

The paper focus on the valorization of soda lime silica waste glass powder in the manufacture of fire protective coatings, for steel structures, based on magnesium phosphate cements (MPCs); waste glass powder is used to substitute part of dead burned magnesia (one of the main components of MPC) or to obtain lightweight aggregate (Gt) by alkali activation with NaOH solution followed by thermal treatment at 900 °C for 1 h. Commercially available expanded perlite is also used in MPCs to replace dead-burned magnesia or as lightweight aggregate. The MPCs mineralogical composition (assessed by XRD), microstructure and adhesion to steel plate (assessed by SEM) as well as the changes which occurs during the thermal treatment (assessed by DTA-TG analyses) are used to explain the fire behaviour and insulation properties of these coatings. The studied MPC coatings adhered very well to the steel plate, even after two consecutive fire tests (direct contact with butane flame for 60 min) and effectively decreased the substrate temperature. The coating thickness plays an important role in decreasing the temperature of the steel substrate; for thinner coatings the rate of temperature increase, in the first 10 min, is 3–4 times higher (depending on the coating composition) as compared with thicker coatings. The presence of lightweight porous aggregates, such as expanded perlite or Gt, combined with the increase of water content of MPC coatings, contributes to the decrease of the steel plate temperature during the fire test.

Static and dynamic mechanical properties of magnesium phosphate cement modified by metakaolin after high-temperature treatment

Magnesium phosphate cement (MPC) is widely used in fields of construction and fire-retardant coatings due to its excellent properties. The hydration properties of MPCs at high temperatures are the points of interests. In this work metakaolin was used to improve workability and mechanical properties of MPC, and static and dynamic mechanical properties and hydration products of the MPC after high temperature treatment were investigated. The results showed that the incorporation of metakaolin in MPC allows extending and importantly tuning the setting time of MPC. Notably, 40% replacement of dead burnt magnesia with metakaolin in MPC could extend the setting time of MPC to about 24 min. While the fluidity of MPC significantly decreased with increasing metakaolin content. Typically, increasing the temperature leads the deterioration of mechanical properties of the MPC. However, static and dynamic strength enhancement was also observed at 600℃. The main hydration products of MPC with metakaolin are struvite (MgNH4PO4·6H2O), unreacted MgO, aluminum phosphate (APH) and possibly amorphous substance containing aluminum. Moreover, the impact resistance of MPC at high temperatures were significantly improved with incorporation of metakaolin.

Development of Magnesium/Calcium Oxalate Cements

Magnesium oxalate cement, a novel alternative to portland cement, can be made at room temperature by reacting dead-burned magnesia and salts of oxalic acid. Since oxalic acid can be made using captured carbon dioxide, oxalate cements may even be carbon negative. However, emissions related with the decarbonation of magnesite at high temperatures make this hard to achieve. This study investigates the effect of replacing magnesia with granulated blast furnace slag on some physical and mechanical properties, as well as the mineralogy and microstructure of oxalate cements. Whewellite and Weddellite are identified when slag is used, in addition to Glushinskite which forms from magnesia. Slag-only mortars undergo faster but less complete reactions and show lower resistance to water than their magnesium oxalate counterparts. An equal-part combination of dead-burned magnesia and slag gives the highest 28-d strength (> 35 MPa), pH~7, and high water resistance.

Synthesis and Characterization of New Composite Materials Based on Magnesium Phosphate Cement for Fluoride Retention.

In this research work, new composite materials based on magnesium phosphate cement (MPC) were developed to evaluate the retention of fluorine from wastewater. This material was prepared with dead burned magnesia oxide (MgO), ammonium dihydrogen phosphate (NH4H2PO4), and some retarding agents. We chose to synthesize with hydrogen peroxide instead of water; alumina and zeolite were also added to the cement. The obtained optimal conditions were studied and analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), BET, and thermogravimetric analysis (TGA). The adsorbents showed a strong ability to remove fluoride from contaminated water, and the best defluoridation capacity was evaluated as 2.21 mg/g for the H2O2 cement. Equilibrium modeling was performed, and the experimental data were presented according to the isotherms of Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich.

Preparation and properties of magnesium phosphate cement with recycled magnesia from waste magnesia refractory bricks

Waste magnesia refractory bricks (WMRBs) are a high-quality magnesia resource with a MgO content of ＞80%. However, numerous WMRBs are not effectively utilized, leading to the waste of magnesia resources. WMRBs can be transformed into recycled magnesia (RM) with a MgO content of ＞82% by simply crushing and grinding the material. Using RM to prepare magnesium phosphate cement (MPC) can significantly cut the preparation cost of MPC and promote the efficient utilization of WMRBs. Herein, the chemical and mineral composition, hydration activity, and micro-morphology of RM are analyzed, and the physico-mechanical properties of MPC prepared with RM are studied. The results show that the chemical and mineral composition of RM is not significantly different from that of dead-burned magnesia (DM), but its hydration activity is remarkably lower than that of DM owing to the larger grain size of periclase and more regular particle morphology of RM. The setting time of MPC prepared with RM (MPCRM) increases significantly, and the hydration temperature rise decreases significantly, which is more conducive to the application of MPC to large-scale engineering. Although the early strength and long-term strength of MPCRM are lower than those of MPC prepared with DM, the 3 h compressive strength of MPCRM exceeds 25 MPa, while its 28-d compressive strength is close to 60 MPa; furthermore, MPCRM has a small drying shrinkage value and good water resistance, which can meet the requirements of repair construction.

Chrome-free qandilite (Mg2TiO4) refractory aggregates: Role of titania source and evaluation of thermal expansion coefficient

As used chrome-based refractories may contain toxic Cr6+, the chrome-free Mg2TiO4 phase is being considered which is known to improve hot strength and resistance to thermal shock and slag corrosion. As in situ Mg2TiO4 formation generates porosity, preformed aggregates are desirable. Dilatometric studies revealed that when dead burned magnesia is combined with industrial anatase instead of rutile, it expanded less (1 % versus 7.2 %) and the reaction occurred earlier (965 ℃ compared to 1120 ℃). After sintering at 1,600 ℃, rutile produced an aggregate with 3.8 % open porosity, whereas anatase led to 0 % porosity and a bulk density of 3.24 g/cm3 with a thermal expansion coefficient (70–1,600 ℃) of 13.5 × 10−6 K−1. Moreover, replacing rutile by anatase eliminates the undesirable MgTiO3 phase. The anatase-based aggregate consisted of a Mg2TiO4 matrix with residual unreacted MgO and an intergranular, refractory CaTiO3 phase. The rutile-based aggregate was deemed unsuitable due to a low-melting microstructure of forsterite contiguous with CaTiO3.

Magnesium Potassium Phosphate Cement-Based Derivatives for Construction Use: Experimental Assessment.

The presented research is focused on the development and testing of the magnesium potassium phosphate cement-based materials (MKPC-based). Firstly, the fresh state properties of the pastes consisting of dead burned magnesia powder, potassium dihydrogen phosphate, setting retarder borax applied in the range of 0–10 wt.%, and batch water were investigated. The aim of testing was to characterize the hydration process in dependence on the borax content. The properties of raw MgO powder were described by chemical composition and particle size distribution. The properties tested in fresh state included shear stress (viscosity), Young’s modulus of elasticity, and temperature; their time dependence was observed. The measurements started immediately after the mixing process. At the age of 14 days, basic structural and mechanical properties of the hardened pastes were obtained. The mixture with 5 wt.% of borax proved to be the most advantageous in terms of setting time, sample integrity, and mechanical strength; therefore, it was chosen as the binder for the following part of the study—MKPC-based mortar development. In the next step, the MKPC paste containing 5 wt.% of borax was supplemented by silica sand aggregate, and the resulting material was marked as a reference. Subsequently, three other mixtures were derived by replacing 100% of quartz sand by lightweight aggregate; namely by expanded glass aggregate, waste rubber from tires, and combination of both in ratio 1:1. The aggregates were characterized by chemical composition (except for the rubber granulate), and loose and compacted powder density. For the resulting hardened composites, basic structural, hygric, strength, and thermal parameters were investigated. The use of lightweight aggregates brought in a considerable decrease in heat transport parameters and low water permeability while maintaining sufficient strength. The favorable obtained material properties are underscored by the fact that magnesia-phosphate is considered to be a low-carbon binder. The combination of magnesia-phosphate binder and recycled aggregate provides a satisfying, environmentally friendly, and thermally efficient alternative to traditional Portland cement-based materials.

Synthesis and evaluation of Mg-Al hydrotalcite formation and its influence on the microstructural evolution of spinel-forming refractory castables under intermediate temperatures

The mechanical properties of in-situ spinel (MgAl2O4)-forming alumina-based castables under intermediate temperatures are of critical importance before the refractory lining system reaches normal operating conditions. The objective of this study is to elucidate the role of the hydrotalcite formed within a fine-grained castables matrix, in which no strength loss of the MgO-bonded alumina-based castables without silica fume was observed. Numerous fundamental studies were conducted to examine the factors influencing hydrotalcite formation within the blended pastes composed of MgO and Al2O3 nanopowders; dead burned or fused magnesia and Al2O3 nanopowder; dead burned magnesia and water-dispersed sol of fumed alumina by using: XRD and DSC-TG-EGA(MS). The XRD, FTIR and 27Al MAS NMR analysis of the hydrotalcite calcination products revealed that the spinel begins to form at temperatures as low as 700 °C. Finally, the physical properties, phase composition and microstructure of the refractory castables bonded with the hydrotalcite decomposition-routed nanostructured spinel were evaluated.

Influence of dipotassium hydrogen phosphate on properties of magnesium potassium phosphate cement

Magnesium potassium phosphate cement (MKPC) is usually prepared from dead burned magnesia (MgO) and potassium dihydrogen phosphate (KH2PO4, KDP). Here, MKPC was produced by mixing KDP and dipotassium hydrogen phosphate trihydrate (K2HPO4·3H2O, DKP) as phosphates. To study the influence of DKP on the hydration process and hardening, pH values, wettability between magnesia and initial solutions, hydration temperature development, initial setting time, fluidity, hydration products, and mechanical properties were measured. The results show that the decrease in the ratio of KDP to DKP leads to an increase in pH and a decrease in wettability, resulting in changes in the hydration process. In addition, setting behavior is also influenced by the ratio of KDP to DKP. When KDP/DKP＞2:1, as the ratio becomes smaller, the initial setting time gets shorter; otherwise, the opposite is true. These differences are attributable to the temperature change during the reaction, the distinction in wettability, and the different pH caused by the various ratios of KDP to DKP. Moreover, XRD analysis shows that the hydrated products are still MgKPO4·xH2O (x = 1 or 6), indicating that hydration products are basically unaffected. TGA analysis signifies that the amount of hydrated products in MKPC is responsible for the changes in the mechanical properties.

Surface modification of magnesium oxide and its effect on the performance of magnesium phosphate cement

In applying magnesium phosphate cement, controlling its setting time is of great importance, and the activity of the main raw material, magnesium oxide, is the key. In this study, dead-burned magnesia was modified with different amounts of water glass and different thermal treatment temperatures, and the effects of MgO on the macroscopic properties and microstructure of magnesium potassium phosphate cement (MKPC) were studied.The results show that the setting time of MKPC prepared with modified magnesia is prolonged, and the increase of the amount of water glass, the longer the setting time is.The setting time of the sample with 15% water glass and 300 °C treatment increased from 4.3 min to 17.5 min, an increase of 307%.The compressive strength results show that the surface modification of MgO decreased MKPC's early strength, which decreased with an increase in the water glass content and a decrease in the thermal treatment temperature.However, when the curing reached 7 days, the samples' compressive strength exceeded that of the reference group, and the strength development was optimum when the content of water glass was 5% with 300 °C treatment. The 28 d strength increased from 65.8 MPa of the reference group to 72.4 MPa, an increase of 10%.Through scanning electron microscopy (SEM), it was found that new phases were formed in the surface modified samples at the later stage of hydration. Combined with the results of energy-dispersive spectroscopy (EDS), it was confirmed that the magnesium silicate hydrate gels (M-S-H) were formed.The mercury intrusion porosimetry (MIP) test results show that these M-S-H gels can refine the pore size of MKPC, and the porosity of the sample with 5% water glass and 300 °C treatment was reduced from 6.6510% to 6.0429%, which explains the improvement in the MKPC compressive strength. Therefore, the surface modification of magnesium oxide can greatly extend its setting time without adversely affecting its later performance.

Microstructural characteristics of refractory magnesia produced from macrocrystalline magnesite in China

Sintered and fused magnesia (FM) produced from the macrocrystalline magnesite in China have attracted attention worldwide for the production of various refractories. Herein, dead burnt magnesia (DBM) with varying compositions was investigated. The results revealed that the periclase crystals of the DBM92 sample were subrounded to rounded euhedral, whereas the periclase crystals of the DBM95 sample were subhedral and idiomorphic. In addition, a significant amount of periclase–periclase bonding with straight boundaries was observed in the DBM97 sample. The periclase crystals of the DBM98 sample with no clear boundaries exhibited the densest packing among the samples. The silicate matrix around the periclase grains of the DBM92 sample contained forsterite and monticellite, whereas that around the periclase grains of the DBM95 sample was mainly composed of monticellite and merwinite. Dicalcium silicate and merwinite were observed in small amounts as interstitial phases in the DBM97 sample. In contrast, only dicalcium silicate, which was concentrated in small triangular pockets, was observed in the densely packed periclase grains of the DBM98 sample. The hot modulus of rupture tests revealed that an alkali-resistant DBM95-based brick can be prepared at a lime–silica ratio of <0.5. The best-quality DBM97-based brick can be prepared at a lime–silica ratio of approximately 2.2, which ensures that dicalcium silicate is the only interstitial phase. The periclase crystals of the two-step FM were significantly larger than those of the one-step FM and exhibited remarkably fewer silicate boundaries. The superior structure and large crystal size of the two-step FM can endow magnesia–carbon bricks with slag corrosion resistance.

Rheological properties of magnesium phosphate cement with different M/P ratios

Magnesium phosphate cement (MPC) with unique advantages of quick setting and high early-strength has been used as grouting materials. The rheological properties of MPC, being vitally important for site operation, need to be studied further. Dead burnt magnesia (MgO) and phosphate (ADP) are the main raw materials for preparing MPC. This study measured the rheological properties with different M/P ratios, and the mechanisms of M/P ratio were studied by characterizing the zeta potential, 1H NMR relaxation signal, water film thickness, mineral changes and electrical impedance. Experimental results indicate that both the yield stress (τ0) and plastic viscosity (η) decrease firstly and then increase gradually with increasing the M/P ratio from 2.5:1 to 4.5:1. Higher M/P ratio decreases the interparticle water film thickness and accelerates the initial hydration rate remarkably, which is responsible for the change in rheological parameters. Low field NMR, TG-DTG analysis and Nyquist plots confirm the quicker initial hydration in MPC with higher M/P ratio. The change in hydration products is very slight during the rheological property measurements. Nyquist plots show that the MPC paste with lower M/P ratios has more compact microstructure and lower porosity at several hours. In a word, initial hydration rate is the most important factor influencing the rheological properties of MPC, and the separation distance among solid particles is the secondary factor.

Study on the basic properties and mechanism of waste sludge solidified by magnesium phosphate cement containing different active magnesium oxide

Magnesium phosphate cement (MPC) containing different active magnesium oxides was used to solidify waste sludge. The fluidity, setting time, unconfined compressive strength, water resistance, shrinkage deformation, spontaneous imbibition, water absorption, thermal conductivity, leachate and particle shape and arrangement of the solidified body were studied and evaluated. Results showed that MPC can be used as a fast sludge curing agent. When the ratio of dead burnt magnesia to light burnt magnesia was 3:7, the strength was the highest. This suggested that appropriately reducing the reaction rate can quantitatively adjust struvite crystals and increase strength. The retarder reduced the reaction rate and increased the strength, which indirectly indicated that the strength was affected by the rate of formation of hydration products. The shrinkage mainly came from the dry shrinkage induced by the evaporation of water, the conversion of free water in the reaction into the bound water in the crystal, and the shrinkage induced by the heat released by the acid-base neutralization reaction. The unsaturated imbibition was mainly divided into the rapid imbibition stage, the unsteady imbibition stage and the equilibrium zone. Imbibition indexes have a linear relationship with the square root of time. For spontaneous imbibition height: the arrangement of regular hexagonal particles ≤ triangular arrangement < spherical arrangement. The equivalent diameters of different particle shapes and the corresponding pore shape correction factors were derived. This study provided a scientific basis for the feasibility of solidified waste sludge as a slope protection, landfill and sustainable green building material.

Assessment of magnesium potassium phosphate cement for waste sludge solidification: Macro- and micro-analysis

The feasibility and performance of eco-friendly and sustainable magnesium potassium phosphate cement (MKPC) to solidify urban river sludge are innovatively evaluated. To probe exhaustively into the mechanical performance and micromechanism of MKPC solidified dredged sludge, the effect of binder content, Mg/P molar ratio, retarder (borax) content and curing age are analyzed through unconfined compressive strength (UCS), mercury intrusion porosimetry (MIP), X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests. The experimental findings indicate the compressive strength is raised owing to an increase in MKPC amount and curing age. The isolation effect of sludge matrix weakens the retarding effect of borax, but reasonable borax content is beneficial to the strength improvement at longer curing age. The UCS of solidified samples with lower molar ratio (Mg/P = 3) increases rapidly at the early age while limitedly in later stage. The UCS reaches the maximum at the molar ratio of Mg/P = 4–5. The XRD and MIP analysis demonstrates that MgKPO4·6H2O (struvite-k) is the major hydration phase produced along with unreacted dead burned magnesia. Struvite-k can effectively fill the large pores (1–10 μm) and cause the transformation of large pores into small pores (0–0.1 μm), which has a good refinement performance for pore structure. The SEM images show that the prismatic or layered struvite-k forms preferentially and its final morphology is intimately connected with the Mg/P molar ratio. When the MKPC dosage and curing age increase, the struvite-k crystals are significantly densified and some microcracks appear due to the surface tension effect during the hydration heat-associated dehydration in crystal growth process. The synthesized crystal is fully developed and the crystal grows more completely under appropriate reaction conditions and reasonable relative concentration of reactants, which produces stronger interconnection between sludge particles and denser microstructure within matrix, contributing to the strength development of solidified sludge. In summary, the mechanical, eco-friendly and cost-efficient benefits could be highly anticipated from substituting MKPC for Portland cement (PC) in sludge solidification.

Environmentally favorable magnesium phosphate anti-corrosive coating on carbon steel and protective mechanisms

Polymer coatings are commonly used to protect carbon steels from corrosion but they are susceptible to weathering and many of them have environmental concerns. Therefore, we studied the possibility of an environmentally favorable inorganic magnesium phosphate cement (MPC) coating for protecting mild steel. A formulation suitable for coating steel was developed by compositional modification [i.e., incremental replacement of dead-burned magnesia (MgO) with magnesium hydroxide (Mg(OH)2)] to a road-repair MPC. This modification yielded an acceptable working time and prevented pore formation at the coating-steel interface. Corrosion monitoring by linear polarization and electrochemical impedance spectroscopy for 14 days found that, the MPC coating substantially increased the linear polarization resistance (Rp) [e.g., day 1: (8.2 ± 1.7) × 103 (nadir value) vs. 495 ± 55 Ω cm−2] and charge transfer resistance (Rct) (e.g., day 1: 9.3 × 103 vs. 3.8 × 102 Ω cm−2). The coated steel underwent neutral sodium chloride (NaCl) salt spray for 2400 h without visible rusting. Immersion for 24 h in liquids simulating the pore fluid indicated that, passivation by the excess MgO in the coating was a major contributor to its anti-corrosive property. Tafel polarization in the liquids found that, corrosion current density (Icorr) followed the rank: 3.5% NaCl solution (6.0 µA cm−2) > 3.5% NaCl solution containing MgO (3.6 µA cm−2) > 3.5% NaCl solution containing fragmented MPC (1.7 µA cm−2), suggesting that a physical barrier effect and dissolved phosphate ions improved its protection. This study shows that, MPC coating is a promising durable and environmentally favorable anti-corrosive material for protecting steel structures in some applications.

Preparation and Application of Self-Curing Magnesium Phosphate Cement Concrete with High Early Strength in Severe Cold Environments.

The early strength of magnesium phosphate cement (MPC) decreases sharply in severe cold environments ≤−10 °C, with the 2 h compressive strength falling to 3.5 MPa at−20 °C. Therefore, it cannot be used as a repair material for emergency repair construction in such environments. In this study, MPC is adapted for use in such cold environments by replacing part of the dead-burned magnesia (M) in the mixture with a small amount of light-burned magnesia (LBM) and introducing dilute phosphoric acid (PA) solution as the mixing water. The heat released by the highly active acid–base reaction of PA and LBM stimulates an MPC reaction. Moreover, the early strength of the MPC significantly improves with the increase in the Mg2+ concentration and the initial reaction temperature of the MPC paste, which enables MPC hardening in severe cold environments. Although the morphology of the reaction products of the MPC is poor and the grain plumpness and size of the struvite crystals are remarkably reduced, the early strength of MPC prepared in the severe cold environment is close to that of MPC prepared under normal temperature. Furthermore, the increases in the early reaction temperature and early strength of magnesium phosphate cement concrete (MPCC) are significantly improved when the PA concentration in the mixing water and the LBM/M ratio are 10% and 4–6% at −10 °C and 20% and 6–8% at −20 °C, respectively. Moreover, self-curing of MPCC can be realized even at −20 °C, at which temperature the 2 h and 24 h compressive strength of MPCC reach 36 MPa and 45 MPa, respectively.

Life cycle assessment of refractory waste management in a Spanish steel works

A life cycle assessment (LCA) is conducted on the current refractory waste management practices in a steel works in Spain producing around 6,000 tonnes of refractory waste in 2018. Management practices included direct reuse of spent magnesia-carbon (MgO-C) bricks, recycling of MgO-C bricks and high-alumina refractories by an external contractor and landfilling of monolithics and isostatic refractories, for which there were yet no established valorization routes. This current situation was compared to a hypothetical scenario where all refractories were disposed in landfill. The LCA included waste management activities (collection, transport, treatment) as well as production of substituted primary materials (dead-burned magnesia, calcined bauxite and new MgO-C bricks) produced in China and Germany. Results are discussed for four indicators: greenhouse-gas emissions, Non-renewable energy demand, land use and water use. Overall, per tonne of produced waste, the current management leads to a reduction or saving of 0.54 tonnes CO2-eq, 3 GJ primary energy from non-renewable sources, the occupation of 10 m2 of land during one year and the abstraction of 5 m3 freshwater. The results also show that reuse, in the particular case of spent MgO-C bricks, leads to higher benefits than recycling by At least a factor four. The robustness of these results is confirmed through the application of a Monte Carlo uncertainty analysis, as well as a sensitivity analysis focusing on a shift from coal to natural gas in the Chinese refractory industry.