Castable matrix,additives and their role on hydraulic binder hydration

The influence of alumina-based castable matrices, dispersants and Li₂CO₃ on the hydration process of different hydraulic binders was evaluated. The experiments were followed by temperature measurements with the time, the oscillatory rheometry technique and the normal force during the penetration of a blade through the material’s surface. By doing these tests, the chemical features of binder hydration, working and demolding time of suspensions could be assessed. The experiments have shown that the alumina-based matrix presented an accelerating effect which is related to its sodium content. Citric acid and ammonium citrate were the dispersants with the highest delaying effect and could be effectively associated with the accelerator (Li₂CO₃) in order to adjust the setting time of the suspensions. Nonetheless, their accelerating and retarding effect varied according to the hydraulic binder.     

Does a tiny amount of dispersant make any change to refractory castable properties?

A growing demand for refractory castables with specific behaviors has given rise to a continuous technological evolution, mainly due to the broad knowledge of hydraulic binders available nowadays. The high alumina cements remain as the most important hydraulic binders for castables. Nevertheless, calcium aluminate bound castables still show a characteristic drop of strength at intermediate temperatures, which could also be affected by the castable chemical additive. Thus, this paper aims to highlight the influence of dispersants on the refractory castable properties with the firing temperature. It was noticed that the hydrates formed during the curing process of castable depends on the dispersing additive used. The FS60, a polycarboxylate ether, induced the AH₃ formation and its decomposition resulted in a more stable hydrate (AH), which increased the splitting strength with the thermal treatment temperature. At a high temperature, the CA₂ and CA₆ formation is also favored in the presence of this additive. However, it did not bring benefits to the castables creep behavior, resulting in a less tough structure.     

Novel drying additives and their evaluation for self-flowing refractory castables

The drying step of dense refractory castables containing hydraulic binders is a critical process, which usually requires using slow heating rates due to the high explosion trend of such materials during their first thermal treatment. Thus, this work investigated the performance of alternative additives to induce faster and safer drying of self-flowing high-alumina refractory castables bonded with calcium aluminate cement (CAC) or hydratable alumina (HA). The following materials were analyzed for this purpose: polymeric fibers, a permeability enhancing compound (RefPac MIPORE 20) and an organic additive (aluminum salt of 2-hydroxypropanoic acid). The drying behavior and explosion resistance of the cured samples were evaluated when subjecting the prepared castables to heating rates of 2, 5 or 20 °C/min and the obtained data were then correlated to the potential of the drying agents to improve the permeability and mechanical strength level of the refractories at different temperatures. The collected results attested that the selected additives were more efficient in optimizing the drying behavior of the CAC-bonded compositions, whereas the HA-containing castables performed better when the aluminum-based salt was blended with a small amount of CAC (0.5 wt%), which changed the binders hydration reaction sequence and optimized the permeability level of the resulting microstructure. Consequently, some of the designed compositions evaluated in this work showed improved drying behavior and no explosion was observed even during the tests carried out under a high heating rate (20 °C/min).     

Self-flowing high-alumina phosphate-bonded refractory castables

Phosphate refractories have a great potential to be applied in petrochemical industries as they present suitable properties at the temperature range used in fluid catalytic cracking units. This study addresses the development of high-alumina self-flowing castables bonded with H₃PO₄ solution (48 wt% concentration) or a mixture of phosphoric acid and monoaluminum phosphate (MAP) solutions, using MgO as a setting agent. Two polyphosphates (Budit 3H and 6H) and citric acid were evaluated as dispersant additives for these castables. The compositions were characterized by measuring their free-flow and temperature evolution over time, working and setting times, cold and hot mechanical strengths, drying behavior and explosion resistance, eroded volume and thermal shock resistance. The results indicated that high flowability (free flow >100%) could be attained when adding the selected polyphosphates to the mixtures, whereas citric acid acted mainly as a retarder agent for the castables’ setting. Moreover, free-flowing compositions with a suitable working time were obtained when combining H₃PO₄+MAP solutions as main binders. The thermo-mechanical tests pointed out that the most promising designed refractory (containing mixture of H₃PO₄+MAP and 0.5 wt% of Budit 3H) presented similar or even a better performance than a benchmark commercial vibratable product used in petrochemical units.     

The effect of SiO2 additions on barium aluminate cement formation and properties

Barium aluminate cements have been synthesized by barium carbonate, alumina, kaolin and colloidal silica as starting materials. The effects of the source of SiO₂ and of firing temperature on phase formation and physical properties of the fired cements have been studied. Cement samples were characterized using XRD, SEM, EDX. The setting time and heat of hydration of cements were also evaluated. The barium aluminate cements were mixed in castables. Cold crushing strengths evaluated, and values compared to those obtained using calcium aluminate cement (Secar 71). Mixtures of BaCO₃ and Al₂O₃ were targeted to produce BaAl₂O₄; which had fast set time, expansive behavior and lower strength compared to samples with SiO₂ additions. SiO₂ additions, regardless of source, resulted in BaAl₂Si₂O₈ (celsian) formation. The prepared samples had short setting times and higher mechanical properties in comparison with standard calcium aluminate cement.     

Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Temperature and common-ion effect on magnesium oxide (MgO) hydration

MgO based refractory castables draw wide technological interest because they have the versatility and installation advantages of monolithic refractories with intrinsic MgO properties, such as high refractoriness and resistance to basic slag corrosion. Nevertheless, MgO easily reacts with water to produce Mg(OH)₂, which is followed by a large volumetric expansion, limiting its application in refractory castables. In order to develop solutions to minimize this effect, a better understanding of the main variables involved in this reaction is required. In this work, the influence of temperature, as well as the impact of the chemical equilibrium shifting (known as the common-ion effect), on MgO hydration was evaluated. Ionic conductivity measurements at different temperatures showed that the MgO hydration reaction is accelerated with increasing temperature. Additionally, different compounds were added to evaluate their influence on the reaction rate. Among them, CaCl₂ delayed the reaction, whereas KOH showed an opposite behavior. MgCl₂ and MgSO₄ presented similar results and two other distinct effects, reaction delay and acceleration, which depended on their concentration in the suspensions. The results were evaluated by considering the kinetics and the thermodynamics of the reaction, and the mechanical damages in the samples that was caused by the hydration reaction.     

Microstructure and phase evolution of alumina-spinel self-flowing refractory castables containing nano-alumina particles

The microstructure and phase composition of alumina-spinel self-flowing refractory castables added with nano-alumina particles at different temperatures are investigated. The physical and mechanical properties of these refractory castables are studied. The results show that the addition of nano-alumina has a great effect on the physical and mechanical properties of these refractory castables. With the increase of nano-alumina content in the castable composition, the mechanical strength is considerably increased at various temperatures. It is shown that nano-alumina particles can affect formed phases after firing. The platy crystals of CA₆ are detected inside the grain boundaries of tabular alumina and spinel grains in samples fired at 1500 °C. CA₆ phase can be formed at lower temperatures (1300 °C) with the addition of nano-alumina particles. As a result of using nanometer-sized alumina particles with high surface area, the solid phase sintering of the nano-sized particles and CA₆ formation can occur at lower temperatures.     

浇注料用新型固态复合结合剂的研究

用自制的5种新型固态复合结合剂进行了浇注料的试验，研究结果表明：这5种固态结合剂所结合的浇注料性能优良，并能解决一般化学结合剂高温下收缩较大的问题。     

Alkali activation of a novel calcium‐silicate hydraulic binder with CaO/SiO2 = 1.1

A quasi‐amorphous low‐calcium‐silicate hydraulic binder, with an overall CaO/SiO₂ (C/S) molar ratio of 1.1, was produced. This cementitious material was then hydrated with aqueous solutions containing 3 wt% alkalis (either NaOH, Na₂CO₃ or Na₂SiO₃). The evolution of the hydration processes of the samples were monitored by compressive strength testing, XRD, FTIR, ²⁹Si and ²⁷Al MAS NMR, isothermal calorimetry and TGA. It was found that the nearly exclusive hydration product formed was a C‐S‐H phase with a semi‐crystalline structure. More importantly, the paste prepared with the Na₂SiO₃ solution developed compressive strength values similar to those of ordinary portland cements (OPC) with faster early age kinetics. In addition, the isothermal calorimetry results indicated that these new hydraulic binders present much lower heat of hydration values compared with a traditional OPC. The results presented here open the possibility of producing cement with a compressive strength comparable to that of OPC but with lower CO₂ emissions during the production process and with lower hydration heat related problems during the production of concrete structures.     

Heat Evolution Tests with Calcium Aluminate Binders and Castables

A simple device is described for the determination of the rate of temperature increase during the hydration of calcium aluminate binders and castables under semiadiabatic conditions. Automatically recorded time-temperature curves are shown for commercial binders of various compositions. The complex relations between the rate of heat evolution and other properties of commercial and synthetic binders were studied by varying such factors as composition, crystal development, and fineness of the binder. The effect of additions of polyelectrolytes was also explored. It was found that, for a given binder composition, the thermal history of the binder and the amount of admixtures influenced the rate of hydration most markedly. Some correlation between heat evolution and strength development was indicated.     

Crystal structure refinement and hydration behaviour of doped tricalcium aluminate

In this paper analytical evidence on crystal structure and hydration behaviour of C₃A solid solutions with MgO, SiO₂, Fe₂O₃, Na₂O and K₂O is given. Samples were prepared using an innovative sol-gel process as precursor, examined by X-ray powder diffraction, infra-red spectroscopy and the crystal structure was refined by the Rietveld method. A significant shift of lattice parameters was found for C₃A solid solutions with SiO₂, Fe₂O₃ or Na₂O but only minor changes were detected for K₂O. The hydration of C₃A solid solutions in the absence of CaSO₄ was accelerated for samples doped with SiO₂ or K₂O and it was retarded in the case of MgO, Fe₂O₃ or Na₂O. The hydration in the presence of CaSO₄ was accelerated when C₃A was doped with K₂O or Na₂O, whereas Fe₂O₃ strongly retarded the hydration. The doping with SiO₂ nearly had no influence on the hydration, the effect of MgO was not straight forward.     

A soft X-ray microscope investigation into the effects of calcium chloride on tricalcium silicate hydration

Calcium chloride (CaCl₂) is one of the most recognized and effective accelerators of hydration, setting, and early strength development in portland cement and tricalcium silicate (C₃S) pastes. The mechanisms responsible for this acceleration, as well as the microstructural consequences, are poorly understood. Soft X-ray transmission microscopy has recently been applied to the study of cementitious materials and allows the observation of hydration in situ over time. This technique was applied to the examination of tricalcium silicates hydrating in a solution containing CaCl₂. It appears that CaCl₂ accelerates the formation of “inner product” calcium silicate hydrate (C-S-H) with a low-density microstructure.     

The role of hydraulic binders on magnesia containing refractory castables: Calcium aluminate cement and hydratable alumina

Previous work by the authors has shown that the effects of calcium aluminate cement (CAC) and hydratable alumina (HA) can modify the magnesia hydration behavior in aqueous suspensions. As a consequence of these studies, the present paper highlights how varying the content of these binders can affect magnesia hydration in refractory castables using pH, apparent volumetric expansion, mechanical strength and porosity measurements and hydration–dehydration tests. Furthermore, as mechanical strength, porosity and refractoriness also play an important role in these materials, binder-free, magnesia-free and magnesia-and-binder-free samples were also tested as references. It was found that the deleterious effects of magnesia hydration can be greatly minimized by the binder and its selection content.     

Binding additives with sintering action for high-alumina based castables

Great efforts have been made recently to totally or partially replace calcium aluminate cement (CAC) by alternative materials in refractory castables, in order to attain an enhanced thermomechanical performance of these ceramic linings at intermediate temperatures (600–1200 °C). Besides that, using additives that induce earlier sintering/densification of the refractory microstructure may also reduce the energy costs derived from the production of pre-formed pieces. Based on these aspects, this work investigated the viability of replacing CAC by calcium carbonate (CaCO₃) or calcium hydroxide [Ca(OH)₂] to ensure a suitable binding action and effective sintering/densification of the designed compositions at intermediate temperatures. Six high-alumina castables containing these alternative additives or their blend were prepared and their green mechanical strength, apparent porosity and Young's modulus evolution with temperature were evaluated within the 30–1400 °C range. After that, the most promising compositions were characterized via X ray diffraction and thermomechanical tests, such as cold and hot modulus of rupture, thermal shock resistance, etc. Although the selected binders did not result in specimens with green mechanical strength values as high as the ones for the cement-bonded materials (2–8 MPa versus ∼18 MPa, respectively), they could be demolded and handled without any problems. CaCO₃ and/or Ca(OH)₂-bonded compositions presented a sintering effect at intermediate temperatures (600–1000 °C) due to the so-called “sintering-coarsening-coalescence” phenomenon. These transformations favored the faster sintering/densification of the tested castables, resulting in samples with improved cold and hot mechanical strength at 900 °C, reaching values within the range of 28–30 MPa instead of 10–13 MPa for the CAC-bonded one. After firing the evaluated compositions at higher temperatures (up to 1500 °C), all compositions presented similar results regarding their modulus of rupture or thermal shock resistance.     

A comparative study of ordinary and mineralised Portland cement clinker from two different production units Part II: Characteristics of the calcium silicates

Portland cement clinkers from two production units were investigated in order to determine the effects of mineralisation on alite and belite; Plant 1: ordinary clinker (P1) and clinker mineralised with CaF₂₊CaSO₄ (P1m); Plant 2: ordinary clinker (P2) and two clinkers mineralised with CaF₂₊CaSO₄ (P2m, P2m′).The polymorphism of alite was studied using synchrotron X-ray powder diffraction (XRD), wavelength 1.5227 Å, and electron diffraction (ED) in a transmission electron microscope. The substitutions of minor elements in alite and belite were determined using electron microprobe analysis. Clinkers P1 and P1m both contained apparent rhombohedral alite (XRD) with an incommensurately modulated structure (ED), while clinkers P2, P2m, and P2m′ all contained monoclinic alite (XRD). The addition of mineralisers in the process caused increased content of fluoride in alite and increased substitution of Si(4+) by Al(3+) and S(6+) in both calcium silicates. The latter effect was most pronounced in clinker P1m due to its high molar SO₃ to alkali oxide ratio (R=2.18).The improved hydraulic activity of P1m compared to P1 was caused by substitutions rather than a change in symmetry. The decreased hydraulic activity of P2m and P2m′ compared to P2 was explained by the high levels of fluorine, which had a retarding effect on the hydration.     

Significant improvement of refractoriness of Al2O3–C castables containing calcium aluminate nano-coatings on graphite

The performance of alumina-carbon castables containing graphite flakes coated by nanosized Ca-doped γ-Al₂O₃ phases has been investigated in terms of refractoriness under load (RUL) and oxidation resistance tests. The coating characteristics and its beneficial effects in castable matrix have been conceived by water-wettability test, differential scanning calorimetry and some physical characteristics. In this regard, a schematic representation of coated graphite has been proposed to elucidate its sustainability in the refractory mass. The comparative gain in performance of the refractory has also been ascertained by scanning electron microscope (SEM) and X-ray diffraction (XRD) studies of the castable matrix. The sol–gel coating overcomes the pitfalls of including uncoated graphites in castables and should be explored for commercial utilization.     

Influence of manganese(II), cobalt(II), and nickel(II) additives in electrolyte on performance of graphite anode for lithium-ion batteries

Manganese dissolution into an electrolyte from the spinel LiMn₂O₄ in the lithium-ion cell has been recently investigated. In order to study the influence of the dissolved manganese species on the lithium intercalation/deintercalation into a natural graphite electrode, the electrochemical behavior of graphite was investigated in 1 mol dm⁻³ LiClO₄ electrolyte solution containing a small amount of Mn(II) by the addition of manganese(II) perchlorate. During the charging process, Mn(II) ions were firstly electroreduced on the electrode around 1.0 V versus Li/Li⁺ followed by irreversible decomposition of the electrolyte and lithium intercalation into the graphite. By microscopic observation of the graphite surface, manganese deposition was confirmed after the charge/discharge test. Due to the manganese deposition, the reversible capacity of the graphite electrode was drastically decreased. Furthermore, the cyclability of the anode was degraded with the amount of the manganese additive increasing. We compared these results with those of the cobalt(II) and nickel(II) additives by dissolving the corresponding perchlorates. Furthermore, we discussed the influence in practical cells based on the consideration of electrochemistry of the deposited metals.     

Effect of hydromagnesite on the hydration of hydratable alumina and properties of corundum-based castables

Hydratable alumina (HA) was premixed with hydromagnesite (BMC) to investigate the BMC impact on the hydration behavior of HA and the thermo-mechanical properties of HA bonded (HAB) castables. The phase composition, microstructure and mass changes of dried HA samples, were characterized by XRD, SEM, and TG, respectively. Flow ability, microstructure, and thermo-mechanical properties of HAB castables were studied. Results indicated that BMC effectively lowered HA hydration rate due to the decreased specific surface area. The hot modulus of rupture strength of castables was improved because the sintering of Al₂O₃ was enhanced by the MgO from BMC decomposition.