Applying colloidal silica suspensions as a binder source for refractory castables has been of great interest in recent years. Nevertheless, the resultant low green mechanical strength of these ...castables has hindered their application in relevant areas. The current paper provides a novel engineering route to improve the mechanical properties of colloidal silica bonded castables before firing. In order to attain this target, small amounts of calcium aluminate cement (CAC) and/or hydratable alumina (HA) were used as gelling agents. A splitting tensile test showed that although using HA resulted in a more significant increase in the modulus of rupture of the samples when compared to CAC, mixing both (CAC+HA) had the most positive impact on the green mechanical properties of the castables, leading to strength levels as high as the reference cement-bonded system (CAC-Ref). XRD and DTG tests were carried out to evaluate the hydration behavior of the selected additives. The results indicated that CAC enhanced the hydration of HA based on the accelerated dissolution of the gel layer formed on HA particles, which was induced by the Al(OH)4− common ion effect. The proposed mechanism was confirmed by a specific conductivity test. Additionally, 29Si solid state NMR spectroscopy was used to determine the effect of the dual additive system (HA+CAC) on the siloxane bond formation. Quantification of Q2, Q3 and Q4 species in alumina–colloidal silica suspensions containing HA, CAC or HA+CAC highlighted that the improved green strength is mainly related to a more effective hydration of HA. Based on the NMR and XHR SEM analyses, HA can be considered as an alternative gelling agent for nano-bonded refractory castables.
This paper evaluated the role played by citric acid and gallic acid in high alumina colloidal silica-bonded castables. The addition of citric acid to three different types of colloidal ...silica-containing compositions led to distinct flow behavior. Vibrating flow measurements were carried out in order to define the castables׳ workability. Zeta potential and specific conductivity tests were also used to analyze the effect of the selected carboxylic acids in alumina-colloidal silica suspensions. Furthermore, other important parameters considered during the evaluation of such additives were: a) pH of the colloidal silica binder and b) pKa values of the carboxylic acid. The higher the pH of the nano-binder solution, the less adsorption of carboxylic acid on the alumina surface will take place. As a result, molecules presenting acidic pKa values such as citric acid are effective additives for colloidal silica binders with pH≤10, whereas others with alkaline pKa values (e.g. gallic acid) can be used with colloidal silica sources presenting higher pH. The use of the mentioned carboxylic acids is required to provide enough workability to the system due to a retarding effect on the setting agent.
Aiming at reducing the temperature of CaO.6Al2O3 (CA6) formation, SiO2, ZnO, and TiO2 were evaluated as mineralizing agents in an Al2O3-CaCO3 based macroporous thermal insulator. For the ...concentrations tested (0.6 mol% to 2.8 mol%), SiO2 presented a small effect in the temperature of CA6 formation. However, ZnO and TiO2 favoured full CA6 generation at 1400 °C, 200 °C below the reference composition. All analysed systems, besides 2.8Si, showed high porosity (> 80 %), suitable mechanical strength and expansion after firing at 1600 °C for 5 h. Softening temperature evaluation highlighted that ZnO- and TiO2-containing compositions could be used at higher temperatures than the SiO2 ones. Thus, compositions containing 2.8 mol% of ZnO or TiO2 were evaluated according to their thermal conductivity (keff), showing lower keff than the reference composition. Therefore, using these mineralizing agents, which are easily available, may provide reductions in the energy input to produce CA6-containing thermal insulators and benefits to all other required properties.
Aiming the in situ formation of CA6 (CaO·6Al2O3) at alumina-based macroporous insulators, distinct Ca2+ sources and contents were used and their effect on some of the refractories properties were ...investigated. Adding CaCO3, Ca(OH)2 or CaO resulted in the decrease of the onset strengthening temperature (TS) and also of the linear shrinkage. However, a higher amount of Ca(OH)2 and CaO could not be used because of their effect on reducing the insulator total porosity. The composition prepared with 12.9 wt% of CaCO3 was the most promising one, leading to an expansion of 0.81 % after firing at 1600 °C for 5 h, TS of 680 °C and low thermal conductivity. These results point out the potential reduction of sintering temperatures and to the possibility of in situ firing the ceramic insulator. These features enable the development of a macroporous refractory composition with a higher thermal insulating effectiveness, which can help industries to decrease their energy demand.
A mechanism to explain the lower onset strengthening temperature induced by CaCO3 in alumina-based macroporous ceramics is proposed, which relies on hydrocalumite-like phase formation during ...processing. Close to 600 °C, such phases are decomposed to lime and mayenite (12CaO·7Al2O3), where the latter, due to its intrinsic nanoporosity and high thermal reactivity, generates bonds between the ceramic particles at ∼700 °C, resulting in microstructure strengthening. Based on this premise, the authors concluded that other Ca2+ sources could act similarly. Indeed, compositions containing Ca(OH)2 or CaO showed the same effect on the onset strengthening temperature, which reinforces the proposed mechanism. The results attained indicated that macroporous insulators could be thermally treated at lower temperatures, just to acquire enough mechanical strength for installation, finishing in-situ their firing process. Besides that, lower sintering temperatures could be used to produce macroporous ceramics that would be applied in low thermal demand environments, e.g. aluminum industries.
Adding pre-foamed colloidal alumina to ultrastable Al2O3-stabilised foams can be a path towards partially counteracting the firing shrinkage of these materials and producing macroporous ceramics with ...smaller pores. Nevertheless, this system still presents a long setting time and high sintering-induced shrinkage, which hinders the production of larger samples and reduces its porosity. In the present work, it was observed that adding calcium aluminate cement suspension (CACS) and CaCO3 (calcite) to the aforementioned system can speed up its solidification kinetics, improve its mechanical strength and reduce its shrinkage after firing, maintaining high porosity and smaller pore sizes. By using these raw materials, samples with an average pore size below 60 μm, total porosity above 70%, and a narrower pore size distribution were attained after thermal treatment at 1600 °C for 5h. Moreover, due to the in situ formation of calcium hexaluminate, their shrinkage after sintering was almost halved (from ~20% to 11%).
The interaction of amino acids (glycine, L-valine, L-isoleucine and L-leucine) with alumina surface was studied to induce partial hydrophobization and production of ultrastable particle-stabilised ...foams. The evaluation of these amino acids was carried out by mechano-quantum simulations followed by experimental tests (foamability, zeta potential, contact angle and foam lifetime measurements). The experimental results agreed with the trends pointed out by the simulations. The selected amino acids interact with alumina particles in aqueous media and in a broader pH range, leading to hydrophobization of surfaces, which was more intense for amino acids with higher molecular mass (isoleucine and leucine). As a consequence, ultrastable foams with a longer lifetime (> 100 h) were produced and the foam microstructure was preserved from ageing phenomena. Moreover, the attained foams were stable at different pH, opening up new possibilities to develop macroporous multiphasic ceramics, which can result in novel materials for thermal insulation at high temperatures.
Macroporous refractory ceramics were prepared with Al2O3-based particle stabilised foams in which ceramic particles were partially hydrophobized with propionic acid or l-leucine. Calcium aluminate ...cement (CAC) as dry powder or stable aqueous suspension were added as binders. Cement addition resulted in solid foams whereby setting took place in less than 24 h. Highly porous samples were attained with leucine-based foams whereas the propionic-acid ones were not stable after CAC addition. For leucine-based foams, fired samples with homogeneous microstructures, both high porosity (~80%) and cold crushing strength (~30 MPa) were attained when CAC aqueous suspension was used instead of CAC dry powder. These results were associated to the better dispersion of cement particles in the foam structure when CAC liquid binder was used and pointed out the advantages of this suspension for producing solid macroporous ceramics based on particle-stabilised foams.
•Microsilica increase mechanical properties without affect acoustic properties.•Higher microsilica percentage, impact in a porosity smaller.•With 5% of microsilica, better is the thermal ...conductivity.•It is possible to obtain a 87.85% of porosity with 0.20% of microsilica.
Gypsum is an ancient material of great abundance that can be used as acoustic and thermal insulation protection in civil construction, especially when water-based foam is added to it, improving these properties. This research presents gypsum foam processing using a direct casting method with additives from natural sources, thus avoiding environment degradation. The results showed that gypsum foam has a potential for thermal and acoustic insulation and is more effective than other materials currently being used. Although, the material mechanical strength decreases when only water-based foam is added, further microsilica (MS) addition improved its mechanical strength without spalling its acoustic absorption and thermal conductivity.
Considering the production of stable CAC aqueous suspensions, this work addressed the action of gluconate anion as a Ca2+ complexing agent and retarder of CAC hydration. Using quantum simulations, ...the complexation energy of gluconate complexes was calculated. The pH range of stability for the complexes was estimated and aqueous suspensions containing CAC and sodium gluconate (NaG), stable up to 4 days at room temperature, were prepared. Afterwards, their hydration reactions were reactivated by adjusting the systems’ pH. Results of solidification kinetics and mineralogical characterisation highlighted that, after reactivation, calcium aluminate hydrates were formed. Thermodynamics simulations indicated that using NaG up to 1 wt% would not be deleterious to the systems refractoriness up to 1700 °C. These systems could be applied in less explored processing routes for CAC-based refractory compositions (e.g. slip casting, direct foaming and additive manufacturing), resulting in innovations to produce advanced refractory ceramics.
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