Strength Of Castables Research Articles

Reconstruction and hydration of hydrotalcite response to thermal activation temperature: Enhancement of properties for magnesia castables

Optimization binding system for refractory castables is significant to enhance the service performance. Hydrotalcite has been considered a promoter for high-temperature performance of basic castables, however, its binding property remains to be improved before practical application. In this work, the thermal activated Mg–Al hydrotalcites were incorporated in magnesia castables, and the mutual influence of pre-calcination temperature on the hydration, microstructure, and strength of castables was investigated. The obtained results indicated that the reconstruction of calcined hydrotalcite took place in the hydration process and effectively motivated the hydrolysis. Hydrate was thus promoted and a relatively dense microstructure of magnesia castables was confirmed by X-ray computed tomography analysis. Hydrotalcite pre-calcinated at 300 °C contributed to the highest early strength for castable, and the high-temperature properties also performed better than that of other pre-calcinated hydrotalcite-adding. The enhancement mechanisms of calcined hydrotalcite were attributed to the two following reasons: (ⅰ) the modified microstructure of magnesia castables from the early stage by hydration process, (ⅱ) the further enhanced sinterability inspired by the appropriate thermal activation effect.

Effect of water‐soluble magnesium lactate on the volume stability of refractory castables containing calcium aluminate cement

AbstractA kind of organic magnesium salt, water‐soluble magnesium lactate, was used as the MgO‐precursor in new forms to identify the particular impact of additive dispersity on properties of calcium aluminate cement (CAC)‐bonded castables, especially the volume stability. Magnesium lactate could dissolve in water in the mixing process, distribute uniformly, and decompose during heating, providing space for counterbalancing the expansion from in‐situ calcium aluminates and MgAl2O4 spinel generation. Hence, in this study, CAC‐bonded castables containing 0–1.2 wt% magnesium lactate were prepared. The volume stability and thermo‐mechanical strength of castables were tested. Results show that the dissolution of magnesium lactate in water distinctly enhances the volume stability of castables. To understand the underlying mechanism of water‐soluble magnesium lactate on the samples, phase compositions and microstructure development of castable matrices were examined.

Microstructure evolution of hydration products and strength change of hydratable alumina‐bonded castables below 1250°C

AbstractThere have been reports that strength of hydratable alumina (HA)‐bonded castables without silica fume drops significantly at 600°C and decreases substantially again at 1000°C. But the strength variation of the HA‐bonded castables during the intermediate temperature range has not been investigated and elaborated from the perspective of phase evolution and microstructural change in the castables. In this work, the relationship between the change in the strength of castables and the microstructural characteristics of the HA‐bonded castables was investigated. The phase and microstructure evolution of HA‐bonded castables between 110°C and 1250°C were investigated by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TG). It has been found that strength drops of the HA‐bonded castables during heating process do not mainly happen at the temperature at which HA hydrates decompose, but at the temperature at which the structure of dehydrated HA hydrates disintegrates.

Effect of CAC content on the strength of castables at temperatures between 300 and 1000 °C

This study investigates the strength progression of calcium aluminate cement (CAC)-bonded castables with a CAC content ranging from 2 to 16 wt% in the temperatures between 300 and 1000 °C. Focusing on the phase changes and microstructure evolution of the hydrates due to heating, we discovered that castables with higher CAC contents have increased strengths between 300 and 800 °C because the morphology and structure of the dehydrated C3AH6 and AH3 remain similar. At 1000 °C, a higher strength of the 16 wt% CAC castable results from the higher degree of coalescence/sintering between the particles formed from the decomposition products of CAC hydrates. The CAC-bonded castables were prepared using tabular alumina aggregates and α-Al2O3 micro-powder as starting materials, and CAC as the binder. The phase compositions and the microstructures of the castable matrices were characterised by X-ray powder diffraction and scanning electron microscopy, respectively.

Difference in pore evolution of calcium aluminate cement-bonded alumina bubble-based castables with micro-sized MgCO3 and Al(OH)3

Micro-sized MgCO3 and Al(OH)3 were used as inorganic porogenic agents for increasing porosity in alumina bubble lightweight castables. The influence of 0–4 wt% micro-sized MgCO3 and 0–8 wt% micro-sized Al(OH)3 additions on the porosity, thermal conductivity and strength of castables after firing was investigated. The results indicate that porosities in castables containing micro-sized Al(OH)3 or MgCO3 after heat-treatment were raised by augmenting micro-sized Al(OH)3 or MgCO3 contents. It is found that MgCO3 favors the porosity formation more than Al(OH)3 in castables because of the different formation behavior of calcium aluminates and MgAl2O4 spinel. The dependence of properties of castables on the micro-sized Al(OH) or MgCO3 addition was discussed with respect to the pore evolution.

Microwave and conventional treatment of low-cement high-alumina castables with different water to cement ratios; Part I. Drying

The aim of this study was to determine the effect of the microwave and conventional drying method on the strength, porosity and composition of low-cement alumina castables with various water to cement ratios (wcr). High-alumina low-cement castable samples were prepared with different w/c ratios: 0.64, 0.75, 0.82 and 1.13. Changes in wcr were effected through volumetric replacement of cement with 0–0,045-mm tabular alumina having a comparable particle size. Water content in all the composition was constant (4,5%). After curing, the samples were dried conventionally in a laboratory electric drier or in a laboratory microwave drier. After drying open porosity and modulus of rupture were determined. The pore size distribution, pore median and tortuosity of the samples were measured by the mercury porosimetry method. Phase composition was determined using X-ray diffraction. The Rietveld method was used for quantitative analysis. It was found that at low wcr (0.62) the main hydrate formed in the castable was C3AH6, which caused a release of a smaller amount of water during the drying process, mainly pore water, resulting in lower open porosity and lower pore size than in the castables with a high wcr (1.13). At a low wcr, the strenght of castable was higher due to a higher amount of hydrates, low porosity and small pore size. On the other hand, at a high wcr, the strength of castable was lower owing to a higher amount of water released in the drying process, which led to loosening the structure of castable. With an increased water-to-cement ratio the degree of CA2 hydration decreased. The temperature rise due to cement hydration probably influenced the kinetics of this process.

The Effect of Different Colloidal Silica Binders on Strengths of Corundum-Mullite Castables

Three kinds of colloidal silica (GS-30, AM and HS-30) were applied as binders to corundum-mullite castables. The apparent porosity and cold modulus of rupture for samples fired at 800 oC and 1000 oC were compared. The fractography of colloidal silica bonded corundum-mullite castables was carried out by slit island method. The microstructure of fracture surface was characterized by scanning electron microscope. The results showed that castables by using imported colloidal silica (AM and HS-30) presented higher strength than those with domestic (GS-30), which was determined by the activity of silica and modified ion. The difference of the size distribution of the salient on the fracture surfaces indicated that the strength of castables was dependent on the bonding among the components.

Phase Composition, Microstructure and Mechanical Properties of Aluminous Cements Containing Magnesium Aluminate Spinel

A new type aluminous cement containing magnesium aluminate (MA) spinel was prepared from mixtures of limestone, magnesian and bauxite at different ratios by sintering process. The phase composition, microstructure and mechanical properties of aluminous cements containing magnesium aluminate spinel were investigated in this paper. According to the XRD results, the magnesium aluminate spinel (MA),calcium monoaluminate (CA) and calcium bialuminate (CA2) are the primary phases of the obtained aluminous cements with a small quantity of calcium silicoaluminate (C2AS) and remained alumina (Al2O3). The results of SEM indicate that the MA mainly exist in the shape of octahedron with the length of about 2-5 μm, and it is agglomerated with the tabular or flaky-shaped CA among the obtained aluminous cements. In addition, the compressive strength of castables containing obtained aluminous cements increase obviously with the content of CA rising or the curing time increasing.