Refractory Aggregates Research Articles

High-strength microporous corundum-mullite refractory aggregates with sub-micron pores using vacuum impregnation treatment

A novel route was proposed to decrease the pore size and enhance the strength of microporous corundum-mullite refractory aggregates, which were fabricated by using the vacuum impregnation treatment of porous Al2O3 powder with silica sol. The effect of the SiO2 content (0–23.6 wt. %) on the phase composition, microstructures and physical properties of the refractory aggregates was studied. It was found that the sub-micron pore structure inside the pseudomorph particles was effectively preserved due to the volume expansion effect of mullite and the mullite sintering neck formation between the α-Al2O3 microcrystallites. With the SiO2 content increasing from 0 to 13.0 wt. %, the pseudomorph particles containing a large number of sub-micron pores formed in the refractory aggregates resulting from the introduction of silica sol. This resulted in the decrease of intra-particle pore size as well as the development of mullite sintering necks between the pseudomorph particles. The strength of the refractory aggregates was therefore enhanced. With a further increase of SiO2 content to 16.4 and 23.6 wt. %, the inter-particle pore sizes increased due to the mullite volume expansion between the pseudomorph particles, which was responsible for the decrease of strength. Overall, the optimized product is the microporous corundum-mullite refractory aggregates with the addition of 13.0 wt. % SiO2, which showed an apparent porosity of 45.4% and a high compressive strength of 56.4 MPa. Significantly, it also had a median pore size of only 1.24 μm and contained a large number of sub-micron pores.

Zinc aluminate (ZnAl2O4) refractory aggregates: Dilatometric sintering studies and thermal expansion coefficient

ZnAl2O4 has recently been reported to enhance thermomechanical properties and slag corrosion resistance of refractory castables. The pore generation and deleterious expansion during in situ ZnAl2O4 formation can limit their amount in a refractory composition. A comprehensive dilatometric study is presented, identifying the optimum raw materials and processing conditions. Preformed ZnAl2O4 refractory aggregates were synthesized from industrial ZnO and reactive Al2O3 by two-stage firing at 1050 ℃ and 1700 ℃ resulting in 0% open porosity. The ZnAl2O4 powder calcined at 1050 ℃ is also a good refractory matrix material as it has better sinterability than preformed refractory-grade MgAl2O4 and calcined alumina. The thermal expansion coefficient of ZnAl2O4 aggregate in the range of 70–1600 ℃ was found to be 10.7 × 10−6 K−1, which is similar to tabular Al2O3 (10.3 × 10−6 K−1) and MgAl2O4 (10.3 × 10−6 K−1), but lower than dead burned MgO (16.4 × 10−6 K−1), creating the possibility of developing thermal shock resistant refractories by utilizing this mismatch.

REINFORCEMENT BONDING PERFORMANCE IN REFRACTORY CASTABLES UNDER ELEVATED TEMPERATURES

Refractory castables, representing a mixture of a refractory aggregate, calcium aluminate cement, ultra-fine particles, and deflocculants, have been created in the mid of past century ’80s for the metallurgy and petrochemical industries. The designed castable binder shows an excellent ability to preserve the material’s mechanical strength in the 600 °C to 1000 °C temperature range. Besides, the micro-scaled silica/alumina activation process increases the strength and sinterability temperature of the castables regarding traditional alternatives. However, a relatively high price (ten times the ordinary concrete cost) limits the structural application of these advanced materials. Therefore, the studies considered reinforcement aspects of these materials are rare. This investigation focuses on the bond performance of ordinary and stainless steel reinforcement in refractory castables. It is a part of the research project on developing structural concrete fire protection systems, employing refractory castables as a protective shell. This experimental program operates a conventional refractory concrete with a 50 MPa target compressive strength and 25wt% of aluminate cement. In addition, the alternative mixture was modified with a 2.5wt% microsilica, doubling the material strength. The pull-out tests of ribbed and smooth bars demonstrate the importance of the ribs’ mechanical interlock, ensuring reinforcement bonding with the refractory material. Moreover, the ordinary reinforcement demonstrates the bonding ability with refractory castables containing microsilica up to the 1000°C temperature, making the proposed fireprotecting system development possible.

Vacuum impregnation treatment of microporous Al2O3-MgAl2O4 refractory raw materials with sub-micron pores and high strength

In the present work, we propose a novel method to decrease the pore size as well as to enhance the strength of microporous Al2O3-MgAl2O4 refractory raw materials, which were prepared by the vacuum impregnation treatment of porous Al2O3 powders with at MgCl2 solution. The effect of the MgCl2 content (0–32.5 wt%) on the phase distribution, microstructures, and physical properties of the refractory raw materials was thoroughly investigated. The results demonstrated that the sub-micron pore structure inside the pseudomorph particles was effectively preserved due to the volume expansion effect of spinel and the spinel sintering neck formation between Al2O3 microcrystallites. With the MgCl2 content increasing from 0 to 11.9 wt%, the pseudomorph particles contained many sub-micron pores resulting from the introduction of the MgCl2 solution, resulting in the decrease of the intra-particle pore size as well as the development of spinel sintering necks between pseudomorph particles. The strength of the aggregates was therefore enhanced. With a further increase of MgCl2 content to 24.2 and 32.5 wt%, the inter-particle pore sizes increased due to the volume expansion and Kirkendall effect associated with the spinel formation between pseudomorph particles, which were responsible for the progressive decrease of the strength. Overall, the optimized samples were microporous Al2O3-MgAl2O4 refractory aggregates with the addition of 11.9 wt% MgCl2, which exhibited an apparent porosity of 45.0%, a high compressive strength of 45.6 MPa, a median pore size of only 1.49 µm, and a high sub-micron pore volume content of 42.5 vol%. Meanwhile, it is possible to obtain the porous Al2O3-MgAl2O4 powders with a large number of sub-micron pores by crushing and sieving the optimized aggregates.

Analisis pasar expanded perlite Indonesia

The availability of perlite is always associated with volcanic activities, so perlite has a huge potential in Indonesia. The use of perlite abroad has developed for various applications, but is still limited in Indonesia. One of the perlite development efforts in Indonesia is an analysis to seek market of the expanded perlite. The purpose of this market analysis is to find out the characteristics and opportunities of the expanded perlite market both domestically and export. Methods of data collection were carried out through direct survey and non-survey, while the processing model and technical analysis used descriptive statistical approaches, trend analysis, and economic analysis models. Expanded perlite is most widely used as a construction raw material, filler, and filter. Domestic market opportunities include insulation or heat in cryogenic tanks, cement in drilling, lightweight brick, mortar, refractory aggregate, plaster, and planting media. The results of this study are expected to be use as an input for the government and stakeholders in formulating their policies so that the potential of perlite can be developed on a prospective basis.

Development of Water Retentive and Thermal Resistant Cement Concrete and Cooling Effects Evaluation.

The high pavement temperature plays an important role in the development of urban heat island (UHI) in summer. The objective of this study was to develop water retentive and thermal resistant cement concrete (WTCC) to enhance the pavement cooling effects. The WTCC was prepared by combining a water retentive material and a high aluminum refractory aggregate (RA) with porous cement concrete (PCC). Water retention capacity test, fluidity test, and compressive strength test were used to determine the composition ratio of the water retentive material. Mechanical performance and cooling effects of WTCC were evaluated by compressive and flexural strength tests and temperature monitoring test. The mass ratios of fly ash, silica fume, cement, and water in the water retentive material were determined as 65:35:15:63.9. The compressive strength and the flexural strength of WTCC after 28 days curing were 30.4 MPa and 4.6 MPa, respectively. Compared with stone mastic asphalt (SMA) mixture, PCC, and water retentive cement concrete (WCC), surface temperature of WTCC decreased by 11.4 °C, 5.5 °C, and 4.1 °C, respectively, and the internal temperatures of WTCC decreased by 10.3 °C, 6.1 °C, and 4.6 °C, respectively. The water retentive material has benefits of strength improvements and temperature reduction for WTCC. Based on the results, WTCC proved to have superior cooling effects and the potential to efficiently mitigate the UHI effects and be used in medium traffic roads.

From magnesite directly to lightweight closed-pore MgO ceramics: The role of Si and Si/SiC

Partial/full replacement of traditional dense refractory aggregates (for furnace lining) with lightweight aggregates is considered to be an effective and promising strategy for energy saving and emission reduction. In this work, lightweight magnesia refractory ceramics with tailored closed porosity were prepared by one-step sintering at 1600 °C from high-purity magnesite added with silicon kerf waste in different ratios, with the emphasis on the evolution of their phase compositions, micromorphologies, and various properties. With the addition of additives, Mg2–xFexSiO4 solid solution was formed at the grain boundaries of the MgO matrix, and then the volumetric expansion effect (interfacial reaction) and activated sintering effect (vacancy defect) promote the decrease of apparent porosity and the increase of closed porosity, respectively. Consequently, MgO ceramics with apparent/closed porosity of 0.6%/6.5%, bulk density of 3.25 g cm−3, and lightweight index of 7.8% were successfully prepared, suitable for the working lining of metallurgical furnaces.

Converting Saprolite (a Low-Grade Unutilized Material) into Refractory Aggregates: A Viable Substitute for Calcined Clay in the Refractory Industry

Converting Saprolite (a Low-Grade Unutilized Material) into Refractory Aggregates: A Viable Substitute for Calcined Clay in the Refractory Industry

Development and Properties of New Mullite Based Refractory Grog.

The presented study is focused on optimization and characterization of a high-alumina refractory aggregate based on natural raw materials—kaolins, claystone, and mullite dust by-product (used to increase the alumina and mullite contents, respectively). In total, four individual formulas with the Al2O3 contents between 45 and 50 wt.% were designed; the samples were subsequently fired, both in a laboratory oven and an industrial tunnel furnace. The effects of repeated firing were examined during industrial pilot tests. Mineral and chemical compositions and microstructures, of both the raw materials and designed aggregates, were thoroughly investigated by the means of X-ray fluorescence spectroscopy, powder X-ray diffraction, and optical and scanning electron microscopies. Porosity, mineral composition, and mullite crystal-size development during the firing process were also studied. Based on the acquired results, the formula with the perspective to be used as a new mullite grog, featuring similar properties as the available commercial products, however, with reduced production expenses, was selected. The quality of grog determines to a large extent the properties of the final product. Hence, optimization of aggregates for specific refractories is of a great importance. The production of engineered aggregates provides the opportunity to utilize industrial by-products.

Microstructures and strengths of microporous MgO‐Al2O3 refractory aggregates using two types of magnesite

AbstractSix microporous MgO‐Al2O3 refractory aggregates were prepared with Al(OH)3 and two different types of magnesite powders via an in‐situ decomposition pore‐forming technique. One magnesite powder contained more CaO (Magnesite C), the other one contained more SiO2 and Al2O3 impurities (Magnesite SA). Effects of magnesite powder type and content (11 wt.%, 36 wt.% and 77 wt.%) on the phase compositions, microstructures and mechanical strengths of the prepared microporous aggregates were investigated by X‐ray diffractometry (XRD), mercury porosimetry measurements, and scanning electron microscopy (SEM), etc. With the addition of 11 wt.% and 77 wt.% magnesite, the type of magnesite had only a small effect on the microporous corundum‐spinel and periclase‐spinel aggregates, while great influence on the microporous spinel aggregates was observed with 36 wt.% addition. This was mainly because of the sintering process with liquid phase. The best microporous MgO‐Al2O3 refractory aggregates, which had an apparent porosity of 39.1%, a median pore size of 3.38 μm and a compressive strength of 66.3 MPa, were prepared by using 36 wt.% Magnesite C. This work has practical significance for the efficient utilization of magnesite and the development of energy‐saving lightweight MgO‐Al2O3 refractories.

Corrosion Mechanisms of Different Refractory Aggregates in Contact with SiO2-MgO-Based Slag

Abstract: To improve the utilization and evaluation of metallurgical solid waste, the investigation of slag has become a focused research subject in the recent years. However, when slag with various compositions was smelted to produce high value-added products in the slag furnace, the SiO2-MgO based slag caused serious damage to the refractory lining material of the furnace compared to the common CaO-Al2O3-SiO2 based slag. The tabular alumina, lightweight alumina, silicon carbide, mullite and brown fused alumina aggregates were selected for testing. The static crucible method was used to investigate the SiO2-MgO based slag corrosion and comparative analysis was performed. The results show that mullite-silicon carbide or tabular alumina-silicon carbide refractories are promising for the use as lining materials of SiO2-MgO slag smelting furnaces.

Preparation and characterization of microporous mullite-corundum refractory aggregates with high strength and closed porosity

Four microporous mullite-corundum refractory aggregates were prepared from Al(OH)3 and kaolinite gangue through an in-situ decomposition synthesis technique. The effect of the compacting pressure (30–150 MPa) on the microstructure and strength were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The higher compacting pressure limited the rearrangement of Al(OH)3 pseudomorphs during the reaction sintering process and promoted the reaction between Al(OH)3 and kaolinite gangue, leading to the formation of closed mullite layers on the surface of Al(OH)3 pseudomorphs. The pore size distributions of all four microporous aggregates were bimodal with closed pores (0.1–1.0 μm) located inside of the Al(OH)3 pseudomorphs, whereas open pores (1.0–10.0 μm) were found between the pseudomorphs. When the compacting pressure was 150 MPa, the microporous mullite-corundum refractory aggregates had a high total porosity of 40.5%, a highly closed porosity of 16.2%, median pore size of only 0.74 μm as well as an excellent compressive strength of 103.2 MPa.

Newly Developed Refractory Aggregates

Abstract: The paper describes the characteristics of the situation in the production and use of aluminosilicate raw materials for the production of refractory materials. It describes the technology and the individual qualities, and discusses the availability and suitability of raw materials. This paper discusses the issue of the fine fraction fireclay, which is not suitable for firing in a shaft furnace, as well as perspectives and characteristics of new refractory aggregates with an Al2O3 content higher than 45%.

Synthesis of MgO–MgAl2O4 refractory aggregates for application in MgO–C slide plate

This work provides an effective method for synthesis of MgO–MgAl2O4 (MgO-MA) aggregates with good thermal shock resistance (TSR) using lightly calcined magnesia and α-Al2O3 micro-powder as the starting materials. The effect of α-Al2O3 micro-powder amount and calcination temperature on TSR and microstructure of the synthesized MgO-MA aggregates was investigated. The optimum amount of α-Al2O3 micro-powder was 15 wt%, and the obtained aggregates had the highest density after calcination at 1650 °C. The synthesized MgO-MA aggregates had excellent TSR which was attributed to the thermal mismatch of MgO and MA grains as well as the pinning effect of MA grains on the periclase grain boundaries. TSR of MgO–C slide plate materials was improved significantly by introducing appropriate amount of the synthesized MgO-MA aggregates.

Preparation and thermal shock behavior of nanoscale MgAl2O4 spinel-toughened MgO-based refractory aggregates

Abstract Nanoscale MgAl2O4 (MA) spinel-toughened MgO-based refractory aggregates were prepared by firing lightly calcined magnesia powder and some nanoscale Al2O3 (≤50 nm) particles at high temperature. The effects of various amounts of nanoscale Al2O3 powder on the phase composition, microstructure, physical properties and thermal shock resistance (TSR) of the prepared MgO-based refractory aggregates were investigated. The results showed that the optimum nanoscale Al2O3 powder addition was 10 wt%, and the nanoscale Al2O3 particles reacted with MgO during firing and formed nanoscale MA creating reactive sintering, which contributed to significant increasing in density and cold strength of the prepared aggregates. The thermal mismatch between nano-MA and MgO could produce microcracks created toughening effect, leading to improvement in TSR of the prepared aggregates. And the formed nano MA grains pinned in MgO grain boundaries and on MgO grain surfaces would lead to microcracks deflection and absorbing much more fracture energy, further improving the TSR. The obtained aggregates were trial used in MgO-C slide plate materials, and thermal shock resistance of the materials was improved.

Thermal investigation of fine alumina powder reinforced Na-metakaolin-based geopolymer binder for refractory applications

Geopolymers have become an alternative binder material for high-temperature applications. The thermal properties of geopolymer products are significantly influenced by the fillers used in their production. In this study, fine α-alumina powders 50% of which are smaller than 6.45 μm (D(0.5) = 6.45 μm) in the ratio of 100 wt.% were added as fillers to relatively high water included (H2O/Al2O3 = 19.6) Na-metakaolin-based geopolymer resin. Although there was a high amount of fine powders included, a composite binder which could be placed in form smoothly by wet casting method was obtained thanks to sufficient water in the resin. The thermal behaviour of the cured composites was examined from room temperature (RT) to 1250 °C. In the conclusion of the study, it was noted that the samples had significant dimensional stability up to 1000 °C with the increasing compressive strength. Investigations showed that the deformation expected to occur with the viscous sintering in the geopolymer matrix between 750 and 1000 °C shifted to higher temperatures due to the refractory effect of the high amount of fine filler. This characteristic revealed that this type of composite binder has a potential to be used alone to produce fine alumina-geopolymer refractory composites or used as a binder for coarser refractory aggregates to produce various refractories.

USING LOCAL IRAQI MATERIALS IN PRODUCTION OF REFRACTORY BONDING MORTAR

The Iraqi kaolin clay was used as a raw material in this research. three mix proportion was applied, (96:4), (93:7) and (90:10) as (kaolin grog: kaolin binding ). The specimens of mortar were burning at 1500ᵒC, after producing a refractory aggregate (grog) by preparation process mixing with sodium silicate solution as adhesive material with 5% by weight of the mix. The mechanical tests were carried (compressive and bonding) strength, durability investigations (thermal shrinkage, and reheating expansion), and the physical investigation was carried (bulk density, specific gravity, apparent porosity and water absorption). It was found the first mix (96:4) recorded the highest compressive strength befor burning and bonding after burning, and the best physical properties after burning at 1500 ºC, with mix proportion (96: 4), while the highest compressive strength after burning were the mix three (90: 10).

Preparation and characterisation of closed-pore Al2O3-MgAl2O4 refractory aggregate utilising superplasticity

ABSTRACTA novel high closed porosity Al2O3-MgAl2O4 refractory aggregate has been successfully fabricated by utilising superplasticity with Al2O3 and MgO as raw materials, SiC as high temperature pore-forming agent. The effects of the addition amounts of MgO and SiC on porosity, sintering behaviours, phase composition, pore size distribution and microstructure of the refractory aggregate have been investigated. The formation mechanism of the closed pore in the refractory aggregate has been discussed. The results showed that the MgO can improve the superplastic deformation ability of Al2O3-based ceramic at high temperature. With the content of MgO and SiC increased, the closed porosity and the pore size increased. The oxidation of SiC improved the sinterability of materials at the initial stage of sintering, and then the released gases due to the further oxidation of SiC promoted the formation of closed pores by motivating the superplastic deformation ability of Al2O3-based materials.

Corrosion and adherence properties of cement clinker on porous periclase-spinel refractory aggregates with varying spinel content

The corrosion and adherence properties of cement clinker on porous periclase-spinel refractory aggregates with varying spinel content were examined using a static crucible test and a sandwich test, respectively. The reaction characteristics of porous periclase-spinel aggregates and cement clinker and the effects of spinel content on the adherence property were investigated through scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and FactSage® thermo-chemical software. It was observed that the spinel content and pore characteristics strongly affected the corrosion results and thus affected the adherence ability of cement clinker on porous periclase-spinel aggregates. With an increase in the spinel content, the amount of glass phase formed from the reaction of the refractory and cement clinker increases because the rate of the spinel dissolution into the cement clinker is higher than that of periclase. The glass phase acts as a bridge between the cement clinker and the aggregate to enhance the adherence property, which depends on the amount, area distribution and viscosity of the glass phase and its penetration in aggregates. When the spinel content is 15–40wt%, the refractory aggregate not only has a high cement clinker resistance but also a high adherence property. Once the spinel content exceeds 50wt%, the skeletal structure of the aggregate will be destroyed, which will lead to a substantial decline in the cement clinker resistance.

Experimental study on separation of valuable refractory aggregate from investment casting ceramic shell waste

In the present study, a processing technique for recycling investment casting ceramic shell waste was proposed to separate valuable refractory aggregate zircon sand. The microstructure and phase constituents of the shell waste and separation process were investigated. The results show that the characteristics of microstructure and phase constituents of the shell waste can meet the conditions for preferentially separating zircon sand, and zircon sand can be separated by gravity separation on a shaking table. The separated zircon sand has good shape and high purity, and can be used for the production of castings and other applications.