Development of Microsilica-Gel Bonded Castable for Nose Ring Application

Refractory castables with improved thermomechanical properties are necessary in the nose ring areas of cement production rotary kilns to withstand thermal cycling and clinker particle abrasion. However, commonly used low- and ultra-low cement castables have limited thermal shock and abrasion resistance due to lime presence in the binders. This study investigates the potential of a microsilica-gel bonded nocement system with a speciality product, SioxXTM-Zero, to improve the thermomechanical properties of refractory castables. The study evaluates the castable’s Cold Modulus Of Rupture (CMOR), Cold Crushing Strength (CCS), Bulk Density (BD), and Apparent Porosity (AP) after curing at 25 °C and firing at 110 °C, 1100 °C, and 1400 °C, as well as Modulus of Rupture (HMOR) at 1400 °C. Additionally, the study assesses the thermal shock and hot abrasion resistance of the castable. A thermodynamic model is utilized to calculate the theoretical liquid phase formation, while phase formation and microstructural features are evaluated using XRD and SEM-BED. The findings of this study could lead to the development of more effective refractory castables for use in the nose ring areas of cement production rotary kilns.

1 Introduction The nose ring area of cement rotary kilns is subjected to constant thermal and mechanical wear due to frequent air cooling and impact and abrasion by cement clinker particles on the lining, leading to cracking and thermal spalling and reduced service life of the castable [1–3]. Conventionally, castables containing Calcium Aluminate Cement (CAC) are used in this area, but their lime content reacts with silica, reducing hightemperature properties [4–5]. Moreover, special precautions need to be taken during the early dry-out process when the presence of cement can lead to explosions if the dehydration of calcium aluminate hydrates is not adequately managed [6]. To overcome these challenges, Low Cement Castable (LCC), Ultra-Low Cement Castable (ULCC) and even No-Cement Castable (NCC) have been explored and developed for nose ring application in cement rotary kiln [7–9]. No-cement systems, such as those based on colloidal silica or microsilica-gel binders, offer potential solutions for fast dry-out and improved high-temperature properties. Colloidal silica is currently one of the most widely used cement-free binders for developing silica-containing no-cement castables. It has superior hot properties as compared to cement-bonded aluminosilicate castables due to mullite formation. Castables bonded with colloidal silica also have high permeability, which reduces the risk of spalling during the dry-out process and enables fast drying. However, colloidal silica binders have limitations related to their long set time, complex set behavior, and inadequate green strength, as well as logistical challenges in handling [10–11]. Microsilica-gel bonded No-Cement Castables (NCCs) have recently gained attention due to their ease of handling, storage, transportation, improved setting behavior, and higher green strength compared to silica-sol bonded NCCs. These castables use microsilica powder as a “dry-version” silica binder, and the bonding mechanism involves cation bridging between negatively charged microsilica particles, controlled by the number of cations and the type of coagulating agent used. When small amount of calcium aluminate cement (approximately 0,5 %) is used as a coagulating agent, Ca2+ (and/or other polyvalent cations) released during cement dissolution react with negative sites on the microsilica surface to form the network. The microsilica-gel bond system contains only a minor amount of cement as a gelling agent, resulting in better hot properties than those of low cement castables. Additionally, replacing the cement binder with a microsilica-gel binder, in combination with a specialty drying agent, enables fast drying of no-cement refractory castables [12]. A new specialty product called SioxXTM-Zero for microsilica-gel bonded NCCs has been recently developed by Elkem, based on its extensive experience with microsilica in refractory castables. The product contains high-grade microsilica as a carrier and is recommended for use at a dosage of approximately 2–3 mass-%. In comparison to two other commercially available dispersants, SioxXTM-Zero was found to have a significant impact on set-behavior and green mechanical properties in microsilicagel bonded NCCs with various aggregates. The use of SioxXTM-Zero provides a welldefined set and short time to final strength, high self-flow, and improved mechanical properties. By combining SioxXTM-Zero with polyvalent cations, the set time can be controlled [13–14]. This paper focuses on the development of microsilica-gel bonded cement-free castables containing SioxXTM-Zero for nose ring applications in cement production rotary kilns, with improved properties compared to an existing product in the market. The performance of this type of NCC was evaluated and compared with that of conventional CAC-containing castables. 2 Experimental 2.1 Mix design The raw materials, composition, and castable formulations used are shown in Tab. 1. To understand the effect of the binder on refractory castable properties, two types of binders were designed: Low-Cement- Bonded (LCC) and microsilica-gel bonded (NCC). Bauxite with 86 % alumina, brown fused, and SiC were used as aggregates, and tabular alumina and reactive alumina were the alumina sources for mullite formation. The LCC material is labeled as NR-LCCC5, and the microsilica-gel bonded material as NR-NCC-SZ. The microsilica and additive product (SioxXTM-Zero) are both from Elkem Silicon Materials, with the microsilica grade being 971U. Since the major composition of SioxX®-Zero is ~70 % silica and ~30 % alumina, the total amount of silica content for both samples was similar at approximately 5 %. 2.2 Sample preparation The dry components were mixed for 4 minutes, water was added, and the castable was wet mixed for an additional 4 minutes. The initial flow of the castables was measured using the drop-table method, following ASTM C1437 and using an ASTM C230 standard cone with dimensions of bottom diameter = 100 mm, top diameter = 70 mm, and height = 50 mm. The flow value was calculated as the percentage increase in the sample’s diameter after 25 drops. The samples were then measured every 5 min to monitor the change in flow with time (flow-decay). The wet mixes were cast into various shapes for Cold Modulus Rupture (CMOR) and Cold Crush Strength (CCS) evaluations using samples measuring 40 mm x 40 mm x 160 mm, and Hot Modulus of Rupture (HMOR) evaluations using samples measuring 100 mm x 100 mm x 25 mm. Samples for hot abrasion resistance were the same dimensions as those used for the HMOR test. Samples were cured at 25 °C (±0,3 °C) and a relative humidity of 75 % (±5 %) for 24 h before firing. After curing, the samples were demoulded and heated to the desired temperatures. The CMOR and CCS tests were conducted at 110 °C for 24 h, 1100 °C, and 1400 °C for 3 h. The HMOR samples were fired at 1400 °C for 24 h before testing. Thermodynamic predictions were made using FactSage 6.2 to determine liquid formation in the samples after firing. 2.3 Physical and mechanical properties measurements The Bulk Density (BD) and Apparent Porosity (AP) were measured according to Archimedes’ method where samples were weighed before and after submerging in aqueous medium under a vacuum. The CMOR and CCS samples were evaluated according to ASTM C133–97 after curing at 25 °C and heating to 110 °C, 1100 °C and 1400 °C. The same method was used to determine the CMOR after thermal cycling of the samples at 1000 °C. The HMOR was evaluated according to ASTM C583 where samples were pre-fired at 1400 °C for 24 h. There were multiple samples evaluated of which the average is reported. 2.4 Thermal shock resistance and hot abrasion resistance measurement To evaluate the thermal shock resistance of the refractory material, the specimens were subjected to thermal cycling. The change in mechanical properties was measured by calculating the CMOR residual. The samples were first pre-fired at 1400 °C for 12 h and allowed to cool. They were then heated to 1000 °C for 30 min and quenched in cool water. This cycle was repeated five times. After the fifth cycle, the Cold Modulus of Rupture Strength (CMORTS) was measured and compared to the initial value (CMORIN) before quenching. To determine the change in mechanical properties due to the thermal cycling, the residual strength (CMORratio) was calculated using the following equation:CMORratio = 100 %*(CMORTS/CMORIN) Before conducting the abrasion resistance test, the samples were measured and weighed to calculate the bulk density. The hot abrasion resistance was then measured at 1000 °C on pre-fired samples that measured 100 mm x 100 mm x 40 mm and had been heated for 24 h at 1400 °C. The samples were heated for 30 min at the test temperature before blasting with SiC abrasive medium for 450 s. During this time, 1000 g of SiC was blasted onto the surface of the sample. The change in weight of the sample before and after blasting was used to calculate the percentage mass loss due to abrasion. The abrasion was calculated by the following equation: A = (M1–M2)/BD, Where BD was the bulk density [g/cm3]. M1 and M2 represented the weight of samples before and after the test, respectively. 2.4 Xay-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM) characterization For phase identification, samples were milled, and powder was pressed for evaluation using a Phillips X’Pert Pro. The XRD analysis identifies the major phases in each of the samples. For SEM analysis, fractured sections of the samples were mounted and prepared for SEM backscatter image analysis. Prior to mounting, the sections were dissolved in HF solution (10 mass-%) for 30 min and dried for 24 h. 3 Results and discussions 3.1 Flowability Fig. 1 displays the initial flow and flow decay over time of both castable compositions. The initial flow value for the LCC castable was similar to that of the NCC, despite a slightly higher water addition in NR-LCC-C5. However, the setting process in LCC appears to be slower than in NCC, as indicated by the high flow value after 120 min. Overall, the working time for both castable compositions is sufficient, up to 120 min. 3.2 Cold Modulus Rupture (CMOR) and Cold Crush Strength (CCS) Fig. 2 summarizes the mechanical properties (CMOR, CCS) of the castables. Both CMOR and CCS of NR-LCC-C5 and NRNCC- SZ increase with firing temperature up to 1100 °C but decline at 1400 °C. NR-LCCC5 shows approximately twice the strength of NR-NCC-SZ after demoulding and drying at 110 °C for 24 h, confirming the benefit of cement bonding (CAC) in castables for improved green and intermediate-temperature compressive strength. However, at 1100 °C, the CMOR of NR-NCC-SZ increases dramatically, surpassing that of the cement-bonded ones. At 1400 °C, both CMOR and CCS of NR-NCC-SZ are higher than the low-cement variant due to more mullite formation and less liquid formation. 3.3 Bulk Density (BD) and Apparent Porosity (AP) The bulk density (BD) and apparent porosity (AP) of the aforementioned samples were measured and depicted in Figs. 3–4 as a function of firing temperatures. Both samples exhibited similar trends in BD and AP. From temperatures ranging between 110 °C to 1000 °C, the BD of microsilica-gel bonded NR-NCC-SZ was slightly higher than the cement bonded NR-LCC-5C. At a firing temperature of 1400 °C, the BD of both samples increased, but the former showed a slightly lower value than the latter. The development of AP varied, which may be attributed to the hydration phase of cement during the dryout process and the formation of liquid with the presence of cement at high temperature. 3.4 Thermal shock resistance, Hot Modulus of Rupture (HMOR), and hot abrasion resistance Thermal shock resistance and high temperature abrasion resistance were performed on both samples. Tab. 2 provides a summary of the CMOR (cold modulus of rupture) values before and after 5 cycles of thermal shock (water-quench method from 1000 °C to room temperature), the calculated residual strength ratio, as well as the abraded volume after hot abrasion resistance at 1000 °C. Although the strength residual ratio for each sample was similar, at approximately 24,5 %, the initial strength before thermal-shock test and the final strength after 5 cycles of thermal shock for NR-NCC-SZ were higher than those of NR-LCC-C5. This suggests that the former exhibited better thermal-shock resistance compared to the latter. As shown in Tab. 2, the microsilica-gel bonded NR-NCC-SZ exhibited excellent hot abrasion resistance, with an abraded volume of 2,37 cm3, which is approximately one-third of the value for the cement-bonded specimen. Photos of the samples after the abrasion resistance test at 1000 °C are shown in Fig. 5. This reveals that the cement-bonded NR-LCC-5C had a larger affected area and volume than the microsilica-gel bonded sample. Overall, the substitution of cement binder with microsilica-gel binder led to improved thermal shock resistance and a significant improvement in abrasion resistance. Additionally, the HMOR of both samples was measured at 1400 °C and listed in Tab. 2. The microsilica-gel bonded NCC exhibited a high HMOR of 6,9 MPa, while the cement-bonded one (NR-LCC-C5) was only 1,8 MPa. This is due to mullite formation in microsilica-gel bonded NCC that increases high-temperature strength [12]. The excellent HMOR of NCC is probably one of the reasons that results in the improved thermal- shock resistance and abrasion resistance compared to NR-LCC-C5. 3.5 Thermodynamic simulation To understand why microsilica-gel bonded NCCs exhibit higher hot properties and significantly improved abrasion resistance than cement-bonded ones, the Equilib module of FactSage 6.2 was first used to calculate the phase composition and phase changes at high temperatures. The predicted phase composition calculated from thermodynamic data is depicted in Fig. 6. It was observed that NR-LCC-C5 containing 5 % calcium aluminate cement had more anorthite (CaAl2Si2O8), less mullite (Al6Si2O13), compared to the microsilica-gel bonded NCC (NR-NCC-SZ), which contained only 0,5 % cement. This indicates higher liquid formation in NR-LCC-C5 at high temperature due to the presence of CaO, which can lower the high-temperature mechanical properties. This explains that why the microsilicagel bonded NCC (NR-NCC-SZ) showed excellent hot properties and improved hot abrasion resistance. It is important to note that the mechanical strength of a castable depends on pore characteristics, ceramic phase formation between the matrix and aggregate, and liquid phase formation. 3.6 Phase composition and microstructure characterization To explore the phase evolution of nose ring castable, XRD phase analysis was carried out for specimens fired at 1100 °C and 1400 °C, and the results are shown in Figs. 7a and 7b, respectively. After firing at 1100 °C (Fig. 7a), NR-LCC-C5 with 5 % cement was mainly composed of corundum and beta-SiC Trace amount of anorthite, cristobalite, and mullite phases were detected. For NR-NCC-SZ, using microsilica-gel as binder, besides the corundum and beta-SiC, cristobalite and mullite phases were verified. Clearly, the amorphous microsilica in both samples was converted cristobalite at this temperature. However, compared to NR-LCC-C5, the intensity of diffraction peak of mullite in NR-NCC-SZ was much stronger, and no anorthite phase was detected. This indicates that the microsilica-gel binder system promotes the formation of mullite. By increasing the firing temperature to 1400 °C (as shown in Fig. 7b), the main mineral phases for both NR-LCC-C5 and NR-NCC-SZ were corundum, beta-SiC and mullite. The amount of mullite formed in the former sample is much less than in the latter. Furthermore, the cristobalite completely disappeared in both samples, and a trace amount of anorthite in the cement-bonded castable was detected. The XRD results in combination with the thermodynamic simulation (Fig. 6) confirmed that more liquid phase would be formed in cement bond refractories, since low melting point phase (anorthite) transformed to liquid phase at high temperature [15]. Fig. 8 shows micrographs of both samples, NR-LCC-C5 and NR-NCC-SZ, after firing at 1400 °C. Numerous mullite crystals were observed in both samples. The mullite phase was identified with EDS, and the grains identified are also marked in Figs. 8b and 8d, respectively. However, the needlelike mullite in NR-LCC-C5 was much smaller than those observed in the microsilica-gel bonded castable (NR-NCC-SZ). Large pores were also observed after the glassy phase was etched away in NR-LCC-C5, whereas the mullite in NR-NCC-SZ was interlocked and closely packed. Specifically, the quantity of mullite in the former was much higher than in the latter. This indicates that little liquid phase is formed in NR-NCC-SZ, whereas a large amount of liquid is formed in NR-LCC-C5 at 1400 °C. The mullite in NR-LCC-C5 is enveloped by the liquid phase, and the strength deteriorates as soon as the liquid phase begins to form. On the contrary, for NR-NCC-SZ, the needle-like mullite crystals provide strength by bridging the aggregates, forming a strong and highly refractory matrix. The microsilica in the both the variants react differently depending on the composition of the matrix. In the case of the NR-LCC-C5, the microsilica reacts preferentially with the CAC to form anorthite whereas the microsilica and sinter reactive alumina would react preferentially to produce mullite which will increase the refractoriness of the material. Naturally, the presence of both anorthite and mullite at 1400 °C for NR-LCC-C5 could explain the high bulk density and low open porosity of the material as previously discussed. Although there was difficult to identify liquid phase with either XRD or SEM backscatter, the thermodynamic prediction of low-temperature liquidus phase like anorthite present in the sample lowers the hot properties of NR-LCC-C5 (Fig. 2). The NRLCC- C5 contained abundant anorthite and less in mullite than the NR-NCC-SZ. Furthermore, the formed mullite in former are enveloped by liquid phase or unstable, this won’t contribute to hot properties. In this case, it is easier to explain that the higher HMOR of the NR-NCC-SZ at 1400 °C is due to the larger mullite grains present in the sample and less liquid phase formed. 4 Conclusions Based on our study comprising flowability, mechanical properties, hot-properties, thermodynamical simulation, XRD and SEM characterisation of microsilica-gel bonded NCCs and low cement bonded LCC for nose rings in cement production rotary kiln, the following concusion can be drawn:

• Microsilica-gel bonded NCC for nose rings application has been successfully development, with improved hot properties, better thermal shock resistance and significantly improvement in hot abrasion resistance.
• The improvement in hot properties of microsilca-gel bonded refractory castable are mainly attributed to the mullite formation between microsilica and alumine fines, as well as limited liquid phases.
• The thermodynamic calculation, XRD characterization and SEM micrographs demonstrates that less mullite was formed and the formed mullite in cement bonded system might be enveloped by liquid phase at high temperature. This can explain that why the NR-NCC-SZ showed improved HMOR, better thermal-shock resistance and less abrasion loss.

Nevertheless, the development of NCCs for nose ring applications may focus on continuous improving their high-temperature mechanical properties and abrasion resistance. One potential approach is to optimize the microstructure of the NCCs, such as by controlling the size and distribution of mullite crystals, to improve their strength and refractoriness. Another approach is to explore new binder systems, such as nanosilica or other advanced binders, that can improve the bonding between the matrix and aggregates and enhance the high-temperature properties. Overall, continued research and development in NCCs for nose ring applications have the potential to improve the performance and lifespan of refractory linings in high-temperature industrial processes.

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