الفهرس 
INTRODUCTION AND OBJECT OF INVESTIGATIONOnly 14 pages are availabe for public view
Abstract
In recent years, there have been strong demands for the processing of no cement refractory castable to overcome on the high cost of HAC and to improve the thermal resistance of castables at high temperature applications. They have received increased attention because of their unique characteristics. Among the established no-cement castable ceramic materials are very fine powder material as hydratable alumina, alpha bonded alumina, very fine reactive MgO powder, colloidal silica, etc, used as a binder material. However, the cost of currently available low and no-cement castables is relatively high. Geopolymers are expected to be beneficial and important for consideration in new the refractory applications. However, little work has been done to investigate the use of geopolymer instead of calcium aluminate cement as a binder in refractory castables. This work investigates and studies the effect of replacement of HAC with one-part geopolymer slag on phase composition, microstructure, physical, mechanical, and thermal properties of produced refractory castables. The outcomes of the present study depict that refractory castables can be designed with 25% replacement of HAC with one-part geopolymer without a significant change in phase composition, microstructure, physical and mechanical properties. However, an increase in the replacement percentage strongly influences the properties of castables, while still attaining acceptable refractory properties, especially at higher temperatures. According to the results, M0, M1, and M2 castable can be applied in the pre-heating, calcinations, and cooling zone in rotary kiln of the cement and chamotte production as well as some zones inside the tunnel furnaces. Also, M3 can be recommended in reactors, reformers, regenerators, heaters, high-temperature vessels in petrochemical and chemical plants, as well as boilers and pipes. Moreover, Monolithic refractory castables are prepared based on slag geopolymer as a binder at different temperatures was studied. The results show that the formulated refractory castables exhibit acceptable physical and mechanical properties. Hibonite phase is formed at 1300C, as shown in XRD and confirmed by SEM, and obviously effect on the CCS of specimens. In addition, krotite and grossite, the main phases formed in calcium aluminate cement, are detected in the XRD. Therefore, slag geopolymer-based refractory castables are recommended to be used in pre-heating zone for tunnel furnaces, cooler in rotary kilns. Furthermore, other applications such as reactors, reformers, regenerators, heaters, high temperature and pressure vessels in petro-chemical industries, chemical plants as well as boilers and pipes.
This investigation includes four main sections:
Section (A): Preparation of Refractory Castables
The batch composition for the unshaped refractory castables was designed using Dinger and Funk model, with a distribution coefficient, q = 0.25. The binder and aggregates (coarse and fine) were dry mixed for 3 min in a rotary drum mixer then water containing NFS was added slowly within a 5 min mixing. In all mixes, the dosage of NFS was 2% by weight of binder. The fresh mixes were poured into cubic steel molds and vibrated for 2 min, and then the surface was smoothed using a trowel. The molded samples were kept covered with a plastic film for 24 h at room temperature to avoid water loss. After de-molding, all samples were submerged in water for 24 h, and then dried at 110 C for another 24 h. The specimens were subjected to firing in an electric muffle furnace for 6 h at different peak temperatures, 850 C, 1100 C and 1300 C with a constant heating rate of 5 °C/min and furnace cooling has been applied to all samples.
Section (B): characterization and Test Methods
The fineness character of slag sample was described by measuring the particle size distribution (PSD). X-ray fluorescence was used to determine the chemical composition of the aggregates and binders was used in the study. Phase identification of the raw materials and refractory castable samples was performed using XRD analysis. The morphology of some selected castable samples was observed through scanning electron microscopy (SEM). Mechanical strength in term of cold crushing strength (CCS) of castables was measured using Control Hydraulic Press. The thermal properties of castables were studied carried out refractoriness under load (RUL), thermal shock resistance (TSR) and permanent linear change (PLC). Archimedes method was used to determine the physical properties of castables through bulk density, apparent porosity, and water absorption for dried and fired castable specimens. All results were based on an average of three specimens.
Section (C): Castables characterization
C.1. Phase Evolution
XRD patterns of one-part geopolymer/calcium aluminate cement based refractory castables dried at 110°C and then fired at different temperatures (850, 1100 and 1300°C) were examined. The corundum (α-Al2O3), cristobalite (SiO2) and mullite (3Al2O3.2SiO2 – A3S2) phases appear as major crystalline phases in all the castables at all firing temperatures. Phase transformation in M1 castable in which 25 wt.% of one-part slag-based geopolymer replaced the HAC is quite similar to that in M0 at higher temperatures. However, the main differences identified between the two castables are the absence of grossite phase and decrease of peaks intensity of calcium aluminate phases (C12A7 and CA) in M1 castables at 850°C, which may ascribe to lower alumina content in slag comparing to HAC. XRD of M2 and M3 castables where one-part slag-based geopolymer replaced 50 and 75 wt.% of HAC, respectivelyed show phase transformation that is fairly similar to M1 at the different temperatures. Finally, the XRD pattern of M4 castables where, one-part slag-based geopolymer is only used as a binder material at different firing temperatures revealed that the cement hydrated phases and zeolites are completely disappeared at 110°C. Interestingly, low melting calcium aluminate phases (C12A7 and CA) and high melting calcium aluminate phase (CA6) are detected at higher temperatures, but the intensity of its characteristic peaks is lower than those of other castables. In addition, the intensity of characteristic peaks of anorthite is progressively increased at higher temperatures.
C.2. Physical Properties
Bulk density, apparent porosity, and water absorption of the different prepared castables were measured at different firing temperatures. The decrease in bulk density as well as increase in apparent porosity and water absorption above 110 C and up to 1100C is ascribed to the evaporation of water and dehydration of hydrated phases at 850C and crystallization of CA2 and C2AS phases at 1100C. While the temperature increases to 1300 °C, M0 castable shows an increase in bulk density together with a decrease in apparent porosity and water absorption which can be interpreted as resulting from the formation of ceramic bond and, hence obtaining a dense structure. On contrary, the bulk density of M2, M3, and M4 castables is continuously and drastically decreases, while the apparent porosity and water absorption values are significantly increased. This phenomenon may attribute to creation of liquid phase at 1300C, by partial or complete replacement of HAC with one-part geopolymer slag binder in the matrix, which might cause formation of open and/or isolated close pores in castables.
C.3. Mechanical Properties
The mechanical behavior of the castable specimens where HAC is replaced by 0, 25, 50, 75, and 100 wt. % of one-part geopolymer slag binder at different temperatures were studied. Obviously, a similar trend toward a decrease in the CCS of castables with further replacement of HAC with one-part geopolymer slag at all temperatures. The highest strength is attained in M0 castables dried at 110 C as a result of high amount of cement hydrated phases. Conversely, M4 castables dried at 110 C shows the lowest value due to complete absence of hydrogarnet and gibbsite phases as well as insufficient geopolymer gel to bind the constituents. Above 110 C and up to 1100C, a well-known phenomenon of refractory castables is observed, strength decline, which is significantly attributable to dehydration and decomposition of hydrated phases as well as to formation of CA2, C2AS, and -CS phases as confirmed in the XRD results. The CCS again increases upon raising the temperature to 1300C, in all castables, because of the sintering progress and the in-situ formation of hibonite phase, which plays an important role in granting strength to the refractory castables.
C.4. Microstructure Investigation
The variations in physical and mechanical properties are often combined with changes in microstructure. To identify the microstructure, the fracture surface of selected castables, was analyzed by SEM. The microstructures of M0, M2 and M4 castables fired at different temperature were examined. Densely packed crystals of hydrogarnet and gibbsite can be seen along with aluminum hydroxide (AH3) gel at 110C in M0 and M2 castables. The presence of AH3 gel in the matrix, indicates incomplete transformation of AH3 into crystalline gibbsite, and consequently will have repercussions on porosity and strength of castables. Neither hydrogarnet nor gibbsite are detected in M4 castable at 110C due to the absence of HAC, but needle-shaped crystals of mullite in the glassy phase of geopolymer are identified. Upon raising the firing temperature to 1100C, there are a tendency for voids formation due to crystallization of relatively large crystals of grossite and gehlenite in the matrix. Also, some liquid phase uniformly present. The high sintering temperature at 1300C, motivate the formation of hibonite phase with a plate-like grains morphologies. Some needle shaped mullite crystals are also noticed and distributed which creating an interlocking structure, therefore will increase the density of castables.
C.5. Thermal Properties
Thermal shock resistance (TSR) of refractory castables can evaluate by two phases of structural damage involving crack initiation and crack propagation which mainly affected by many factors such as the shape and structure of the sample, grain size distribution, strengths, modulus of elasticity, thermal expansion, thermal conductivity, thermal diffusivity, etc. the thermal shock of the fired castables based HAC only as a binder has a high thermal shock resistance because of high intensity peak of hibonite. Regarding the number of thermal shock resistance (TSR) of castables was decreased by increasing the percentage of slag cement as a binder. On the other hand, castable based on alkali-activated slag only as a binder could withstand more than 15 cycles in quenching in water without damage or sign of cracking. Also, RUL of castables is mainly affected by the chemical composition distribution, size and morphology of crystalline, the amount of the glassy phases, the apparent porosity of the structure and etc. The good refractoriness for of M0 induced formation of corundum, cristabolite, grossite, mullite and formation of hibonite (CA6) phase which is the most alumina-rich intermediate phase of the CaO- Al2O3 system which has a significant effect on the thermal properties. T0.5 and T1 values are 1250C and 1460 C; respectively for M1 patch, in which one-part slag based geopolymer replaced 25% (wt.%) of HAC, which is lower than M0 (1300C and 1550 C ) which attributed to decrease the peak intensity of hibonite . For M2 and M3 castables where one-part slag-based geopolymer replaced 50 and 75 (wt., %) of HAC, respectively showed continuous decrease in T0.5 and T1values by decreasing the HAC content. It is a fundamentally that with a decrease in the content of HAC the thermal properties will significantly be affected. Moreover, The PLC was measured in terms of contraction or expansion percentage. Clearly, M0 castable shows slight linear shrinkage, 0.09% and 0.6%after firing at 1100 C and 1300 C, respectively. By contrast, other castables exhibit permanent linear expansion at 1300 C and the percentage of liner expansion is significantly affected by an increase in replacement percentage of one-part geopolymer slag in the matrix. Overall, the results revealed that alkali-activated slag cement is a promising binder for refractory castables at high temperature applications.
Section (D): Cost Estimation and Applications
The designed refractory castables with partial or complete replacement of HAC with one-part geopolymer slag show acceptable properties to be used for various refractory applications. Overall, the results revealed that alkali-activated slag cement is a promising binder for refractory applications at high temperatures. On the other hand, the applications and cost estimations and results proved that M1 is the best batch because of high wide applications and cheaper than other batches.
REMARK CONCLUSION
• The results revealed that no significant change in phase composition, microstructure, and physico-mechanical properties of refractory castables by replacement of 25% of HAC with one-part geopolymer slag.
• M0 = 6803.2 E.P is the highest cost in all batches because it’s containing 100% HAC as a binder.
• By replacement HAC with slag binder the cost was reduced M1= 6718.2 E.P and M2=6701.2 E.P but by reducing the amount of HAC to 75% and 100% the cost conversely was increased because of the amount of bauxite aggregates was increased.
• The designed refractory castables with partial or complete replacement of HAC with one-part geopolymer slag show acceptable properties to be used for various refractory applications.
• Overall, the results revealed that alkali-activated slag cement is a promising binder for refractory castables at high temperature