THE
REFRACTORIES ENGINEER
Technical Paper
Table 1: Composition (mass%) of refractory castables
is the % increase of the diameter of the fresh mix measured 90 seconds after removing the cone.
Cold modulus of rupture (CMOR) and cold crushing strength (CCS) at room temperature after demoulding, hot modulus of rupture (HMOR) and refractoriness under load (RUL) were also measured. The HMOR testing apparatus (Isoheat, UK) is equipped with a pre-heating chamber that keeps 10 samples at test temperature. The dried samples (25x25x150mm) were heated at 300 ºC /hr. The moulds were kept at 20ºC and RH>90% for 24hrs before de-moulding, then dried at 110ºC for 24hrs. Refractoriness under load (RUL) was measured on cylinders with a height of 50 mm and a central bore of 12.5 mm diameter, as described in ISO R1893 (1970). The constant load on the test samples is 0.2MPa. The equipment is designed in accordance with ISO R1893 (1970) and the maximum testing temperature is 1800ºC.
Lab-scale explosion resistance testing according to Chinese Standard YB/ T4117-2003 were carried out for all mixes. 50mm cubes are placed into a furnace heated to a preset temperature. The cubes are inspected after 30 minutes exposure. The temperature at which cracks start to form or explosive spalling occurs is reported as the explosion resistance.
The samples after HMOR testing at 1400 and 1500°C were used for further characterization. XRD with CuKα radiation was used for the mineralogical phase analysis (x’Pert Pro, Philips, Netherlands). A Scanning Electron Microscope (SEM, Quanta 400, FEI Company, USA) was used for examination of the microstructure on polished surfaces after etched in HF solution to remove the glassy phase.
3. RESULTS AND DISCUSSIONS 3.1 Flowability and setting behaviour
Self- and vibration-flow values are summarised in Fig. 1. It was observed that the specimen MSZ-7 showed highest self-flow value (94%), and the flowability of microsilica-gel bonded NCCs dropped when the same vol% of microsilica was replaced by reactive alumina. Compared to MSZ- 7, the self-flow value of castable SOL-7 was much lower (only 47%), while those of castables ULCC-7 and LCC-7 were fine, 79% and 75%, respectively.
As gel-bonded castables do not give good temperature readings using exothermic curves, the propagation of ultrasound was used to monitor the setting and hardening process. As stiffness and speed of sound are closely related the increase of velocity indicates end of working time and initial set. Fig. 2 shows the ultrasonic velocity development for the mixes. All NCCs had quick and “defined” setting as also seen for LCC-7, while ULCC-7 showed long set-time. The significance of “defined” here is that once the castable starts to set, the strength development is fast and the time to reach its final value (green-strength) is short. Time to final value
Fig. 1: Self-flow and vibration-flow of microsilica-gel/silica-sol NCCs, ULCC and LCC
Fig. 2: Setting-behaviour of microsilica-gel/silica-sol bonded NCCs, ULCC and LCC
of NCCs was in the order of 5 to 6 hours. The silica-sol bonded castable with SioxX-Flow exhibited slower set-behaviour than the microsilica-gel bonded NCCs. Furthermore, the silica-sol bonded NCC never reached the strength level of the SioxX-Zero containing mixes.
Fig. 3 shows the green cold crushing strength (CCS) and cold modulus of rupture (CMOR) (24hrs at >90%RH and 20°C). The green CCS of MSZ-7 was approximately 8.4MPa, about four times higher than that of SOL-7 (which had less than 2MPa). With decreasing microsilica content, the green strength decreased slightly. Even though the green strength of
September 2019 Issue
23
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