www.ireng.org 3.2. Evaluation of NCC and LCC for dry-gunning
The mixes listed in Table 1 were first evaluated according to the lab procedures. The NCC-G mix showed similar good consistency and stickiness as the LCC-G mix.
Figure 4 shows the development of ultrasonic velocity in the lab-scale mixes as a function of time from mixing. As stiffness and speed of sound are closely related, the increase in velocity indicates end of working time and initial set. As seen, the microsilica-gel bonded NCC-G exhibits almost as fast gelling and hardening process as LCC-G, even though the final velocity was lower.
Technical Paper
Figure 6: CMOR of cut specimens after firing at various temperatures
both NCC-S and LCC-S are at the same level. Concerning the dry-gunned specimens, the CCS is much lower at given temperature. CCS of the NCC-G is 40-50MPa at temperatures >1000ºC, which is only about half of NCC-S. As shown in Figure 6, CMOR follows as expected the same trend as CCS.
The CCS and CMOR of shotcreted specimens are about double that of the dry-gunned ones while the water addition is about half. Obviously, it is very important for refractory producers to consider various factors in order to choose the right installation techniques and the right binder system.
Figure 4: Setting and hardening of NCC-G and LCC-G 3.4 Hot-properties 3.3 Mechanical strength
The sprayed panels were prefired at 600°C before cutting with the exception of NCC-G, which was fired at 1000°C since it was too weak to cut. The cut specimens were fired for 12hrs at 1000, 1200, 1300 and 1400°C before testing. CCS and CMOR of NCC mixes are summarized in Figure 5 and 6, and the LCC-S and LCC-G are included for comparison.
HMOR results are shown in Figure 7. At 1000ºC, the HMOR of NCC-S is ~15MPa, significantly higher than that of LCC-S. With increasing temperature, up to 1400ºC, HMOR for LCC-S drops continuously, probably due to liquid formation between microsilica and cement. For NCC-S, alumina, silica and calcium (from the 0.5% cement added as coagulating agent) form a liquid phase that causes the material to soften and loose strength up to around 1200°C. A similar HMOR strength development is observed for NCC-G and LCC-G.
As seen in Figure 7, the HMORs for NCC-S and NCC-G reach their lowest value at 1200ºC. At higher temperatures NCC-S and NCC-G have better hot strength than LCC-S and LCC-G. At 1400ºC, the HMOR of NCC-S is ~3,5MPa, about 100% higher than LCC-S. The improved HMOR is attributed to mullite formation, which is in line with our previous findings11
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Figure 5: CCS of cut specimens after firing at various temperatures
Surprisingly, the CCS of NCC-S is higher than LCC-S at firing temperatures in the range of 600-1200°C, as seen in Figure 5. The CCS of microsilica-gel bonded NCC-S fired at 1000°C is ~85MPa, while the LCC-S only reaches ~60MPa. The better performance can perhaps be attributed to 0.5% lower water addition in the NCC-S. At temperatures above 1200°C, the CCS of
Figure 7: HMOR of cut specimens at various test temperatures January 2018 Issue ENGINEER THE REFRACTORIES 21
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