Technical Paper 3.5 SEM characterisation


Cut specimens of LCC-S and NCC-S were chosen for further SEM characterisation, in order to look at the bond mechanism of microsilica-gel bonded NCC compared to LCC at elevated temperatures. Figure 8 and 9 show micrographs after HMOR testing at 1200 and 1400°C, respectively. At 1200°C, needle-like mullite is observed in the matrix of LCC-S while only trace amount of mullite is found in the microsilica-gel bonded NCC-S. This is probably attributed to the formed liquid phase from cement and microsilica at 1200°C that facilitates local mullite formation in LCC-S, which leads to higher HMOR compared to NCC-S (See Figure 7).


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were tested. Table 2 shows the lab-scale explosion test results of both “wet” and “dried” samples.


Temp. (o 300 350 400 500 600


1000 1200


C)


Wet (20o Without √ x


C/24 hrs)


EMSIL-DRY √ √ √ √ x


Dried (110o Without


C/24 hrs) EMSIL-DRY


√ √


√ √


Table 2: Explosion resistance of microsilica-gel bonded NCC-S* with and without EMSIL-DRY √: passed; x: failed


Figure 8: SEM micrographs of etched fractured surfaces of cut specimens fired at 1200°C, a) LCC-S and b) NCC-S


All “dried” samples show excellent explosion resistance and pass the test at 1200°C. The good performance is attributed to a stable bond phase and the low amount of residual water in the bond phase. When the “wet” samples were tested, good explosion resistance was achieved for the microsilica-gel bonded NCC containing drying agent (EMSIL-DRY). Without drying agent, the specimens survived the test at 300°C, and exploded at 350°C. With EMSIL-DRY the specimen passed the test at 500°C.


Figure 9: SEM micrographs of etched fractured surfaces of cut specimens fired at 1400°C, a) LCC-S and b) NCC-S


At 1400°C, numerous mullite crystals are observed in both LCC-S and NCC-S, as shown in Figure 9. However, the difference is that the needle- like mullite in LCC-S are single crystals after the glassy phase has been etched away, but the ones in NCC-S are interlocked and closely packed. This indicates that little liquid phase is formed in NCC-S whereas a large amount of liquid is formed in LCC-S at 1400°C. The mullite in LCC-S is embraced by the liquid phase and the strength deteriorates as soon as the liquid phase begins to form. On the contrary, for NCC-S, the needle- like mullite crystals provide strength by bridging the aggregates, forming a strong and highly refractory matrix.


3.6 Explosion resistance of cast NCC


In order to cast samples that could be de-moulded after one day, an additional accelerator was added in NCC-S (see Table 1), and the mix was labelled NCC-S*. The microsilica-gel bonded NCC-S* was used in a further investigation to better understand the drying behaviour. Lab- scale explosion resistance tests were conducted according to Chinese Standard YB/T4117-2003. Cast 50mm cubes were cured for 24hrs at room temperature and 100%RH before de-moulding. The freshly de-moulded samples are labelled “wet” and samples further dried for 24 hrs at 110°C are called “dried”. Samples both with and without drying agent (EMSIL-DRY)


Figure10: ~400kg block of NCC-S* with EMSIL-DRY after testing at 850°C


To further understand the drying behaviour, a larger block, 600x600x350mm (~400kg), of microsilica-gel bonded NCC-S* containing EMSIL-DRY was produced. The block was de-moulded after one day and put into the furnace for testing. The procedure was heating from 20 to 850°C at 50°C/hr; then cooling from 850 to 20°C at 50°C/hr. The block performed perfectly. Figure 10 shows the block after the test. A 400kg block of microsilica-gel bonded NCC-G with additional retarder was also cast and tested using the same procedure as NCC-S*. After testing, the ~400kg NCC-G block remained intact and looked similar to NCC-S* in Figure 10.


This demonstrates that the performance of microsilica-gel bonded NCCs was significantly improved by using EMSIL-DRY which contributes to fast dewatering and consequently improved explosion resistance. It indicates that a true rapid heating is possible for industrial-scale production.


4. Conclusions


Microsilica-gel bonded NCC and cement bonded LCC were successfully shotcreted and dry-gunned and the properties were compared. The


22 ENGINEER THE REFRACTORIES January 2018 Issue


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