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Technical Paper
Figure 7: Impact of warehouse aging of industrial packed castables on flow
Figure 8: Impact of warehouse aging of CA470 TI on setting (EXO start 1), plastic vs. paper bag. Measurement for CA-470 TI in plastic bags: phase 1 (0-21 and 40-108 months) cured in the lab and phase 2 (40-108 months) cured in a temperature cabinet at 20°C
Previous studies investigated the aging behaviour of warehouse-stored industrially packed castables. The paper of Gierisch et al. [10] included comparison between a low cement castable containing dispersing alumina ADS/W (LCC-ADS/W) and another one containing polyacrylate and citric acid as additives (LCC-PACA). The dispersing alumina system showed storage stability over a period of 9 months whereas LCC-PACA showed a decrease in flow after 7 months which resulted in no flow after 9 months. The castable was still usable but only at a 0.4% higher water demand. The test series was continued and subsequent measurements confirmed the storage stability over 23 months for the dispersing alumina containing castables. Figure 7 shows the flow properties for LCC with ADS/W and CA- 14 M respectively CA-470 TI and LCC with polyacrylate/citric acid and CA-14 M.
Aging behaviour of silica fume low cement castables in the warehouse has been tested over a period of one year by Schmidtmeier et al. [11]. As additive systems dispersing alumina M-ADS/W, sodium tripolyphosphate with and without citric acid and polyphosphate with and without citric acid were used. An aging trend was observed for all additive combinations tested. The aging of castable containing M-ADS/W remained within more narrow limits especially with regard to EXO Max which stayed well below 24 hours. The phosphate containing castables showed a short working time which can be retarded by small additions of citric acid, taking into account that the main reaction and the start of strength development were strongly retarded. In addition small additive dosages increase the risk of overdosage and insufficient homogenisation which negatively affects the placing properties. When aged material is used for refractory installations problems during on-site installation or service life are inevitable, as castable properties have changed, and in the worst case scenario will no longer meet the requirements needed for the installation or the application.
Ambient conditions such as humidity also have an impact on aging. Some castable components, e.g. cement binders, show hygroscopic behaviour and need to be protected against moisture pick up. Schmidtmeier et al. [12] investigated the aging behaviour of calcium aluminate cement packed in plastic bags by storing them in a warehouse for 40 months. The cement was regularly tested in a castable application and the castable properties remained stable over the entire storage period. The test
series was
continued for up to 9 years without showing any aging effect. Figure 8 shows the EXO setting start of cement packed in plastic bags vs. paper bags
tested in a test castable containing 5% cement.
This packaging concept could also be an option for refractory castables for better moisture protection during storage resulting in a longer shelf life.
Conclusion Castable producers and users have to deal with fluctuating ambient conditions and other uncertainties during on-site installations. Nevertheless lab investigations showed that different measures can be taken to increase the robustness of castable behaviour and therefore the reliability of castable installations.
Raw material selection is the one essential element to achieve that target. Dispersing Aluminas provide a tool to steer the castable set time in general, and in addition leads to more robust castble behaviour at different ambient temperatures and variations in water addition compared to conventional additive systems such as phosphate/citric acid. Dosage and homogenisation is easy and more reliable than for additives which have to be added in very small amounts. A series of tests with industrial mixed and packed castables showed a shelf life of at least one year for castables containing dispersing aluminas as additives. Reliable setting behaviour is guaranteed when using CA-470 TI as binder. The cement behaves less sensitively with regard to temperature variations, raw material inconsistencies and trace impurities. Furthermore improved flow is achieved in silica fume castables. The risk of castable overwatering can be eliminated by using E-SY alumina as a matrix component. It provides short wet-out times and enables low energy mixing at very low water demands.
As important as raw material selection are precautions taken prior to the installation. The lab trials showed that the temperature of the dry mixed castable is an important influencing factor for the setting behaviour. Therefore in winter time the storage of the castable in a pre-heated warehouse at a temperature above 15°C for at least two days prior to usage, is advisable to avoid very long setting times. An excellent moisture protection is provided by plastic bags which can be used not only for cement but also for pre-mixed castables. It would provide additional safety during storage even if material is unintentionally stored outdoors for a period of time.
For refractory installations, stable castable processing and product
November 2017 Issue
ENGINEER THE REFRACTORIES
15
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