Microsilica in Refractory Castables- Surface Properties and ... - Elkem

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$ageing of refractory castables; which interactions exist and ... - Elkem$

densely packed castable systems where cement is present. The calcium and aluminate cations from the cement dissolve and attack the otherwise fluid bond phase. To prevent this coagulation, the calcium ions have to be prevented from absorbing on the microsilica. This may either be done by having a pH below 5, but as this may affect setting; addition of a proper amount of a polyelectrolyte or a surface active agent is a common stratagem. The surface active agent, normally termed deflocculant (in Portland cement based systems termed plasticiser or superplasticiser), is believed to absorb to the surfaces thus preventing the absorption of calcium but it also creates an equal, normally negative charge of the particles of the bond system. As the particles then will repel each other, casting can take place at reduced water additions. Deflocculants commonly used are polyphosphates, e.g. sodium hexametaphosphate (Calgon) at approximately 0.2 wt% addition level, and polyacrylates (Darvan 811D, 0.05 wt %). The deflocculating mechanisms involved falls normally into two main groups or combination thereof: 1) Steric stabilisation, the deflocculant attaches to the fines and the size of it prevents agglomeration of particles 2) Electrostatic stabilisation, the deflocculant creates an equal electrical charge on the surface of the fines. Electrostatic repulsion prevents particles from agglomeration. 3) Combination of 1) and 2): The recent years a new group of deflocculants have emerged on the market containing both steric groups and electrical charges which gives a “double” deflocculating effect (typically Castament FS20 from BASF). Below, in Figure 3, the structure of such a deflocculant is sketched. And in Figure 4 the deflocculating action is visualised. Figure 3. Structure of a combined steric and electrostatic deflocculant. A typically polycarboxylate “backbone” with polyether sidechains. Figure 4.Visualisation of the deflocculating mechanism of a compound like the one presented in Figure 3.
The use of deflocculants enables us to maintain flow of refractory castables for some time, sufficient to place the castable before setting (gelling) commences. Sometimes setting may get adversely affected though, giving unacceptably long or short set times. This has been explained with several mechanisms. For long set time with polycarboxylate ethers, one explanation is that the surface of the cement get “shielded” by the side chains and that longer side chains consequently retard more than shorter 2 . If deflocculants are added to microsilica slurry its effect can be followed by measuring the resulting Zeta-potential of the microsilica. Castament FS 20 at 0.05% addition levels (approximately 0.3 mg/m 2 ref Fig. 6 and 7) Table 1 Standard LCC castable composition. [wt%] Microsilica (18 or 24 m 2 /g) 8 CA-14 [wt%] 6 White Fused Alumina. 3-5mm 10 White Fused Alumina. 0,5-3 mm 32 White Fused Alumina. 0-0,5mm 16 White Fused Alumina. -74 micron 12 Calcined alumina: CT 9FG 16 Deflocculant: Castament FS20 0,01-0,1* Water [wt%] 4.15 * Standard dosage, 0.05% Zeta-potential (mV) 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Surfactant content (mg/m2) FS20 Darvan 811D Calgon Figure 6. Zeta potential of microsilica 971 as a function of deflocculant addition represented as addition per surface area. In Figure 6 the effect of deflocculant additions to a 10% microsilica (971) on zeta-potential is shown for three different deflocculants. Calgon is a sodium polyphosphate, Darvan is a sodium polyacrylate and FS20 is polycarboxylate ether as described above. For both Darvan 811D and Calgon, additions to the microsilica slurry result in an increased negative surface charge, zeta potential. This increased zeta-potential strengthens repulsion between particles and stabilise the slurry. For the combined deflocculant FS20, the electrostatic effect is less pronounced but this was expected as the steric contribution is considerable in this type of deflocculants. Based on the zeta potentials alone, without taking the steric factor into consideration, one could expect that the Darvan would give best flow when used in a castable and that the Castament would show much poorer performance. This is not the case however; in real life performance is quite similar for the Darvan and the To check the effect of the Castament FS20 on flow of a castable, we employed a standard LCC mix (Table 1) used for testing in our (Elkem) laboratories, where the amount of deflocculant was taken as the variable. Two different microsilica qualities with surface area of 18 and 24m 2 /g were used. In Table 1, the composition of this mix is given. In Figure 7, the resultant self-flow values are shown as a function of deflocculant addition per microsilica surface area (mg/m 2 ). The two microsilica qualities gives very similar results when expressed as a function of surface area, only the ultimate self flow differs somewhat. Although the lower self flow was obtained with the microsilica with the higher surface area, and conclusions in the direction of lower flow at higher surface area would be tempting, we do not have data proving that this actually is the case. Self-flow (%) 100 90 80 70 60 50 40 30 20 10 0 24 m 2 /g 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 FS20 dosage (mg/m 2 ) 18 m 2 /g Figure 7. Self flow of white fused alumina based LCC as a function of deflocculant addition. Apart from the flow, the set is the other factor of high significance when producing and using
Page 1 and 2: MICROSILICA IN REFRACTORY CASTABLES
Page 3: Based on theoretical and experiment
Page 7 and 8: measurements may be around 3-5% dep
Page 9: Or, expressed slightly differently,

Microsilica in Refractory Castables- Surface Properties and ... - Elkem

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