THE MAKING OF DOLOMITE BRICK AND A STUDY OF THEIR PROPERTIES 1

This investigation deals with the making and the properties of dolomite brick. A thorough review of the literature shows that there are many improvements possible in the making of dolomite brick and some of the principal faults are pointed out. In this investigation it was shown that 200‐mesh dolomite mixtures of a composition 9–0–6M (9% Fe 2 O 3 –0% Al 2 O 3 –6% SiO 2 –85% dolomite); 6–3–6M (6% Fe 2 O 3 –3% Al 2 ) 3 –6% SiO 2 –85% dolomite); and 2–4–4M (2% Fe 2 O 3 –4% Al 2 O 3 –4% SiO 2 –90% dolomite) can, if calcined to cone 20 down, be made into better brick than any previously described. The advantages of this higher fired material are (1) it has been more completely shrunk than that fired at lower temperatures and therefore does not shrink as much when refired in the form of brick, (2) in the use of aqueous binders less trouble is encountered due to slaking than with the lower fired material. In a continuation of the binder studies a thorough investigation was made of molasses, tar, epsom salts, water and carbonated water. No conditions were found where water, carbonated water, bindex or epsom salts, could be satisfactorily used. Using 20‐mesh (2–4–4M) calcine and 15 and 20% of molasses as a binder it was found that the brick always squatted considerably at about cone 15 down. This was shown to be inherent in the material itself and not due to a migration of any part of the material or a softening of the molasses. The most successful brick were made using 20‐mesh D9‐0‐6M composition (9% Fe 2 O 3 –0% Al 2 O 3 –6% SiO 2 –85% dolomite 200‐mesh calcined to cone 20 down), plus 13% of a one to three dextrin‐water mixture as binder, and 20‐mesh S6–3–6M (6% Fe 2 O 3 –3% Al 2 O 3 –6% SiO 2 –85% dolomite 200‐mesh calcined to cone 20 down), plus 12% of a 5% solution of sodium silicate and in each case fired to cones 16 to 18 down. These brick are dense, well shaped and refractory. Cone fusion temperatures of calcines 9–0–6M, 6–3–6M and 2–4–4M were made in an especially constructed oxyacetylene furnace. The results of the tests show that the above compositions have cone fusion temperatures above cone 40 (2010°C) down. In connection with this work it was also shown that an electric induction furnace, where graphite is used as the resistor, is not reliable for cone fusion tests due to the strong reducing atmosphere. In a load test on brick D9–0–6M and S6–3–6M it was found that when they are heated to 1350°C and held at 1350°C for one hour under a load of twenty‐five pounds per square inch they show only a small compression. Brick D9–0–6M shows a compression of 3% while S6–3–6M shows a compression of 0.9% neither brick showing any tendency to crack or squat. In a spalling test these brick (D9–0–6M and S6–3–6M) were shown to possess the characteristic fault of magnesite in that they do not withstand sudden heat change without spalling. On heating one hour a t 1350°C and then exposing to room temperature they crack badly. The use of less flux was unsuccessful as the material thus prepared shows strong hydration which is fatal in that it causes the material to disintegrate. Dolomite compositions 9‐0‐6M, 6‐3‐6M and 2 4 4‐M, flux violently with silica and fire brick but are inactive toward magnesia and chrome brick. It is, however, possible to burn the dolomite on fire clay brick if a thick layer of magnesia brick grog is used. Since it is necessary to grind the dead‐burned dolomite before it can be made up into brick i t is necessary to know the relative tendency toward slaking of the various size particles. It was found that the finer the calcined dolomite is ground the more rapidly the slaking takes place and that it is in some cases very appreciable for 20‐mesh material as used in making brick.