In most cases, refractories are designed for the highest achievable density. The more solid they are the better they resist slag penetration, gas penetration, molten metal penetration and chemical attack. If they are too solid they are prone to thermal spalling.

To get them as solid as possible, we rely on “Grading” or particle size distribution. (See section 69) the coarser the aggregate, the better for high density, and then we add finer aggregates to fill the gaps between them, etc. down to sub-micron particles. The finer the particles, the more influence they have on rheology.

Finer particles are more expensive than coarse aggregates. Silica fume is generally the finest particles in refractories, and to get them into the spaces between the larger particles requires intensive mixing.

The first stage of mixing is dry mixing in the factory. Many kinds of mixers can be use, but with insulation materials, the lower density materials tend to stay in the top of the mixer and denser materials gather at the bottom. This can lead to segregation such that the first materials discharged go into the first bag, containing too many heavy particles, and the lost bag, has too many lightweight materials. If each bag is made as on batch, the bag has the perfect composition. With handling, segregation can occur in the bag, so if you take a scoop of materials from the top of a bag of insulation material, it can differ from the last scoop from the bottom of the bag.

The next stage of mixing is wet mixing before installation. Generally, pan mixers are used on site. When you add the right prescribed amount of water, the mix usually appears to be too dry. After a while you can observe it getting “wetter”. Products with high silica fume contents show this to a surprising extent. Operators unfamiliar with them always add extra water and ruin the properties. Manufacturers sometimes specify a mixing time of 10 minutes. Some manufacturers refuse to sell their ultra-high strength castables and insist on making the casting themselves. Products with the highest grade silica fumes require not only long mixing times but high speed, high shear mixers.

For most materials, planetary mixers, as used by bakeries, give the best results. Some materials work best with a procedure called “paste and let down”. A portion of the powder first and mixed until a paste is formed. Good dispersion is easily achieved in paste from. Then the balance of the water is added in very small amounts until a pourable or paintable consistency is reached.

“Rolls-Royce” castables like Dump mixes on the other hand are super user friendly, and can be mixed satisfactorily in a concrete mixer. They should be discharges directly into the mould or furnace. They flow slowly but continuously and release all bubbles without assistance. Don’t try to transport them in buckets, as they stick to the sides. They are very cohesive and barely wet your hands in handled.

Another mixing technique is called “pudding”. This is similar to the process of mixing concrete on the slab, where you place the mix on the slab with a hollow in the centre, pour the water into the hollow and gradually add the dry concrete into the puddle. There is very little shear involved. In a bakery mixer, place all the water into the bowl, then add half of the powder. Mix until fluid and then add half of the remainder of the powder, i.e. ¼ of the total. Repeat the procedure adding 1/8 of the powder, then 1/16 and so forth and reach the final consistency faster than if you add all the powder to all of the water.

Vermiculite is a very soft aggregate. If you follow the manufacturers instructions the mix will take a long time to wet up. What actually happens is that the vermiculite particles get squashed by the mixer and occupy less volume, ruining most of the insulation value.

Dilatant materials have a lot of equal sized fine particles and surface tension create a phenomenon called dilatancy. This property is familiar in wet beach sand. When you step on it, it goes stiff and the water film is sucked dry. These materials can be fluid but go hard if you push a spade into them. They cannot be pumped.

Other types of mixers are:

Z blade mixer. It is low speed but high shear.

Ribbon blenders are mostly used for continuous feed and discharge.

Cone blenders are used in some industries but not for refractories.

Watch out for segregation. Horizontal shafted mixers like Z blade and ribbon blenders are less prone to segregation. There are various hybrid mixers which are optimal for specific materials.

Very old mixers are several times more robust than modern mixers.