Comparison of Refractory Precast Bricks and Unfired Machine-pressed Bricks

Comparison of Refractory Precast Bricks and Unfired Machine-pressed Bricks

Refractory bricks prefabricated by castables are also called castable prefabricated blocks or refractory precast blocks. They are prepared as follows: add water to castables and mix well; pour the mixture into moulds; shape by vibration or self-flowing; after shaping, cure at room temperature for 1-3 days (can be adjusted according to the climate and temperature); when the green blocks have some strength, demould; then naturally dry the green blocks (one day－one month, adjustable according to the climate, temperature and size of the precast blocks); at last, bake the dried blocks in a drying furnace for several days to get the final products.

Machine-pressed bricks were shaped by machine-pressing the mix which is composed of aggregate, powder, binder, additive, and a small amount of water. Since the bricks contain a very low amount of water, they can be dried and fired rapidly. For unfired bricks, direct low temperature drying is OK.

The above mentioned methods to produce refractories are two conventional production ways for shaped refractories. They have their advantages and disadvantages. Generally speaking, unfired machine-pressed bricks have more advantages than precast bricks.

1）Unfired machine-pressed bricks:

Unfired machine-pressed bricks are characterized by the short production period and high efficiency for the high production efficiency, high yield, good quality, high green brick production rate of the press, requiring no curing and firing, short drying time, and less influence by weather and temperature. Machine-pressing endows the bricks with neat corners and edges, as well as accurate dimensions, facilitating the lining construction and promoting the yield rate. The machine-pressed bricks are dense and uniform, so they have high strength, low porosity, good corrosion resistance and stable integrated properties. Without firing, the unfired machine-pressed bricks are also environmental friendly and energy-saving. The disadvantage of unfired machine-pressed bricks is the press investment which increases the initial investment but in long terms a low cost.

2）Precast bricks:

Compared with unfired machine-pressed bricks, precast bricks have longer production period but less stable product quality. When casting bricks, 5%-8% water is needed. Casting and curing highly depend on the weather and temperature. The solidification and strength change from time to time. The in-mould and demould curing periods are both long. Besides, drying and baking are important procedures during production. During baking, free water and combined water in the brick release; so for blocks of big size or high density, cracks and spalling may happen due to the vapor pressure and thermal stress. Thus, a proper baking schedule is required. So the production period is several times of that of the machine-pressed bricks and green bricks after demoulding or baking usually get rejected for the deformed shape or failed size. Precast bricks have higher porosity than the machine-pressed bricks leading to worse corrosion resistance and lower strength. Precast bricks require more moulds and longer production time, so the actual cost is not low.

If producing standard high performance refractories used in serious conditions, unfired machine-pressed bricks are obviously a better choice than precast bricks. The binder of unfired machine-pressed bricks is the key point. The magnesium aluminate binder series produced by our company are different from the common resin binder or phosphate binder. They are environmental friendly, carbon-free, high-strength, and have no low-melting materials formed at high temperatures, which are a good choice for the production of high grade refractory unfired machine-pressed bricks.