First, the components are mixed in an appropriate weight ratio (e.g. cement/polymer/water/plasticiser = 80/4/15/1). The produced loose non-cohesive mass is then homogenized and compacted in equipment which applies high shear forces (e.g. a twin-roll mixer). In this step the characteristic microstructure forms which is associated with the resulting properties of the material. The produced compact mass can be further moulded using high-pressure techniques (e.g. calendering, injection-moulding or extrusion) to the required shape according to the intended application (e.g. a slab or load-bearing section). A product prepared according to this method, i.e. through complex hydration reactions without the need for heat processing, hardens spontaneously at any temperature from 0 to 90 °C, and within 48 hours it achieves considerable strength, particularly tensile strength and bend strength of even more than 50 MPa.
Fig. 1: The working part a of twin-roll mix at high-shear mixing
Shaped refractory materials are produced by ceramic technologies which use heat processing to form a ceramic bond in a material. The material is heated to the sintering temperature. This energy-consuming process is mostly associated with the production of greenhouse gases arising from the combustion of fossil fuels, e.g. natural gas.
Non-shaped refractory materials are moulded after mixing with a hardening ingredient (mostly water) on the site of the installation of brickwork. A typical example of non-shaped refractory materials are aluminate cements. Once the aluminate cement has been mixed with water, the cement solidifies and hardens as a consequence of hydration reactions of the sintered minerals, particularly the minerals CaO.Al2O3 and 12CaO.7Al2O3, producing crystalline aluminate hydrates hexagonal in character (e.g., 2CaO.Al2O3.8H2O, CaO.Al2O3.10H2O and 4CaO.Al2O3.13-19H2O) and, later, cubical (3CaO.Al2O3.6H2O).When the hydrated system hardens it is exposed to high temperatures during the fulfilment of the function of a refractory material. All the above-mentioned aluminate hydrates gradually dehydrates at temperatures of 100 – 350 °C, which is connected with considerable decrease in volume. The result is the creation of local tensions in the material, which are released by the occurrence and spread of fissures. The compactness and function of the materials is thus considerably compromised. A research team of the Faculty of Chemistry at the Brno Institute of Technology has successfully dealt with the issue of both shaped and non-shaped refractory materials. Compared to shaped materials, production of this material does not require a process of thermal treatment like in most of non-shaped refractory materials.
However, compared to non-shaped materials which are also formed on the basis of aluminate cement, fissures do not occur in this material when it is exposed to high temperatures. The reason is that there is a difference between the bonding connective mechanism in aluminate concretes and this material, where formation of aluminate hydrates is considerably suppressed. The role of connective phase is not only played by aluminate hydrates, it is primarily provided by a polymer. If the material is exposed to high temperatures, at first the aluminate hydrates present dehydrate gradually till temperature reaches approx. 350 °C. Concurrently, the gradual degradation and pyrolysis of organic material starts. If polyphosphate is used as a polymeric ingredient, it dehydroxylates gradually. This is associated with some temporary decrease in strength. At high temperatures, the ceramic bond starts to form by the sintering of clinker minerals present in the cement and strength once again increases.
The advantage of a product prepared this way and developed at the Faculty of Chemistry at BUT is the option of direct installation in high-temperature equipment as a refractory element. Moreover, compared to concretes made of aluminate cement, this material does not show any shrinking after the dehydroxylation of hydro-aluminates that would result in the in occurrence of fissures, and compared to heat-resistant ceramics, it does not need the high energy-consuming procedure of thermal processing. After mixing and a short hardening, the raw materials are installed directly in high-temperature equipment with the expectation of resistance at a temperature of 1300 – 1500 °C, depending on the composition of the starting mixture.
The invention of high-shear mixed refractory materials has been protected with a utility model and a patent application has been filed.
Photo: archives of the Centre for Material Research, Faculty of Chemistry, BUT.
