Examples | Thermal shock resistance of refractories | Dehydration and sintering of refractory castables

Dehydration and sintering of refractory castables

 Use of SonicByte^TM for non destructive real-time characterization of the mechanical properties variation of refractory castables during dehydration and sintering

Refractory castables

Unlike bricks, refractory castables contain calcium aluminate cement that develops hydraulic bonds, in presence of water, at room temperature. The hydrates generated during this process form a solid three-dimensional network around the aggregates. The water used to form these hydrates is called “combined” or “chemical” water. To get the adequate workability (or consistency) with such refractories prior to casting, an excess of water is required to fill the interstices between their aggregates and to form a film of liquid on their surface. This excess water, reaching more than 95% of the total water content of the castable, is called “uncombined” or “free” water.

Mechanical properties variation of refractories on firing

During the first heating of refractory castables, the free water is evaporated up to 100°C (212°F). Between 100 to 550°C (212 to 1022°F), the dehydration (or lost of combined water) takes place. These water losses cause a reduction of the castable mechanical properties. To develop ceramic bonds into such materials, the later are fired at a temperature of at least 1000°C (1832°F). Between 550 and 1000°C (1022 and 1832°F), the aggregates into such materials are almost only linked by relatively weak mechanical bonds.

Real-time monitoring of changes in mechanical properties of castables using the SonicByte^TM

The real-time monitoring of changes in mechanical properties of refractory castables during their dehydration and their sintering is possible through SonicByte^TM, by measuring their hot modulus of elasticity.

For example, Figures 1 and 2 show the elastic modulus variation of an alumino-silicate refractory castable during its dehydration and sintering, respectively. The elastic modulus measurements were performed during the heating and cooling periods of the tests pursued by means of the set-up shown on Figure 3.

 Fig. 1: Flexural elastic modulus variation during the castable dehydration.

 Fig. 2: Flexural elastic modulus variation during the castable sintering.

Fig. 3: Set-up used for the hot modulus of elasticity measurements using SonicByte^TM.

This set-up includes an electric furnace, electromagnetic impactors as well as alumina rods and tubes. The rods are connected to the impactors while tubes, acting as waveguides, are connected to the SonicByte^TM microphones. The arrangement of the rods and tubes allow to impact the sample and to collect its acoustic signals under three vibration modes: longitudinal, flexion and torsion.

Note that the elastic modulus values shown on Figures 1 and 2 can be used by SonicByte^TM to estimate in real-time the modulus of rupture and the compressive strength variations of the castable during its dehydration and sintering. The SonicByte^TM indeed includes a calculation algorithm allowing such estimation with an accuracy higher than 95% (see: Use of SonicByte^TM for estimating the physical and mechanical properties of refractories).