BACKLightweight Castables - used primarily for their thermal insulating properties behind dense castables, or in certain conditions as working hot face linings. Their inherent low density significantly reduces structural loading.
Gunning Castables - modified versions of the above castables allowing them to be placed effectively using pneumatic guns. Many grades are dual purpose allowing both casting and gunning without change. Installation of gunning grade castables is discussed in IP/002.
Special Purpose Castables - this grouping covers the modern range of low cement castables and castables with less conventional binders and are discussed in other procedures.
This procedure covers the installation of dense and lightweight castables (groups 1.1 and 1.2 of the above) by casting.
PRE-INSTALLATION
Storage.
Castables should be stored in a dry, well-ventilated area and held off the floor on pallets. If stored outside, the bags must be protected from rain or dripping water by a fixed cover. If they are further protected by plastic sheeting, ensure that there is sufficient ventilation underneath the plastic sheet to prevent water condensing on the bags. Avoid storing in areas of high humidity.
Pallets should not be stacked more than 3-4 pallets (6 tonne) high when in storage. This should be reduced to 2-3 pallets (2 tonne) when storing lightweight castables. This is to prevent the consolidation and caking of material on the bottom rows of the pallet.
Most Thermal Ceramics Australia castables have a finite shelf life called a "nominal shelf life". The nominal shelf life is half of the maximum expected shelf life of the material. When quoting shelf life, a nominal shelf life of 6 months is used. Product that is older than the nominal shelf life should be checked for setting properties prior to use.
Signs of aging are longer setting times and reduced strengths. All bags have the date of manufacture printed on them to aid in determining the age of the material. Always use the oldest materials first and if aged beyond the nominal shelf life, have the material tested before use.
Surface Preparation Prior to Refractory Placement. The work surface shall be substantially free of dust, scale, oil, water, dirt and loose foreign material and shall be cleaned by sandblasting if necessary.
Formwork.
Formwork shall be constructed from strong materials that resist water absorption. Generally steel formers are favoured for this job because of their inherent strength and non-absorbent nature. The surface of the formwork must be lightly but thoroughly oiled or greased to facilitate removal from the face of the casting. The surface of the formwork should be uniform and smooth to give a good surface finish to the casting. It also aids in formwork release.
Formwork must be watertight, so all joints and holes must be sealed. The use of vibrators during casting will cause liquids and fines to 'bleed' through cracks in the formwork. This liquid will be rich in cement and can thus leave a weakly bonded material adjacent to the leaking joint causing detrimental results.
Where refractory concrete is to be cast vertically, the formwork shall generally have a rise of not more than 1000mm, with provision for additional rises to be mounted rapidly to ensure the continuous casting of panels or sections between designated joints.
Anchors.
Both ceramic and metallic anchors are appropriate for use with conventional castable refractories. This subject is treated in a separate procedure (IP/008).
INSTALLATION
Water.
The water used for mixing shall be clean and of drinkable quality - potable water. Since the amount of water added to the castable does more to affect the properties than any other factor, it must be accurately measured.
Water amounts are shown on the back of the monolithic bags, but these are intended as a guide only. Actual amounts will depend upon field conditions and shall adhere to that recommended on the manufacturers data sheet. This may only be varied at the manufacturer's representative's direct instruction.
Temperature.
Ambient, casting water and material temperatures can significantly affect the setting time of castables. In very cold conditions, the bags should be kept in warmer storage, though not less than 15(C, for at least 48 hours before use.
The ideal temperature of mixed castables lies between 15(C - 30(C. When cast in place until thoroughly dried, the castable and environment should preferably not fall below 10(C and certainly not below 15(C as they will take longer to set. If the castable freezes before the hydraulic set is completed, the ultimate strength of the material can be reduced by as much as 60-70%.
In very hot weather, the bags should be kept in cool storage and the outer steel shell and formwork should be cooled with water sprays. When spraying, care needs to be taken to ensure that no cooling water seeps into the casting cavity. If it does, it must be dried before casting commences. Cool water should be used for mixing. At high temperatures, the time available to place the material is dramatically shortened; e.g. at 40(C the working time can be as little as 2 minutes.
Mixing.
The object of mixing is to combine the dry materials and water into a homogenous mass. Mixing shall be done in a high intensity paddle type mixer of sufficient capacity to ensure thorough mixing (not completely full). The level of dry product in the mixer must not be above the centre paddle shaft. These mixers incorporate rotating shafts with paddles attached. The paddles, which put a high level of work energy into the mixing operation, must have no more than 15 mm clearance from the mixer walls. They assure a rapid, thorough mix and virtually clean themselves from batch to batch.
The mixer and tools used in casting must be clean. Some substances found in dirty mixers combine with the cement in castables and can cause flash setting or otherwise lower the ultimate strength of the castable. The mixer should also be washed periodically during the course of mixing to prevent build up of material.
Do not mix more material than can be placed within 15 minutes from the start of mixing and do not combine different types of material. Add the dry castable in full bag lots. Add three quarters of the mixing water along the entire length of the mixer to ensure rapid, even mixing.
After the mix becomes uniform in colour, the remainder of the water is added in small increments to bring the mix to the correct consistency. Do not use more than the recommended amount of water without direct authorisation from the manufacturer's representative.
Discharge at least two buckets of product from the mixer to clear the discharge gate. This material shall be immediately returned to the mixer and re-mixed. The discharge gate should be cleaned out after every mix to prevent build-up of old material in this region.
If stainless steel fibres are to be added to the castable, they should be added after all the refractory and water has been added to the mixer. The fibres should be introduced by sprinkling them into the mixer or preferably by shaking them through a wire basket. If the fibres are already contained in the castable, the water addition shall be made on the basis of the powder weight, not the bag weight. The weight of fibres present in the mix needs to be subtracted from the bag weight before calculating the casting water needed. Care needs to be taken here since the addition of fibres will cause a lowering of the mix fluidity. NO EXTRA WATER needs to be added to compensate for this loss in fluidity.
Lightweight castables should be mixed for approximately 1-2 minutes. Dense castables should be mixed for a minimum of 4 minutes. Excess mixing generates heat causing a decrease in the setting time and reducing the strength of the set castable. Insufficient mixing will result in a non-homogenous batch and again reduce the strength.
"Ball in Hand" test. This often mentioned test is a useful guide to proper casting consistency. Tossed 15-30 cm in the air, a ball of properly mixed castable should adapt to the shape of the hand when it is caught. The ball should not flow through the fingers or break apart. Breaking may indicate insufficient water in the mix. Flowing through the fingers may indicate that the mix is too wet. In contrast, when testing insulating castables, they are wetter and thus will flow through the hand when thrown in the air and caught. This test is unsuitable for the thixotropic, vibratory cast, low cement materials.
Placement.
The work of placement of the mixed refractory castable covers several functions: to position and consolidate freshly mixed material within 15 minutes of it leaving the mixer; to work in a manner which minimises material segregation; to fill all voids, particularly around obstructions and in corners and to eliminate all air bubbles.
Once application has started, it shall proceed without interruption until the lining of the part or section concerned is completed. Consolidation of dense castables is effected by means of a medium to high frequency vibrator, applied either internally or externally. Most dense castables require a high frequency vibrator (minimum rated speed of 14000 VPM when immersed in the mix). High frequency is desirable as a lower water content can then be used, causing an increase in strength, density and abrasion resistance. A poker type vibrator (internal) gives maximum strength and densification in the finished casting and assists flow around anchors and into recesses.
High consolidation is desirable in dense castables but density is not required in insulating castables, which also have a greater tendency for segregation. In these materials an external vibrator or hand rodding should be used.
Vibrate (or rod) until the material has settled. The surface should have a wet appearance and air bubbles rising to the surface will cease. Best results are achieved by inserting the poker to a depth of no greater than 300-400 mm, keeping it vertical, and letting the vibrator move slowly through the castable mass at its own speed. DO NOT force the vibrator through the material as this will cause pockets of air to be left behind. The vibrators size should be suited to the size of the section being cast - generally a 50-65 mm diameter vibrator is used for large casts, whilst a 23-35 mm diameter vibrator can be used in smaller casts. Smaller vibrators require a higher VPM rating. When removing, the vibrator should be drawn upwards very slowly through the castable so it does not leave any holes or channels behind. The mix is too stiff if holes are left behind.
When vibrating around anchors and other obstructions, add fresh material from one side and allow the vibrator to feed the material around and under the obstruction to avoid the formation of air pockets. For deep cavities requiring successive layers of castable mix, the poker should pass through the freshly added mix into the previously cast material to ensure a homogenous lining free of laminations.
Over vibration will tend to segregate water from the mix, weakening the cast and should thus be avoided.
Multi-layer Linings
Where an insulating lining has previously be installed, it should be coated with an impermeable membrane to prevent premature moisture loss from the fresh castable into the insulating layer. The membrane used should burn out at low temperature when the drying cycle is started. Alternatively, the backup layer may be dampened prior to casting by lightly spraying with water.
Joints.
There are two types of joints used in casting:
Construction Joints
These joints are designed to break the job up into conveniently sized panels. Adjoining panels are cast up against each other without leaving any gaps using the set face of previously cast sections as the formwork edge. Panels are generally 1.0 - 1.5 m². This size can generally absorb stresses without cracking. The construction joint can be regarded as placing a crack where you require it, rather than leaving the castable to stress relieve itself in a random crack pattern.
Expansion Joints
For these joints adjacent panels are separated during installation by materials such as cardboard or plastic that will burn out during firing and leave a gap, or by high temperature materials such as ceramic fibre which remain in the gap, but have some degree of recovery after compression.
Finishing.
When finishing the exposed surface of the refractory casting, it is important to avoid excessive trowelling of the surface. This draws water to the surface and gives rise to a fine finish of cement-rich segregated material which is easily dislodged under alternate heating and cooling. It also seals the surface retarding the escape of moisture during dry-out.
Curing
The purpose of curing is to prevent the premature loss of moisture from freshly cast refractory concrete during the chemical changes associated with hydration of the calcium aluminate binder. When mixed with water, an exothermix hydration reaction takes place that drives off water at an early stage.
Loss of water from the surface of the cast before the cement is fully hydrated results in a weaker cast. To prevent this, various methods of moist curing should be used.
Formwork should be left in place for a minimum of 24 hours and exposed refractory concrete either lightly sprayed with water, covered with plastic sheeting, or sprayed with a concrete curing compound. The concrete curing compound forms an impermeable membrane to prevent moisture loss during curing but will burn out at low temperatures during firing.
The lower the ambient temperature, the longer the curing time required for proper strength generation. External heating (using a small space heater or similar) may be required to keep the ambient temperature high enough for effective curing (and also casting).
After moist cure, formwork can be stripped away carefully, being wary not to damage the lining as this occurs. A further 24 hours drying is generally allowed before the lining is fired to further increase the final strength.
FIRING
Detailed firing schedules are separate from this procedure. They are available from any Thermal Ceramics Australia office or representative.
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