Instructions for the use of Darley Castables
(Heat Resistant Concretes)
Refractory castables consist of suitably graded
aggregates and hydraulic cements in proportions formulated to
achieve certain desired properties for the particular end use.
They require only the addition of clean water either by hand or
mechanical mixing or during pneumatic gunning to form a heat
resistant concrete piece or structure.
Refractory castables can either be cast between
formwork, cast in-situ, gunned, trowelled or cast into moulds to
make special shapes Coarse, regular or fine mixes are
available to suit the method of application and end use selected.
As most castables have negligible shrinkage, the moulds, formwork,
etc., can be made to exact size. Mould should be watertight and
structurally strong and treated with a suitable release oil or
grease to facilitate stripping.
Preparation of Surfaces
Steelwork should be reasonably free from scale and
timber should be clean and smooth. For repairs to an existing
brickwork or castable structure the area should be cleaned by
chipping off slagged or affected areas (See Note 1. below) and then
thoroughly damped to prevent absorption of mixing water from the
newly applied castable.
Mixing is best done in a paddle mixer, but it is
essential that all vessels and implements are clean. Contamination
by lime or Portland cement will have a detrimental effect on
setting time and properties. For small casts, mix with a shovel
on a clean surface. For mechanical and hand mixing, water is
worked into the material gradually until its texture changes from
“harsh” to “workable”. The correct water addition is achieved when
a handful of mix can be formed into a ball and be tossed 30 – 40
cm into the air and caught as a cohesive mass. If
too little water is present, the “ball” will break up.
If too much, the “ball”
will slump (See Note 2. below). Where vibration is used to
assist placement, slightly lower water contents will give
satisfactory results. Excess water reduces strength. Thoroughly
mix before placing and never mix more than can be properly handled
inside 20 to 30 minutes.
Ideally temperature of mixing water will be 10ºC
to 25ºC. If site ambient temperatures exceed 25ºC, chilled
water should be used. Where site ambients reach 35ºC or higher,
loss of workability may be encountered.
The refractory concrete can be consolidated by
rodding, tamping or vibration. Finish off the surface to the
correct profile with a trowel or screeding level. Do not overwork
and/or slick to a wet smooth finish. For restoration of vertical
surfaces in particular a fine mix of the appropriate grade of
castable is recommended. (It is applied with a trowel in the same
manner as cement-sand plaster rendering on walls). Alternatively
it can be cast behind formwork (See Note 3. below).
Pneumatic gun placement procedures vary from
product to product. Details of typical variables, nozzle air
pressure and predamping levels, are available on request.
Curing and Drying
After the initial set, cover with damp bags,
polythene sheeting, fine water spray, or apply a curing membrane
etc., to prevent loss of mixing water. Cure for 24 hours, then
remove covering. Air dry for 48 hours. Drying can be prompted by
using radiant heat.
If a piece of cold steel when placed against the
drying structure for 20 – 30 seconds exhibits moisture droplets on
the contact surface, pore water is still present and drying should
A slow fire can be applied and the temperature
raised at the rate of 50ºC approx per hour per 25mm thickness
of lining until working temperature is reached.
Castables cast in a hot environment or against a
hot structure or denied a proper curing and/or drying treatment
will not generate the same properties as at ambient temperatures.
- Always try to provide a “key” for the new
material to adhere to, e.g. an enlarged joint.
- When mixing, it is best to reserve a small
amount of dry mix. If excess water is added, a correction can
be made by adding in the dry mix and proceeding as described
above. (If all the mix is committed and excess water is
added, the result will be a segregated, under strength casting).
- Light weight insulating castables are
unsuitable for the trowelling vertical repair technique.
The Curing and Firing of Refractory Castables
Densecretes and Litecretes
Installed refractory linings and castings containing
calcium aluminate cement binders should be cured prior to drying.
This applies to both cast and gun placed material. Curing is
necessary to prevent moisture loss during the setting of the material.
If the moisture loss is allowed to occur, the refractory material
will be weaker due to an incomplete hydration process of the cement
Curing time should be at least 24 hours and maybe
undertaken by several methods:-
- Covering the refractory concrete with wet
Hessian bags which must be kept wet
- Covering with plastic sheeting
- Spray the surface with a curing compound
- Spray the surface with a fine water spray
Drying of Densecretes and Litecretes
After curing the lining should be air dried for up
to 24 hours. If the lining is greater than 250mm, curing should be
24 hours. If Q.T. fibre is present in the refractory concrete, this
step may be reduced to 8 to 12 hours for linings greater that 100 mm.
Firing of Densecretes and Litecretes
Raise the temperature of the lining to 100ºC to
150ºC at a rate of 15 to 25ºC per hour and hold for 8 to
12 hours depending upon the lining thickness. Ensure steaming ceases
before heating the lining further.
Increase the temperature to 500 to 600ºC at a
rate of 25 to 50ºC per hour and hold for 1 hour per 25mm of
lining thickness; and
Increase the temperature to the final working
temperature plus 25ºC if possible at a rate of 50 to 100ºC
per hour and hold for 1hour. Never exceed the maximum service
temperature of the refractory castable.
- The holding time is dependent upon the lining
thickness. For each 25mm thickness above 200mm an extra holding
time of one hour per step is recommended.
- Preheating should be carried out as slowly as
possible and with a good control over temperature. Refractory
castables usually have a dense structure which does not allow the
water to escape easily. A too rapidly increased temperature may
create steam formation in the structure of the castable, thus
causing spaulling of the layers from the surface and/or cracks.
During preheating sufficient ventilation should be provided so
that the water vapour can escape.
- Temperatures are the lining temperatures
measured on the surface of the lining.
- It is of critical importance to execute a
uniform and graduated drying out procedure with professional
standards and practice. This schedule is accordingly made
available without warranty of any kind. The firm or person
executing the drying out procedure is solely responsible for any
damages resulting to the refractories or other items for which
Darley Refractories Australia Pty Ltd disclaims all liability.
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