Proper Refractory Mixing
Good refractory practices are necessary to achieve proper refractory installation. It all begins with mixing. As discussed in the article “The Lost Art of Mixing Refractory,” published in the February 2008 issue of Insulation Outlook, it is common to find workers without previous experience mixing refractory at power plants.
If the workers installing the refractory have limited experience, they might not be able to react or adjust to a refractory mix that is not the correct consistency for the application. At a power plant, for example, the refractory mix brought to the point of installation was too wet (i.e., it had too much water) for the trowel application required. The lack of experience of those doing the installation meant that they did not know to wait a few minutes to allow the refractory to set up (or stiffen) before application. They were unaware that refractory has a pot life (the time recommended by the manufacturer between mixing and installation). The workers could have waited a suitable period (5 to 10 minutes) to get a better install with less mix falling off the vertical wall surface.
The refractory mix described above was mixed improperly. The extra water in the mix means that the refractory will have less strength and be more prone to failure even though it could be troweled onto the vertical wall surface (with difficulty).
Mixing refractory is not only about adding water to the mix using a hose and mixing it just enough until it looks wet—like mixing concrete. Mixing refractory requires a bit more. Properly mixed refractory must:
- Have a carefully calculated, measured, and/or weighed amount of water
- Never have water added directly from a water hose
- Be mixed for a certain amount of time
- Be mixed in a paddle mixer or by hand, vigorously
- Be mixed to the consistency matching the application of usage
To mix refractory properly, one must know the type of application intended for its use. The most common applications are casting, pouring, and troweling of refractory.
- Casting and pouring applications are used for installing large amounts of refractory and usually require forms to hold the refractory in place. The mix consistency should include just enough water so the material will flow into the formed area. Using the ball-in-hand test, the mix should flatten out on the palm when tossed into the air.
- Troweling application is used for hand-applied areas requiring thin linings. The mix consistency should have just enough water added so the material is sticky and will stay in place without falling off a vertical surface. Again, using the ball-in-hand test, the mix should flatten out on the palm when tossed into the air.
The following suggestions are listed to help those with less experience get a better refractory mix:
- Never use a cement mixer. Refractory mixed in a cement mixer will not have the strength characteristics expected by design. A cement mixer should only be used for adding the initial amounts of water to the dry mix. After all the water is added, the wetted mix must be emptied into a pan or tub and vigorously hand mixed for the total time required. Refractory needs the action created by the paddle or by the rigorous stroke of hand mixing to get the proper reaction required between the water and cement.
- Check the size (capacity) of the paddle mixer to be used and figure one bag fewer than the size or quantity of bags the mixer is designed to mix. Paddle mixers are usually based on capacity (two, four, or six bags are common sizes). The paddle mixers are very temperamental and can jam easily, especially if they are not used every day. Using one fewer bag will eliminate the tendency to jam.
- Calculate the amount of water recommended by the refractory manufacturer. This is normally printed on the back of the refractory bag and will be either a percentage of weight or a measurement (quarts, pints, gallons). The recommended amount of water on the bag is usually for casting, which should be noted if the application is to be troweled. Figure 1 gives an example of such a calculation. This type of chart or conversion calculation can be used for developing or establishing the
- Pour expected water amounts into clean buckets or pails, following the procedures in “The Lost Art of Mixing Refractory” (Insulation Outlook, February 2008). The first mix/batch will establish the exact amount of water needed, so keep a measured or weighed gallon of water off to the side in case more water is needed to reach the consistency the application requires. Remember, you can never be too accurate in establishing the initial water quantity. (There is no shame in using a bathroom scale to measure initial water amounts.)
- Test for proper mix consistency using the ball-in-hand method and note the exact amount of water used (or not used); then, refill the buckets or pails with the total amount of water used to complete the first batch/mix.
- Place a mark on the outside or drill small holes in the pails or buckets. These pails or buckets then can be used to ensure mix/batch consistency. If properly marked or labeled (with refractory material name and application), and stored so they remain clean, they can be reused for similar work.
- Add the water, following the procedures in “The Lost Art of Mixing Refractory” (Insulation Outlook, February 2008). Keep in mind that a properly mixed refractory material most often will appear to be on the thick side. That is the desired consistency.
- Mix the refractory based on good refractory procedures, such as those in “The Lost Art of Mixing Refractory.” Avoid adding too much water, even though that may make the mix easier to handle. Too much water robs the refractory material of its strength.
- Use a watch or timer when mixing refractory, because over-mixing or under-mixing also robs the refractory material of its strength.
Mixing refractory properly means using the exact amount of water–not guessing or approximating. Proper mixing means mixing for the correct amount of time, providing the refractory to the point of installation in the right consistency, and having the mix retain its consistency until the installation is complete. Anything less is an improper mix and is usually the root or contributing cause of refractory failure.
Paying attention to the mixing process and keeping the consistency of each batch will help ensure that a proper refractory installation is off to a good start. Though refractory may be among the smallest components on a steam-generating boiler, it is one of the most important for keeping the fire inside the box and for energy efficiency. To achieve better refractory installations, review refractory mixing practices to avoid refractory failures and application problems.