Refractory mortar, also known as refractory clay or fire mortar, is a material used in furnace construction to fill the joints between refractory bricks, bonding them together and providing the masonry with a certain degree of integrity, strength, and airtightness. Therefore, the use of refractory mortar has a significant impact on the quality of the masonry, the heat loss of the electric furnace, and the furnace shell temperature.
How to solve the problem of high-alumina refractory mortar drying too quickly on the construction site?
If high-alumina refractory mortar dries too quickly on the construction site, it will prevent the refractory bricks from forming a bond, affecting the construction progress. There are several solutions.
- First, check the ambient temperature at the construction site. Is the high-alumina refractory mortar not watered enough or not mixed evenly? If the mortar is too thick or has poor water retention, it will dry too quickly. Therefore, if the site temperature is too high, the mixed high-alumina refractory mortar should be covered with a damp cloth and not left exposed.
- Secondly, if too much high-alumina refractory mortar is mixed in a single batch and exposed for too long, the surface will form a skin and lose water, also causing it to dry too quickly and affecting the construction progress. It is best to mix small batches at a time, using them within approximately 30 minutes.
- If the temperature and mixing are normal, 0.2-0.5% hydroxypropyl methylcellulose (HPMC) can be added to slow down water loss. Alternatively, 0.5-1% dextrin or methylcellulose can be added to increase viscosity and delay drying, allowing sufficient time for refractory bricklaying. However, excessive binder should be strictly avoided to prevent decreased strength and cracking at the joints later on.
- If the construction site is in a mountainous area with inconvenient transportation, or if the supplier adds excessively long transport times for the binder, add approximately 20% drinking water when mixing the high-alumina refractory mortar, let it stand for 2-3 minutes, and then add the remaining 5-10%. A small amount of household detergent can be added during mixing to increase the viscosity of the high-alumina refractory mortar and prevent delays in construction. (This method is only applicable when the construction site is in a remote mountainous area with inconvenient transportation.)
In summary, the above are all remedial measures. Ideally, the manufacturer should conduct proper process proportioning and testing during production to ensure that the high-alumina refractory mortar powder is above 180 mesh, has strong adhesion, and does not delay on-site construction time.
Can high-alumina refractory mortar still be used after long storage?
High-alumina refractory mortar is a jointing material. If it has been stored for a long time, its usability needs to be assessed. Individually packaged binders that have been stored for more than 6 months cannot be used and must be replaced. If there is no binder, you can judge by smell; if there is no odor, you can re-mix the binder and use it. If the binder was added directly and stored for more than 6 months, a test can be conducted. If it can bond two refractory bricks, it can be used; otherwise, add a reasonable proportion of binder and test again. If successful, it can be used.
If there is no individually packaged binder, you can take a small amount and smell it. If there is an odor, the binder was already added to the refractory powder during production. If there is no odor, it proves that the binder was not added. If they were not mixed together, you can re-mix the binder and use it.
If the binder was added directly during the production of the high-alumina refractory mortar and the storage time has exceeded 6 months, a simple test should be conducted on-site. Weigh 1 kg of refractory powder, add 20-25% drinking water, and stir the refractory powder into a slurry. Take two refractory bricks of the same material, apply a portion of the refractory mortar to one brick, smooth it out, and then place the second brick on top. Rub the two bricks back and forth about fifteen times. If the two bricks stick together, the high-alumina refractory mortar is still usable.
If the high-alumina refractory mortar cannot bond the two bricks, add some binder in a proper ratio. Then conduct another on-site test using the above method. If the bricks bond together, it is ready for use.
The binder in high-alumina refractory mortar is for promoting adhesion of the slurry. After being baked at high temperatures, the binder essentially disappears. Therefore, the use of high-alumina refractory mortar that has been stored for a long time depends on its condition.
Requirements and Selection of Refractory Mortar for Refractory Brick Laying
Requirements for refractory mortar when laying refractory bricks:
(1) It should be able to be mixed with water to a certain consistency, filling uneven parts of the brick without causing cracks.
(2) It should have properties similar to the brick body (e.g., refractoriness, softening temperature under load).
(3) It should have good airtightness after drying and at high temperatures.
(4) It should have certain mechanical strength and good airtightness.
Selection of Refractory Mortar for Laying Refractory Bricks
Commonly used refractory mortars include clay-based, high-alumina, silica-based, and magnesia-based refractory mortars. Clay-based refractory mortar should be used when laying clay refractory bricks. Clay-based refractory mortar is composed of clay clinker and binding clay.
High-alumina refractory mortar should be used when laying high-alumina bricks. When laying high-alumina bricks containing 60%-70% Al₂O₃, the refractory mortar used consists of 75%-85% calcined alumina and 10%-20% raw clay. When laying high-alumina bricks containing 48%-55% Al₂O₃, high-refractory clay-based refractory mortar can also be used.
When laying silica bricks, silica refractory mortar with a SiO₂ content of 90%-93% should be used. Its particle size distribution should be: no more than 3% of particles remaining on a 1.0 mm sieve, and no less than 80% of particles passing through a 0.20 mm sieve.
When laying magnesia bricks, magnesia refractory mortar should be used. Its magnesium oxide content should be no less than 78%, its silicon oxide content no less than 6%, and its caustic soda content no more than 2%. The particle size distribution of the magnesia refractory mortar is: 100% smaller than 1 mm; no less than 97% smaller than 0.5 mm; and no less than 50% smaller than 0.125 mm.
Fine-grained refractory mortar is typically used for building electric furnaces.
