by Brett Fleury
Gypsum is a mineral that has been used in building materials for centuries. However, gypsum concrete as an underlayment is still misunderstood despite its growth in popularity over the last 40 years. As a result, several myths about using gypsum in this way have become canonized. This article aims to expose these myths and provide information about the benefits of using gypsum-based concrete underlayments.
Gypsum composition
Gypsum is a mineral of crystalline structure composed of calcium, sulfate, and two molecules of water (CaSO4-2H2O, also known as calcium sulfate dihydrate). It is a very common mineral found around the world—right behind its more plentiful cousin, limestone (calcium carbonate). The process of cooking gypsum is known as ‘calcination.’ During this process, 75 percent of the water is removed from the mineral, leaving calcium sulfate hemihydrate. This process is reversible. When water is added to calcium sulfate hemihydrate, hydration begins and crystal growth occurs, returning it to its original mineral form—gypsum. It is this process that allows gypsum to offer characteristics other binders cannot.
First, as a result of crystal growth from the hydration process, the gypsum expands and eliminates shrinkage cracks normally associated with traditional cements. Second, as the gypsum crystals begin to grow, they ‘lock’ themselves into surfaces that do not have a coarse enough profile for traditional cements to cling to. Lastly, gypsum cements are not negatively affected by the depth of the pour—single-lift applications up to 76 mm (3 in.) can be simply achieved.
Recaptured gypsum
Recaptured gypsum (FGD gypsum) is a by-product of the process used to clean combustion gases from fossil-fuel-burning power plants. This process greatly reduces emissions of sulfur dioxide, which contribute to the formation of acid rain if not removed from the atmosphere.
The use of this recaptured gypsum eliminates the need to dispose of the material in landfills as a solid waste. Gypsum products manufactured from FGD gypsum can reach up to 90 percent recycled content—far beyond the recycled contents of portland cement (PC) or high alumina cement (HAC).
Gypsum products made with recaptured gypsum may also assist in obtaining credits for recycled content or regional material use under the Leadership in Energy and Environmental Design (LEED) rating program. Across the full lifecycle, gypsum-based materials will have fewer negative impacts, such as air pollution, when compared to PC- or HAC-based products.
Myth number one
Gypsum self-leveling cements are soft, chalky, and do not meet industry standards for commercial floor coverings.
Truth
Gypsum concrete has been installed and specified since the late 1970s. Original formulas were robust and applied correctly for many years. When a market for gypsum materials spawned as a result of building code enforcement of floor–ceiling fire-breaks, competitive formulations made their way to market. However, because the technologies were nearly identical, applicators had very few opportunities to differentiate their services. Ultimately, when competition became fierce, applicators would extend their coverage rates by adding more water and sand to their mix. This practice, however, has an extremely negative affect on the quality of the slabs being poured. By the late ‘90s, the average gypsum concrete floor was producing compressive strengths in the range of 6890 to 10,335 kPa (1000 to 1500 psi)—well below today’s requirement of 20,670 kPa (3000 psi) for commercial installations per ASTM F710-11, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring.
So why does adding more water and sand have such a negative affect on the finish strengths of a gypsum concrete slab?
Water
The amount of water required for hydration of the hemihydrate is consistent. Every 45 kg (100 lbs) of pure hemihydrate powder requires 8.5 kg (18.6 lb) of water (often called ‘theoretical water’) to convert into gypsum. However, this is not enough water to create an adequate fluid slurry. In some cases, 34 to 40 kg (75 to 90 lb) of water may need to be added to create acceptable flow characteristics.
The more water used to create a slurry (greater than the actual hemihydrate theoretical need), the weaker the hardened mass will be. This is because the excess water has no effect on the hydration process and just takes up space. Once the mass has ‘set up’ and is fully hydrated, it is imperative the hardened mass begin to dry. The excess water, sometimes referred to as ‘water of convenience’ ultimately has to leave the system through natural evaporation or through forced drying. Once the water evaporates from the hardened mass it leaves behind tiny air voids. The more air voids, the lower the strength, be it compressive, flexural, or tensile strength. It is important to note—this is counter to PC or HAC. These materials require water for an extended amount of time while the cement sets. This is often referred to as moist curing. Gypsum cements only require water to form new crystals as explained earlier. Once formed (usually within a few short hours), the system no longer requires water.
The second story apartment above me has been creaking and popping very loudly. A inspection from the landlord says the gypcrete or concrete under lament is broken to pieces. I was able to see under the carpet and in fact is was broken very badly. the building was built in 1999 and it looks to me the cracks have been there a very long time. My question is could the mixture or some kind of sealer been missed to cause such cracking? I want to prove that this issue has been put off for years. Thanks
yes. USG has the only U/L classified/specified repair program for all U/L Fire & Sound assemblies.
Gypsum is the best! I’m doing my house in it and next the yard next! It’s amazing anything. John Paul, mail box, automobile. Wow. Very happy, yes indeed! If everyone would Gypsum we might all become Egyptian! You know the saying my friends. Yes!
Hi! We are trying to level a recessed floor (house built in 1992 with concrete slab). Half of the front of the house is recessed, and the other half is all concrete with carpet on top. We were advised to use 3″ of Gypcrete to level the floors so consistent engineered wood floors can be laid.
1. can you pour 3″ gypcrete (600 sq. ft area)
2. What precautions do we need?
3. can we glue engineered wood floors (7″ wide pieces) to the gypcrete?
Help! I’m nervous :)
Hello, Can you tell me if 1 1/2″ of Gypsum Concrete is good enough of a Sound Barrier for the Condo floor above me? I moved in a few months ago and the Noise above me is Unreal, walking on the floor, vacuuming, shutting cupboards even flushing the toilet. I would like to prove they missed something. Appreciate any help you can offer. Thanks so much
If there is 1.5″ of it above your unit, and you are hearing their noises loud and clear, then clearly it is not good enough. I’d hope that there is a foam pad and carpet at least on top of the floor above you. Could you have fiberglass batt insulation glued to the bottom of the floor above you – or some other similar insulation- mineral wool maybe? If there is a ceiling cavity you might also put insulation on top of your ceiling. Check out Sonex foam. Maybe Sonex on the bottom of floor above, plus on top of your ceiling, plus on the bottom of your ceiling. Try sonex-online dot com and probably other sites. Good luck. If money is tight, I’d go thick batts of insulation but be careful not to insulate things like lights that might build up heat and start a fire – possibly even wiring – and don’t insulate a water pipe such that it is essentially outdoors and might freeze and break. Get a contractor if you need help.
In addition to my previous reply, I will offer this suggestion: offer to buy your neighbors above a thick, heavy rug to go on their floor. OR offer to buy them some cushy soft slippers, sandals, etc. Just a thought. I hate neighbor noises.
Better yet, move to where there is nobody above you. ;)
hi
thanks for content. can gypsum be poured ontop of a concrete slab?
Can this product be poured over a cracked and uneven concrete basement floor?
Hi Sylvia,
There are a few options you could take. I would recommend something like a non Gypsum, Self-Leveling Cementitious Underlayment.
The cracks may eventually come through, but you can certainly make the floor more level.
Can I place a concrete overlay on the original gypcrete floor? We really want to put finished concrete in our kitchen. There was parquet and old linoleum which has all been removed. Now it is just gypcrete. Thx
I recently moved into a 55+ cooperative. when I vacuum (weekly) I get a very fine gray powder, along with carpet fibers in the vacuum. I get a sandwich baggie filled with dust and fibers.
My concerns/questions are:
1. Is this gray powder hazardous to my health? (does the product contain and known cancer causing agents?)
2. What is the process for fixing this issue?
3. Can laminate flooring be placed over gypcrete?
Thank you.
We think we have the same problem. Have you received any answers?
Please reply. I am recovering from lung cancer and my new apartment floors are doing the same thing. Am I living in a danger zone? I wear a mask every day because of the powder visibly seen on my carpet from the gypcrete turning to powder again.
I’m glad I read this web page. Think I will avoid gyp crete!
Can gypcrete be used to level an outdoor patio slab and be tiled over? I have a 50×25 patio that slopes the wrong direction by 3″ in 25′. Looking for affordable solution other than bags of Rdex or self leveler$$$$.
Some one can talk me where I ca take orientation o get certificate for floor self leveling gypsum I’m in Los Angeles California thanks
I need to know of a self leveling concrete I can use on top of a slab that will allow me to do a mirror finish epoxy floor. I was told you can’t do this on top of a self leveler and that the concrete has to be etched or scarified.
When they built my house in 2018 a radiant floor was installed in gypcrete and tile was laid on top of the gypcrete. Subsequently, the bathroom tile has cracked in two straight lines: one in front of the cabinets with granite counter tops and a second where a Kohler soaking tub is. Evidently, there was settling/flexing as the cracks run perpendicular to the joists. These cracks run the length of the bathroom. I asked the project manager who ran the job how the tile should be removed. Obviously, he wants nothing to do with it since the house is no longer under warrantee. I was told that you can’t get the tiles up without damaging the radiant floor (gypcrete). I believe he is full of it. Therefore, I am seeking expert advise before I speak to contractors. FYI – The counter top cannot be moved so the tiles will have to be cut.
Can the tiles be scraped up and the mortar chipped off as you would on a concrete floor?
Any advice you can provide will be very helpful.
Hi Brett, I read your article and wanted to follow up with you to learn about which types of gypsum can be stained and polished to provide a decorative wear surface. Is this feasible for a basement project?