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% 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 inches) 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% 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 #1

Gypsum self-leveling cements are soft, chalky and do not meet industry standards for commercial floor coverings.


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 or 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 1990s, the average gypsum concrete floor was producing compressive strengths in the range of 6,890 to 10,335 kPa (1,000 to1,500 psi) — well below today’s requirement of 20,670 kPa (3,000 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 effect on the finish strengths of a gypsum concrete slab?


The amount of water required for hydration of the hemihydrate is consistent. Every 45 kg (100 pounds) of pure hemihydrate powder requires 8.5 kg (18.6 pounds) of water (often called “theoretical water”) to convert into gypsum. However, this is not enough water to create adequate fluid slurry. In some cases, 34 to 40 kg (75 to 90 pounds) of water may need to be added to create acceptable flow characteristics.

The more water used to create 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 introduction of an aggregate such as sand to the hemihydrate slurry will reduce the strength of the hardened mass because it spreads the new hydrated gypsum crystals apart. Sand also prevents the new hydrated gypsum crystals from growing into each other. Further, the sand does not chemically bond to the newly formed crystals; instead, the sand becomes trapped and locked within the crystal matrix.

Since sand is added to most gypsum cements, special attention must be paid to the quantity and type of sand that will help the slab meet the designed specification. One way to maximize how sand integrates with the slurry is to make sure the particles range in size and shape. Sand that is too angular and flat is not as good as sand that has a rounded shape. Sands that have a more rounded shape tend to “roll” more than flat-shaped sand. This is important when trying to control the flow and leveling characteristics of the slurry.

Ultimately, given the rules previously described, one can deduce that excess water and the resultant negative effects of crystal growth will result in soft or chalky gypsum concrete floors.

In the last 15 years, advancements in formulations and more stringent installation procedures have improved the quality of gypsum-based levelers to the point where installed characteristics can exceed those of traditional PC and HAC products. Gypsum levelers in today’s market can be mixed to meet or exceed industry standards as being a suitable substrate for all types of floor coverings. Current technology can create hydrated products achieving compressive strengths that approach 137,800 kPa (20,000 psi), compared to approximately 34,450 to 41,340 kPa (5,000 to 6,000 psi) of normal PC and HAC products. This strength can be attained in as little as a few hours or days, not several weeks.

Myth #2

Gypsum-based underlayment will “melt” when exposed to water.


Gypsum is soluble in water at a rate of 2 g/L (1 oz./3.64 gal) of water. This means a solution rate of 2 g/L of water, 0.45 kg (1 pound) of gypsum would require the equivalent to about 227 L (60 gal) of water.

For example, if 227 L of water were poured onto a floor that was being supported by a gypsum underlayment, such as in a bathroom or kitchen, in no way would 0.45 kg of gypsum just ‘melt’ away, especially considering the floorcovering would prevent most of the water from coming into contact with the gypsum underlayment.

If the floorcovering had not been applied, the majority of the water would probably be absorbed by the gypsum underlayment. Even if the water is completely soaked into the gypsum underlayment, erosion will not occur because the water solution will be saturated at a rate of 2 g/1 L and will just sit in the mass. Assuming the water source is eliminated, the underlayment will dry. The water will evaporate, leaving the gypsum which had been in solution to reside within the interstitial spaces of the crystal structure of the mass.

Myth #3

Gypsum concrete is not a hydraulic cement.


ASTM C219-07, Standard Terminology Relating to Hydraulic Cement defines hydraulic cement as: a cement that sets in water and is capable of doing so underwater.

Gypsum completely fulfills this definition, and it is proper to call gypsum-based self-levelers a “gypsum cement” (as it comes from the bag) and a “gypsum concrete” (after it is sets up as a floor underlayment).

Features and benefits of poured gypsum floors

For most applications, gypsum levelers do not require shot blasting or scarification of the substrate or existing floor covering prior to application because gypsum concrete expands slightly during the hydration process. This is very different than the shrinking normally associated with traditional concrete as it cures. Some gypsum cements can even be stained and polished to provide a decorative wear surface.

Poured gypsum floors achieve high compressive strengths in hours versus a typical 28-day cure cycle for traditional cements. In wood-framed construction, gypsum can be poured directly on the wood without wire lath reinforcement at 19 mm (3⁄4 inch) minimum thickness to achieve a one-hour fire rating. PC requires a minimum thickness of 38 mm (11⁄2 inches) to achieve a one-hour rating. Over hollow-core concrete planks, gypsum underlayments can achieve up to four-hour fire ratings at half the thickness of Portland-based concrete. Gypsum levelers, when used in combination with sound mats, provide a sound transmissions class (STC) and impact insulation class (IIC) that will exceed the requirements of Section 1207 of the International Building Code (IBC). Gypsum levelers can also be poured up to 76 mm (3 inches) in a single lift as opposed to approximately 25 mm (1 inch) for traditional cement levelers.


Gypsum concrete is a favorable material to use as an underlayment for a variety of reasons. It requires less preparation, has excellent flow properties, can be poured deeper and offers potential for better crack resistance. Some products manufactured with FGD gypsum can reach 90 percent recycled content and achieve higher compressive strengths than other materials. In most cases, a gypsum underlayment can meet or exceed the characteristics of a PC- or HAC-poured floor at a fraction of the installed cost.