Gypsum concrete is a lightweight, pourable building material made primarily from calcium sulfate (gypsum) mixed with water and sometimes sand or other aggregite. It’s used almost exclusively as a floor underlayment, creating a smooth, level surface over wood or concrete subfloors before finished flooring goes down. Unlike traditional Portland cement concrete, gypsum concrete doesn’t shrink or crack as it cures, which makes it especially popular in multifamily housing, hotels, and commercial buildings where flat floors and sound control matter.
What Gypsum Concrete Is Made Of
The key ingredient is calcium sulfate hemihydrate, a processed form of gypsum mineral. When mixed with water, it rehydrates and hardens into a solid mass. Most commercial gypsum concrete blends contain roughly 75% gypsum binder by mass, with the remaining portion made up of Portland cement and sometimes supplementary materials like silica fume that improve strength and durability. Sand is often added as an aggregate to increase density and compressive strength.
The chemistry is straightforward: gypsum absorbs water and crystallizes into a rigid structure. This reaction happens fast. Depending on the product, gypsum concrete can support foot traffic in as little as 90 minutes after pouring, though it needs much longer to fully dry before finished flooring is installed over it.
How It Differs From Portland Cement
The biggest practical difference is shrinkage. Portland cement shrinks as it cures, which can cause cracking and requires control joints. Gypsum concrete does not shrink and does not crack under normal conditions. This means it can be poured in large, continuous areas without the joints and hand-finishing work that Portland cement demands.
Gypsum concrete is also self-leveling. Once poured, the mixture flows into a flat plane on its own, requiring only a straightedge (called a darby) to set thickness and uniformity. Portland cement underlayments typically need more hands-on finishing. Gypsum concrete is lighter, too. Loose gypsum has a density around 70 pounds per cubic foot, compared to standard concrete at 140 to 150. That lower weight puts less structural load on the building framing, which is a real advantage in upper-story applications.
The tradeoff: gypsum concrete is not structural. It cannot bear loads on its own. It also cannot get wet after curing. Prolonged moisture exposure will soften and degrade it, which is why it’s only used indoors and never on ground-level concrete slabs unless a vapor barrier sits directly beneath the slab.
Where It’s Typically Used
Gypsum concrete shows up most often as a thick-poured underlayment in multistory buildings. It gets poured over wood structural panel subfloors or existing concrete floors, creating a smooth, level base for carpet, hardwood, vinyl, tile, or other finish flooring. Pour thicknesses typically range from 3/8 inch to 3 inches in a single application.
It’s also widely used in radiant floor heating systems. The poured gypsum encases the heating tubes or cables, locking them in place and eliminating the rattling and noise (called tube chatter) that can happen with loosely secured tubing. Because gypsum conducts heat reasonably well and forms tight contact around the tubing, it helps distribute warmth evenly across the floor surface.
Fire Resistance
Gypsum is naturally fire-resistant because the crystal structure contains chemically bound water. When exposed to flame, that water releases as steam, absorbing heat and slowing the temperature rise on the other side of the assembly. Gypsum concrete is used in UL-rated fire-resistant wall and floor assemblies that achieve ratings from 1 to 4 hours depending on thickness and construction details. A 2-inch application over metal lath can provide a 1-hour fire rating, while a 4-inch application can achieve a full 4-hour rating. These ratings make gypsum concrete a practical choice for meeting fire codes in multifamily and commercial buildings.
Sound Control Performance
One of the strongest selling points for gypsum concrete in apartment buildings and condos is noise reduction. Sound performance in floor-ceiling assemblies is measured by two ratings: STC (Sound Transmission Class), which covers airborne noise like voices and music, and IIC (Impact Insulation Class), which covers footfall and dropped objects. Well-designed floor-ceiling assemblies using gypsum concrete can achieve both STC and IIC ratings above 60, which reduces sound transmission roughly twice as much as building codes require. The mass of the gypsum layer, combined with its density and the way it decouples from the structural framing, all contribute to this performance.
Installation and Drying
Gypsum concrete is mixed on-site or delivered by a pump truck and poured directly onto the prepared subfloor. The material stays workable for 30 minutes to an hour depending on the product, giving installers time to spread and level it. It hardens quickly, supporting foot traffic within 90 minutes to two hours.
Hardening and drying are two different things, though. The surface firms up fast, but the moisture inside needs to evaporate before any finish flooring can go on top. Drying time depends on thickness, ventilation, temperature, and humidity, but it commonly takes several days to a few weeks. For resilient flooring installations, the moisture level in the underlayment needs to drop below specific thresholds, typically 75% relative humidity or lower when tested with an in-situ probe. Rushing this step and installing flooring over a still-damp underlayment can trap moisture and cause adhesive failure, mold, or damage to the finish flooring.
Limitations to Know About
Gypsum concrete is not suitable for exterior use, wet areas, or anywhere it will be exposed to standing water. It’s not a structural material and cannot replace load-bearing concrete. It also should not be installed over concrete slabs sitting directly on the ground unless there is a functioning vapor barrier beneath that slab, because ground moisture will migrate upward and degrade the gypsum over time.
The surface of cured gypsum concrete is relatively soft compared to Portland cement. It can be scratched or gouged by heavy point loads, which is why it’s always covered with a finish flooring material rather than left exposed. Some finish flooring types also require a primer or sealer over the gypsum surface before adhesive application, since gypsum is slightly alkaline and porous.
ASTM F2419 is the industry standard governing the installation of thick-poured gypsum concrete underlayments in commercial buildings. It covers preparation, pouring procedures, and surface requirements before resilient flooring goes down. Project specifications often add requirements beyond this baseline standard, so installation details vary by building.