Gypsum-concrete has been a growing segment for underlayment for the past three decades. It can be found in many residential and few commercial applications. My sources tell me there is over a billion square feet in use today. Gypsum underlayment is mostly found in multiple-use dwellings. Normally, it is installed over a wood subfloor and is usually ¾-inch deep or greater. The gypsum product is generally pumped-in and is a self-leveling product. Because it is pumped into the structure, the amount of water used in the mix, to make it pumpable, is very important. When the installation is in progress the manufacturers of the gypsum product recommend the underlayment installers take periodic samples to be sent in for analysis for strength. These should be available to the flooring contractor to determine the quality of the underlayment.

The more water used in the mix, the weaker the material. One of the most common ways to determine the strength of the underlayment is to see how dusty the surface is. The more dust the weaker the surface.
Moisture testing
There are several methods of moisture testing of gypsum underlayment. The oldest method is the use of a 24″ x 24″ mat or taped down plastic.

Another method is the use of a moisture meter with needle probes (shown). The reading should be 5% or less for glued down materials.

The newest method is the use of a hygrometer probe with a special protective sleeve; this meets the ASTM F-2170 specifications.

The calcium chloride test method (ASTM F-1869) is not a recommended method for testing a gypsum underlayment. The reason is because gypsum is so porous the ambient humidity will affect the calcium chloride test.
Preparation skim coat completed
There are several important factors when selecting a patching material for gypsum underlayment. It is best to use a patching compound that is designed for this purpose and a lot of the patching compound manufacturers make products specifically designed for this purpose. If you use a Portland-based patch there are two factors that may change your mind. First, Portland-based patch has a different expansion/contraction coefficient which can create a shear, causing a delamination between the two surfaces. Second, you can develop a chemical reaction between the calcium sulfate (Gypsum) and the Portland-cement, creating a crystalline growth known as ettringite. Ettringite appears like an alkaline deposit but is somewhat different.
Handling cracks
Many times when the gypsum underlayment is installed with too much water or is allowed to dry too fast there is some cracking that takes place. The method used to overcome this is to flow a low-viscosity epoxy, which is flowed into the cracks bonding them back together.
Priming
It is recommended by the manufacturers of gypsum underlayment to use a primer about two hours prior to the start of the installation of resilient materials. Priming can be done with a Poly-vinyl-acetate (PVA) or Acrylic primer. The primer does several things to make the installation much easier. First, it serves as a surface hardener, increasing the density on the top surface of the underlayment. Next, it allows the adhesive to perform as it is intended. The primer is to be rolled, sprayed or mopped on at a spread rate of about 300 sq. /ft. per gallon. Without a primer the adhesive will dry too fast and its bond will be compromised. Failure to prime can result in an adhesive sticking to the dust or laitance on the surface, which also results in bond failure. Primers such as Armstrong S-185 and Mapei Primer L diluted 3 parts water to 1 part primer will seal the gypsum.
Weakness beneath the hardened surface
This illustrates the end result of a properly primed surface. Note how soft the gypsum is below the surface of the hardened surface.

Gypsum is severely affected by moisture. When it comes into contact with moisture the calcium sulfate (gypsum) particles will expand and soften. For this reason it is not recommended for use directly over concrete or any where moisture is anticipated.

The other thing that gypsum is not designed for is heavy rolling loads, as they will create a sure delamination.

Gypsum concrete is a fact of life and it requires a special type of preparation to ensure a successful installation.

Thanks to JJ Haines