helps to concentrate the calcined gypsum at the surface. Previous formulations US patent 4,075,374 lists the by-weight formulation as 10 parts pressure calcined gypsum, 38-48 parts sand, and 4-10 parts water. 0.03 to 0.1 parts of a latex emulsion, such as Dow Latex 460, were also added. To prevent foaming, a defoamer such as WEX was added to the latex at a concentration of 0.2%. It was stated that gypsum calcined at atmospheric pressure produced poor results due to it having flaky particles, and that gypsum calcined under a pressure of 15-17 psi produced better results because it had denser, crystalline particles. Later it was found that this original formulation expanded too much and in some instances floors cracked. US patent 4,159,912 describes changes made so that the expansion was greatly reduced. In that formulation, 5-8% of Portland cement was added to reduce the expansion. The latex emulsion and antifoaming agent were no longer necessary as the concrete was strengthened by the Portland cement. It was found that atmospheric calcined gypsum could be used for the majority of the calcined gypsum if it was ball milled to change the texture. The proportion of sand was also changed, so that it was in a 1:1.3 to 1:3 ratio with the calcined gypsum. This resulted in a runnier mix, but the set up time was not changed. Advantages and disadvantages Gypsum concrete is lightweight and fire-resistant. A 1.5-inch slab of gypsum concrete weighs 13 pounds per square foot versus 18 pounds per square foot for regular concrete. Even though gypsum concrete weighs less, it still has the same compressive strength as regular concrete, based on its application as underlayment or top coat flooring. A 7-man work crew can lay 4–6 times as much gypsum concrete in a work day as regular poured Portland cement. This is due to the ease of leveling the very runny gypsum concrete versus normal concrete. In addition, if the wooden subfloor is first coated in a film of latex, the adhesion between the subfloor and the concrete is much better than the adhesion obtained with “normal” concrete. A further benefit is that nails can be driven through the cement into the subfloor without it chipping. The cost of gypsum concrete is comparable to regular concrete, ranging from $1.75 per square foot to $6.00 per square foot. Regular concrete ranges from $2.50 to $4.50 per square foot. History