The Great Drywall of China (Part 1)

Before we examine the Chinese drywall issue in the next blog, I thought I would give a little background on how the material is produced – it gives some insight into the current situation.

The basic component of sheetrock is gypsum (calcium sulfate, CaSO4). Gypsum comes from one of two sources – it can either be mined in its raw form or collected as a by-product of coal-powered electricity generation and other processes. As part of environmental regulation, coal-powered utilities are required to remove (“scrub”) sulfur dioxide (SO2) vapor from their exhaust gases. In order to do this, finely ground lime (CaO) or limestone (CaCO3) is mixed with water to form a slurry, which is sprayed into the power plant’s exhaust stacks. The calcium in the slurry materials chemically binds with the sulfur from the exhaust gases to form calcium sulfite (CaSO3). This material is commonly referred to as Flue Gas Desulfurization (FGD) by-product. In many cases the sulfite is further oxidized to produce more marketable calcium sulfate (CaSO4).

Because this form of material comes from a production process, it is known as “synthetic gypsum.” Both mined gypsum and FGD/synthetic gypsum are nearly chemically identical, so many domestic sheetrock companies work closely with coal-powered utilities to collect and utilize the material. This has economic and environmental benefits for both parties. Prior to use, FGD gypsum is routinely screened for impurities such as fly ash which can discolor sheetrock products. Domestic manufacturers of sheetrock use about one and a half million tons of synthetic gypsum a year, which accounts for about 7% of its overall gypsum use.

Other manufacturing processes produce different forms of synthetic gypsum. Titanogypsum is a by-product of the paint industry and is considered safe for use in sheetrock products. Other safe synthetics include fluorogypsum and citrogypsum. Some synthetic gypsums, such as phosphogypsum, are considered unsuitable for reuse due to the potential presence of radon and radionuclides.

To produce sheetrock, the gypsum, water and additives are mixed together and fed between continuous layers of paper. The mixture quickly bonds mechanically and chemically to the paper as it sets up into its solid “board” shape. Panels are then cut and moved through a dryer to remove trace moisture.

Given the fact that sulfur is one of the basic elements of sheetrock compound, it is easier to understand how unsuitable sulfur materials could have made their way into the Chinese production process. Visit the Gypsum Association and FGD Products websites for more information on synthetic gypsums and sheetrock.

This blog was written for Paul LaGrange’s BuildWrite website and was originally posted on March 27, 2009.

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