CHARACTERIZING SYNTHETIC GYPSUM FOR WALLBOARD MANUFACTURE 

P. J. Henkels J.C. Gaynor 

USG Corporation Research Center 

700 N. Highway 45 

Libertyville, IL 60048 

FGD Synthetic Gypsum, Gypsum Wallboard, Test Methods 

United States Gypsum Company 

125 S. Franklin 

Chicago, IL 60606 

Keywords: 

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United States Gypsum Company (USGC) has developed specifications and guidelines covering 

the chemical and physical aspects of synthetic gypsum to help predict end use acceptability in 

wallboard manufacture. These guidelines are based in part on past experiences with natural ad 

synthetic gypsum. Similarly, most wallboard manufacturers in North America have developed 

their own guidelines based in part on its unique history and particular experiences with synthetic 

gypsum. While there are similarities between manufacturers' guidelines, differences do exist. 

This paper discusses the importance of selected parameters contained in the FGD gypsum 

guidelines. In most cases, the parameters are equally relevant to other synthetic gypsums and the 

naturally occurring gypsum mineral as well. 

USGC's general guidelines of FGD gypsum along with guidelines from the German Gypsum 

Association are listed in Table One. The guidelines are not an all inclusive list of every gypsum 

property important to the wallboard manufacturer. The guidelines serve as a starting point in 

negotiating sales agreements between the wallboard manufacturer and synthetic gypsum 

supplier. The product specification agreement at USGC is tailored to each individual source 

depending in large part to the percent usage at the wallboard facility, type of synthetic gypsum 

and capabilities of the supplier. 

GYPSUM CHARACTERIZATION 

Gypsum Purity. Obviously, gypsum (CaS0p2H20) purity is an important attribute. Purity is 

key in manufacturing wallboard. Purity of mined natural rock varies widely usually from 80 - 

96%. High purity is desired because lower weight gypsum board may be produced. Generdly, 

a higher purity is desired for synthetic gypsum compared to natural rock in wallboard 

manufacture. Usually, high purity synthetic gypsum can be reasonably obtained by the supplier. 

Therefore, a higher purity synthetic gypsum will have increased value. 

Impurities found in synthetic as well as natural gypsum can be quite detrimental to the wallboard 

produced. The higher purity reduces the chance of deleterious effects from these impurities. 

Purity can be determined by several test methods including Differential Scanning 

CalorimetqfThermal Gravimetry (DSCfWA), X-Ray Fluorescence Spectroscopy (XRF) and 

SO3 analysis. 

Free Moisture. Many synthetic gypsums discharged from by-product manufacturers are in the 

form of a wet cake. The free or surface moisture of the cake is usually in the range of 6 - 25%. 

Natural mined rock will vary from 0 - 3% moisture. Often, the amount of synthetic gypsum that 

a wallboard plant can blend in with the natural rock will be dictated by the thermal capacity of 

the rock drying system. High moisture reduces the amount of synthetic gypsum that can be 

blended. This reduces its value. 

Often a wallboard manufacturer will enter a sales agreement with a supplier prior to the 

availability of the synthetic gypsum. When the actual free moisture is significantly greater than 

contracted, processing capacity problems will arise. This could be due to both thermal drying 

limits and because of the more challenging handling characteristics of high moisture gypsum. 

High moisture gypsum has a greater tendency to stick and build up on conveying equipment. 

Free moisture of natural and synthetic gypsum materials are determined using a simple oven 

weight loss method per ASTM C471. In addition, heat and moisture determining balances are 

used. 

Impurities. The type and quantity of impurities have the greatest impact on qualifying the use 

of a synthetic and natural gypsum. The guidelines list the predominant impurities found in FGD 

gypsum. Discussion of selected impurities follow. 

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Residual Carbonates. Unreacted limestone (Ca/MgCO,) is the predominant impurity found in 

many synthetic gypsum sources. Limestone is a common impurity in natural gypsum as well. 

Fortunately, limestone remains chemically inert through the board conversion process. 

However, increased wear on processing equipment results when encountering high amounts of 

limestone (Mohs value 3 - 4) since it is a harder substance than gypsum (Mohs value 1.6 - 2). 

Limestone quantity can be determined through XRF oxide analysis of calcium and magnesium in 

conjunction with carbon dioxide (CO,) analysis by coulometric titrimeby. Alternatively, COz 

can be quantified through DSC scans. 

Flyash. One concern with flyash in FGD gypsum is the chemical variability associated with 

buming different fuel sources (e.g. various coals and OrimulsionTM). Flyash can affect paper to 

core bond during wallboard manufacture. The variability can cause intolerable problems in 

wallboard conversion. Also, flyash laden with silica and iron causes increased wear on process 

equipment. 

An important concern with flyash is the amount of trace elements that may accompany it. This 

can raise serious industrial hygiene issues. Trace elements and the analysis will be discussed in a 

later section. 

Flyash is easily detected using a scanning electron microscope (SEM). By ping image analysis, 

an estimate of the amount of flyash present can be established. Fly ash can also be calculated by 

determining the mass balance around the scrubber and dust collection system. Figure One is an 

SEM photo of flyash impurity in a FGD gypsum sample. 

Silica (Si02). Silicon dioxide is an important impurity from an industrial hygiene perspective 

and a process issue for all mineral industries. Silica is common in both natural and synthetic 

gypsums. It can be part of a clay, flyash or quartz impurity. 

High quantities of respirable (0 - 4 microns) silica could present an industrial hygiene issue. 

Crystalline silica or quartz is a very hard substance (Mohs value 7). Even low amounts (1-2%) 

can cause dramatic accelerated wear on gypsum processing equipment. 

Amorphous silica is also contained in a variety of clays. Clays negatively impact the amount of 

water required to form a fluid slurry and thus, create a higher thermal demand to drive off the 

excess water in the board conversion process. 

Silica can be quantified using XRF. X-Ray diffraction @RD) is used to identify whether the 

SiOz present is amorphous or crystalline in nature. In addition, ASTM C-471 describes a wet 

chemistry method to determine SiO, and insoluble matter: 

Calcium Sulfite (CaS0,.l/ZH20). FGD systems may be designed to produce unoxidized 

gypsum or calcium sulfite instead of calcium sulfate dihydrate. This material is always 

landfilled. Sulfite impurities are a much greater concern at the gypsum producing FGD systems 

because they lead to scaling and other processing problems. The fine particle size of sulfite will 

cause cake washing and dewatering problems. Calcium sulfite is an unwanted impurity in any 

gypsum. 

Thermal analysis and XRF will detect sulfite above 0.1%. Titration procedures are considered a 

better method to determine sulfite. A titration procedure is listed in EPRI-Method 40 (EPRI CS3612. Project 1031-4. Final Report dated July, 1984). 

Soluble Salts. Soluble salt impurities are one of the most important parameters affecting the 

physical properties of gypsum wallboard. Salts are a common impurity in natural and several 

different types of synthetic gypsums. It is common practice to mine around natural gypsum rock 

seams high in salt content. Chloride salts are common in natural and some synthetic gypsums. 

In addition, high amounts of magnesium salts can end up in some synthetic gypsums. 

Magnesium can originate from limestone sources used to neutralize waste acid and in 

desulfurization systems. 

Salts readily go into solution when the calcined gypsum (stucco) is mixed with water and other 

additives in the board mixer. During the drying of the gypsum board in the kiln the salts migrate 

to the paper - core interface and interrupt the paper to core bond. Salts are very hydroscopic and 

cause moisture to deposit in the critical bond area of the board. On exposure to high moisture 

from joint finishing and wallpaper products the drywall paper can detach itself from the core. 

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The four soluble salt ions typically monitored are magnesium (Mg2+), potassium (K+), sodium 

(Na+), and chloride (Cl-). Soluble salts are analyzed by several different methods. Atomic 

absorption is used for the determination of magnesium. Atomic emission is used for the 

determination of potassium and sodium. Ion-selective electrode (ISE) is used for the 

determination of chloride. ISE can also be used for sodium and potassium determinations. 

Table Two shows a typical soluble salt analysis for a synthetic gypsum sample. Total salts are 

based on a mathematical reconstruction. This mathematical reconstruction is based on the 

theoretical solubility of the ions. 

rrnnr _...__ unnu~~~~. WL----- Trace eiements and pH are evaluated on all natural and synthetic gypsum 

sources for industrial hygiene purposes. Trace elements normally evaluated are shown in Table 

Three. Trace elements are unwanted impurities. They arise from flyash contamination and 

increase process equipment wear. Also, trace elements are commonly found in most naturally 

occurring minerals including gypsum and limestone. 

Trace elements can be determined through several methods including Atomic 

AbsorptionEmission (AA) with a graphite fiunace option, wet chemistry methods, Inductive 

Coupled Plasma (ICP) and XRF. 

Organic impurities. Although not listed in the FGD guidelines, organic impurities even at 

trace (ppm) levels can have a dramatic effect on gypsum board operations. Organics are 

unwanted impurities in gypsum. The effects of organics found in natural and synthetic sources 

are observed during gypsum rehydration in the board conversion process. They can easily cause 

the rehydration time to dramatically lengthen and cause the board line to slow down and reduce 

production. Organics adversely affect crystal growth and reduce strength development of the 

gypsum core. 

Residual organic impurities in synthetic gypsum from waste acid neutralization processes such as 

citric and lactic acid production can strongly retard the hydration of gypsum stucco. 

Organic impurities can be identified through several methods including coulometric titrimetry, 

Infrared Spectroscopy (IR), Nuclear Magnetic Resonance (NMR) and High Performance Liquid 

Chromatography (HPLC). 

Physical Properties. Although chemically the same, there can be significant physical property 

differences between natural and synthetic gypsums. It is well publicized that there are significant 

differences in particle size and shape between natural roller milled ground gypsum rock (land 

plaster) and most synthetic gypsums. 

Particle Sue. Most synthetic gypsums are formed in a solution environment supersaturated with 

respect to gypsum and accompanied with vigorous agitation. Under these conditions, the 

gypsum particles precipitate fairly uniformly in particle size and shape. The particular size and 

shape depends on the process conditions of the unit operations. USGC FGD guidelines list a 20 

micron minimum median particle size. More commonly, synthetic gypsums have median 

particle size is in the 35 - 45 micron range. 

Many synthetic gypsums have narrow or monolithic particle size distributions. Typical roller 

milled natural gypsum has a much broader particle size distribution. Figure Two shows SEM 

photomicrographs of natural and FGD gypsum. Figure Three shows a comparative particle size 

histogram between natural and a FGD gypsum. 

A monolithic particle size distribution with a large mean particle size (40 - 60 microns median) 

for synthetic gypsum is preferred even though it creates processing challenges at the wallboard 

facility. Large particles (low surface area) will readily dewater. Low free moisture is a desired 

quality because of lower drying costs. Also, as free moisture is reduced through dewatering, the 

quantity of water soluble impurities will be reduced. 

In general, synthetic gypsums that have poor dewatering characteristics also have high surface 

moisture and high surface area. In addition, the material has more challenging material handling 

properties and causes increased amounts of process water to be used during the continuous 

casting of gypsum board. The later point causes a higher thermal demand in drying the gypsum 

board. 

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Therefore, it is preferred that the gypsum supplier design the synthetic gypsum system to 

produce gypsum particles that will readily dewater. Despite engineering design challenges at the 

wallboard plant, synthetic gypsums with large particles are desired because they impart good 

I dewatering properties. 

Particle size is measured using laser scattering or sedimentation technique. The two methods do 

not always yield the same results. Figure Five shows particle size distribution for natural and 

laser technique yields a higher mean particle size then sedimentation. 

j synthetic gypsums using laser light scattering and sedimentation. Note that for both samples the 

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Surface area is another method to measure particle fineness. The Blaine surface area technique is 

a common measuring method used for powders. The Fisherm subsieve sizer is used as well. 

The surface area of natural roller milled gypsum is typically 2000 - 3000 cmz/gram. Wallboard 

grade FGD gypsum is commonly below 1000 cmz/gram. 

Aspect Ratio. Some wallboard manufacturers will include an aspect ratio guideline (usually 

1Ol:l - 20:l:l maximum) on particle shape. The aspect ratio guideline is usually put into 

product specifications to insure against contending with problems associated with needle-like 

gypsum crystals in processing, calcining and wallboard manufacture. Since gypsum particles 

retain their shape after calcination, high aspect ratio particles can cause an increase in water 

demand during board conversion. 

The synthetic gypsum producer needs to concern itself with aspect ratio because of its influence 

on internal processing issues. Needle shaped gypsum particles are more difficult to dewater than 

large blocky shaped crystals. Thus, under the same processing conditions, gypsum with a high 

aspect ratio will contain higher surface free moisture and water soluble impurities. It may also 

be more expensive to process in order to meet finished product guidelines. Hence, the material 

will have reduced value. 

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The aspect ratio of natural gypsum is generally 1: 1. The particle shape of gypsum can easily be 

determined using the SEM. 

Bulk Density. Large monosized gypsum particles of many FGD gypsums tend to exhibit a high 

bulk density. Often, the synthetic gypsum is to be purchased sight unseen by the gypsum 

company. This can present engineering and process challenges to the wallboard manufacturer in 

designing suitable material handling systems due to the high hulk density. Loose bulk density of 

natural gypsum is about 50 pcf. The bulk density of synthetic gypsums can vary widely from 

less than 40 pcf to over 70 pcf. Table Four lists bulk densities of selected natural and synthetic 

gypsums used by USGC. Bulk density is easily measured through the weighing of a known 

volume of material. 

SUMMARY 

Over the years of using natural and synthetic gypsums, USGC has developed and employed 

several methods of characterizing natural and synthetic gypsum. Synthetic and natural gypsum 

are chemically the same. The kind and amount of impurities will vary from source to source for 

both natural and synthetic gypsums. Chemical and physical property testing techniques are the 

same for natural and synthetic gypsum. It is the physical properties that commonly distinguish 

synthetic !?om natural gypsum. 

In general, synthetic gypsum has a narrow particle size distribution. Natural gypsums are 

ground and have a much broader particle size distribution. Other physical properties differences 

such as surface area, bulk density and aspect ratio are related to the narrow particle size 

distribution of synthetic gypsum. 

Synthetic gypsum is received at the wallboard plant in a wet cake form. It is important to the 

wallboard manufacturer that the gypsum be as low as possible in fiee moisture within the 

capability of the supplier. Lower free moisture reduces wallboard manufacturing costs. 

REFERENCES 

1. P. J. Henkels and J.C. Gaynor, "Characterization of Synthetic Gypsum", The Fourth 

International Conference on FGD and Other Synthetic Gypsum, May 1995. 

J. W. Barber, B. A. Hudgens, C. D. Byers and V. R. Nathan, "Characterization of 

synthetic Gypsum", The Second International Conference of FGD and Chemical 

%mun, May 1991. 

2. 

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TABLE ONE FGD GYPSUM GUIDELINES 

TABLE TWO SAMPLE SOLUBLE SALT ASSAY 

TABLE THREE TRACE ELEMENTS 

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TABLE FOUR BULK DENSITY 

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FIGURE ONE SEM Photograph of Flyash Impurity in Synthetic Gypsum 

FIGURE TWO SEM Photographs of FGD and Natural Gypsum 

Natural Gypsum 

FIGURE THREE 

Particle Size Distribution 

NatYrsl and FGD Gyprum 

Pahcle Sue (muons) 

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FIGURE FOUR Laser Scattering vs. Sedigraph Data 

0 1 10 100 1000 

Particle Size (microns)