CHARACTERIZING SYNTHETIC

### GYPSUM FOR

### WALLBOARD

### MANUFACTURE
P. 

### J. Henkels J.C. Gaynor USG Corporation Research Center 700N. Highway 45 Libertyville, 

### IL 60048 FGD Synthetic Gypsum, Gypsum Wallboard,TestMethods United States Gypsum Company 125S.Franklin Chicago,

### IL 60606 Keywords:



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# --.----.. 111 1 KUUUL 11UI1 United States Gypsum Company (USGC) has developed specificationsand 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 guidelinesbased 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 equallyrelevant to other syntheticgypsums and 

### the naturally occurring gypsum mineral 

### as well.
USGC's general guidelinesof FGD gypsum along with guidelinesfrom the German Gypsum Associationare listed in Table One. The guidelines are not an all inclusivelist of every gypsum property importantto the wallboard manufacturer. The guidelines

### serve

### as a startingpoint in negotiating sales agreements between the wallboard manufacturerand synthetic gypsum supplier. Theproduct specification agreement at USGC is tailoredto each individual source depending in large part to the percent usage at the wallboardfacility,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 syntheticgypsum can 

### be reasonably obtainedby the supplier. Therefore, a higher purity syntheticgypsum will have increased value. Impurities found in synthetic as well 

### as natural gypsum can be quite detrimentalto the wallboard produced. The 

### higherpurity reduces the chance of deleterious effectsfrom these impurities. Purity can be determined by several test methods including Differential Scanning CalorimetqfThermal Gravimetry (DSCfWA), X-Ray Fluorescence Spectroscopy (XRF) and 

### SO3analysis.
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. 

### Thisreduces 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 greatertendency 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 guidelineslist the predominant impurities found in FGD gypsum. Discussion of selected impuritiesfollow. 569Residual 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 substancethan gypsum (Mohs value 1.6

## - 2). Limestone quantity can be determined

### through

### XRF oxide analysisof calcium and magnesium in conjunction with carbon dioxide (CO,) analysis by coulometric titrimeby. Alternatively, COz can be quantified throughDSC 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 raiseserious industrial hygiene issues. Trace elements and the analysis will be discussedin a later section. Flyash is easily detected using a scanningelectron microscope 

### (SEM). By pingimage analysis, an estimateofthe amount of flyashpresent 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 

### SEMphoto of flyashimpurity in a FGD gypsum sample. Silica 

### (Si02). Silicondioxide is an importantimpurity from an industrial hygiene perspective and aprocess issue for all mineral industries. Silicais commonin both natural and synthetic gypsums. It can be partofa clay, flyash or quartz impurity. High quantities ofrespirable(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 acceleratedwear on gypsum processing equipment. 

### Amorphous silica 

### is 

### also containedin 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 methodto 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 considereda better method to determine sulfite. A titrationprocedure is listed in EPRI-Method 40 

### (EPRI CS3612. Project 1031-4. Final Reportdated 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 differenttypes of synthetic gypsums. It is common practiceto 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 additivesin the board mixer. During the drying ofthe gypsum board in the kiln the salts migrate to the paper 

## - core interface and interrupt the paper to corebond. Salts are very hydroscopicand cause moisture to deposit in the criticalbond area of the board. On exposure to 

### high moisture 

### from joint finishing and wallpaper productsthe drywall paper can detach itself from the 

### core. 570 

### J

.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 ofchloride. 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 ofthe 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 impuritiesin synthetic gypsum from waste acid neutralizationprocesses such as citric and lactic acid production can stronglyretard 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 significantphysical 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 formedin 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 inthe 

### 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 ofwater 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 challengingmaterial 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. 571Therefore, 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 scatteringor sedimentation technique. 

### The two methods do not always yield the same 

### results. Figure Five shows particle size distribution for natural and 

### lasertechnique yields ahigher mean particle sizethen sedimentation. j synthetic gypsums using 

### laser light scattering and sedimentation. Note that forboth samples the 

# 1

I I 

# t' 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 

### rollermilled gypsum is typically 

### 2000 

## - 3000 cmz/gram. Wallboard grade FGD gypsum is commonlybelow 

### 1000cmz/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 needsto 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. 

# \

1



### 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 engineeringand process challenges to the wallboard manufacturerin designing suitable material handling systems due to the high hulk density. 

### Loosebulk 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 ofmaterial. 

### SUMMARY Over the years of using natural and synthetic gypsums, 

### USGC has developed and employed several methods of characterizingnatural and synthetic gypsum. Synthetic and natural gypsum are chemically the same. The 

### kind and amount of impuritieswill 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 distributionof 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 capabilityofthe supplier. Lower 

### freemoisture reduces wallboard manufacturing costs. 

### REFERENCES 1. P. J. 

### Henkels and J.C. Gaynor, "Characterization of Synthetic Gypsum", The 

### Fourth International ConferenceonFGD 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.

572TABLE 

### ONE FGD 

### GYPSUMGUIDELINES

TABLETWO SAMPLE SOLUBLE SALTASSAY

TABLE THREE TRACE ELEMENTS 

# I Zinc 

# 1

513TABLE FOUR BULK DENSITY 

## G

\ FIGURE

### ONE SEM Photograph

### ofFlyash Impurityin Synthetic Gypsum

FIGURE

### TWO SEM Photographs

### ofFGD and Natural Gypsum Natural Gypsum

FIGURE THREE Particle Size Distribution

NatYrsland FGD Gyprum

Pahcle Sue (muons)

574 FIGURE

### FOUR LaserScattering vs. SedigraphData

0 1 10 100 1000 Particle Size (microns)