Analysis of Gypsum Moisture Content

Gypsum has been used as a building material for several centuries and is ever present in modern buildings. With a demand initially driven by the mass structural devastation of WWII, wallboard is a product that transformed the use of gypsum as a building material.

However, the wallboard industry has witnessed a decline in demand in recent times, pushing the manufacturers to develop high quality products at a low cost.

For that reason, manufacturers have begun to recycle scrapped products resulted from production process failure. In addition, wallboard is now being collected from building destruction sites to make new products by mixing it with unused gypsum.

Although these innovations lower raw material costs, they demand more monitoring of production processes and stringent quality checks of products before reaching out to customers.

Issues with Traditional Measurements of Purity, Free Moisture and Bound Moisture

Since it is possible to quantify the water content per gram, measuring the purity of the gypsum is an easy task. Conventional techniques of purity analysis involve heating the material and quantifying the change in mass to yield accurate measurement of purity.

However, these methods need long analysis times and ovens that have high operating costs, making them less desirable for manufacturers. Moreover, in-test analysis is not possible with these methods.

Free moisture content represents the amount of water that is evolved off upon heating the material but is not chemically bound to the plaster. It needs to be measured and removed to quantify the gypsum purity as it may considerably affect the bound moisture measurement. The conventional measurement involves low heat for at least 2hr to as high as 24hr in an oven.

Bound moisture represents the amount of water chemically bound with the primary component, calcium sulfate. The bound moisture can be quantified after completely removing the free moisture content in the sample. This measurement is traditionally completed by applying high temperature to the sample placed in an oven for 4hr.

Rapid Loss-On-Drying Technology

The Computrac® MAX® 5000XL developed by AMETEK Brookfield Arizona is a high temperature rapid loss-on-drying analyzer that can heat samples to 600°C and start testing at room temperature. This makes it a suitable instrument for testing gypsum for free and bound moisture.

In addition, the Computrac® MAX® 5000XL yields real time measurements during analysis, thus allowing for optimization of testing criteria. The MAX® 5000XL is also capable of testing free and bound moisture concurrently, thus yielding the gypsum purity free from any calculation error caused by technicians performing the test.

Experimental Procedure

A 4’X4’ piece of 3/8” thick drywall was cut into small 1” square segments and then blended for 40s utilizing a kitchen-aid coffee grinder. The blended sample was then stored in a mason jar. The reference testing to determine free moisture, bound moisture and fiber content of wallboard as well as free moisture and bound moistures of gypsum was performed as per the procedures.

In MAX® 5000XL testing, both wallboard and gypsum were analyzed using the same sample parameters. The instrument settings for measuring free moisture content were as follows:

Sample Size - 7.0g +/- 1.0g
Idle Temperature - 40°C
Testing Temperature - 50°C
Pan Tare - Standard
Sample Tare - 3 seconds
Ending Criteria - Rate, 0.0500%/minute

The analysis was initiated when sample chamber achieved a temperature of 40°C. The MAX® 5000XL tared the pan and prompted the operator to add the sample. The wallboard analysis involved tilting the glass jar to 90°and rotating it to allow loading of controlled amounts of material into the sample pan.

The material consisted of both gypsum filler, and wallboard paper. The synthetic gypsum material was too wet and clumpy to be placed on the sample pan in this way. Hence, a spatula was used to spread it uniformly on the pan. After completing the free moisture analysis, the instrument immediately started bound moisture analysis using the following parameters:

Testing Temperature - 230°C
Ending Criteria - Rate, 0.0500%/minute

Since the bound moisture analysis was linked to the free moisture analysis, other parameters were not used. The wallboard material was additionally analyzed for determining the fiber content using the following parameters:

Testing Temperature - 600°C
Ending Criteria - Temp → Rate, 0.0200%/minute
Ash Rate - 7.0%/minute

Experiment Results

The results obtained from the two methods are summarized in Tables 1 and 2. The MAX® 5000XL testing results show good agreement with conventional testing results in the case of free and bound moisture for both products, as well as ash content for the wallboard.

The purity factor equation built into the MAX® 5000XL determined the purity of the gypsum as 92.35% for the wallboard, and 97.71% for the synthetic gypsum.

Table 1. Comparison of Testing Methods for Wallboard.

Wallboard Testing
  Free Moisture Testing Bound Moisture Ash Content (Fiber)
  MAX® 5000XL Convection Oven MAX® 5000XL Convection Oven MAX® 5000XL Muffle Furnace
Average Result 0.46 0.58 19.33 18.94 8.00 9.14
median test time (mm:ss) 4:49 120:00 23:01 120:00 7:39 120:00

Table 2. Comparison of Testing Methods for Synthetic Gypsum.

Synthetic Gypsum Testing
  Free Moisture Testing Bound Moisture
  MAX® 5000XL Convection Oven MAX® 5000XL Convection Oven
Average Result 8.02 8.33 20.45 20.34
median test time (mm:ss) 24:02 120:00 7:44 120:00

Figures 1 and 2 show the graphs generated by the MAX® 5000XL for both free and bound moisture. As can be observed, it is possible to monitor the testing for both free and bound moisture to ascertain that the testing conditions adequately drive off all of the free and the bound moisture for each analysis.

In addition, it is possible to adjust the testing criterion to optimize the testing conditions. In-situ measurement is not possible with conventional methods of measurement.

Real Time Measurement of Free Water using the MAX® 5000XL.

Figure 1. Real Time Measurement of Free Water using the MAX® 5000XL.

Real Time Measurement of Bound Moisture for Synthetic Gypsum using the MAX® 5000XL.

Figure 2. Real Time Measurement of Bound Moisture for Synthetic Gypsum using the MAX® 5000XL.


The results clearly demonstrate the advantage of using rapid loss on drying technology over conventional methods for determining moisture content and purity of gypsum and wallboard products. The MAX® 5000XL shortens analysis times from 4-6 hr to below 1 hr on an average. The interface and real time data provided by the MAX® 5000XL facilitates a comprehensive profile of the material.

Furthermore, the possibility of adjusting testing criteria can help obtaining optimized total results. The MAX® 5000XL high temperature ashing features enable analyzing reclaimed or recycled materials. These updated features allow manufacturers to lower material cost without compromising the product quality.


Arizona Instrument

This information has been sourced, reviewed and adapted from materials provided by AMETEK Brookfield Arizona

For more information on this source, please visit AMETEK Brookfield Arizona



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