Gypsum, derived from the Greek word meaning plaster, has been used as a building material dating back to 9000 B.C.E.
As a building material, the use of gypsum was revolutionized in 1894 when Augustine Sackett began sandwiching gypsum plaster between wool paper, creating the first version of the wallboard in the United Kingdom.
Following the initial iteration of Sackett’s processing, wallboard began being used for both residential and commercial construction. Despite its versatility, early quantities were limited, and further advancement was needed to make it a viable replacement for more established building materials. This changed however following the mass structural devastation of WW II, when Western Europe was in dire need of materials and resources to rebuild. The wallboard industry exploded as demand for this product grew. Recently, demand for wallboard has decreased, pushing the industry toward rapid innovation required to provide a high quality product at a low cost to consumers. To address this issue manufactures have started to recycle products that were previously scrapped due to failures in the manufacturing process. Additionally, wallboard is now being reclaimed from building destruction sites and mixed with unused gypsum to make new products. These innovations reduce raw material costs, but requires more monitoring of manufacturing processes, and quality checks prior to materials being brought to market.
Calcium Sulfate dihydrate (CaSO4∙ 2 H2O) is the primary component used in wallboard, along with other fillers. Since the amount of water can be quantified per gram, the purity of the gypsum is easy to measure. Traditional methods of purity analysis is achieved by heating the material and measuring the change in mass. These methods accurately determine the purity, but require long test times and ovens that are expensive to operate. This translates into longer processing times for manufacturers, as well as high operating costs. Additionally, these outdated methods do not allow for in-test analysis.
Free Moisture Measurement
This is the amount of water that will evolve off of the material, but is not chemically bound to the plaster. In order to determine the purity of the gypsum in the wallboard, this must be measured and removed, since it may significantly impact the bound moisture measurement. The measurement is traditionally accomplished by low heat for a minimum of 2 hours and as long as 24 hours in an oven.
The quantity of water that is chemically coordinated with the calcium sulfate is referred to as bound moisture. Once all of the free moisture has been driven off of the sample, the bound moisture can be measured. This is significant because the amount measured will directly correlate to the purity of the gypsum in the wallboard. Traditionally, this analysis takes over 4 hours and uses an oven at a high temperature for analysis.
Rapid Loss-On-Drying Technology
Significant advancement has been made to improve traditional loss-on-drying analytical techniques. Arizona Instrument manufactures the Computrac® MAX® 5000XL, a high temperature rapid loss-on-drying analyzer that is capable of heating samples to 600°C, and can start testing at room temperature, making it an ideal candidate for testing gypsum for free and bound moisture. Additionally, the analyzer provides real time measurements during analysis, and testing criteria can be optimized. The MAX® 5000XL also can test for free and bound moisture simultaneously, and can output the purity of the gypsum following the test, removing any calculation error by technicians conducting the analysis.
Sample Preparation – A 4’X4’ piece of 3/8” thick drywall was purchased for testing. The board was cut into small 1” square segments, keeping the wallboard paper and gypsum filler together, then blended for 40 seconds using a Kitchen-Aide coffee grinder. Once the sample was blended it was transferred to a mason jar for storage.
Free moisture – Five aluminum sample pans were cleaned and dried prior to testing. The sample pans were labeled and weighed, and then 15.0g +/- 0.5g of processed wallboard was added to the pan. The pans were transferred to a convection oven and heated to 45°C for 2 hours. The pans were removed from the oven and stored in a desiccator for 30 minutes. After the pans reached room temperature they were reweighed. The change in mass was recorded and reported as free moisture loss.
Bound Moisture – Following the free moisture was determined the samples were placed in a convection oven set a 230°C for 2 hours. They were then moved to a desiccator for 30 minutes to cool. After they reached room temperature, the sample pans were weighed and the bound moisture was calculated.
Fiber Content – Untested and processed material was used to determine the fiber content of the paper. Empty crucibles were cleaned and heated to 600°C for 2 hours prior to testing, to ensure no debris was present. Once the crucibles had returned to room temperature, 15.0g +/- 0.5g of sample was added to the crucibles and placed in the furnace for 2 hours at 600°C for testing. The crucibles were moved to a desiccator were they were allowed to cool to room temperature. Following cooling, the samples were reweighed and the total fiber content was calculated using the following equation:
Free Moisture – Three aluminum sample pans were cleaned and dried prior to testing. The sample pans were labeled, weighed, and then 10.0g +/- 0.1g of synthetic gypsum was added to the pan. The pans were transferred to a convection oven and heated to 45°C for 2 hours. The pans were removed from the oven and stored in a desiccator for 30 minutes. Once the pans were at room temperature they were reweighed. The change in mass was recorded and reported as free moisture loss.
Bound Moisture – Following the free moisture determination the samples were placed in a convection oven set a 230°C for 2 hours. They were then moved to a desiccator for 30 minutes to cool. Once at room temperature, they sample pans were weighed and the bound moisture was calculated.
MAX® 5000XL testing
WALL BOARD AND GYPSUM
Both materials were tested using the same sample parameters for free moisture the instrument settings were:
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 test was started when the sample chamber reached 40°C. The instrument tared the pan and prompted the user to add sample (see screen shot below).
For wallboard testing, the glass jar was tilted 90° and rotated to allow controlled amounts of material to land on the sample pan. The material contained both gypsum filler, and wallboard paper. The synthetic gypsum material was too wet and clumpy to add to the sample pan in this manner. Instead it was spread evenly on the pan using a spatula. Once the free moisture testing was complete the instrument immediately began testing for bound moisture. The parameters for testing were:
Testing Temperature – 230°C
Ending Criteria – Rate, 0.0500%/minute
Because the test was linked to the free moisture test the other parameters are not used.
The wallboard material went through an additional test to determine the fiber content. The parameters for this test were:
Testing Temperature – 600°C
Ending Criteria – Temp → Rate, 0.0200%/minute
Ash Rate – 7.0%/minute
The ending criterion was chosen to ensure that the instrument would not stop prematurely as the temperature increased to the ashing temperature of the paper portion of the wallboard. Additionally, the rate that the temperature increased was controlled by the ash rate. If the ash rate went above the 7.0%/minute testing limit then the temperature would not increase until it returned to below 7.0%/minute. This is the default setting for testing above 300°C.
From the tables above the MAX® 5000XL shows correlation to traditional testing methods when determining both free and bound moisture for both products, as well as ash content for the wallboard. Using the purity factor equation that is built into the MAX® 5000XL it was determined that the purity of the gypsum was 92.35% for the wallboard, and 97.71% for the synthetic gypsum.
From the graphs the testing for both free and bound moisture can be monitored to ensure that the testing parameters sufficiently drive off all of the free and the bound moisture for each respective test. Additionally, the testing criterion is able to be adjusted, which allows for optimized testing conditions. In-situ measurement is not available for traditional methods of measurement.
For determining moisture content and purity of gypsum and wallboard products, rapid loss on drying shows strong correlation when compared to traditional loss-on-drying techniques. Additionally, the performance of the MAX® 5000XL reduces testing times from 4-6 hours to less than 1 hour on average. The interface and real time data also allows for a complete profile of the material as well as testing adjustments that may be required for optimizing total results. Also, analysis of reclaimed or recycled materials can be accomplished using the MAX® 5000L high temperature ashing features, allowing manufactures to reduce material cost without sacrificing the quality of product.