Gypsum wallboard has been used for over 100 years as a barrier to the spread of fire in residential and commercial structures. The gypsum molecule, CaSO 4 •2H 2 O, provides two crystalline waters that are released upon heating providing an endothermic effect. Manufacturers have recognized that the source of the gypsum ore is a factor that affects all aspects of its performance; thus, it is hypothesized that the impurities present in the gypsum ore are the causes of the performance differences. Differential Thermal Analysis/Thermogravimetric Analysis (DTA/TGA) and X-ray Diffraction (XRD) were used to compare and characterize samples of gypsum ore representing sources of natural, synthetic from a Flue Gas Desulfurization process (FGD) and blends thereof. The hemihydrate phase of representative natural, FGD, and reagent grade calcium sulfate were rehydrated with distilled water and evaluated by DTA/TGA. Analysis of the data shows distinct areas of similarity separated by the conversion to anhydrite ~250°C. Compositional reconstructions based on DTA/TGA and XRD data were compared and although, the results were comparable, the DTA/TGA suggests thermally active compounds that were not detected by XRD. Anhydrite, silica and halite were reported by XRD but were not thermally reactive in the temperature range evaluated by DTA/TGA (ambient to 1050°C). The presence of carbonate compounds (e.g., calcite and dolomite) were indicated by XRD and estimated from the thermal decomposition reaction ~700°C.

Calcium sulfate dihydrate is the primary ingredient in passive gypsum drywall fire barrier systems. The endothermic properties of calcium sulfate dihydrate are well known and exploited in these systems. Researchers in fire modeling have extensively studied the endothermic properties of gypsum up to 500°C. This study reports on the influence of selected chemical and mineral constituents on dimensional and thermal property changes in gypsum casts in excess of the critical crystal dehydration temperature (between 500° and 1000°C). Gypsum from natural and flue gas desulfurization (FGD) sources used in commercial operations were evaluated for changes by differential thermal analysis/thermogravimetric analysis (DTA/TGA), Fourier transform infrared (FTIR) spectroscopy, Raman spectra analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Petalite and Gerstley borate were added to a basic formula used in commercially manufactured wallboard for fire barrier systems. The analysis of the data identified significant changes in density and hardness of the gypsum casts related to the addition of Petalite and Gerstley borate.

The core composition of gypsum wallboard is calcium sulfate dihydrate (CaSO4·2H2O) with varying impurities, additives, or both. This study compares two commercially calcined sources of hemihydrate (CaSO4·½H2O) from a natural source and a synthetic by-product of flue gas desulfurization and neutralization to reagent grade hemihydrate. Two common ingredients, borax and kaolin, are mixed into a slurry with distilled water. The analysis supports the hypothesis that minor components in the cast have an effect on the high temperature performance of gypsum casts. The analysis is enhanced when the differential thermal analysis, thermogravimetric analysis, and dilatometry data are combined to study the changes in density versus heat flow. Specifically, the thermal performance is affected by (1) the impurities found in hemihydrate sources; (2) during the fluidization phase, the reaction of borax with free Ca++ ions to form new borate salts that melt at lower temperatures; and (3) the intercalation of these and other ions with kaolin, providing thermal stability by reducing the formation of thermally active salts.

Summary Slabs of cast calcium sulfate dihydrate have been used for decades as a heat transfer barrier in commercial and residential construction and are assessed in accordance with the parameters defined by ASTM C1396. The study described herein hypothesizes that minor impurities have significant effects on the high‐temperature performance of these casts. Five thermocouples are cast into a 20‐mm‐thick slab at approximately 4‐mm intervals are measured between ambient and approximately 1000°C in a furnace. Thermal diffusivity and inertia are estimated from this temperature profile. This study compares natural, flue gas desulfurized (FGD) and reagent grade hemihydrates that have been converted into the dihydrate form with distilled water; the common additives kaolin and borax are used to modify the cast. The thermocouple data allow an effective thermal diffusivity (α′) and an effective thermal inertia (I′) to be calculated. The hemihydrate source and kaolin content are found to affect the high‐temperature performance; the FGD source increases the thermal inertia, and kaolin inhibits the formation of other borate compounds through intercalation.