Two common approaches to estimating heat capacity are to 1) calculate Cp from a measurement of thermal diffusivity and knowledge of the density and thermal conductivity of the FRM, or 2) measure Cp directly using a differential scanning calorimeter (DSC). The former is often complicated by the dynamic nature of FRMs, as they typically lose significant mass during the measurement time. An exciting recent development for the latter method is the availability of commercial simultaneous thermal analysis (STA) units. These units permit the simultaneous monitoring of heat flow and mass during exposure to a (high) temperature regime. With conventional DSC, only the heat flow is measured and to obtain the specific heat per unit mass of material that is the required input for thermal performance models, the results need to be adjusted by mass measurements (TGA) made on a companion sample. The advantages of making both measurements simultaneously on the same material specimen are obvious. In addition, newer commercial STA units may allow for larger sample volumes/masses (on the order of 1 g as opposed to the 50 mg to 100 mg typical of most DSCs). This is especially important for typical spray-applied FRMs that may exhibit a microstructural heterogeneity on the scale of millimeters. For FRMs whose mass composition is exactly known, an alternative approach is to calculate the FRM heat capacity as a mass-weighted average of the heat capacities of the component materials. Of course, this requires that Cp data as a function of temperature are available for each component.
To obtain quantitative Cp data (via ASTM E1269 for example 2), the typical procedure is to use a sapphire or other reference specimen to obtain a correction factor (graciously named the "calorimetric sensitivity" in the ASTM E1269 standard) under the same operating conditions as those used for the test specimen. Due to typical mass mismatch between the reference and sample pans, further corrections may be needed based on the known tabulated heat capacities of aluminum or gold (pans) as a function of temperature. A typical set of DSC curves for a spray-applied FRM are provided in Figure 3. The presence of several endothermic peaks is clearly indicated. The binder component of this particular FRM is portland cement-based and the first two peaks (around 100 ºC) correspond to the loss of bulk water and (loosely) bound water from gel-like hydration products respectively, the third peak (near 400 ºC) to the loss of chemically bound water from calcium hydroxide, and the fourth peak (near 650 ºC) most likely to the loss of carbon dioxide from carbonated reaction products. This material exhibited about a 10 % mass loss during exposure up to 700 ºC. By integrating the area under these peaks, the corresponding enthalpies of reaction could be estimated. However, with the small sample size employed in this experiment (< 10 mg), a quantitative interpretation is hindered by the previously mentioned heterogeneity of the material, e.g., most likely a representative volume was not sampled in this specific DSC measurement.