John P. Sanders

Kinetic analyses using simultaneous TG/DSC measurements: Part I: decomposition of calcium carbonate in argon

Abstract Dynamic kinetic analyses are performed on CaCO3 using simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC) measurements in both wet and dry flowing Ar. Differences in the results are discussed in terms of the property measured and the possible influence of the atmosphere on the structural nature of the solid product, CaO. Although the quality of the DSC results was inferior compared with those based on TG, it is clear that they yield different results in subsequent kinetic analyses using a wide range of numerical methods. The presence of moisture in the atmosphere produced slightly lower values of the Arrhenius parameters with less variation during the course of the reaction. Reservations are described concerning the general interpretation of such results.

Kinetics of the oxidation of magnetite using simultaneous TG/DSC

Kinetics of the oxidation of magnetite (Fe3O4) to hematite (a-Fe2O3) are studied in air using simultaneous TG/DSC. The mechanism is complex and the differences between the kinetic conclusions and Arrhenius parameters based on either TG or DSC are discussed. As in our previous work on CaCO3 [1], the determination of a satisfactory baseline for the DSC results adds considerable uncertainty to those kinetic results. Consequently the calculations based on the TG data are considered superior. Solid state reactivity varies from one source of material to another and the results are compared for two different commercial samples of magnetite, both presumably prepared by wet chemical methods. These materials are much more reactive than the material studied previously [2], which had been coarsened and refined at high temperatures. In that earlier study, the metastable spinel, g-Fe2O3, was formed as an intermediate in the oxidation to the final stable form, a-Fe2O3. The exothermic reaction of the gamma to alpha form of the product during the oxidation process destroys the direct comparison between the TG and DSC results, since the former only detects the change in mass of the sample and not the crystallographic transformation. The TG results, however, represent the true oxidation process without superposition of the structural aspects.

Application of thermal analysis in preservation and restoration of historic masonry materials

In part A, thermal analysis techniques were discussed to characterize masonry building materials from significant historic properties in the Charleston, South Carolina, area. In part B, thermal analysis confirms various modes of long-term degradation of the materials through environmental exposure. This research provides evidence of diagenesis within materials usually considered as chemically inert. Masonry mortars used in the nineteenth century include those composed of lime and sand in the early part of the period, with the use of natural (manufactured) cement in conjunction with the mid-century fortification of the harbor. Mortars with both types of binders have been found subject to chemical alteration by long-term reactions with intruding soluble salts to form phases rich in sulfur (primarily from ground salt intrusion) or rich in chloride (primarily from sea water intrusion). The general direction of reactions may be toward formation of smectite minerals found in salt water estuaries. Further, evidence of alkali–silica reaction was found in a mortar exposed to sea water. Differences were found in reactions between lime mortars and those containing natural cements in regard to reaction phenomena. Mineral phases as a result of chemical alteration were also found in “underfired” clay bricks. Thermal analysis techniques confirm the identities of new phases within the historic masonry materials. The results suggest new phases formed within the masonry elements have the potential to cause deterioration through salt crystallization phenomena.

Test Method for Determining the Efflorescence Potential of Masonry Materials Based on Soluble Salt Content

A single test method that is capable of determining the efflorescence potential for all materials in a masonry system is needed. The need for preconstruction testing is referenced in ASTM C1400-1—“Standard Guide for Reduction of Efflorescence Potential in New Masonry Walls”—but the standard acknowledges that there is not a suitable test method for all masonry materials. In the past, soluble salt measurements have been used to quantitatively access the efflorescence potential of masonry materials. To determine the water-soluble salt content of masonry materials, a simple leaching procedure was developed to remove the salts followed by quantification of the water-soluble salt content by ion chromatography. A series of modified efflorescence tests using salt solutions and real masonry materials has been used to determine the significance of water-soluble salt measurements. A clear threshold for soluble sulfate content that indicates a high potential for the development of visible efflorescence has been identified. Sulfate salts are the most common water-soluble compounds associated with efflorescence complaints.

Improving the Thermal Resistance of Brick Masonry Systems

Finite element modeling was used to relate the coring or cell configuration of brick to the heat flow through brick masonry wall systems. Core or cell configurations that minimize thermal bridging through the brick thickness substantially reduced the heat flow through the masonry. The effect of bed depth on heat flow was also investigated. Larger units with optimized coring designs resulted in substantial reductions in heat flow. Heat flow through the mortar joint was also compared to heat flow through the brick. Wall designs that minimize the mortar joint thickness also reduce the heat flow. From these simulations, it is clear that a masonry system can be optimized to reduce heat flow. The effect of core or cell filling with various materials was also studied. Insulating materials reduce heat flow primarily by displacing mortar in the cores or cells.

Ceramics, Glass, and Electronic Materials

This chapter discusses the potential applications of thermal analysis and calorimetry to the study of inorganic materials. The techniques have tremendous possibilities, and can also be misleading if the investigator does not focus on the specific property being monitored. In most cases, assumptions must be made in relating the apparent mass change, heat absorbed or evolved, and dimensional change to some event at the atomic/molecular level. Complementary studies using more direct techniques, such as X-ray diffraction, microscopy, and various forms of spectroscopy, are frequently necessary. Particular care is required when interpreting observed evolution or absorption of heat, because physical processes, as amorphous to crystalline transformations and sintering, may accompany chemical reactions.

Statistical Analysis of Flexural Tensile Strength of Clay Masonry as a Function of Void Area

Extensive bond wrench testing of clay brick with different void areas has recently been performed. The data set is very comprehensive, testing different types of brick, brick with low and high initial rate of absorption, different mortar types, and voids ranging from 22 to 36 %. All pairwise combinations within each data set were compared with the U-test. The U-test tests for statistically significant difference between means of two data sets. The U-test is distribution-free, is nearly as statistically efficient as the t-test, can handle different sample sizes, and is robust against different variances. Approximately one-half of the comparisons showed no statistical difference in bond strength with different void areas. Of those that showed statistically significant difference, an essentially equal number showed higher bond strength with lower void area as those pairs that showed lower bond strength with lower void area. From the statistical testing, it is clear that flexural tensile strength is not a function of brick void percentage for the range of voids tested.

The effect of void area on brick masonry performance

Higher void (hollow) brick offer the potential for energy savings, decreased raw material usage and reduced environmental impact. These advantages are related to the movement toward “green” building materials. The National Brick Research Center has performed an extensive study comparing the wall system performance of hollow and solid brick. Hollow masonry units are defined by ASTM C 652 while solid masonry units are specified in ASTM C 216. The key differences between these two specifications are related to the permissible void area and face shell thickness. The objective of this work was to determine what effect, if any, increasing void area might have on important aspects of the performance of brick masonry. The effect of decreasing face shell thickness was also evaluated. For this study, the performance of several sets of comparison extruded brick was measured. These sets of comparison brick represent a range of manufacturers and, thus, a range of physical properties. Water penetration, flexural bond strength, and compressive strength were measured on each type of brick and used as indicators of potential performance in a wall. Additionally, mortar usage as a function of void area was studied. Based on the results of testing from these sets of comparison brick, increasing void area or decreasing face shell thickness did not result in increased water penetration or decreased flexural bond strength.

Chapter 5 – Ceramics, Glass, and Electronic Materials

This chapter discusses the potential applications of thermal analysis and calorimetry to the study of inorganic materials. The techniques have tremendous possibilities, and can also be misleading if the investigator does not focus on the specific property being monitored. In most cases, assumptions must be made in relating the apparent mass change, heat absorbed or evolved, and dimensional change to some event at the atomic/molecular level. Complementary studies using more direct techniques, such as X-ray diffraction, microscopy, and various forms of spectroscopy, are frequently necessary. Particular care is required when interpreting observed evolution or absorption of heat, because physical processes, as amorphous to crystalline transformations and sintering, may accompany chemical reactions.

Kinetic analyses using simultaneous TG/DSC measurements

Abstract Dynamic kinetic analyses are performed on CaCO3 using simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC) measurements in both wet and dry flowing Ar. Differences in the results are discussed in terms of the property measured and the possible influence of the atmosphere on the structural nature of the solid product, CaO. Although the quality of the DSC results was inferior compared with those based on TG, it is clear that they yield different results in subsequent kinetic analyses using a wide range of numerical methods. The presence of moisture in the atmosphere produced slightly lower values of the Arrhenius parameters with less variation during the course of the reaction. Reservations are described concerning the general interpretation of such results.