At times other methods are useful for supplementing petrographic findings and explaining the performance of a material. A number of different tools are available to us but we find that spot chemistry, bulk chemistry and x-ray diffraction are those we use most commonly.
Spot chemistry has a long-standing history in petrography as a set of methods that can detect the presence of various chemical elements in a compound. These tests are qualitative - they detect elements but don't give quantitative information on their concentration. The fizzing reaction that occurs when dilute hydrochloric acid is dropped on limestone (indicating the presence of the carbonate ion) is an example of a spot chemical test that is commonly used by geologists in the field. One can use these tests to rapidly and economically detect the presence of a wide range of chemical elements and compounds. We most often use spot tests to detect the presence of chloride, sulfate, carbonate, sodium, magnesium, potassium and calcium in secondary deposits. We often use these methods to sift through and select key samples that we investigate further with other analytical methods as discussed below.
Bulk chemistry refers to the chemical analysis of a relatively large sample that is crushed and sieved to provide a homogenized material. These analyses are generally done by traditional wet chemistry, spectrometer-based or photometric methods. We generally use bulk chemistry to detect chloride levels in concrete (and demonstrate the applications of deicer salts in scaling investigations) and to document the migration of a chemical front through a material in projects that involve chemical attack. We also use bulk chemistry for mortar investigations via ASTM C1324, Standard Test Method for Analysis of Hardened Masonry Mortar and for checking the consistency of different batches of materials.
X-ray diffraction (XRD) This method characterizes the crystallographic structure of compounds. It is one of a few analytical techniques (optical microscopy is another) that can unequivocally identify minerals. We generally use XRD to identify secondary deposits, to quantify the concentration of minerals in a material, or to detect minerals in materials such as plasters that present are very fine-grained and difficult to work with optically.