WHAT CAUSES ROOFING GRANULES MINERAL BASE TO RUST AND METHODS FOR DETERMINING THE ROCKS RUST POTENTIAL

Pigment-coated mineral rocks are commonly used as color granules in roofing applications to provide aesthetic as well as protective functions to the asphalt shingles. Roofing granules typically comprise crushed and screened mineral materials, which are subsequently coated with a binder containing one or more coloring pigments, such as suitable metal oxides. Roofing granules are typically produced by mixing mineral particles with the coating materials in a drum or pan coater prior to drying or curing of the color coating in a kiln with extremely high temperature. In general, many rocks which are opaque, dense, and properly graded by screening for maximum coverage can be used conventionally in roofing granule process. Most stone sources have a small percentage of iron as a part of the stone’s chemical composition. For the most part the iron content is not encapsulated in the stone thus allowing for traces of rust to leach from the stone. Subject: The roof discoloration caused by the rust of the base rock is one of the reasons most serious customer complaints. The rust can appear during the curing process or throughout shingle lifetime weathering. The article seeks to answer the question what substance in a base rock can cause the rust and subsequent mineral degradation. Research objectives: discoloration, appearance of stains and streaks of rust on polymer-bitumen roofs is one of the most frequent causes of complaints from buyers. All this can be avoided by a deliberate choice of rocks, as a mineral carrier for the production of granules, that are not prone to “rust” and degradation. The reasons for the color change and the methods of choosing the rock least affected by this factor are described in this article. Materials and methods: The research has been done by comparing mineralogical and petrographic properties of granule base rocks and their color change values between calcined and incalcined granules. The rock samples underwent Differential thermal analysis in order to define a fraction of weak minerals in each of the rocks. Results: The study shows that all test samples changed its color after calcining at operation temperature 537°. Several minerals that change their colors and collapse during heating at operational temperature. Some of the mineral display rust, which is iron hydroxide. There are minerals with encapsulated iron that were stable during the calcining process and some collapsed and become sours of iron hydroxide leaching on the colored granules surface during the outdoor-weathering test. Conclusions: In the result of the study we can make a statement that the weakest minerals that collapses at operation temperature of the granule process and creates the iron hydroxides that can freely leak out of rock body are sulfides. It recommended to define the sulfides content in rock in order to prevent the rusting and breakdown issue.