Advancements in Process Development and Mechanical Properties of Recycled Glass Building Materials Through Experimentation and Analytical Modeling

Abstract

Over the past 50 years U.S. consumers and municipalities have made efforts to decrease landfill waste and reduce raw material extraction through recycling. The rate of recycling has risen tenfold over that time period and many materials, such as paper and aluminum, can be efficiently and effectively recycled into new products. However, this is often not the case for glass. Currently, over 11 million tons of post-consumer glass enters the municipal solid waste stream annually and the majority is collected for recycling. Unfortunately, less than 30% of the collected glass is successfully recycled into new products, while the remaining ~7.5 million tons is ultimately sent to landfills. A new, energy efficient glass recycling technology is being developed at EnVitrum, Inc., a UW spinout based in Seattle. The process uses waste glass as the primary feedstock in the manufacture of high recycle-content building materials. This technology is compatible with post-consumer and post-industrial waste glass and may provide a commercial outlet for the vast quantities of post-consumer glass that cannot be recycled using current technology. Moreover, due to reduced process temperatures and densification times, the materials produced through this process would have lower embodied energy and shorter production cycle times than traditional products. This would be of great benefit to potential manufacturers and in addition, the end-products resulting from this process would provide the construction industry with a sustainable material alternative. During the process, materials are consolidated through cold isostatic pressing and then densified at temperatures up to 55% lower than the melting temperature. Initial studies conducted by Marchelli and Prabhakar at EnVitrum, showed that densification through warm working was found to be compatible with common contaminants present in the glass waste stream and the process resulted in porous materials that possessed up to 95% recycle content. In 2012, an exploratory study was undertaken by Prabhakar and the results indicated that overall the physical and mechanical behavior of the material could be predictably calibrated by modification of process parameters (primarily densification temperature). However, the results presented were preliminary and it was hypothesized that inconsistencies stemmed from issues associated with processing. Therefore, advanced improvements of the process will be needed. The purpose of the research described herein was to i) investigate improvements to material processing conditions that may have affected the preliminary results, ii) characterize the physical & mechanical behavior with respect to peak densification temperature, and iii) compare the mechanical properties to zero porosity values and evaluate the suitability of established porosity-property correlations. Through this work, several improvements were made to the formation and densification processes. As a result, it was determined that the EnVitrum process is compatible with post-consumer waste glass and high strength materials can be created at lower temperatures than indicated by previous investigations. Most importantly, the findings agree with Prabhakar’s earlier assertion that the mechanical behavior of the material could be predictably calibrated by modification of process parameters and it was found that materials with sufficient strength for a variety of construction applications could be produced.