A study on combination of geopolymer and cold-pressing technique to recycle fine BOF slag as compressed paving units

Basic oxygen furnace (BOF) slag is in general very difficult to reutilize due to its low volume stability. Around 1.6 million tons of BOF slag is generated annually in Taiwan. The coarse BOF slag (>4.75 mm) was mainly reused as aggregate for asphalt concrete. But the recycling of the remaining fine BOF slag (more than 600,000 tons per year), however, was very difficult. A new concept for stabilization and recycling of fine BOF slag using geopolymer (GP) combined with cold-pressing techniques to produce high-value green building materials (i.e., compressed paving units, CPUs) was tested and developed in this study. The modified densified mixture design algorithm (DMDA) was also adopted to design the geopolymer-based composites. Herein four alkali solutions with different concentrations were prepared and tested. Furthermore, more than 75% (by weight) of fine BOF slag was designed for the geopolymer-based composite mixtures. The test results show that the compressive strength of developed CPUs produced with 6M and 8M alkali solution at the age of 1-d and 3-d was more than 50 and 65 MPa, which meet the national standards CNS 13295 Class B and A, respectively. The compressive strength of CPUs produced with 4M alkali solution at the age of 1-d, 3-d, and 7-d was more than 45, 50, and 65 MPa, which meet the national standards CNS 13295 Class C, B, and A, respectively. Other properties of developed CPUs also conformed to the requirements of CNS 13295 Standard. The combination of geopolymer and cold-pressing technique, therefore, is not only a new and practicable concept to recycle fine BOF slag, but also reduces the energy consumption and CO2 emission as well.