Influence of coal mining by-products on the new blended cement properties

The authors of this paper report the possibility of obtaining a metakaolin based pozzolanic material from coal mining. This is a priority line of European environmental policies on the by-products recycling as a raw material due to its environmental benefits. In 2009, global coal production was 6.9 million tonnes [7.6 million tons], which produced the coal mining by-products of about 10-15% of coal production. Most of these by-products accumulate in landfills for many decades, with the resulting economic, social, and environmental challenges. These starting by-products, due to their characteristics, are inert materials without pozzolanic properties. However, once activated under controlled thermal conditions, they have good qualities as active addition, which may be based on metakaolin. This research work presents, the scientific and technical viability of obtain new eco-efficient blended cements based on the thermally activated coal mining by-products (ACMW). Different instrumental techniques were used to evaluate different scientific aspects (composition, mineralogy, and pozzolanic properties), as well as on the rheological and mechanical behaviour of blended cements up to 20% of replacement of ACMW. The results show that theses by-products once calcined at 600°C, shows high pozzolanic activity in Ca(OH)₂/ACMW systems, forming metastable hydrated phases similar to those obtained by natural metakaolin. However, an important difference was found in the LDH structures, as a consequence of the presence of calcite in the raw by-products. Also, the results obtained had good behavior as blended cements with ACMW up to 20% of replacement of OPC following the chemical, physical, and mechanical requirements of European standard.