Thermally treated coal mining waste as a supplementary cementitious material - Case study from Bogdanka mine, Poland

The presented study deals with the comprehensive analysis of coal gangue from the Bogdanka mine (Lubelski Wegiel "Bogdanka" S.A.) in terms of its properties in both raw and activated states, and with the evaluation of the functional parameters of blended cement pastes containing 10 wt %, 15 wt%, and 20 wt% of thermally treated mining waste. The assessment of the raw material included chemical, physical, mineralogical, and morphological characterization, thermogravimetry, and pozzolanic activity tests. The motivation for the research was the intention to utilize a significant amount of waste from the Bogdanka mine with low additional energy consumption for processing. For this purpose, the temperature of 600 degrees C was chosen for its thermal activation based on the simultaneous thermal analysis (STA) measurement. The activated coal gangue was tested in the same way as the raw materials and then applied in the blended cement pastes as a partial cement substitute. The cement pastes were evaluated in the fresh state (flow table test), and also at the age of 7, 28, and 90 days. The structural, microstructural, mechanical, and hygric parameters were investigated and compared with the plain cement paste. Furthermore, the development and rate of the pozzolanic reaction were studied using simultaneous thermal analysis with mass spectrometry (STA-MS), X-Ray diffraction (XRD), isothermal calorimetry, and assessment of strength activity index (SAI). The performed analyses gave evidence of the pozzolanic activity of thermally activated coal mining waste (ACMW) from the Bogdanka mine and its effectiveness in cement blends. The slightly dropped hydration degree was later compensated by prolonged curing time and progress in the pozzolanic reaction. The blending of Portland cement with the examined ACMW did not negatively affect the rheology and setting time of fresh blended pastes. Considering the hardened cement blends' structural, microstructural, and mechanical parameters, ACMW can be safely used as a part of a PC-based blended binder for up to 20% replacement of cement mass.