Performance of copper slag contained mortars after exposure to elevated temperatures

Performance of copper slag contained mortars after exposure to elevated temperatures is studied in this paper. Portland cement (PC) based mortar with copper slag replacements of 0, 5%, 10% and 15%, as well as NaOH activated copper slag mortar with NaOH concentrations of 6 M, 8 M, 10 M and 12 M were manufactured. Compressive strength and flexural strength of the mortar specimens were tested at room temperature and after exposure to 200 °C, 400 °C, 600 °C, 800 °C, 1000 °C and 1200 °C, respectively. Paste specimens were further prepared to measure their chemical and physical changes with temperature by using X-ray diffraction (XRD), thermal gravity (TG)/differential scanning calorimeter (DSC) and scanning electron microscopy (SEM)/SEM-energy dispersive spectrometer (EDS) techniques. The results show that the strength of PC mortars containing copper slag increased within 200 °C, but after that their strength declined continuously. While the strength of alkali activated copper slag mortars increased dramatically both within 200 °C and after 800 °C. Amorphous Fe(OH)3/Fe(OH)2 gels could be formed in the hydration products of PC containing copper slag at room temperature and of alkali activated copper slag at a higher temperature. For alkali activated copper slag, after 600 °C, Fe2O3 was formed as a result of the oxidation of Fe2SiO4 and/or the decomposition of Fe(OH)3/Fe(OH)2. With the further increase of temperature, Fe2O3 could have melted in a Na-rich environment. The melted matters could sufficiently flow through the pores and cracks in the matrix. When the matters cooled down, they could fill in the pores and cracks and then make the matrix much dense to provide high strength.