Influence of elevated temperature on the microstructures and mechanical properties of cement-based mortar with sands and refractory materials

In this study, the influence of elevated temperature on the mechanical properties of the cement-based mortar with sands and refractory materials was investigated at 200, 400, 600, 800, and 1000 °C. Four different samples were prepared: two samples are general cement-based mortars with a cement to sand ratio of 1:1 and 1:3 (11C and 13C), and the other two are fire-resistant mortars that utilize silica fume/alumina and geopolymer as refractories (FRC and FRG). From microstructural investigation, it was shown that the matrices of FRC and FRG with refractory materials remained as comparatively denser structure with less defects than 11C and 13C with sands at high temperatures. To investigate pyrolysis behavior at elevated temperatures, thermogravimetry and differential thermal analysis were conducted. As a result, minimum weight loss was seen for the FRG sample at high temperatures, while there is seen relatively large weight loss for the 11C and 13C. The compressive strength was observed as 50, 41, 64 and 31 MPa for the pristine 11C, 13C, FRC and FRG samples, respectively, and it decreased to 9.0, 5.2, 15 and 9.7 MPa when exposed to 1000 °C owing to the increased defect structures. FRC exhibited superior compressive stress for all temperatures. On the other hands, FRG showed the lowest compressive stress below 600 °C, while the compressive stress above 800 °C surpasses that for the general cement-based mortars (11C and 13C). Comparably better thermal stability of FRG at very high temperatures seem to be due to geopolymer, which provide denser microstructures and the least weight losses.