Refractory products based on the pore reduced cement technique
The present study was aimed at an investigation of the use of the pore reduced cement (PRC) technique on the fabrication and properties of calcium aluminate-based refractories. Physical and chemical performances were compared with those of conventionally prepared (unpressed) refractory castables. The influence of firing on two types of calcium aluminate cements; (Ciment Fondu and Secar 80), mixed with aggregates such as firebrick and alpha-alumina, were observed. Most of the pressed samples based on Ciment Fondu cracked and melted during firing to 1250oC. Pressed samples based on Secar 80, however, tended to crack less with increased tabular alumina content. An appropriate weight proportion of tabular alumina and Secar 80 was found to be 70 to 30. The physical properties of Secar 80 blended with (i) tabular aluminas (20-μm and 250-μm) only and (ii) round (RMA325) and 250-μm tabular alumina were investigated before and after firing to 1350oC. Pressed tabular alumina mixes containing various percentages of aggregate showed very little change in bulk density (3000 kg/m3) and flexural strength (23 MPa), but after firing, flexural strengths increased considerably as the fine tabular alumina content increased (up to 50 MPa). Microstructural evidence indicated that the interlocking structure occurring from the crystallisation of hexagonal plates of calcium hexaluminate including partial sintering of fine alumina grains could be the origin of the increased strength on firing. Although unpressed samples show similar structures, they are more porous. On firing, unpressed samples tended to expand (0.43 to 0.64%) less than pressed samples (1.32 to 1.71%) because the phase transformation from CA2 to CA6 occurred which in pressed samples leads to expansion whereas in unpressed products, expansion is accommodated within available porosity. Abrasion resistance increases as strength increases. Fracture toughness increases with the coarse tabular alumina content increases. Similar improvements in bulk density and flexural strength are observed when round alumina (~ 44μm) and tabular alumina (250-μm) are blended. The thermal shock resistance of pressed samples was also found to be better than for unpressed samples as shown by the higher relative strength after the test. The chemical durability of unpressed and pressed samples based on the slag test was investigated. Pressed samples resist slag penetration better than the unpressed samples because of the smaller pores and reduced pores connectivity. Otherwise, chemical interaction between cement and slag was similar. In conclusion, refractory products based on the PRC technique have shown considerable improvements in both physical and chemical performance, especially green strengths which are substantially higher than for conventional castables, making them less susceptible to handling damage prior to service, i.e. during transport and installation.