TY - THES

T1 - Fracture mechanical characterisation of model castables with wedge splitting tests

AU - Gull, Raphaël

N1 - embargoed until null

PY - 2020

Y1 - 2020

N2 - Castables are important refractory products, which may be shaped at the costumer site. Compared to burnt bricks castables get their heat treatment in service, out of this reason the ceramic bond is developing during application and binders are necessary to achieve a significant green strength. The main binder used is Calcium Aluminate Cement (CAC), but in recent years, new binders such as Calcium Magnesium Aluminate (CMA) cement were developed. Not only the choice of the binder, but also the raw materials and the grain size distribution as well as the addition of spinel has an influence on the behaviour at different temperatures of the castables in operation. Therefore, in this thesis, the influences of Tabular Alumina (TA) or White Fused Alumina (WFA) as raw material the influences of the type of spinel addition and the influences of CA- or CMA-cement on the mechanical behaviour of castables was investigated. This is important for castables because the first heating causes dehydration, new phase formations and at sufficiently high temperatures a change of the bond type. For this reason, 110°C, 600°C and 1500°C have been chosen as temperatures for heat pre-treatment. The RFDA (Resonant Frequency and Damping Analyser) was applied for the determination of the Young’s modulus. The results were interpreted with regard to phase changes and the microstructural evolution. Important parameters to investigate the failure behaviour of castables are the specific Fracture Energy (Gf), the Brittleness number (B) and the Notch Tensile Strength (σNT). These parameters were determined with the Wedge Splitting Test (WST) coupled with Digital Image Correlation technique (2P-DIC (two Part Digital Image Correlation)). Compared to standard DIC the 2P-DIC allows for an more accurate evaluation of the crack pattern on the sample surface. An influence of the CAC on the brittleness and on the mean crack length at 15% of maximal horizontal force after drying of the castables was highlighted by these investigations. Furthermore, it was recognised that especially at 600°C, which is a relevant temperature range for the pre-treatment of purging plugs, the choice of raw material is decisive. If WFA is used the brittleness decreases and mean crack length is shorter at the end of the WST. Also, at high temperatures, the choice of raw material has an important influence on the Young's modulus, in case WFA is used the Young’s modulus is lower compared to castables with TA. The selected CaO content for castables after sintering is also decisive, as it significantly influences the brittleness. With a higher amount of CaO the brittleness of the castables increases. With raising temperatures, the determined crack length decreases and the deviation from the vertical symmetry line decreases. This was determined by 2P-DIC. Not only the choice of raw material, the binder or spinel addition have a significant influence on the failure behaviour but also the heat pre-treatment. As the heat treatment temperatures rise, the castables reach higher strength and fracture energy values, but the brittleness also increases.

AB - Castables are important refractory products, which may be shaped at the costumer site. Compared to burnt bricks castables get their heat treatment in service, out of this reason the ceramic bond is developing during application and binders are necessary to achieve a significant green strength. The main binder used is Calcium Aluminate Cement (CAC), but in recent years, new binders such as Calcium Magnesium Aluminate (CMA) cement were developed. Not only the choice of the binder, but also the raw materials and the grain size distribution as well as the addition of spinel has an influence on the behaviour at different temperatures of the castables in operation. Therefore, in this thesis, the influences of Tabular Alumina (TA) or White Fused Alumina (WFA) as raw material the influences of the type of spinel addition and the influences of CA- or CMA-cement on the mechanical behaviour of castables was investigated. This is important for castables because the first heating causes dehydration, new phase formations and at sufficiently high temperatures a change of the bond type. For this reason, 110°C, 600°C and 1500°C have been chosen as temperatures for heat pre-treatment. The RFDA (Resonant Frequency and Damping Analyser) was applied for the determination of the Young’s modulus. The results were interpreted with regard to phase changes and the microstructural evolution. Important parameters to investigate the failure behaviour of castables are the specific Fracture Energy (Gf), the Brittleness number (B) and the Notch Tensile Strength (σNT). These parameters were determined with the Wedge Splitting Test (WST) coupled with Digital Image Correlation technique (2P-DIC (two Part Digital Image Correlation)). Compared to standard DIC the 2P-DIC allows for an more accurate evaluation of the crack pattern on the sample surface. An influence of the CAC on the brittleness and on the mean crack length at 15% of maximal horizontal force after drying of the castables was highlighted by these investigations. Furthermore, it was recognised that especially at 600°C, which is a relevant temperature range for the pre-treatment of purging plugs, the choice of raw material is decisive. If WFA is used the brittleness decreases and mean crack length is shorter at the end of the WST. Also, at high temperatures, the choice of raw material has an important influence on the Young's modulus, in case WFA is used the Young’s modulus is lower compared to castables with TA. The selected CaO content for castables after sintering is also decisive, as it significantly influences the brittleness. With a higher amount of CaO the brittleness of the castables increases. With raising temperatures, the determined crack length decreases and the deviation from the vertical symmetry line decreases. This was determined by 2P-DIC. Not only the choice of raw material, the binder or spinel addition have a significant influence on the failure behaviour but also the heat pre-treatment. As the heat treatment temperatures rise, the castables reach higher strength and fracture energy values, but the brittleness also increases.

KW - Gießmassen

KW - white fused alumina

KW - tabular alumina

KW - CAC

KW - CMA

KW - Keilspalttest (WST)

KW - castables

KW - white fused alumina

KW - tabular alumina

KW - CAC

KW - CMA

KW - Wedge Splitting Test (WST)

M3 - Master's Thesis

ER -