Fireclay brick strength at high temperature: an ordinary fireclay brick has a cold crushing strength of about 950 kg/cm^2. After exposure to around 1500°C, the crushing strength may fall to approximately how low (kg/cm^2)?

Introduction / Context:
Refractory selection and lining design require realistic expectations of strength retention at temperature. Cold crushing strength (CCS) is measured at room temperature, but service at elevated temperatures can drastically reduce brick strength due to microstructural changes, glassy phase development, and thermal damage. This item tests recognition of the order-of-magnitude reduction for ordinary fireclay bricks after severe thermal exposure near 1500°C.

Given Data / Assumptions:

• Cold CCS ≈ 950 kg/cm^2 for an ordinary fireclay brick.
• Exposure temperature ≈ 1500°C (very high for fireclay class).
• Question asks for “as low as” value—worst-case indicative figure.

Concept / Approach:
At very high temperatures, fireclay bricks may experience softening of the glassy matrix, grain-boundary weakening, and increased porosity from thermal cycling. Consequently, hot strength (or residual room-temperature strength after exposure) can drop dramatically to a small fraction of the original CCS. Among the options, 150 kg/cm^2 reflects a severe yet plausible lower bound in line with typical data charts for ordinary (not super-duty) fireclays subjected to extreme temperatures near 1500°C.

Step-by-Step Solution:
Baseline: CCS_cold ≈ 950 kg/cm^2.Thermal exposure: approach 1500°C → matrix softening, microcracking.Result: strength may fall to a small fraction (on the order of 10–20%).Pick closest “as-low-as” figure: 150 kg/cm^2.

Verification / Alternative check:
Refractory property tables often show large drops in compressive strength for clay-bonded bricks above their safe working range, reinforcing that a reduction to ≤ 200 kg/cm^2 is reasonable as a lower-bound benchmark after such exposure.

Why Other Options Are Wrong:
450 or 250: could occur at lower temperatures; do not represent the “as-low-as” extreme queried.65: excessively pessimistic for ordinary fireclay in many datasets.900: essentially unchanged—unrealistic after 1500°C exposure.

Common Pitfalls:
Confusing hot modulus of rupture with cold crushing strength trends.Assuming linear decline with temperature; degradation can accelerate near softening ranges.

Final Answer:
150