Firing (burning) of refractories: which of the following changes does NOT occur during proper kiln firing and maturation of refractory bricks?

Introduction / Context:
Firing (burning) converts formed, dried green refractories into strong, service-ready products. Thermal treatment drives chemical and microstructural changes: dehydration, sintering, phase development, and vitrification where appropriate. This item asks you to identify which change does not belong to normal, successful firing.

Given Data / Assumptions:

• Conventional ceramic firing cycles are used for fireclay/high-alumina refractories.
• Crushing strength refers to cold crushing strength measured after firing.
• “Water of hydration” includes chemically combined water in clays/hydrated phases.

Concept / Approach:
As temperature rises, physically adsorbed water is removed during drying, then chemically bound water is expelled (dehydroxylation/dehydration). Sintering densifies the body; glassy phases can form (vitrification) and strengthen the microstructure. Stable high-temperature crystalline phases (e.g., mullite) develop, improving refractoriness. These processes normally increase, not decrease, mechanical strength compared with the unfired state.

Step-by-Step Solution:
Water removal: dehydration and dehydroxylation occur upon heating.Phase evolution: kaolinite → metakaolin → spinel/mullite; glass may form.Property outcome: sintering/vitrification raise cold crushing strength.Hence, “decrease in crushing strength” is not a characteristic outcome of proper firing.

Verification / Alternative check:
Standard QC curves show cold crushing strength increasing with peak firing temperature (until over-firing or bloating). Failures or over-firing can reduce strength, but that is not the intended, normal change.

Why Other Options Are Wrong:
Removal of water: expected during firing.Vitrification: typical in many refractories; extent depends on recipe.Stable mineral development: key to refractoriness (e.g., mullite).

Common Pitfalls:
Confusing abnormal defects (over-firing, bloating, thermal shock) with the nominal trend; mixing drying stage with firing though both remove water at different bonding states.

Final Answer:
Decrease in crushing strength.