Binding agents for refractory mixes: silica bricks are susceptible to basic slags. Which of the following is not used as a binding material by itself when preparing refractory mixes?

Introduction / Context:
Refractory mixes often include binders to impart green strength for handling, shaping, and initial firing. Appropriate binders vary with chemistry (acidic/basic), service temperature, and curing mechanism. Understanding what constitutes a binder—and what does not—prevents processing defects and premature failure.

Given Data / Assumptions:

• Common binders: phosphates (e.g., aluminium phosphate), hydraulic lime/cement, gypsum (plaster of Paris), sodium silicate, and chemical-setting systems.
• Vehicle: water facilitates mixing and workability but is not itself a chemical binder.

Concept / Approach:
A binder chemically sets or hardens to create interparticle bonding prior to firing. Water is a mixing medium; it assists plasticity and dissolves/activates true binders but does not create bonds by itself after drying (it evaporates, leaving weak, friable bodies). In contrast, aluminium phosphate forms phosphate bonds, lime hydrates/carbonates, plaster of Paris hydrates to gypsum, and sodium silicate sets via carbonation or dehydration, all producing binding phases.

Step-by-Step Solution:

Verification / Alternative check:
Processing guides specify minimum water additions with binder solids; without binders, dried shapes lack green strength, confirming water’s non-binding role.

Why Other Options Are Wrong:

• Aluminium phosphate: chemical binder forming phosphate networks.
• Lime: hydraulic/air-setting binder in basic systems.
• Plaster of Paris: sets by hydration to gypsum, providing early strength.
• Sodium silicate: chemical binder that hardens through CO2 gassing or drying.

Common Pitfalls:
Equating plasticity from water with strength; overdosing water, which increases drying shrinkage and cracking.

Final Answer:
Water