Floor finishing practice in building construction: Rubbing a floor with oxalic acid is primarily done to make the surface…

Introduction / Context:
During finishing of stone, terrazzo, or mosaic floors, the final appearance and maintainability depend on the polishing sequence and chemicals used. Oxalic acid is a traditional finishing agent used after mechanical grinding to refine the surface and enhance reflectivity. This question tests practical site knowledge about what oxalic acid rubbing actually achieves on a floor surface.

Given Data / Assumptions:

• Substrate is typically terrazzo, mosaic, or marble, already mechanically ground.
• Oxalic acid is applied as an aqueous solution and rubbed with a polishing pad or machine.
• Objective is to improve the final look and clean minor stains of iron or cement scum.

Concept / Approach:
Oxalic acid is a mild organic acid that chelates certain metal ions (notably iron) and reacts slightly with the calcium-rich cement matrix or carbonate stone. In controlled amounts, it helps close micro-scratches formed during grinding, cleans surface efflorescence/scum, and enhances the optical smoothness, increasing specular reflectance. The result is a glossy, mirror-like appearance often called the “final shine.” It is not a structural process; durability and thickness uniformity are governed by mix design and installation, not by oxalic acid rubbing.

Step-by-Step Solution:
Recognize the operation stage: post-grinding finishing and stain removal.Identify chemical action: oxalic acid lightly reacts and cleans, allowing finer burnishing.Link surface micro-smoothness to increased gloss (higher reflectance).Conclude that “glossy” best describes the primary purpose of oxalic-acid rubbing.

Verification / Alternative check:
Standard terrazzo/marble finishing schedules list successive abrasive grits followed by oxalic-acid polishing or crystallization to produce a bright sheen. Site QS/BOQ descriptions also specify “final oxalic-acid polishing” for gloss.

Why Other Options Are Wrong:
Free from voids: Voids are controlled by mix, compaction, and grouting, not by acid rub.Durable: Durability relates to materials and curing; acid rub does not significantly change mechanical properties.Uniform thickness: Achieved during laying/grinding, not by chemical polishing.Non-slip: High gloss tends to reduce friction; anti-slip finishes are different treatments.

Common Pitfalls:
Overusing oxalic acid (can etch, discolor, or weaken the very top layer); assuming it seals voids or adds structural strength.

Final Answer:
Glossy (polished with a bright sheen).