Cement chemistry — primary cause of efflorescence on cement-based surfaces Efflorescence on cement surfaces is mainly caused by an excess of which constituent leading to soluble salts that migrate and crystallize on drying?

Introduction / Context:
Efflorescence appears as whitish salt deposits on concrete or masonry surfaces. It is primarily a transport and crystallization phenomenon driven by moisture movement carrying dissolved salts to the surface, where they precipitate as water evaporates.

Given Data / Assumptions:

• Portland cement–based materials with exposure to moisture.
• Presence of soluble ions in cement, water, or aggregates.
• Open pore structure or microcracking that permits moisture migration.

Concept / Approach:

Higher alkali contents (sodium and potassium compounds) increase the risk of soluble salts forming alkali carbonates/bicarbonates. Moisture transports these salts to the surface, creating efflorescence upon drying. Alumina, iron oxide, and silica are relatively insoluble in hydrated cement. Magnesium compounds can contribute to other forms of deterioration (e.g., brucite formation, sulphate attack) but are not the principal cause of efflorescence in typical cases.

Step-by-Step Solution:

Verification / Alternative check:

Standards often limit equivalent alkalis in cement to control ASR; the same alkalis are implicated in efflorescence propensity when moisture movement is present.

Why Other Options Are Wrong:

• Alumina, iron oxide, silica are not present as highly soluble phases causing surface salt blooms.
• Magnesium oxide relates more to expansion/soundness issues than efflorescence.

Common Pitfalls:

• Ignoring external salt sources (groundwater, deicing salts) which also drive efflorescence.

Final Answer:

Alkalis.