Cement composition — which chemical constituent is present in the smallest percentage by mass? Considering the main oxides in ordinary Portland cement, the minimum percentage among the following is typically that of:

Introduction / Context:
Ordinary Portland cement consists mainly of lime (CaO), silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3), and smaller amounts of other oxides. Their typical ranges help identify conformity and predict performance.

Given Data / Assumptions:

• Typical OPC composition ranges.
• Comparative, not exact, values are needed.

Concept / Approach:

In OPC, CaO is the largest fraction (about 60–65%), followed by SiO2 (about 17–25%), Al2O3 (about 3–8%), Fe2O3 (about 0.5–6%), and MgO (up to about 5%). Among the listed oxides, iron oxide is generally the smallest proportion on average across common cements.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturer's datasheets and standards list similar ranges confirming Fe2O3 as the smallest among the major oxides.

Why Other Options Are Wrong:

• CaO and SiO2 are major components; Al2O3 and MgO exceed Fe2O3 in many OPCs.

Common Pitfalls:

• Confusing trace constituents with major oxides; the question refers to the principal chemical ingredients.

Final Answer:

Iron oxide (Fe2O3).