In basic building materials chemistry, the typical chemical composition of ordinary Portland cement is best represented by which of the following combinations of raw materials?

Introduction / Context:
This question covers basic knowledge of building materials, especially cement. Ordinary Portland cement is one of the most widely used construction materials in the world. Understanding its main chemical composition helps in appreciating how it sets and hardens, and why certain raw materials are required in its manufacture.

Given Data / Assumptions:

• The question mentions chemical composition of cement in terms of raw materials.
• Options include different combinations of limestone, clay, gypsum and sand.
• We are considering ordinary Portland cement, not special varieties.

Concept / Approach:
Ordinary Portland cement is manufactured by heating a mixture of limestone and clay in a kiln to form clinker, which is then ground with a small amount of gypsum. Limestone provides calcium oxide, while clay provides silica, alumina and some iron oxide. Gypsum is added mainly to control the setting time of cement. Therefore, the best representation of the composition of cement among the choices is the combination of limestone, clay and gypsum.

Step-by-Step Solution:

Verification / Alternative check:
Civil engineering and materials science textbooks describe cement manufacture in two basic stages. First, limestone and clay are crushed and heated to form clinker. Second, the clinker is ground with 2 to 5 percent gypsum. Flow diagrams of cement plants always show these three components. Sand is not usually listed as one of the main raw materials in the cement itself, although it is widely used as an aggregate in concrete. This confirms that limestone, clay and gypsum together represent the composition of ordinary Portland cement correctly.

Why Other Options Are Wrong:

• Limestone and clay only: This combination ignores the necessary addition of gypsum for controlling setting time.
• Limestone and gypsum only: This leaves out clay, which is the major source of silica and alumina.
• Clay and gypsum only: This does not include limestone, the main source of calcium oxide, so it cannot represent cement composition.
• Sand, clay and gypsum only: Sand is not a typical raw material inside the cement composition, and limestone is missing.

Common Pitfalls:
Some learners confuse the composition of cement with the composition of concrete. Concrete is made from cement, sand, aggregates and water, but this question focuses only on the cement itself. Another frequent mistake is to forget the role of gypsum, which is added in smaller amounts but is critical for controlling setting time. Keeping the two stage process in mind, clinker from limestone and clay plus gypsum, makes it easier to remember the correct combination in exams.

Final Answer:
The chemical composition of ordinary Portland cement is best represented by Limestone, clay and gypsum.