In limestone terminology for building and cement raw materials, “dolomite” refers to a limestone that contains carbonate of magnesia (MgCO3) up to approximately what percentage by mass? Choose the closest standard figure.

Introduction / Context:
Limestones vary from high-calcium to magnesian and fully dolomitic. The magnesia content governs burning behavior in lime kilns, cement raw mix balance, and suitability for specific construction purposes. This question checks whether you recall the threshold typically associated with dolomitic composition.

Given Data / Assumptions:

• “Dolomite” indicates the mineral CaMg(CO3)2 as a dominant phase.
• We are asked about MgCO3 percentage in a dolomitic limestone.
• Approximate rounded values are acceptable for exam purposes.

Concept / Approach:

Pure dolomite mineral theoretically contains about 45.7% MgCO3 and 54.3% CaCO3 by mass. Hence, limestones termed “dolomitic” approach this level of magnesian carbonate. Therefore, the nearest rounded figure is 45% MgCO3.

Step-by-Step Solution:

Verification / Alternative check:

Materials handbooks cite approximately 45% MgCO3 content for dolomite, distinct from magnesian limestones that contain substantially less.

Why Other Options Are Wrong:

15–35% MgCO3 denotes magnesian limestone rather than fully dolomitic composition.

Common Pitfalls:

Confusing MgO percentage with MgCO3; mixing up “dolomitic limestone” (high MgCO3) with slightly magnesian limestone.

Final Answer:

45%