Setting control of cement — identify the admixture that increases setting time Which addition is commonly used during cement manufacture to control and increase the initial setting time, preventing flash set during mixing and placing?

Introduction / Context:
Control of setting time is essential to ensure adequate working time for transporting, placing, and finishing concrete. Cement chemistry allows selective adjustment of setting behavior by adding small quantities of specific compounds during grinding.

Given Data / Assumptions:

• Ordinary Portland Cement.
• Focus on additions at the cement mill, not jobsite admixtures.

Concept / Approach:
Gypsum (calcium sulfate) is interground with clinker to regulate the hydration of tricalcium aluminate (C3A). Without gypsum, C3A reacts rapidly, causing flash set. Gypsum moderates this reaction, increasing the initial setting time to practical values while allowing normal strength development. Other listed chemicals are not used for this purpose in cement manufacture; for example, calcium chloride is a set accelerator in concrete, not a cement-setting retarder.

Step-by-Step Solution:

Verification / Alternative check:
Mill certificates and cement standards specify SO3 content limits precisely to achieve desired setting behavior.

Why Other Options Are Wrong:

• Hydrogen peroxide: not a setting regulator in cement manufacture.
• Calcium chloride: accelerates set in concrete; not added to cement grinding.
• Sodium oxide: present as alkali but not used to control set.

Common Pitfalls:
Confusing concrete admixtures with cement intergrinding additions; misinterpreting “increase setting time” versus “accelerate.”

Final Answer:
Gypsum