Gypsum used in cement manufacturing acts as:
ā³ Understanding Admixtures in Concrete
Admixtures are materials other than cement, water, and aggregates that are added to concrete immediately before or during mixing. They are used to modify the properties of fresh or hardened concrete. This question deals with identifying the specific function of gypsum.
š Detailed Analysis of Admixture Types
(a) accelerator
An accelerator does the opposite of what gypsum does. It speeds up the rate of hydration, reducing the setting time and increasing the rate of early strength gain. Calcium chloride is a common example of an accelerator.
(b) air entraining agent
This type of admixture introduces millions of tiny, stable air bubbles into the concrete. This primarily improves the concrete's resistance to damage from freezing and thawing cycles. It is not the function of gypsum.
(c) plasticizer
Also known as water-reducers, plasticizers are used to increase the workability (fluidity) of fresh concrete without adding more water, or to maintain workability while reducing the water content to increase strength.
(d) retarder
This is the correct answer. A retarder is an admixture that slows down the chemical reaction of hydration, thereby lengthening the setting time of the concrete. Gypsum is the classic example of a retarder that is inter-ground with cement clinker. It specifically slows down the hydration of CāA to prevent a "flash set," giving workers adequate time to place and finish the concrete.
š Common Admixtures and Their Functions
| Admixture Type | Primary Function | Common Example(s) |
|---|---|---|
| Retarder | Slows down setting time | Gypsum, Sugar |
| Accelerator | Speeds up setting time and strength gain | Calcium Chloride, Sodium Chloride |
| Plasticizer | Increases workability / Reduces water | Lignosulfonates, Sulfonated Melamine |
| Air Entraining Agent | Improves freeze-thaw durability | Vinsol resin, Darex |
š” Study Tips
- Gypsum = Retarder = Time: This is a fundamental concept. Always link gypsum with slowing down the initial set to provide more working time.
- Know the Opposites: Understand that a retarder (like gypsum) is the direct opposite of an accelerator (like calcium chloride).
- Categorize Admixtures: Mentally group admixtures by their function: those that affect setting time (retarders/accelerators), those that affect workability (plasticizers), and those that affect durability (air entrainers).