LESSON 210 min read
Gypsum & Grinding Aids
Interground gypsum as a sulfate retarder against C₃A flash setting, and chemical grinding aids.
In this lesson: Interground gypsum as a sulfate retarder against C₃A flash setting, and chemical grinding aids.
Gypsum: The Set Controller
Without gypsum, Portland cement would be unusable. The aluminate phase (C₃A) hydrates almost instantaneously when mixed with water, causing flash set — the concrete stiffens within minutes. Gypsum (CaSO₄·2H₂O) is interground with clinker at 3–5% to control this.
The Sulfate-C₃A Reaction
When cement contacts water, gypsum dissolves and reacts with C₃A to form ettringite (calcium sulfoaluminate hydrate) — a needle-like crystal that coats C₃A particles, slowing their hydration. This delays setting from minutes to hours, giving workers time to place and finish concrete.
Chemical Equation
C₃A + 3CaSO₄·2H₂O + 26H₂O → C₃A·3CaSO₄·32H₂O (ettringite)
Ettringite formation: The sulfate from gypsum reacts with C₃A to form a protective coating, delaying hydration and preventing flash set.
Safety · Gypsum Optimization
Too little gypsum → flash set (C₃A hydrates too fast). Too much gypsum → false set (excessive ettringite causes premature stiffening) and potential sulfate attack in hardened concrete. The optimal SO₃ content is typically 2.0–3.5%, determined by ASTM C563 optimization.
Chemical Grinding Aids
Grinding aids are organic compounds (typically triethanolamine (TEA), diethylene glycol (DEG), or propylene glycol) added at 0.01–0.05% by weight of cement. They work by adsorbing onto particle surfaces, reducing surface energy and preventing agglomeration (particles sticking together).
This improves grinding efficiency by 10–20% and increases Blaine fineness at the same energy input. Grinding aids also reduce mill coating (material build-up on grinding media and liners) and improve cement flowability in silos and transport.
This improves grinding efficiency by 10–20% and increases Blaine fineness at the same energy input. Grinding aids also reduce mill coating (material build-up on grinding media and liners) and improve cement flowability in silos and transport.
Pro Tip · Operator Insight
Grinding aid dosage is a powerful, cheap optimization lever. Increasing dosage by 0.01% can increase Blaine by 100–200 cm²/g with no additional energy. But excessive dosage can cause set retardation and air entrainment — always test in the lab first.
Flash Set (No Gypsum)
- Stiffens in 1–5 minutes
- Caused by rapid C₃A hydration
- Large heat evolution
- Cannot be remixed
- Unusable concrete
False Set (Excess Gypsum)
- Stiffens in 5–30 minutes
- Caused by excessive ettringite
- Minimal heat evolution
- Can be remixed (reversible)
- Workable after remixing