Chemical Admixtures

Chemical admixtures are added to concrete in small doses (0.1–2% by cement weight) to modify setting, workability, air content, or durability. They are distinct from SCMs (which replace cement) and additives (which are interground at the cement plant).

Air-entraining admixtures (typically vinsol resin or synthetic surfactants) stabilize microscopic air bubbles (10–1000μm) in the concrete paste. These bubbles provide pressure relief when water in the paste freezes and expands — without air voids, freezing water cracks the paste.

Required for all concrete exposed to freeze-thaw cycles. Typical air content: 4–8% depending on aggregate size and exposure severity. Specified by ASTM C260.

Water-reducing admixtures reduce the water needed for a given workability, or increase workability at a given water content. They work by dispersing cement particles (which naturally clump due to electrostatic attraction), freeing trapped water.

Lignosulfonate-based water reducers are the oldest and cheapest — byproducts of paper manufacturing. They reduce water by 5–10%. They also retard setting and can entrain some air, which may or may not be desired.

PCE superplasticizers are the modern high-range water reducers. They use a comb-like polymer structure: a negatively charged backbone adsorbs onto cement particles, and side chains (polyethylene oxide) create steric hindrance — a physical barrier that keeps particles apart.

This is far more effective than electrostatic dispersion. PCEs reduce water by 20–40%, enabling self-consolidating concrete (SCC) that flows without vibration, or high-strength concrete at very low W/C ratios (0.25–0.35).