Admixtures are chemicals added to a concrete mix, beyond cement, aggregate, and water, to change how the concrete behaves — either during placement or over its long-term service life. For watertight concrete specifically, the relevant categories are superplasticizers, which improve flow and workability so the mix compacts densely with fewer voids for water to travel through, and pore-blocking or crystalline agents, which chemically reduce the concrete's permeability from within.

A dense, well-compacted concrete matrix with minimal voids is genuinely the first line of defence against water penetration, before any coating or membrane is even considered — poorly compacted concrete full of honeycombing and voids will let water through no matter how good the surface waterproofing is. This is why superplasticizers, which aren't marketed as 'waterproofing' products at all, often do more for a structure's water-tightness than people expect.

Crystalline and pore-blocking admixtures work on top of that dense base, physically filling remaining capillary pores as they react with residual mix water. They're most valuable on structures that are difficult or expensive to access for repair once built — underground tanks, foundation walls, retaining structures — where a self-healing capacity within the concrete itself is worth more than it would be on an easily accessible, easily re-coated surface.

None of these admixtures replace proper joint sealing. Superplasticizers and crystalline agents both improve the concrete body itself, but construction joints, movement joints, and pipe penetrations are discontinuities in that body and still need waterstops or sealants specifically designed for joints — an admixture can't waterproof a gap that was never continuous concrete to begin with.