Additives and fillers are integral to enhancing the properties of concrete. Pozzolans and blast-furnace slag are additives or admixtures due to their reactions with calcium hydroxide released during cement hydration. Fillers, which are finely ground and similar in fineness to Portland cement, improve concrete attributes such as workability density, and reduce capillary bleeding or cracking. Some fillers possess hydraulic properties or participate in benign reactions within the cement paste.

The role of fillers is to promote Portland cement hydration, providing nucleation sites, a role often played by fly ash and titanium dioxide, which contribute to the microstructure of the resultant cement paste. Fillers can be natural or processed, with consistent properties, notably fineness. They should not increase water demand or adversely affect concrete's durability and strength.

Fillers and additives are mostly physically mixed with cement and must be compatible. For example, some very fine cement particles are essential for early strength. Inert materials like hydrated lime or normal-weight aggregates are added to improve workability, not strength. Additives that release hydrogen in the presence of alkalis or calcium hydroxide are used in making gas concrete or aerated concrete, providing thermal insulation. Gas-forming admixtures like hydrogen peroxide generate oxygen bubbles in aerated concrete mixes.