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.
Fillers and additives are fine-grained materials with a fineness similar to Portland cement.
Pozzolans and blast-furnace slag, used as cementitious additives, improve concrete qualities like workability and density while reducing bleeding and cracking.
Fillers can boost the hydration process of Portland cement by providing nucleation points forming calcium carbonate. These nucleation sites are observed in concrete with fly ash and titanium dioxide particles.
Calcium carbonate merges with the calcium silicate hydrate phase, favorably altering the structure of the cement paste once hydrated.
Fillers may also be derived from natural or processed inorganic substances, and they mainly exert a physical effect, such as hydrated lime or aggregate dust.
Typically, these filler materials serve as workability enhancers in grouts and masonry mortar. Color pigments are also considered inert additions.
When powdered zinc or aluminum is introduced in the presence of calcium hydroxide, it produces hydrogen gas. These materials, known as gas-forming admixtures, create aerated concrete.
Hydrogen peroxide, another example, releases oxygen bubbles retained within the sand-cement mixture to produce aerated concrete.
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