Page 190 - Handbook of Thermal Analysis of Construction Materials

174 Chapter 4 - Introduction to Concrete Admixtures

acids and their salts, and organic salts of sulfonated hydrocarbons are
examples of air-entraining agents.
The air-entraining action is known to involve adsorption at the air-
water or solid-water interfaces. The soluble surfactant ions are adsorbed on
cement particles making them hydrophobic, so that as bubbles are gener-
ated during mixing, they adhere to the cement. This process stabilizes
bubbles, preventing their coalescence. Lowering of surface tension may
also stabilize bubbles.
Non-ionic air-entraining agents have no appreciable effect on the
rate of cement hydration. Even if some of them possess retardant behavior,
the amounts used are such that the effects are not significant. It is known that
anionic agents such as sodium vinsol resin react with calcium hydroxide to
form a precipitate of calcium salt. When air-entraining agents are used with
other admixtures, the interaction between them and the cement has to be
considered for compatibility purposes. At higher dosages, the anionic air-
entrainers may retard the C S hydration. [82]
3
Many factors influence the amount of entrained air. They include
dosage, slump, aggregates, temperature, inclusion of other admixtures,
chemical composition of cement, mixing, vibration, etc.
Generally, a strength loss occurs in air-entrained concrete. As it
also allows the reduction of w/c ratio, the loss in strength may partly or
wholly be offset.

6.0 MINERAL ADMIXTURES

Mineral admixtures are finely divided materials that are added in
relatively large amounts as replacement of cement and/or of fine aggregates
in concrete. Some of them also possess self-cementitious properties in
addition to being pozzolanic. A pozzolanic material is one that is able to
react with calcium hydroxide forming a cementitious material. Natural
pozzolans such as volcanic tuffs, earths, trass, clays, and shales (raw or
calcined), and industrial products such as fly ash, slag, silica fume, red mud,
and rice husks are the main source of mineral admixtures. Extensive work
has been carried out on fly ash, slag, and silica fume.
The mineralogical composition, particle morphology, size, and
physical make-up of the mineral admixtures control their reactivity to a
relatively larger extent than their chemical composition.