CHEMICAL ADDITIVES AND SURFACE TREATMENTS FOR CONCRETE

By W. Serkin, Rocla Research Department

Reproduced from Technical Journals (No. 26 November 1964) of Rocla Concrete Pipes Limited and Rocla Stoneware Pipes Limited with the kind permission of the publishers - The Certificated Engineer July 1965.

Architects and engineers tend, with some justification, to view additives with caution. People have sometimes in the past made extravagant claims concerning additives, and disappointed users have become understandably sceptical. However, there are many cases where additives, which have been produced by experienced manufacturers, and which are based on sound scientific principles, can confer distinctly improved properties to concrete.

According to ASTM definition C 125 all constituents of concrete other than cement aggregates and water, which are added to a concrete mix immediately before or during the mixing procedure, are classified as 'additives.' Additions of pozzolana or hydrate of lime, however, do not constitute 'additives,' because they have cementitious properties.

'Additives' in the practical meaning of the ASTM definition are artificially made chemical compounds.

The chemical industry has, during several decades, produced several materials and processes which have been proved to confer special properties to concrete, which the latter does not sufficiently possess even when made by the best normal procedures of cement and concrete technology.

In this discussion, we will deal with certain rather commonly used and practically helpful additives.

Air-entrainers and plasticisers

About 20 years ago, it was found that finely-divided air distributed throughout concrete conferred useful special properties, including greater plasticity for given water-cement ratio, and superior resistance to damage by frost. Air entrainment has often been discussed in this 'Journal.'

The extremely small, evenly distributed, non-interconnected air bubbles throughout the body of concrete improve workability, reduce the water requirement, reduce or eliminate bleeding and segregation and improve elasticity, resistance to frost and even, to some small degree, resistance to fire and sulphate attack.

To bring this about, very small quantities of an air-entraining compound are added to the gauging water. The optimum amount of entrained air is considered to be 3 to 5% by volume of the mix.

Entrained air plays a part similar to that of the fine sand in the mix; an amount of the latter, corresponding with the volume of entrained air, should normally be omitted from the mix to counteract possible losses of strength caused by the introduction of the 'elastic aggregate'-air.

Another type of chemical additive which tends to impro e workability of concrete mixes is the 'plasticiser' group. There is a fundamental difference between the principles on which air-entrainers and plasticisers act. While a plasticiser works chiefly on the cement and very finest sand particles in the mix, and air-entrainer exerts its ·effect on the larger particles of the mix.

This is demonstrated by making a cement/water paste of 'normal consistency' with the addition of an air entrainer; the latter will not produce a spectacular water-reducing effect. On the other hand, if a plasticiser is added to the gauging water for making the cement/water paste, normal consistency can be attained with a noticeable reduction of the water ratio (e.g. by 5 to 7%). If coarse sand alone-without any addition of cement is mixed with water containing an air-entrainer, such mixture will show all symptoms characteristic of the air-entraining effect.

Air-entrainment requires agitation of the concrete mix; the air is 'caught' during stirring by the coarser aggregate. A plasticiser does not rely upon agitation; it merely renders the gauging water 'wetter' and, by doing so, reduces the water requirements for a given degree of plasticity and workability.

Finally, while an air-entrainer will only be fully effective if the concrete mix contains a substantial quantity of water, a plasticiser works effectively with rather dry or earth-moist mixes.

Neither of the two above-mentioned additives will chemically interfere with the setting and hardening of the cement. In any case, the solids content of the additives is insignificantly small, concerning the quantity of cement (normally a fraction of 1 per cent of cement weight).

Chemical additives having an accelerating effect on the setting of portland cement. The 'initial set' of cement is measured by the time interval that elapses between the addition of the gauging water and the stiffening of the cement paste. Accelerators shorten that interval. The hardening of a cement paste is practically independent of the initial set. A cement may show a rather slow initial set but a relatively fast subsequent evolution of strength.

In general, modern Portland cements set rather slowly in the initial stages, as required by standard specifications. This eliminates the risk of too early a stiffening of a concrete mix, during its mixing or its transport from the mixer to the site of use. But, under certain conditions, for example, at very low temperatures which slow down the setting rate, the use of an accelerator is sometimes desirable.

Amongst the various chemicals which will speed up initial set (e.g. alkalies, common salt and others), calcium chloride is the most suitable. As distinct from other materials, calcium chloride, applied at a ratio not exceeding 2% per weight of cement, does not lead to efflorescence nor other detrimental results. It does not impair the strength of the concrete.

The possibility of small calcium chloride inclusions in reinforced concrete causing rusting of reinforcements has been widely studied. Experience has shown that steel reinforcements will not corrode, if they are well covered with good concrete, provided calcium chloride content is limited to 2% of cement weight.

Most of the acceptable accelerators are based-in more or less modified form-on calcium chloride.

Integral waterproofing compounds

Waterproofing additives have their main use in cement mortars where they are effectively used to repel moisture penetration. There is some risk in using them in the main concrete body, because load-carrying concrete, of course, requires a certain minimum period of moist curing.

If concrete dries out-even superficially-too early, and if it contains an integral water-proofer, re-wetting will be difficult or impossible. In other words, the more efficient the integral water-proofer is, the riskier will be its use where a premature drying of the concrete surface is possible. This consideration does not apply to rendering, or to jointing mortars where maximum strength evolution is not so critical as in concrete. Properly compounded integral water-proofers are therefore not objectionable for cement mortars and cement-lime mortars.

Chemical additives to retard setting

Although compounds of this type may somewhat lower concrete strength, they are occasionally useful as, for example, in cases where concrete is cast under very hot weather conditions or where surface exposure of aggregate is desired for decorative purposes. The possible strength-reducing effect may be counteracted by the simultaneous use of a plasticiser, which will reduce the water requirement of the mix and so will raise the ultimate strength.