November_1970 — Page 30

How chemicals can

aid

construction

by R.G.H. Boyes, MICE

WITH the growth of chemical knowledge and experience, the real and potential applications for chemical materials and products in building and civil engineer- ing are much greater than is probably realised. Though it may add to the in- formation burden of the designer and builder, a study of some of the main properties and typical uses of chemi- cals may on many occasions provide the solution to difficult or day to day problems, improve the technical quali- ty of the work, and reduce costs.

Cellulose ethers, water soluble chemical derivatives of purified alkali cellulose, are particularly adaptable, from the variety of types available and the effects that their properties can produce, to constructional use. In fact, they find applications in most indus- tries, being found in food and pharma- ceutical products and they even help to give a whiter wash, when used in synthetic detergents.

Some of the important grades and proterties of cellulose ethers are as follows:

Depending upon the manufacturing process and the degree of refinement in manufacture generally, a number of types and forms may be obtained from which the correct grade is selected. There are two main groups, ionic and non-ionic cellulose ethers.

Viscosity

The main commercial example of the first group is sodium carboxy- methyl cellulose, usually abbreviated as CMC. The non-ionic group includes hydroxyethyl-cellulose (HEC), methyl cellulose and its modifications and ethyl hydroxyethyl-cellulose. Anions, some cations, and certain metal salts and acids affect ionic cellulose ethers whilst non-ionic forms remain stable over wide range of pH values and in the presence of common building materials such as cement and plaster and acids and strong alkalis.

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medium and high viscosity types. Thus low viscosity grades are often employ- ed in such situations as grouting or drilling where water retention is need- ed, but high viscosity would increase pumping requirements and lower the obtainable solids concentration.

High viscosity grades are used where thickening power is the main criterion, such as in certain adhesive formulations. Cellulose ethers may be supplied suitable for low speed and hand mixing or for mixing by high speed shearing, the former would for instance be supplied for incorporation in mortars, whilst the latter would be used in collodial grout mixes. Solu- tions are best made up in water of a neutral or alkaline nature, taken as a general rule.

As supplied, cellulose ethers are white powders which are dissolved in cold or hot water to form solutions, rather than suspensions of solids in water as with clays or cement. CMC solutions are generally thixotropic, though again adaptability is such that special non-thixotropic forms are avail- able. High viscosity non-ionic forms are generally pseudoplastic.

Thixotrophy is a time dependent change in consistency, the viscosity varying from a gel to a fluid and back as agitation or shear is applied and re- moved. With pseudoplasticity this merely means that this change in con- sistency occurs very rapidly, and is more marked the greater the degree of agitation. Thixotrophy is desirable where other materials are suspended in solution. Engineers should not be put off chemicals by the often long, but usually easily explained, terms used in describing their properties.

An important reason for using cel lulose ethers is the high water reten- tion or low 'fluid loss' characteristic. The higher the solution viscosity the higher its water retentivity. Greater amounts of a low viscosity grade are All forms are available as low, more effective than smaller amounts

of a higher viscosity grade for a given viscosity, but cost must be taken into consideration.

Through this property, cohesion of water based mix is retained longer, the fluidity improved and segregation and bleeding reduced as well as water loss to porous materials such as aerat- ed concrete blocks, or evaporation from mortars, plasters and stuccos in hot weather.

Stability

When used neat, cellulose ether solutions may have a limited life, as they are broken down in time by bacteria from soil and other sources. Thus, if required to resist such attack for periods longer than a week or so, a preservative should be employed and an on-site trial carried out if possible. Preservatives include organo-mercurial agents added at 0.01 per cent., and substituted phenols at up to 0.1 per cent. of the cellulose ether.

'Protective colloid' is a term for a useful property which can be likened to a protective film formed around the individual particles carried by a cellu- lose ether solution, thus keeping the particles separate through repulsion of other particles.

Two examples of this in practice are the inclusion of a number of sepa- rate colours within a single paint form- ulation to give a multi-coloured coat- ing when applied, and protection of drilling muds, grouts and diaphragm wall trench slurries against flocculation by salt water or other electrolytes.

Non-ionic forms are slightly surface active. The action is often only slight, but the surface tension of water can be reduced from 72 to 50 dynes/sq. cm. by one available form. Thus small quantities of air can be entrained, and this should be remembered during de- sign of mortar mixes, etc.

A number of minor misunderstand- ings may occur when chemicals are employed in construction. The first is

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that test methods have been devised for industries other than building and civil engineering and the instruments and properties are not always easily understood in, or always relevant to, the building industry.

A useful simple test for water re- tention in building materials has been suggested by British Celanese, Ltd. A small cylinder containing the mix is placed over a filter paper on which are drawn a number of concentric circles, and mounted upon a glass plate. The