coloured, against the virtually colour less gel formed by the aluminium salt. The CMC content may vary from some 0.2-4.0 per cent., whilst the gel may be varied from soft to rigid either by in. creasing the proportion of salt or by addition of further materials such as gelatin and sugar.
If the reaction is too quick, retard- ers such as disodium phosphate will in- hibit gelation, and use of acids such as hydrochloric or fumaric acid will shorten gelation time. The gels are
Left photograph shows a normal CMC solution and demonstrates its thixotropic property; (right) a special CMC solution which has no thixotropic and is used where clear, smooth solutions are needed. Photographs are by courtesy of Hercules Powder Co., Ltd.
outward spread and amount of water lost in given times can then be easily measured.
A second difficulty is that of cost. It is not easy for an industry used to low cost bulk materials such as cement to visualise that expensive materials such as cellulose ethers, can be econo- mical. Because they are used in such small quantities, however, for instance in reducing friction in concrete pump- ing mixes about lb. non-ionic cellu- lose ether would be used per cu. yd. of the concrete, the improvements can often be made at little practical cost.
Stabilisation
CMC/water solutions will react with certain salts such as aluminium or ferrous salts to precipitate rigid to soft gels acting as temporary grouts. Such grouts may be prepared by premixing the dry powders or by dissolving the CMC into solution prior to, or after, forming the salt solution. The life of such a grout may be extended to many months by addition of preservatives.
Where the grout has to be identi- fied in the soil, ferrous salts would be preferred as the gel formed is amber
Far East BUILDER, November 1970
resistant to acids, but will slowly be dissolved by alkalis.
An example of stabilisation using CMC was that of tunnel constructed in unstable ground. The tunnel was de- signed for an in-situ concrete lining and, prior to concreting, a CMC/salt gel was placed in the void between formwork and soil to prevent soil from falling and filling the space. The gel was soft and easily displaced by pres- sure of the plastic concrete during the lining operation.
The CMC/aluminium reaction was also suggested and employed to solve a problem which developed recently where a large tunnel machine was being employed to bore through sand- stone beneath a river estuary. In parts, high water flows were encountered, and these caused a build-up of very wet slurry in the void carved but by the cutters directly ahead of the machine face, on each occasion that the machine was stopped. On starting up, this very wet slurry was deposited on the conveyor belts, causing slipping and blockages on the belts.
It was quickly determined by a site trial that a small amount of CMC
powder, dispersed in a matrix of high alumina cement and thrown into the void ahead of the face directly before starting up and then stirred in by the cutters for about a minute, gelled and dried up the water initially present so that a virtually dry sand was brought
out.
This method was employed with success on further portions of the tun- nel. It is possible that where econo- mics justify it, other wet wastes of suitable composition could be dried out in this way for transportation as solid materials.
Cellulose ethers have been supplied on occasions for intermixing with soils during formation of temporary earth- fill dams, giving greater impermeability to the earth. With a preservative add- ed, a life of up to about one year could be expected. CMC is suggested
a thickener for lignosulphonate solutions, when used at a concentra- tion of 0.2-1.0 per cent. in water, and in dry parched soils, cellulose ethers may help by retaining water, thus act- ing as a mulch.
In one proprietary hydraulic seed- ing formulation, CMC is used in com- bination with a synthetic resin diluted in water where, again, it acts as a thickener and allows the film produc- ed to be employed on steeper slopes a as well as promoting the action of soil bacteria and replacing humus. About 12.5-25Kg (27.5-55lb.) of CMC are used in the mix per 2000 sq. m. (2392 sq. yd.) of area seeded.
Friction reduction
The highly viscous cellulose ether solutions with their water retention. and homogeneity under presure are well suited for forming slippery, lubri- cating films. Used in less happy parts for controlling crowds by the 'banana peel' principle, they can be employed in construction for reducing friction in such operations as thrust boring, jack- ing tunnel segments against clay or in sinking caissons.
Here the ability to form the solu- tion by hand stirring is a useful site attribute. The action is different from that of other alternatives such as ben- tonite as friction will be restored when enzyme action destroys the solution properties.
In caisson sinking, for example, an oversize hole is cut out initially by a V-shaped cutting edge around the cais- son base. The void between soil and caisson sides is filled by a clay slurry, usually bentonite. A solids concentra-
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