November_1970 — Page 32

tion of 8-11 per cent bentonite may be employed to prevent travel around the cutting edge. Where frictional grip must be restored after sinking, grout holes are drilled from inside and ce- ment grouting carried out to displace the gelled slurry.

It is suggested that this operation would be made much easier if a major proportion of the bentonite clay solids were replaced by a high viscosity cellu- lose ether at a very small proportion. Natural degradation of the cellulose ether constituent, or breakdown by ar- tificially introduced enzymes, would produce a low viscosity solution more easily displaced by grout.

Preliminary tests have shown that a life of at least 21⁄2 weeks may be attain. ed by thin layers of cellulose ether solutions in topsoil layers very active in soil bacteria. This technique might also enable drainage trenches to be dug out without the danger of bentonite gel blocking the gravel fill, or water wells to be constructed without reduc- tion of water extraction due to the filter cake gel.

Oil well drilling

An improvement in the flow characteristics of grouts based upon cement and cement/sand can be made using non-ionic cellulose ethers, enabl- ing pumping pressures to be reduced and with lower segregation and bleed- ing under pressure. Considerable ex- perience in this application has been built up through oil well drilling use, where cement slurries with high water retention properties are needed and used in porous formations.

Carboxymethylchydroxyethyl cel lulose (CMHEC) has been the particu- lar type used for reducing water losses to open strata. The thickening and setting times of the cement are affect- ed, cellulose ethers in general having a retarding effect on this property. CMC is also employed and as an example the initial set of an oil well cement at 49C (120F) without CMC was 2hr. 27min., with 0.2 per cent CMC this was increased to 4hr. 12min.

It was suggested that optimum re- tardation in these conditions was given by CMC concentrations of between 0.06 and 0.15 per cent of dry cement weight. An upper limit of 0.3 per cent. CMC was placed as tests have shown that 0.5 per cent. would destroy or strongly inhibit the cement setting properties. Based upon this use, cellu lose ethers have been investigated as a means of increasing the pumpability of

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concrete by Britain's Building Re- search Station.

Additions of methyl cellulose of about 0.1 per cent. of the cement weight are said to have been used in the major part of this investigation, and it was concluded that cellulose ethers as thickening additives be used for mixes of 1:6 to 1:10 cement: ag- gregate ratio.

The effect of the cellulose ether solution was to provide a cohesive matrix for the aggregate skeleton, which might allow more difficult ag- gregates such as crushed rock or lime- stone to be pumped more easily or normal mixes to be employed at lower water contents. A degree of retarda- tion was noted, but was not consider- ed to be significant.

Due to their stability to acids, non- ionic grades can be employed to thick. en and bind acidic solutions. They are employed in this way in paint strip- ping formulations where they enable the remover to adhere to vertical sur- faces and are also tolerant to the wide number of other additives included in the product. They are also used in acid fracturing of rocks and acidising of oil wells.

In the latter use, oil well forma- tions composed of limestone or dolo- mite are treated with the thickened acid to enlarge the pores and to de- velop connecting channels by dissolv- ing out parts of the formation. It was developed as early as 1894 by Frasch and Dyke in the USA. The cellulose ether solution prevents segregation during travel down the tube under pressure and then enables deeper and more effective penetration of the lime- stone, preventing the acid from merely forming deep fissured zones in the im- mediate vicinity of the borehole wall. Thus the formation is actually broken down by acid and other materials in- jected under pressure. Sand is often in- cluded as filler to keep the fracture from closing.

In oil well drilling, CMC is used as a 'fluid loss' agent by preventing floccu- lation of the drilling mud in the pre- sence of salt, gypsum and anhydrite (impure gypsum) and an undue viscosi- ty rise in these situations, which would lower carrying capacity and increase pumping pressures. The filter cake of high solids content mud formed at the borehole wall is improved in quality and permeability and being thinner, forms less of a barrier to movement of the drill stem. A similar improvement against flocculation in saline water against flocculation in saline water

would be noted in bentonite slurries for trench excavation support and bet- ter wall retention capability.

The properties of improved flow and workability, better water reten- tion and cohesion, ease of mixing and compatibility with a wide range of components have led to cellulose ethers finding very wide applications in the building industry.

When used in cement mortars in quantities from 0.25-0.5 per cent. by weight of cement the non-ionic cellu- lose ethers give water retention and workability to the mix through in- creased cohesion. If they were not used finer sands would be employed to make the mix more workable, but these do not produce such a good mortar as the harsher sands made pos- sible by the additive. This latter fact should offset the additional cost of the cellulose ether. In comparative tests bond strengths of mortars containing methylcellulose were some 3000lb. load against 1200-2000lb. load for Portland cement lime mortars.

Cellulose ethers are compatible with air entraining agents and are re- commended for use with these and aluminium powder to improve the bubble structure. The non-ionic grades, being mildly surface active, will entrain air themselves, i.e. the air con- tent of a freshly mixed Portland ce- ment lime mortar was about 5 per cent., of a masonry cement mortar about 17-20 per cent. against 20-24 per cent. for a methylcellulose mortar.

A similar improvement is stated to be gained in aerated cement blocks whilst with p.f.a. aggregate, an addi- tional advantage is that any low densi- ty particles of unfired carbon present do not accumulate at the surface, thus giving an improved appearance. Pro- prietary additives for sand/cement grouts for filling cable conduits may combine air entraining agents or alumi- nium powder with cellulose ether, leading to better placement charac- teristics and strength.

Many cement and gypsum based products already benefit from the in- clusion of cellulose ethers. Water re- tention and thickening are the main advantages again and lead to the elimi- nation of the need for pre-wetting the base, better adhesion and the ability to form thin films by the increased cohe- sion and the reduction in losses to the substrate, particularly where absorbent materials such as aerated blocks are used, a slightly longer application period through the slight retardation

Far East BUILDER, November 1970