Method of Making Flexible Cementitious Materials

This invention relates to a method of making flexible materials.

In British Patent Application No. 1,512,781 there is disclosed, inter alia, a method of building employing concrete structures and including the steps providing between at least two adjacent concrete structures a load bearing portion comprising a concrete composition containing a flexibilizing agent in the form of an elastomeric polymer, the proportion of the flexibilizing agent when calculated as polymer in the composition when set being 2 to 50% by weight. That invention is applicable to buildings, in particular tunnels, where adjacent concrete structures which are cast in situ or precast abut one another and there is a tendancy for spalling to occur. Spalling is the undesirable splitting, splintering or chipping of the concrete which tends to occur when the concrete is subjected to high local stresses. It is particularly likely to occur in tunnels, where there is great pressure from above and the joints between the concrete structures go into compression as the gap between them deforms.

The present invention is a result of efforts to perfect a material for use between adjacent concrete structures, in particular in tunnels, although it may of course have other applications.

It is proposed to make a flexible cementitious material by mixing a flexibilizing agent in a latex with cement and, preferably, another dessicant, and subjecting the material to pressure after initial setting of the cement, whereupon the material acquires flexibility and improved cohesion while retaining strength.

The compression is preferably performed by rolling, and appears to have the functions of "knitting" the flexibilizing agent, filling any voids there may be and crushing the initially set cement. As a result, the brittle characteristic which would otherwise be apparent is removed and flexibility is acquired and is maintained even though secondary setting of the cement may occur later on.

Preferably, as mentioned, a suitable secondary dessicant in addition to cement is employed.

The cement and dessicant can be added to the latex together, or in either possible succession.

We have found that the latex, cement (and possible secondary dessicant) mixture should, after shaping for instance into boards be initially cured for about 20 to 40 days, for instance 28 days, in conditions of high relative humidity, e.g. 90 to 100%. The compression operation preferably follows a subsequent period of 5 to 10, e.g. 7 days, in which the material is stored under ambient conditions.

As previously mentioned, the materials referred to in British Patent Application No. 1,512,781 require the flexibilizing agent when calculated as polymer in the set composition to be 2 to 50% by weight. In the present Specification different criteria are adopted, and it is suggested that mixtures of, by weight, 15 to 28% latex, 20 to 30% dessicant, 45 to 55% cement with up to 6% of other ingredients are suitable. One particular mixture which may be suitable comprises 22% latex, 50% cement, 24% dessicant and 4% other ingredients. The cement may be Portland, high aluminous sulphate resisting cement or any other appropriate cement. Examples of dessicants are silica gel, a calcium sulphate plaster (for example a hemi-hydrate plaster containing if necessary an accelerator of the rate of setting of the plaster), barium or calcium oxide, calcium chloride, or orthoester (for example trimethyl orthoformate) or a molecular sieve, or quicklime. Among the other ingredients there may be mentioned light oils, stabilisers, antioxidents and surfactants. The light oils can be added to slow down the action of the secondary dessicant. The antioxidents may be of substituted or hindered phenol type, e.g. 2,2'-methylene bis (4-methyl-6-tertiary butyl phenol). The surfactants may be anionic surfactants, e.g. alkyl aryl sulphonates or straight chain alkyl sulphonates.

The flexibilizing agent can be an elastomeric polymer, natural or synthetic and is initially in the form of an aqueous dispersion or latex. Butyl rubber is one possibility. A latex with 30-70%, preferably 40-50% solids is most appropriate. Non-ionic stabilisers (for example polyoxyethylene esters of phenol or long chain aliphatic alcohols) may be usefully added among the other ingredients to prevent premature precipitation of the latex.

The normal setting behaviour of the cement may be modified by impurities or additives. Thus accelerators or retarders may be required. Examples of accelerators are sulphuric acid, potassium and ammonium sulphate while borax is one possible retarder.

The material can, when mixed, be cast, moulded extruded or calendered and then subjected to the initial setting operation. The material can be so prepared in any suitable shape and size, for instance in the form of boards of 0.03 to 10 cm thick.

Good results have been achieved by initially adding lime, cement and the other ingredients to latex in a high powered high speed mixer, casting the mixture into moulds to form boards, storing them for 28 days at nearly 100% relative humidity, then storing them for 7 days in ambient conditions, and then passing them between nip rolls. The resultant material contains a lot of moisture so that secondary setting can occur, but is remarkably flexible, without cracking, and can be subjected to great stress and compressive strain without breaking down.

In addition to the above described process, the invention provides materials made by such process.