Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
1999-11-10
2001-06-19
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C428S404000, C428S920000
Reexamination Certificate
active
06248166
ABSTRACT:
This invention relates to a method of treatment of shell sand and to the use of shell sand thus prepared in cementitious products.
In this specification the term “shell sand” includes also coral sand, and other finely-divided particles of basically calcareous material originating from marine animals such as coral, molluscs and the like. The chemical composition of shell sand is typically a mixture of calcium carbonate (calcite, aragonite and vaterites) and organic matter, plus sea salt. It is lower in density than mineral (silica) sand and is made lighter by an inherent porosity.
Shell sand has previously been used in cementitious compositions such as lightweight concretes and non-slip surfaces. This invention is based on the fact that a suitably processed shell sand gives an unexpected and very valuable result. The shell sand used in previous applications has been finely ground to a desired sieve size and then heated to around 350° C. to drive off the water and decompose any organic material present. However, this heating also has the effect of removing much of the natural elasticity and porosity of the shell sand. It is an important feature of this invention that the elasticity and porosity be retained to a high degree, because it has been found that such shell sand can be used in the preparation of a cementitious composition which has a high degree of fire- and heat-proofness.
The invention therefore provides a method of preparation of a shell sand comprising heating the shell sand at a temperature of less than 300° C. for a time sufficient to remove substantially all of the water and organic content while maintaining substantially all of the inherent porosity of the shell sand. Such a treatment regime has not been previously used and it results in a shell sand ready for use which has unique parameters and properties. The invention therefore also provides a dried shell sand having a particle size of 15 mm maximum and a porosity of from 10-25% by volume. Shell sand currently available generally has a particle size of 1 mm maximum and a porosity of no higher than 5%.
The process of preparing the shell sand according to this invention consists of heating at low temperatures for suitable times, in contrast to the processes of the known art. Prior to the heating stage, the shell sand is optionally washed with fresh water to remove the sea salt and some of the organic matter. In many cases, it is not important to do this, but it should be done where there will be use of the shell sand in conjunction with ferrous metals, for example, steel reinforcing used in conjunction with concrete or cement, as the salts can cause corrosion, leading to early failure.
The shell sand is preferably heated to a temperature of no higher than 300° C. It is surprising that, at this temperature, only 50 degrees lower than that used by the art, a considerable difference in retained porosity is observed. Preferably, the temperature should not exceed 250° C. The advantages of the present invention can be given at much lower temperatures, but it is preferred to go no lower than 95°-120° C., 100° C. having been found to be an especially useful maximum temperature in this lower range. Naturally, the lower the temperature the higher the degree of porosity retained, but the longer the heating time required. The heating may be carried out by any convenient method known to the art, one convenient apparatus being a tumble dryer which is pre-warmed to an appropriate temperature.
The time of heating needed may be readily determined for any given temperature and any given shell sand by the skilled person by means of simple experimentation. In the case of the maximum temperature of 300° C., the heating time should be no longer than 10 minutes, and shorter times of the order of 3-7 minutes may be used (5 minutes is a useful time in this range). Should lower temperatures be desired or necessary, the time may be longer, up to one hour being typical. It is possible to heat at these temperatures for longer, but the economics of the process then become less favourable.
When the heating is finished, the shell sand is, if necessary, reduced to the correct size. Ideally, the particle size should be no greater than 15 mm, preferably no greater than 6-10 mm, most preferably with most of the particles of size no greater than 8 mm. The shell sand produced by this process is a light, airy material with a pore volume of from 10-25%, preferably between 15% and 25%. The heating hereinabove described will drive off at least most, and in many cases all, of the water in the shell sand, and the material will feel dry. However, in some cases there may remain sufficient water such that the shell sand cannot be mixed with cement in a dry mortar composition without an undesirable degree of reaction taking place on storage. The removal of any final water may be achieved by art-recognised methods, using gentle heating (for example, at a temperature slightly above room temperature), preferably combined with agitation or forced ventilation. Thus, for example, when tumble drying is used, the heating hereinabove described can be followed by a period of tumbling at low temperature.
The shell sand can be employed in cementitious compositions. The invention therefore provides a cementitious composition, comprising at least one hydraulic binder and a shell sand as hereinabove described. Such cementitious compositions can, for example, be concretes, which can be cast or otherwise fabricated into blocks, pipes or other desired forms. The presence of the shell sand hereinabove described confers excellent flexibility and a surprisingly high degree of fire- or heat-proofness on the hardened compositions.
The hydraulic binder may be any hydraulic material known to the art. It may, for example, be Portland cement, high alumina cement, slag cement, sulphate-resistant cement or any of the other standard types, or mixtures of any of these types. The selection of a suitable cement or cements is completely within the skill of the art, readily determinable for every application.
Aggregate is a normal component of cementitious compositions and any aggregate known to the art may be used in the cementitious compositions of this invention. Aggregates used in the art range from very fine sands to coarse rock fragments, the sands being used in mortars and the coarser aggregates in concretes.
To the cementitious compositions may be added any of the known admixtures in art-recognised quantities to achieve expected effects. Examples include, but are by no means limited to:
workability-improving and water-reducing additives, such as &bgr;-naphthalene sulphonate-formaldehyde condensate (BNS), lignosulphonates and carboxylates;
accelerators, such as calcium chloride (where the chloride is not objectionable) and triethanolamine;
reinforcing fibres, such as glass, steel or plastics fibres, typically of the order of 10-20 mm in length.
Particularly good workability-improving additives for use in conjunction with this invention are the alkoxylated styrene-maleic anhydride copolymer types described in European Published Application 0 306 449, German Offenlegungsschrift 41 42 388 and PCT Application WO 97/39037, and the carboxylated acrylic polymer types described in European Published Application 0 753 488.
An especially useful application of the shell sand of this invention is in the manufacture of mortar compositions. Such compositions have become of great interest to the tunnelling industry in particular, in the aftermath of the investigation of several fires in tunnels, especially in the Channel Tunnel, where it has been found that concrete lining is not so impervious to fire as was previously assumed, and that severe and sometimes potentially disastrous cracking can result. The invention provides a fire- or heat-resistant mortar composition, comprising at least one hydraulic binder and a shell sand as hereinabove described.
The invention also provides a method of providing a concrete substrate with fire or heat resistance, comprising the application thereto of a mortar comp
Fish & Richardson P.C.
Marcantoni Paul
MBT Holding AG
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