Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
2001-05-15
2002-05-14
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S737000, C106S823000
Reexamination Certificate
active
06387173
ABSTRACT:
The invention relates to a concrete composition and a method for the preparation thereof. The invention further relates to the use of the prepared concrete composition. The invention especially relates to a highly fluid concrete composition containing a silica sol and a polycarboxylate superplasticiser.
BACKGROUND OF THE INVENTION
Concrete is an artificial material commonly used as a building material, in which aggregates or ballast of suitable sizes are bound together by a matrix of at least one hydraulic binder, which usually is of cement type, e.g. Ordinary Portland Cement. Concrete mixtures may also contain various additives to change the properties of the mixture e.g. to modify setting and hardening conditions, or to modify rheological properties in order to attain a desired workability or fluidity of the mixture.
When a concrete mixture is to be cast, it is necessary that the concrete is maintained in fluid form to keep it workable until the casting moment. It is thus of great importance that the concrete mixture has a certain degree of workability and that it does not start to harden or set in advance. Today's concrete producers and building contractors often face the problem of too short workability time of the concrete. Sometimes, the transport of the concrete from the concrete factories to the building sites takes too long time so that the concrete loses its flowability.
It is a further concern that the cement particles and the aggregates should not segregate. Segregation usually results in an undesirable or even uncastable concrete.
Previous methods to prepare fluid concrete mixtures have involved vibration energy input after placing resulting in expensive installation and operating costs due to indispensible compaction and vibration equipment and extensive need of control personnel. Various attempts have been made to overcome these problems. Rheological modifiers such as cellulose ethers, welan gum, silica fume, and precipitated silica and acrylic copolymer dispersions have been added to concrete mixtures to obtain a vibration-free fluid concrete mixture with sufficient workability. These modifiers, however, have been shown to increase the viscosity of the concrete mixture over time thereby decreasing the workability.
EP-A-0931 030 discloses a concrete mixture comprising aggregates, hydraulic binder, and a silica sol having a specific surface area below 200 m
2
/g. EP-A-0931 030 aims at improving the compressive strength by providing a silica sol to a concrete mixture wherein the silica sol particles have a relative standard deviation higher than 30%. According to EP-A-0931 030, superplasticisers of e.g. sulphonated naphtalene-formaldehyde resin, sulphonated melamine-formaldehyde resin, or sulphonated melamine-urea-formaldehyde resin may be added to the concrete mixture. These types of superplasticisers only provide improved workability for a very limited period of time far too low to be acceptable under ordinary working conditions as the mixture must be vibrated to keep it in essentially fluid workable form. These conditions are far below the demand of the concrete industry of today, especially for self-compacting concretes.
U.S. Pat. No. 6,008,275 discloses a cementitious mixture comprising low-grade cement, e.g. Ordinary Portland Cement partially replaced with pozzolanic cement containing e.g. fly ash, slag, and natural pozzolans. The cementitious mixture also comprises a polycarboxylate polymer water reducer and an accelerator.
The present invention effectively provides a non-bleeding high fluidity concrete composition, i.e. a concrete that does not require any energy input or only some energy input after placement in moulds, and a method for preparation thereof.
THE INVENTION
According to the present invention, a new concrete composition have been provided in order to solve the above-mentioned problems. The new concrete composition comprises a hydraulic binder, aggregates, water, silica sol and a superplasticiser being a polycarboxylate.
It has been surprisingly noticed that the concrete composition of the present invention can attain a workability time of about 1-2 hours or more, depending on how much silica sol and polycarboxylate superplasticiser is added, essentially without any static bleeding, i.e. without any substantial segregation of cement particles and aggregates or separation of water in the static non-vibrated aqueous concrete mixture. This is due to the combination effect of a polycarboxylate superplasticiser and a preferably aqueous silica sol which together are capable of increasing the workability of the concrete mixture while suppressing bleeding.
By workability time is here meant the time elapsed between the mixing moment of a superplasticiser, in order to increase the workability, with a concrete mixture and the point of time when the workability has decreased to the initial workability level, i.e. before addition of the superplasticiser. Furthermore, it has surprisingly been noticed that the concrete composition according to the present invention also provides an improved compressive strength compared to compositions not containing a polycarboxylate superplasticiser or compositions containing a polycarboxylate but not containing a silica sol. The admixture of both a polycarboxylate and a silica sol improves the compressive strength due to the synergistic effect that arises when the two components are added to the concrete mixture.
The hydraulic binder may be any cement, e.g. Ordinary Portland Cement (OPC), blast furnace slag cement or other classes of cement comprising slag, fly ash or other cements as described in the literature, e.g. U.S. Pat. No. 6,008,275.
Aggregates are preferably composed of stones, gravel and sand, and commonly having an average particle diameter range from about 0.01 to about 100 mm. In this context, mortar, only containing aggregates with a maximal particle size of from about 2 to about 4 mm, e.g. sand, and a hydraulic binder should also be mentioned. For simplicity, mortar is here meant to be incorporated in the term concrete.
The silica sol added to the concrete mixture suitably comprises silica sol particles having an average particle diameter range from about 2 to about 200 nm, preferably from about 3 to about 100 nm. It has been found that silica sol particles effectively prevent bleeding due to their capability of binding water molecules, in contrast to e.g. silica fume which has a hydrophobic action and a low specific surface area. The silica sol particles also provide good adhesion to the cement particles and to the aggregates.
By the term “silica sol” is herein also comprised aluminium-modified silica sols. Aluminium modified silica sols, sometimes also referred to as aluminate modified silica sols, can be prepared by adding an appropriate amount of aluminate ions, Al(OH)
4
−
, suitably of a diluted sodium or potassium aluminate solution, suitably from about 0.05 to about 2, preferably from about 0.1 to about 2 Al atoms
m
2
surface area of the silica particle, to a conventional non-modified silica sol under agitation and heating. The aluminium-modified silica particles comprise inserted or exchanged aluminate ions, creating aluminosilicate sites having a fixed surface negative charge. The pH of the aluminium-modified silica sol can be adjusted, preferably by means of an ion exchange resin, suitably to a pH ranging from about 3 to about 11, preferably from about 4 to about 10. The aluminium modified silica particles suitably have an Al
2
O
3
content of from about 0.05 to about 3 wt %, preferably from about 0.1 to about 2 wt %. The procedure of preparing aluminium modified silica sol is further described e.g. in “The Chemistry of Silica”, by Iler, K. Ralph, pages 407-409, John Wiley & Sons (1979) and in U.S. Pat. No. 5 368 833.
According to one embodiment of the invention, the silica sol particles of the concrete composition suitably have a specific surface area of from about 50 to about 1200 m
2
/g, preferably from about 300 to about 1000 m
2
/g, and most preferably from about 500 to about
Bergqvist Hans
Greenwood Peter
Skarp Ulf
Akzo Nobel N.V.
Marcantoni Paul
Parker Lainie E.
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