Compositions: coating or plastic – Coating or plastic compositions – Alkali metal silicate containing
Patent
1990-09-05
1992-09-15
Dixon, Jr., William R.
Compositions: coating or plastic
Coating or plastic compositions
Alkali metal silicate containing
106600, 106602, 106603, 106624, 106631, C04B 1204
Patent
active
051474590
DESCRIPTION:
BRIEF SUMMARY
This invention relates to cementitious products and more particularly to a method of making shaped and hardened cementitous products by curing or firing compositions comprising reactive fillers and silicates. The term `cementitious products` is used herein to refer to products made from cementitious compositions, i.e. compositions which are mouldable or otherwise shapable and which can be hardened, for example by curing or firing.
The development of high strength cementitious products has been the focus of much research activity. Methods which have been proposed for making such products include the addition of fibrous and/or organic polymeric materials to uncured cementitious compositions and the treatment of conventional cementitious compositions to reduce the pore volume so as to give so-called macro-defect free cementitious products (see EP-A-0021682). A combination of these techniques has been described in EP-A-0055035; however the macro-defect free (mdf) cementitious products described in this latter specification suffer from the problem that the polymer contained therein has a tendency to leach out of the cured cement on contact with water with consequent reduction of the strength of the product. In addition the cured products in general have poor heat stability even at temperatures as low as 80.degree. C.
Various attempts have been made to solve the problems of poor water-stability and poor thermal stability which have been encountered with mdf cementitious products. These have included treatment with boric acid (GB-A-2168292), burning out of the polymer followed by contact with water (GB-A-2168293) and careful control of the nature of the calcium silicate (GB-A-2162165). In addition it has been proposed to cross-link the polymers to hinder leaching out of the polymer (GB-A-2181726). None of these methods has however been entirely satisfactory.
We have now surprisingly found that compositions comprising one or more reactive fillers (e.g. pozzolanic materials) and one or more alkali metal silicates can be used to make high strength mdf cementitious products having both good water- and thermal stability. It has not hitherto been appreciated that the abovementioned compositions could be satisfactorily subjected to the high shear mixing processes required to make mdf products and that they could subsequently be successfully shaped and hardened, e.g. by forming into cohesive sheets and curing or firing, to obtain mdf cementitious products having advantageous properties.
According to one feature of the invention, therefore, there is provided a mdf cementitious product made by curing or firing of a cementitious composition comprising water, at least one reactive filler (e.g. a pozzolanic material) and at least one water soluble alkali metal silicate, not more than 4% of the total volume of the product being made up of pores of maximum dimension exceeding 100 microns. Preferably these pores do not form more than 2%, more preferably not more than 0.5%, of the total volume of the product. The strength of the product is greater the smaller the proportion of relatively large pores and thus it is preferred that not more than 2% of the total volume is made up of pores of maximum dimension exceeding 50 microns, more preferably 15 microns. It is also preferred that the total pore volume of the product does not exceed 20% of the total volume including pores, more preferably not exceeding 10% of the total volume.
According to a further feature of the invention, there is provided a process for the preparation of a cementitious product according to the invention as hereinbefore defined which comprises milling a cementitious composition as hereinbefore defined, by means of high shear mixing, shaping the milled composition thereby obtained and then hardening the compositon by curing or firing.
The term "reactive filler" as used herein is intended to include filler materials capable of reacting chemically with an alkali metal silicate to form a cementitious composition or product.
Suitable reactive fillers are for example
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Young, J. F., Macro-Defect-Free Cement: A Review, Mat. Res. Soc. Symp. Proc., vol. 179, pp. 101-121 (1991).
Durey Christine A.
Lynn Michael E.
Dixon Jr. William R.
Green Anthony J.
Marley Tile AG
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