CA1296847C - Cementitious composition comprising particulate silica - Google Patents

Abstract

ABSTRACT
A cementitious composition and, by extension, an article of manufacture and a curable composition of particulate or comminuted silica, an alkali-metal silicate and, optionally, an alkyl silicate and/or ester catalyst is disclosed. The silica, (80-75%, 80-95%, 80-92.5%), is preferably sand, either pure silica sand for a white effect, or naturally/artificially coloured for colour effects. The alkali metal silicate, (20-25%, 20-5%, 20-7.5%), may be either sodium and/or potassium. The alkyl silicate is preferably ethyl silicate and the ester catalysts are generally esters of long chain organic, (preferably unsaturated aliphatic), carboxylic acids. Ester catalysts usually employed in foundry sand moulding may also be used.
The addition of an alkyl silicate and/or ester catalyst (5-15%) appear(s) to give added strength to the product over that by the alkali-metal silicate alone. The alkali-metal and alkyl silicate combination, moreover, is useful for the prevention of rough surface texture. The preferred compositions shown in parentheses above for silica and alkali-metal silicate indicate the amounts used in the absence of an additive and in the presence of the alkyl silicate and/or ester catalyst(s) respectively. The curable composition/article of manufacture is prepared by mixing an amount of 5-20% of finely ground or comminuted "flour" of silica with the balance of the mix to exclude voids, occlusions or rough surface texture. The alkali-metal silicate may be added or prepared in situ by mixing the sand with caustic alkali. After mixing all the constituents together and reaction, if any, has ceased, the mixture is then allowed to cure for several hours at temperatures up to 200°C. to stabilise and increase the strength of the composition. Cementitious compositions of the above type have particular advantages, as they are resistant to the acid constituents in the urban atmosphere, finding applications in the manufacture of building units, such as various tiles, paving materials and special plastering compounds.

Description

, SILICATE-BONDED SILICA MATERIALS

This invention relates to silicate-bonded silica materials, and more particularly to structural and masonry units and building components produced from such materials including wall, floor, roof and ceiling tiles~ p~ving materials and special plastering (in situ and masonry unit application) compounds "Artificial" and "reconstituted" stone has long been made by man, many buildings dating from the `10 Imperial Rome epoch being mainly constructed of a kind of concrete which was faced with marble. However, in this latter half of the twentieth century, concrete has proved to be less attractive as a building material because of the high atmospheric pollution obtaining in today's towns and cities, which conditions can destroy concrete, and even such natural materials as marble and sandstone, in the course of time.
AU-B-155,386 describes fast-setting acid-proof mortars which contain i) sodium silicate, ii) finely ground silica, sand, or diorite dust and iii) partly condensed organic silicates (e.g. hydrolysed ethyl silicate). The weight ratios of these components is such that the mortars have very short setting times, typically a matter of minutes, and produce l'granite-like masses." GB-A-256,258 discloses hardenable mixtures of various silicates and a water-glass solution.

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`, It is an object of the present invention to overcome the above and other disadvantages of conventional building materials by the provision of "artificial stone" which is extremely resistant to acidic and other atmospheric pollutants, and which has the physical strength and other mechanical characteristics to enable it to be used in buiIding construction.
It is a further object of the present invention to utilise, for the above purpose, one of the Earth's most abundant elements, that is to say, silicon, to produce silicate-bonded silica materials.
According to one aspect of the present invention there is provided a curable composition for the manufacture of building blocks, structural units and preformed concrete-like materials suitable for internal or external use, said composition comprising 80-95% by weight silica and correspondingly 5-20% by weight of sodium or potassium silicate, and including also ethyl silicate present in a proportion of from 5-15qo by weight of the total weight of silica and sodium or potassium silicate.
The invention also provides a method for preparing a building product suitable for use as a ; 25 building block, structural unit or similar material for either internal or external use, said product being long term resistant to rainwater and atmospheric gases, comprising the following steps:
(a) preparing d water-glass composition comprising 80-95% by weight silica and correspondingly 5-20% by weight of either sodium or pot~ssium silicate, and including also ethyl silicate present in a proportion o~ from 5-15% by weight of the total weight of silica and sodium or potassium silicate, and ~y, :~$~

(b) forming an article from said composition in a mold and allowing said composition to set thereby producing said article.
The silica may be any suitable readily available silica sand or mixture of sands. A
preferred silica sand is a pure white silica sand, very large deposits of which have been ascertained to exist, particularly in Australia. One such deposit which has been identified is of very high purity, having a Fe203 content of less than 0.008%; the virtually iron-free nature of this raw material being of great natural advantage in avoidance of discolouration, streaking, etc, due~ to weathering; and producing pristine white units, or combined with additives e.g. pigments, to give subtle shading effects. However, other sands of various colours can also be employed to give different shades of colour or different surface effects to the final moulded product.
In some embodiments of the invention, a predetermined percentage of silica sand is firstly ground to a smaller particle size (e.g. 100~300 mesh), and in some cases finely comminuted silica "flour" is employed in the sand mix. Usually, no more than about S-20% by weight of the fine sand is used in the sand mix. This flour tends to prevent any voids or occlusions occurring in the finished blocks, and may also reduce the quantity of binder required in the manufacturing process. It has been found that when no fine sand is included in the sand mix the surface of 3 the moulded product has a rough texture. However, incorporation of even 10% fine silica material is sufficient to substantially eliminate most of this surface roughness. The presence of ethyl silicate and an alkali metal silicate al 50 9 i ves quite a smooth surface texture.

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The alkali metal silicate is selected from sodium silicate or potassium silicate, preferably the latter (preferred SiO2:K20 ratio in the range of 1.7:1 to 1.4:1). In some cases, it may be of advantage to use mixtures of sodium silicate and potassium silicate.
Sodium silicate has for many years been extensively used in the field of metal-founding as a binder. Spray-on protective coatings have also been proposed, composed of curable silicate compositions containing a phosphate hardener coated with a reaction product of a metal aluminate and/or metal borate.
It is envisaged, however, that a sodium ' silicate binder may lack the necessary strength characteristics, particularly for the manufacture of pre-cast units, and in consequence potassium silicate binders are highly preferred.
Various proposals for the production of potassium silicate binders and "waterproof" protective coatings have been made; for instance Weiand et al.
have worked on cold hardening refractory binders prepared from alkali metal (with particular reference to potassium) silicate with various solutions. Weather resistant coatings prepared by employing potassium water glass have been proposed by Strobonov et al., modified potassium silicate binder in potassium aluminosilicate-bonded material by Korneev et al., while optimal values for the potassium/silica ratio of potassium silicate protective coatings have been investigated by Perlin et al., and potassium silica 3 solutions by Murashkevick.
The sodium or potassium silicate is employed in amounts of from 5-20% by weight of the moldable composition, but usually requires thermal curing at . ..
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temperatures of up to about 200C for several hours.
Ideally, the amount of sodium or potassium silicate is kept to a minimum in the moldable mix to prevent staining caused by alkali leaching.
The sodium or potassium silicate can be added to the moldable composition or can be formed in situ by mixing the sand with an alkali, such as sodium or potassium hydroxide, whereby the corresponding alkali metal silicate is precipitated and apparently functions as a binder for the particulate silica sand, and is allowed to cure at ambient temperature. Thermal curing , up to 200C tends to strengthen the product and to reduce the amount of free alkali present, presumably due to reaction with the silica present.
The ethyl silicate is used in amounts of about 5-15% by weight of the moldable composition (e.g. 5-10%
potassium silicate and 5-15% ethyl silicate). The addition of ethyl silicate appears to provide added strength to the product when compared with potassium silicate alone.
Thermal curing at temperatures of up to 200C
; for several hours increases the strength.
Other compounds may be included in the moldable composition as required e.g. ester catalysts (typically esters of long chain organic carboxylic acids), additional binders, plasticizers and fillers. As ; fillers there are used the usual inert substances, which may be incorporated for special purposes such as colour or because it is a readily available local ;~ 3 material, e.g. bauxite, lime, kaolin and other clays.
According to the present invention the ~components of the moldable composition are admixed together to form an homogeneous mixture, with water added to the degree necessary to assist moldability to the desired shape.

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If desired, the molded shape may be cured entirely at ambient temperatures, since elevated temperature curing is not required to provide a product with excellent mechanical properties. However, thermal curing at temperatures of up to 200C for several hours may increase the strength of the product and reduce the possibility of water slumping or the leaching of alkali from the final product The moldable compositions of the present invention can be molded into a variety of shapes, including blocks, panels, sheets, tiles and the like.
The final product is densely compacted and non-porous, and resistant to acid and alkali corrosion (e.g. from "acid rain"). Suitable pigments can be added during the manufacture to produce a range of coloured finishes. The surfaces of the product may be glazed or given any desired surface finish treatment.
Although conventional mortars may be employed in ; the laying of blocks manufactured according to the invention, suitable mortars and the like may also be produced from the moldable mixtures of khe present invention, with or without plasticizers or other additives.
It will be seen that a particular advantage of the invention lies in the fact that the materials can be cured at ambient temperature just as are the much-inferior concretes. Thus, masonry blocks, panels, tiles, etc., can be pre-cast cheaply and easily and then transported to a buiIding site; on the other hand, large slabs, structural units and the like may just as cheaply and rapidly be produced in situ (with or without reinforcing).

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~ ,, , - 7 - ~ _ b~7 Molded products were prepared from a moldable composition prepared by weighing out the necessary quantities of material, mixing them, and molding the resultant composition to the desired shape and curing the molded shape at either ambient or elevated temperature. The final products presented a good finish and a pleasing appearance and had good strength (15-40 MPa compressive strength), and were stable to rain, sunlight and general weathering conditions.
Resistance to water slumping was tested by placing the molded product in boiling water for at least four (4) hours. Resistance to acid corrosion was tested by placing the molded product in concentrated (4M) hydrochloric acid for at least several hours.
The following non-limiting examples are intended to illustrate the invention.

A mixture was prepared comprising 90% by weight of silica (80% sand, 10%-100 mesh silica, 10%-300 mesh silica), 10~ by weight of K60 potassium silicate and 7.5% by weight of ethyl silicate (based on total weight); with sufficient added water to assist moldability.
The mixture was cured at room temperature for twenty-four (2~) hours. The cured samples exhibited good strength and were not affected by hot water or 4M
hydrochloric acid.

~ E~AMPLE _ Example 1 was repeated, except that samples were cured at 100C and 200C. The products exhibited an increase both in hardness and in strength.

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95/0 silica (80% sand, 10%-lO0 mesh silica, 10%-300 mesh silica), 5% K60 potassium silicate and 7.510 ethyl silicate (based on total weight).
The mixture was cured at room temperature for twenty-four (24) hours. The cured samples exhibited good strength and were not affected by hot water or 4M
hydrochloric acid.

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Claims (4)

1. A curable composition for the manufacture of building blocks, structural units and preformed concrete-like materials suitable for internal or external use, said composition comprising 80-95% by weight silica and correspondingly 5-20% by weight of sodium or potassium silicate, and including also ethyl silicate present in a proportion of from 5-15% by weight of the total weight of silica and sodium or potassium silicate.

2. A composition according to claim 1 wherein said silica comprises a mixture of silicas, from 5-20% by weight of the mixture being comminuted silica flour.

3. A method for preparing a building product suitable for use as a building block, structural unit or pre-formed concrete-like material for either internal or external use, said product being long term resistant to rainwater and atmospheric gases, comprising the following steps:

(a) preparing a water-glass composition comprising 80-95% by weight silica and correspondingly 5-20% by weight of either sodium or potassium silicate, and including also ethyl silicate present in a proportion of from 5-15% by weight of the total weight of silica and sodium or potassium silicate, and (b) forming an article from said composition in a mold and allowing said composition to set thereby producing said article.

4. A method according to claim 3 wherein said silica comprises a mixture of silicas, from 5-20% by weight of the mixture being comminuted silica flour.