FIELD OF THE INVENTION

The present invention relates to a method for producing calcium silicate hydrate coated particles. The invention also relates to the use of said particles for accelerating hydration reaction and strength development of cementitious products.

BACKGROUND

In the cement and construction industries early-setting, high-strength cements are required. Particularly the Portland-type cements have been under development in order to achieve even greater compressive strengths and higher rates of strength development. Typically, the producing of producing pre-cast, pre-stressed concrete products with high compressive strengths often requires at least sixteen hours, usually even more time for developing the desired strength. Additionally, the construction and repair of highways, bridges, and freeway overpasses requires many days and even weeks of time before these structures set to sufficient compressive strengths to support their anticipated loads so that they may be utilized as designed. The resulting delays form a significant component of construction costs.

In the construction of concrete buildings, in which the cement matrix is cast into forms, days of curing time are necessary to allow the cement to develop sufficient strength before the forms may be removed. Such delays result in lost revenues for property owners.

Many cement-based products are also mass manufactured in high-throughput factory situations. The time required for cement to set completely and develop sufficient strength adds to the cost and difficulty of manufacturing.

The most commonly used accelerant for setting cement is calcium chloride. However, it is widely known in the art that calcium chloride is incompatible with steel-reinforced cements due to its tendency to corrode the steel reinforcement over time in the presence of water and oxygen.

Besides calcium chloride, other accelerants have been proposed . Examples of such accelerants are alkaline compounds (sodium hydroxide, potassium hydroxide, ammonia, sodium or potassium carbonate, and sodium or potassium silicate), alkaline or alkaline earth metal nitrates, nitrites or carboxylates, which are all chloride-free with reduced corrosiveness compared to calcium chloride.

There are several well-known organic materials used in cementitous composites and concrete applications. These so-called admixtures include cellulose ethers for rheology modification, lignosulfonate or naphthalene sulfonates as water reducing agents and superplasticisers, polymer latexes or emulsions for modifying water absorption properties or improving flexibility, as well as other organic admixtures known in the art. One typical effect of these admixtures is retarding the setting time of the cement. Often, an accelerant is used to counteract this retarding effect. Accelerants known in the art are corrosive and/or expensive relative to ordinary Portland cement and can add significant cost to a concrete or cement composite formulation. Attempts to reduce the C0 ₂ emissions of cement have also increased the use of supplementary materials in blended cements. Their drawback is slow early age strength development.

Patent US 5,709,743 discloses a calcium silicate hydrate based accelerant that does not cause corrosion and is as effective as calcium chloride. This material is prepared by hydrating Portland cement to form a calcium silicate hydrate (CSH) material.

This CSH material is then finely ground into "crystallization seeds" which accelerate the cure of cement cubes when added in an aqueous suspension to cement.

Grinding or milling the calcium silicate particles to a specific particle size (as determined by sedimentation volume) is essential for the invention described in US

5,709,743.

Early-age hydration of ordinary Portland cement was studied with semi-adiabatic calorimeter in the presence of limestone and calcium-silicate-hydrate (CSH) coated limestone in the publication Vehmas, T., Kronlof, A: A Study of Early-age Ordinary Portland Cement Hydration According to Autocatalytic Reaction Model, In : The Nordic Concrete Federation (ed.). Proceedings of XXI Nordic Concrete Research Symposium. Hameenlinna, Finland, 2011. Oslo: Norsk Betongforening. P. 269 - 272. (The Nordic Concrete Federation Publication No. 43, 1/2011.)

Based on the above it can be seen that there exists a need to provide improved methods for accelerating strength development of cementitious products. SUMMARY

An object of the invention was to provide new means for accelerating strength development of cementitious products. A further object of the invention was also to provide a method for accelerating strength development of cementitious products.

A still further object of the invention was to provide calcium silicate hydrate coated particles and the use thereof.

The present invention relates to a method for producing calcium silicate hydrate coated particles.

The method for producing calcium silicate hydrate coated particles comprises the steps where an aqueous mixture is formed of water, substrate and at least one Si- compound to obtain molar amount of Si from O. lxlO ^"4 to lOxlO ^"4 mol/m ² of the substrate, followed by agitation, and at least one Ca-compound is added to the aqueous mixture in an molar amount of 0.5 - 5 x Si molar amount, followed by agitation to yield an aqueous suspension comprising calcium silicate hydrate coated particles.

The invention also provides the use of calcium silicate hydrate coated particles, particularly to the use thereof for accelerating strength development of cementitious products.

The invention also provides a method for accelerating strength development of cementitious products.

A further embodiment provides a composition comprising said calcium silicate hydrate coated particles, obtainable by the above method.

The characteristic features of the invention are presented in the appended claims.

DEFINITIONS

Unless otherwise specified, the terms, which are used in the specification and in the claims, have the meanings commonly used in the field of inorganic chemistry, particularly in the field of cement and concrete chemistry and industry. Specifically, the following terms have the meanings indicated below. The term "Portland Cement" includes, but is not limited to, Ordinary Portland Cement (OPC), Off-white Portland Cement, White Portland Cement (WPC), White Ordinary Portland Cement (WOPC) and blended cement. Portland Cement includes calcium silicate based cements typically used in construction applications, including OPCs, sulfate tolerant OPCs, fast OPCs, off-white Portland cement, and blended cement.

The expression "accelerating hydration reaction" is understood to mean accelerating formation of calcium silicate hydrates.

The expression "accelerating strength development" is understood to mean hardening, setting and curing.

The expression "calcium silicate hydrate coated particles" refers to solid particles and/or agglomerates coated with calcium silicate hydrate.

The expression "cementitious product" refers to products comprising cement.

Unless otherwise noted, all percentages are by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates scanning electron microscope images of uncoated calcite (1A) and CSH coated calcite surface (IB) . DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on studies relating to coating of substrate particles with a thin layer of calcium silicate hydrate (CSH). When exposed to an aqueous media, the coated particles act like calcium silicate hydrate. According to studies with semi-adiabatic calorimeter, early age hydration of ordinary Portland cement in the presence of substrate particles, such as limestone and calcium silicate hydrate coated limestone followed the principles of autocatalytic reaction and the rate controlling phase was the initially nucleated CSH. Addition of limestone did not increase the reaction rate nearly as much as CSH coated limestone, which had a massive impact to the hydration reaction rate. Scanning electron microscope images of uncoated calcite (1A) and CSH coated calcite surface (IB) are illustrated in Fig. 1. It was now surprisingly found that only the carefully selected parameters provide coated particles with desired morphology and an average layer thickness suitable for acting in an efficient and economical way in the designed uses. The method for producing calcium silicate hydrate coated particles comprises the steps where an aqueous mixture is formed of water, substrate and at least one Si- compound to obtain molar amount of Si from O. lxlO ^"4 to lOxlO ^"4 mol/m ² of the substrate, followed by agitation, and at least one Ca-compound is added to the aqueous mixture in an molar amount of 0.5 - 5 x Si molar amount, followed by agitation to yield an aqueous suspension comprising calcium silicate hydrate coated particles.

According to one preferable embodiment of the invention the method for producing calcium silicate hydrate coated particles comprises the steps where,

- in the first step the specific surface area of substrate particles is measured,

in the second step an aqueous mixture is formed of water, substrate and at least one Si-compound to obtain molar amount of Si from O. lxlO ^"4 to lOxlO ^"4 mol/m ² of the substrate, followed by agitation,

- in the third step at least one Ca-compound is added to the mixture obtained in the second step, in an molar amount of 0.5 - 5 x Si molar amount of the first step, followed by agitation to yield an aqueous suspension comprising calcium silicate hydrate coated particles.

The substrate particles are coated with calcium silicate hydrate (CSH) in the method.

Said substrate comprises substrate particles. Said substrate particles may be selected from mineral materials, suitably from limestone, calcium carbonate, clay, talc, fly ash, cement, gypsum, titanium dioxide, silicates, organic pigments, slag, ground granulated blast furnace slag, slags from metal manufacturing processes, silica dust, metakaolin, natural pozzolans and calcined oil shale. The substrate is suitably finely divided particulate material.

The specific surface area of the substrate (substrate particles) is suitably measured using methods and equipment typically used for particle size distribution measurements, such as methods based on laser diffraction, microscopy, SEM, Coulter counter etc. In the present application particle size distributions were measured with a Beckman Coulter LS particle size analyzer. The specific surface area (SSA) was calculated from the particle size distributions. This method ignores the smallest surface structure details accessible for example to nitrogen molecules in the BET method (method based on gas absorption on solid surface and often used for specific surface area measurements) and it results in smaller SSA values. Because the CSH layer is much thicker than the layers of nitrogen molecules, the CSH layer ignores small details of the molecule size as well. Other methods ignoring small details below the size of approximately 10 nm can also be used. Examples of such methods include methods for the determination of particle size distribution (X-ray sedimentation etc.) as well as methods for the determination of specific surface area, based on air permeability through a bed of particles. In said method the air permeability specific surface of a powder material is a single-parameter measurement of the fineness of the powder. The specific surface is derived from the resistance to flow of air (or some other gas) through a porous bed of the powder.

So-called Blaine method commonly used in cement industry, based on air permeability is also a suitable method as long as the fineness of the material tested lays within the fineness range of cement.

The Si-compound is a water-soluble compound selected from sodium silicate, potassium silicate, silicic acid, sodium metasilicate, potassium metasilicate and any combinations thereof. Suitably Na ₂ (Si0 ₂ )3,3 is used.

Said water-soluble compound is suitably present in the aqueous solution as sodium metasilicate, potassium metasilicate and/or waterglass. These compounds are readily soluble in water.

Preferably the molar amount of Si is from 0.8xl0 ^"4 to 1.5xl0 ^"4 mol/m ² of the substrate, particularly preferably from l .OxlO ^"4 to 1.3xl0 ^"4 mol/m ² of substrate. The specific surface area of the substrate is used for the determination of the needed amount of Si.

The Si concentration is from 100 to 10000 μΜ, suitably 400 - 1000 μΜ, particularly suitably 600 - 800 μΜ in the second step.

In the second step an aqueous mixture is formed of water, substrate and at least one Si-compound, and the components may be mixed in any order or simulatneously. Suitably the Si-compound is mixed with water and the substrate is added to the obtained mixture.

In the second step the agitation is carried out for a sufficient period of time to ensure wetting and even distribution of the particles, for more than one second, preferably from 0.5 to 5 min, particularly preferably from 0.5 to 1.5 min. Suitably vigorous agitation is used for ensuring sufficient mixing of the components.

In the third step the agitation is carried out for more than one second, preferably from 0.5 to 20 min, particularly preferably from 2 to 8 min. In this step the reaction of the Ca-compound and the Si-compound occurs in order to precipitate a layer of calcium silicate hydrate on the substrate particles.

Preferably the molar amount of the Ca-compound is 1.5 - 1.9 x Si molar amount, particularly preferably 1.6 - 1.8 x Si molar amount.

The Ca-compound may be selected from sparingly soluble and readily soluble Ca- salts. Preferably the sparingly soluble solid Ca-compounds are used whereby the desired heterogenic reaction (heterogenic nucleation) occurs readily. In the case readily soluble Ca-compounds are used the addition of the Ca-compound is carried out in step by step wise.

The Ca-compound is selected from calcium chloride, calcium nitrate, calcium formate, calcium acetate, calcium bicarbonate, calcium bromide, calcium citrate, calcium chlorate, calcium fluoride, calcium hydroxide, calcium hypochloride, calcium iodate, calcium iodide, calcium lactate, calcium nitrite, calcium oxalate, calcium sulphate, calcium sulphate hemihydrate, calcium sulphate dihydrate, calcium sulphide, tricalciumsilicate, Portland cement and any combinations or impure forms thereof and any combinations thereof. Preferably Ca(OH) ₂ is used.

The reactivity and particle size distribution of the Ca-compound, such as Ca(OH) ₂ effects the needed reaction time, and thus the time may be adjusted according the used calcium source. The temperature in the method may range from 5 to 100°C, suitably from 5 to 70°C, preferably from 15 to 25°C.

Suitably normal atmospheric pressure is used in the method. If desired, in order to increase the coating thickness, the method may be repeated one or more times with coated filler particles as the substrate. In the repeated treatment it is not necessary to use supersaturated Si-solution. Alternatively, for increasing coating thickness any materials comprising soluble Ca and Si may be added to the suspension after the earlier described primary calcium silicate coating. The amount of the added material can be from 0.01 to 10% of the substarte weight, preferably from 0.1 tol% of the substrate weight, and the addition can be repeated multiple times. Suitably Portland Cement may be used.

Suitably any mixing tank or reactor may be used for the method, equipped with means to provide efficient/vigorous agitation.

The method may be carried out as a batch method, semi-batch method or continuous method.

If desired, the product may also be subjected to drying using methods known as such in the art. A powder product can be obtained from the aqueous product by for example spray-drying or drying in a fluid-bed dryer.

The product (calcium silicate hydrate coated particles) may be in the form of dry products or aqueous products. Said aqueous products may be form of slurries, pastes, fresh mortar or fresh concrete. The product may optionally comprise additives known in the art, such as viscosity enhancer polymers, setting retarders, any formulation components typically used in the field of construction chemicals, such as defoamers, air entrainers, dispersants, retarders, shrinkage reducers, redispersible powders, other hardening accelerators, anti-freezing agents and/or anti-efflorescence agents.

Thus the invention also provides a composition for accelerating strength development of cementitious products, comprising calcium silicate hydrate coated particles and at least one additive selected from viscosity enhancer polymers, setting retarders, defoamers, air entrainers, dispersants, retarders, shrinkage reducers, redispersible powders, hardening accelerators, anti-freezing agents and anti-efflorescence agents. Suitably said composition is an aqueous composition.

The invention comprises the use of the product obtainable by the method of the present invention, in building material mixtures containing cement, gypsum, anhydrite, slag, preferably ground granulated blast furnace slag, fly ash, silica dust, metakaolin, natural pozzolans, calcined oil shale, calcium sulphoaluminate cement and/or calcium aluminate cement, suitably in building material mixtures which comprise substantially Portland Cement, such as OPC as a hydraulic binder, optionally in combination with one or more additives. The product is contained or included in the building material mixture preferably at a dosage of 0.05 to 10 weight-% with respect to the cement (OPC) weight.

The method may suitably be realized on-site, close to the substrate source or in a dedicated plant, for example at concrete element plant.

The method can also be utilized for example in the manufacture of cementitious products, concrete, as well as in the precipitation or flocculation of particulate material in aqueous streams, such as mine slurries comprising finely divided stone materials.

The method according to the invention has several advantages.

For providing the desired average layer thickness of 1-100 nm, preferably 2-20 nm, particularly preferably 5-9 nm of the calcium silicate hydrate coating on the substrate, it is necessary to use a supersaturated Si solution having saturation around 10-fold. Higher saturation results in a coating with inferior morphology where the coating is not evenly divided on the substrate. Because the specific surface area of the substrate is high, the amount of the precipitating or crystallizing material needed for the coating is high, and very large liquid volumes for providing around 10-fold supersaturation are required . This would make the method practically impossible to be carried out in any reasonable industrial scale.

However, when the average thickness of the coating layer was predetermined (for example as 8.4 nm) and the necessary amount of the coating material was calculated therefrom as the molar amount of Si mol/m ² of substrate, as well as the corresponding amounts of the reagents needed and by following the described procedure, particles with desired coating could be obtained even though the amounts of reactants present correspond to the supersaturation of several hundred folds (100 - 10 000, preferably 600 fold) without the need to use large liquid volumes. Thus practically no waste is formed with the method .

The coating is directed to surface of the substrate particles, and it was surprising that with such high concentrations and high supersaturation (even 600-fold) a coating with desired morphology was obtained rapidly and easily with heterogeneous precipitation and heterogeneous nucleation. Particularly, in a preferable embodiment where only the Si-compound was dissolved in the second step and the Ca-compound was added as solid material (powder) in the fourth step, the solution in the beginning of the reaction was not saturated, but the saturation increased with the dissolving of the Ca-compound (Ca(OH) ₂ ) and at the same time precipitation took place heterogeneously on the surfaces of the substrate, readily available because of vigorous agitation. The product (calcium silicate hydrate coated particles) was precipitated or crystallized simultaneously with the dissolving of the Ca-compound, resulting in that the solution never reached the situation of too high supersaturation which would affect negatively on the morphology of the product.

For example particles having average particle size of less than 10 μιη may be obtained, with a "hairy" coating, the average layer thickness of the coating being typically from 1 to 100 nm, suitably from 2 to 20 nm.

Practically any solid particles may be coated with calcium silicate hydrate, thus also particulate waste materials may be utilized. This results in that, for example in the concrete industry, the use of cement can be decreased. The coated materials may find valuable use in the manufacture of cemetitious products, particularly as accelerants for developing strength of the products and as replacing traditional fillers at least partly.

Also C0 ₂ emission can be decreased in the concrete manufacture (when the coated particles are used as cement replacement), and the curing times of cementitious products can be decreased.

The following examples are illustrative of embodiments of the present invention, as described above, and they are not meant to limit the invention in any way.

EXAMPLES EXAMPLE 1

Manufacture of calcium silicate hydrate (CSH) coated limestone particles Desired average coating thickness was selected as 8.4 nm. The specific surface are of the substrate (limestone powder) was determined. Then a solution was formed of Na ₂ (Si0 ₂ )3,3 and water to achieve a concentration of 700 μΜ Si and the molar amount of Si being 1.166*10 ^"4 mol/m ² substrate (limestone), the substrate was added and the mixture was agitated vigorously for 1 min. Then solid powdery Ca(OH) ₂ was added . The molar amount of it was calculated as follows: Ca(OH) ₂ = 1.7 x Si molar amount + 0.005 mol/l x water volume. Mixing was continued for 10 minutes. According to conductometric measurements the reaction took place in 3 minutes and the formation of the coating was followed by observing the flocculation as the coated product flocculated very readily. The method was carried out at 20°C temperature. SEM (scanning electron microcopy) was used for verifying the coating.

EXAMPLE 2

Manufacture of calcium silicate hydrate (CSH) coated quartz particles

The procedure of example 1 was repeated using quartz powder as the substrate. The method was carried out in a similar way and a similar product was obtained.