Patent Publication Number: US-2020299202-A1

Title: Core-shell expanding agents and their use in cementitious systems

Description:
The invention relates to coated inorganic expanding agent particles, a process for preparing the coated particles, and their use in cementitious systems. 
     Cementitious systems harden with a slight loss in volume (shrinkage). As a consequence thereof, crack formation may occur which impairs the strength of the concrete. In the oil and gas industry bonding between set cement and casing or formation is reduced so that a small gap or microannulus may form at the cement/casing and/or cement/formation interface, in particular under the thermal and mechanical stress that occurs in connection with oil and gas drilling operations. Under these circumstances an effective zonal isolation cannot be achieved. 
     To avoid the shrinkage expansive cements have been used which are hydraulic cements that, after setting, increase in volume. The increase in volume is often achieved by adding expanding agents such as fibers, magnesium oxide or calcium oxide. However, the use of such expanding agents is problematic since the expanding agents known in the art begin to hydrate and thus begin to expand as soon as they contact water. Therefore, the expanding agent can in many cases not be added to the cement slurry because the expanding agent is going to expand too early, i.e. before the cement slurry is positioned at the desired location leading to significant disadvantages. 
     Various efforts to delay expansion have been suggested. Coating of metal oxide particles with non-hydratable or previously hydrated minerals such as metal carbonates, hydroxides and hydrates was suggested in U.S. Pat. Nos. 4,332,619, 5,741,357, EP2169027A1; but these materials can be difficult to prepare and have had only limited success. Also the coating of core particles with water glass has been known, resulting in silica-coated particles. 
     FR 710 302 discloses a hardening accelerator for cement which is obtained by coating sand, gravel or a pozzolan with water glass and converting the water glass to a hydrated silicic acid gel. 
     U.S. Pat. No. 5,118,527 discloses a method of coating aluminum flakes which are used as an expanding agent with a film, e.g., sodium silicate, which is sensitive to the water and/or alkaline nature of wet grout. 
     WO 2013/023949 which describes a powder having a calcium oxide core and a shell formed of amorphous silicon dioxide. The powder is produced by injecting a hydrolysable or oxidizable calcium compound, a hydrogen-containing fuel gas and an oxygen-containing gas in a combustion zone igniting the mixture at 900-1200° C. and introducing a hydrolysable or oxidizable silicon compound in the combustion zone at a temperature of 700-750° C. This results in a nano-scale product in which the thickness of the coating is in the lower nanometer range. 
     At ambient temperatures, a silica coating is slowly dissolved in the alkaline environment of mortar or grout and, thus, effectively delays the exposure of the expanding agent cores to the water in order to control the expansion on-set time. However, at higher temperatures of, e.g., more than 60° C., the silica coating often fails to delay the expansion on-set time. 
     CN 101705741 discloses a phase-change aggregate which is incorporated in a cement-containing phase-change heat insulating layer. The phase-change aggregate has a phase-change core material and a shell material in a weight ratio of 2-4:1-1.5. The phase-change core material contains 55-60 weight parts of paraffin or saturated fatty acid, 30-35 weight parts of expanded perlit particulate and 5-15 weight parts of silica sintering material or slag. The shell material is formed from 40-45 weight parts of aluminum sulfate, 50-55 weight parts of sodium silicate and 3-5 weight parts of emulsifier. The phase-change aggregate is said to have good water resistance and permanent anti-aging properties. 
     The problem underlying the invention was therefore to provide expanding agents that show a delayed expanding effect also at higher temperatures. 
     The invention relates to coated inorganic expanding agent particles comprising a core of an inorganic expanding agent and a sol/gel-formed coating comprising a mixed oxide of two or more metals and/or metalloids. 
     The term “coated particle” refers to a particle comprising the expanding agent as a core which is at least partially coated with a sol/gel-formed coating. The term also refers to a particle comprising a sol/gel-formed matrix in which particles (i.e., a plurality of cores) of the expanding agent are dispersed throughout a sol/gel-formed matrix. Thus, the particles may comprise a single-core arrangement or configuration, i.e., a single core at least partially contained within a sol/gel-formed coating and/or may comprise a “matrix-island” arrangement or configuration in which the expanding agent is distributed at least partially within each particle as small islands within a matrix of the encapsulating sot/gel-formed coating, e.g., the expanding agent is dispersed as a discontinuous phase within the continuous coating. Unless stated otherwise, reference to a coated particle includes both single core-shell and preferably matrix-island arrangements. 
     Preferably, the inorganic expanding agent accounts for not less than 90% by weight of the core(s). Typically, the core(s) are essentially free of organic matter. 
     An expanding agent is any inorganic material, which, when added to hydraulic cement, tends to increase the volume of the set cement. The expanding agent is an agent that is capable to swell in contact with water, e.g., a hydratable material that hydrates with water to form a product having a larger volume relative to the unhydrated material. 
     In an embodiment, the expanding agent comprises a metal oxide which hydrates to form an expanded product. Examples include calcium oxide, magnesium oxide, strontium oxide, barium oxide, or combinations or mixed oxides thereof. In another embodiment, the expanding agent comprises calcium oxide, magnesium oxide, or a combination thereof. In an embodiment, the expanding agent comprises calcium oxide and magnesium oxide in any mixing ratio. Specifically, the inorganic expanding agent may comprise not less than 90% by weight, relative to the weight of the inorganic expanding agent, of at least one of calcium oxide and magnesium oxide. Thus, the inorganic expanding agent may comprise (i) not less than 90% by weight of calcium oxide, (ii) not less than 90% by weight of magnesium oxide, or (iii) not less than 90% by weight of a combination of calcium oxide and magnesium oxide. In an embodiment calcium oxide and magnesium oxide are used in a molar mixing ratio of 1:100 to 100:1, or 1:20 to 20:1 or 1:10 to 10:1 or 1:5 to 5:1. In a further embodiment, the mixture of calcium oxide and magnesium oxide is obtained from dolomite so that the molar mixing ratio is 1:1. It is advisable to avoid contact of the expanding agent with water or humidity in order to keep the hydrated amount of the expanding agent thereof as low as possible. 
     In another embodiment the expanding agent is selected from calcium sulfate hemihydrate or anhydrite, sodium sulfate, magnesium sulfate, and sodium chloride. 
     In another embodiment the expanding agent is a silicate which is capable to expand in contact with water. Suitable silicates are phyllosilicates such as clays, Examples include bentonite, montmorillonite, smectite, nontronite, beidellite, perlite or vermiculite. 
     Suitably, the inorganic expanding agent is selected from calcium oxide, magnesium oxide, strontium oxide, barium oxide, and mixed oxides thereof, calcium sulfate hemihydrate, anhydrite, sodium sulfate, magnesium sulfate, phyllosilicates, and mixtures of two or more thereof. 
     Typically, upon contact with an excess of 1 M NaOH the coated inorganic expanding agent particles are capable to expand in volume by at least 100%, preferably at least 180%, of the non-expanded volume, when contacted with an excess at after 5 days, as determined by the expansion quick test described in the experimental section that follows. 
     The expanding agent core (single core or dispersed cores within a matrix) may have an average particle size d(50) in the range from about 1 μm to about 100 μm, preferably in the range from about 10 μm to about 90 μm, in particular from about 20 μm to about 60 μm. 
     Generally, the coated Inorganic expanding agent particles according to the invention have a particle size d(50) In the range from about 50 μm to about 1000 μm, in particular from about 60 μm to about 300 μm and preferably from about 60 μm to about 200 μm, as determined by SLS. 
     Generally, the sot/gel-formed coating has a thickness in the range from about 0.5 μm to about 50 μm, preferably in the range from about 1 μm to about 30 μm. The coating thickness may be detremined by visual inspection via scanning electron microscopy (SEM). 
     The coated inorganic expanding agent particles according to the Invention comprise a sol/gel-formed coating comprising a mixed oxide of two or more metals and/or metalloids. In the following, the term “metal(loid)” denotes a metal and/or metalloid. The sol/gel process is a wet-chemical technique. A precursor solution or dispersion (sol) is deposited on the inorganic expanding agent cores to form a film. The sol evolves gradually towards the formation of a gel-like network. Formation of the mixed metal(loid) oxide involves connecting the metal(loid) centers via oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal(loid)-oxo or metal(loid)-hydroxo polymers in solution. 
     The sol/gel formed oxides are known to the skilled person and, e.g., described in Sol-gel science: The physics and chemistry of sol-gel processing, C. J. Brinker, G. W. Scherer, 1990, Academic Press Inc. The sol/gel formed oxides may be selected from oxides of various metal(loid)s, e.g., silicon, aluminum, boron, alkaline earth metals, such as magnesium, or transition metals, such as zirconium, titanium or zinc. 
     It has been found that the sol/gel-formed coating of mixed oxides exhibits an increased temperature and/or chemical stability in alkaline environment as compared to a sol/gel-formed coating of a single oxide. Without wishing to be bound by theory it is assumed that the presence of a dopant metal(loid) oxide facilitates the sintering or densification of the dried gel to a glass-like amorphous substance at a temperature below the liquidus for the glass. 
     Preferably, the sol/gel-formed coating comprises a mixed oxide of silicon and at least one other metal and/or metalloid selected from aluminum, boron, titanium, zirconium and zinc. In preferred embodiments, the weight percentage of other metal(loid)s to the sum of silicon and other metal(loid)s in the sol-gel-formed coating is from 0.1 to 15.0, preferably from 1.0 to 8.0, calculated on an elemental basis (i.e., calculated as silicon or metal(loid)). Especially preferred sol/gel-formed coatings comprise a borosilicate, an aluminosilicate or a mixed borosilicate/aluminosilicate. 
     In preferred embodiments, the coated inorganic expanding agent particles according to the invention have undergone a heat treatment at a temperature in the range of from 100° C. to 1000° C. or 200° C. to 800° C. or 200° C. to 500° C. or 300° C. to 400° C. 
     The sol/gel-formed coating is a protective coating which delays release of the expanding agent also in alkaline systems like cementitious systems. The release of the expanding agent is achieved by slow dissolution or chemical degradation of the sol/gel-formed coating in alkaline medium (pH Z 10) such as in cementitious compositions or by permeation of water through the sol/gel-formed coating or by infiltration through imperfections of the coat such small cracks. Expansion of the expanding agent may also rupture the coat allowing a better contact of the expanding agent with water. The release of the expanding agent may also be effected by the action of mechanical forces such as shearing forces. The delay in release of the expanding agent may range from 0.5 h to 48 h or 3 h to 12 h. The release of the expanding agent is also delayed at higher temperatures such as 60° C. to 100° C. This means that the particles may be added into a cementitious composition prior to positioning and/or setting of the cement slurry without the risk of a premature release of the expanding agent. When the expanding agent is finally released it combines with water to form a hydrated product having a larger volume relative to the unhydrated expanding agent. Due to the higher volume of the expanding agent the volume of the set cement is increased so that small gaps and cracks in the set cement will be filled or at least reduced. 
     The coated inorganic expanding agent particles according to the invention can be prepared by various methods. Preferably, the coated inorganic expanding agent particles according to the invention are prepared by a method comprising
         (a) providing particles of an inorganic expanding agent,   (b) depositing a sol from sol/gel-forming precursors on the particles of the inorganic expanding agent to form a film,   (c) gelling the sol.       

     Optionally, the method further comprises
         (d) subjecting the coated particles to a heat treatment.       

     The expanding agent may be used in powder form. In an embodiment, the particles of the expanding agent have an average particle size d(50) in the range from about 1 μm to about 100 μm, preferably in the range from about 10 μm to about 90 μm, in particular from about 20 μm to about 60 μm, prior to coating, as determined by static light scattering (SLS). 
     A sol from sol/gel-forming precursors is deposited on the particles of the inorganic expanding agent to form a film. Deposition of the sol on the particles of the inorganic expanding agent particles may in some instances involve the agglomeration of smaller inorganic expanding agent particles around a larger inorganic expanding agent particle. The smaller particles will eventually become embedded in a sol/gel-formed matrix around a core of a larger inorganic expanding agent particle ( FIG. 1 ). 
     According to the invention, the sol/gel-forming precursors are such that they are capable to form a mixed oxide of two or more metal(loid)s. Typically, the sot/gel-forming precursors comprise at least a first sol/gel-forming component of a first metal(loid) and a second sot/gel-forming component of a second metal(loid). The sol/gel-forming precursors may be hydrolytic or non-hydrolytic sol/gel forming components. It will be appreciated that it is not necessary for each of the components if taken alone to be capable of forming a sol/gel as long as their mixture can form a sol/gel. 
     In a typical hydrolytic sot/gel processing, the sot/gel components are contacted with the inorganic expanding agent particles in the presence of water. Non-hydrolytic sols may be made in a similar manner, but likely essentially in the absence of water. 
     The sol/gel forming components may be selected from alkoxides, oxides, carbonates, formats, acetates, sulfates, nitrates, halides of various metal(loid)s, with alkoxides, oxides, acetates, nitrates being preferred. The metalloid) alkoxides can have the general formula M(OR) x , wherein M is any metal(loid) from a metal(loid) alkoxide which, e.g., may hydrolyze and polymerize in the presence of water. R is an alkyl radical of 1 to 30 carbon atoms, which may be straight chained or branched, and x has a value equivalent to the metalloid) ion valence. Metal(loid) alkoxides such as Si(OR) 4 , Ti(OR) 4 , Al(OR) 3 , Zr(OR) 3  and Sn(OR) 4  may be used. Specifically, R can be the methyl, straight-chain, or branched ethyl, propyl or butyl radical. Further examples of suitable metal alkoxides can include Ti(isopropoxy) 4 , Al(isopropoxy) 3 , Al(sec-butoxy) 3 , Zr(n-butoxy) 4  and Zr(n-propoxy) 4 . 
     The sol/gel components used in the sols may also comprise colloidal metal oxides, preferably those colloidal metal oxides which are stable long enough to be able to combine them with the other sol/gel components. Such colloidal metal oxides may include, but are not limited to, SiO 2 , Al 2 O 3 , MgO, ZrO 2 , TiO 2 , SnO 2 , B 2 O 3 , La 2 O 3 , Sb 2 O 5  and ZrO(NO 3 ) 2 . SiO 2 , Al 2 O 3 , B 2 O 3 , and ZrO 2  may be preferably selected. Further examples of the at least one sol/gel forming component include aluminum hydroxide sols or gels, aluminumtri-sec-butylat, AlOOH-gels and the like. 
     The sol/gel components used in the sols may also comprise water-soluble alkali metal silicates. 
     The sot may be a solution, colloidal solution or dispersion, preferably an aqueous solution, colloidal solution or dispersion. Deposition of the sol on the particles of the inorganic expanding agent particles is achieved by applying, in particular by spraying. It is advantageous to use the fluidized bed technology or the spouted bed technology for depositing the sol onto the inorganic expanding agent particles. The particles of the inorganic expanding agent are placed into the fluidized bed or spouted bed apparatus and the apparatus is put into operation. As soon as the content of the apparatus has reached the operating temperature, the sot/gel-forming precursors are applied. The sol/gel-forming precursors are used in an amount such that the weight ratio of inorganic expanding agent to the mixed oxide formed from the sol/gel-forming precursors is in the range from 15:1 to 1:2, preferably from 10:1 to 1:1. 
     In an embodiment, the first sot/gel-forming component is a silica precursor. The term “silica precursor” means a compound that can be converted to silica either by heating the precursor during the drying step or by using a conversion agent. The silica precursor may be an alkali metal silicate or a precondensed alkali metal silicate that can be further condensed and converted to the silica network by said heating or conversion agent. 
     In an embodiment, the silica precursor is a water-soluble alkali metal silicate (here water-soluble means an alkali metal silicate having a solubility of 100 g alkali metal silicate per 1 L water). In another embodiment, the alkali metal silicate is selected from compounds having the empirical formula m SiO 2 .n M 2 O, where M stands for Li, Na, K, and NH 4 , and also mixtures thereof, preferably for Na and K. The molar ratio m:n (also called “modulus”) is advantageously 0.5 to 4.0, preferably 1.0 to 4.0 and more particularly 2.0 to 4.0 or 2.6 to 4.0 or 2.3 to 3.5. The alkali metal silicate is preferably water glass, more preferably a liquid water glass, and more particularly a sodium or potassium water glass. Use may also be made, however, of lithium or ammonium water glasses, and also of mixtures of the said water glasses. 
     Water glasses in the above modulus range are commercially available as aqueous solutions. The solids contents of the aqueous water glass solutions are generally 20% to 60% by weight, preferably 30% to 50% by weight. 
     In another embodiment the silica precursor is a dialkoxysilane, trialkoxysilane or tetraalkoxysilane (the alkyl group in said silane is preferably a C 1 -C 4  alkyl group), The silanes are applied in a solvent that can be water and/or water miscible solvents (e.g. acetone). 
     The silica precursor is used as an aqueous solution which preferably contains the precursor in a concentration of 5% to 40% by weight, in particular 5% to 30% by weight. 
     Suitably, the second sol/gel-forming component Is selected from a sol/gel-forming compound of aluminum, boron, titanium, zirconium and/or zinc, preferably from boric acid, borax, an aluminum salt and/or an alkali metal aluminate. 
     Gelation may in some instances self-initiate, for example, when water is present in the inorganic expanding agent particles or the solvent, or via rapid drying, such as spray drying. In other Instances, gelation must be initiated, which can be achieved in a number of ways. 
     Gelation is dependent on a number of factors such as the amount of water present, because water is required for the hydrolysis and condensation reaction. Other factors include temperature, solvent type, concentrations, pH, pressure and the nature of the acid or base used. 
     Typically, either acidic or basic catalysts may be applied, particularly in hydrolytic sol/gel processes. Suitable acids are water-soluble (water-soluble means an acid having a solubility of a 100 g per 1 L water) inorganic and organic acids having a pKa of ≤5. Examples for such acids are hydrochloric acid, hydrobromic acid, sulfuric acid, phosphoric acid, nitric acid, methane sulfonic acid, toluene sulfonic acid, benzene sulfonic acid, formic acid, acetic acid, etc. The concentration of the acid is advantageously ≤1.5 N, in particulars ≤1.2 N. Suitable bases include, for example, sodium hydroxide, ammonia and carbonate as well as organic amines. Suitable catalysts in non-hydrolytic sol/gel processes can include anhydrous halide compounds, for example, BCl 3 , NH 3 , AlCl 3 , TiCl 3  or mixtures thereof. 
     If the precursor is an alkoxysilane, for example tetraethoxysilane, the conversion is initiated by the addition of water, preferably of acidic or alkaline pH (for example, pH 3-4 or 10-11) and a water-miscible organic solvent such as ethanol. 
     The pH can be achieved in a number of ways, for example, by adding an acid or base to the sol/gel-forming precursor solution or by adding the sol/gel-forming precursor solution to the acid or base, or by adding a metal oxide to the solution than adjusting pH with acid or base. 
     In a particularly preferred embodiment, a first solution containing an acid-catalyzed sol/gel-forming component is prepared at a pH at which the acid-catalyzed sol/gel-forming component is stable. Typically, the acid-catalyzed sol/gel-forming component is a silica precursor such as a water-soluble alkali metal silicate. The pH of the first solution may be in the range of from 10 to 12. A second solution containing a base-catalyzed sol/gel-forming component is prepared at a pH at which the base-catalyzed sol/gel-forming component is stable. Typically, the base-catalyzed sol/gel-forming component is boric acid, or a sulfate or nitrate of a metal such as aluminum. The pH of the second solution may be in the range of from 1 to 5. The first solution and the second solution are then combined and deposited onto the inorganic expanding agent particles. Deposition onto the Inorganic expanding agent particles preferably is carried out by spraying, with the following options:
         (1) The first solution and the second solution are mixed before spraying. Preferably, the pH of the mixed solutions is still in the alkaline range, i.e. in the range from 8 to 10. This results in a sol/gel-forming precursor solution in which the condensation is advanced but not yet completed. This precursor solution is then applied to the particles of the inorganic expanding agent and the condensation to the extended network is completed at the drying temperature.   (2) Only a part of the second solution is added to the first solution prior to the application of the precursor solution to the inorganic expanding agent particles. The pH of the mixed solution is still in the alkaline range, i.e. in the range from 10 to 12. This achieves a partial condensation of the sol/gel-forming precursors which is less advanced relative to (1) above. The precursor solution is then applied to the particles of the inorganic expanding agent by spraying it into the operating fluidized bed or spouted bed apparatus. A second part of the second solution can then be applied simultaneously with or subsequently to the mixed sol/gel-forming precursor solution by spraying it into the operating fluidized bed or spouted bed apparatus.   (3) The first solution is applied to the particles of the inorganic expanding agent and simultaneously the second solution is applied by spraying them into the operating fluidized bed or spouted bed apparatus. This includes that the second solution is applied at any time after the application of the first solution has already begun. In an embodiment of option (3) the total of the first solution is applied first and the total of the second solution is applied subsequently and either immediately after the first solution has been applied or after a short period of time. In a further embodiment of option (3) the first solution and the conversion agent are applied intermittently beginning with the first solution. First solution and second solution may be applied immediately after each other or after a specific period of time.       

     In a further embodiment, steps (b) and (c) can be carried out two or more times in order to obtain particles having a thicker coating. 
     It is critical to avoid premature swelling of the inorganic expanding agent. This has to be achieved by control of the amount of water available at any moment of the process. In this regard it is advantageous to use the fluidized bed technology or the spouted bed technology. In an embodiment, a drying step is carried out simultaneously with and/or subsequent to step (b) and/or step (c). The drying step allows control of the excess amount of water contained in the sol/gel forming precursor solutions and the water formed by polycondensation of the sol/gel forming precursors. Also, the drying assists in the gelation of the sol deposited on the particles of the inorganic expanding agent. Suitably, the drying step is carried out at a temperature in the range from about 40° C. to about 120° C., preferably about 50° C. to about 100° C. 
     The spouted bed technology and, in particular, the fluidized bed technology for drying the particles can be carried out in a conventional apparatus such as those commercially available from Glatt GmbH, 79589 Binzen, Germany. The drying conditions are selected such that the temperature of the particles is maintained in the range from 40° C. to 120° C., in particular 50° C. to 100° C. during the drying operation, i.e. the drying is done such that the exit temperature of the drying gas (normally air) is in the range from 40° C. to 120° C. To achieve this the supply gas temperature is, in general, selected in the range from 60° C. to 200° C., in particular 80° C. to 140° C. whereas the other parameters will have to be selected depending on the type and size of the apparatus and the batch size. 
     For feeding the reagents, conventional nozzles can be used such as a two-substance nozzle or a three-substance nozzle. 
     The coated particles are dried to a water content of 0.5% to 5% by weight. 
     After the drying of the coated particles is finished they are preferably subjected to a heat treatment in the range from 100° C. to 1000° C. or 200° C. to 800° C. or 200° C. to 500° C. or 300° C. to 400° C. In one embodiment the heat-up rate is 1 to 5° C./min. In a further embodiment the heating time is from 0.5 h to 12 h or 1 h to 6 h or 1 h to 4 h or 2 h to 4 h. After the heat treatment the particles are cooled down. In an embodiment the cool down phase is from 0.5 h to 24 h or 0.5 h to 12 h or 1 h to 8 h or 2 h to 8 h or 4 h to 8 h. Any apparatus can be used for the heat-treatment, such as a rotary kiln or calciner, conveyor belt furnace, etc. 
     Depending on the amount of sol/gel-forming precursors and the heat treatment temperature and time it is possible to design the properties of the particles according to the needs of the user. This means that the delay in release of the expanding agent can be controlled by the amount of coating and by the heat treatment. Within the above-mentioned boundaries it can be said that the higher the amount of coating and/or the longer the heating time, the slower the heating rate and the slower the cool down time, the longer is the delay of the release of the expanding agent. 
     The invention further relates to compositions comprising the particles of the invention and a binder. In an embodiment the composition may be a cementitious composition or construction chemical composition comprising the particles of the invention and a hydraulic binder, such as cement. The cementitious composition may be in solid form or in the form of a cement slurry. Depending on the intended use the composition may comprise additives such as hardening accelerators, such as calcium silicate hydrate, retarders, dispersing agents etc. According to an embodiment, the composition is a building material composition which comprises a hydraulic binder such as cement, in particular OPC (ordinary Portland cement), high alumina cement, a latent hydraulic binder or a non-hydraulic binder such as gypsum, including α- and ß-hemihydrate or anhydrite. 
     Portland cements are generally preferred for use in accordance with the present invention, and Portland cements of the types defined and described in API Specification For Materials And Testing For Well Cements, API Specification 10, 5th Edition, dated Jul. 1, 1990 of the American Petroleum 25 Institute and their equivalents are particularly preferred. API Portland cements including classes A, B, C, G and H can be utilized with API classes G and H being preferred and class H being the most preferred. 
     According to a further embodiment, the construction chemical composition comprises a geopolymer binder. The geopolymer binder may be selected from latent hydraulic binders, pozzolanic binders and/or alkali-activated aluminosilicate binders, and mixtures thereof. 
     The latent hydraulic binders may be selected from industrial or synthetic slags, for example blast furnace slag, granulated blast furnace slag, ground granulated blast furnace slag, slag sand, ground slag sand, electrothermic phosphorus slag, steel slag, and mixtures thereof, and the pozzolanic binders may be selected from amorphous silica, for example precipitated silica, pyrogenic silica and microsilica, finely ground glass, fly ash, for example brown-coal fly ash or mineral coal fly ash, metakaolin, natural pozzolans such as tuff, trass and volcanic ash, natural and synthetic zeolites, and mixtures thereof. 
     Aluminate cement (“high-alumina cement”) comprises about 20% to 40% by weight CaO, up to about 5% by weight SiO 2 , about 40% to 80% by weight Al 2 O 3  and up to about 20% by weight Fe 2 O 3 . 
     The slags may be both industrial slags, i.e. waste products from industrial processes, and synthetically reproduced slags. The latter slags provide the advantage that they are always available in consistent quantity and quality. 
     A latent hydraulic binder may be a binder in which the molar ratio of (CaO+MgO):SiO 2  is of between 0.8 and 2.5, preferably between 1.0 and 2.0. 
     Blast furnace slag, a typical latent hydraulic binder, generally comprises 30% to 45% by weight CaO, about 4% to 17% by weight MgO, about 30% to 45% by weight SiO 2  and about 5% to 15% by weight Al 2 O 3 , for example about 40% by weight CaO, about 10% by weight MgO, about 35% by weight SiO 2  and about 12% by weight Al 2 O 3 . The cured products generally have the properties of hydraulically cured systems. 
     “Blast furnace slag” is a waste product of the blast furnace process. “Slag sand” is granulated blast furnace slag, and “ground granulated blast furnace slag” is finely pulverized slag sand. The ground slag sand varies, according to origin and processing form, in its particle size and grain-size distribution, with the particle size affecting the reactivity. As a characteristic variable for the particle size, the figure known as the Blaine value is employed, which is typically in the order of magnitude of 200 m 2  kg −1  to 1000 m 2  kg −1 , preferably between 300 m 2  kg −1  and 500 m 2  kg −1 . 
     Electrothermic phosphorus slag is a waste product from the production of phosphorus by electrothermic means. It is less reactive than blast furnace slag and comprises about 45% to 50% by weight CaO, about 0.5% to 3% by weight MgO, about 38% to 43% by weight SiO 2 , about 2% to 5% by weight Al 2 O 3  and about 0.2% to 3% by weight Fe 2 O 3 , and also fluoride and phosphate. Steel slag is a waste product from various steel manufacturing processes, with a highly varying composition (see Caijun Shi, Pavel V. Krivenko, Delia Roy, Alkali-Activated Cements and Concretes, Taylor &amp; Francis, London &amp; New York, 2006, pp. 42-51). 
     Amorphous silica is preferably an X-ray-amorphous silica, i.e. a silica which exhibits no crystallinity in a powder diffraction procedure. The amorphous silica for example comprises SiO 2  in an amount of at least 80% by weight, preferably at least 90% by weight. Precipitated silica is obtained industrially via precipitation processes starting from water glass. Depending on the manufacturing method, precipitated silica is also called silica gel. Pyrogenic silica is generated by reacting chlorosilanes, such as silicon tetrachloride, in an oxyhydrogen flame. Pyrogenic silica is an amorphous SiO 2  powder with a particle diameter of 5 nm to 50 nm and a specific surface area of 50 m 2  g −1  to 600 m 2  g −1 . 
     Microsilica is a by-product of silicon or ferrosilicon manufacture and comprises amorphous SiO 2  powder. The particles have diameters of about 0.1 μm. The specific surface area is about 15 m 2  g −1  to 30 m 2  g −1 . In contrast, commercial silica sand is crystalline and has comparatively larger particles and a comparatively lower specific surface area. 
     Fly ashes are for example formed in operations including the combustion of coal in power stations. Class C fly ash (brown coal fly ash) comprises, according to WO 08/012438, about 10% by weight CaO, whereas class F fly ash (mineral coal fly ash) comprises less than 8% by weight, preferably less than 4% by weight, and typically about 2% by weight, CeO. 
     Metakaolin may be formed in the dehydrogenation reaction of kaolin. Kaolin releases bound water at 100° C. to 200° C., dehydroxylation occurs at 500° C. to 800° C., with collapse of the lattice structure and formation of metakaolin (Al 2 Si 2 O 2 ). Pure metakaolin, accordingly, comprises about 54% by weight SiO 2  and about 46% by weight Al 2 O 3 . 
     Further pozzolanic binders are for example shown in Caijun Shi, Pavel V. Krivenko, Della Roy, Alkali-Activated Cements and Concretes, Taylor &amp; Francis, London &amp; New York, 2006, pp. 51-63. Testing for pozzolan activity can take place in accordance with DIN EN 196 Part 5. 
     The above-mentioned alkali-activated aluminosilicate binders may comprise latent hydraulic and/or pozzolanic binders as defined above and also alkaline activators, such as aqueous solutions of alkali metal carbonates, alkali metal fluorides, alkali metal hydroxides, alkali metal aluminates and/or alkali metal silicates, such as soluble water glass, magnesia cement, phosphate cement, or calcium oxide. 
     The construction chemical compositions may additionally contain additives such as glycols, polyalcohols, aminoalcohols, organic acids such as citric acid or tartaric acid, sugars, melasses, organic and inorganic salts, polycarboxylate ethers, naphthalene sulfonate, melamine-formaldehyde polycondensates, lignin sulfonate, as well as mixtures thereof. Further suitable additives are defoamers, water retention agents, pigments, fibers, dispersion powders, wetting agents, retarders, hardening accelerators, such as calcium silicate hydrate, complex forming agents, aqueous dispersions and rheology modifiers. 
     In a further embodiment, the construction chemical compositions may be selected from compositions for concretes such as on-site concrete, finished concrete parts, pre-cast concrete parts, concrete goods, cast concrete stones, concrete bricks, in-situ concrete, sprayed concrete (shotcrete), ready-mix concrete, air-placed concrete, concrete repair systems, industrial cement flooring, one-component and two-component sealing slurries, screeds, filling and (self)-levelling compositions, such as joint fillers or self-levelling underlayments, adhesives, such as building or construction adhesives, thermal insulation composite system adhesives, tile adhesives, renders, plasters, adhesives, sealants, coating and paint systems, in particular for tunnels, waste water drains, splash protection and condensate lines, screeds, such as anhydrite or hemihydrate or cement based screeds, mortars, such as dry mortars, sag resistant, flowable or self-levelling mortars, drainage mortars, or repair mortars, grouts, such as joint grouts, non shrink grouts, or wind-mill grouts, anchor grouts, EIFS grouts (Exterior Insulation Finishing Systems), or swelling explosives. 
     In a further embodiment, the construction chemical composition may be a well cementing composition or slurry comprising cement and the particles of the invention. They are useful for strengthening a well bore (oil well, gas well or water well), in particular for the exploration and/or recovery of oil, gas or water. 
     In a further embodiment, the composition comprises a binder which may be selected from polyurea silicate compositions. The polyurea silicate composition is obtainable by reacting a mixture comprising (i) at least one isocyanate component having at least two isocyanate groups per molecule, (ii) at least one alkali metal silicate, and (iii) water. These compositions are disclosed in WO 20141096349 which is incorporated herein by reference in its entirety, in particular, page 2, 3 rd  paragraph to page 6, 3rd paragraph, page 7, full paragraphs 1 to 3 and the claims. Construction chemical compositions comprising said polyurea silicate composition are useful for strengthening a well bore (oil well, gas well or water well), in particular for the exploration and/or recovery of oil, gas or water. The construction chemical compositions are further useful for coatings, adhesives, sealants, flooring, and injection resins. 
     In a further embodiment, the composition comprises polymer dispersions as a binder. Such compositions are useful for coatings and adhesives. In an embodiment, the expanding agent is present in the composition at a concentration greater than or equal to about 0.1 weight percent based on the total weight of hydraulic cement present, or greater than or equal to about 0.5 weight percent, or greater than or equal to about 1 weight percent, and less than or equal to about 25 weight percent, or less than or equal to about 20 weight percent, or less than or equal to about 15 weight percent, or less than or equal to about 10 weight percent, or less than or equal to about 5 weight percent, based on the weight of hydraulic cement present. For example, the slurry may comprise from 0.1 to 20 weight percent of the coated particles, or from 0.5 to 10 weight percent of the coated particles, or from 1 to 5 weight percent of the coated particles, based on the weight of the hydraulic cement. 
     A further embodiment Is a method to cement a subterranean well having a borehole disposed through a formation, comprising: (I) preparing a cement slurry comprising water, hydraulic cement, and the coated particles of the invention and combining the capsules with water and hydraulic cement; (ii) placing the slurry in an annular region of the well around a tubular body; (iii) hardening the slurry to form an at least partially set cement; and (iv) hydrating the expanding agent to form an expanded set cement within the annular region. In embodiments, the method further comprises forming bonds between the expanded set cement, the tubular body and a wall of the borehole, and/or isolating a zone of the formation adjacent the expanded set cement. 
     The invention also relates to the use of the particles of the invention for preparing a well cementing slurry by combining the particles with water and cement. 
     In accordance with the above, the invention provides the following embodiments: 
     The attached drawings and the following examples illustrate the invention without limiting it. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of the production process for coated inorganic expanding agent particles. 
         FIG. 2  shows representative SEM images of the coated inorganic expanding agent particles after calcination. a. with silica shell (Ref.), b. with Si/Al-oxide (sample 6), c. with Si/B-oxide (sample 5) and d. with Si/B/Al-oxide (sample 7). 
         FIG. 3 a    shows cement expansion profiles of the inventive coated inorganic expanding agent particles in comparison with silica-coated expanding agents (Reference) for the full test period (&gt;48 h);  FIG. 3 b    shows expansion profiles of the initial 8 h. 
     
    
    
     The particle size d(50) Is determined by static light scattering (SLS) using a Malvern Mastersizer 2000 (Malvern Instruments GmbH, Germany). The software utilized for measurement and evaluation is the Malvern software package belonging to the instrument. The measurement principle is based on laser diffraction by measuring the intensity of light scattered as a laser beam passes through a particulate sample. This data is then analyzed to calculate the size of the particles that created the scattering pattern. 
     Quick Test for Determining the Expansion of the Expanding Agent Particles: 
     3 g of the particles to be tested are placed in glass containers having a diameter of 2 cm and a height of about 8 cm. The particles are then dispersed in 10 ml of a 1 M NaOH by shaking and allowed to settle. The height of the sediment is determined. Thereafter, the particles are allowed to expand at 60° C. The height of the expanded particles is measured after a 5 days. 
     In the examples the following sodium water glasses were used: 
     NaSi 37/40: 8% Na 2 O, 26.8% SiO 2  and 65.2% H 2 O; modulus 3.4; solids content 34.8%. 
     NaSi 40/42: 8.8% Na 2 O, 29.5% SiO 2  and 61.7% H 2 O; modulus 3.4; solids content 38.3%. 
     EXAMPLE 1. SOL/GEL COATING WITH MIXED PRECURSOR SOLUTION 
     A basic precursor solution was prepared by diluting 75 parts of NaSi 40/42 with 25 parts of water with stirring. An acidic solution was prepared with boric acid or sodium aluminate in the concentrations and amounts given in tables 1 below. The acidic solution was added slowly with vigorous stirring to the basic precursor solution. The data are given in tables 1 below. 
     CaO/MgO particles (mixture in a molar ratio of 1:1) having a particle size d(50) of 40 μm and a particle size distribution of (d(90) 200 μm/d(10) 2 μm) were placed into a pilot fluidized bed apparatus in which the particles were heated to 65° C. The basic precursor solution was sprayed onto the particles via bottom spray (air inlet temperature: 120-130° C.; outlet temperature: 65° C.; gas volume: 47 m 3 /h; drying gas velocity: 0.9-1.0 m/s; nozzle gas volume: 4-7.5 m 3 /h; nozzle pressure: 3.3 bar). The dried particles had a water content of &lt;1% by weight. 
     All the coated particles described in the examples were calcined as following method. The coated particles were placed into ceramic crucibles and heated in a muffle furnace. The particles were heated within 2 h to the selected temperature (325° C.-350° C.). The selected temperature was maintained for 3 h and thereafter the particles were allowed to cool to room temperature within 3 h to 9 h. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 CaO/ 
                   
                   
                   
                   
                   
               
               
                 Sam- 
                 MgO 
                 Water glass 
                 NaSi 
                 coating 
               
               
                 ple 
                 in kg 
                 solution 
                 in g 
                 in g 
                 relative 
                 Acidic Solution 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Ref. 
                 2 
                 NaSi 40/42 
                 1810 
                 500 
                 20% 
                 0.5M HCl 1448 g 
               
               
                   
                   
                 75% 
               
               
                 1 
                 2 
                 NaSi 40/42 
                 1810 
                 500 
                 20% 
                 4% boric acid 
               
               
                   
                   
                 75% 
                   
                   
                   
                 1448 g 
               
               
                 2 
                 2 
                 NaSi 40/42 
                 1810 
                 500 
                 20% 
                 4% boric acid 724 
               
               
                   
                   
                 75% 
                   
                   
                   
                 g + 1.3% NaAlO 2   
               
               
                   
                   
                   
                   
                   
                   
                 724 g 
               
               
                   
               
               
                   2)  The boric acid and sodium aluminate were mixed before spraying 
               
            
           
         
       
     
     EXAMPLE 2. SOL/GEL COATING BY ALTERNATING SPRAY OF PRECURSOR SOLUTIONS 
     A basic precursor solution was prepared by diluting 90 parts of NaSi 37/40 with 10 parts of water with stirring (sample 3) or using neat NaSi 37/40 (sample 4). The acidic solution was prepared with boric acid in the concentrations given in table 2 below. The basic precursor solution (BS) and the acidic solution (AS) were sprayed alternately in the following order: 40% of BS, 50% of AS, 40% of BS, 50% of AS, and 20% of BS. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 CaO/ 
                   
                   
                   
                   
                   
               
               
                 Sam- 
                 MgO 
                 water glass 
                 NaSi 
                 coating 
               
               
                 ple 
                 in kg 
                 solution 
                 in g 
                 in g 
                 relative 
                 Acidic Solution 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 3 
                 2 
                 NaSi 37/40 
                 1590 
                 499 
                 20% 
                 4% boric acid 
               
               
                   
                   
                 90% 
                   
                   
                   
                 1400 g 
               
               
                 4 
                 2 
                 NaSi 37/40 
                 1410 
                 492 
                 20% 
                 4% boric acid 
               
               
                   
                   
                 100% 
                   
                   
                   
                 1400 g 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 3. SOL/GEL COATING BY SIMULTANEOUS SPRAY OF PRECURSOR SOLUTIONS 
     A basic precursor solution was prepared by diluting 90 parts of NaSi 37/40 with 10 parts of water with stirring (sample 5) or diluting 75 parts of NaSi 37/40 with 25 parts of water with stirring (samples 6 and 7). The acidic solution was prepared with boric acid or sodium aluminate in the concentrations and amounts given in table 3 below. 
     The basic precursor solution and the acidic solution were sprayed simultaneously via the bottom nozzle (spraying upwards) and via the top nozzle (spraying downwards) respectively. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 CaO/ 
                   
                   
                   
                   
                   
               
               
                 Sam- 
                 MgO 
                 water glass 
                 NaSi 
                 coating 
               
               
                 ple 
                 in kg 
                 solution 
                 in g 
                 in g 
                 relative 
                 Acidic Solution 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 5 
                 2 
                 NaSi 37/40 
                 1590 
                 499 
                 20% 
                 3.5% boric acid 
               
               
                   
                   
                 90% 
                   
                   
                   
                 1590 g 
               
               
                 6 
                 2 
                 NaSi 37/40 
                 1890 
                 495 
                 20% 
                 10% aluminum 
               
               
                   
                   
                 75% 
                   
                   
                   
                 sulfate 1890 g 
               
               
                 7 
                 2 
                 NaSi 37/40 
                 1890 
                 495 
                 20% 
                 3% boric acid 
               
               
                   
                   
                 75% 
                   
                   
                   
                 945 g 
               
               
                   
                   
                   
                   
                   
                   
                 10% al. sulfate 
               
               
                   
                   
                   
                   
                   
                   
                 945 g (total 
               
               
                   
                   
                   
                   
                   
                   
                 1890 g) 
               
               
                   
               
               
                   7)  The boric acid and aluminum sulfate solutions were mixed before spraying 
               
            
           
         
       
     
     EXAMPLE 4. EXPANSION TEST OF THE CORE-SHELL EXPANDING AGENTS 
     The expansion test for concrete was carried out in accordance with DIN EN ISO 10426-5 by making a well cement slurry including 10% by weight of cement (bwoc) of the particles of the invention, 0.5% (bwoc) Liquiment K3F (dispersant, suffonated acetone resin), 0.6% bwoc Polytrol FL34 (fluid loss additive, modified polyacrylamide) and a small amount &lt;0.1% by volume defoamer (triisobutylphosphate). 
     The percentage expansion over the time is shown in  FIG. 3 . As seen in  FIG. 3 , all core-shell expanding agents with a sot/gel-formed coating comprising a mixed oxide of B and Si, Al and Si or B/Al and Si showed retarded on-set time of expansion as compared to the reference sample with silica shell.