WO2022071797A1

WO2022071797A1 - Magnesia powder

Info

Publication number: WO2022071797A1
Application number: PCT/NL2021/050589
Authority: WO
Inventors: Hossein Karimi; Qingliang YU; Jos BROUWERS
Original assignee: Technische Universiteit Eindhoven
Priority date: 2020-09-29
Filing date: 2021-09-29
Publication date: 2022-04-07

Abstract

The present invention relates to magnesia powder having a density of 2.8 to 3.6 g/cm³, a crystal grain size of smaller than 200 Å, a slit pore size after calcination smaller than 20 nm, loss on ignition of smaller than 40%, the powder comprising an amount of particles in which a particle size of greater than 50 μm is 5% by weight or less, and the amount of particles in which a particle size of greater than 25 μm is 50% or less.

Description

Title: Magnesia powder

Description

Background

The present invention relates to magnesia powder, a method for producing magnesia powder, the use of magnesia powder and articles comprising magnesia powder.

Commercially produced magnesium oxide (commonly referred to as magnesia or periclase) is not mined directly, as periclase itself is relatively rare in nature and its hydration product brucite [Mg(OH)2] occurs in only a limited number of commercially viable geological formations. MgO is instead generally obtained either by a dry route from the calcination of mined magnesite deposits (MgCOs) or by a wet route from solutions of magnesium-bearing brines or seawater. The dry route for MgO production typically requires the crushing of magnesite before calcination through the process of MgCOs-> MgO + CO2. Higher-grade MgO requires careful selection of MgCOs- bearing rocks or pretreatment due to Fe2Os, AI2O3, CaO, and SiO2 impurities, which can adversely affect the refractory usage of MgO. MgO is typically used in the refractories industry to form linings and bricks, principally because of its high melting point. Caustic-calcined/light-burned MgO finds a range of applications in agriculture, the paper and pharmaceuticals industries, fire proofing, and many more.

In large structural applications, where shrinkage is often observed as a result of cooling after an initial exothermic hydration reaction rather than autogenous or drying shrinkage of the cement hydrates, conventional shrinkage-compensating cements are unsuitable, as shrinkage occurs long after the desired expansive products have formed. For this purpose, MgO expansive cements have been gaining momentum. These rely on the expansive hydration of MgO to Mg(OH)2.

US 4,039,345 relates to an improved shrinkage compensating Portland cement concrete and mortar compositions containing essentially an admixture of an expansive Portland cement, mineral aggregate, a styrene-butadiene- 1 ,3 copolymer , a non-ionic surfactant, an anionic surfactant, a polyorganosiloxane foam depressant and alkali resistant glass fibers to provide restraint against expansion, wherein the expansive Portland cement is Type K expansive cement. Drying shrinkage of concrete is its volume change due to moisture loss, caused by the outward movement of water from its surface. If drying shrinkage is not properly mitigated, it causes concrete structures to crack over time. In order to offset strains caused by drying shrinkage in cement composites, traditionally, four types of shrinkage compensating admixtures (SCAs), namely Type K (a blend of calcium sulfoaluminate and calcium sulfate), Type M (a blend of calcium aluminate cement and calcium sulfate), Type S (a blend of tricalcium aluminate cement and calcium sulfate), and Type G (a blend of calcium oxide and aluminum dioxide) have been used. Three of these SCAs (Type K, Type M, and Type S) contain sulfoaluminates which produce ettringite after hydration, thereby resulting in swift expansion while the remaining SCA (Type G) contains calcium oxide which produces swift expansive product, calcium hydroxide, after hydration. The working mechanism of these SCAs is based on producing swift expanding hydration products, which by proper restraining, offset drying shrinkage strains. Since these SCAs have been introduced, their high energy demand for production and the instability of their hydration products have raised many concerns. Hence, introducing more sustainable SCAs is of paramount importance.

In an article from 2002 written by H. Russel, R. Stadler, H. Gelhardt, “Shrinkage-Compensating Concrete Made with an Expansive Component”, is reported that Type G expansive components, similar to other SCAs such as Type K admixtures, swiftly start expanding at very early age and stop expanding in less than a week. On the contrary, magnesia-based expansive components react slowly and cause expansion in cement composites after a few weeks. Due to this lower expansion capacity, unlike calcium oxide in Type G expansive components, magnesia has never been used as an SCA in concrete. In order for magnesia to be used as an SCA, it has to be capable of producing swift expansion, preferably in less than a week, to be restrained by rebars and offset contraction caused by drying shrinkage.

There is a need of a magnesia powder which swiftly expands in cement composites and produces expansion in less than a week.

Objects of the invention

It is an object of the present invention to provide a magnesia powder that swiftly expands in cement composites and produces expansion in less than a week.

The present invention relates to a magnesia powder having a density of 2.8 to 3.6 g/cm³, a crystal grain size of smaller than 200 A, a slit pore size after calcination smaller than 20 nm, loss on ignition of smaller than 40%, the powder comprising an amount of particles in which a particle size of greater than 50 .m is 5% by weight or less, and the amount of particles in which a particle size of greater than 25 .m is 50% or less.

In an embodiment of the magnesia powder according to the present invention more than 80% of mesopore volume is made up by pores smaller than 20 nm, preferably the peak of log differential pore volume appears in mesopore sizes smaller than 15 nm.

The purity of the magnesia powder according to the present invention may range from 70% to 99,99% by weight.

In an embodiment of the magnesia powder according to the present invention the particle size distribution satisfies the relationship (1), wherein Dx shows the size at which x% of the particle-size distribution falls below, Di is the size of the particles between Dmin and Dmax, and q is between 0.25 and 0.5,

The present invention also relates to a method for producing magnesia powder by thermal decomposition of a magnesium compound, wherein the magnesium compound is chosen from the group of nesquehonite, magnesite, brucite, dolomite and landsfordite, or any combination thereof.

In an embodiment of the present method for producing magnesia powder nesquehonite is thermally decomposed at a calcination temperature less than 500°C, more preferably between 350°C and 450°C is used.

In an embodiment of the present method for producing magnesia powder magnesite is thermally decomposed at a calcination temperature less than 650°C, more preferably between 500°C and 600°C is used.

In a further aspect, where CaCCh at high percentage is present in the magnesite-bearing rocks, a calcination temperature less than 950 °C may be used to obtain a mixture of CaO and magnesia, in which CaO also contributes to the overall expansive behavior. Dolomite is a carbonate mineral composed of calcium magnesium carbonate, ideally CaMg(CC>3)2, is more abundant than magnesite-bearing rocks. However, for dolomite the present inventors found that higher calcination temperatures, e.g. 700 and 800 °C, are needed. The percentage of the compounds is ideally around 50-50 in a dolomitic rock. Dolomite is characterized by its nearly ideal 1 :1 stoichiometric ratio of magnesium to calcium. Natural dolomites often contain impurities such as Mn, Fe, and other divalent cations that replace either Mg or Ca in the lattice.

In an embodiment of the present method for producing magnesia powder brucite is thermally decomposed at a calcination temperature less than 500°C, more preferably between 250°C and 450°C is used.

In an embodiment of the present method for producing magnesia powder the thermal decomposition takes place in air, an inert gas, such as nitrogen gas or argon gas, or in vacuo, at a thermal gradient of less than 5 degrees per minute, preferably less than 2 degrees per minute, more preferably less than 1 degree per minute at temperatures above 300°C for nesquehonite, at temperatures above 450°C for magnesite, and at temperatures above 200°C for brucite.

The present invention also relates to use of magnesia powder as a shrinkage compensating admixture in cement composites and geopolymers,

In an embodiment of the present invention the shrinkage is chosen from the group of chemical shrinkage, autogenous shrinkage and drying shrinkage of concrete. In a further aspect the magnesia powder is applied at a dosage of 0.5% to 20% of cement content to concrete.

In an embodiment of the present invention magnesia powder is used for eliminating the formation of cracks in solid articles, such as concrete slabs, floors and roads, by exerting expansion.

The present invention also relates to a solid article chosen from the group of concrete rebars, cement composites rebars and geopolymer rebars comprising magnesia powder showing an expansion between 0.03% and 1.0%, measured according to ACI 223. In a further aspect the magnesia powder is applied at a dosage of 0.5% to 20% of cement (or binder) content to concrete or geopolymer.

In a further aspect a dosage is used of at least 0.5 wt.% of cement or binder content, preferably a dosage of at least 2.5 wt.%, more preferably a dosage of at least 5.0 wt.% and a maximum of at most 20 wt.%, preferably a maximum of at most 17.5 wt.%, more preferably a maximum of at most 15.0 wt.%. In an aspect magnesia powder meets one or more parameters chosen from the group of a density of 2.8 to 3.6 g/cm³, a crystal grain size of smaller than 200 A, a slit pore size after calcination smaller than 20 nm, loss on ignition of smaller than 40%, an amount of particles in which a particle size of greater than 50 .m is 5% by weight or less, and an amount of particles in which a particle size of greater than 25 |_im is 50% or less.

The following examples, wherein all parts and percentages are to be taken by weight, illustrate the present invention.

Figure 1 shows mineral crystalline phases in different samples of magnesia, measured by XRD (P: periclase, Q: quartz, M: magnesite, C: calcite).

Figure 2 shows morphology of the particles in the samples of magnesia, obtained by ESEM: (a) MgO-l; (b) MgO-ll; (c) MgO-lll, and (d) MgO-IV.

Figure 3 shows experimental adsorption and desorption isotherms of N2 (at 77 K) on the samples of magnesia: (a) MgO-l; (b) MgO-ll; (c) MgO-lll, and (d) MgO-IV.

Figure 4 shows the BJH pore size distribution obtained for the nitrogen adsorption at 77K on the samples of magnesia: (a) MgO-l; (b) MgO-ll; (c) MgO-lll, and (d) MgO-IV.

Figure 5 shows the expansive properties of mortars containing magnesia, cured under water.

1.1. Materials

The present research involved analyzing four types of magnesia, all produced by calcining magnesite. The time required for 5.00 g of each of these samples of magnesia to neutralize a diluted solution of 100 mL of 1.0 N acetic acid in 300 mL of deionized water at 25±1°C, measured according to [12], was considered as their acid reactivity time and is shown in Table 1. In general, the samples covered a wide area of swift acid reactivity from 200 to seven seconds. More specifically, MgO-l had ‘slowest’ acid reactivity (200s) while MgO-ll gave ‘medium’ acid reactivity (145s). On the other hand, both MgO-lll and MgO-IV ‘swiftly’ reacted with acid (10s and 7s, respectively).

Table 1. Acid reactivity time of magnesia samples, measured according to [12],

The cement CEM I 52.5R, provided by ENCI (the Netherlands), having Blaine surface area of ca. 527 m²/kg, was used to examine the influence of the magnesia samples on the volume change of standard mortars. The chemical composition of the cement CEM I 52.5R was measured by X-ray fluorescence spectroscopy (XRF, Malvern Panalytical) and is listed in Table 2. A CEN standard sand was used in making standard cement mortars [23],

Table 2. Chemical composition of the cement CEM 1 52.5R, measured by XRF.

1.2. Experimental methodology

1.2.1. Loss on ignition and Chemical composition

Loss of ignition (LOI) is the reduction in weight of as-received material at 1000 °C due to releasing free moisture, chemically combined water, carbon dioxide, sulfur dioxide, etc. The difference between the original mass of 3 g of the magnesia samples, passing a No. 100 (149 pm) sieve and the final mass of the sample after being ignited in a muffle furnace to 1000°C until constant mass was obtained, was used to calculate LOI, according to ASTM C25 [24], Next, the residues were blended with non-wetting agent (LiBr) and flux (UBO4 and U2BO7). Then, the blends were fused at 1050°C in a fluxer (classisse leNeo) and cast in molds to obtain fused beads. Finally, the fuse beads were analyzed by X-ray Fluorescence (PANalytical Epsilon 3, OMNIAN method) to measure chemical compositions.

1.2.2. Phase identification, crystallinity, and average crystallite size

A D4 ENDEAVOR X-ray Diffractometer from Bruker was used to examine the crystalline structure. A DIFFRAC.EVA software was employed for identifying Phases and determining crystallite sizes. The Scherrer equation, as below, was used to calculate the average crystallite size.

where AQ is the breadth of a particular peak, is the wavelength in the pattern employed and k is a constant.

1.2.3. Environmental scanning electron microscopy (ESEM)

The morphology of magnesia particles was investigated using a FEI quanta 600 environmental scanning electron microscope. Before the investigation, in order to improve the conductivity of sample surface, all the samples were sputtered with an approximately 15 nm layer of gold. The micrographs were taken using both secondary and back-scattered electron detectors (MIX mode) at 5 kV with a spot of 3.0.

1.2.4. Physisorption isotherms and BET surface area

A complete physisorption isotherm was measured for all samples of magnesia. The tests were conducted at cryogenic temperature of boiling point of liquid nitrogen (77 K), using a Micromeritics TriStar II analyzer. The adsorption isotherms were plotted using

where V^a is the amount of adsorbate, m^s the mass of solid, p the actual adsorbing gas pressure, p° the saturation pressure of the adsorbing gas at T, and T the thermodynamic temperature [25], The adsorption isotherm was used to calculate the BET surface area.

1.2.5. Barrett-Joyner-Halenda method (BJH)

The characterization of the porous structure of materials by BJH method involves the application of the Kelvin equation

<^v") where y is the surface tension, v^l the molar volume of the liquid (i.e., the condensed adsorptive), r_K the Kelvin radius, R the gas constant, T the thermodynamic temperature, and ^p/_p0 as used previously. The computation procedure for pore size distribution by BJH method can be found in [25],

In this study, the Frenkel-Halsey-Hill (FHH) equation for multilayer analysis was utilized for calculating the adsorbed layer remaining on the pore walls in each step of the BJH method, which gives the following formula for nitrogen adsorbed at 77 K:

where t is the thickness of multimolecular layer (nm), and ^P //p as used previously.

1.2.6. Expansive properties of mortars containing magnesia

As listed in Table3, mortars containing magnesia were produced to assess their expansive performance in cement composites. In addition, a commercially available Type G shrinkage-compensating admixture was also used to compare and contrast its swift expansive behavior with that of magnesia samples.

The mortars were prepared using a Hobart mixer. The mixing procedure started with blending cement and expansive admixtures (magnesia samples or Type G admixture). Next, water and binder were mixed at low speed for 1 min. Then, the mixer was stopped for 90s, during which the mortar adhering to the bottom, walls, and stirrer was removed. Finally, the mixing was continued at the high speed for 2 min.

The mixtures were used to cast three replicate prism specimens, 40 x 40 X 160 mm³. Six hours after casting, the samples were demolded, and the strain measurement pins were installed on two parallel sides of the specimens. After measuring the distance between pin on both sides, the samples were stored and cured under water at 20°C.

Table 3. Recipe of mortars with different samples of magnesia and Type G expansive admixture.

2. Results

2. 1. 1. Loss on ignition and chemical composition

The chemical composition and loss on ignition of different samples of magnesia are listed in Table 4. As all these sample were produced by thermal decomposition of magnesium compounds at temperatures far below 1000°C, the higher loss on ignition may be connected to a lower degree of heat treatment of the parent solid to make these types of magnesia. The main difference in the chemical composition of MgO-lll and MgO-IV is that the loss on ignition of MgO-lll is lower and its magnesia percentage is higher. Both MgO-l and MgO-ll have about 90% magnesium oxide.

Table 4. Chemical composition and loss on ignition of the samples of magnesia, measured according to ASTM C25 [24].

2. 1.2. Phase identification and average crystallite size

Main crystalline phases of the samples of magnesia are shown in Figure 1. All the samples mainly consist periclase. Besides, small concentrations of magnesite, calcite, and quartz are traceable in the samples. These data are consistent with what measured in the previous step by the XRF and were shown in Table 4. For example, what observed in Table 4 for MgO-IV, regarding the high loss on ignition and small concentration of oxides other than magnesium oxide, are in line with the existence of magnesite in it in Figure 1. In addition, what listed in Table 4 for MgO-l and MgO-ll, regarding the small concentrations of silicon dioxide and calcium oxide, are in correspondence with the existence of quartz and calcite in the samples.

The peaks of periclase in Figure 1 have different breadths and heights, which are indications of different crystal grain sizes in the samples. Table 5 reports the average crystal grain sizes of the samples. The MgO-IV has the smallest crystallite sizes. The average crystallite sizes in MgO-IV is 71.7% of that in MgO III. It is also smaller than 30% of MgO-l and MgO-ll. This data is agreement with the data regarding the LOI of the samples in Table 4, as higher LOI usually corresponds with lower heat treatments. Lower heat treatments result in less sintering in the samples and smaller crystallite sizes.

Table 5. Average crystal grain size of different samples of magnesia.

2. 1.3. Environmental scanning electron microscopy (ESEM)

Figure2a-d show the morphology of the particles of the samples of magnesia under an environmental electron microscope. The MgO-l and MgO-ll look similar as their morphology suggests a very high degree of sintering in the samples. This observation is in line with their high value of LOI in Table 4 and their big crystallite sizes. On the other hand, although particles of MgO-ll I show some degree of sintering, the micrographs of particles in MgO-IV give no indication of sintering. These data are in keeping with the lower LOI and smaller crystallite sizes of MgO-IV and suggest lower heat treatment temperatures in this sample.

2. 1.4. Physisorption isotherms and BET surface area

The physisorption isotherms of the different types of magnesia can be seen in Figure 3a-d. The physisorption isotherm of each sample consists of an adsorption and a desorption isotherm. The general shape of the adsorption isotherms of three samples of magnesia, namely MgO-l, MgO-ll, and MgO-ll I are similar — they consist of three regions: (1) a region, concave to the p/p° axis, at the beginning, (2) a linear region in the middle, and (3) a region convex to the p/p° axis at the end. The first concave region is a result of high interaction between the adsorbate and the spots on the adsorbent with the highest energies. The more the adsorbates occupy highly energetic spots of the adsorbent, the higher the curve plateaus out. By the beginning of the linear region, a monolayer of adsorbate has already covered the adsorbent. The gradual shift from the first to the second stage of the isotherms indicates the possibility of an overlap in the monolayer and multilayer adsorption of adsorbate to the adsorbent. The upsurge at the end of these adsorption isotherms is a result of the bulk condensation of the adsorbate to a liquid.

The physisorption isotherm of MgO-IV consists of three regions, too: (1) a region, concave to the p/p° axis, at the beginning, (2) a linear region in the middle, and (3) an almost flat region at the end. Although the beginning and the middle region of MgO-IV are similar, the flat region is an indication of limited mesopore sizes in the sample.

2. 1.5. Barrett-Joyner-Halenda method (BJH)

Figure4a-d represent the pore size distribution of magnesia samples, calculated by BJH algorithms for the nitrogen adsorption at 77K. Three magnesia samples, namely MgO- I, MgO-ll, and MgO-lll show a broad range of mesopores from 2 nm to 5 nm. While most of the pore volume in MgO-l and MgO-ll is in mesopores bigger than 20 nm, most of the pore volume in MgO-lll is in the pores between 5 nm and 20 nm. By contrast, for the most part, the pores in MgO-IV are smaller than 10 nm and the pore structure is uniform.

2. 1.6. Expansive properties of mortars containing magnesia

Figure 5 illustrates the expansive properties of mortars containing magnesia. The MgO-l and MgO-ll do not produce expansion at early age. This property can be attributed to their low reactivity as a result of sintering as was also observed in their pore size distribution curves in Figure 4. The MgO-lll produces a small amount of expansion at the early age, but the amount of this expansion is small due to sintering. The sintering in this sample showed itself in pore size distribution curves of Figure 4 in the form of having significant pore volume in almost all the mesopore sizes. The MgO-IV expands very fast and produces a high degree of expansion thanks to low sintering and high surface energy as was noticeable in Figure 4 from its uniform mesopore size distribution.

Claims

1. A magnesia powder having a density of 2.8 to 3.6 g/cm³, a crystal grain size of smaller than 200 A, a slit pore size after calcination smaller than 20 nm, loss on ignition of smaller than 40%, the powder comprising an amount of particles in which a particle size of greater than 50 .m is 5% by weight or less, and the amount of particles in which a particle size of greater than 25 .m is 50% or less.

2. Magnesia powder according to claim 1 , wherein more than 80% of mesopore volume is made of pores smaller than 20 nm, preferably the peak of log differential pore volume appears in mesopore sizes smaller than 15 nm.

3. Magnesia powder according to claim 1 , wherein the particle size distribution satisfies the relationship (1 ), wherein Dx shows the size at which x% of the particle-size distribution falls below, Di is the size of the particles between Dmin and Dmax, and q is between 0.25 and 0.5,

4. A method for producing magnesia powder according to any one or more the preceding claims by thermal decomposition of a magnesium compound, wherein said magnesium compound is chosen from the group of nesquehonite, magnesite, brucite, dolomite and landsfordite, or any combination thereof.

5. A method according to claim 4, wherein nesquehonite is thermally decomposed at a calcination temperature less than 500°C, more preferably between 350°C and 450°C is used.

6. A method according to claim 4, wherein magnesite is thermally decomposed at a calcination temperature less than 650°C, more preferably between 500°C and 600°C is used.

7. A method according to claim 4, wherein dolomite is thermally decomposed at a calcination temperature less than 950 °C, more preferably between 700°C and 800°C is used.

8. A method according to claim 4, wherein brucite is thermally decomposed at a calcination temperature less than 500°C, more preferably between 250°C and 450°C is used.

9. A method according to any one or more the claims 4-8, wherein the thermal decomposition takes place in air, an inert gas, such as nitrogen gas or argon gas, or in vacuo, at a thermal gradient of less than 5 degrees per minute, preferably less than 2 degrees per minute, more preferably less than 1 degree per minute at temperatures above 300°C for nesquehonite, at temperatures above 450°C for magnesite, and at temperatures above 200°C for brucite.

10. The use of magnesia powder according to any one or more of claims 1-3 or obtained according to the method according to any one or more of claims 4-8 as a shrinkage compensating admixture in cement composites and geopolymers by wherein the magnesia powder is added at a dosage of 0.5% to 20% of cement (or binder) content to concrete or geopolymer.

11. The use according to claim 10, wherein the dosage added is at least 0.5 wt.% of cement or binder content, preferably a dosage of at least 2.5 wt.%, more preferably a dosage of at least 5.0 wt.% and a maximum of at most 20 wt.%, preferably a maximum of at most 17.5 wt.%, more preferably a maximum of at most 15.0 wt.%.

12. The use according to any one or more of claims 10-11 , wherein said shrinkage is chosen from the group of chemical shrinkage, autogenous shrinkage and drying shrinkage.

13. The use according to any one or more of claims 10-12 for eliminating the formation of cracks in solid articles, such as concrete slabs, floors and roads, by exerting expansion.

14. A solid article chosen from the group of concrete rebars, cement composites rebars and geopolymer rebars comprising magnesia powder according to any one or more of claims 1-3 or obtained according to the method according to any one or more of claims 4-9, showing an expansion between 0.03% and 1 .0%, measured according to ACI 223 wherein the magnesia powder is added at a dosage of 0.5% to 20% of cement (or binder) content to concrete or geopolymer.

15. A solid article according to claim 14, wherein the dosage added is at least 0.5 wt.% of cement or binder content, preferably a dosage of at least 2.5 wt.%, more preferably a dosage of at least 5.0 wt.% and a maximum of at most 20 wt.%, preferably a maximum of at most 17.5 wt.%, more preferably a maximum of at most 15.0 wt.%.