NL2030357B1

NL2030357B1 - Use of mineral waste in the production of ceramics

Info

Publication number: NL2030357B1
Application number: NL2030357A
Authority: NL
Inventors: Berend Baas Jan
Original assignee: Vitrifex Ceram Glazes
Priority date: 2021-12-30
Filing date: 2021-12-30
Publication date: 2023-07-06
Also published as: WO2023126534A1

Abstract

The invention concerns the use of fly ash in the preparation of ceramic articles. The invention provides a method forthe manufacture of ceramic articles, wherein the weight ratio of fly ash to clay in the ceramic body mixture is at least 70/30. The invention further concerns the ceramic articles 5 obtained by the method. The ceramic articles according to the invention have excellent properties in terms of drying shrinkage, firing shrinkage, density, water absorption and frost resistance, they do not exhibit efflorescence and are suitable to be subjected to glazing.

Description

Use of mineral waste in the production of ceramics

Field of the invention

[0001] The present invention is in the field of the manufacture of clay-based articles, particularly in the field of clay articles manufactured by extrusion or moulding. The invention involves the use of a clay mixture comprising fly ash and clay for the manufacture of ceramic articles, as well as the process to manufacture a ceramic article using this ceramic body mixture as input material and the ceramic articles obtained from this process.

Background art

[0002] The process of manufacturing ceramic materials from clay and other materials is broadly known in the field of construction. Fly ash is a mineral waste resulting from coal combustion that is composed of the particulates that are driven out of coal-fired boilers together with the flue gases.

This type of ash differentiates from ash that falls to the bottom of the combustion chamber of the boiler, which is called bottom ash.

[0003] The components of fly ash vary considerably depending on the source and composition of the coal being burned, but all types of fly ash fly ash include substantial amounts of silicon dioxide (SiO2) (both amorphous and crystalline), aluminium oxide (AlzO3) and calcium oxide (CaO), the main mineral compounds in coal-bearing rock strata. Fly ash can also comprise other components in trace concentrations such as arsenic, beryllium, boron, cadmium, chromium, hexavalent chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium, along with very small concentrations of dioxins and PAH compounds, and also unburnt carbon.

[0004] In the past, fly ash was generally released into the atmosphere, but air pollution control standards now require that it be captured prior to release by fitting pollution control equipment. In order to avoid accumulation of this ash into landfills, fly ash has been used for decades in the construction industry, initially in the manufacture of cement later in the production of ceramic articles. However, there are issues linked to the use of fly ash in ceramic article manufacture. In particular, fly ash usually originates from oxidative combustion, which results in a fly ash with a very high carbonate content (approximately 50 wt%); this high concentration of carbonates prevents its use in the manufacture of ceramic articles, mainly linked to the weight loss of fly ash when subjected to thermal treatment above 900 °C, common in the ceramic industry.

[0005] There have been some intents in the prior art to maximize the use of fly ash in the manufacture of ceramic materials in the construction industry. For instance, the international patent application WO 2008/017082 describes a method for the preparation of a starting material for the production of clay mixtures, wherein said method comprises adding fly ash together with firstly added sodium silicate and then added phosphoric acid. However, no experimental evidence for the industrial applicability of the resulting ceramic bodies is presented, for instance about the properties resulting from these high levels of fly ash in ceramic articles.

[0006] One of the most prominent efforts to incorporate fly ash in ceramics for construction purposes is that of EP2660219; this patent application relates to a method for obtaining a ceramic body mixture by mixing clay and fly ash with a percentage of ash equal to or lower than 50 wt%, adding phosphoric acid and grinding the mixture; and adding hydrated sodium silicate {NazO)n(SiOz}m(H20) and grinding the mixture. However, this application teaches away from using higher percentages of fly ash because the properties of the resulting ceramic article would not match those of a conventional ceramic articles for construction.

[0007] Additionally, efflorescence of water soluble calcium salt in natural clays, mainly in the form of calcium sulphate is known to cause the formation during drying of a white surface layer of bricks, also known as scumming. This is a known issue in the brick and tile industry using natural clays.

To solve this issue, the industry demands clays of as low as possible content of free (i.e. water soluble) calcium. To prevent the formation of scumming when using clays with free calcium, a small wt% of barium carbonate (BaCO:s) is added to the clay. The clay mixture is "aged" as it is called for up to four weeks before being processed, during which time calcium sulphate reacts with barium carbonate forming calcium carbonate and barium sulphate [reference], both of which are not water soluble (see e.g. E. Brownell. Scum Development. In: Structural Clay Products, Applied Mineralogy 9, chapter 5.6 (Springer-Verlag, 1976), and US4226635). This way, the free calcium is trapped in the body, preventing its efflorescence.

[0008] The present invention provides in the need for ceramic articles suitable for the construction industry with a high percentage of fly ash material, thus maximizing the use of fly ash in the construction industry.

Summary of the invention

[0009] The inventor has developed a process for the manufacture of ceramic articles by using a ceramic body mixture comprising fly ash and clay, wherein the weight ratio of fly ash to clay in the mixture is at least 70/30. The resulting ceramic articles from the process have physical properties that enable them to be used in the construction industry. The inventor found that especially the combination of a highly plastic clay such as bentonite and/or ball clay and biomass-based fly ash results in a ceramic body mixture with enhanced plasticity which enables its use in the production of ceramics. The present invention offers advantageous effects over the prior art as it enables the use of high content of fly ash, while eliminating the need to chemically modify the fly ash (e.g. by addition of reactants).

[0010] The invention can be defined according to the following list of preferred embodiments: 1. Use of a ceramic body mixture comprising fly ash and clay, wherein the weight ratio of fly ash to clay in the mixture is at least 70/30, for the manufacture of ceramic articles. 2. The use according to embodiment 1, wherein the weight ratio of fly ash to clay in the mixture is in the range of 80/20 to 95/5. 3. The use according to embodiment 1 or 2, wherein the fly ash comprises soluble salts in the range of 0.5 to 10 wt®%.

4. The use according to any one of embodiments 1 to 3, wherein the fly ash is biomass-based fly ash. 5. The use according to any one of embodiments 1 to 4, wherein the clay is bentonite. 6. The use according to any one of embodiments 1 to 5, wherein the ceramic body mixture further comprises water and 0.01 to 1 wt% of a detergent based on total water. 7. Method for producing a ceramic article, comprising (a) obtaining a ceramic body mixture as defined in any one of embodiments 1 -6; (b) subjecting the resulting ceramic body mixture to a shaping step and (c) firing the molded ceramic body mixture at a temperature in the range of 800 to 1300 °C, to obtain the ceramic article. 8. The method according to embodiment 7, wherein the firing is carried out at a temperature in the range of 1000 to 1250 °C. 9. The method according to embodiment 7 or 8, wherein the shaping step is done by moulding, dry-pressing or extrusion. 10. The method according to any of embodiments 7 to 9, wherein the fly ash is incubated with water prior to mixing with other ingredients to obtain the mixture of step (a), and the process further comprises drying the ceramic body mixture of step (a), and crushing and/or grinding the dried ceramic body mixture prior to step (b). 11. The method according to embodiment 10, wherein the water content during the incubation is in the range of 1 — 80 wt%, based on the amount of fly ash. 12. The method according to any of embodiments 10 or 11, further comprising applying a glaze coating to at least part of the surface of the ceramic article obtained in step (c). 13. Ceramic article obtainable by the process according to any one of embodiments 7 to 13. 14. Ceramic article according to embodiment 14, which is a brick or a tile.

Detailed description

[0011] The inventors have developed ceramic body mixtures comprising fly ash and clay, wherein the weight ratio of fly ash to clay in the mixture is at least 70/30, for the manufacture of ceramic articles. Ceramic body mixtures are known in the art as the precursor to a ceramic article such as a brick. The ceramic body mixtures can be formed into a desired shape, such as a brick. After drying and firing in an oven, a solid ceramic brick is obtained with optimal properties to be used in the construction industry.

[0012] First and foremost, the present invention provides for the use of a ceramic body mixture comprising fly ash and clay, wherein the weight ratio of fly ash to clay in the mixture is at least 70/30, for the manufacture of ceramic articles, as well as the process to manufacture a ceramic article using this ceramic body mixture as input material and the ceramic articles obtained from this process (e.g. bricks). Any characteristic disclosed herein for the process according to the invention equally applies to the product according to the invention and vice versa.

The ceramic body mixture

[0013] The basis of the present invention is the ceramic body mixture that is developed by the inventor. The ceramic body mixture comprises fly ash and clay in a weight ratio of at least 70/30. In a first aspect, the present invention concems the use of this ceramic body mixture in the manufacture of ceramic articles such as pottery, constructions materials such as bricks, walls and floor tiles, cooking pots, art objects, dishware, furniture such as tableware, smoking pipes and musical instruments, preferably bricks, walls or tiles.

[0014] Fly ash is a completely non-plastic material and therefore lacks the unique design properties of clay. To improve its plasticity for its use in the ceramic industry, the current state of the art relies on the use of additives such as sodium silicate or phosphoric acid, with the consequent financial and environmental costs added to the process.

[0015] Surprisingly, the inventor found that the high amounts of water-soluble salts in biomass- based fly ash (when compared to classic pulverized coal fly ash) enables the manufacture of ceramic articles from this fly ash with enhanced physical properties. Biomass-based fly ash refers to fly ash that originates from biomass and may be obtained by the combustion of biomass.

Biomass-based fly ash may also be referred to as biomass fly ash. The inventor hypothesizes that these salts behave like electrolytes, thus increasing the shaping behaviour of the final mixture of fly ash - clay and having a favourable effect on the plasticity of the mixture, thus reducing or even removing the need to add further additives to the mixture. The presence of soluble salts with biomass-based fly ash enables the generation of a mixture with high concentration of fly ash and adequate plasticity to be use in the production of materials in the ceramic industry. The high content of soluble salts in biomass-based fly ash makes this type of fly ash not appropriate for the production of concrete, wherein a low content of soluble salts is required. Therefore, the present invention enables the use of these fly ash in the construction industry overall. Thus, in a preferred embodiment, the fly ash is biomass-based, preferably it comprises or even is biomass-based fly ash. In an especially preferred embodiment, the biomass-based fly ash is biocarbon fly ash, which results from the burning of carbonized biomass, e.g. by heat-treatment or pyrolysis of biomass.

[0016] In a preferred embodiment, the fly ash comprises water-soluble salts. Preferably, the content of water-soluble salts in the fly ash is between 0.5 and 10 wt%. More preferably, the content of water-soluble salts in the fly ash is between 0.5 and 5 wt%, most preferably between 1 and 3 wt%. Preferably, the fly ash is biomass-based fly ash, most preferably biocarbon fly ash. The content of water-soluble salts in biomass-based fly ash depends on the percentage of biomass in the fuel mixture used the incineration process. The water-soluble salt content of biomass-based fly ash also depends on the weather and the seasons where the crops are collected, so care needs to be taken in adjusting the soluble salts concentration of the fly ash if needed.

[0017] Clays suitable for ceramic body mixtures are known in the art. In a preferred embodiment, the clay used for the ceramic body mixture is selected from the group of clay minerals, preferably kaolinite, smectite and/or illite. Preferably, the clay used for the ceramic body mixture is fire clay, stoneware clay, red fire clay (iron oxide containing), ball clay and/or bentonite. Most preferably, the clay used for the ceramic body mixture is a highly plastic clay such as bentonite and/or ball clay.

The high plasticity of bentonite enables obtaining a mixture accepting even higher amounts of biomass-based fly ash while keeping a higher overall plasticity of the mixture. Bentonite is also available in so-called recycled form as recycling material from greenhouse horticulture; therefore, a shaped brick from 100 wt% (recycled) waste material can be obtained from the process according to the present invention. In addition, adding a highly plastic clay such as bentonite and/or ball clay in combination with a more standard and cheaper type of clay such as red-firing brick clay could be advantageous regarding the plasticity of the ceramic body. Furthermore, bentonite is very useful in increasing the firing shrinkage and density and reducing the water absorption of the ceramic articles. In view thereof, articles containing bentonite have improved properties, such as improved frost resistance. Thus, in a preferred embodiment, the clay comprises bentonite, preferably at least 25 wt% bentonite, more preferably at least 50 wt% bentonite or even at least 80 wt% bentonite, based on total clay in the ceramic body mixture. In a most preferred embodiment, the clay in the ceramic body mixture is bentonite.

[0018] The weight ratio of fly ash to clay in the ceramic body mixture is at least 70/30. In other words, the ceramic body mixture comprises at least 70 wt®% fly ash. In a preferred embodiment, the mixing ratio of fly ash to clay in the ceramic body mixture is higher than 70/30. Preferably, the weight ratio of fly ash to clay in the ceramic body mixture is at least 75/25, more preferably in the range of 80/20 to 99/1, even more preferably in the range of 85/15 to 95/5, most preferably in the range of 90/10 to 95/5. These higher ratios enable obtaining ceramic articles with acceptable physical properties while avoiding an excessive density, while the content of fly ash, a waste stream which is now put to valuable use, is maximized.

[0019] In addition to the clay and fly ash, the ceramic body mixture may comprise further components. Typically, at least 60 wt% of the total weight of the ceramic body mixture is formed by the clay and fly ash, preferably 65 — 95 wt%, more preferably 70 — 90 wt%. The ceramic body mixture may further comprise water and additives known in the art. In a preferred embodiment, the ceramic body mixture further comprises water, and optionally one or more detergents.

[0020] In a preferred embodiment, the water content of the ceramic body mixture is in the range of 1 to 40 wt%. Preferably, the water content of the ceramic body mixture is in the range of 10 to 35 wt%, more preferably in the range of 15 to 30 wt%, even more preferably in the range of 22 to 28 wt%. This percentage enables a ceramic body mixture with appropriate physical properties to facilitate the shaping step.

[0021] In a preferred embodiment, the ceramic body mixture comprises one or more detergents.

Preferably, the amount of detergent added to the ceramic body mixture is in the range of 0.01 to 1 wt% of the total mass of the ceramic body mixture, more preferably 0.01 — 0.5 wt% of the total mass of the ceramic body mixture, even more preferably 0.03 — 0.3 wt%, most preferably 0.05 — 0.15 wt% of the total mass of the ceramic body mixture. Any detergent known in the art is useful in this respect, although optimal results are obtained with anionic surfactants. The function of the detergent is to reduce the surface tension of the water in the ceramic body mixture and thus improves the plastic properties of the ceramic body mixture. The ceramic body mixture may also comprise minor amounts of other additives, such as binders and/or plasticizers as known in the art. The total content of additives typically is not more than 5 wt%, such as in the range of 0.1 — 2 wt%.

The process

[0022] In a second aspect of the invention, the present invention relates to a method for producing a ceramic article, comprising at least the following steps: (a) obtaining a ceramic body mixture as defined above; (b) subjecting the resulting ceramic body mixture into a shaping step and (c) firing the shaped ceramic body mixture at a temperature in the range of 700 to 1250 °C, to obtain the ceramic article.

[0023] Such a process for producing ceramic articles is well known in the art, and can be performed in any way known in the art. Even though the present invention mainly resides in the composition of the ceramic body mixture, certain aspects of the process as further defined below are especially suitable for the producing ceramic articles from the ceramic body mixture according to the invention.

[0024] In a preferred embodiment, the shaping step (b) is carried out by moulding, dry-pressing or extrusion, preferably by moulding or extrusion, most preferably by extrusion. Such shaping of ceramic body mixtures is well-known in the art, and can be performed in any suitable way. The exact nature of the shaping step depends on the desired shape and nature of the ceramic article that is being produced.

[0025] In a preferred embodiment, the ceramic body mixture is conditioned prior to the firing step {(c). Conditioning preferably involves a drying step which is preferably carried out at a temperature in the range of 20 °C — 100 °C for a period of time in the range of 6 — 72 hours. The skilled person will understand that the optimal drying conditions vary based on several factors, such as the availability of residual heat from the firing step (c) for drying, the size of the article etc. More preferably, the clay product is dried at a temperature in the range of 30 °C — 99°C for 10 — 48 hours, most preferably at a temperature in the range of 50°C — 90°C for 20 — 36 hours. Preferably, the product is dried in a drying chamber as in known in the art.

[0026] In a preferred embodiment, the firing step (c) is carried out at a temperature in the range of 800 to 1300 °C, more preferably in the range of 900 to 1250 °C, even more preferably in the range of 1000 to 1250 °C, even more preferably in the range of 1070 to 1250 °C, most preferably in the range of 1100 to 1200 °C. In particular, these higher ranges of firing temperature maximizes properties such as the firing shrinkage, loss on ignition (LOI) while minimising the water absorption of the resulting ceramic article.

[0027] The inventor surprisingly found that the reduction of scumming was a factor of the time the ceramic body mixtures were incubated (or "aged") at ambient temperature before allowing them to dry, even when no other material is added to react with the free calcium. Subsequently, the inventor found that when only the fly ash itself is mixed with water and conditioned, prior to the addition of the other materials of the ceramic body mixture, surface scumming of the fired ceramic end product is prevented. Without willing to be bound by any theory, it is hypothesized that free calcium is trapped in the wet fly ash by pozzolanic reactions, resulting in zeolitization of the fly ash. In the wet state, free calcium in the fly ash may react with dissolved silicate (and/or aluminate) ions to form calcium {alumino)silicates, which form a gel-like material with a molecular sieve structure and properties similar to zeolites (cation binding/absorption properties). Silicate, as well as aluminate ions, can be dissolved from any aluminosilicate material in water when the pH of the water is very alkaline (212). The inventor surprisingly found that the free alkali/alkali earth content of biomass- based fly ashes is such that in a wet state the pH is raised above 13,enabling silica and alumina present in the fly ash to dissolve, thus initiating and maintaining the pozzolanic reaction at ambient temperature. Furthermore, longer incubation times of the fly ash with water results in higher levels of zeolitization, eventually trapping all free calcium, thus resulting in the production of a ceramic article wherein scumming is reduced or even entirely absent. During the incubation in the presence of water, the fly ash will harden, most likely as a result of the pozzolanic reactions and zeolitization.

The resulting materials is very brittle, and can be easily crushed or ground.

[0028] Thus, in a preferred embodiment, the process according to the invention includes an incubation step for the fly ash, where in the fly ash is incubated with water before the addition of other materials to complete the ceramic body mixture. Preferably, the incubation time is at least 1 day, more preferably at least 6 days, even more preferably at least 18 days and most preferably at least 32 days. Even though there is no limit to the incubation time, from a practical perspective, it is preferred the incubation lasts at most two years, more preferably at most 1 year. The water content in the mixture of fly ash and water is between 1 and 80 wt%, more preferably between 5 and 60 wt%, even more preferably between 10 and 50 wt®%, even more preferably between 15 and 45 wt®%, most preferably between 30 and 40 wt%. In a preferred embodiment, the incubated material is subsequently crushed (for example, in a jaw crusher) and/or ground (for instance in a rolling mill), either prior to or together with the mixing with the other materials that make up the ceramic body.

[0029] The process according to the invention may further comprise a glaze coating step, wherein a glaze coating is applied on at least part of the surface of the fired ceramic article. Such glaze coatings and the application thereof on ceramics are known in the art, and can be performed in any suitable way. Surprisingly, glaze coatings could be applied on ceramic articles according to the invention, even if a scumming layer would be present. The inventor has shown that this scumming layer is invisible after application of the glaze layer, probably because it is dissolved therein.

The obtained ceramic article

[0030] The invention further concerns the ceramic articles that are obtainable or obtained by the process according to the invention. These articles are characterized by a high content of fly ash.

Because of the negligible loss on ignition of the ceramic body mixture, i.e. both the fly ash and the clay, the ceramic articles are characterized by the same contents of fly ash and clay as the ceramic body mixture defined above. Thus, the ceramic articles according to this aspect of the invention can also be defined as having a fly ash content of at least 70 wt%, based on total weight of the ceramic article. The process according to the invention enables the production of any type of ceramic article, although the production of bricks or tiles is preferred.

[0031] The ceramic articles according to the invention have excellent properties, while being prepared from substantial amounts of fly ash. Their performance in terms of drying shrinkage, firing shrinkage, density and water absorption are in the same range as for conventional ceramic articles not containing any fly ash. Also, the ceramic articles according to the invention do not exhibit any problems associated with scumming or efflorescence. Additionally, the ceramic articles according to the invention may be subjected to a coating step, wherein a glaze coating is applied to the ceramic article in a subsequent process. Tests by the inventor have shown that even if the ceramic product exhibits scumming, a glaze coating can be applied that will render the scumming invisible.

Without being bound to a theory, it is believed that the scumming layer is very thin and will dissolve into the applied glaze.

Description of the figures

[0032] Figure 1 shows the level of scumming present on the surface of ceramic bodies as a function of the incubation time of fly ash in wet state.

[0033] Figure 2 shows the level of scumming present on the surface of ceramic bodies as a function of the wt% of water on which the fly ash was incubated.

Examples

[0034] The following examples are intended to illustrate the invention.

Example 1

[0035] In this example, the correlation between the properties of the ceramic article (drying shrinkage, firing shrinkage, Loss on Ignition (LOI), density and water absorption) and the type of clay used, the mixing ratio of flying ash (FA) and clay, and the firing temperature in the kiln is investigated.

[0036] The biomass fly ash used in the experiment resulted from the combustion of a fuel mixture with a mixing ratio of about 90 wt% biomass and 10 wt% coal. The clay materials are indicated in

Table 1 and typical examples of their type used in the field. The dry materials were weighed and the mixture homogenized, followed by the addition of water (25 wt%). The resulting wet ceramic body was kneaded, pressed into a tile mould by hand, and cut into individual test tiles. In each test tile, three identical sets of markings at 65.00 mm distance were carved into the wet body prior to drying. The test tiles were dried in open air at ambient temperature for 5 days. Subsequently, the tiles were fired in an electric kiln (fully oxidative kiln atmosphere) to the top temperature indicated in Table 2, at a rate of 150 °C per hour, and a dwell time of 20 minutes at the top temperature. The kiln with its content was subsequently cooled to ambient temperature without controlled cooling.

[0037] Drying shrinkage values were obtained by measuring the distance between the markings after drying, and calculating the percent difference with respect to the set distance of 65.00 mm in the wet state. Table 1 depicts the correlation between the type of clay used and the mixing ratio with the flying ash (FA) that was used to manufacture the ceramic article with the drying shrinkage (in vol%) of the obtained articles.

Table 1: Drying shrinkage of the ceramic articles.

Dryin

FA/clay (vol%) 90/10 0,63%

Ball clay 80/20 1,21% 70/30 1,83% 90/10 1,11%

Bentonite 80/20 1,34% 70/30 2,65% 90/10 0,54%

Stoneware clay 80/20 0,98% 70/30 1,30%

[0038] Firing shrinkage values were obtained by measuring the distance between the markings after firing, and calculating the percent difference with respect to the distance in the dried state, and

Loss on Ignition (LOI) values were obtained by determining the mass of the tiles before firing (in the dried state) and after firing (in the fired state), and calculating the percent change between both values. Table 2 depicts the correlation between the type of clay used, the mixing ratio with the flying ash (FA) and the firing temperature used to manufacture the ceramic article, with the firing shrinkage (in vol%) and LOI (wt%) values of the articles during the manufacturing process.

Table 2: Firing shrinkage and LOI values of the ceramic articles. eee oe ee wo | a | |e em | | aw | a stoneware

[0039] Density values were obtained by dividing the calculated volume of each tile (from the dimensions of each tile) by the mass of each tile in the fired state, and water absorption values were obtained by calculating the weight percent gained for each fired tile before and after having been submerged in water at ambient temperature for 24 hours. Table 3 depicts the correlation between the type of clay used, the mixing ratio with the flying ash (FA) and the firing temperature used to manufacture the ceramic article, with the density (in kg/m® and water absorption values (wt%) of the obtained article.

Table 3: Density and water absorption values of the ceramic articles. eee ee stoneware

[0040] As evidenced in Tables 1 — 3, the behaviour of the properties of the inventive ceramic articles is in line with traditional (i.e. 100 wt%) clay bodies, which supports their applicability in the construction industry.

[0041] Especially interesting features arising from the data are the jump in firing shrinkage with bentonite at 1150°C , which demonstrates that even a small addition of bentonite (even at a FA/clay ratio of 90/10) suffices to provide high firing shrinkage properties. Additionally, these conditions (FA/bentonite ratio of 90/10 and firing at 1150 °C) also provide for fired bodies with a relatively low density. This lower density results into ceramic articles with better properties in terms of weight (lower overall weight of a brick built wall) and better thermal insulating properties.

Example 2

[0042] In this example, the frost resistance of the ceramic articles according to the invention is investigated. The frost resistance was determined according to this test which is standard practice in the field: The fully water-logged tiles from the water absorption test in Example 1 were placed in air-tight sealed plastic bags to prevent free-drying, and were subsequently subjected to 20 freeze- thaw cycles by freezing over night at -20 °C, followed by thawing at ambient temperature. After each freeze-thaw cycle, the tiles were investigated for the formation of internal cracks (by tapping), and visually for any surface damage. After 20 freeze-thaw cycles, the tiles were still free from cracks and surface damage, indicating good frost resistance.

Example 3

[0043] In this example, the correlation between the formation of efflorescence (scumming) and the time and wt% of water on which the fly ash was incubated is investigated.

[0044] Water was added water to a batch of fly ash to arrive to a concentration of 25 wt®% of water, and samples were taken after a certain number of successive incubation days to analyse the presence of scumming. The effect of this incubation time on the scumming of the ceramic bodies is shown in Figure 1, with the numbers on the ceramic bodies indicating the number of days the fly ash was incubated in the wet state. As can be observed on the scraped (to remove the calcium efflorescence) lower left corner of the first two tests in the dried state post firing, the ceramic body is red-coloured underneath the white scumming, confirming that this white effect appears only on the surface. Thus, after 1 a or 4 days of incubation, the white layer is clearly visible, while after six days of incubation, the efflorescence starts to disappear on the fired body. After 18 days of incubation the white layer is no longer present, and the situation remains the same if more incubation days are used, as illustrated by the test after 32 days of incubation. This demonstrates that scumming can be reduced by increasing the incubation days of fly ash in wet state.

[0045] Subsequently, the effect of the amount of water added the fly ash on the scumming of the ceramic article was investigated. Water was added to the fly ash, ranging from 15 to 40 wt®%, and the mixture was incubated for seven days. The results are shown in Figure 2. It appears from the result shown in Figure 2 that every water wt®% tested can be used to obtain a reduction on scumming.

Claims

Conclusions

Claims 1. Use of a ceramic masterbatch containing fly ash and clay, the weight ratio of fly ash to clay in the mixture being at least 70/30, for the manufacture of ceramic articles.

The use according to claim 1, wherein the weight ratio of fly ash to clay in the mixture is in the range of 80/20 to 95/5.

The use according to claim 1 or 2, wherein the fly ash contains soluble salts in the range of

0.5 to 10% by weight.

The use according to any one of claims 1 to 3, wherein the fly ash is derived from biomass.

The use according to any one of claims 1 to 4, wherein the clay is bentonite.

The use according to any one of claims 1 to 5, wherein the ceramic masterbatch further contains water and 0.01 to 1% by weight of a detergent, based on the total weight of water.

A method of manufacturing a ceramic article comprising (a) providing a ceramic masterbatch as defined in any one of claims 1 to 6; (b) subjecting the ceramic masterbatch to a shaping step; and (c) firing the molded ceramic masterbatch at a temperature in the range of 800 to 1300°C to obtain the ceramic article.

The method of claim 7, wherein the baking is performed at a temperature in the range of 1000 to 1250°C.

The method according to claim 7 or 8, wherein the shaping step is performed by molding, dry-pressing or extrusion.

The method according to any one of claims 7 to 9, wherein the fly ash is incubated with water before mixing with the other ingredients to obtain the ceramic masterbatch of step (a), and wherein the method further comprises drying the ceramic - masterbatch of step (a) and crushing and/or grinding the dried ceramic masterbatch prior to step (b).

The method according to claim 10, wherein the water content during the incubation is in the range of 1-80% by weight based on the amount of fly ash.

The method according to claim 10 or 11, further comprising applying a glaze layer to at least a portion of the surface of the ceramic article obtained in step (c).

A ceramic article obtainable by the method according to any one of claims 7 to 13.

The ceramic article of claim 14, which is a brick or a tile.