NO860083L - Reinforcing fibers treated with silica dust. - Google Patents

Description

The present invention concerns the treatment of fibers or other reinforcing bodies, which are usually used in cement and concrete materials, plastics, rubber and similar products. In particular, the invention relates to reinforcement bodies, which are usually used in facade panels of cement and concrete materials, roof panels and other non-load-bearing products produced by evacuation after casting or other conventional systems, or in the well-known Hatschek or Magnani machines.

Generally, different types of reinforcement are used in cement and concrete products, such as glass or organic fibers rather than asbestos because of the health risk the use of asbestos entails. However, it has been shown that the performance characteristics of such compositions are not entirely satisfactory. Practical use, especially under hot and wet climate conditions, inevitably leads to deteriorated mechanical properties, which entails a significant reduction in strength and an almost complete loss of ductility.

The deterioration is associated with at least two mechanisms. Deterioration in the shorter term is thought to be mainly due to the deposition of massive hydration products of cement, especially calcium hydroxide, in the spaces between and on the surfaces of the individual filaments that the fiber yarns are made of. The longer-term deterioration is usually associated with attack from aggressive alkaline solutions present or formed in the cement matrix.

Glass fibers are particularly sensitive to early attacks. The deterioration of the glass fibers is particularly associated with the formation of calcium hydroxide crystals in the narrow parallel spaces between the individual filaments in the yarns, which usually consist of approx. 200 single filaments. These spaces between the individual filaments in the glass fiber yarns are narrow openings, which are only a few ^urns wide and which only occur between filaments which can have a thickness of approx. 12 µm.

These spaces are usually not penetrated by the cement particles. However, deposits in the interstices of calcium hydroxide that appear during the cement hydration will tend to stick the filaments together, which results in a reduced ability to withstand bending stresses without breaking the filaments. Microscopic damage to the surface of the fibers due to chemical attack from, for example, alkalis in the cement can significantly reduce the strength of the fibers and can impair their reinforcing properties. The use of large E-glass fibers in concrete and cement materials is under current conditions not satisfactory as these fibers are subject to deterioration due to chemical attack within a relatively short period of time. Some improvement is achieved by alkali resistant (AR) glass fibers, which contain zirconium (ZrC^) to provide resistance to alkali attack. With the same purpose, various polymer coatings have been used to protect reinforcing fibers and other reinforcing materials, but none of these have proved completely satisfactory.

It has now been shown that a significant reduction in the rate at which strength and ductility are lost over time can be achieved by pre-treating the reinforcing bodies, such as glass or organic fibers with a slurry of microsilica. The pre-treatment can be carried out in any conventional manner, such as by spraying or roller application to the reinforcing bodies with a slurry of microsilica, but the best results are obtained by immersing the glass fiber rovings in a bath consisting of an aqueous slurry of microsilica containing a dispersant which contributes to that the individual silica particles penetrate between the individual filaments and into the small, microscopic spaces between the individual filaments in the glass fiber yarns.

Each of the known dispersants for finely divided solid particles in water can be used and excellent results have been obtained by using one or more water reducing agents in concrete and cement materials, as a dispersant in the microsilica slurry. A conventional water reducing agent will, when used in the microsilica fiber slurry, have the advantage that it tends to contribute to the dispersion of the microsilica and the reinforcing fibers in the finished product. According to the present invention, the reinforcing bodies can be added to the product matrix in the form of a pretreatment slurry of microsilica containing the reinforcing bodies, or the reinforcing bodies can be pretreated with a slurry of microsilica after which the reinforcing bodies are dried and, if desired, treated with heat, and then added to the product matrix in dry form. The present invention is particularly aimed at pre-treatment of fibers that are used as reinforcing bodies in a product matrix, e.g.

including Portland cement, but it is clear that such treated fibers can also be used in matrices composed of plastic, rubber or other substances or combinations of substances or in any desired way. E.g. a web or a mat of fibers can be treated with an aqueous slurry of microsilica and then used for such purposes as separators in accumulator batteries or as filters for industrial liquids.

Water is the best liquid for suspending the microsilica, but if desired, the microsilica can be suspended in any suitable organic liquid that can serve as a carrier and dispersant for the microsilica. Aqueous solutions or suspensions of conventional water-reducing agents in concrete can be used, especially when the pre-treated fibers are to be used in a matrix of concrete and cement materials. The amount of microsilica in the slurry can be varied depending on the particular fiber to be used. Good results can be obtained with glass fibers using an aqueous slurry containing from about 5.0 to about 80.0% by weight of microsilica and preferably from 0.5 to approx. 40.0% by weight of a dispersing agent, such as one or more of the conventional water-reducing agents in concrete. From approx. 40 to approx. 60% by weight of microsilica and from approx. 1 to approx. 20% by weight dispersant in the aqueous slurry is preferred. In all cases, the liquid in the slurry does not amount to less than approx. 15% by weight.

Conventional water-reducing agents are solutions or dispersions of chemical substances which, when added to concrete, soften the concrete for a period of time so that (1) normal workability can be achieved in a concrete with a lower ratio between water and cement than could otherwise be used or (2) so that an extremely workable "flowable concrete" (which is essentially self-healing without undesirable side effects, such as water separation, poor durability, poor wear resistance and seepage) can be obtained, or (3) a combination of (1) and (2).

Water reducing agents are well-known additives to concrete. The commercial materials generally available include: 1. Hydroxylated carboxylic acids and their salts and modifications and derivatives thereof, 2. Carboxylic hydrates, polysaccharides and sugar acids, 3. Amines and their derivatives, polymeric compounds, such as cellulose ethers and silicones, 4. Lignosulfonic acids and their salts, as well as modifications and derivatives thereof,

5. Melamine compounds,

6. Naphthalene compounds,

Some of the above-mentioned conventional water-reducing agents can act as dispersants for the microsilica in the aqueous slurry and help to disperse the microsilica and the reinforcing fibers in the matrix of concrete and cement materials, regardless of whether this is carried out with dried, pre-treated fibers or supplied in liquid form in the pre-treatment slurry of microsilica and fibres. The best results for use in cement and concrete production are achieved by using water reducing agents belonging to the aforementioned categories (4), (5) and (6). The preferred material in category (5) is a conventional sulphonated condensate of melamine and formaldehyde sold under the designation Melment or Rescon HP, and the preferred material in category (7) is a sulphonated condensate of naphthalene and formaldehyde sold under the designations Mighty, Lomar D or PSP-R and N. The preferred material in category (4) is a lignosulfonate sold under the name Borresperse-Na. The conventional water reducing agents in categories (5) and (6) are known as superplasticizers or broadly acting water content reducing agents for concrete and these are used as dispersants in the aqueous slurry of microsilica in the pre-treatment of glass fibers to achieve the best results.

It has been shown that the aqueous slurry of microsilica containing one or more water-reducing agents during the pre-treatment of reinforcing fibers will produce a much denser micro-structure, increase the strength and density of the mortar or concrete by several orders of magnitude when added to the mortar and concrete, and the resulting product has enhanced resistance to frost blasting.

The microsilica used for pre-treatment of the reinforcing bodies according to the present invention is amorphous silica which is obtained as a by-product in the production of ferrosilicon or silicon metal products in electric arc furnaces. Microsilica is a pozzolan, i.e. it combines with lime and moisture at ordinary temperatures to form substances that have cement properties. The main component is silicon dioxide (SiG^), which is usually present in an amount of at least 60% by weight, but the best results are obtained according to the present invention when the content of SiG^ is at least approx.

85% by weight.

An amorphous microsilica which is particularly well suited for use according to the present invention appears as a by-product in the production of silicon metal or ferrosilicon in electric reduction furnaces. During these processes, relatively large amounts of silica are formed, which are extracted from filters or other collection devices. Such microsilica can be purchased from Elkem a/s, Norway.

The analysis and the physical data for typical samples of microsilica corresponding to this description appear in the following table: The production of silicon involves the reduction of silicon dioxide (e.g. quartz) with carbon. Iron is added if ferrosilicon is to be produced. Part of the product from this reduction of silicon dioxide can be reoxidized in the vapor phase (e.g. in air) to form the fine, particulate microsilica, which is useful in this connection. Although dust collected from an electric furnace producing ferrosilicon containing at least 85% silicon is preferred, dust collected from an electric furnace used to produce 50% ferrosilicon can also be used in connection with the present invention.

It is possible to produce the amorphous microsilica not as a by-product, but as a main product by suitably adjusting the reaction conditions. Amorphous microsilica of this type can also be produced synthetically without reduction and reoxidation.

The amorphous silica used in accordance with the present invention essentially consists of microscopic, spherical particles. The spherical shape together with the fineness and the pozzolanic activity make it surprisingly applicable according to the present invention.

For example, the amorphous microsilica particles can consist of at least 60 to 90% by weight SiO- and have a density of 2.2 - 2.25 g/cm 3 and a specific surface area of 18 22 m 2 /g, since the particles in the substantially all are spherical and where at least 90% by weight of the primary particles have a particle size of less than 1 µm. Of course, there can be variations in these values. For example, the distribution of the particle sizes can be adjusted, as it is possible to remove coarser particles during sorting.

The amorphous microsilica can be dark gray due to its carbon content. However, this carbon can be burned off, for example at temperatures of around 400°C. It is also possible to modify the silicon and ferrosilicon production processes so that microsilica is produced in a relatively white form, which in other respects is completely identical to the normally produced gray silica. Actually, the modification in the process consists in reducing the amount of coal or eliminating the coal from the charge. The second consequence of this modification is a change in the ratio of the amount of silica produced in relation to the amount of silicon and ferrosilicon. In other words, this means that the ratio between silica and silicon or ferrosilicon is higher in the modified process.

The term microsilica used here is to be understood as the powdered, amorphous silica that results from a process in which silica is reduced and the reduction product is oxidized in the vapor phase in air. The term microsilica also includes the same type of amorphous silica produced synthetically without reduction and reoxidation. It is most appropriate that the microsilica according to the present invention is produced from the flue gas that occurs during the production of silicon metal or ferrosilicon

in electric reduction furnaces.

Fig. 1 is a schematic illustration of an apparatus used for testing the flexural strength properties of cement and concrete products made with glass fibers that have been pre-treated with microsilica according to the present invention. Fig. 2 shows a sample for examination of the microstructure.

Below are specific examples to illustrate further advantages of pre-treatment of glass fibers with an aqueous slurry of microsilica according to the present invention and these examples must not be regarded as limiting the scope of the invention stated in the claims.

Example 1

In this example, conventional type E and AR glass fiber rovings were cut into lengths of approx. 10 cm, weighed and then immersed in a slurry of microsilica supplied by Elkem Chemicals Inc. under the designation EMSAC-N. The slurry of microsilica of the type EMSAC-N contained 48.0% by weight of microsilica. A typical analysis of the microsilica appears above on pages 8 and 9. The aqueous slurry of EMSAC-N microsilica contained 1.44 wt% dry solids of lignosulfonate dispersant (Borresperse-Na) and 0.32 wt% formalin as a biocide. The amount of water was 50.2% by weight.

After approx. After 15 minutes the rovings were removed from the EMSAC-N bath and oven dried and then weighed again to record the increase in weight due to the embedded microsilica particles. The weight increase was approx. 50% by weight in the glass fiber yarns of both type E and AR.

Some of the treated rovings were pulled apart and the individual yarns were mounted in and examined in a scanning electron microscope. It was quite obvious that the microsilica particles effectively penetrated into the spaces between the filaments. and in effect formed a fused, continuous layer engulfing the entire yarn. It was found that many particles of microsilica also adhered to the surface of the outer filaments.

Other tests were carried out by allowing yarn of 10 cm to be immersed in the EMSAC-N pre-treatment bath for a period of approx. 60 minutes. The extension of the time period did not result in significantly increased penetration of the amount of microsilica embedded in the yarns.

Example 2

Samples were cast in blank molds by filling the bottom third of the mold with a concrete mixture, after which the mold was vibrated. The pre-treated glass fibers were placed in place in the mold using a holder and the upper two-thirds of the mold were then pressed into the correct position over the lower part. This arrangement was used to ensure that the glass rovings had an accurate position in all samples. The top two thirds were then filled with the concrete mixture and compacted by vibration.

All specimens were removed from the mold after one day and cured at 20°C in lime water for at least seven days. This provided an initial state reference in the subsequent aging test of the composition's properties. The amount of lime water was sufficient to cover the blanks completely in all cases.

The geometry of the blanks is shown in Fig. 1 and it is shown that the glass fiber rovings 10 were manually placed at a height of 35% above the surface 12 with the greatest traction on the blank with the specified dimensions. In cases where a single fiberglass roving is used, it made up a volume percentage of approx. 0.4 of the subject. When two fiberglass rovings are used, the volume percentage of the reinforcement was approx. 0.8%. All specimens were constructed for testing in a known manner in a four-point bending test with supports as shown at 14 in Fig. 1 and with the stated dimensions. The load on the test piece at 16 was continuously recorded by means of a load cell 18 and the output signal from the load cell was transferred to the Y axis of the X-Y recorder 20. The deflection at the center of the test piece was continuously recorded by means of an LVDT 22 connected to the recorder's 20 X axis. This gave a continuous record of the load versus the deflection. The rate of load increase was kept at 0.015 mm/min.

Specimens from the same mixtures were specially molded into bars containing eight parallel glass fiber yarns arranged as shown in Fig. 2 at 24. The test bars had the dimensions shown and were split as shown at 26 to show the interface between glass fibers and concrete matrix. Some test bars were also split to expose the surface perpendicular to the yarns. The split specimens were mounted after being coated with a 300 Å thick layer of a gold-palladium alloy and were examined in a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDXA) to investigate the microstructure of the specimens.

A number of series of corresponding specimens were prepared and tested for flexural strength as follows: Series A-A with a single roving of ordinary AR glass was used in a Portland cement matrix mixed according to the method of ASTM C 305 at a water-cement ratio of 0, 35.

Series B - as series A except that 10% of the Portland cement in the matrix was replaced by an equivalent amount by weight of microsilica. The ratio of water to cement plus microsilica was 0.35.

Series C - as Series A except that the AR fiberglass rovings were immersed in a bath consisting of a slurry of EMSAC-N microsilica containing the dispersant for 15 minutes and then air-dried for approx.

15 minutes before it was placed in the mold.

Series D - as series C except that 10% by weight of the Portland cement in the base mass was replaced by an equivalent amount by weight of microsilica. The ratio of water to cement plus microsilica was 0.35.

Series E - as series A except that two rovings of AR glass fibers were used in these test pieces.

Series F - like series D, except that two rovings of AR glass fibers were used in these test subjects.

The reinforced glass fibers accounted for approx. 0.4% by volume of the test pieces from series A - D and 0.8% by volume of the test pieces in series E and F. In each case, the test pieces were produced in the mold as described above in connection with example 2. After the test pieces was removed from the molds, each specimen was cured at 20°C in lime water for 7 days to provide an initial condition reference. Another set of specimens of Series A-F were subjected to an accelerated aging for a further 14 days in lime water at 50°C, and a third set of specimens of Series A-F were subjected to accelerated aging in lime water at 50°C for a further 28 days. The specimens were tested for flexural strength with the following results. Table III shows the average of the quantitative parameters based on the individual stress-deformation curves that emerged during the bending test. Six corresponding test subjects of each type and age were tested. Based on table III, it can be seen that the specimens in series A with untreated AR glass fibers after 28 days of accelerated ageing, have practically lost their strength after cracking and are only able to withstand loads at very small deflections. There had been a drastic drop in strength even after only 14 days of accelerated ageing. The pretreatment of the fibers with microsilica significantly reduced the undesirable effects of accelerated aging in the specimens in series C, D and F. The reduction in the maximum stresses after cracking and in ductility was significantly less in the specimens containing glass fibers that had been pretreated with microsilica. The maximum values after cracking for the specimens that contained pre-treated glass fibers were still clearly defined as sharp peaks on the curves after 28 days of accelerated ageing, while for the specimens in series A with untreated glass fibers the peaks for the maximum values after cracking were reduced to insignificant increases after the same accelerated aging treatment. Equally important is that pre-treatment of the glass fibers with microsilica improves the toughness of the specimen, which is shown by the area under the load-deflection curve between the deflection at the first crack and the deflection at the point where the maximum value after the crack formation has been exceeded and the load has dropped to 75% of the maximum value as stated in the last column of table III. As shown in Table III, samples treated show a significant improvement over samples containing them

ordinary untreated AR glass fibers used in series A. After 28 days of accelerated ageing, the toughness after crack formation on the specimens in series D was 8 times as great as the corresponding value for the specimens in series

A.

The properties of the test specimens in Series F compared to the reference specimens in Series E were outstanding. There was no quantitative drop in any of the measured parameters after a full 28 days of accelerated aging of the specimens in series F. It was completely surprising and unexpected to find that the strength at first crack formation in the specimens in series F increases from 896.4 to 1103 N/ cm 2 and the maximum value after crack formation was increased from 1138 N/cm 2 to 1448 N/cm 2. The area corresponding to first crack formation of the specimens in series F was increased from 1.36 to 2.48 Nem, and the parameter for the toughness after crack formation was increased from 38.4 to 39.5 Easy. The pre-treatment of the glass fibers with microsilica according to the present invention thus prevents the mechanical deterioration of the mechanical properties of the composite material, which would otherwise take place in glass fiber cement composite systems during ageing.

The samples were split and examined to determine the microstructure and the characteristics of the fibers after the accelerated aging of 28 days. In the case of the specimens in series A, it was found that most of the glass filaments had broken at the fracture surface rather than by being pulled out of the matrix before breaking, as such failure to pull out fibers before breaking shows that the composite material has become brittle. It was also observed that the microscopic spaces between the filaments contained massive deposits of calcium hydroxide. In the case of the specimens in series C, the ratio after 28 days of accelerated aging between the filaments that break without being pulled out was significantly smaller than in the case of the specimens according to control series A subjected to the same treatment, which, when split, showed that these specimens had greater ductility than the specimens in series A. With respect to the test pieces in series D and F, it was found that most filaments were pulled out to a significant extent before use, even after accelerated aging for 28 days. There were no calcium hydroxide deposits in the microscopic spaces between the filaments in the specimens from series D and F and there were no calcium hydroxide deposits surrounding the yarns. It is presumed that the microsilica that was present in the microscopic spaces between the filaments and around the glass fiber yarns reacts to form a relatively less massive and cohesive calcium silicate instead of the massive calcium hydroxide that is usually deposited in such spaces when no microsilica is present. As a result of this, the individual filaments in a yarn are not firmly joined together in such a yarn so that they can no longer be bent or pulled out individually, and the composite material therefore retains its toughness and ductility.

It must be understood that the quantity of pre-treated fibers according to the present invention and the specific type of fiber which is supplied to cement or concrete matrices, rubber or plastic matrices or combinations thereof, may be the quantities and types that are usually used in commercial practice. It will also be understood that each of the conventional additives can be added to the liquid slurry according to the present invention, depending on the product for which the slurry of fibers and liquid is intended, provided that the additive does not interfere with the penetration of microsilica into the cavities between the fibers or with the microsilica particles that adhere to the surface of the fibers.

It must also be understood that the claims cover all changes and modifications of the preferred embodiments according to the present invention, which have been chosen for illustration only and which do not constitute a deviation from the spirit and scope of the invention.

Claims (19)

1. Method for improving the reinforcement properties of fibres, characterized in that a liquid slurry of microsilica is prepared and that the fibers are brought into contact with the microsilica in this slurry. 2. Method according to claim 1, characterized in that the microsilica is suspended in water. 3. Method according to claim 2, characterized by comprising a step where one or more dispersants are added to the slurry before it is applied to the fibres. 4. Method according to claim 1, characterized by including immersing the fibers in the liquid microsilica slurry. 5. Method according to claim 1, characterized in that the dispersant is a water-reducing agent. 6. Method according to claim 1, characterized in that the fibers belong to the group comprising glass fibres, organic fibres, coal and metal fibers or combinations thereof. 7. Method according to claim 1, characterized in that the fibers have a shape like a web. 8. Method according to claim 1, characterized by further comprising the step of adding the microsilica-treated fibers to a cement-based material. 9. Reinforcement body, characterized in that it has microsilica on its surface to improve its durability properties when used for reinforcement in products manufactured with a content of such bodies. 10. Liquid slurry of microsilica, characterized in that it contains reinforcing bodies for reinforcing a matrix product. 11. Reinforcing body according to claim 9, characterized in that the bodies are selected from the group containing glass, organic, coal and metal fibers or combinations thereof. 12. Glass fibers for reinforcing cement materials, characterized in that they are pre-treated by applying a slurry containing microsilica. 13. Additive for cement and concrete materials, characterized in that it comprises an aqueous, liquid mixture of microsilica, a dispersant and reinforcing fibers where the microsilica is present in the additive in an amount of from approx. 5 to approx. 80% by weight, and the dispersant is present in an amount from approx. 0.5 to approx. 40% by weight. 14. Additive according to claim 13, characterized in that the dispersant is a water-reducing agent. 15. Additive according to claim 13, characterized in that the microsilica is produced from the smoke from an electric furnace which is used for the production of ferrosilicon or silicon metal. 16. Hardened cement or concrete material containing conventional components, characterized in that they comprise reinforcing fibers which are at least partially surrounded by microsilica to protect the fibers against loss of ductility. 17. Material according to claim 16, characterized in that the reinforcing fibers are glass fibers with microsilica particles on the surface and with microsilica particles located in the spaces between the reinforcing fibers. 18. Glass fibres, characterized in that they contain 5 - 150% by weight of microsilica based on the weight of the glass fibres. 19. Web of glass fibers, characterized in that they contain microsilica placed between the individual fibers of the web.