Research: Aggregate Uses - Canadian Wollastonite

Wollastonite has proven useful in a wide range of industrial applications with a composition that is low in carbon and high in of silicon and calcium.

We have summarized the research studies we have collected to date under the following linked headings:

Cement & Concrete, Ceramics, and Refractory Applications Research

1. Cement & Concrete

1.1 Sample High Strength Cement Compositions

1.2 Mechanical Properties Of High Performance Cement Binders Reinforced With Wollastonite Micro-Fibres

Abstract: Factors that influence the flexural strength characteristics of the cement-water and the cement-silica fume-water systems reinforced with inorganic wollastonite micro-fibres are discussed. The effect of the wollastonite micro-fibres on matrix pore structure was investigated using mercury intrusion porosimetry and helium gas pycnometry. Fibre-matrix interaction was also examined using the conduction calorimetry method. Results indicated that natural wollastonite micro-fibres are effective in improving both pre-peak and post-peak load behaviour. The results are discussed in terms of composite theory and matrix microstructure.

Conclusions: The experimental results obtained in this investigation support the following conclusions:

Natural wollastonite micro-fibres are effective in modifying the flexural strength characteristics of hydrated cement and cement-silica fume composite matrices.
Incorporation of the wollastonite micro-fibres into cement and cement-silica fume matrices improves both the pre-peak and post-peak load-deflection response resulting in higher flexural strength and better ductility characteristics.
Incorporation of wollastonite micro-fibres into the cement matrix modifies the microstructure of the cement-based composite systems.
Porosity, pore size distribution and solid density of the hydrated phases were significantly different in the cement- wollastonite and in the cement-silica fume-wollastonite composite systems.
Porosity determined by mercury intrusion porosimetry is generally greater than that determined by the helium gas pycnometry.
Wollastonite micro-fibres and silica fume addition appear to promote pore discontinuity in cement systems.
It is suggested that on the basis of conduction calorimetry measurements there is no apparent chemical interaction between the wollastonite micro-fibres and the cement matrix during the hydration process.
Natural wollastonite micro-fibres are an effective reinforcement for improving the flexural strength characteristics of the cement systems. The relatively lower cost of the wollastonite micro-fibres compared to other micro-fibres may result in more widespread use of this natural mineral in the construction industry.
See related topics and documents: Mechanical properties of high-performance cement binders reinforced with wollastonite micro-fibres – nrcc-34968.pdf

1.3 Repair And Rehabilitation – A New Market For Wollastonite Micro-Fibres?

Abstract: The deterioration of North American infrastructure has provided new challenges and opportunities for research in the area of new construction materials. The emergence of performance-based codes underlines the strategic importance of developing materials that have an increased service life. Advances in fibre-reinforced cement technology have spawned the invention of numerous innovative products, including high performance materials for repair and new construction applications. One such new product is wollastonite, in the form of micro-fibres, which appears to be a cost- effective and durable material for cement reinforcement.

Conclusions: Cement systems containing wollastonite micro-fibre reinforcement can be used in various applications such as sidewalks, highway bridges and repairs can be effected that obtain better mechanical performance and longer service life than can be achieved with more commonly used materials. As well, the microstructural modification of incorporating wollastonite micro-fibres can increase the chloride resistance of cement and concrete structures so that damage caused by corrosion of the reinforcing steel is reduced.

See related topics and documents: Repair_Rehabilitation_FP.htm

1.4 Marble Slurry Dust And Wollastonite – Inert Mineral Admixtures For Cement Concrete

Abstract: Engineering characteristics of concrete mixes incorporating marble slurry dust (marble waste), wollastonite micro fibres (natural mineral), fly-ash and silica fume were studied in the laboratory. Workability of the concrete mixes was found to diminish as the proportion of Marble Slurry Dust (MSD) increases from 30-60 per cent as partial replacement of sand, the fine aggregate. However, the workable mixes prepared using super plasticizers have highly enhanced compressive strength and improved flexural strength. Further, to such concrete mixes when silica fume is incorporated as partial substitute of cement and wollastonite as that of sand, there is a sharp increase in the flexural strength of concrete. Improvements in compressive and splitting tensile strength were also observed. Water absorption, drying shrinkage and abrasion were found to decrease with the addition of these fine mineral admixtures and concrete so prepared remains unaffected by sulphate waters and alternate freezing and thawing.

Conclusions

Sand can be partially replaced by marble slurry dust in making cement concrete mixes.
Optimum quantity of sand that can be replaced by marble slurry dust is around 40 per cent.
Replacement of sand by marble dust results in increased water requirement for the desired workability of concrete mix. Keeping same amount of water per cubic meter of concrete, the workability can be achieved by using super plasticizer doses.
Remarkable improvements in compressive strength can be achieved by substitution of 30-40 per cent sand by marble dust. Improvement in flexural strength were also observed.
Reduction in water absorption and abrasion were also found when marble slurry is incorporated in concrete mixes by partial replacement of sand.
When 10% cement is replaced by silica fume & 10% fine aggregates (sand and marble dust) arc replaced by wollastonite, there is vast improvement in the flexural strength of concrete.
The concrete mixes containing silica fume, wollastonite and marble slurry dust are durable against alternate freezing -thawing and sulphate attack.
See related topics and documents: Marble slurry dust and wollastonite admixtures for cement.pdf

1.5 Investigations On Pastes And Mortars Of Ordinary Portland Cement Admixed With Wollastonite And Microsilica

Abstract: Wollastonite is abundantly available in Rajasthan, Tamil Nadu, Uttarakhand, and Andhra Pradesh states of the Indian Union as a low-cost material. In this study, investigations were made on pastes and mortars to evaluate its potential as a new material for admixing with ordinary portland cement with or without microsilica. Its physical and chemical properties were analyzed. Wollastonite consists of 45.6% of CaO and 48% of SiO2, mostly in amorphous form. It has an average specific surface area of 842.7 m2 /kg and retention on 45-micron sieve of 3.20%. When ground to fine powder, it attains an average particle size of 4 microns which is about 4.5 times finer than ordinary portland cement. Scanning electron microscope images show that wollastonite particles were solid, acicular in shape, and have rough surfaces. Several cementitious mix proportions of ordinary portland cement, wollastonite, and microsilica were investigated for normal consistency, initial and final setting time of paste, and compressive strength of mortar. Test results indicate that the mortar, which contains 82.5% cement, 10% wollastonite, and 7.5% microsilica, as cementitious material attains the highest compressive strength. The mortar, which contains 77.5% cement, 15% wollastonite, and 7.5% microsilica, as cementitious material achieves compressive strength higher than the conventional OPC mortar along with rendering maximum cement replacement for better economy of concrete work. It was observed that the compressive strength of mortar varied logarithmically with the days of moist curing and linearly with the proportion of admixing. Suitable predictive models are presented accordingly.

1.6 An AC Impendence Spectroscopy Study Of Freezing Phenomena In Wollastonite Micro-Fibre Reinforced Cement Paste

Abstract: A.C. impedance spectroscopy (ACIS) methods were used in studies designed to further understanding of the mechanism of frost action occurring within neat Portland cement paste and cement paste reinforced with wollastonite micro-fibres. An impedance measuring system was coupled with thermomechanical instrumentation to provide real-time impedance spectra and length change data simultaneously on the same specimen during a freezing and thawing cycle. Analysis of the ACIS data provided insight on the relationship between the behavior and mechanism of frost action within the paste systems studied. A mechanistic model (involving interfacial phenomena between liquid water – ice – C-S-H surfaces) based on interpretation of impedance spectra is put forward. In this context the importance of a parameter associated with the depression of the high frequency arc in an impedance spectrum is discussed.

The role of the wollastonite micro-fibre addition in enhancing the durability of cement paste to freezing-thawing is described.

Conclusions:

Low-temperature impedance spectra provide useful information for studies of freezing-thawing phenomena in microporous materials.
The ratio of resistance values at the completion and initiation of the water-ice transition has value as a durability index. Its magnitude corresponds to the amount of freezable water in the system.
A parameter related to the depression angle of the high-frequency arc in an impedance spectrum provides information related to the nature of mass transport below the triple point in a water-saturated microporous medium.
Wollastonite micro-fibre reinforcement of cement binders appears to improve resistance to a freezing-thawing cycle up to 35 percent when present in amounts exceeding 10 percent by volume.
See related topics and documents: An AC impedance spectroscopy study of freezing phenomena in Wollastonite micro-fibre reinforced cement paste.pdf

1.7 Effect Of Wollastonite Micro-Fibre Aspect Ratio On Reinforcement Of Portland Cement-Based Binders

Abstract: Reinforcement of Portland cement-based binders with natural wollastonite micro-fibres has been further investigated with emphasis on the effect of fibre geometry on property modification. Wollastonite micro-fibres were separated into five different size groups before addition to cement and cement-silica fume matrices. The aspect ratio (length/width) of the wollastonite micro-fibres was determined by scanning electron microscopy technique. Systematic experimentation including flexural tests, mercury intrusion porosimetry, helium gas pycnometry, and isopropyl alcohol saturation measurement showed that, at constant micro-fibre content, the flexural strength and total porosity of the composites remained essentially unchanged and independent of the aspect ratio of the wollastonite micro-fibres. However, the volume of coarse pores, the pore size distribution, the flexural toughness and overall ductility characteristics of the hydrated cement and cement-silica fume matrices were observed to change systematically as the aspect ratio of wollastonite microfibres was increased. A discussion of the differences in the observed properties is presented.

Conclusions: The experimental results obtained in this investigation appear to support the following conclusions.

The flexural strength of cement and cement-silica fume matrices reinforced with 11.5% by volume of wollastonite micro-fibres is independent of the aspect ratio of wollastonite micro-fibres.
The pore structure in the hydrated cement-wollastonite and cement-silica fume-wollastonite composite systems varies slightly by the addition of wollastonite micro-fibres having different aspect ratio.
The overall total porosity in the hydrated cement-wollastonite and cement-silica fume-wollastonite composite systems as determined by mercury intrusion, helium gas pycnometry and isopropyl alcohol saturation methods is also independent of the aspect ratio of wollastonite micro-fibres.
The pore volume versus pore size curves for both cement-wollastonite and cement-silica fume-wollastonite composite systems have slightly different characteristics when the aspect ratio of wollastonite micro-fibres is increased. The volume of coarse pores in the range of 0.5 – 0.1 μm and the volume of very small pores in the region 0.01 – 0.005 μm increase slightly as the aspect ratio of wollastonite micro-fibres in the composite matrices increases.
Wollastonite micro-fibres having relatively large aspect ratio added to cement and cement-silica fume matrices appear to promote the formation of more large pores in both cement -wollastonite and cement-silica fume-wollastonite composites.
Portland cement based-binders reinforced with wollastonite micro-fibres having larger aspect ratio are tougher and more ductile than similar binders reinforced with similar fibres having lower size aspect ratio.
See related topics and documents: Effect Of Wollastonite Micro-Fibre Aspect Ratio On Reinforcement Of Portland Cement-Based Binders.pdf

1.8 Stability Of Portland Cement-Based Binders Reinforced With Natural Wollastonite Micro-Fibres

Abstract: The stability of Portland cement-based binders reinforced with natural wollastonite micro-fibres was investigated for hydration periods up to one year. The wollastonite micro-fibres imbedded in the hydrated cement paste were examined employing a scanning electron microscopy technique. Composite specimens were also periodically evaluated by flexural strength testing and microstructural characterization including mercury intrusion porosimetry, helium gas pycnometry, and isopropyl alcohol saturation measurement. The amount of Ca(OH)2 in the hydrated matrices was also determined by differential scanning calorimetry. Wollastonite micro-fibres imbedded in hydrated cement-silica fume matrices remained stable after prolonged hydration and exhibited no surface or bulk deterioration. The flexural strength and overall pore structure of the Portland cement-based binders reinforced with wollastonite micro-fibres also remained essentially unchanged and unaffected. Flexural toughness and the post peak deflection, however, were observed to decrease with hydration time. The amount of Ca(OH)2 in the hydrated matrices decreased slightly at advanced hydration times. The observed behaviour is discussed.

Conclusions

The experimental results obtained in this investigation appear to support the following conclusions.

Natural wollastonite micro-fibres are relatively stable in cement composite systems and apparently unaffected by exposure to prolonged hydration in calcium hydroxide solution.
The flexural strength of the cement-silica fume matrices reinforced with 11.5% by volume of wollastonite microfibres is unchanged and unaffected by prolonged hydration in calcium hydroxide solution up to one year.
The flexural toughness and post-peak deflection of the cement-silica fume and cement-silica fume-wollastonite composites decreases linearly with increase of hydration time.
The total porosity in the hydrated cement-silica fume and cement-silica fume-wollastonite composite systems determined by mercury intrusion, helium gas pycnometry and isopropyl alcohol saturation methods decreases slightly with increase of hydration time.
Pore volume in the 1.0 – 0.001 μm region of the cement-silica fume and cement-silica fume-wollastonite composite systems appears to be unchanged with increase of hydration time.
Wollastonite micro-fibres may affect the stoichiometry and the rate of formation of C-S-H.

See related topics and documents: NRCC-36074.pdf

1.9 Flexural Strength And Microstructure Of Cement Binders Reinforced With Wollastonite Micro-Fibres

Abstract: An investigation of cement composite behaviour showed that a flexural strength in excess of 28 MPa (4080 psi) can be readily attained in normal cement paste by incorporating an optimum amount of silica fume and 11.5% by volume of natural wollastonite micro-fibres. Systematic experimentation showed that modification of the flexural strength and the microstructure in hydrated cement and cement-silica fume matrices by addition of natural wollastonite micro-fibres is strongly controlled by the proportion of the various components. A discussion based on results obtained in bending tests, mercury intrusion porosimetry, helium gas pycnometry, and isopropyl alcohol saturation measurement, is presented in terms of composite theory and difference in observed properties.

Conclusions: The experimental results obtained in this investigation appear to support the following conclusions.

The flexural strength characteristics of hydrated cement and cement-silica fume matrices can be maximized by incorporating an optimum amount of natural wollastonite micro-fibres and silica fume in the composite mixture. A maximum flexural strength in excess of 28 MPa (4080 psi) can be attained when a composite mixture consisting of 11.5% by volume of wollastonite micro-fibres and 5.2% silica fume is prepared with a water/cement+silica fume ratio of 0.35 and 1% superplasticizer solution and hydrated in calcium hydroxide solution for a period of 28 days.
Improvement of the pre-peak and the post-peak load-deflection response by the incorporation of wollastonite microfibres was observed in composite systems with and without the presence of silica fume. This resulted in higher flexural strength and better ductility characteristics.
The microstructure of the cement-based composite systems is significantly modified by the incorporation of wollastonite micro-fibres and silica fume. The characteristics change depending on the composition of the mixture.
Very different total porosity, pore size distribution and threshold pore diameter characteristics are observed in hydrated cement-wollastonite and cement-silica fume-wollastonite composite systems.
Total porosity in the hydrated phases determined by mercury intrusion porosimetry is generally greater than that determined by the helium gas pycnometry.
Total porosity in the hydrated phases determined by isopropyl alcohol saturation method is also greater than that determined by the helium gas pycnometry; the values are close to those determined by the mercury intrusion porosimetry method.
Addition of wollastonite micro-fibres and silica fume appears to promote pore discontinuity in cement systems. This suggests that permeability may be significantly reduced and durability increased.

See related topics and documents: NRCC-34034.pdf

1.10 Influence Of Wollastonite On Mechanical Properties Of Concrete

Abstract: Studies were made on cement concrete and cement-fly ash concrete mixes incorporating wollastonite as partial substitute of cementitious material and sand respectively. Improvements in compressive (28-35%) and flexural strength (36-42%) at 28 and 56 days respectively were observed by incorporation of wollastonite (10%) in concrete mixes. By incorporation of wollastonite, reduction in water absorption, drying-shrinkage and abrasion loss of concrete, and enhancement in durability against alternate freezing-thawing and sulphate attack were observed. Because of high concrete strength and abrasion resistance, a better utilization of concrete cross section is possible. Alternatively, thickness of pavement slab can be reduced by incorporation of wollastonite micro-fibres in concrete mixes.

See related topics and documents: Influence of wollastonite on mechanical properties of concrete.pdf

1.11 Laboratory Study on Use of Wollastonite Micro-Fibers for Abrasion Resistance of Pavement Quality Concrete

Abstract: Deterioration of concrete surfaces occurs due to various forms of wear such as erosion, cavitations, and abrasion due to varying degrees of exposures. Deterioration of concrete surfaces in concrete pavements is most significantly contributed by abrasion wear. Abrasion wear occurs due to repeated rubbing, scraping, skidding of wheel tyre on the concrete pavement surface. Concrete abrasion resistance is markedly influenced by a number of factors including concrete strength, aggregate properties, water-cement ratio, microstructural features of cement matrix, pore structure of surface zone and surface finishing. The paper presents an initial part of an on-going research to evaluate the performance of hardened concrete surface with respect to abrasion resistance using wollastonite micro-fibres as a new material. A concrete specimen of water-cement ratios of 0.4 was made using ordinary Portland cement (OPC), and OPC admixed with wollastonite micro-fibres. A total of five concrete mixes were considered with varying proportions of wollastonite micro-fibres. Admixing of wollastonite micro-fibres improves the flexural strength significantly irrespective of days of moist curing, but decreases the compressive strength beyond 20 percent replacement level. Marked improvement in resistance to abrasion was observed for wollastonite micro-fiber admixed concrete up to 30 percent of its admixing with OPC. The abrasion resistance is more pronounced for admixed concretes with prolong curing duration.

1.12 Experimental Investigation of Micro-Reinforced, Alkali-Activated Ground Granulated Blast Furnace Slag with Wollastonite

The alkali activation of ground granulated blast furnace slag with a potassium-based solution has demonstrated production of a cementating material with early compressive strength higher than concretes based on ordinary Portland cement. This strength can be increased by reinforcing the paste with wollastonite micro-fibers. This paper presents the results obtained when an alkali-activated slag paste was reinforced with two types of wollastonite, NYAD G and wollastocoat. Compressive strength, setting time, and microscopic structure were analyzed as a function of type of wollastonite and percentage of wollastonite added. All properties were compared to potassium-based alkali-activated slag without wollastonite. Based on a 28 day compressive strength curve, it was determined that the optimal loading of wollastonite in slag generated an average compressive strength of 55.1 MPa, an increase in strength of 60% from the wollastonite-free sample. Higher loading of wollastonite was shown to significantly reduce the setting time of the samples.

1.13 Factors Affecting the Setting Time and Compressive Strength of Alkali Activated Ground Granulated Blast Furnace Slag Reinforced with Wollastonite

Abstract: We conducted a Design of Experiments to analyze and optimize the effect of the SiO2/K2O , H2O/K2O molar ratios of a Potassium base activating solution, slag/solution, Wollastonite/Slag mass ratios and mixing amount in the compressive strength and setting time of alkali activated Ground granulated Blast Furnace Slag. It was found that the total amount mixed had little to no effect in the compressive strength and the setting time. The setting time was largely affected by the slag/solution mass ratio followed by the SiO2/K2O molar ratio in the activating solution. For the compressive strength the larges correlation was found in the joint effect of the slag/solution and Wollastonite/slag mass ratios. The optimum formulation had seventh day compressive strength of 8000 Psi (55 Mpa) and setting time of 36 minutes was found when the activating solution had molar ratios of SiO2/K2O =1, H2O/K2O =10 and mass ratios of slag/solution = 1.75 and Wollastonite/Slag=0.37.

1.14 Clinker based on Wollastonite

Reducing CO2 emission and energy consumption are main drivers in the research and development of new, alternative cementitious binders. One interesting option are cement clinker which are based on the mineral wolllastonite. This clinker has a lower calcium/silicate ratio than alite or belite in Ordinary Portland cement clinker.

HeidelbergCement filed a patent on a method of producing clinker containing wollastonite which is based at least on 20 % by mass of secondary industrial by-product and wastes as SiO2 and CaO in the raw material. A raw meal is produced by grinding or granulation, mixing and homogenization which is burned at temperatures of 900 to 1500 °C forming a clinker containing at least 40 % by mass of wollastonite or para-wollastonite. Alternative fuels could be used partially or completely as energy source for the burning process. The clinker can be used to produce wollastonite cement or wollastonite-composite cements containing additional inorganic mineral components.

Link to the Patent Application: Verfahren zur Herstellung von Wollastonit-haltigen Bindemitteln

NOTE: Patent non defensible due to pre-published work on the reduction of CO2 Emissions from Cement Manufacturing Process by partly substituting Silicate for Limestone (see report below)

See related topics and documents: http://www.heidelbergcement.com/global/en/htc/News/index.htm

1.15 Reduction Of C02 Emission From Cement Manufacturing Process By Partly Substituting Silicate For Limestone

Abstract: The cement manufacturing industry is one of the major energy intensive industries, and furthermore, plenty amount of carbon dioxide is released from the calcination of limestone, CaC03, In Japan, four percent of total carbon dioxide emission is come from cement manufacturing plants. In the previous study, tne possibilities of utilization of weathering of alkaline or alkaline earth silicate rocks were demonstrated as a measure for carbon dioxide problem. The amount of the silicate resources was also discussed that silicates are abundant in the crust. In the present paper, the possibilities of utilization of calcium silicate rocks, wollastonite, CaSi03 as a substitute for limestone in cement manufacturing plants are discussed.

First of all, the maximum amount of wollastonite substitutively used was determined to be 440 kg and reduced amount of limestone to be 365 kg (30%) for l ton production of cement clinker, by adjusting the amounts of main components of cement product from wollastonite with JIS (Japan Industrial Standard). In case of the cement from wollastonite, most of the silica, ca. 220 kg/t-cement was supplied from wollastonite. In the course of evaluation, silica from coal ash was not taken into account. Thus the carbon dioxide from limestone calcination was reduced by 150 kg/t-cement (30%), from 510 to 360 kg/t-cement.

Secondly the process heat balance was evaluated. Necessary amount of process heat is to be supplied from coal combustion. It was also assumed that the coal contained no ash. In the standard cement manufacturing process, the heat for limestone calcination is necessary in addition to the heat for temperature increase from room temperature to flue gas temperature of 359°C or clinker temperature of 1450°C. The reaction heat from wollastonite to clinker is thought to be small because of similar chemical structure to each other. Thus the calcination heat is reduced in case of the cement production using wollastonite, leading to the reduction of amount of coal needed. The reduction of carbon dioxide emission also reduces the sensible heat of gas. Thus the carbon dioxide emission is further reduced by more than 40 kg/t-cement.

Totally, the carbon dioxide from the cement process was reduced by 190 kg/t-cement (24%) at least from 800 to 610 kg/t-cement by substituting wollastonite for 30% of limestone.

Finally, the quality of the produced cement from wollastonite was experimentally evaluated. In the most of analytical results, no remarkable difference was observed between wollastonite substituted alternative cement and conventional cement. Rather, the strength test, texture analysis and free lime analysis for the wollastonite substitute showed better results than the conventional sample, The present results indicated that the sufficient clinker formation reaction is expected even when wollastonite is partly substituted for limestone.

1.16 Solidia Cement (TM) – properties and performance results

Chemical properties and performance results of Solidia Cement™

Written by Global Cement staff
Thursday 19 December 2013

US: Solidia Technologies has reported the chemical properties, manufacture and performance qualities of a sustainable cement that can reduce the carbon footprint of cement and concrete products by up to 70%.

Solidia Cement™is made from the same raw materials and equipment as OPC, but is adaptable to a wide variety of cement formulations and production methods, offering a sustainable and performance-enhancing alternative.

Solidia Cement clinker is produced at 1200°C, approximately 250°C lower than OPC clinker. The cement is a non-hydraulic material that is composed primarily of low-lime-containing calcium silicate phases such as wollastonite / pseudowollastonite (CaO.SiO2) and rankinite (3CaO.2SiO2). The setting and hardening characteristics are derived from the reaction between CO2 and the calcium silicates. During the carbonation process, calcite (CaCO3) and silica (SiO2) form and are responsible for the concrete strength development.

Concrete products produced with Solidia Cement are manufactured using the same mixing and forming processes as OPC-based concrete and sequester up to 300kg of CO2/t of cement. The reduced CO2 emissions, combined with the ability of the cement to sequester CO2 during concrete curing, renders a CO2 footprint (associated with both the manufacturing and use) that is reduced by up to 70%.

“For over 50 years, scientists have tried to cure concrete with CO2 knowing the resulting product would be stronger and more stable. Solidia Technologies is the first to make this commercially viable. Our current focus is testing additional applications with an even wider variety of concrete formulations and manufacture methods to facilitate adoption across the globe,” said Solidia Chief Technology Officer Nicholas DeCristofaro, who co-authored the paper with principal scientist Sada Sahu.

Solidia Concrete™will be explored in a companion paper that is due to be released in January 2014.

2. Ceramics

2.1 Science For Ceramic Production – Wollastonite As A New Kind Of Natural Material

Abstract: Translated from Steklo i Keramika, No. 9, pp. 15 – 17, September, 2001.
An analysis of literature and patent data on the problem of producing and using wollastonite and wollastonite-based materials is carried out; the results of studying the properties of heat-insulating articles produced by the vacuum pumping technology are summarized. The theoretical possibility of using natural wollastonite as a material for the production of heat-insulating articles is demonstrated.
Conclusion: All this corroborates the principal possibility of using natural wollastonite in making heat-insulating ceramics. Articles based on this material can be recommended for the lining of casting and pouring equipment in melting aluminium melts, which provides for savings in power and metal consumption in transportation and casting of aluminium melts and ensures high quality of the cast metal.

See related topics and documents: Wollastonite as a new kind of natural material.pdf

2.2 Lime – Alumina -Silica Ceramic Processing Incorporating Wollastonite (CaO•SiO2)

Abstract: Improvements in mechanical and physical properties of advanced ceramics have been undermined by high processing costs and the reputation of unpredictable reliability, resulting in limited use of these materials in technical, industrial, and recreational applications. It was felt that a lower cost ceramic with desirable properties would open up new areas of application, traditionally dominated by hard or temperature-resistant metals. One potential technique was to use a naturally occurring mineral to develop new ceramic microstructures using a simple processing technique, reducing both raw material and production costs. The system chosen for potential development was an alumina (A1203) ceramic modified using 20 wt% naturally occurring wollastonite (CaO-SiO2). Green body production was carried out using wet processing which was optimised using rheology; the best conditions were found to be a pH of 3.0 using 1500 ppm of BETZ 1190 (dispersant) processed at about 0.35 volume fraction solids in a water-based suspension. Thermal analysis techniques revealed that the best sintering technique involved liquid phase sintering for fifteen minutes at 1500 C, followed by an air quench. This glass-ceramic was then devitrified using a heat treatment of 1032 C for 4 h followed by 1200 C for 4 h. The resulting structure, 68 wt% (60 vol%) alumina, with the remainder consisting of gehlenite and anorthite, had high indentation toughness (4.8 MPa√m) and a bend strength of 120 MPa; properties which lend it to economical anit-abrasion applications.

Conclusions

Wollastonite can be crushed to a smaller grain size using a shatterbox mill with an alumina mortar/pestle. Mean particle size of the grains was reduced from 30 µm to 4.5 µm after 10 minutes of crushing. Longer crushing times did not significantly reduce the size as the material began to heat up and cake against the sides of the pestle. Crushing in this way also reduced the aspect ratio of the larger grains from 10:l down to 3:l.
Alumina slips containhg 0.45 p grains were optimized at a pH of 4.0 and with a maximum green density of 2-74 g/cc (69 %TD). These samples were fired to 99 %TD at 1520 C for 8 hours with a grain size of 3µm.
Combining the material in an aqueous suspension produced a slip with poor pH control due to the alkaline nature of wollastonite. The window of pH dispersion, based on viscosity tests, was found to change fkom pH 2-6 for alumina to pH 1-3 for alumina + wollastonite.
Use of a polyrnenc dispersant was required to improve dispersion stability. Several anionic polyelectrolyte dispersants were tested: Emphos CS361 , Dolapix PC-75, Darvan 821A and Darvan C, the latter being the most effective. The concentration was optimized at 30 ppm, with an optimum green density of an alumina + 10 vol% wollastonite body of 55.6 %TD.
Subsequent testing using a high concentrations (>1000ppm) of a cationic polyelectrolyte, BETZ 1190, showed higher green densities of the alumina + 10 vol% wollastonite system were achievable: 2.49 g/cc = 65 %TD at 1500 ppm BETZ 1190, at pH 2.3.
Using F*A*C*T thermodynamic software and database, it was determined that above 1400 C the system would contain only crystalline alumina and a liquid phase at equilibrium. The amount of liquid was dependent upon the amount of wollastonite in the body and therefore, to maintain body mucture during liquid phase sintering, it was decided to limit the wollastonite to 20 wt%. As well, the LPS temperature was set at 1500 C to ensure appearance of the liquid phase without producing an excessive amount.
The thermodynarnic software also predicted the appearance of gehlenite and anorthite in a crystallised product.
A simple DTA apparatus was prepared and a DTA scan of green alumina + wollastonite bodies showed the transformation of wollastonite to pseudo-wollastonite at about 1100 C, followed by a small undefined endothermic peak at 1250 C, and finally an alumina dissolution peak after the appearance of a liquid phase at 1370 C.
Air-quenching of the alumina + 20 wt% wollastonite system from 1500 C formed a CAS (lime-alumina-silica) glass. This material was measured by DTA and showed evidence of a crystallisation peak at 1280 C.
Nucleation heat treatments over a broad temperature range produced noticeable changes in the DTA crystallisation peak (peak height, peak width, and peak shift to lower temperatures). Using these observations it was determined that optimum bulk (homogenous) nucleation occurred at 1032 C. The glass phase could be subsequently crystallised by heat treating at 1200 C.
Thus the optimised furnace schedule was: i) heat to 1500 C, ii) maintain for 15 minutes, iii) cool quickly to <900 C, iv) heat to 1032 C for four hours, v) quickly ramp to 1200 C, vi) maintain 1200 C for four hours, vii) cool to room temperature.
Electron probe microanalysis showed that the parent glass in the quenched CAS ceramic consisted of 39% alumina + 29% calcia + 32% silica. Anorthite and gehlenite crystallisation products in the devitrified CAS ceramic were too fine to be verified with this method.
X-ray diffraction of the products showed that the oniy crystalline phase present after liquid phase sintering was alumina. After nucleation and crystallisation gehlenite and anorthite were found with the alumina in the devitrified product.
Scanning electron microscopy of a polished liquid phase sintered CAS sample showed no observable grain structure. After nucleation and crystallisation the alumina grains were clearly visible along with a significant amount of porosity (14 %). As well as brighter phase along grain boundaries is the crystallization products (gehlenite and morthite). Images of a non-polished surface before and after the crystallisation treatment showed a distinct appearance of a dendtritic product between alumina grains where the CAS glas was formerly present.
Thermal expansion data showed that the devitrified CAS ceramic had slightly lower thermal expansion than alumina between 450C and 1050 C. For example at 600 C the CTE for alumina and the devitrified CAS is 9.0×10-4 and 8.1×10-6 K-1 respectively.
This result, combined with strength data implies that alumina and the devitrified CAS have similar thermal shock resistance, although more extensive testing would be required to verify this.
Ultrasonic measurements of elastic modulus and Poisson’s ratio showed good agreement with materiais of the same class given in various literature sources. Alumina had a dynamic Young’s modulus of 389 GPa compared to 170 GPa for the devitrified CAS.
Vickers hardness (10 kg load) of the alumina was 14.6 GPa, while the devitrified CAS was 9.35 GPa.
Using meaçurements of cracks generated at Vickers indents under defined loads, it was found that the devitrified CAS has a toughness of 4.80 MPa m 1/2 compared to 3.08 MPa m 1/2 for the alumina. This 56% improvement in toughuess was show to be principally due to the new shape of the alumina grains combined with the porous microstructure.
Flexural strength using a four point load showed very high strength for the alumina (498 MPa) compared to the devitrified CAS (121 MPa).
The results of the wet processing experiments showed that suitable homogeneous bodies consisting of alumina and wollastonite could be formed using slip casting with the aid of a polymenc dispersant.
It was found that the liquid phase sintering (LPS) technique allowed for slip cast
bodies with low green density, since the liquid diffusion during sintering removes most porosity. The finai porosity of 14% reflects the change in the microstructure during devitrification (the lower density CAS glass forming into higher density crystalline phases).
Given the above mentioned properties, it was suggested that this new reduced-cost material is ideal for applications in which abrasive wear resistance is critical. Indeed, using two wear relations it was shown that the devitrified CAS has much better wear resistance than common alumina ceramics.

See related topics and documents: Lime – Alumina -Silica Ceramic Processing Incorporating Wollastonite.pdf

3. Refractory Applications

3.1 Natural Wollastonite – Based Refractory Materials And Products For Aluminium Industry

Introduction: Wollastonite – calcium metasilicate with the CaSiO3 composition – is now a basic material for aluminum casting equipment in western countries. It is associated with the combination of the following characteristics of wollastonite:

it is not moistened with aluminum;
it is inert to aluminum;
it has low thermal conductivity;
it is nontoxic.

Observation: These ceramic materials offer promise as the thermoprotective materials for lining thermal units with operating temperatures up to 1000oC. The tests of the specimens from composite ceramic materials with silica fibers revealed practically limitless heat resistance of the specimens under cyclic heating, nonwettability with aluminum melt, lack of cracks under repeated immersion in aluminium melt