Repair and Rehabilitation

- A New Market For Wollastonite Micro-Fibres?

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.

Wollastonite is a natural mineral, calcium meta-silicate (β-CaO-SiO₂), and is generally available in the shape of acicular particles for commercial applications. Natural wollastonite micro-fibres are fibres that are very fine with diameters mostly in the range of cement particles (about 25-40 mm) and with lengths of about 0.04-0.6 mm (Figure 1).

There has been a major increase in the world's production and consumption of wollastonite. Production has more than doubled between 1986 and (Figure 1) 1993 from approximately 145,000 tonnes to more than 365,000 tonnes. The market share has also changed from being U.S.-dominated (mainly NYCO Minerals Inc.) to a highly competitive situation due to new production coming on-line. This expansion includes ore produced in India, Finland and, most significantly, in China. Two Canadian companies, Orleans Research Inc. and Ram Petroleums Ltd., have plans to start production in 1996.

The largest current market for wollastonite is that of ceramics, followed by plastics, asbestos substitution, metallurgy and paints. Among these applications, plastics, asbestos substitution and paints generally require high aspect ratio wollastonite in order to obtain extra hardness, flexural strength and impact resistance. Its use in ceramics and metallurgy, however, does not require reinforcing-grade. Here, the chemical composition is the primary concern.

Recent research efforts in the development of high performance cement-wollastonite composites has opened up a prospective market for wollastonite in repair and new construction. Research in the Materials Laboratory at the National Research Council's Institute for Research in Construction (IRC) indicates that the mechanical and microstructural properties of Portland and non-Portland cement binders can be significantly improved with the addition of wollastonite micro-fibres to make up a small percentage of the composite material.

Wollastonite is an attractive reinforcement for brittle construction materials because it is readily available and is considerably less expensive than steel or carbon micro-fibers.  (The latter two materials are effective in improving the tensile strength characteristics of cement-based materials). These factors make wollastonite-reinforced systems an attractive alternative to more traditional reinforcing materials for use in repair and new construction markets.

 Figure 1. A scanning electron microscopy (SEM) photo of natural wollastonite micro-fibers.

Figure 2. Flexural strength (at 28 days) relative to wollastonite micro-fibre content (by volume) of cement composite systems containing 5.2% of silica fume. The water/cement ratio is 0.35.

Some of the advantages of using wollastonite micro-fibres as reinforcement are listed below:

Flexural Strength: Improvement in flexural strength can be obtained by incorporating an optimum amount of natural wollastonite micro-fibres and silica fume in the cement composite mixture. For example, a maximum flexural strength in excess of 28 MPa ( 4080 psi) is 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 (Figure 2).

Ductility: Improvement of the pre-peak and the post-peak load- deflection response by the incorporation of wollastonite micro-fibres is achieved in composite systems both with and without the presence of silica fume.

Microstructural Modification: The microstructure of cement-based composite systems is significantly modified by the incorporation of wollastonite micro-fibres and silica fume. The addition of wollastonite micro-fibres and silica fume appears to promote pore discontinuity in cement systems, which has positive implications for improved durability of these products.

Water Permeability: By adding wollastonite micro-fibres, lower values of the water permeability constant are obtained than can be achieved with pure cement paste, or with binders that have steel or carbon micro-fibres systems (Figure 3).

Chemical Stability: Wollastonite micro-fibres are chemically stable in hydrated cement and cement-silica fume composite matrices. They appear to be unaffected by exposure to pro- longed hydration in calcium hydroxide solution. Expectations for long-term durability of cement- wollastonite composite systems appear to be warranted.

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. .

Figure 3. Permeability constant (water) for cement paste and pastes containing 5% (by volume) of wollastonite, steel, and carbon micro-fibers respectively

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