Effect of Wollastonite Powder on the Properties of Cement Mortar

Wollastonite is a naturally occurring chain silicate mineral, with its main component, Ca₃(Si₃O₉), accounting for over 96%, and also containing minor amounts of magnesium oxide, aluminum oxide, and iron oxide. The basic unit of the wollastonite crystal structure is a composite single chain formed by the connection of CaO octahedral columns and a SiO₂ framework. This special crystallographic structure allows wollastonite to retain its fibrous morphology even after specialized processing. Fibrous wollastonite powder can replace short asbestos fibers or glass fibers in building materials to enhance impact resistance, flexural strength, wear resistance, and dimensional stability. Studies have shown that natural wollastonite commonly occurs as fibrous, radial, acicular (feathery), or massive aggregates, with fibrous or needle-like structures being the most prevalent. RANSINCHUNG et al. analyzed the particle morphology of wollastonite powder using scanning electron microscopy, as shown in Figure 1. The images reveal that the wollastonite particles exhibit a needle-like structure.

Wollastonite powder is an ultrafine material containing 85%–95% amorphous reactive silica, with an average particle size of less than 0.1 μm. It can fill the pores in the cement matrix, thereby increasing the density of the paste. Wollastonite powder is classified as a reactive material. According to the study by Jiang Yufeng et al., the activity of wollastonite powder in mortar was evaluated based on the compressive and flexural strength indices of wollastonite-cement mortars, following the methodology outlined in GB/T 1596-2017 (“Determination of the Activity Index of Fly Ash for Use in Cement and Concrete”). Mortar specimens with wollastonite powder were compared to control specimens, and the ratios of compressive and flexural strengths were used to assess the material's reactivity. The results showed a compressive strength index of 72.9% and a flexural strength index of 110% for the wollastonite-modified mortars, indicating that the incorporation of wollastonite powder can significantly enhance the flexural strength of cement-based materials and improve their brittleness. This improvement is attributed to the partial participation of wollastonite in the hydration process of cement over time.

During hydration, continuous, flocculent deposits form on the surface of wollastonite particles, which help enhance flexural strength. Additionally, the elongated morphology of wollastonite particles can form an interwoven network structure within the cement paste, acting as a micro-fiber reinforcement and further improving flexural performance. Therefore, the combination of hydration reaction and micro-fiber reinforcement provided by wollastonite particles significantly increases the flexural strength of mortar specimens.

RANSINCHUNG et al. tested the consistency of wollastonite-cement mortars, finding that wollastonite reduces mortar fluidity. WAHAB et al. reported that partial replacement of cement with wollastonite has little effect on the initial setting time of cement mortars. In experiments where wollastonite replaced cement at 10%, 20%, and 30% by mass, the control mixture exhibited an initial setting time of 150 minutes. Only at the highest replacement level of 30% did the initial setting time show a slight increase, reaching a maximum of 5%.

ÖZ et al. prepared high-performance mortars with partial cement replacement by wollastonite powder, demonstrating that wollastonite can improve the impermeability of cement mortars. For replacement rates of 0% and 12%, chloride ion penetration of the high-performance mortar mixtures was highest at 28 and 90 days. Cement specimens containing wollastonite exhibited good resistance to chemical attack, primarily due to the chemical composition and crystalline structure of natural wollastonite. As a calcium silicate mineral, wollastonite is similar to calcium silicate hydrates formed under hydrothermal conditions and features long silicate chains extending along the crystallographic axis, providing high strength and chemical stability. This structural characteristic underlies the enhanced chemical resistance observed in cement-based materials containing wollastonite powder.

In conclusion, wollastonite powder acts as a functional admixture that effectively improves the performance of cement mortars. The main findings are as follows:

1. Ultrafine wollastonite powder exhibits certain reactivity, and its fibrous morphology can significantly enhance flexural strength and flexural-to-compressive strength ratio within a suitable replacement range, improving the brittleness of cement-based materials.

2. Due to the fineness and elongated particle shape, the flowability (spread) of mortars decreases as the wollastonite content increases under constant mix proportions, indicating that the use of wollastonite in concrete may require higher dosages of superplasticizer.

3. Cement specimens containing wollastonite demonstrate excellent chemical resistance, which can effectively improve the durability of cement mortars.