How discarded diatomite marls became a new source of wollastonite
In discussions about sustainability in mining and materials science, the word waste often appears as an inevitable by product. Tailings, overburden and discarded sediments are usually treated as liabilities that must be stored, stabilized or simply hidden from view. From an economic perspective, they represent cost. From an environmental perspective, they represent risk. Rarely are they seen as opportunity.
Yet this classification depends less on chemistry than on convention.
A material becomes waste not because it lacks useful properties, but because, at a particular moment, no one has found a practical use for it.
This simple observation motivated our recent work on diatomite quarry residues in southeastern Spain. While visiting extraction areas near Hellín, we were struck by the volume of marly sediments accumulating in spoil tips. These materials were routinely separated from the commercial diatomite fraction and treated as discard. At the same time, the Spanish ceramic industry continued importing wollastonite, a calcium silicate mineral widely used to improve ceramic performance.
The juxtaposition was difficult to ignore. On one side, large piles of unused material. On the other, imported raw minerals fulfilling a similar chemical function.
Our study set out to explore whether the “waste” could replace the import.
The full research article is available here in Minerals Engineering:
https://www.sciencedirect.com/science/article/pii/S0366317521000376?via%3Dihub
The idea was conceptually straightforward. Diatomaceous marls contain two components that are particularly interesting for ceramic chemistry. The diatomite provides reactive silica, while the associated carbonates supply calcium. Together, these elements are precisely those required to form wollastonite, CaSiO₃, a mineral known to promote liquid phase formation during sintering and to enhance dimensional stability and mechanical strength in ceramic bodies.
In other words, the raw ingredients were already present. The question was whether they could be combined efficiently through thermal treatment.
Instead of complex synthesis routes or chemical additives, we chose a simple approach that would be realistic for industrial application. The marl was calcined at controlled temperatures to encourage solid state reactions between silica and calcium phases. Mineralogical analysis showed that at around 1050 degrees Celsius wollastonite became the dominant crystalline product. Lower temperatures resulted in incomplete transformation, while higher temperatures increased energy consumption without clear benefit.
The result was not an exotic laboratory material but a functional industrial mineral produced through a single thermal step.
However, producing wollastonite in the laboratory is only half the story. The more important question is whether it performs adequately in real ceramic formulations. Industrial minerals are valuable only if they improve processing and product quality under manufacturing conditions.
For this reason, we incorporated the synthetic wollastonite directly into whiteware ceramic bodies, partially replacing conventional calcium carbonate or natural wollastonite. The goal was not to demonstrate theoretical feasibility but to test practical behavior during shaping, drying and firing.
What we observed was encouraging. The ceramic bodies remained workable during processing and, after firing, exhibited mechanical strength and whiteness comparable to reference compositions. Most notably, the presence of wollastonite promoted sintering at slightly lower temperatures. The effective firing range shifted downward by roughly sixty degrees Celsius.
At first glance, sixty degrees may seem modest. In an industrial kiln operating continuously and processing large volumes, this difference translates into substantial energy savings and a measurable reduction in emissions. When multiplied across the scale of regional ceramic production, the environmental and economic implications become significant.
Equally important is the supply perspective. Spain imports most of its natural wollastonite. Substituting even part of that demand with locally derived material reduces transport, dependence on external sources and exposure to price fluctuations. What began as quarry residue becomes part of a more resilient and regional value chain.
Of course, the process was not without challenges. The synthetic powder displayed a relatively narrow particle size distribution and contained soluble salts that affected slurry rheology during preparation. These issues required adjustments in deflocculants and processing parameters. Yet such adaptations are typical in ceramic manufacturing and do not represent fundamental obstacles. They are engineering details rather than conceptual barriers.
Stepping back, the broader lesson of this work goes beyond wollastonite itself.
It highlights how strongly our definition of waste depends on perspective. Materials discarded during extraction often retain useful chemical or mineralogical properties. In many cases, they simply do not match the specifications of the primary product and are therefore overlooked. With modest additional processing, however, they can become secondary resources.
This shift in viewpoint is central to the idea of a circular economy. Instead of separating materials into categories of valuable and worthless, we begin to see extraction sites as complex geochemical inventories. Residues are not merely leftovers. They are potential feedstocks waiting for the right application.
From a sustainability standpoint, this approach has multiple advantages. It reduces the need for new extraction, lowers transport distances and makes better use of material already disturbed by mining activities. It also aligns environmental responsibility with economic logic, which is often the most reliable path to long term adoption.
For researchers working at the interface between geology, materials science and industry, this kind of cross disciplinary thinking can reveal opportunities that are easy to miss within narrower frameworks. Sometimes innovation does not require discovering a new mineral or developing a novel technology. Sometimes it simply requires looking again at what we already have.
In this case, a marl heap at the edge of a quarry turned out to contain the ingredients for an industrial mineral that the country was importing from abroad. The transformation from discard to resource was not dramatic. It required only heat, analysis and a willingness to question assumptions.
That modest shift in perspective may be one of the most valuable outcomes of the study.
Further details and experimental results are described in the published article in Minerals Engineering:
https://www.sciencedirect.com/science/article/pii/S0366317521000376?via%3Dihub
More of my research on industrial minerals, resource efficiency and sustainable mining can be found on my Springer Nature author profile:
https://www.springernature.com/gp/authors/antonio-alonso-jimenez