A scientist at the University of Granada (UGR) has shown that adding volcanic ash to clayey soil during the brick-making process makes bricks less porous and more resistant. The work by Giuseppe Cultrone, a researcher at the UGR's Department of Mineralogy and Petrology, has been published in the journal Ceramics International, and is one of the few investigations carried out to date on the effectiveness of volcanic ash in the manufacture of ceramic products.
The deposition and accumulation of large quantities of volcanic ash, such as the one currently occurring on the island of La Palma, can cause significant damage to infrastructures and buildings, such as reducing the traction of vehicles on roads and aircraft on airport runways, or the collapse of houses due to the weight of particles on roofs. It is also the possible cause of contamination of aquifers and damage to crops.
For this reason, disposal operations, which are undoubtedly expensive, must be undertaken to ensure the maintenance of urban areas. Also, human exposure to volcanic ash is potentially dangerous because it can lead to respiratory problems and/or eye and skin irritations. And if this ash is not removed, health complications can arise from the re-mobilisation of the particles by wind and vehicular traffic.
In addition to the problems mentioned above, another point to consider is how to dispose of this type of waste. "Currently, volcanic ash is disposed of in landfills and there is no clear legislation regarding its recycling. The European Union classifies volcanic ash under the generic group of "municipal waste" and more specifically as "street cleaning waste". Several studies have addressed the reuse of volcanic ash as an additive in the production of various construction materials, especially mortar and concrete. However, its effectiveness in the manufacture of ceramic products has rarely been tested,' explains Giuseppe Cultrone, author of the study.
Etna ash
In this research, the UGR scientist has studied the addition of volcanic ash from the Etna volcano (Sicily, Italy) to a clayey soil from the province of Granada in order to evaluate the quality of new bricks from the physical and mechanical points of view.
For this purpose, bricks were made by adding 10 and 20% by weight of volcanic ash to the clayey soil and fired between 800 and 1100 ºC. The firing resulted in changes in the mineralogy of the pieces and vitrification of the clay matrix in which the ash particles partially melted. The addition of volcanic ash reduced the amount of mixing water required in the production process by up to 14%.
From a physical point of view, the addition of volcanic ash reduced the porosity of the bricks, a trend that intensified as the ash content increased. The bricks became more compact and their compressive strength increased as the firing temperature increased. The addition of volcanic ash reduced both the compactness and the mechanical strength of the fired pieces. However, the measured values remained above those recommended for these materials in construction works.
Bricks made with volcanic ash proved to be more durable than the same bricks without this additive against deterioration caused by salt crystallisation and were more resistant the higher the ash content. The firing temperature also increased the durability of the bricks, especially at 1100 °C, due to the high vitrification of the pieces.
This study has shown how volcanic ash has great reuse potential for the ceramic industry. "On the one hand, its addition would reduce the consumption of non-renewable resources such as clay raw materials and, on the other hand, its removal from landfills would benefit the environment. Therefore, volcanic ash should no longer be considered as a waste to be disposed of, but rather as a resource in the production of new efficient and durable building materials contributing to the development of a circular economy," says Cultrone.
