Alkali-activated materials have long been dealt with at the Institute of Materials Chemistry, Faculty of Chemistry, BUT, thanks to national and international basic and applied research projects. “It is mostly seen as an alternative to Portland cement-based materials, especially for applications requiring high chemical resistance or resistance to high temperatures. Furthermore, the main advantages include that alkaline-activated materials are more environmentally friendly and sustainable in production. Especially because it is produced from industrial waste and secondary products. So we don't waste natural resources like limestone. It is the main raw material for cement production, and in addition, its decomposition produces carbon dioxide,” Vlastimil Bílek lists the benefits.
One of the major problems with these materials, however, is that they do not contain the common organic admixtures used in traditional concrete. “Conventional additives, such as superplasticizers, which even a small dose will significantly thicken the concrete, are designed for conventional cement. Thanks to them, we can add less water and get more strength and durability. Concrete can also be used to build complex massive structures such as skyscrapers or bridges. However, these plasticizers usually do not work with alkaline-activated materials. We wanted to get deeper into which substances work, which don't, and why,” explains Lukáš Kalina, about the aim of their three-year project under the auspices of the Grant Agency of the Czech Republic.
The multidisciplinary team of researchers, which included experts from the Institute of Physical and Consumer Chemistry of the FCH (Faculty of Chemistry) BUT or Faculty of Civil Engineering of the BUT, focused primarily on understanding the behaviour of organic additives with their binder. “We have shown that three fundamental aspects of how ingredients work and don't work are pivotal. Their chemical stability, solubility in the activation solution and adsorption on the precursor surface. For example, we have found that if we activate the slag with sodium hydroxide, a certain type of plasticizer works very well. But the moment even a tiny amount of silicate ions appears in the activation solution, the plasticizer suddenly stops working completely,” says Vlastimil Bílek. This finding was also part of the articles recently published by prestigious journals Cement and Concrete Research and Construction and Building Materials.
According to Vlastimil Bílek and Lukáš Kalina, a better understanding of the behaviour of alkali-activated materials will help make them more competitive. “If we want to compete with conventional Portland cement-based concrete with alkali-activated materials, we need to find suitable additives that would fundamentally change the characteristics of this non-traditional type of concrete,” Kalina points out.
However, according to Vlastimil Bílek, there are already interesting applications in the world. “In Brisbane, Australia, for example, an airport runway is made of alkali-activated slag and fly ash. In our country, commercial use is much smaller,” he says, adding that European legislation in particular prevents more widespread use. “Concrete standards do not foresee the complete absence of Portland cement as a binder,” he adds. Architects and designers, who have more confidence in cement, are also cautious, according to him. “But standardisation and testing is already being worked on globally,” Bílek points out.
They themselves plan to continue the research and possible application. “To some extent, we are already following up this project with another one, primarily at the Faculty of Civil Engineering of the BUT. We have also been approached by a company that would like to work with us on the production of alkali-activated parts. I don't want to be completely specific yet, but it looks promising,” says Lukáš Kalina, adding that greater industrial production is their ultimate goal. “We can research and test something here in the laboratory, but if it doesn't make it into practice, it's half the point,” Vlastimil Bílek concludes.