The photo is sourced from gruntovozov.ru
Every year, Russian coal-fired TPPs produce about 22 million tons of ash and slag waste, whose accumulated volume has already reached 2 billion tons. One of the options for the disposal of this waste is the production of construction materials: high-calcium ash and slag can be used as cement, while low-calcium ash and slag can serve as aggregates in concrete mixtures or as a component for the synthesis of geopolymers. The latter are formed by the interaction of aluminosilicate raw materials (including coal ash) with an alkaline agent – a sodium hydroxide solution or liquid glass.
However, not all ash can produce materials of the required quality after coming in contact with an alkaline agent. The scientists from St. Petersburg State University and the Kola Science Centre RAS have attempted to estimate how to solve this challenge more efficiently, whether by adding calcium carbonate (calcite, CaCO3) to ash or by jointly processing ash and calcite in a mill. Experiments have shown that the combined use of these two methods can make the geopolymer mixture much stronger than if used separately. This synergistic effect could be compared to the combination of clay and reeds in the construction of dwellings in ancient times: clay walls reinforced with reeds were much stronger than walls built with clay alone or with reeds alone.
To understand the nature of this process, the scientists studied the impact made by the addition of magnesium, strontium and barium carbonates, i.e., calcium’s neighbours in the periodic system. They found that the key factor is the ability of carbonate to react with an alkaline agent – a sodium hydroxide solution. While magnesium carbonate interacts with alkali most actively, strontium and barium carbonates are practically inert. Calcite occupies an intermediate position, which gives the geopolymers the greatest efficiency.
“By subjecting both ash and calcite to machine processing in a mill at the same time, we don’t just mix them together and reduce particle size. When a particle splits, the chemical bonds holding it together are torn apart and a fresh surface rich in active centres is exposed. As a result, the ash becomes more reactive and dissolves more intensively in alkali,” Alexander Kalinkin, one of the authors of the study and doctor of chemical sciences, is quoted as saying by St. Petersburg State University.
