The production of PC is the third most energy-demanding industry in the world, immediately behind the production of aluminium and steel, where to produce one ton of cement approximately 3.7 GJ of energy is needed. Production of cement simultaneously increases production of CO₂, so that in the production of 1 ton of Portland cinder a total of 1 ton of CO₂ is produced; 0.55 t is released during limestone calcification and a remaining 0.4 t is caused by the combustion of fuel. Emissions of carbon dioxide along with other gases create the greenhouse effect, which most probably is generating climate change due to global warming.

The scientific team at the Faculty of Chemistry, Brno University of Technology, has been engaged in the development and subsequent application of environmentally friendly and economically efficient cement-free concrete produced using secondary raw materials such as cement works stone-dust, power-plant fly-ash and blast-furnace cinder. However, it must be pointed out that today blast-furnace cinder is a scarce secondary material and finds its use in many applications.

The actual principle underlying the production of this non-traditional composite is the process of alkaline activation, in which the reaction in alkaline conditions of a mixture of blast-furnace cinder and power-plant fly-ash results in the transition of the siliceous and aluminous component to a solution with the subsequent formation of the binding phases of CSH (CASH) gel. Solutions of sodium (potassium) hydroxide and water glass are used as the alkaline activators. The high price of alkaline activators, which have limited the widespread use of these materials, has been resolved by partially replacing sodium hydroxide with cement works stone dust. Thanks to research activities it was verified that the considerable content of alkalis and free lime found in cement-works stone-dust can be used for alkaline activation; cement-works stone-dust is currently without use. Owing to its price and the realistic possibility to process this material in an environmentally sound way, cement-works stone-dust has proved to be an excellent alkaline activator for the activation of pozzolans.

What are pozzolans?

Pozzolans are natural or industrial siliceous, aluminosiliceous materials or mixtures of these materials. After being mixed with water, Pozzolans will not harden by themselves. However, if finely ground they react in the presence of water at normal temperature with dissolved calcium hydroxide to produce compounds of calcium silicates and calcium aluminates, which then cause gradually increasing solidity. Pozzolans, in principle, must contain active silicon dioxide and aluminium dioxide. The content of active aluminium dioxide must be at least 25 wt %.

The composition of alkali-activated concrete, which has been used successfully by the research team from the BUT Faculty of Chemistry, is shown in the following table, where the amount of each component is stated in kilograms and re-calculated to a 1 m³ volume of concrete. As aggregate, 3 fractions (0/4, 4/8, 8/16) from the Halámky quarry were used. The table also shows the approximate prices of each raw material, based on which the resulting price of the concrete can be compared against commercially produced concrete. The prices of aggregate and water are the same for both types of concrete. The final price of the newly-developed concrete is thus approx. 1,400 CZK per 1 m³ (without VAT), which is today the normal price of structural concrete C8/10. Moreover, in its production certain amounts of secondary raw materials (fly-ash and cement works stone-dust) were processed, which means the concrete can be called environmentally friendly. The resulting mixture was then used to produce a foot-slab (see the title image). Mixing concrete can be performed in the standard facilities of “concrete plants”, where all pilot-plant experiments were carried out.