Well-implemented, highly-used, world-wide-known building products like cement, concrete and ceramics are made mostly from raw materials and at very high temperatures. Because alkali-activated materials can be made from waste, in just a few hours to a few days, and at temperatures below 100 °C, they represent a potential future material in the building sector. Precursors used in alkali activation are different slags (well cherished are the ground granulated blast furnace slags), ashes (especially fly ashes) and wastes containing enough Si and Al in the amorphous content, which should be mixed with alkali in a way that avoids efflorescence [1]. However, the basic material for research of alkali-activated materials represents metakaolin, usually, a non-waste material used also in the conventional building industry, i.e. metakaolin represents a reference precursor to the secondary materials used in the alkali activation synthesis.

The beauty of the building materials is that it is possible to mix into them any waste, even non-building materials, and they still fit the primary function and at the same time gain additional highly desired properties if the proportions of the used materials are well-chosen, materials well enough milled [2] and homogenous [3], and curing performed at the correct conditions [1].

The use of energy and time of curing at temperatures below or around 100 °C can be decreased by the replacement of surface heating with volumetric heating. Namely, alkali-activated metakaolin from our previous study [4] irradiated by microwaves of frequency 2.45 GHz at 100 W [5,6] results in a temperature raise to 40 °C in 1 min, and 80 °C in 4 min, i.e. dissolution of precursor in alkali-media is enhanced instantly. Therefore, early compressive strength was with irradiation for 1 min at 100 W compared to the sample cured solely at room conditions almost 100% higher.

Considering all the above, metakaolin was mixed with melamine, a fire-retardant material, in different mass ratios, well-mixed with Na-silicate solution, moulded into silicone moulds, and then after immediately irradiated with microwaves with the frequency 2.45 GHz at 100 W for 1 min to speed up the reaction, curing and hardening. The replacement of melamine with metakaolin was evaluated primarily through changes in the mechanical strengths, where melamine does not contribute additional value, and secondarily through the ability to influence fire, before metakaolin would be replaced by secondary material that does not have a loss on ignition at 550 °C close to 0 (organic content present in the precursor) and therefore represents a more difficult system for evaluation.

[1] Horvat, B. et al. 30 (2019)
[2] Horvat, B. et al. Materials 13, 2227 (2020)
[3] Horvat, B. et al. in 3rd International Conference on Technologies & Business Models for Circular Economy: Conference Proceedings 11–50 (University of Maribor Press, 2022)
[4] Horvat, B. et al. Ceramics International S0272884222045461 (2022)
[5] Horvat, B. et al. in (2022)
[6] Horvat, B. et al. in 5th International Conference on Technologies & Business Models for Circular Economy: Conference Proceedings 9–24 (University of Maribor Press, 2023