Phosphate-based geopolymers are a type of ceramic-like material that is most often synthesised from a highly reactive aluminosilicate precursor such as metakaolin or fly ash with phosphoric acid. According to current scientific knowledge, due to dealumination that occurs under the influence of low pH, two phases are formed, silicate (SiO2·SiO2) and aluminophosphate (AlO2·PO2). This type of ceramic-like material is often called silico-aluminosilicate geopolymer ceramic. The advantage of the acidic synthesis pathway compared to the traditional alkaline pathway is in avoiding the use of free alkaline cations to balance the negative charge on the [AlO4]- tetrahedron. Namely, [PO4]+ in the [AlO4]- environment behaves as a cation that balances the negative charge on the structure. Due to the acidic synthesis pathway, the newly formed silico-aluminosilicate structure provides better resistance to acids, while due to non-alkaline conditions, it has reduced efflorescence compared to alkaline analogues. Also, many studies attribute better mechanical and thermal properties to phosphate geopolymers at elevated temperatures due to the formation of aluminophosphate mineral – berlinite (AlO2·PO2), which is an isostructural analogue of quartz (SiO2·SiO2) and which acts as a filler, reducing stresses within the material. The main parameter in reaction for phosphate-based geopolymers is the Al:P atomic ratio with an optimal value of 1. Therefore, in this work, the atomic ratio Al:P is set to 1.1 and the water-to-binder ratio, as a measure of the workability of geopolymer resin, is set to 0.85. We also investigated the feasibility of preparing porous phosphate-based geopolymer ceramics from metakaolin and phosphoric acid via replica shaping. The polyurethane (PU) template was infiltrated by the geopolymer resin, with the addition of fine quartz sand to improve mechanical properties, until a homogeneous green body was achieved. Green bodies were cured in sealed moulds at 80 ? for a few hours in order to solidify their shape before being subjected to heat treatment. Porous geopolymer ceramics were obtained by heat treatment at 600 °C at a speed of 1 °C min–1, which enabled the combustion of PU, after which it was additionally calcined at 1100 °C for 6 h. The samples were investigated using thermal, (micro)structural and spectroscopic methodology, consequently proving better thermal properties.

Acknowledgements:
This work has been funded by the projects UIP-2019-04-2367 and PZS-2019-02-1555 by the Croatian Science Foundation, and KK.01.2.1.02.0316 by the European Regional Development Fund.