Red-clay ceramic powders as geopolymer precursors: Consideration of amorphous portion and CaO content

Geopolymers are conventionally prepared from metakaolin, fly ash, or blast furnace slag. In this paper, geopolymer synthesis using two different red-clay ceramic powders and a varying amount of Na2O/SiO2 activator is presented. Experimental results show that despite of the high content of crystalline minerals and lower concentration of amorphous matter, both tested precursors enable to prepare geopolymers with satisfactory mechanical properties. The discussion of measured values of compressive strength with respect to SiO2/Al2O3 and Al2O3/(Na2O + K2O) ratios in the prepared geopolymer mixes reveals that the consideration of only amorphous portion of ceramics is appropriate. A higher content of CaO in the precursor is found to cause a broader pore size distribution and higher porosity of resulting geopolymers, which is due to the formation of calcium silicate hydrates.