Comparative analysis of long-term and high temperature performances of OPC based high strength mortar and silica fume based high strength geopolymer mortars

The effects of Class C fly ash (FA) contents on the performance of silica fume (SF) based high strength geopolymer mortars (HSGM) subjected to high temperatures up to 1000 ºC are investigated. The percentages of FA substitution by SF are 10 %, 15 %, 20 % and 25 % by weight. The alkali activators used consist of sodium silicate (SS) and sodium hydroxide (SH) and are used in mixtures with SH/SS ratios of 0.3, 0.4, 0.5 and 0.6. In order to compare SF based HSGMs, ordinary Portland cement (OPC) based high strength mortar (HSM) as a control mortar is also produced with the same dosage and water content. The results at environmental temperature show that higher mechanical properties are obtained from SF based HSGMs compared to OPC based HSM. The optimum replacements of Class C FA and SH/SS ratios are 15 % and 0.3 or 0.4 in terms of mechanical properties. At 28 days, SF based HSGMs with flexural strength (ffs) of 15 MPa and compressive strength (fc) of 100 MPa can be produced without thermal curing. High reductions in the mechanical properties are seen on the OPC based HSM and SF based HSGMs subjected to high temperatures. In addition, SF based HSGMs with fc values above 25 MPa can also be obtained after exposure to 1000 ºC. Alterations in the microstructure of OPC based HSM and SF based HSGMs under the influence of high temperatures are also examined with XRD, FTIR, SM, and FESEM/EDX analyses. Particularly, a spongy structure with volumetric expansion is seen with the formation of the glassy phase in the matrix of SF based HSGMs subjected to a temperature of 1000 °C.