Experimental study of mechanical properties of G-UHPC against sodium sulfate attack at elevated temperature

Geopolymer based ultra-high performance (G-UHPC) would be used as mass concrete in sulfate corrosion environments. The temperature effects caused by the huge hydration heat or the high ambient temperature significantly affect its properties. The objective of this study is to investigate the mechanical properties of GUHPC against the sulfate attack at the elevated temperature. The influences of CaO, Al2O3, and SiO2/Al2O3 molar ratio were analyzed in detail. A series of experiments including compressive, flexural, SEM, XRD, and FTIR tests were conducted to explore the compressive strength, flexural strength, the microstructure, and the reaction products of G-UHPC subjected to sulfate attack at elevated temperature. The results revealed that the G-UHPC with a lower CaO (& LE;0.2 %), higher Al2O3 (>0.18 %), and higher Si/Al (>4.0 %) exhibited superior sulfate corrosion resistance at elevated temperature but deteriorated mechanical properties. The degradation of mechanical properties of G-UHPC with higher CaO subjected to sulfate attack at elevated temperature was due to the substitution of Na+ for Ca2+, which rendered C-(N)-A-S-H easier to exhibit a decalcification to form N-(C)-AS-H. The higher Al2O3 negatively affected the polyreaction and this negative effect was enhanced by sodium sulfate solution. The higher Si/Al ratio (>4.5 %) could enhance the mechanical properties of the corroded GUHPC. The elevated temperature seems harmful to the resistance of G-UHPC against sodium sulfate attack.