Strength characteristics and electrochemical impedance spectroscopy study of red mud-coal metakaolin geopolymer in a hydrochloric acid environment

Abstract

In this study, the compressive strength and electrochemical impedance spectroscopy (EIS) of red mud - coal metakaolin geopolymer (referred to as RM-CMK geopolymer), soaked in a high concentration (pH value of 2) hydrochloric acid solution were tested. The compressive strength of the RM-CMK geopolymer increased first and then decreased with the increasing immersion time in a hydrochloric acid solution environment. When the immersion time was 14 d, the compressive strength reached a maximum of 39.3 MPa. The EIS results showed that the Nyquist diagrams were composed of a small capacitive arc in the high-frequency region and large capacitive arc in the medium-frequency region. The radius of the capacitive arc in the medium frequency region, the total impedance value and the phase angle increased first and then decreased with the increasing immersion time. The equivalent circuit model of the RM-CMK geopolymer in the hydrochloric acid solution environment was determined to be Rs(Q1(Rct1W))(Q2Rct2). The compressive strength and electrochemical parameters of the RM-CMK geopolymer in hydrochloric acid solution environment had a good linear correlation. In addition, combined with scanning electron microscopy (SEM), the reasons for the strength and EIS change of the RM-CMK geopolymer in the hydrochloric acid solution were explained from a microscopic point of view.