AC impedance spectroscopy interpretation of the hydration behavior for cement mortar containing fly ash and nano-metakaolin

Abstract

The partial replacement of cement with fly ash (FA) has the potential to reduce industrial waste and carbon emissions. FA-cement systems exhibit lower strength during the early hydration period, which limits their applications. Nano-metakaolin (NMK) is known to enhance early strength, yet its effect on the hydration and microstructure development of FA cement systems remains unclear. This study utilizes the alternating current impedance spectroscopy (ACIS) method to investigate the hydration properties and microstructural evolution of NMK/FA cement mortar. The suitability of the ACIS method was validated through the use of TGA, XRD, and MIP tests. A suitable equivalent circuit model R_s(CPE₁(R_ct1W₁))(CPE₂(R_ct2W₂)) was determined to analyze the ACIS data. Furthermore, the relationships between strength and ACIS parameters of NMK/FA cement mortar were established. The results indicate that NMK promotes the formation of hydration products and refines the pores of FA cement paste, particularly during the 7–14 d. The ACIS observations are consistent with the conclusions derived from XRD and TGA. FA decreases the bulk resistance of cement mortar, whereas NMK significantly increases it, especially during the 7–14 d period. A synergistic pozzolanic reaction between NMK and FA occurs within the 7–14 d period, which is crucial for the long-term performance and durability of NMK/FA cement systems. When 5 % NMK is added to the cement paste with 30 % FA, the pores larger than 1 μm significantly decrease. Compared to FA30N0, the compressive strength of FA30N5 cement mortar increased by 12.9 %, 35.4 %, 20.0 %, and 8.3 % at 3, 7, 14, and 28 d, respectively. Furthermore, a logarithmic relationship exists between compressive and flexural strengths, the diffusion impedance coefficient and the bulk resistance of the NMK/FA cement mortar.