Abstract
Understanding the hydration kinetics of cement paste is essential for adjusting the early-age performance of concrete. Low-field nuclear magnetic resonance (LF-NMR) has emerged as an innovative technique to evaluate cement hydration progress by analyzing the evolution of transverse relaxation time (T(2)) signals. This study provides insights into the influence of a super-absorbent polymer (SAP) as an internal curing agent and calcium oxide (CaO) as an expansive agent (EA) on LF-NMR spectroscopy of cement paste for the first time. The chemical compositions of the cement and CaO-based EA were determined by X-ray fluorescence, while the morphological characterizations of the cement, SAP and CaO-based EA materials were characterized by scanning electron microscopy. Based on the extreme points in the first-order derivatives of the T(2) signal maximum amplitude curve, the hydration progress was analyzed and identified with four stages in detail. The results showed that the use of the SAP with a higher content retarded the hydration kinetics more evidently at a very early age, thus prolonging the duration of the induction and acceleration stages. The use of the CaO-based EA shortened the induction, acceleration and deceleration stages, which verified its promotion of hydration kinetics in the presence of the SAP. The combination of 3 wt% SAP and 2 wt% CaO consumed more water content synergistically in the first 100 h by hydration reactions. These findings revealed the roles of SAP and CaO-based EA (commonly adopted for low-shrinkage concrete) in adjusting hydration parameters and the microstructure evolution of cement-based materials, which would further offer fundamental knowledge for the early-age cracking control of concrete structures.