Bayer red mud (RM)-based geopolymers are economical and ecofriendly alternatives to cement because of their superior performance. This study investigated alkali-activated cementitious materials by combining RM, fly ash (FA) and slag, and the mixtures were used to produce ecofriendly composites. The influence of the Si/Al molar ratio (3.30–3.79) on the initial properties (setting time and flowability) and hardened properties (compressive strength, drying shrinkage and water permeability) of the composite materials was studied. The Na₂O content was fixed at 4 wt%, and the thermal activation temperature was 800 °C. The phase evolution and geopolymerization mechanism of the effect of the initial Si/Al molar ratio on the material properties was investigated by FTIR, XRD, TG–DTG and SEM–EDS. The results of M1.2Si333 indicated that the compressive strength of the blends can reach 33.5 MPa at 28 days, with a drying shrinkage rate of 1.20%. Compressive strength decreases, while drying shrinkage increases with a higher initial Si/Al ratio. Microstructural analyses revealed that a low Si/Al ratio and alkali activator modulus enhance the dissolution of precursors to form C–(A)–S–H gels, which increase the compressive strength. The results promoted the application of RM-based geopolymer-engineered cementitious composite and enhanced the resource efficiency of the bauxite residue.

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 (T₂) 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 T₂ 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.