Mechanism underlying effect of expansive agent and shrinkage reducing admixture on mechanical properties and fiber-matrix bonding of fiber-reinforced mortar

• Published in: Cement and concrete research 2023-10, Vol.172, p.107247, Article 107247
• Main Authors: Aghaee, Kamran, Han, Taihao, Kumar, Aditya, Khayat, Kamal H.
• Format: Article
• Language: English
• Subjects: ITZ; Microstructure; SRA; Thermodynamic simulation
• ISSN: 0008-8846; 1873-3948
• DOI: 10.1016/j.cemconres.2023.107247

Expansive agent (EA) and shrinkage reducing admixture (SRA) are utilized to reduce shrinkage and risk of cracking in concrete. EA compensates shrinkage by initial expansion, and SRA reduces surface tension in the pore fluid. Although EA and SRA effectively reduce shrinkage, they can impair micro-structure of concrete at high contents. The shrinkage reduction effect of EA and SRA is well known; however, there is limited knowledge about their negative effect on microstructure and fiber matrix interfacial transition zone (ITZ). The current study explores the effect of using 10 % CaO-based EA, 2 % SRA, and their combination on mechanical, shrinkage, and microstructural properties of fiber-reinforced mortar (FRM) made with 0.5 % steel fiber. The hydration products are characterized by X-ray diffraction and derivative thermogravimetric. Backscatter scanning electron microscopy, energy dispersive spectroscopy (EDS), and micro-hardness experiments are employed to characterize the microstructure of fiber-matrix ITZ at two distances of 50 and 100 μm surrounding the fibers. Experiment results indicate that the quality of ITZ and fiber-matrix bonding is impaired by using either EA or SRA. The highest drop is obtained by the combination of EA and SRA. The element analysis by EDS line and point scanning reveals the presence of longer silicate chain and stronger fiber-matrix ITZ in the reference mixture. Micro-hardness measurements confirm higher mechanical properties at the ITZ between 50 and 100 μm surrounding the fibers. Thermodynamic simulations and phase assemblages interpret the influence of EA and SRA on mechanical properties and shrinkage.