Influence of combined substitution of slag and fly ash in improving the pore structure and corrosion resistance of foam concrete mixtures used for reinforced concrete applications

Foam concretes (FCs) have been used for several light-weight applications demanding low strength; however, their use for reinforced concrete (RC) applications is limited since high macrovoid amounts result in weaker matrices. The high w/c and overlapping of macrovoids have been identified as critical factors constituting a permeable microstructure in FC systems. Since corrosion performance is a critical aspect in the design of RC members and that the durability parameters of concrete, such as its porosity, pore size distribution, and permeability, also play a vital role in restricting the diffusion of chloride ions, particularly in the cover region of members, it is imperative to determine these properties and thoroughly understand the material behaviour or characteristics. In the current study, five FC mixtures (GF0 to GF4) were produced using ground granulated blast furnace slag (GGBS) and fly ash (FA) as a replacements for cement and sand, respectively, at different dosages, considering both economic and environmental aspects. The principal objective of the study is to determine the pore and permeability related properties of produced FCs, assess their corrosion performance, and ascertain their suitability for RC applications by comparing their performance with that of conventional M25-grade concrete. Also, the influence of the combined effect of GGBS and FA addition and the individual effect of FA addition were assessed. The research findings showed that FCs had a higher water absorption capacity and porosity compared to M25 concrete. In addition, the inclusion of GGBS and FA resulted in a decrease in water absorption by 8%–19% and a decrease in porosity by 23%–36% compared to the control FC mixture. The Mercury Intrusion Porosimetry (MIP) results indicated a substantial enhancement in gel pores, threshold, and critical pore diameter, while the size of the larger capillary pore decreased by 90%–95% when GGBS and FA were added compared to the control FC mixture. The scanning electron microscopy (SEM) analysis and binary images from the image analysis revealed the presence of uniformly distributed and distinct macrovoids formed by foam. These macrovoids were predominantly found within the size range of 7.1μm – 100 µm. The addition of GGBS and FA particles to FC mixes resulted in a significant improvement in the chloride ion permeation (CIP) class, reducing it from 'High' to 'Very Low' compared to M25 concrete. The Accelerated Corrosion Test (ACT) results obtained from FCs demonstrated a significant increase in cracking time ranging from 30% to 323%, as well as a notable decrease in mass loss ranging from 56% to 85% when compared to M25 concrete. The Fourier transform infrared (FTIR) and energy dispersive spectroscopy (EDS) analysis results revealed the presence of corrosion products, including feroxyhyte, akageneite, and Friedel's salt crystal, in both FCs and M25 concrete, indicating the occurrence of corrosion by the chloride ions present in the NaCl solution outside the specimen. Overall, the porosity, CIP, and corrosion resistance of FC with GGBS and FA are superior to those of the M25 concrete, making it suitable for use in RC applications.