Chemical Mechanisms Involved in the Coupled Attack of Sulfate and Chloride Ions on Low-Carbon Cementitious Materials: An In-Depth Study

Abstract

This study aims to analyze the individual and combined chemical attacks of sulfate and chloride ions on cementitious materials and assess the efficiency of some selected additives (fly ash, blast furnace slag, and metakaolin) in countering this combined attack. This research is conducted in the context of construction in marine environments, where reinforced concrete structures are often subject to significant challenges due to early exposure to sulfate and chloride ions. This early exposure results in concrete expansion, cracking, and, ultimately, the corrosion of steel reinforcements. Nevertheless, the interaction between sulfate ions, chloride ions, and the cementitious matrix remains poorly understood. Previous research has drawn conflicting conclusions, with some suggesting that sulfate ions mitigate chloride attacks, while others have come to the opposite conclusion. During this study, experimental investigations were conducted by immersing powders obtained from crushed ordinary Portland cement (CEM I) paste specimens, as well as binary, ternary, and quaternary blends, in sulfate, chloride, and sulfate–chloride solutions over the course of 25 days at an early age. Results from different characterization techniques (thermogravimetric analysis, Fourier Transform Infrared spectroscopy, Raman spectroscopy, etc.) indicate that chloride ions delay the formation of ettringite, while the presence of sulfate ions accelerates the chloride attack by limiting the formation of Friedel’s salt. The Mercury Intrusion Porosimetry test confirmed these results by showing a pronounced increase in specimens’ porosity after exposure to solely sulfate after 25 days, compared to the ones exposed to both sulfate and chloride ions. Furthermore, the incorporation of multiple additives, particularly in ternary and quaternary blends, demonstrates the enhanced durability of the studied samples. This was confirmed by a Fourier Transform Infrared spectroscopy analysis, which indicated a delayed ettringite formation in these mixtures. This delay was further affirmed by the complete depletion of sulfate ions in the sulfate solutions upon contact with powders derived from the 100% CEM I paste.

Prediction of the Compressive Strength of Vibrocentrifuged Concrete Using Machine Learning Methods

“…Chlorine and sulfur ions penetrate into the thickness of concrete through places of increased permeability (micro-cracks, capillary, and open pores) and enter into chemical reactions with the formation of new unstable compounds. These crystalline compounds, having a slightly larger volume compared to the volume of the initial phase, cause internal stresses and destroy concrete [8,9,47,48]. The number of wetting and drying cycles varied from 1 to 500 cycles.…”

New Methods for Assessing External Sulfate Attack on Cement-Based Specimens

“…After one month of curing in water, the specimens were immersed (to a depth of 1 cm) for a period of 24 weeks in a solution of Na 2 SO 4 at a concentration of 15 g per liter, which was demonstrated by several studies to be the most appropriate for ESA testing [10,11]. The immersion depth of the specimens was only 1 cm, corresponding to a partial immersion usually referred to as semi-immersion.…”

“…Hydrate compounds react with residual anhydrous compounds and sulfate ions during this process. As the two processes coexist, it is imperative to analyze both the physical and chemical aspects of ESA [10,11].…”

Long-Term Effects of External Sulfate Attack on Low-Carbon Cementitious Materials at Early Age

“…When it comes to external sources, sulfate, a pervasive agent contributing to the deterioration of concrete structures, is inevitable due to its presence in various external sources where constructing RC structures is a necessity. Upon contact with RC structures, sulfate ions penetrate the cementitious matrix through the porous network, initiating chemical reactions with hydrated cementitious materials to produce ettringite and/or gypsum [5][6][7][8][9]. This process induces internal stresses, leading to expansion and cracking in the structure.…”