# New Methods for Assessing External Sulfate Attack on Cement-Based Specimens
## 1 Introduction  
Reducing CO2 emissions in concrete production is a critical priority for sustainable construction practices. With the rising awareness of environmental impacts, there is a growing demand for innovative technologies and processes that can minimize the carbon footprint of concrete [1, 2]. This includes exploring alternative binders such as fly ash, slag, and calcined clays, as well as developing more energy-efficient kiln technologies [3, 4]. Additionally, the use of fiber optic technology in structural health monitoring systems for civil engineering infrastructure offers a way to reduce CO2 emissions. By enabling real-time, remote monitoring of the structural integrity of bridges, buildings, and other critical infrastructure, fiber optic sensors can help identify and address potential structural issues before they become major problems [5]. This proactive approach can lead to more efficient maintenance and repair strategies, ultimately extending the lifespan of infrastructure and reducing the need for energy-intensive construction activities associated with major repairs or replacements. Furthermore, by providing accurate and timely data on structural performance, fiber optic monitoring systems can optimize the use of materials and resources, minimizing the environmental impact of construction and maintenance activities. Reinforced concrete (RC) structures are susceptible to a variety of destructive processes when exposed to aggressive agents [6, 7]. External sulfate attack (ESA) is a significant threat to structures, compromising their durability [8]. During ESA, cement-based materialsundergo a chemical attack resulting in the formation of aluminates due to the ingress of sulfate ions via a transport process [9]. In this regard, ESA appears to be the result of two processes. The first step is the transfer of sulfate ions from the external environment into the cement matrix, followed by the leaching of calcium ions from the cement matrix. There are many parameters that influence this process, including the porosity and permeability of the matrix. Second, the chemical composition of the cementitious material determines the chemical processes involved. 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]. In ESA, pores or cracks are partially or completely filled by expanding products (ettringite, gypsum), resulting in changes in the microstructure, microcracking, and loss of strength [12]. The main cause of microstructural changes in cementitious materials exposed to ESA is the formation of ettringite and/or gypsum within capillary pores and smaller gel pores [10,13,14]. During the early stages of attack, ettringite crystals accumulate in voids. As ESA progresses within the material, the crystals invade smaller capillary and gel pores, causing excessive expansion and damage. For this reason, macroscopic and microscopic evaluation methods appear to be useful in assessing the damage mechanisms associated with ESA [13]. The progress of ESA is influenced by several factors. These factors include the cement's chemical composition, including its tricalcium aluminate (C\({}_{3}\)A) content, the presence of minerals, and the water-to-cement ratio. In addition to pH, exposure time, and temperature, other variables such as the type of ions associated with the sulfate ions can also affect the attack and transport process. The physicochemical aspects of ESA and their destructive effects on cementitious materials have been extensively investigated in numerous studies [15, 16, 17, 18, 19]. However, few experimental studies have considered the combined effect of exposure conditions, cement type, and water-to-cement ratio on cementitious materials in order to describe the ESA mechanisms on the real exposed structures [20, 21, 22].