# Microstructure of Mortar with Ballast Waste as a Cement Replacement
## 1 Introduction  
The utilization of construction and demolition waste produced in cement manufacturing is one of the challenges facing modern society. One of the least well-known types of waste is that generated after the useful life of ballast, which serves as a substrate for railway tracks. The design and operation of railway systems must comply with both technical and legal requirements. Today, two main types of railway substructures are used worldwide: ballasted track systems and ballastless track systems. A ballasted track consists of a pair of rails, sleepers, and fastening systems supported by a ballast layer. Its main advantage lies in its low construction cost and effective drainage performance, although regular maintenance is always required [1]. In contrast, ballastless tracks feature a concrete roadbed in which the sleepers are embedded--commonly known as a slab track [2]. Mineralogically, ballast waste consists of silicates and kaolinite clay minerals [3]. Most of this waste is either used as inert material or stored in waste dumps or landfills, presenting significant technical, economic, and environmental challenges. With continued use, ballast becomes rounded and loses the properties that make it suitable for supporting the weight of the track structure, ultimately turning into inert waste that is sent to landfills. However, different studies have shown that this waste canbe repurposed, particularly as supplementary cementitious material (SCMs), replacing cement in different proportions for cement and concrete production. The incorporation of various types of waste into cement--such as fly ash, slag, sewage sludge, plant waste, and, more broadly, construction and demolition waste, including ballast waste--is a widely accepted practice. Supplementary cementitious materials (SCMs) are multiphase heterogeneous materials whose properties are influenced by the characteristics of each component [4]. This necessitates the study of the microstructure of the construction element, which has been carried out in different works using mercury intrusion porosimetry (MIP), one of the most widely used techniques for determining pore size distribution, from capillary pores to air voids in cement paste samples [5]. X-ray diffraction (XRD) can be employed for qualitative and semiquantitative analysis of the components present in powdered samples [6]. Scanning electron microscopy (SEM) provides microstructural information on the sample surface through high-resolution imaging [7]. Additionally, nuclear magnetic resonance (NMR), a non-destructive technique, can be used to examine extremely fine components such as C-S-H gels and capillary pores [8; 9]. The pore structure is one of the most critical characteristics of cement-based materials, directly affecting mechanical properties, transport properties, and durability. The microstructure of these materials has been characterized through various properties, including permeability [10; 11], thermal conductivity [12], strength [13], carbonation [14; 15; 16], freeze-thaw resistance [17], chloride attack resistance [18], porous structure [19; 20; 21], pore connectivity [22], hydration [23; 24; 25], damage evolution [26], and deterioration [27], along with physical properties such as mass diffusivity [28], among others. Recently, X-ray micro-computed tomography (\(\upmu\)-CT) has contributed significantly to the study of porous structures in materials [29], as it does not require special sample preparation. This aspect is particularly important for studies involving Portland cement-based materials, where many conventional sample preparation procedures can alter the structure of the cement paste. The role of additives in cement-based systems has been extensively studied, not only for sustainability and cost-effectiveness, but also due to the advantages these materials provide in the final cement product [30]. The cost and environmental impact of cement production are constrained by sustainability concerns, and also by the benefits associated with pozzolanic binders added to cement.