The amount of water is essential to explain the shrinkage process. It is determined by the water-cement ratio, but also by the moisture content. Water in hydrated cement paste occurs in various forms: chemically bound, interlayer, adsorbed, free water and water vapor. During cement hydration, the balance between various forms of water changes. For example, the amount of physically bound water diminishes, and the moisture content changes at similar environmental conditions. Moisture changes occur first in the larger pores and then in the smaller pores. The air pores are larger than the hydration pores (capillary pores, gel pores) and have a greater effect on strength and permeability properties, while the cement hydration pores have a greater effect on shrinkage. At early ages, when composite properties are not fully developed, higher shrinkage influences the development and propagation of cracks. These (micro)cracks represent weak points and affect durability. To reduce shrinkage and bridge the cracks, fibers are added to the cementitious composite. Depending on the type and geometry, the fibers can reduce shrinkage and bridge the cracks. To reduce the environmental impact of fiber production, the use of waste fibers in cementitious composites is proposed. The main objective of this study was to investigate the influence of alkali-resistant glass fibers from production waste on the development of the microstructure of cementitious composites, i.e., the influence on the total porosity. The pore content was determined on fresh cementitious composites after mixing, while the total porosity of the material was measured after 28 days using a mercury intrusion porosimeter. Results were complemented by X-ray computed microtomography (micro-CT). The influence of production waste fibers is presented as a function of length and fiber content. In addition to the reference mixture, results were also compared with mixtures containing factory-produced fibers.