## 1 Introduction  
The reuse of waste sludge water from CP in concrete mixes allows the reduction of waste production and represents a possible new view of this waste as a secondary raw material. In general, if we want to find use for waste material, in our case waste sludge water, we need to investigate its properties with regard to the area of use. As indicated in the studies, researchers most often encounter major problems, such as a higher share of fine particles, the content of substances showing acidic reaction, and a high content of sulphates [1,2]. These waters need to be chemically treated or diluted by adding pure mixing water to comply with the optimum composition of the concrete mixture according to the given formula and also to meet the limits stipulated by the standard [3,4]. However, it has to be taken into account that a higher share of fine particles requires more mixing water to maintain the water-cement ratio for proper hydration, as shown by the studies [5; 6; 7; 8]. Knowing the interactions of the individual components of cement composites, the desired properties of the resulting composites can be achieved. The knowledge of interactions does not only apply to waste mixing water, but can also be applied to other waste products, as suggested by the studies [9; 10; 11]. In fact, it is estimated that a truck with the volume of 9 m\({}^{3}\) carries approximately 300 kg of returned concrete a day. This remaining concrete is generally discharged into large containers and, after hardening, the concrete can be crushed and used as recycled aggregate for further construction. After emptying the rest of fresh concrete, the agitation truck is washed with large amount of water, 700-1300 litres per truck, and the scattered mass settles in large sedimentation tanks [12; 13]. Due to the large amount of suspended substances and high alkaline conditions, untreated sludge water cannot be legally discharged into municipal sewer systems [1]. Water is an important component of concrete. It participates in the hydration of cement, but also contributes to the workability of fresh concrete. The quality of mixing water is therefore crucial for the properties of fresh concrete, including the strength and durability. Most standards specify that water used as mixing water must be clean and free from oils, acids, alkalis, organic matter, or other harmful substances [14; 15]. Complete recycling of sludge in concrete plants is considered because of its great benefits in terms of the reduction of the costs of disposal, as well as the protection of the environment. The use of washing water from concrete plants for the preparation of fresh concrete was studied by [PERSON] et al., and they focused their attention on an experimental study in order to determine compressive strength, sulphate resistance, workability, and setting times of cement mortar [16]. A study on the use of sludge from concrete plants has found that concrete mixed with sludge water containing residual cement tends to show shorter setting time and lower fluidity [1; 17]. In the study \"Properties and possible recycling of solid waste from ready-mix concrete\", the use of a fine fraction of the waste slurry (adding solid waste) led to a reduction in the compressive strength [18]. [PERSON] et al. used washing water from a concrete plant mixed with water from a water main for the production of concrete and mortar. Their results have shown that after 28 days, the compressive strength of most samples was higher than 96% of the reference samples of concrete [12; 19]. Apart from that, dry sludge can be used as binder or filler for the production of concrete mixture, following a study on the use of sediment from water management tanks [20]. Because this material has natural fineness, it can be used as the product replacing very commonly used limestone-cement fillers. This helps to preserve natural resources, save energy (crushing and grinding is a very energy-intensive process), and recycle waste [21].