**1. Introduction**

Progressive urbanisation is indispensable in any developing society. With it, sewage treatment is becoming an increasing problem. The result of treatment is a large quantity of sewage sludge (SS), the amount of which corresponds only 1–3% of the volume of flowing sewage. However, with increasing urbanisation and economic progress, the amount of generated waste is starting to become a serious problem. EU regulations require us to dispose of this waste properly, due to its specific properties. This type of waste contains heavy metals and pathogenic organisms, which can have a highly negative impact on the environment and human health. In order to protect the environment and individuals, and to minimise the volume of sludge generated, it is subjected to thermal treatment. However, the ash produced by the thermal disposal of sludge (SSA) still contains heavy metals. The use of this ash and other wastes in cement materials seems to make the most sense due to its high immobilisation properties [1–6]. The idea of this type of utilisation of sludge seems to be highly appropriate, since, as the research of the authors of the study [7] demonstrates, these composites have the ability to almost completely immobilise heavy metals. Heavy metals content in ash depends primarily on the urbanisation and industrialisation of the area. The amount of fly ash resulting from the thermal disposal of sewage sludge and contaminated with heavy metals is a major problem in many developing countries.

Literature reports on the physical and mechanical properties of hardened cement composites containing SSA are quite widely described [8–12], as well as in our previous

**Citation:** Adamczyk, M.; Zdeb, T.; Tracz, T. Physico-Chemical Properties of Sewage Sludge Ash and Its Influence on the Chemical Shrinkage of Cement Pastes. *Mater. Proc.* **2023**, *13*, 26. https://doi.org/10.3390/ materproc2023013026

Academic Editors: Katarzyna Mróz and Izabela Hager

Published: 15 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Faculty of Civil Engineering, Cracow University of Technology, ul. Warszawska 24, 31-155 Cracow, Poland **\*** Correspondence: marcin.adamczyk@pk.edu.pl

studies [13], while the number of publications on their durability is limited [13,14]. So far, it is difficult to find publications devoted to the problems of chemical and autogenous shrinkage of cement binders containing ash formed after thermal disposal of sewage sludge.

The reaction of cement with water reduces the volume of the mixture. This reduction can be calculated from the density of substrates and products of cement hydration. Volume changes of this nature are called chemical shrinkage or contraction, and they involve a smaller volume of water in the hydrated phases than its volume in the liquid phase. Contraction depends on the mineral composition of the cement. The C3A phase shows the greatest contraction, while the C2S phase shows the least [15]. Yodsudjai et al. [16] demonstrated that varying levels of C3A contained in different types of cement play a major role in the behaviour of cement paste during chemical shrinkage. For this reason, replacing cement with SSA, and thus reducing C3A, may result in a reduction of chemical shrinkage. In addition, due to the high content of hematite in the ash in question, its hydration can also affect the volume of the reaction products of the blended cement paste.
