polymers Article Selected Properties of Cement Bound Spruce and Larch Bark Bio-Aggregates Johannes Urstöger 1 , Marius Cătălin Barbu 1,2 , Thomas Pacher 1 , Alexander Petutschnigg 1,3 , Johannes Jorda 1 and Eugenia Mariana Tudor 1,2, *   Citation: Urstöger, J.; Barbu, M.C.; Pacher, T.; Petutschnigg, A.; Jorda, J.; Tudor, E.M. Selected Properties of Cement Bound Spruce and Larch Bark Bio-Aggregates. Polymers 2021, 13, 4438. https://doi.org/10.3390/ polym13244438 Academic Editor: Nektaria-Marianthi Barkoula Received: 22 November 2021 Accepted: 13 December 2021 Published: 17 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Forest Products Technology and Timber Construction Department, Salzburg University of Applied Sciences, Markt 136 a, 5431 Kuchl, Austria; jurstoeger.htw-m2021@fh-salzburg.ac.at (J.U.); cmbarbu@unitbv.ro (M.C.B.); tpacher.htw-m2021@fh-salzburg.ac.at (T.P.); alexander.petutschnigg@fh-salzburg.ac.at (A.P.); johannes.jorda@fh-salzburg.ac.at (J.J.) 2 Faculty of Furniture Design and Wood Engineering, Transylvania University of Brasov, B-dul. Eroilor nr. 29, 500036 Brasov, Romania 3 Institute of Wood Technology and Renewable Materials, University of Natural Resources and Life Sciences (BOKU), Konrad Lorenz-Straße 24, 3340 Tulln, Austria * Correspondence: eugenia.tudor@fh-salzburg.ac.at Abstract: The aim of this study is to investigate the suitability of spruce and larch bark for the production of cement-bonded composites. At the beginning of this research, the curing behaviour of the admixtures was quantified with temperature profiles when testing spruce, larch, pine and poplar bark, to determine the compatibility between the components of the bio-aggregates, to analyse the cement curing and to establish which bark species should be successfully included in cement bonded composites. Considering the results, it was observed that the average densities of 600–700 kg/m 3 of bio-aggregates are 40–55% lower than that of established products on the market, although spruce and larch bark are in a similar range. The situation is different for the compressive strength, as larch bark showed up to 30% higher values than spruce bark. This study revealed also different hardening characteristics of the two cement types used as binders for spruce and larch bark. The results of this study demonstrated that tree bark of Picea abies and Larix decidua Mill. can be successfully utilized for the production of a cement-bonded composite material. Keywords: tree bark; spruce; larch; cement-bonded bark-based composite materials 1. Introduction Using lignocellulosic resources combined with cement as raw materials for construc- tion is an interesting topic of research. Wood-cement composites (WCC) or wood chip concrete consist of cement, water and wood chips. WCC are used for the production of water- and fire-retardant as well as sound-absorbing materials for exterior construction applications. These are included in the group of wood-cement panels (WCP) made from wood chips or particles of ligno-cellulosic residues or fibres [1]. Cement-bonded wood- based materials have been manufactured industrially for decades and include products such as particleboard, fibreboard, wood wool panels, as well as sound insulation elements and hollow blocks for primary use in construction [1]. WCC are characterized by high weather resistance, protection against insect and fungal attack, fire resistance and sound- absorbing properties [2–4]. These composites have no formaldehyde emissions due to the inorganic binder and can be manufactured with a certain amount of recycled mate- rial [5]. Due to its exothermic reaction behaviour, the setting of cement can be monitored by measuring the hydration temperature. The curves consist normally of three phases: (a) initial temperature rise, (b) dormant period (temperature remains constant or slight decreases) and (c) cement hardening [6]. WCC have been used since the beginning of the 20th century, mostly as sawdust concrete and wood wool cement bonded panels [1,7–9]. Polymers 2021, 13, 4438. https://doi.org/10.3390/polym13244438 https://www.mdpi.com/journal/polymers