The discovery of the Magnesium oxychloride cement (MOC) was made more than 150 years ago, in 1867, by a French civil engineer, inventor and chemist named Stanislas Sorel. From the name of the inventor, the MOC cement is also called Sorel cement [11]^[1]. MOC belongs to a special type of cement, characterized by rapid setting, strong binding ability and high strength in normal air condition [12]^[2]. The main components of MOC are caustic burned magnesia powder and magnesium chloride hexahydrate (MgCl₂_6H₂O) mixed under ambient temperature and pressure [13]^[3]. The main stable oxychlorides at ambient temperature are the so-called “phase 3” and “phase 5”, whose formulas can be written as 3Mg(OH)₂·MgCl₂·8H₂O and 5Mg(OH)₂·MgCl₂·8H₂O, respectively, or, equivalently, Mg₂(OH)₃Cl₄H₂O and Mg₃(OH)₅Cl 4H₂O [14]^[4].

MOC cement attracted much attention due to the fact that, in many ways, it exhibits properties superior to standard Portland cement, and among the low-carbon cement materials, it exhibits considerable potential in replacing Portland cement [15]^[5]. The main advantages that make the MOC cement superior to ordinary cement are referring to lower CO₂ emission, short setting time, the lack of humid curing, low alkalinity and excellent mechanical strength [16]^[6]. Because the phase composition could be regulated through adjusting the molar ratio of precursor [17,18]^[7][8], MOC cement provides a series of promising cementitious substrates [19]^[9]. Furthermore, it exhibits rheological properties that enable the material to flow into irregular cavities [20]^[10]. It also exhibits comparatively high flexural strength (≥4 MPa), compressive strength (≥69 MPa) and elastic modulus (70–85 GPa) [21]^[11].

2. Perspective of Using Magnesium Oxychloride Cement (MOC) and Wood as a Composite Building Material

In the effort of reducing the CO₂ emissions generated by the construction industry, solutions that can decrease the use of concrete and better use of wood products are welcome. Wood is a sustainable building material with many advantages and multiple-use possibilities. Wood is an inhomogeneous material (anisotropic) made up of a large number of plant cells organized into specialized tissues, also called anatomical elements, which are very diverse [66]^[12]. They differ in their lifetime and tree functions, shape and size, position in the tree and quantity or number. Many of the cells die during the life of the tree, retaining only the role of ensuring the mechanical strength of the tissues in its composition. The way in which the anatomical elements observable with the naked eye are grouped is called the macroscopic structure [5]^[13]. A big challenge of the building industry is to better manage the waste resulting from the production of building materials in various building stages. In [67]^[14], it was shown that “proper utilization of sawdust in concrete will conserve the environment by reducing the use of natural resources, reducing the volume of waste material, and reducing CO₂ emissions”. Waste can be incorporated into the concrete mixture in the form of flying ash. The studies presented in [68]^[15] reveal that “the incorporation of fly ash can enhance the workability or fluidity, retard the setting time, and improve the water resistance of the MOC mortars”. Other subject addresses refer to its rheological properties [69]^[16], the effect of increasing the resistance of the MOC cement composites under the water attack [70]^[17], the influence on the compressive strength [46]^[18], the improvement of the water resistance [71]^[19] or the anticorrosion effect for the reinforcement [72]^[20]. Several other researchers identified the advantages and possibility of combining wood and concrete. In the paper of [73]^[21], the researchers presented a new material, Wood-Crete, indicating the possibility of using wood waste and concrete products for in-fills for wall panels and hollow blocks or for thermal insulating material. The good properties of the wood wool–cement composite material are highlighted by [74]^[22], which shows that “the material presents excellent mechanical, chemical and biological properties. However, the understanding of its mechanical behavior is rather limited”. The incompatibility between wood and cement is analyzed by [6]^[23], and it is proven that “the alkaline hydrolysis was found as the most effective treatment for the suppression of inhibitory substances and the highest decrease in mechanical properties of resulting composites”. There is obviously a need to find a proper balance in the wood-and-cement composition, which is quite difficult to determine. Wood–cement composites are now being investigated and made industrially in many countries in the world, mostly in the form of panels because of their excellent exterior properties [75]^[24]. The main difficulty for wood–cement composites manufacturing is the chemical incompatibility between wood and cement; in most cases it is the ordinary Portland cement that inhibits cement setting and hardening [7]^[25]. The inhibitory substances mainly include some sugars, part of hemicelluloses and their degradation products. The inhibitory degree is affected by many factors, including wood species, location, part of the tree, season during wood-cutting, wood/cement ratio, type of cement, storage condition and other factors [76]^[26]. Studies have shown that wood-MOC composites prepared with a higher wood fiber content had a lower thermal conductivity, higher bending resistance, higher residual bending resistance after exposure to high temperatures and water immersion and a better noise reduction effect. Even if the water absorption increased with an increase in wood fiber content, it could still be considered low [48]^[27]. The proprieties of the MOC cement make it suitable to be used in the composition of building elements with good sound absorption levels [76]^[26]. Other identified solutions propose the use of MOC cement and wood to produce lightweight composite building materials [8]^[28] through the process of extrusion. At the same time, MOC cement can be used with great results as an alternative to the adhesive solutions used in wood building materials. Several studies are addressing this issue and proposed different solutions, as in [75]^[24], where it is proposed to use MOC cement inorganic adhesives are presented as an effective and sustainable binder for plywood applications. The study [24]^[29] focused on the preparation of an eco-friendly and high-performance MOC-based formaldehyde-free adhesive based on an organic–inorganic hybrid structure, showing that MOC cement deposited on the fibers’ surface was connected by the hydrogen bonds in wood, which improved the mechanical properties of wood-based composites. In addition, the use of MOC improved the flame retardancy of the plywood. By using MOC cement in the composition of wood panels, studies showed a decrease in greenhouse gas emissions. In [7]^[25], it was shown that “greenhouse gas emissions associated with wood-MOC board incorporating incinerated sewage sludge ash was 71% lower compared to plywood production, and comparable to the resin-based particleboard. Moreover, the human toxicity of the wood-MOC board was 58% lower compared to the conventional resin-based particleboard production due to the latter uses a large amount of highly toxic organic resin”. Other approaches in using MOC cement combined with wood can be identified in [8]^[28], where it is shown that “the specific dry densities of the wood–MOC cement composites were close to 1.0 and they were nailable like hard natural wood. Their flexural strength decreased as temperature increased. By replacing 50% sawdust in weight by perlite, the composite exhibited less die swell and better performance in resisting high temperature”. The research results indicate that the field of magnesium oxychloride cement has been presenting more and more interest in the last years. The annual evolution of the number of published articles indicates a big increase in the last five years. An interesting aspect is a fact that in the last five years, from 2017 to 2021, 109 articles were published, three times more than the total number of 33 articles published before 2017. Considering that in the last years the awareness of the impact of human activities on the environment is more and more felt and that world leaders enroll in making efforts to reduce the negative impact and reduce the global warming phenomenon, sets the premises of a future increase in the interest in more eco-friendly solutions to build. The increased interest in MOC cement as building materials led to the dissemination of the results in various journals. From this point of view, the journals with the biggest number of publications are Constructions and Building Materials and Materials. In the co-citations analysis, besides the journal Construction and Building Materials, which is the biggest on the map, journals such as Concrete and Cement Research and Concrete & Cement Composites are other journals highlighted as being more influential in the research field. An interesting aspect revealed in this study is that more than 65% of the total scientific production is produced by researchers from China. In the second place from the number of papers, but far away, are the researchers from the Czech Republic. From the main subjects addressed in the field of MOC cement, the trend topic analysis revealed that magnesium oxychloride cement is only a keyword that was most frequently used in the year 2020 and that the first keyword mostly used was cement in 2014. In the last year, other words are becoming more and more used alongside the MOC cement, such as “composite” or “graphene”. Both of the two directions present a big interest and a big potential to generate new building materials that will decrease the negative impact of the construction industry on the environment.