Co-Doped Magnesium Oxychloride Composites with Unique Flexural Strength for Construction Use

In this study, the combined effect of graphene oxide (GO) and oxidized multi-walled carbon nanotubes (OMWCNTs) on material properties of the magnesium oxychloride (MOC) phase 5 was analyzed. The selected carbon-based nanoadditives were used in small content in order to obtain higher values of mechanical parameters and higher water resistance while maintaining acceptable price of the final composites. Two sets of samples containing either 0.1 wt. % or 0.2 wt. % of both nanoadditives were prepared, in addition to a set of reference samples without additives. Samples were characterized by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, and energy dispersive spectroscopy, which were used to obtain the basic information on the phase and chemical composition, as well as the microstructure and morphology. Basic macro- and micro-structural parameters were studied in order to determine the effect of the nanoadditives on the open porosity, bulk and specific density. In addition, the mechanical, hygric and thermal parameters of the prepared nano-doped composites were acquired and compared to the reference sample. An enhancement of all the mentioned types of parameters was observed. This can be assigned to the drop in porosity when GO and OMWCNTs were used. This research shows a pathway of increasing the water resistance of MOC-based composites, which is an important step in the development of the new generation of construction materials.


Introduction
The recent advances in the research of the magnesium oxychloride cement (MOC) [1,2] show a promising direction in the field of alternative construction materials. As nonhydraulic binder prepared from light-burnt magnesia, MOC is usually considered as an environmentally sustainable and energy efficient construction material [3]. The material itself provides high values of compressive and flexural strength [4,5]; it is fireproof [6] and it has very short setting time [7]. Its ability to bond a wide range of additives makes it possible to enhance its properties by using various fillers and to create materials with very specific mechanical, physical, chemical and functional properties, which are applicable in many branches of the construction industry. It is possible to use great volumes of secondary fillers, such as diatomite [8], fly ash [9,10], glass powder [11], wood aggregates [12,13], lunar regolith simulant [14] or others [15], in order to create an eco-friendly material, while maintaining the high mechanical strength and stiffness.
The production of MOC is quite simple and depending on the molar ratio of the raw materials and the conditions of the synthetic procedure, such as temperature, a variety of compounds in the system MgO-MgCl 2 -H 2 O can be synthesized [16,17]. The most common and used phase of this system is MOC phase 5 (also known as , whose formula is 5Mg(OH) 2 ·MgCl 2 ·H 2 O. This phase crystallizes in the form of needle-shaped whiskers [18]. The interlocking of these crystals is one of the possible causes of the high strength of the binder [19]. The properties of MOC are also influenced by its ability to absorb CO 2 from the atmosphere [20,21] which results in the formation of magnesium carbonate phases on the surface of MOC.
The main issue of MOC is its low water resistance [22,23]. The decomposition of MOC in the water environment and the reaction of the present phases with water which forms brucite have been previously described in the literature [24,25]. During this process, the mechanical strength decreases rapidly and leads to leaching of magnesium chloride, which creates a corrosive environment. Therefore, such material is not applicable in combination with steel reinforcement [26,27]; alternative reinforcing materials must be researched [28]. The issue of the low water resistance was previously studied by many researchers and various approaches have been suggested. Li et al. [29] have described the process of using phosphoric acid and soluble phosphates in the amount of 0.5-1.0 wt. % of the MOC phase, resulting in a very significant increase in the water resistance. Similarly, citric acid [30] and tartaric acid [31,32] also showed improvement on the water resistance of MOC. The use of fly ash as an additive for increasing the water resistance was described by Wu et al. [33]; however, the influence of the dosage of the fly ash on the mechanical properties of the prepared composites was negative and quite significant. Another approach was used by He et al. [34], who suggested the use of incinerated sewage sludge ash. The disadvantage of this method is in the varying composition of the ash in different locations and, therefore, different effects on the water resistance. A possible solution to this problem lies in the area of the carbon-based nanomaterials.
The general use of carbon-based nanomaterials in the construction composites is based on the addition of a small amount of the nanomaterial into the matrix while obtaining large enhancement of the material properties. This originates in the extraordinary properties of the C-based nanomaterials, namely their high mechanical strength [35], great electrical and thermal conductivity [36], and outstanding optical properties [37]. Graphene and its derivatives belong to the group of 2D carbon nanomaterials. The functionalization of graphene and formation of its derivative has been thoroughly studied in the past years. It can be modified with sulfur, fluorine, boron, nitrogen and other elements [38][39][40][41][42][43]. The multi-walled carbon nanotubes (MWCNTs) and their derivatives are representatives of the 1D group of carbon nanomaterials. Analogically to graphene, MWCNTs can be modified to create many derivatives with various functional groups [44][45][46][47][48].
The use of graphene and carbon nanotubes in ordinary construction composites is limited by their difficult dispersion in water, which is caused by their hydrophobicity [49]. The hydrophobic nature of these materials can be affected by their functionalization, and therefore the dispersion of such materials in the composite matrix is much easier. The process of functionalization of these nanoadditives is usually based on their reaction with strong acids and formation of their oxidized analogues [50,51]. The oxidized analogues contain hydroxyl (-OH), carboxyl (-COOH) and other groups, which can react with the matrix itself and contribute to the mechanical properties of the final composite [52,53].
The use of carbon-based nanoadditives in MOC as a mechanical and physical propertiesenhancement agent has been previously described in the literature. It has been proven that a very small amount of graphene (G), graphene oxide (GO), multi-walled carbon nanotubes (MWCNTs) or their oxidized analogues (OMWCNTs), can improve the mechanical and physical properties of the MOC matrix [54,55].
Using GO and OMWCNTs as additives in MOC should result in the formation of various types of interactions between their atoms and the magnesium ions in the MOC matrix. This is caused by the reaction between such ions and the oxygen-containing groups of the oxidized nanoadditives [56]. In the previous studies, the interactions between magnesium ions and GO have been divided into two groups: (1) bridging the edges of the sheets through carboxylate chelates to the metal and (2) intercalating between the basal planes through either weak alkoxide or dative bonds from carbonyl and hydroxyl groups; however, it has been shown that the latter type are weak and can be removed by water rinsing. Moreover, as GO contains reactive epoxy groups, its exposure to Lewis acidic divalent metal ions such as Mg 2+ may lead to ring-opening of such groups. During this process, new types of C-O bonds may form [57,58]. The interactions between OMWCNTs and magnesium ions are mainly chemical; however, the electrostatic interactions and sorption-precipitation interactions also occur at a lower rate. The origin of such interaction is caused by the nature of the oxygen-containing groups, which induce negative charge on the OMWCNT's surface, resulting in donation of the single pair of electrons from the oxygen to the magnesium ions [59].
In this study, a combined influence of graphene oxide and OMWCNTs on the properties of MOC matrix was evaluated. The novelty of the paper lies in fact that the effect of co-doping of MOC matrix by a blend of 1-D and 2-D oxidized carbon-based nanoadditives has not been analyzed yet, although it can bring potential improvement in technical and functional parameters of multi-scale MOC composites for construction use. Based on the required mechanical and material properties, according amounts of both nanoadditives were chosen and used for the preparation of a novel composite based on reactive magnesia. The basic micro-and macro-structural parameters, as well as the mechanical, hygric and thermal properties, were studied in order to obtain a complex insight into the problems of the nano-doping of MOC with oxidized forms of carbon-based nanoadditives.
Micrographs of OMWCNTs and GO can be seen in Figure 1. Elemental composition of both nanoadditives is shown in Table 1. The obtained microstructure of OMWCNTs and Graphene Oxide (GO) is shown in Figure 2.  MgCl 2 ·6H 2 O was dissolved in tap water. GO and OMWCNTs were dosed and dispersed together in the part of the prepared MgCl 2 water solution using a homogenizer UltraTurrax T-18 (IKA, Staufen im Breisgau, Germany). This first step of homogenization took 5 min and was realized at 11,000 rpm. The obtained suspension was transferred to a planetary type mixer (ELE) with the rest of MgCl 2 solution, and the MgO powder was added. The second homogenization in ELE took 5 min. The fresh mixtures were then cast into steel molds with dimensions of 40 mm × 40 mm × 160 mm, unmolded after 24 h, and cured in the air atmosphere laboratory at temperatures of (23 ± 2) • C and relative humidity of (50 ± 5)%. After 27 days, the samples were submitted to the particular tests.
For the fresh mixtures, rheological parameters such as dynamic viscosity η (Pa·s) and shear stress τ (Pa) as functions of velocity gradient were measured in order to document possible effect off MOC co-doping on the changes of its workability. The measurement itself was performed by the use of the rotation viscometer HAAKE Viscometer E (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a rotary spindle R3 type.
X-ray powder diffraction (XRD) was carried out using a Bruker D2 Phaser (Bruker AXS GmbH, Karlsruhe, Germany), a powder diffractometer with Bragg-Brentano geometry, applying CuKα radiation (λ = 0.15418 nm, U = 30 kV, I = 10 mA) and 5 rpm rotation. The step size was set to of 0.02025 • (2θ) and the overall data were acquired in the angular range of 5 • -80 • . To evaluate the obtained data, X'Pert HighScore Plus software (2017, PANalytical, Almelo, The Netherlands) was used.
To study the surface morphology and microstructure of the prepared samples, scanning electron microscopy (SEM) was used (Tescan MAIA3, TESCAN Brno, s.r.o., Brno, Czech Republic).
The elemental composition and mapping were characterized using an energy-dispersive spectroscopy (EDS) analyzer (X-Max150, Oxford Instruments, High Wycombe, UK) with a 20-mm 2 SDD (silicon drift detector) detector and AZtecEnergy software (Oxford instruments, High Wycombe, UK). The sample was set on a carbon conductive tape to ensure the conductivity of the experiments. For both SEM and SEM-EDS analysis, the electron beam was set to 10 kV.
To study the used carbon-based nanomaterials, HR-TEM was performed using an EFTEM Jeol 2200 FS microscope (Jeol, Tokyo, Japan). A 200 keV acceleration voltage was used for the measurement. The sample preparation was attained by drop-casting the suspension (1 mg·mL −1 in isopropyl alcohol) on a TEM grid (Cu; 200 mesh; Formvar/carbon) and then drying in a vacuum dryer at 25 • C and p/p0 = 0.2.
The BET specific surface was measured using a sorption analyzer NOVAtouch LX2 (Quantachrome Instruments, Boynton Beach, Florida, USA). The sample was outgassed for 10 h at 100 • C under high vacuum. A nitrogen-cooled (77 K) detector was used for the evaluation of the results using BET (Brunauer, Emmett and Teller) and Kelvin equations. The Quantachrome software was used for evaluation of measured data and to recalculate the measured value to m 2 per 1 g.
To verify the distribution and presence of applied oxidized nanoadditives and to reveal the main precipitated phases and bonds among the chemical forming the hardened structure of the researched composites, the MIR (mid-infrared) spectra were collected using ATR (attenuated total reflection) technique. The MIR spectra were obtained after 32 scans that were conducted using the FT-IR spectrometer Nicolet 6700 (Thermo Fisher Scientific, Waltham, MA, USA), which operates with spectral resolution 4 cm −1 at the spectral range 4000-400 cm −1 . The analyzed samples were cut from the original prisms, dried in a vacuum drier and then mechanically crushed and homogenized with a ball grinder MM 400 (RETSCH, Haan, Germany). Finally, they were pulverized in an agate mortar.
The tests of the hardened composites were done for 28 days matured samples. The experimental campaign was proposed in such a way to evaluate the co-doping effect of the oxidized nanoadditives on the structural, mechanical, hygric, and thermal parameters on MOC-based matrix. For each composite mixture, at least 5 samples were subjected to the particular tests. To measure the dry bulk density, the casted prisms 40 mm × 40 mm × 160 mm were dried in a vacuum drier at 50 • C until their constant mass was achieved. The dry bulk density ρ b (kg·m −3 ) was then measured as prescribed in the EN 1015-10 [60]. The fragments extracted from the inner part of the crushed prisms were analyzed in a helium pycnometer Pycnomatic ATC (Porotec, Hofheim, Germany) to determine their specific density ρ s (kg·m −3 ). The total open porosity was calculated from the bulk density and specific density values [61]. The composites microstructure was studied on a mercury porosimetry principle using porosimeters of Pascal series, Pascal 140 and Pascal 440 (Thermo Fisher Scientific, Waltham, MA, USA). The pore size was evaluated from the mercury pressure using the Washburn-Laplace equation and the cylindrical capillaries were assumed. The investigated parameters were pore size distribution curves, total pore volume, average and modal pore diameter and pore surface area. The dry samples mass in microstructural analysis was about 2 g. The compressive strength f c (MPa), flexural strength f f (MPa), and the dynamic modulus of elasticity E d (GPa) were the tested mechanical parameters. The mechanical strength tests were performed in compliance with the EN 1015-11 [62]. The flexural strength was measured on the casted prisms in a three-point bending strength test arrangement. On the rest of broken prisms, the compressive strength was assessed. The loaded cross section in the uniaxial strength tests was 40 × 40 mm 2 . The stiffness of MOC composites was characterized by the dynamic modulus of elasticity, which was tested by the Vikasonic apparatus (Schleinbinger Geräte, Buchbach, Germany) operating on a frequency of 54 kHz. The specimens were 40 × 40 × 160 mm prisms and were measured along their longitudinal axis. A capillary absorption test was used to evaluate the water transport and storage in the examined composites and to quantify the effect of nanoadditives usage on the deceleration and retardation of water ingress. The measured parameters were water absorption coefficient A w (kg·m −2 ·s −1/2 ), 24-h water absorption W (kg·m −2 ), and 24-h water absorption W a (wt. %). These were obtained in accordance with the EN 1015-18 [63]. The samples used in the water absorption experiment were prisms with a size of 40 × 40 × 160 mm. As the nano-doping of the MOC matrix affects its porous structure, and thus heat transport and storage, thermal conductivity λ (W·m −1 ·K −1 ), thermal diffusivity a (m 2 ·s −1 ), and volumetric heat capacity c v (J·m −3 ·K −1 ) of the matured samples were tested using a Hot Disk TPS 1500 (Hot Disk AB, Göteborg, Sweden) apparatus equipped with the Kapton-insulated sensor having a radius of 6.4 mm. The temperature of the thermal parameters test was (23 ± 2) • C. Before the measurement, the prismatic samples having a size of 40 × 40 × 50 mm were dried in a vacuum at 50 • C. The used experimental techniques and conducted test are together with the expanded combined uncertainties of the particular investigated parameters summarized in Table 3.

Results and Discussion
We prepared three sets of samples and we compared their properties.  The effect of the application of nanoadditives on the rheology of fresh composites mixtures was studied using a rotation viscometer. The dynamic viscosity (dash lines) and the shear stress plotted in the dependence on the velocity gradient are graphed in Figure 4. As only minimum differences in investigated rheological parameters were recorded, the effect of the incorporation of GO and OMWCNTs can be considered as negligible, which enables one to design and develop multiscale composites with improved functional and technical properties. As used nanoadditives have no adverse effect on the workability of fresh composites, it is possible to incorporate different types of fillers, aggregates and reinforcing fibers in composite mixture that would bring other advanced parameters of the materials on nano-doped MOC basis. To confirm the phase composition of the hardened composites, XRD analysis was used. The diffraction patterns of the three types of samples (see Figure 5) are all similar due to the presence of the two required crystalline phases-MOC Phase 5 (ICDD 04-014-8836) with the main reflection at 2θ = 11.9 • and MgO (ICDD 04-007-5693) with its characteristic reflection at 2θ = 40.9 • , which works as a filler in the samples. The carbon-based nanomaterials used as dopants are not visible in the diffraction patterns due to their low content. The SEM was used in order to study the morphology of the prepared set of samples. The micrographs show in Figure 6 show a compact structure of the samples without cracks. For the sample MOC-REF, the typical needle-shaped crystals were visible, especially in the area of air-bubbles which formed in the suspension during mixing. The presence of needle-shaped crystals in the samples containing GO and OMWCNTs was also confirmed; however, these crystals had less distinct shape with blurred edges and, overall, were shorter and wider than the ones of the reference sample.     The basic structural parameters of 28-days samples are summarized in Table 5. The mean values from the measurement of 5 samples of each examined composites are presented together with their expanded combined uncertainties. The co-doping of the control matrix by GO and OMWCNTs resulted in the increased bulk density and specific density; this densification led to the significant drop in porosity, which was approx. 15.8% for MOC-GO-OMWCNT-0.1 and 28.9% for MOC-GO-OMWCNT-0.2, respectively. Quantitatively, the researched composites exhibited really low porosity; thus, their high mechanical strength and durability may be tanticipated. In Table 6, the micro-structural parameters obtained by the mercury intrusion porosimetry (MIP) analysis are presented. The cumulative and incremental pore size distribution curves are displayed in Figures 9 and 10. MIP microstructural data gives evidence of the densifying and solidification effect of co-doping of MOC matrix by C-based nanoadditives. For all examined materials, the shape of the cumulative pore size distribution curves was almost similar. In the case of the incremental pore size distribution, the differences in pore size were more distinctive, especially in the pore diameter range from 0.015 µm to 0.084 µm. These pores can be classified according to the IUPAC terminology [68] as macro-pores (>50 nm) and meso-pores (2-50 nm). Based on the pore size classification established in cement research, these pores are responsible for mechanical strength, permeability and, thus, frost resistance [69]. In this respect, one can simply estimate improvement in the mechanical performance and reduced water ingress by the co-doping of the MOC matrix.   The accessed mechanical parameters are presented in Figure 11. All materials exhibited high strength and stiffness that corresponded well with the nature of the MOC matrix [70,71] and its low porosity. It is evident that the pore structure had a great impact on material's physical properties; thus, the lower the porosity and the smaller the pore number and the radii are the better the mechanical performance of the composite will be. As the compressive strength was affected by the nanoadditives to a low extent only, the combination of a high strength of graphene oxide [72,73], the bridging and interlocking effect of OMWCNTs [74], and the micro-pores filling resulted in a great improvement of flexural strength and toughening of the co-doped composites. The increase in the flexural strength was approx. 55.3% for MOC-GO-OMWCNTs-0.1 and 92.1% for MOC-GO-OMWCNT-0.2. In this respect, one must consider that only low dosage of nano-dopants was used, and further enhancement in the mechanical performance can be anticipated in the case of a higher content of nanoadditives incorporated in the composite mixture. The water transport and storage properties obtained for the hardened composites are given in Table 7. In the comparison with the capillary porous materials [75][76][77], the water absorption coefficient and 24-h water absorption values were much lower; they were further reduced by the application of the nanoadditives. For example, the drop in A w was 24% for MOC-GO-OMWCNT-0.1 and~28% for MOC-GO-OMWCNT-0.2. Accordingly, 24-water absorption was reduced by 25% and 39%, respectively. Similarly, as in the case of mechanical parameters, the reduction in water ingress was assigned to the drop in the volume of pores accessible for water, total reduced pore number and an average pore size. The limited wettability of the nano-doped composites gives presumption of their durability in terms of moisture-induced damage and allows one to classify the applied carbon-based dopants as water repellant admixtures for the MOC matrix. In combination with inorganic fillers, e.g., on active SiO 2 basis, advanced, multifunctional MOC composites can be further designed and developed, meeting the technical, functional and durability criteria of the construction materials intended for specific application purposes. Table 7. Hygric parameters of 28 days matured composites.

24-h Water Absorption
W a (wt.%) The thermal parameters measured on the transient hot disk technique principle are introduced in Table 8. Quantitatively, the values of the thermal conductivity were high, typical for dense inorganic materials, such as low porous concrete [78]. High thermal conductivity of MOC materials has already been reported in our previous studies, see, e.g., [79]. The assessed thermal properties represent results of two parallel effects: (i) dropped porosity and, thus, increased density that enhances itself for heat transport and storage, (ii) unique properties of carbon-based nanoadditives that, in respect to their morphology, size and defects [80], exhibit high thermal conductivity and heat capacity [81,82]; they are temperature dependent and show changes in dimensionality [83]. Although it is impossible to distinguish which effect was the dominant in enhancement of the analyzed thermal parameters, co-doping nanoadditives have served as controlling agents of heat transport and storage, respectively. This might find application in MOC composites with low heat conductive phase change materials that could be effectively heated by the bridging effect of OMWCNTs and dispersed GO particles.

Conclusions
In this research, the combined effect of oxidized 1D and 2D carbon nanoadditives on the MOC Phase 5 was studied. The following advanced findings of the analysis of the 28-days hardened samples were highlighted: (i). the micro-and macro-structural parameters analyses showed the base of the enhancement caused by the carbon-based nanoadditives that significantly decreased the porosity, which was reduced by~15.8% for MOC-GO-OMWCNT-0.1 and~28.9% for MOC-GO-OMWCNT-0.2; (ii). the great drop of porosity led to the increase in the mechanical parameters, where the flexural strength values increased by 55.3% and 92.1% for the samples containing 0.1 and 0.2 wt. % of each nanoadditive, respectively; (iii). as the volume of pores decreased due to their filling and interlocking with the nanoadditives, the water transport and storage properties were significantly lowered; thus, the improved water resistance of the MOC matrix can be anticipated-this effect of the nanodopants is crucial for the MOC-based composites, which are usually considered as materials with very poor water resistance; (iv). the materials thermal conductivity was quite high due to the decreased porosity, but also as a result of adapting the thermal conductivities of the used GO and OMWCNTs, which are generally very high.
Based on the summarized results it was concluded, the used nanoadditives have significant potential in the field of design and the development of advanced and multifunctional MOC-based composites. However, the homogenization and ideal dispersion of the nanodopants must be improved in order to get even greater enhancement of the composite properties.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.