Study on Preparation and Properties of Intrinsic Super-Hydrophobic Foamed Magnesium Oxychloride Cement Material

applied sciences

Article Study on Preparation and Properties of Intrinsic Super-Hydrophobic Foamed Magnesium Oxychloride Cement Material

Jiaxin Huang, Shaojin Ge, Hongning Wang and Ruoyu Chen *

College of Petrochemical engineering, Changzhou University, Changzhou 213164, China; [email protected] (J.H.); [email protected] (S.G.); [email protected] (H.W.) * Correspondence: [email protected]; Tel.: +86-519-8633-0580

Received: 27 October 2020; Accepted: 16 November 2020; Published: 17 November 2020

Abstract: As we all know, magnesium oxychloride foamed cement material has poor water resistance, leading to a decline in application value. In our research, tetraethylorthosilicate (TEOS) and triethoxy-1H, 1H, 2H, 2H-tridecylfluoro-n-octylsilane (FAS) were pre-cohydrolyzed to prepare the overall super hydrophobic magnesium oxychloride cement (MOC) foamed material, and its structure and performances were systematically studied. The results show that adding organosilane can make it have overall hydrophobicity under the premise of maintaining the compressive strength. Mechanical abrading and chemical corrosion tests show its good engineering durability. The maximum moisture absorption rate dropped by 16.2%, and the quality can be restored to 98.1% of the original quality after dehumidification. All these properties show that the hydrophobic foamed magnesium oxychloride cement has potential engineering application value.

Keywords: magnesium oxychloride cement; foamed material; overall super hydrophobic; engineering durability; moisture absorption and dehumidiﬁcation

1. Introduction Magnesium oxychloride cement (MOC) is a gas-hardening gel material made of activated magnesium oxide powder and a certain concentration of magnesium chloride solution [1]. It has a short setting time, low alkalinity, fire resistance, heat resistance, and other properties [2]; after 28 days of curing, the compressive strength can reach 80–100 KPa and even 200 KPa after modification [3]. The above properties indicate that magnesium oxychloride cement is particularly suitable for foamed cement substrate and has application potential in light insulation board, composite wall panels, and other fields. The pore structure is an important property of foam materials. It also determines bulk density, heat preservation, heat insulation, and sound insulation performance [4]. On the other hand, its compressive strength is much lower than that of dense bulk materials [5]. In the actual use process, the continuous pore structure of the porous material leads to its extremely high water absorption, which further reduces the insulation effect, especially in moist and rainy areas. In addition, the water absorbed by the pore structures causes secondary damage such as a freezing fracture and mold propagation [6]. The setting time of ordinary Portland foam cement is long, the pore structure is unstable, and the volume water absorption rate increased rapidly after 72 h of blistering [7]. Magnesium oxychloride foam cement is made of magnesium oxychloride gel material as a matrix, thus it also inherits the disadvantage of poor water resistance of magnesium oxychloride cement. The main strength phase of magnesium oxychloride cement is the 5Mg(OH) MgCl 8H O (five phase), which easily changes from a needle-like 2· 2· 2 network structure to a loosely packed layered structure after blistering [8–10]. Phosphoric acid and other additives can significantly improve the water-resistance of magnesium oxychloride cement [11].

Appl. Sci. 2020, 10, 8134 ; doi:10.3390/app10228134 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 8134 2 of 15

However, the natural hydrophilicity of inorganic materials still cannot prevent water vapor from adhering to the inside and the outside of the porous material and gradually eroding its gel structure. It has been found that, by controlling the pore structure, minimizing the connection between pores can significantly reduce the capillary water absorption of foamed cement [12]. Due to the high dopability of magnesium oxychloride cement, the addition of graphene and graphene oxide [13], Magnesium Oxychloride 5Mg(OH) MgCl 8H O[14], granulated Scrap tires andexpanded glass [15] can also increase 2· 2· 2 its water resistance, but this is limited. The introduction of hydrophobic materials into foam materials can significantly improve the water absorption of foam cement. In the research of conventional silicate and sulfoaluminate foam materials, by dipping polystyrene into the material surface [16], silane-modified graphene oxide (GO) coating [17], organosilane coating [18], etc., can significantly reduce water absorption. The advantage of the surface dipping method is that the method is simple and does not affect the hydration reaction process of the cement material, but because the surface hydrophobic layer is only a few hundred nanometers, the hydrophobic layer wears easily. Therefore, improving the moisture resistance of foamed materials has become a key issue in the field of thermal insulation building materials. If the porous material is given overall hydrophobic properties, it can avoid the reduction of hydrophobicity due to surface wear and long-term use. Adding organosilane to the cement system and using the hydrophobicity of the organosilane hydrolysate can achieve the overall hydrophobicity of Portland cement materials [19], but the hydration reaction of MOC materials is quite diﬀerent from that of Portland cement. According to our knowledge, there is no literature report on the hydrophobicity of MOC materials. In this study, we used H2O2 as a blowing agent and introduced triethoxy-1H, 1H, 2H, 2H-tridecylﬂuoro-n-octylsilane (FAS) and tetraethyl orthosilicate (TEOS) into magnesium chloride solution. The co-hydrolyzed product was used as a hydrophobic agent and mixed with activated magnesium oxide to prepare an overall hydrophobic magnesium oxychloride cement foaming material. Finally, we systematically studied phase composition, pore size distribution, overall hydrophobicity, engineering durability, moisture resistance, dehydration performance, and other properties.

2. Materials and Methods

2.1. Experimental Materials The three raw materials of magnesium oxychloride cement are magnesite powder, magnesium chloride, and water. The lightly burnt magnesite powder is produced in Haicheng City, Liaoning Province, and its chemical composition is shown in Table1. The mass percentage of activated magnesium oxide (α-MgO) is 63%, and the average particle size is 75 µm. Magnesium chloride hexahydrate (MgCl 6H O 2· 2 purity = 98 wt %) and triethoxy-1H, 1H, 2H, 2H-tridecyl fluoride-n-octyl silane (FAS) are analytically pure reagents sourced from Aladdin Chemistry Test Agent Co., Ltd. Tetraethyl orthosilicate (TEOS), phosphoric acid (H3PO4), and 30% hydrogen peroxide (H2O2) are analytically pure reagents sourced from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China.

Table 1. The chemical compositions of the magnesia powder.

Components MgO SiO2 CaO Fe2O3 Al2O3 Loss on Ignition Mass fraction: % 84.27 6.25 1.68 0.28 0.57 6.95

2.2. Preparation of Foamed Hydrophobic Magnesium Oxychloride Cement Test Block In this study, the initial raw material ratio of magnesium oxychloride cement was determined to be MgO:MgCl2:H2O = 6.5:1:13 (molar ratio) [20], and MgO was calculated as α-MgO. Firstly, we pre-hydrolyzed the organosilane in magnesium chloride solution, mixed magnesium chloride hexahydrate and distilled water at a molar ratio of MgCl2:H2O = 1:13, then added H2O2, FAS, and TEOS; the amount of H2O2 added was 4% to 12% of the mass of the magnesium oxide powder. Then, FAS and TEOS were added, and the amounts of FAS were 0.5%, 1%, 1.5%, 2%, and 2.5% of the mass of magnesium Appl. Sci. 2020, 10, 8134 3 of 15 oxide powder. TEOS was added according to the molar ratio of FAS and TEOS with n: m (n = 0.5, 1, 1.5, 2, 2.5, m = 6, 8, 10, 12, 14). The mixed solution was stirred at 60 ◦C for 8 h to obtain the treated MgCl2 solution. The H3PO4 was then mixed with the treated MgCl2 solution. The amount of H3PO4 added was 1% of the total MgO. This was mixed with magnesium oxide powder and stirred well to form magnesium oxychloride cement slurry. The slurry was cast in steel mold of 20 20 20 mm and 40 40 160 mm, × × × × vibrated, compacted, and cured at room temperature for 1 d. It was then demolded and air-cured. After curing for 1–28 d, the hydrophobic foamed magnesium oxychloride cement represented by F-MOC was ﬁnally obtained. N-MOC means that the blank control foamed magnesium oxychloride cement was added with H2O2, but FAS and TEOS were not added.

2.3. Characterization and Test Methods of Samples

2.3.1. Characterization of Pore Structure The pore structure of porous materials is an important parameter to understand the performance of porous materials. At present, the methods for characterizing the pore structure of foam cement include optical method [21], mercury intrusion method [22], soaking medium method [23], etc. In this study, the optical image method was used to analyze the pore size distribution of foamed cement. Firstly, we selected the flat surface of the F-MOC and the N-MOC samples and then used a high-power magnifying glass to capture the surface morphology of the sample to obtain a photo. We then captured 9 regional planes (A1, A2, A3, A4, A5, A6, A7, A8, A9) with an area of 0.2 0.2 cm on the flat surface. × We calculated the diameter of the holes in the 9 areas and classified the pore size interval (each 70 µm was classified as an interval). Finally, the pore size distribution of the entire sample surface was estimated based on the pore size of the statistical area.

2.3.2. Bulk Density Test According to the JG/T 2,662,011 “Foam Concrete” standard, we first used an electric heating constant temperature drying oven to dry the samples at 60 ◦C. When the samples were measured at intervals of 4 h, the quality difference did not exceed 0.01 g. We then turned off the electric heating constant temperature drying oven, cooled the test piece to room temperature, and weighed it. The mass was accurately measured to 0.01 g, and the bulk density was calculated according to Formula (1).

ρ = M/V 103 (1) × where ρ is the bulk density in kilograms per cubic meter (kg m 3), accurate to 0.01 g; M is the weight · − of the sample under dry conditions (g); V is the volume of the sample (cm3).

2.3.3. Compressive Strength Test According to JC/T 1062-2007 “Standard Test Method for Mechanical Properties of Foamed Concrete Blocks”, the strength was measured after 28 days using a sample with a size of 100 100 100 mm. × × Six specimens of each group were measured, and the average value was calculated.

2.3.4. X-Ray Diffractometer (XRD) Test The crystalline phases of F-MOC and N-MOC samples were identified by a D/max-2500/PC X-ray diffractometer (XRD, CuKα, λ = 0.154 mm, tube voltage of 40 kV, tube current of 100 mA, made in Japan). Scanning at a speed of 3◦/min and a scanning angle of 10 to 65◦, a powder sample was prepared by crushing the cuboid specimens and passing the powder through a sieve with a 75 µm mesh. The XRD data were quantitatively analyzed, and the relative contents of the five-phase in F-MOC and N-MOC were calculated. Appl. Sci. 2020, 10, 8134 4 of 15

2.3.5. X-Ray Photoelectron Spectroscopy (XPS) Test Thermo ESCALAB 250XI electronic spectrometer was used for high-resolution X-ray photoelectron spectroscopy (XPS) testing of F-MOC and N-MOC sample powders. Acquisition parameters were as follows. total acquisition time: 1 min 8.0 s; number of scans: 1; analyzer mode: CAE: pass energy 150.0 eV; energy step size:1.000 eV; number of energy steps:1361.

2.3.6. Fourier Infrared Spectrometer (FTIR) Test American Nicolet-460 Fourier transform infrared spectrometer was used for testing. The experimental method was as follows: the samples were processed by KBr tableting method. The formed samples were dried under an infrared drying lamp to remove the absorbed water and then placed in a Fourier 1 transform infrared spectrometer for testing. The test wavelength range was 400–4000 cm− followed by infrared analysis.

2.3.7. Water Contact Angle, Mechanical Abrading, and Chemical Corrosion Test The test method for measuring the water contact angle of the sample was to select a foamed cement sample with a flat surface and then use a contact angle measuring instrument that met the HARKE-SPCA standard to measure. The F-MOC was polished by a grinding machine with a grinding thickness of 0–10 mm, and the water contact angle was measured once for each 1 mm of grinding. The F-MOC sample was soaked in solutions of 0.1 mol/L NaCl, 0.1 mol/L NaOH, and 0.1 mol/L HCl for three days, then taken out and dried in an electric heating constant temperature drying oven. Then, the water contact angle was measured again.

2.3.8. Moisture Resistance and Dehydration Performance Test First, we put F-MOC and N-MOC in an electric heating constant temperature drying oven to dry to constant weight. Then, the samples were put in a constant temperature and humidity desiccator with a temperature of 30 ◦C and a relative humidity of 100% to absorb moisture. The samples were taken out after 120 h, moisture was wiped oﬀ the surface with a damp cloth, and then sample masses were immediately weighed to the nearest 0.0001 g. We calculated the moisture absorption rate of the sample mass according to Formula (2) and took the average value.

m m ω = − 0 100% (2) m0 × where ω is the mass moisture absorption rate (%), m0 is the mass of the sample after drying (g), and m is the mass of the sample after wiping oﬀ the moisture (g). Then, thesamplewasdriedinanovenat30 ◦Ctoremovethewatervapordistributedinthepores, andits mass was taken out and weighed every 20 min to monitor the evaporation of water in the whole process. A total of 120 min was monitored, and mass dehumidification rate was calculated according to Formula (3). F-MOC and N-MOC samples with different bulk densities were selected for parallel experiments.

mn mn+1 ω1 = − 100% (3) mn+1 × where ω1 is the mass dehumidification rate (%), mn is the mass when weighed after the nth dehumidification (g), and mn is the mass when weighed after the (n + 1)th dehumidification (g) (n 0). +1 ≥ Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 15 dehumidification (g), and mn+1 is the mass when weighed after the (n + 1)th dehumidification (g) (n ≥ 0).

3. Results and Discussion

3.1. The Bulk Density and Pore Structure of Foam Material

From the bulk density of F-MOC and N-MOC obtained with different H2O2 content in Figure 1, it can be seen that the bulk density of F-MOC obtained with the same H2O2 content was greater than that of N-MOC. The standard deviation of the bulk density of F-MOC and N-MOC shows that, although the conditions for preparing the samples were the same, the bulk density of the samples was different, and the stability was poor when only H2O2 was used as the foaming agent. After adding FAS and TEOS, the standard deviation was reduced, and the stability was significantly improved. To a certain extent, the disadvantages of poor stability of the chemical foaming method can be improved. When preparing the hydrophobic foamed MOC material, FAS with MgO mass of 0.5%–2.5 wt % was added, and the content of TEOS was (0.5–2.5):6 based on the molar ratio of FAS to TEOS. The mass of H2O2 was 10 wt % of the mass of MgO, and a series of foamed MOC materials were obtained. The bulk density of the material was maintained at 410–415 kg·m−3, indicating that the addition of FAS had little effect on the bulk density. Based on hydrophobicity and economy, in subsequent experiments, the FAS addition amount was selected to be 1% of the mass of MgO to explore the influence of H2O2 and TEOS content on bulk density. From the three-dimensional graph of the influence of H2O2 and TEOS content on bulk density in FigureAppl. Sci. 20202, ,it10 , 8134can be seen that, when the TEOS content was fixed and5 of the 15 H2O2 content increased from 4 wt % to 12 wt %, the bulk density decreased from 1200 kg·m−3 to 300 kg·m−3, thus the bulk density3. Results change and Discussiond greatly. When the content of H2O2 was fixed, n(FAS):n(TEOS) increased from 1:6 to 3.1.1:14, The Bulkthe Densityrelative and Pore content Structure ofof Foam TEOS Material increased, and the bulk density of the sample increased, with an average increase of 70 Kg·m−3. This indicates that TEOS may cause partial From the bulk density of F-MOC and N-MOC obtained with different H2O2 content in Figure1, decompositionit can of be H seen2O that2 during the bulk densitythe pretreatment of F-MOC obtained of with the the MgCl same H2 2solution.O2 content was The greater results than show that the that of N-MOC. The standard deviation of the bulk density of F-MOC and N-MOC shows that, although content of H2O2 had the greatest impact on the bulk density, followed by the content of TEOS, while the conditions for preparing the samples were the same, the bulk density of the samples was different, the content ofand FAS the stabilityhad no was effect. poor when only H2O2 was used as the foaming agent. After adding FAS and TEOS, the standard deviation was reduced, and the stability was significantly improved. To a certain extent, the disadvantages of poor stability of the chemical foaming method can be improved.

Figure 1. Comparison of foamed magnesium oxychloride cement (F-MOC) and magnesium oxychloride Figure 1. Comparisoncement without 2H-tridecylfluoro-n-octylsilaneof foamed magnesium (FAS) oxychloride or tetraethylorthosilicate cement (TEOS)(F-MOC (N-MOC)) and magnesium bulk density under different H2O2 content (FAS content was fixed at 1% of the mass of MgO; TEOS was oxychloride addedcement according without to the molar2H-tridecylfluoro ratio of FAS to TEOS-n- ofoctylsilane 1:6). (FAS) or tetraethylorthosilicate (TEOS) (N-MOC) bulk density under different H2O2 content (FAS content was fixed at 1% of the mass of When preparing the hydrophobic foamed MOC material, FAS with MgO mass of 0.5%–2.5 wt % MgO; TEOSwas added,was added and the according content of TEOS to the was (0.5–2.5):6molar ratio based of on FAS the molar to TEOS ratio ofof FAS 1:6) to. TEOS. The mass of H2O2 was 10 wt % of the mass of MgO, and a series of foamed MOC materials were obtained. The bulk density of the material was maintained at 410–415 kg m 3, indicating that the addition of · − FAS had little effect on the bulk density. Based on hydrophobicity and economy, in subsequent experiments, the FAS addition amount was selected to be 1% of the mass of MgO to explore the influence of H2O2 and TEOS content on bulk density. From the three-dimensional graph of the influence of H2O2 and TEOS content on bulk density in Figure2, it can be seen that, when the TEOS content was fixed and the H 2O2 content increased from 4 wt % to 12 wt %, the bulk density decreased from 1200 kg m 3 to 300 kg m 3, thus the bulk density · − · − changed greatly. When the content of H2O2 was fixed, n(FAS):n(TEOS) increased from 1:6 to 1:14, the relative content of TEOS increased, and the bulk density of the sample increased, with an average increase of 70 Kg m 3. This indicates that TEOS may cause partial decomposition of H O during · − 2 2 the pretreatment of the MgCl2 solution. The results show that the content of H2O2 had the greatest impact on the bulk density, followed by the content of TEOS, while the content of FAS had no effect. Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 15

Appl. Sci. 2020, 10, 8134 6 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 15

Figure 2. Three-dimensional graph of the influence of H2O2 and TEOS content on bulk density (FAS Figurecontent 2. waThree-dimensionals fixed at 1% of MgO graph mass) of the. inﬂuence of H2O2 and TEOS content on bulk density (FAS contentFigure was 2. Three ﬁxed- atdimensional 1% of MgO graph mass). of the influence of H2O2 and TEOS content on bulk density (FAS Generally,content wa thes fixed density at 1% gradeof MgO of mass) foam. concrete with good thermal insulation properties is 300– 1200 Generally,Kg·m−3 [24]the Therefore, density this grade study of foamselected concrete MOC samples with good within thermal the above insulation-mentioned properties density is 3 300–1200Generally, Kg m− [the24] Therefore,density grade this studyof foam selected concrete MOC with samples good withinthermal the insulation above-mentioned properties density is 300– range for subsequent· experiments and analysis. When adding the same content of H2O2, the pore 1200 Kg·m−3 [24] Therefore, this study selected MOC samples within the above-mentioned density rangesize distribution for subsequent of F- experimentsMOC and N and-MOC analysis. is shown When in Figure adding 3 the (F- sameMOC contentbulk density of H2O wa2,s the 540 pore kg·m size−3, distributionrange for ofsubsequent F-MOC and experiments N-MOC is shownand analysis. in Figure When3 (F-MOC adding bulk the density same was content 540 kg of Hm2O3,2, N-MOC the pore N-MOC bulk density was 301 kg·m−3). The pore size of F-MOC was small, and ·the− pore size bulksize density distribution was 301 of kgF-MOCm 3). and The N pore-MOC size is ofshown F-MOC in Figure was small, 3 (F- andMOC the bulk pore density size distribution was 540 kg·m was−3, distribution was very uniform,· − mainly concentrated in 210 ± 25 μm. On the contrary, N-MOC had a veryN-MOC uniform, bulk mainly dens concentratedity was 301 inkg·m 210−3). 25Theµm. pore On thesize contrary, of F-MOC N-MOC was hadsmall, a larger and porethe sizepore and size larger pore size and a wider distribution.± The most easily achieved pore size distribution was 490 ± distribution was very uniform, mainly concentrated in 210 ± 25 μm. On the contrary,µ N-MOC had a a25 wider μm, distribution.which indicates The mostthat the easily addition achieved of poreFAS- sizeTEOS distribution copolymer was can 490 make25 them, pore which size indicates of the larger pore size and a wider distribution. The most easily achieved pore size± distribution was 490 ± thatfoamed the additionMOC foam of FAS-TEOS smaller. copolymer can make the pore size of the foamed MOC foam smaller. 25 μm, which indicates that the addition of FAS-TEOS copolymer can make the pore size of the foamed MOC foam smaller.

Figure 3.3. F-MOCF-MOC and and N-MOC N-MOC aperture aperture test test interval interval diagram diagram and and aperture aperture distribution distribution digram(F-MOC digram(F- −3 −3 bulkMOC density bulk density was 540 was kg 540m kg·m3, N-MOC, N-MOC bulk bulk density density was 301was kg 301m kg·m3). ). Figure 3. F-MOC and· N−-MOC aperture test interval diagram· and− aperture distribution digram(F- MOC bulk density was 540 kg·m−3, N-MOC bulk density was 301 kg·m−3). We chosechose F-MOCF-MOC withwith aa bulkbulk densitydensity ofof 500–600 500–600 kg kg·mm −33 toto compare compare with N-MOCN-MOC with similar · − bulk density (Figure 44).). Although Although F F-MOC-MOC and and N N-MOC-MOC ha hadd similar similar bulk bulk densities, densities, the the pore pore size size of We chose F-MOC with a bulk density of 500–600 kg·m−3 to compare with N-MOC with similar ofN- N-MOCMOC wa wass significantly signiﬁcantly larger larger than than that that of F of-MOC. F-MOC. The The N-MOC N-MOC sample sample had hada large a large pore pore area areaand bulk density (Figure 4). Although F-MOC and N-MOC had similar bulk densities, the pore size of andmany many solid solid parts, parts, but butthe thepores pores of ofthe the F- F-MOCMOC foam foam material material we werere very very evenly distributeddistributed N-MOC was significantly larger than that of F-MOC. The N-MOC sample had a large pore area and throughout the the material. material. A Acommon common disadvantage disadvantage of the of thechemical chemical foaming foaming method method is the is wide the widepore many solid parts, but the pores of the F-MOC foam material were very evenly distributed poresize distrib size distributionution of the of foam the foam[25]. In [25 our]. In research, our research, adding adding FAS-TEOS FAS-TEOS copolymer copolymer to the toMOC the MOCfoam throughout the material. A common disadvantage of the chemical foaming method is the wide pore foamsystem system could could significantly signiﬁcantly improve improve the stability the stability and andthe uniformity the uniformity of the of thepores. pores. It shows It shows that that,, in size distribution of the foam [25]. In our research, adding FAS-TEOS copolymer to the MOC foam system could significantly improve the stability and the uniformity of the pores. It shows that, in Appl.Appl. Sci. Sci. 20202020, 10, 10, x, 8134FOR PEER REVIEW 7 7of of 15 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 15 the same bulk density of foamed MOC material, an increase in the number of independent pores on Fthein-MOC the same same results bulk bulk indensity densitybetter ofthermal offoamed foamed insulation MOC MOC material, material, performance an an increase increase [26]. in in the the number number of of independent independent pores pores on FF-MOC-MOC results in better thermal insulation performance [[2266].].

Figure 4. Comparison of the morphology of F-MOC and N-MOC with a similar bulk density ( the bulk density of F-MOC and N-MOC is 500–600 kg·m−3). Figure 4. ComparisonComparison ofof thethe morphology morphology of of F-MOC F-MOC and and N-MOC N-MOC with with a similar a similar bulk bulk density density (the ( bulk the bulkdensity density of F-MOC of F-MOC and N-MOCand N-MOC is 500–600 is 500 kg–600m kg·m3). −3). 3.2. Mechanical Properties and Phase Composition· of −Super-Hydrophobic Foamed MOC Materials 3.2. FigureMechanical Mechanical 5 shows P Propertiesroperties the corresponding and P Phasehase C Compositionomposition compressive of S Super-Hydrophobicuper strength-Hydrophobic of F-MOC FoamedFoamed samples MOC MaterialsM ofaterials different bulk density.Figure It can 55 showsshowsbe seen thethe from correspondingcorresponding the figure that compressivecompressive, when the strengthstrength bulk density ofof F-MOCF- MOCof the samplessamples samples ofof drop didifferentffpederent from bulk −3 −3 1200density. kg·m It It can canto aboutbe be seen seen 300 from kg·m the , thefigure figure samples that that,, 'when compressive the bulk strength density alsoof the dropped samples from droppeddrop aboutped from 10 3 3 MPa1200 to kg 0.8m MPa−3 to. aboutCompared 300 kg withm −3the, the compressive samples’ compressive strength of strength N-MOC also corresponding dropped from to aboutsimilar 10 bulk MPa 1200 kg·m· − to about 300 kg·m· − , the samples' compressive strength also dropped from about 10 density,MPato 0.8 to MPa. the0.8 MPadecrease Compared. Compared wa withs small thewith or compressive theeven compressive negligible, strength andstrength of it N-MOC wa ofs similarN corresponding-MOC to corresponding the compressive to similar to bulksimilarstrength density, bulk of MOCdensity,the decrease foamed the decrease wasmaterials small wa with ors small even similar or negligible, even bulk negligible, density and it wasandin the similarit waliterature.s similar to the It compressiveto can the reachcompressive the strength compressive strength of MOC of strengthMOCfoamed foamed standard materials materials required with similar with in the similar bulk application density bulk densityof in thermal the literature. in theinsulation literature. It can materials reach It can the [2reach7 compressive]. The the XRD compressive analysis strength ofstrengthstandard F-MOC standard requiredand N-MOC inrequired the in application Figure in the application6 ofalso thermal found of insulation thatthermal the insulationmain materials diffraction [materials27]. The peaks XRD [27 ].of analysisThe them XRD we of analysis F-MOCre the same,ofand F- N-MOC MOCindicating and in FigureNthat-MOC the6 also inaddition Figure found thatof6 alsoFAS the found mainand TEOS dithatffraction thehad main no peaks effect diffraction of themon the werepeaks phase the of composition same, them indicatingwere theof MOC.same,that the The indicating addition five-phase ofthat FASrelative the and addition of TEOS F-MOC hadof FAS calculated no eandffect TEOS on by the jade ha phase dsoftware no composition effect wa ons 55.9% the of phase, MOC. and that composition The of five-phase N-MOC of waMOC.relatives 59.4%, The of whichfive F-MOC-phase wa calculateds relative only a ofdecrease by F jade-MOC softwareof calculated 3.5%. wasThe by 55.9%,P5 jade phase software and wa thats retainedwa ofs N-MOC55.9% as, andthe was mainthat 59.4%, of strength N which-MOC phase,waswas only59.4%, indicating a decreasewhich that wa of thes 3.5%.only decrease Thea decrease P5 in phasecompressive of was3.5%. retained The strength P5 asphase thewas main wathes result strengthretained of phase,theas theporous indicatingmain structure strength that ratherphase,the decrease than indicating the in changes compressive that theof the decrease strength five-phase. in was compressive the result ofstrength the porous was structure the result rather of the than porous the changes structure of ratherthe five-phase. than the changes of the five-phase.

Figure 5. The compressive strength and the bulk density of F-MOC. Figure 5. The compressive strength and the bulk density of F-MOC . Figure 5. The compressive strength and the bulk density of F-MOC. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 15 Appl. Sci. 2020, 10, 8134 8 of 15

Figure 6. X-ray patterns of F-MOC and N-MOC. Figure 6. X-ray patterns of F-MOC and N-MOC. Figure 6. X-ray patterns of F-MOC and N-MOC. 3.3.3.3. TheThe PresencePresence ofof OrganosilaneOrganosilane in in MOC MOC Foam Foam Materials Materials 3.3. The Presence of Organosilane in MOC Foam Materials TheThe comparisoncomparison ofof thethe FourierFourier infraredinfrared testtest ofof F-MOCF-MOC andand N-MOCN-MOC inin FigureFigure7 a7a showed showed that that 1 −1 thethe characteristic characteristicThe comparison C-F C- peakF ofpeak the that Fourierthat should should infrared have have appeared test appear of nearF-MOCed 1170 near and cm 1170 −N-[MOC 28cm] was [2in8 coveredFigure] was covered7a by showed the infrared by thatthe peakinfraredthe characteristic of the peak magnesium of the C- Fmagnesium peak oxychloride that oxychlorideshould cement have itself. cement appear However, itself.ed near However, in1170 the cm XPS in−1 the[2 energy8 ]XPS wa s spectrumenergy covered spectrum testby the of Figuretestinfrared of7 b,Figure itpeak was 7bof found, theit was magnesium that found the characteristic that oxychloride the characteristic peak cement of the peak Fitself. element of However,the appearedF element in the at appeared 690 XPS eV energy [ 29at], 690 indicating spectrum eV [29], thatindicatingtest theof Figure F element that 7b the, enteredit F was element found the entered MOC that foamthe the characteristic MOC material. foam material.peak of the F element appeared at 690 eV [29], indicating that the F element entered the MOC foam material.

(a) (a)

(b) FigureFigure 7. 7.Fourier Fourier infraredinfrared test test ( a(a)) and and XPS XPS(b) spectrum spectrum ( b(b)) of of F-MOC F-MOC and and N-MOC. N-MOC. Figure 7. Fourier infrared test (a) and XPS spectrum (b) of F-MOC and N-MOC. Appl. Sci. 2020, 10, 8134 9 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 15

The pHpH of of the the magnesium magnesium chloride chloride solution solution was about was 6.3 about to 6.5. 6.3 In thisto 6.5. near-neutral In this environment,near-neutral the hydrolysis of FAS and TEOS was relatively slow. However, when adding H O , the content of environment, the hydrolysis of FAS and TEOS was relatively slow. However, when2 2 adding H2O2, hydroxyl radicals in MgCl solution increased [30], and hydroxyl radicals significantly promoted the content of hydroxyl 2radicals in MgCl2 solution increased [30], and hydroxyl radicals significantlythe hydrolysis promote and thed condensation the hydrolysis of and silane the [ 31condensation], thus FAS and of silane TEOS [ were31], thus added FAS to and the magnesiumTEOS were addedchloride to solution. the magnesium The first hydrolysis chloride couldsolution. make The it more first fully hydrolysis hydrolyzed. could Due make to the it consumptionmore fully of hydroxyl radicals, part of H O was decomposed in the pre-hydrolysis stage. The more TEOS hydrolyzed. Due to the consumption2 2 of hydroxyl radicals, part of H2O2 was decomposed in the pre- content there was, the more H O was consumed. As TEOS increased, this resulted in the addition hydrolysis stage. The more TEOS2 2 content there was, the more H2O2 was consumed. As TEOS of the same H O . The content of F-MOC in the foamed material was higher than that of N-MOC. increased, this2 result2 ed in the addition of the same H2O2. The content of F-MOC in the foamed Whenmaterial the wa magnesiums higher chloridethan that solution of N-MOC. containing When organosilane the magnesium was mixed chloride with solution active MgO, containing the pH valueorganosilane of the MOCwas mixed system with increased active MgO, to 8–9.5 the [ 32pH], value and the of alkalinethe MOC environment system increase furtherd to 8 promoted–9.5 [32], andthe polycondensationthe alkaline environment reaction offurther the pre-co-hydrolyzed promoted the polycondensation product of TEOS reaction and FAS. of Studies the pre have-co- hydrolyzedshown [33], product under normal of TEOS conditions, and FAS. FAS Studies and have TEOS shown can be [33 copolymerized], under normal into conditions, silica spheres FAS withand TEOS40–200 can nm fluoroalkylbe copolymerized chains. Theinto silicasilica spheresspheres with with fluoroalkyl 40–200 nm chains fluoroalkyl were uniformly chains. The dispersed silica spheresthroughout with the fluoroalkyl MOC gelling chains system, were resultinguniformly in dispersed the overall throughout hydrophobicity the MOC of the gelling foamed system, MOC resultingmaterial (Figurein the overall8). hydrophobicity of the foamed MOC material (Figure 8).

Figure 8. TriethoxyTriethoxy-1H,-1H, 1H, 2H, 2H-tridecylﬂuoro-n-octylsilane2H-tridecylfluoro-n-octylsilane (FAS) (FAS),, and tetraethyl orthosilicate (TEOS) co-hydrolysisco-hydrolysis mechanism.mechanism.

3.4. Hydrophobicity Hydrophobicity of F Foamedoamed MOC M Materialsaterials When waterwater was wa droppeds dropped on on the surfacethe surface of the of N-MOC the N- (weMOC added (we aadd smalled amounta small of amount methylene of methyleneblue to the waterblue to for the easy wat observation),er for easy theobservation), water immediately the water penetrated immediately the sample penetrate throughd the the sample holes, throughand after the adding holes,TEOS and after copolymer adding TEOS withFAS copolymer and FS with samples, FAS and the sameFS samples, volume the of same water volume droplets of watercould droplets stay on the could material stay on on the the material surface, o andn the the surface, contact and angle the ofcontact water angle droplets of water on the dr surfaceoplets on of the surface material of was the greatlymaterial increased. was greatly increased. In FigureFigure9 ,9, the the FAS FAS content content of Aof1 A–A1–5Awas5 wa 1%,s 1%, n (FAS): n (FAS): n (TEOS) n (TEOS)= 1:8; = when1:8; when the content the content of H2 Oof2 Hincreased2O2 increase fromd 4%from to 4% 12%, to the12%, contact the contact angle angle changed. change Thed. results The results show show the bulk the densitybulk density of F-MOC of F- 3 3 was reduced from about 1200 Kg m to about−3 200 Kg m . The water−3 contact angle increased first and MOC was reduced from about 1200· − Kg·m to about· 200− Kg·m . The water contact angle increased firstthen and decreased. then decrease When thed. HWhen2O2 contentthe H2O was2 content 10%, the wa waters 10%, contact the water angle contact reached angle the highest reache 154d the◦. highestB1–B 154°.5 were the changes in contact angle when the additional amount of H2O2 was fixed at 10%, and theB1–B TEOS5 were content the changes increased in contact from nangle (FAS): when n (TEOS) the addition= 1:6 toal n am (FAS):nount of (TEOS) H2O2 =wa1:14.s fixed The at result 10%, andshowed the TEOS that the content water increase contact angled from increased n (FAS): firstn (TEOS) and then = 1:6 decreased, to n (FAS):n reaching (TEOS) a maximum= 1:14. The of result 142◦ showedat n (FAS): that n (TEOS)the water= 1:8. contact angle increased first and then decreased, reaching a maximum of 142° Cat1 n–C (FAS):5 were n the (TEOS) fixed additional= 1:8. amounts of H2O2, which was 10%, n (FAS):n (TEOS) = 1:8; the FAS contentC1–C increased5 were the from fixed 0.5% addition to 2.5%.al amount Whens the of FASH2O2 content, which increasedwas 10%, fromn (FAS):n 0.5% (TEOS) to 1%, the= 1:8 water; the FAScontact content angle increase rangedd from from 140 0.5%◦ to to 156 2.5%◦ with. W ahen larger the increase.FAS content When increase the FASd from content 0.5% continued to 1%, the to waterincrease, contact the water angle contact range angled from remained 140° to above 156° 150with◦ and a larger did not increase. continue When to increase. the FAS content continued to increase, the water contact angle remained above 150° and did not continue to increase. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 15

Appl. Sci. 2020Summarily,, 10, x FOR PEERthe addition REVIEWal amount of H2O2 was 10%, n (FAS):n (TEOS) = 1:8. When the FAS10 of 15 content was 1%, the F-MOC obtained had the highest water contact angle and had the best hydrophobic effect. The pores of the material provided suitable roughness on the surface of the Summarily, the additional amount of H2O2 was 10%, n (FAS):n (TEOS) = 1:8. When the FAS Appl. Sci.material.2020, 10 ,Thus, 8134 low surface energy fluorinated alkyl chain and certain roughness make the10 of 15 content was 1%, the F-MOC obtained had the highest water contact angle and had the best hydrophobic effect of F-MOC the best. hydrophobic effect. The pores of the material provided suitable roughness on the surface of the material. Thus, low surface energy fluorinated alkyl chain and certain roughness make the hydrophobic effect of F-MOC the best.

FigureFigure 9. The 9. waterThe water contact contact angle angle of di offferent different ratios ratios of F-MOC of F-MOC ((A 1(()–(A1)A–(5A)5 are) are FAS FAS content content of of 1%, 1%, n n (FAS): n (TEOS)(FAS):= 1:8,n (TEOS) the addition = 1:8, the of addition H2O2 increased of H2O2 increased from 4% from to 12%, 4% (Bto1 )–(12%,B5 )(B were1)–(B5 when) were thewhen additional the amountadditional of H2O amount2 was fixed of H at2O2 10%, was andfixed the at TEOS10%, and content the TEOS increased content from increased n (FAS): from n (TEOS)n (FAS):= n1:6 to (TEOS) = 1:6 to n (FAS): n (TEOS) = 1:14, (C1)–(C5) were the additional amounts of H2O2 fixed at 10%, n (FAS): n (TEOS) = 1:14, (C1)–(C5) were the additional amounts of H2O2 fixed at 10%, n (FAS):n (TEOS) = 1:8,n the (FAS) FAS : n content (TEOS) increased= 1:8, the FAS from content 0.5% increased to 2.5%). from 0.5% to 2.5%). Figure 9. The water contact angle of different ratios of F-MOC ((A1)–(A5) are FAS content of 1%, n (FAS):Through n (TEOS) a series= 1:8, ofthe tests, addition the samples of H2O with2 increased the largest from water 4% tocontact 12%, angle(B1)–( Bwere5) were selected when (A the is Summarily, the additional amount of H2O2 was 10%, n (FAS):n (TEOS) = 1:8. When the FAS content additionalF-MOC, B amountis N-MOC). of HWater2O2 was conta fixedct angle at 10%, could and reach the 156° TEOS, as iscontent shown increased in Figure 10afrom. As n shown(FAS): inn was 1%, the F-MOC obtained had the highest water contact angle and had the best hydrophobic effect. (TEOS)Figure 10b= 1:6, theto n sample (FAS): wasn (TEOS) mechanically = 1:14, (C cut1)– (toC 5retain) were the the internal additional incision amounts of the of foamH2O2 material.fixed at 10%, The The pores of the material provided suitable roughness on the surface of the material. Thus, low surface nwater (FAS) contact : n (TEOS) angle = 1:8,of all the points FAS content on the increasedsurface of from the foam 0.5% materialto 2.5%). exceeds 150°,and the solution energywill fluorinated always remain alkyl on chain the surface and certain of the roughnessF-MOC without make pen theetration hydrophobic as shown eff inect Figure of F-MOC 10c.which the best. Throughachieved the a series inherent of tests,hydrophobic the samples and ha withs more the advantages largest water in applications contact angle than the were hydrophobic selected (A is FF-MOC,-MOC,modification B is is N N-MOC).-MOC). of surface Water Water coating conta contact andct angledipping. angle c couldould reach reach 156° 156,◦ as, as is is shown shown in in Figure Figure 10a 10. a.As As shown shown in Figurein Figure 10b 10, theb, the sample sample was was mechanically mechanically cut cutto retain to retain the internal the internal incision incision of the of foam the foam material. material. The Thewater water contact contact angle angle of all of allpoints points on onthe the surface surface of ofthe the foam foam material material exceeds exceeds 150°,and 150◦,and t thehe solution will always remain on the surface of the F-MOCF-MOC without penetrationpenetration as shown in Figure 1010cc.which.which achieveachievedd the inherent hydrophobic and hashas more advantages in applications than the hydrophobic modificationmodification of surface coating and dipping.

(a) (b) (c)

Figure 10. F-MOC hydrophobic effect: (a) is the water contact angle test graph, (b) is the overall hydrophobic effect graph of F-MOC, and (c) is the comparison graph of F-MOC and N-MOC.

3.5. Engineering Durability of Foamed MOC Materials Engineering durability is a key issue in the application of materials, thus the long-term stability of foamed hydrophobic(a) materials in complex (environmentsb) is very important. Therefore,(c) we studied the influence of mechanical polishing and chemical corrosion on the hydrophobic Figureproperties 10. FF-MOCof-MOC the samples. hydrophobic The result effect eﬀect: of: ( (theaa)) is ismechanical the the water water grindingcontact contact angle angleexperiment test graph, is shown ( b) is in the Figure overalloverall 11. hydrophobic eeffectﬀect graphgraph ofof F-MOC,F-MOC, andand ((cc)) isis thethe comparisoncomparison graphgraph ofof F-MOCF-MOC andand N-MOC.N-MOC.

3.5. Engineering Engineering D Durabilityurability of F Foamedoamed MOC M Materialsaterials Engineering durabilitydurability isis aa keykey issue issue in in the the application application of of materials, materials, thus thus the the long-term long-term stability stability of offoamed foamed hydrophobic hydrophobic materials materials in complex in complex environments environments is very is important. very important. Therefore, Therefore, we studied we studiedthe influence the influence of mechanical of mechanical polishing andpolishing chemical and corrosion chemical on corrosion the hydrophobic on the propertieshydrophobic of propertiesthe samples. of Thethe resultsamples. of the The mechanical result of the grinding mechanical experiment grinding is shown experiment in Figure is 11shown. With in the Figure increase 11. of the thickness of mechanical grinding, the water contact angle of the sample did not decrease significantly, indicating that the fluorine-containing alkyl chains were evenly distributed in the sample, Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 15 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 15 With the increase of the thickness of mechanical grinding, the water contact angle of the sample did With the increase of the thickness of mechanical grinding, the water contact angle of the sample did not decrease significantly, indicating that the fluorine-containing alkyl chains were evenly not decrease significantly, indicating that the fluorine-containing alkyl chains were evenly Appl.distributed Sci. 2020, 10in, 8134the sample, giving the material overall hydrophobicity. The result of immersion11 of 15in distributed in the sample, giving the material overall hydrophobicity. The result of immersion in NaCl, NaOH, and HCl solutions in Figure 12 showed that the material still maintained a high water NaCl, NaOH, and HCl solutions in Figure 12 showed that the material still maintained a high water contact angle, indicating the chemical stability of the foamed hydrophobic MOC material in harsh givingcontact the materialangle, indicating overall hydrophobicity. the chemical stability The result of ofthe immersion foamed hydrophobic in NaCl, NaOH, MOC and material HCl solutions in harsh environments such as acid-base and salt solutions, proving that composite materials have potential in Figureenvironments 12 showed such that as the acid material-base and still salt maintained solutions, a high proving water that contact composite angle, indicatingmaterials thehave chemical potential value in engineering applications. stabilityvalue in of engineering the foamed hydrophobicapplications. MOC material in harsh environments such as acid-base and salt solutions, proving that composite materials have potential value in engineering applications.

FigureFigure 11.11. TheThe influence inﬂuence of of different diﬀerent mechanical mechanical polishing polishing thickness thickness on the on thewater water contact contact angle angle of F- Figure 11. The influence of different mechanical polishing thickness on the water contact angle of F- ofMOC F-MOC.. MOC.

Figure 12.12. The eeffectffect of soakingsoaking inin didifferentfferent solutionssolutions onon thethe chemicalchemical stabilitystability ofof F-MOC.F-MOC. Figure 12. The effect of soaking in different solutions on the chemical stability of F-MOC. 3.6.3.6. The MoistureMoisture AbsorptionAbsorption RateRate andand DehumidificationDehumidification RateRate ofof FoamedFoamed MOCMOC MaterialMaterial 3.6. The Moisture Absorption Rate and Dehumidification Rate of Foamed MOC Material TheThe thermalthermal conductivity conductivity of airof inair the in thermal the thermal insulation insulation material wasmaterial only 0.02was w only/(m K). 0.02 When w/(m·K). water The thermal conductivity of air in the thermal insulation material was only· 0.02 w/(m·K). vaporWhen enteredwater vapor the cavity enter ofed foamed the cavity cement, of foamed the thermal cement, conductivity the thermal of waterconductivity was 0.59 of w water/(m K), wa ands 0.59 its When water vapor entered the cavity of foamed cement, the thermal conductivity of water· was 0.59 thermalw/(m·K), insulation and its thermal performance insulation was greatly perfor reduced.mance was Therefore, greatly hygroscopicity reduced. Therefore, and water hygroscopicity content had w/(m·K), and its thermal insulation performance was greatly reduced. Therefore, hygroscopicity aand significant water content impact onhad the a thermalsignificant conductivity impact on of the foamed thermal cement. conductivity The test environmentof foamed cement. was to simulateThe test and water content had a significant impact on the thermal conductivity of foamed cement. The test theenvironment temperature wa ands to the simulate humidity the of temperature the material; theand temperature the humidity was of 30 the◦C, material the relative; the humidity temperature was environment was to simulate the temperature and the humidity of the material; the temperature 100%,was 30 and °C the, the moisture relative absorptionhumidity wa rates tested100%, withand differentthe moisture densities absorption of F-MOC rate and tested N-MOC with isdifferent shown was 30 °C, the relative humidity was 100%, and the moisture absorption rate tested with different indensities Figure 13 of. TheF-MOC results and show N-MOC that, under is shown the same in Figure density, 13. the The hygroscopic results show rate of that F-MOC, under was the less same than densities of F-MOC and N-MOC is shown in Figure 13. The results3 show that, under the same thatdensity, of N-MOC. the hygroscopic When the bulkrate of density F-MOC was wa approximatelys less than that 1200 of Kg N-mMOC.− , it wasWhen difficult the bulk for density water vapor was density, the hygroscopic rate of F-MOC was less than that of ·N-MOC. When the bulk density was toapproximately enter the sample 1200 due Kg·m to the−3, smallit was pore difficult size. Afterfor water cleaning vapor the surface,to enter thethe moisture sample absorptiondue to the ratesmall of approximately 1200 Kg·m−3, it was difficult for water vapor to enter the sample due to the small F-MOCpore size. and After N-MOC cleaning was the close, surface, as it only the moisture decreased absorption by 1.7%, but rate when of F- theMOC bulk and density N-MOC decreased was close, to pore size.3 After cleaning the surface, the moisture absorption rate of F-MOC and N-MOC was close, 300as it Kg onmly− decreaseddue to the by increase 1.7%, but of thewhen specific the bulk surface density area, decreased the moisture to 300 absorption Kg·m−3 ratedue ofto F-MOCthe increase was · −3 13.3%,as it whileonly decreased that of N-MOC by 1.7%, was but up when to 29.5%. the bulk density decreased to 300 Kg·m due to the increase Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 15

of the specific surface area, the moisture absorption rate of F-MOC was 13.3%, while that of N-MOC

wasAppl. upSci. 20to20 29.5%., 10, x FOR PEER REVIEW 12 of 15

of the specific surface area, the moisture absorption rate of F-MOC was 13.3%, while that of N-MOC

Appl.was Sci.up 2020to 29.5%., 10, 8134 12 of 15

Figure 13. Moisture absorption rate of F-MOC and MOC with a different bulk density.

The dehydration rate of F-MOC and N-MOC is as follows in Figure 14. Dehydration Figure 13. Moisture absorption rate of F-MOC and MOC with a diﬀerent bulk density.−3 experimentsFigure of F- 13.MOC Moisture and N absorption-MOC with rate aof bulk F-MOC density and MOC of approximately with a different bulk600 Kg·mdensity . after being blisteredThe dehydrationfor 120 min rateat 30 of °C F-MOC showed and that N-MOC the quality is as follows of F- inMOC Figure could 14. Dehydrationbe restored to experiments 98.1% of the of original quality, while N-MOC only could be restored to 83.2% of3 the original quality. The high F-MOCThe and dehydration N-MOC with rate a bulk of densityF-MOC of and approximately N-MOC is 600 as Kg followsm− after in beingFigure blistered 14. Dehydration for 120 min dehydration rate of F-MOC indicates that the water in the pores· of the foamed material−3 was the atexperiments 30 ◦C showed of F that-MOC the and quality N-MOC of F-MOC with a could bulk bedensity restored of approximately to 98.1% of the 600 original Kg·m quality, after whilebeing N-MOCwaterblistered that only for adh 120 couldere mind be toat restored 30the °C surface, showed to 83.2% which that of the the wa original qualitys easy quality. ofto Fevaporate-MOC The c highould and dehydrationbe remove,restored whileto rate 98.1% of the F-MOC of lowthe indicatesdehydrationoriginal quality, that rate the waterwhileof N-MOC inN- theMOC indicates pores only of thecthatould foamed some be restored of material the water to was 83.2% in thethe of waterpores the thatoriginalhad adhered been quality. immersed to the The surface, in high the whichMOCdehydration wasgelling easy rate material to of evaporate F-MOC. Internally, indicates and remove, we that know while the thatwater the the low in mechanical dehydrationthe pores of properties ratethe foamed of N-MOC of materialMOC indicates mate wasrials that the somedecreasewater of that the significantly wateradhere ind theto in pores thelong surface,- hadterm been blisters which immersed [34wa],s which ineasy the to MOCalso evaporate indicates gelling material.and that remove,F-MOC Internally, materialswhile wethe know havlowe thatbetterdehydration the resistance mechanical rate ofto properties Nwater-MOC vapor indicates of MOC and materials thatcan somemaintain decrease of the their water signiﬁcantly initial in the strength pores in long-term ha duringd been blisters longimmersed-term [34], whichwaterin the alsovapor.MOC indicates gelling that material F-MOC. Internally, materials havewe betterknow resistancethat the tomechanical water vapor properties and can maintainof MOC their mate initialrials strengthdecrease duringsignificantly long-term in long water-term vapor. blisters [34], which also indicates that F-MOC materials have better resistance to water vapor and can maintain their initial strength during long-term water vapor.

Figure 14. The dehydration rate of F-MOC and MOC (F-MOC and N-MOC bulk density are both Figure 14. The dehydration3 rate of F-MOC and MOC (F-MOC and N-MOC bulk density are both around 600 Kg m− , blistering time is 120 Min, the temperature is 30 ◦C). around 600 Kg·m· −3, blistering time is 120 Min, the temperature is 30 °C). 4. Conclusions 4. Conclusions InFigure this study,14. The TEOS dehydration and FAS rate were of prehydrolyzedF-MOC and MOC in (F MgCl-MOC2 solution and N-MOC containing bulk density H2O2 asare a both blowing −3 agentInaround and this then 600study, mixedKg·m TEOS, withblistering and MgO FAStime powder iswe 120re toMin,prehydrolyzed further the temperatu hydrolyze inre MgClis and30 °C)2 synthesize solution. containing hydrophobic H2 foamedO2 as a magnesiumblowing agent oxychloride and then mixed cement. with The MgO properties powder to of further cement hydrolyze were characterized and synthesize and comparedhydrophobic to ordinaryfoamed4. Conclusions foamedmagnesium magnesium oxychloride oxychloride cement. cement. The Theproperties results areof thecement following: were characterized and compared to ordinary foamed magnesium oxychloride cement. The results are the following: 1. InFirstly, this westudy, pre-hydrolyzed TEOS and FAS TEOS we andre FASprehydrolyzed in the MgCl 2insolution. MgCl2 solution When the containing MgO and theH2O MgCl2 as 2a blowingsolutions agent wereand then mixed, mixed the with pH ofMgO the powder system roseto further to weakly hydrolyze alkaline, and which synthesize further hydrophobic catalyzed foamedthe hydrolysismagnesium condensation oxychloride of TEOScement. and FAS,The andproperties the XPS resultsof cement demonstrate were characterized that the ﬂuoroalkyl and comparedchain to was ordinary successfully foamed introduced magnesium into oxychloride F-MOC. The cement. presence The of results TEOS madeare the the following: F-MOC normal pore size distribution curve obtained by adding the same H2O2 foaming agent content thinner than the N-MOC, and the pore structure of the material was more uniform. Appl. Sci. 2020, 10, 8134 13 of 15

2. The overall hydrophobic F-MOC with a bulk density of 300 kg m 3–1200 kg m 3 and a water · − · − contact angle of 156◦ was prepared by doping with an organosilane. The XRD result and the calculation of the relative content of five-phase show that the five-phase of F-MOC was the main strength phase, as its relative content was only 3.5% lower than that of N-MOC. Based on compressive strength retention, it prevented moisture intrusion and increased its resistance to water. It fundamentally solved the poor water-resistance of the foamed MOC. 3. With decreasing bulk density, although the moisture absorption rate of F-MOC increased, it was less than that of N-MOC. When the bulk density was 300 Kg m 3, the moisture absorption rate of · − F-MOC was 13.3%, while in that of N-MOC, the moisture absorption rate of MOC was as high as 29.5%. In the high-temperature dehydration experiment, the F-MOC mass could be restored to 98.1% of the original mass, while the N-MOC mass could only be restored to 83.2%, indicating that the moisture invading the sample was more evaporated in the form of free water and did not participate in the MOC hydration reaction, effectively preventing the strength loss caused by the collapse of the pore structure caused by hydration. 4. The influence of mechanical polishing and chemical corrosion on the hydrophobic properties of F-MOC show that the composite material has good engineering durability and broad application prospects.

Author Contributions: Conceptualization, R.C. and H.W.; methodology, R.C.; software, S.G.; writing—original draft preparation, J.H.; project administration, R.C. and H.W.; funding acquisition, R.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors gratefully acknowledge financial support from National Natural Science Foundation of China (21571024) and Major Special Science and Technology Projects in Qinghai Province (2018-NN-152). Conflicts of Interest: The authors declare no conflict of interest.

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