Effect of Phosphoric Acid and Soluble Phosphate on the Properties of Magnesium Oxychloride Cement

Abstract

This study investigates the effects of phosphoric acid (H₃PO₄), potassium dihydrogen phosphate (KH₂PO₄) and sodium dihydrogen phosphate (NaH₂PO₄) admixtures on the setting time, compressive strength and water resistance of magnesium oxychloride cement (MOC). MOC samples incorporating different admixtures are prepared, and their hydration products and microstructures are studied via X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicate that the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ reduces the initial and final setting times and decreases the compressive strength. However, the compressive strength of MOC is higher than 30.00 MPa with the addition of 2.0 wt.% phosphoric acid and its phosphate after 14 days of air curing. The water resistance of modified MOC slurries is significantly improved. The softening coefficient of MOC with 2.0 wt.% H₃PO₄ is 1.2 after 14 days of water immersion, which is 3.44 times higher than that of the neat MOC. The enhancement in water resistance is attributed to the formation of amorphous gel facilitated by H₃PO₄, KH₂PO₄ and NaH₂PO₄. Furthermore, the improvement in water resistance is manifested as H₃PO₄ > KH₂PO₄ > NaH₂PO₄.

1. Introduction

Magnesium oxychloride cement was invented by the French scientist, Sorrell, and is also known as Sorrell cement (abbreviated as MOC) [1,2,3]. As a special type of cement, its main hydration products at room temperature are 5Mg(OH)₂·MgCl₂·8H₂O (abbreviated as 518 phase) and 3Mg(OH)₂·MgCl₂·8H₂O (abbreviated as 318 phase) [4]. Compared with ordinary Portland cement, MOC is an air-hardening cementitious material [5]. It has many advantages such as early strength, high strength, fast setting time, wear resistance, corrosion resistance, fire resistance, strong bonding force, low alkalinity, etc. [6,7,8,9,10]. In addition, it has good compatibility with the environment, so it can replace artificial polymer materials in the preparation of disposable products. But its disadvantages are also obvious [2,10]. Firstly, MOC has poor water resistance when immersed in water, and its strength decreases [11]. Secondly, MOC easily absorbs moisture, returns to brine and frosts in humid environments, which affects its appearance and use. Thirdly, the MOC product is prone to warping and deformation, resulting in product scrapping [12,13]. Among these, the biggest drawback is the poor water resistance of MOC.

The hydration reaction of MOC is mainly carried out in a ternary system composed of magnesium oxide, magnesium chloride and water, and its formation can be represented by the following equation [14]:

5MgO + MgCl₂ + 13H₂O→5Mg(OH)₂·MgCl₂·8H₂O(1)

3MgO + MgCl₂ + 11H₂O→3Mg(OH)₂·MgCl₂·8H₂O(2)

From a thermodynamic perspective, the 518 phase is a metastable phase, and the 318 phase is a stable phase. The 518 phase tends to transform into the 318 phase. However, due to its higher strength and better water resistance, it is hoped that more 518 phases will be generated in the system [15]. As the main hydration products of MOC, the 518 and 318 phases are highly prone to water absorption and hydrolysis, as shown in the following equations [16,17]:

5Mg(OH)₂·MgCl₂·8H₂O +H₂O→Mg(OH)₂↓ + Mg²⁺ +Cl⁻ + H₂O(3)

3Mg(OH)₂·MgCl₂·8H₂O +H₂O→Mg(OH)₂↓ + Mg²⁺ +Cl⁻ + H₂O(4)

After hydrolysis, the framework structure in MOC will transform to the layered structure of Mg(OH)₂, leading to the loose structure and resulting in a significant decrease in strength [12]. There are many voids and pores inside the structure and crystallization contact points due to the multiphase porous structure of MOC [11]. From the thermodynamic perspective, the stability is relatively low and the solubility is high at the contact point of crystallization [18]. When it encounters water, it will dissolve first, and the numerous pores provide channels for water intrusion. Therefore, these structural characteristics also determine the water resistance of MOC [19].

The poor water resistance of MOC severely limits the development of the industry. At present, adding modifiers to MOC has become the most effective method [11,12,20,21,22]. Adding modifiers ensures that the hydration product crystal phase can stably exist in water. The modifiers form a water-resistant protective film on the surface of the hydration product. Or the modifiers fill the pores in the MOC that reduce the movement channels of water molecules and hydrolysis ions. Modifiers can prevent adverse phenomena such as moisture absorption, halogenation, frost and deformation [2,19,23].

The commonly used modifiers include inorganic modifiers, organic modifiers and composite modifiers [24]. From current research, although many positive measures have been proposed and achieved good results, the problem of the poor water resistance of MOC has not been fundamentally solved [25].

To improve the water resistance of MOC, various methods have been adopted. Among them, the most effective method is the incorporation of admixtures and supplementary materials to cementitious materials [26,27,28]. It is reported that the compressive strength of MOC decreases by only 10% and 15% when 1% soluble phosphate [29] and 30% fly ash [30] were added to the MOC, respectively, after 28 days of immersion in water. After immersion in water for 28 days, the compressive strength of MOC decreased by up to 20% with only 1% phosphoric acid addition, but the improvement in water resistance was significant, facilitating a 50% increase in the strength retention coefficient [12]. Additionally, different scholars have conducted relevant research and reports on the mechanism of adding phosphoric acid and its salts to MOC to improve water resistance [31,32].

In conclusion, phosphoric acid and soluble phosphate are the main modified materials currently used to improve the water resistance of MOC [11,12,33,34]. However, the majority of studies primarily concentrate on the modification of MOC through phosphate. In contrast, there is a scarcity of research that examines and compares the effects of modification on MOC using phosphoric acid versus soluble phosphate. In this paper, in order to compare and analyze the effects of phosphoric acid and soluble phosphate on the performance of MOC, the same amount of phosphoric acid and its soluble salt were added to MOC. The influence of phosphoric acid and its soluble phosphate on the properties of MOC was analyzed.

2. Materials and Methods

2.1. Raw Materials

The light-burnt magnesia used in the experiment was obtained from Liaoning Province, China. The activity of light-burnt magnesia was tested by direct hydration [35] at 105 °C and 101.3 MPa containing 50.0 wt.% active magnesium oxide (MgO_a). The bischofite used in this experiment was mainly composed of hygroscopic magnesium chloride hexahydrate (MgCl₂·6H₂O), which was provided by Jiayoumeiye Ltd. (Xining, Qinghai Province, China). The phosphoric acid (H₃PO₄), potassium dihydrogen phosphate (NaH₂PO₄) and sodium dihydrogen phosphate (KH₂PO₄) used in this experiment were analytically pure. The chemical composition of light-burnt magnesia is listed in Table 1.

2.2. Specimen Preparation

The molar ratio of active MgO to MgCl₂ to H₂O was fixed at 7:1:19.8 for the preparation of MOC paste. Magnesium chloride solution was prepared firstly by dissolving bischofite in water. Then, the light-burnt magnesia was added in the solution to form fresh slurry. Meanwhile, H₃PO₄, KH₂PO₄ and NaH₂PO₄ were added as admixtures into the slurry for the incorporation of 0.0 wt.%, 0.5 wt.%, 1.0 wt.% and 2.0 wt.% (by weight of light-burnt magnesia); the detailed quantities are presented in Table 2. The MOC samples were labeled as n%H₃PO₄/KH₂PO₄/NaH₂PO₄-W-14D, where n% indicated the addition of H₃PO₄/KH₂PO₄/NaH₂PO₄, and W indicated that the sample was soaked for curing. The mixture slurry was then cast into steel molds with the size of 20 × 20 × 20 mm. Samples were finally covered with plastic sheets to prevent evaporation and initially cured for 24 h at room temperature. Specimens were then removed from the molds and further cured for another 14 days and further immersed in water for another 14 days.

2.3. Specimen Analysis

Setting times were determined using the Vicat test at 20 ± 2 °C [36]. The compressive strength of different specimens was measured by a material testing machine (MTS, SANYU, SYE3000B) with a maximum force of 300 kN and a loading rate of 1.52 mm min⁻¹. More than three samples with different compositions and ages were tested. The 80 µm powders were prepared by crushing the samples for X-ray diffraction (XRD) analysis. Diffraction patterns were obtained using an X’Pert Pro diffractometer (PANalytical, 2θ = 5~70°, Cu Kα radiation, λ = 0.15406 Å). The Rietveld method was then used to quantify crystal phase compositions via analysis of the diffraction patterns with Topas 4.2 software [37].

The water resistance of MOC slurries was evaluated for various periods of time after being cured in air for 14 days. The compressive strength of each specimen was then measured. The strength retention coefficient (I) of each MOC paste after different immersion ages was calculated [37].

I = R_d/R₀(5)

where R_d and R₀ represent the compressive strength of the MOC paste after immersion in water for d days and being cured in air for 14 days without subsequent immersion, respectively.

The micro-morphologies of MOC slurries with 14 days air curing and immersion in water for different times were characterized via scanning electron microscopy (SEM, JSM-5610LV).

3. Results

3.1. Setting Time of MOC Slurries

The setting times of MOC slurries with different additions of H₃PO₄, KH₂PO₄ and NaH₂PO₄ are shown in Figure 1a–c.

Compared with neat MOC paste, the initial setting times and final setting times of MOC slurries increased with the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄. Meanwhile, with the increasing addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ from 0.0 wt.% to 2.0 wt.%, the initial setting times and final setting times of MOC slurries obviously increased. For example, the initial setting times and final setting times of MOC slurries increased by 164.94% and 160.00% with the 0.5 wt.% addition of H₃PO₄. The initial setting times and final setting times of MOC slurries increased by 272.47% and 256.42% with the 1.0 wt.% addition of KH₂PO₄. The initial setting times and final setting times of MOC slurries increased by 351.69% and 313.68% with the 2.0 wt.% addition of NaH₂PO₄. As a result, the addition of H₃PO₄, NaH₂PO₄ and KH₂PO₄ significantly prolonged the setting time of MOC.

The comparation of three types of MOC in terms of setting time is shown in Figure 1d. With the addition of 2.0 wt.% H₃PO₄, the initial setting time and final setting time of MOC were 1771 min and 1965 min, respectively. The initial setting time and final setting time of MOC were 1790 min and 1974 min with the addition of 2.0 wt.% KH₂PO₄. When adding 2.0 wt.% NaH₂PO₄, the initial setting time and final setting time of MOC were 1739 min and 1963 min. Among them, the addition of H₃PO₄ and KH₂PO₄ had similar effects on prolonging the setting time of MOC, while NaH₂PO₄ had the smallest effect on delaying the setting time of MOC. When the addition dosage was all 2.0 wt.%, the delayed effect of H₃PO₄, KH₂PO₄ and NaH₂PO₄ on the setting time was as follows: KH₂PO₄ > H₃PO₄ > NaH₂PO₄.

3.2. Compressive Strength of MOC Slurries

The compressive strengths of MOC with different dosages of H₃PO₄, KH₂PO₄ and NaH₂PO₄ are shown in Figure 2a–c.

From Figure 2a, the 3-day and 14-day compressive strength of MOC samples with 0.5 wt.%, 1.0 wt.% and 2.0 wt.% H₃PO₄ added are 41.00 MPa, 32.16 MPa and 15.93 MPa and 55.45 MPa, 47.66 MPa and 32.86 MPa, respectively. Compared with the neat MOC sample, the compressive strength of the MOC samples with 0.5 wt.%, 1.0 wt.% and 2.0 wt.% H₃PO₄ addition decreased by 3.96%, 24.67% and 62.68% at 3 days, respectively. The compressive strength at 14 days decreased by 16.1%, 27.89% and 50.28%, respectively. Therefore, the addition of H₃PO₄ reduces the compressive strength of MOC. And the decrease in compressive strength becomes more pronounced with the increased content of H₃PO₄.

A shown in Figure 2b, the 3-day compressive strength of MOC samples with the KH₂PO₄ addition amounts of 0.5 wt.%, 1.0 wt.% and 2.0 wt.% were 51.36 MPa, 34.24 MPa and 20.20 MPa, respectively. The 14-day compressive strength was 53.37 MPa, 40.11 MPa and 43.80 MPa, respectively. Compared with the neat MOC sample, the compressive strength of the MOC samples with 0.5 wt.%, 1.0 wt.% and 2.0 wt.% KH₂PO₄ addition decreased by 10.31%, 19.79% and 52.68% at 3 days, respectively. The compressive strength at 14 days decreased by 19.25%, 39.31% and 33.73%, respectively. Therefore, adding KH₂PO₄ also reduces the compressive strength of MOC.

The compressive strengths of MOC samples with NaH₂PO₄ addition at a dosage of 0.5 wt.%, 1.0 wt.% and 2.0 wt.% were 41.00 MPa, 32.16 MPa and 15.93 MPa at 3 days and 55.45 MPa, 47.66 MPa and 32.86 MPa at 14 days, respectively. Compared with the neat MOC specimens, the compressive strength of the specimens with NaH₂PO₄ added at 0.5 wt.%, 1.0 wt.% and 2.0 wt.% decreased by 17.57%, 12.11% and 43.38%, respectively, at 3 days. The compressive strength at 14 days decreased by 19.81%, 25.72% and 33.73%, respectively. Therefore, adding NaH₂PO₄ also reduces the compressive strength of MOC.

As a result, the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ obviously decreases the compressive strength. Additionally, the decreasing effect of H₃PO₄, KH₂PO₄ and NaH₂PO₄ on the compressive strength is more serious with the increasing dosage. However, the compressive strength of MOC was higher than 30.00 MPa with the addition of 2.0 wt.% phosphoric acid and its phosphate after 14 days of air curing.

3.3. Water Resistance of MOC Slurries

The softening coefficients of MOC slurries with different dosages of H₃PO₄, KH₂PO₄ and NaH₂PO₄ are shown in Figure 3 at different immersion ages. It is clear to see that the softening coefficients of MOC slurries were all higher than neat MOC after the incorporation of H₃PO₄, KH₂PO₄ and NaH₂PO₄. That is to say that the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ can enhance the water resistance of MOC.

As can be seen from Figure 4, the addition of 1.0 wt.% H₃PO₄ obviously improved the water resistance of MOC after 14 immersion days. The softening coefficient of MOC with 2.0 wt.% H₃PO₄ was 1.2 after water immersion for 14 days, which was 3.44 times higher than the neat MOC. Additionally, we can clearly see that the optimal dosage of H₃PO₄ is 2.0 wt.% for the MOC.

As illustrated in Figure 4, the softening coefficients of MOC samples incorporated with KH₂PO₄ and NaH₂PO₄ after a 14-day immersion period are comparable. Furthermore, their softening coefficients exhibited a trend of initially increasing followed by decreasing as the inclusion amount rose. The optimal concentrations of KH₂PO₄ and NaH₂PO₄ were determined to be 1%. Through the comparation, it was found that the enhancement effects of H₃PO₄, KH₂PO₄ and NaH₂PO₄ on the softening coefficient of MOC were different. Overall, adding H₃PO₄ had the best effect on the softening coefficient. The water resistance manifested as H₃PO₄ > KH₂PO₄ > NaH₂PO₄.

3.4. XRD Patterns

The XRD patterns of MOC with H₃PO₄, KH₂PO₄ and NaH₂PO₄ cured in air for 14 days are shown in Figure 4a–c, and the corresponding quantitative mineralogical compositions are presented in Figure 4d.

The primary crystallized hydration product present in all of the MOC specimens was the 518 phase, regardless of the incorporation of H₃PO₄, KH₂PO₄ or NaH₂PO₄. The 518 phase contributes predominantly to the mechanical strength of MOC [37,38,39]. In addition, compared with the neat MOC sample, the content of the 518 phase in the MOC specimens with the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ was relatively low. This proves that the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ can reduce the early mechanical strength of MOC. However, the content of the 518 phase significantly increased after 14 days of immersion in water, indicating a significant improvement in its water resistance.

3.5. SEM Images

The microstructure morphology has a direct impact on the mechanical properties of MOC slurries. Figure 5 shows SEM images of neat MOC, MOC with 2.0 wt.%. H₃PO₄, MOC with 2.0 wt.% KH₂PO₄ and MOC with 2.0 wt.% NaH₂PO₄ after 14 days of air curing. From Figure 5a, it can be seen that the 518 phase in the MOC without admixtures was covered with slender rod-shaped crystals, with clear edges and coarse shapes in structure. The rod-shaped crystals interlaced and overlapped with each other, resulting in the higher strength of the MOC. After modification with H₃PO₄, the 518 crystals transformed into block and flake shapes, with wide and smooth shapes and blurred edges. After modification with KH₂PO₄, the 518 crystals of the MOC changed to sheet-like shapes, with wide and smooth shapes and gradually blurred edges. After modification with KH₂PO₄, the 518 crystals in the MOC also became large and smooth. However, compared with the H₃PO₄ MOC and KH₂PO₄ MOC, the 518 phase crystals of the KH₂PO₄ MOC had relatively clear edges and lower edge blurring.

Figure 6 shows the SEM images of neat MOC, 2.0 wt.% H₃PO₄ MOC, 2.0 wt.% KH₂PO₄ MOC and 2.0 wt.% NaH₂PO₄ MOC soaked in water for 14 days. From Figure 6a, it can be seen that after being soaked in water for 14 days, the surface of the matrix was mainly composed of a large number of leaf shaped crystals, with loose and porous structures. Water invaded the interior of the cement along the pores, decomposing the 518 phase into Mg(OH)₂ and MgCl₂, resulting in poor mechanical properties of MOC after immersion [29,30], and showing poor water resistance. In Figure 6b, it is found that the matrix surface of MOC with H₃PO₄ added was mainly composed of a large number of sharp rod-shaped crystals after being soaked in water for 14 days. The rod-shaped crystals overlapped and crosslinked with each other, indicating that the addition of H₃PO₄ changes the morphology of MOC. By comparing Figure 5 and Figure 6, we can observe the microstructure changes in MOC samples before and after immersion with H₃PO₄, KH₂PO₄ and NaH₂PO₄. It can be seen that the structural morphology of MOC underwent certain changes after immersion. But the structure mainly contained rod-shaped crystals. It can be observed that amorphous substances attached to the crystals on the surface and filled some gaps between the crystals, thus preventing the contact between internal crystals and water. This can thereby improve the water resistance of MOC.

4. Discussion

On the whole, the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ prolonged both the initial and final setting time of MOC (Figure 1). The acid group of H₃PO₄, KH₂PO₄ and NaH₂PO₄ can absorb on the surface of MgO particles to form Mg₃(PO₄)₂. This further restricts the hydration of MgO particles. H₃PO₄, KH₂PO₄ and NaH₂PO₄ also form coordination bonds with Mg²⁺ in phase 5, which delays the hydration and setting time of MOC cement. Although H₃PO₄, KH₂PO₄ and NaH₂PO₄ delay hydration, the hydration products are similar. XRD data show that phase 518 was formed in the cement formed by H₃PO₄, KH₂PO₄ and NaH₂PO₄. The retarded setting time match the results in the literature [40,41].

H₃PO₄, KH₂PO₄ and NaH₂PO₄ reduce the initial and later strength of MOC. Other researchers also have found that additives reduce the strength of MOC before 28 days [10,32]. The reason for the decreased compressive strength is the slowing down of the hydration rater of MOC. Additionally, the content of 518 phase in MOC specimens with the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ was relatively low, which proves the decrease in compressive strength. The compressive strength of MOC is directly related to its microstructure. And the contribution of blocky and flaky crystals to strength is much smaller than that of fibrous and needle-like crystals. Therefore, the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ significantly reduced the early compressive strength of MOC. The reason for this is that the addition of H₃PO₄, KH₂PO₄ or NaH₂PO₄ provides a certain amount of [PO₄]³⁻. The addition of [PO₄]³⁻ can transform the 518 crystals into low-crystallinity gel crystals, thereby decreasing the compressive strength of MOC [12].

The incorporation of phosphoric acid and phosphate into the MOC system demonstrated enhanced water resistance, likely attributable to the ionization reactions of phosphoric acid and phosphate within the MOC matrix. The specific ionization reaction is as follows:

H₃PO₄→H⁺ +H₂PO₄⁻(6)

H₂PO₄⁻→H⁺ +HPO₄²⁻(7)

HPO₄²⁻→H⁺ +PO₄³⁻(8)

Magnesium oxide undergoes a hydration reaction when exposed to water. The specific hydration reaction is as follows:

MgO + H₂O→Mg(OH)₂(9)

Mg(OH)₂→Mg²⁺ +2OH⁻(10)

The H⁺ generated from Equations (6)–(8) undergoes a neutralization reaction with the OH⁻ generated from Equation (10), as shown in Equation (11). The resulting PO₄³⁻ and Mg²⁺ generated from Equation (10) form a magnesium phosphate phase, as shown in Equation (12) [39]:

H⁺ +OH⁻→H₂O(11)

2PO₄³⁻ + 3Mg²⁺→Mg₃(PO₄)₂(12)

Due to the fact that the ionization equilibrium constant of HPO₄²⁻ is smaller than that of H₂PO₄⁻, when the initial molar concentration is the same, the H⁺ concentrations of H₃PO₄, KH₂PO₄ and NaH₂PO₄ in MOC are H₃PO₄ > KH₂PO₄ > NaH₂PO₄. KH₂PO₄ and NaH₂PO₄ are both strong electrolytes that are highly soluble in water. And the stability of K⁺ and Na⁺ is K ⁺ >Na⁺.

Research [33] shows that after immersion in H₃PO₄ solution, the 518 phase crystals are significantly reduced, and a large amount of phosphate (MgHPO₄·3H₂O) is generated. The generated MgHPO₄·3H₂O is slightly soluble in water, and its encapsulation on the surface of the sample can block the contact between water and 518 phase crystals, effectively preventing 518 phase hydrolysis and improving water resistance. Therefore, the phosphate generated by H₃PO₄ and Mg²⁺ in cement paste can be the main reason behind the prevention of the hydrolysis of 518 crystals. The magnesium phosphate formed by it covers the surface of the crystals, which can effectively block the invasion of water and improve the water resistance of MOC. Due to the fast setting and hardening rate of MOC, it is difficult for [PO₄]^3- and Mg²⁺ to migrate on the surface of the hydration product crystal phase, and it is difficult for a crystal phase to form with phosphate. The surface of the hydration product can be amorphous and irregularly distributed. In addition, the amount of H₃PO₄, KH₂PO₄ and NaH₂PO₄ is very small, so magnesium phosphate cannot be detected by XRD and other methods when added internally. In addition, after soaking in water for 14 days, the content of 518 phase increased relatively. Another reason for this may be that under this experimental design ratio, there was an excess of active magnesium oxide. When soaked in water, secondary hydration occurred, resulting in an increasing trend of 518 phase content in MOC.

Compared with the neat specimen, the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ changed the morphology of the main crystal 518 in MOC. Before soaking in water, the 518 phases in neat MOC appeared to have columnar shapes with clear edges and were interlaced with each other, exhibiting great compressive strength. After adding H₃PO₄, KH₂PO₄ and NaH₂PO₄ to MOC, the 518 crystals began to shrink in shape. It is suspected that this may be due to the hydrolysis of H₃PO₄, KH₂PO₄ and NaH₂PO₄, which provides [PO₄]³⁻ and reacts with Mg²⁺ in MOC to generate amorphous magnesium phosphate that covers the surface of 518 crystals. As magnesium phosphate salts are insoluble substances, their solubility is much lower than that of 518 crystals. This contributes to the good water resistance of MOC.

5. Conclusions

In this paper, to compare the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ as additives on MOC, the effects on the setting time, compressive strength, water resistance, phase composition and microscopic composition of MOC are examined. The main conclusions are as follows:

1. The addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ has a serious retarding effect on the initial setting time and final setting time of MOC. The delayed effect of H₃PO₄, KH₂PO₄ and NaH₂PO₄ on the setting time is as follows: KH₂PO₄ > H₃PO₄ > NaH₂PO₄.

2. The addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ obviously decreases the compressive strength. The decreasing effect of H₃PO₄, KH₂PO₄ and NaH₂PO₄ on the compressive strength is more serious with the increasing dosage. However, the compressive strength of MOC is higher than 30.00 MPa with the addition of 2.0 wt.% phosphoric acid and its phosphate after 14 days of air curing.

3. The addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ can enhance the water resistance of MOC. The optimal dosage of H₃PO₄ is 2.0 wt.% for the MOC, and the optimal concentrations of KH₂PO₄ and NaH₂PO₄ are determined to be 1%. The water resistance is manifested as H₃PO₄ > KH₂PO₄ > NaH₂PO₄.

4. The primary crystallized hydration product present in all of the MOC specimens is the 518 phase, regardless of the incorporation of H₃PO₄, KH₂PO₄ and NaH₂PO₄.

5. The addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ can transform the 518 crystals into low-crystallinity gel crystals, thereby improving the water resistance of MOC.

Author Contributions

All authors discussed and agreed upon the idea and made scientific contributions: writing—original draft preparation, Q.H. and W.Z.; experiment designing, Y.D. and S.W.; experiment performing, J.Z. and Z.Y.; data analysis, Q.H. and W.Z.; writing—review and editing, Q.H., B.C. and W.Z. All authors have read and agreed to the published version of the manuscript.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Setting time of different MOC slurries ((a) H3PO4, (b) KH2PO4, (c) NaH2PO4, (d) Setting time comparation of three MOC slurries).

Figure 2. The compressive strength of MOC slurries ((a) H3PO4, (b) KH2PO4, (c) NaH2PO4).

Figure 3. The softening coefficient of MOC slurries.

Figure 4. XRD patterns and crystalline phase contents of different MOC slurries cured in air for 14 days ((a) H3PO4, (b) KH2PO4, (c) NaH2PO4, (d) The crystalline phase contents of three MOC slurries).

Figure 5. SEM images of MOC air cured for 14 days with H3PO4, KH2PO4 and NaH2PO4.

Figure 6. SEM images of MOC immersed in water for 14 days with H3PO4, KH2PO4 and NaH2PO4.

Table 1

Chemical composition of light-burnt magnesia.

Component	MgO	MgOa	CaO	Al2O3	SiO2	Fe2O3	LOI
Mass fraction/wt.%	84.82	42.22	1.60	1.58	6.01	0.65	5.44

Table 2

The amount of modifier used in MOC.

Label of MOC	H3PO4	KH2PO4	NaH2PO4
W-14D	0	0	0
n% H3PO4-W-14D	0.5	0	0
	1.0	0	0
	2.0	0	0
n% KH2PO4-W-14D	0	0.5	0
	0	1.0	0
	0	2.0	0
n% NaH2PO4-W-14D	0	0	0.5
	0	0	1.0
	0	0	2.0

References

1. Sorrel, S. On a new magnesium cement. Comptes Ren-Dus-Acad. Des. Sci.; 1867; 65, pp. 102-104.

2. Deng, D.H.; Zhang, C.M. The formation mechanism of the hydrate phases in magnesium oxychloride cement. Cement Concr. Res.; 1999; 29, pp. 1365-1371.

3. Montle, J.F.; Mayhan, K.G. Magnesium oxychloride as a fire retardant material. JFF/Fire Retard. Chem.; 1974; 1, pp. 243-254.

4. Sugimoto, K.; Dinnebier, R.E.; Schlecht, T. Structure determination of Mg₃(OH)₅Cl₄H₂O (F5 Phase5) from laboratory powder diffraction data and its impact on the analysis of problematic magnesia floors. Acta Crtstallogr. B; 2007; 63, pp. 805-811. [DOI: https://dx.doi.org/10.1107/S0108768107046654]

5. Li, X.; Qiu, R.; Xue, F.; Fang, L.; Cheng, F. Effects of unreactive MgO and impurities in light burned MgO on the hydration process and performance of base magnesium sulfate cement. Constr. Build. Mater.; 2020; 240, 117854. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2019.117854]

6. Zhou, W.G.; Ye, Q.Q.; Shi, S.Q.; Fang, Z.; Gao, Q.; Li, J.Z. A strong magnesium oxychloride cement wood adhesive via organic–inorganic hybrid. Construct. Build. Mater.; 2021; 297, 123776. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2021.123776]

7. Zhou, X.M.; Li, Z.J. Light-weight wood-magnesium oxychloride cement composite building products made by extrusion. Construct. Build. Mater.; 2012; 27, pp. 382-389. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2011.07.033]

8. Hao, Y.H.; Li, Y.R. Study on preparation and properties of modified magnesium oxychloride cement foam concrete. Construct. Build. Mater.; 2021; 282, 122708. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2021.122708]

9. Gomes, C.M.; Garry, A.L.; Freitas, E.; Bertoldo, C.; Siqueira, G. Effects of rice husk silica on microstructure and mechanical properties of magnesium-oxychloride fiber cement (MOFC). Construct. Build. Mater.; 2020; 241, 118022. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2020.118022]

10. Deng, D.H. The mechanism for soluble phosphates to improve the water resistance of magnesium oxychloride cement. Cement Concr. Res.; 2003; 33, pp. 1311-1317. [DOI: https://dx.doi.org/10.1016/S0008-8846(03)00043-7]

11. Huang, Q.; Zheng, W.X.; Xiao, X.Y.; Dong, J.M.; Wen, J.; Chang, C.G. A study on the salt attack performance of magnesium oxychloride cement in different salt environments. Construct. Build. Mater.; 2022; 320, 126224. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2021.126224]

12. Li, Y.; Li, Z.J.; Pei, H.F.; Yu, H.F. The influence of FeSO₄ and KH₂PO₄ on the performance of magnesium oxychloride cement. Construct. Build. Mater.; 2016; 102, pp. 233-238. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2015.10.186]

13. Aiken, T.A.; McPolin, D.; Russell, M.; Nanukuttan, S.; Bagnall, L. Exposure of magnesium oxide boards to various conditions for extended durations. Constr. Build. Mater.; 2021; 302, 124429. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2021.124429]

14. Osio-Norgaard, J.; Hayes, A.C.; Whiting, G.L. Sintering of 3D printable simulated lunar regolith magnesium oxychloride cements. Acta Astronaut.; 2021; 183, pp. 227-232. [DOI: https://dx.doi.org/10.1016/j.actaastro.2021.03.016]

15. Zhou, Z.; Chen, H.; Li, Z.; Li, H. Simulation of the properties of MgO-MgCl₂-H₂O system by thermodynamic method. Cement Concr. Res.; 2015; 68, pp. 105-111. [DOI: https://dx.doi.org/10.1016/j.cemconres.2014.11.006]

16. Li, S.T.; Liu, S.R.; Du, Y.S.; Huang, Q.; Qu, W.H.; Zheng, W.X. Effect of mineral-generated lithium slag on the properties of magnesium oxychloride cement. Crystals.; 2023; 13, 513. [DOI: https://dx.doi.org/10.3390/cryst13030513]

17. Huang, Q.; Zheng, W.X.; Xiao, X.Y.; Dong, J.M.; Wen, J.; Chang, C.G. Effects of fly ash, phosphoric acid, and nano-silica on the properties of magnesium oxychloride cement. Ceram. Int.; 2021; 47, pp. 34341-34351. [DOI: https://dx.doi.org/10.1016/j.ceramint.2021.08.347]

18. Li, J.Q.; Li, G.Z.; Yu, Y.Z. The influence of compound additive on magnesium oxychloride cement/urban refuse floor tile. Construct. Build. Mater.; 2008; 22, pp. 521-525. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2006.11.010]

19. Chau, C.K.; Li, Z. Microstructures of magnesium oxychloride. Mater. Struct.; 2008; 41, pp. 853-862. [DOI: https://dx.doi.org/10.1617/s11527-007-9289-y]

20. Zheng, W.; Xiao, X.; Wen, J.; Chang, C.; An, S.; Dong, J. Water-to-cement ratio of magnesium oxychloride cement foam concrete with caustic dolomite powder. Sustainability; 2021; 13, 2429. [DOI: https://dx.doi.org/10.3390/su13052429]

21. Záleská, M.; Pavlíková, M.; Pivák, A.; Marušiak, Š.; Jankovský, O.; Lauermannová, A.-M.; Lojka, M.; Antoncík, F.; Pavlík, Z. MOC Doped with Graphene Nanoplatelets: The Influence of the Mixture Preparation Technology on Its Properties. Materials; 2021; 14, 1450. [DOI: https://dx.doi.org/10.3390/ma14061450] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33809728]

22. Chandrawat, M.P.S.; Yadav, R.N.; Gupta, P.; Dagar, N. Impact of sodium silicate as an additive on the properties of magnesium oxychloride cement (MOC). Intern J. Chem. Sci.; 2011; 9, pp. 581-588.

23. Ge, M.L.; Ye, Q.Q.; Kang, H.Q.; Li, J.Z. Research progress of admixture-modified magnesium oxychloride cement. Bull. Chin. Ceram. Soc.; 2022; 41, pp. 1202-1210.

24. Li, K.; Wang, Y.S.; Yao, N.N.; Zhang, A.M. Recent progress of magnesium oxychloride cement: Manufacture, curing, structure and performance. Constr. Build. Mater.; 2020; 255, 119381. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2020.119381]

25. He, P.; Hossain, M.U.; Poon, C.S.; Tsang, D.C. Mechanical, durability and environmental aspects of magnesium oxychloride cement boards incorporating waste wood. J. Clean. Prod.; 2019; 207, pp. 391-399. [DOI: https://dx.doi.org/10.1016/j.jclepro.2018.10.015]

26. Misra, A.K.; Mathur, R. Magnesium oxychloride cement concrete. B Mater. Sci.; 2007; 30, pp. 239-246. [DOI: https://dx.doi.org/10.1007/s12034-007-0043-4]

27. Chuang, L.I.; Hongfa, Y.U.; Ying, L.I. Study on moisture absorption and efflorescence properties of magnesium oxychloride cement materials. Bull. Chin. Ceram. Soc.; 2011; 30, pp. 373-378.

28. Huang, Q.; Zheng, W.X.; Dong, J.M.; Wen, J.; Chang, C.G.; Xiao, X.Y. Influences of different bischofite on the properties of magnesium oxychloride cement. J. Build. Eng.; 2022; 57, 104923. [DOI: https://dx.doi.org/10.1016/j.jobe.2022.104923]

29. Lu, H.P.; Wang, P.L.; Jiang, N.X. Design of additives for water-resistant magnesium oxychloride cement using pattern recognition. Mater. Lett.; 1994; 20, pp. 217-223. [DOI: https://dx.doi.org/10.1016/0167-577X(94)90090-6]

30. Wu, C.Y.; Zhang, H.F. The effects of alumina-leached coal fly ash residue on magnesium oxychloride cement. Adv. Cem. Res.; 2013; 25, pp. 254-261. [DOI: https://dx.doi.org/10.1680/adcr.12.00019]

31. Chau, C.K.; Chan, J.; Li, Z.J. Influences of fly ash on magnesium oxychloride mortar. Cement Concr. Compos.; 2009; 31, pp. 250-254. [DOI: https://dx.doi.org/10.1016/j.cemconcomp.2009.02.011]

32. Deng, D.H. Effect of the phosphate anion on the stabilization of hydrate in magnesium oxychloride cement. J. Build. Mater.; 2002; 5, pp. 9-12.

33. Tan, Y.; Liu, Y.; Grover, L. Effect of phosphoric acid on the properties of magnesium oxychloride cement as a biomaterial. Cement Concr. Res.; 2014; 56, pp. 69-74. [DOI: https://dx.doi.org/10.1016/j.cemconres.2013.11.001]

34. Hu, C.; Xu, B.; Ma, H.; Chen, B.; Li, Z. Micromechanical investigation of magnesium oxychloride cement paste. Construct. Build. Mater.; 2016; 105, pp. 496-502. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2015.12.182]

35. Sglavo, V.M.; Genua, F.D.; Conci, A.; Ceccato, R.; Cavallini, R. Influence of curing temperature on the evolution of magnesium oxychloride cement. J. Mater. Sci.; 2011; 46, pp. 6726-6733. [DOI: https://dx.doi.org/10.1007/s10853-011-5628-z]

36. GB/T 1346-2011 Test methods for water requirement of normal consistency, setting time and soundness of the Portland cement; Standardization Administration (SAC) of the People’s Republic of China: Beijing, China, 2011.

37. Liu, P.; Dong, J.M.; Chang, C.G.; Zheng, W.X.; Liu, X.Q.; Xiao, X.Y.; Wen, J. Preparation of low-cost magnesium oxychloride cement using magnesium residue byproducts from the production of lithium carbonate from salt lakes. Materials; 2021; 14, 3899. [DOI: https://dx.doi.org/10.3390/ma14143899]

38. Chang, C.; An, L.; Zheng, W.; Wen, J.; Dong, J.; Yan, F.; Xiao, X. Research and engineering application of salt erosion resistance of magnesium oxychloride cement concrete. Materials; 2022; 14, 7880. [DOI: https://dx.doi.org/10.3390/ma14247880]

39. Chen, X.F.; Wang, L.M. Research on the mechanism of water resistance of magnesium oxychloride cement with phosphoric acid. Concrete; 2018; 2, pp. 68-71.

40. Chen, X.; Zhang, T.; Bi, W.; Cheeseman, C. Effect of tartaric acid and phosphoric acid on the water resistance of magnesium oxychloride (MOC) cement. Construct. Build. Mater.; 2019; 20, pp. 528-536. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2019.04.086]

41. Yu, H.F.; Li, S.T.; He, Q.Y.; Ding, X.Q. Research on long term strength and water resistance of MgO–SF–FA–MgCl₂–H₂O system cementitious materials. J. Chin. Ceram. Soc.; 2000; 28, pp. 33-37.