Sustainable Calcination of Magnesium Hydroxide for Magnesium Oxychloride Cement Production

The $\mathrm{Mg}{(\mathrm{OH})}_2$ and ${\mathrm{MgCO}}_3$ precursors ($<5\%$ impurities), a commercial MgO powder derived from calcined dolomite used as a commercial reference, and magnesium chloride hexahydrate crystals (${\mathrm{MgCl}}_2·6{\mathrm{H}}_2\mathrm{O}$) of 98% purity were all obtained from Magnesia, Lüneburg, Germany. The chemical characteristics of the ${\mathrm{MgCO}}_3$, $\mathrm{Mg}{(\mathrm{OH})}_2$, and commercial magnesium oxide (C-MgO) were analyzed by wavelength dispersive X-ray fluorescence (WD-XRF) spectrometry (Axios mAX, Malvern Panalytical, Worcestershire, UK), and are listed in Table 1. The following abbreviations are used to denote the different MgO powders: MgC for MgO derived from ${\mathrm{MgCO}}_3$; MgH for MgO derived from $\mathrm{Mg}{(\mathrm{OH})}_2$; and C-MgO for the reference commercial MgO. In addition, the calcination temperature and calcination duration were added to the latter abbreviations. For example, MgH-650/1 refers to MgO from the calcination of $\mathrm{Mg}{(\mathrm{OH})}_2$ at 650°C for 1 h.

The preparation of MgO was based on the calcination of $\mathrm{Mg}{(\mathrm{OH})}_2$ ${\mathrm{MgCO}}_3$ precursor powders under a range of calcination temperature and calcination time combinations. In a typical synthesis, $50.00\pm 0.005\mathrm{g}$ of precursor powder was weighed into a ceramic crucible, transferred to a 3-L-capacity furnace operating under static air (Carbolite, Hope, UK) and calcined at temperatures of 650°C or 850°C for either 1 or 2 h at a ramp rate of $20°\mathrm{C}/\min$. The calcination conditions of the ${\mathrm{MgCO}}_3$ and $\mathrm{Mg}{(\mathrm{OH})}_2$ are included in Table 2.

The required amount of ${\mathrm{MgCl}}_2·6{\mathrm{H}}_2\mathrm{O}$ was first added in deionized water and, using a magnetic stirrer, dissolved until a clear solution was obtained. Selected samples of MgO based on chemical and physical properties were carried forward for the production of the MOC paste. Batches of typically 1.0 kg were prepared using a laboratory bench-top mixer by combining the ${\mathrm{MgCl}}_2·6{\mathrm{H}}_2\mathrm{O}$ solution and MgO powder for 5 min. $\mathrm{MgO}/{\mathrm{MgCl}}_2$ molar ratios of 3, 5, and 9, along with a constant $\mathrm{MgO}/{\mathrm{MgCl}}_2$ ratio of 10, were used in the preparation of all samples. The water to solids ratio (w/s) was initially fixed at 0.22 for all compositions studied. However, to achieve adequate and consistent paste rheology, the water content was altered as necessary. Following mixing, three prismatic $20\times 20\times 160\text{-}\mathrm{mm}$ specimens were cast for each sample, with vibration compaction being implemented to reduce the air-void content as much as practically possible. After casting, the specimens were air-cured for 7 days at a temperature of $25°\mathrm{C}\pm 1°\mathrm{C}$ and relative humidity of $60\%\pm 5\%$.

The $20\times 20\times 160\text{-}\mathrm{mm}$ specimens were utilized to conduct the three-point flexural testing first. Then, the broken remains were used for the compression testing. Both tests were carried out using a 50-kN universal testing machine (Instron 5960, High Wycombe, UK) at a loading rate of $12.7\mathrm{mm}/\min$. The XRD patterns of all samples were obtained using a Bruker D8-Advance X-ray diffractometer (Madison, Wisconsin) with Cu $\mathrm{K}\alpha$ radiation ($\lambda =1.5418Å$), with the operation voltage and current maintained at 40 kV and 40 mA, respectively. SEM images were obtained with a JEOL JSM-6330F (JOEL, Tokyo) operated at a beam energy of 20.0 kV. Particle-size analysis and specific surface area measurements were performed using a Mastersizer 2000 (Malvern Instruments, Worcestershire, UK). The rate at which MgO reacted with a dilute solution of acetic acid was used as a measure of its activity. An excess of MgO was used so that at the end of the reaction, the solution turned from acidic to basic and was detected by a color change employing a phenolphthalein indicator.

The major chemical compositions of the commercial MgO and the MgO obtained from calcination of ${\mathrm{MgCO}}_3$ and $\mathrm{Mg}{(\mathrm{OH})}_2$, analyzed by WD-XRF, are presented in Table 3. Chemical analysis of the commercial MgO (C-MgO) indicated a $96.21\%\pm 0.1\%$ MgO content, with calcium, silicon, iron, aluminum, and sulfur oxides appearing as the major impurities. Regarding the calcination of ${\mathrm{MgCO}}_3$ at the two different temperatures of 650°C and 850°C, i.e., samples MgC-650/2 and MgC-850/2, the quantity of MgO obtained was practically the same at 90.18% and 91.48%, respectively. The calcination temperature of $\mathrm{Mg}{(\mathrm{OH})}_2$ did not induce any change in the content of MgO produced because calcination at 650°C and 850°C for 1 h resulted in 98.99% and 98.91% of MgO, respectively. Increasing the calcination time from 1 to 2 h for samples of $\mathrm{Mg}{(\mathrm{OH})}_2$ calcined at 650°C or 850°C did not induce any significant change in the MgO content either, consistently remaining in the range of 98.60%–98.99%. What was apparent was the significant difference in MgO content between the calcined ${\mathrm{MgCO}}_3$ and $\mathrm{Mg}{(\mathrm{OH})}_2$. An increase of 8.8% in MgO content was measured for the $\mathrm{Mg}{(\mathrm{OH})}_2$ calcined at 650°C for 2 h (MgH-650/2) compared with the ${\mathrm{MgCO}}_3$ calcined at the same conditions, i.e., MgC-650/2.

Finally, a prolonged temperature hold of $\mathrm{Mg}{(\mathrm{OH})}_2$ calcination did not affect the efficiency of $\mathrm{Mg}{(\mathrm{OH})}_2$ transformation to $\mathrm{MgO}·\mathrm{Mg}{(\mathrm{OH})}_2$ calcined for 1 h (MgH-850/1) compared with $\mathrm{Mg}{(\mathrm{OH})}_2$ calcined for 6 h (MgH-850/6) produced an almost identical MgO content, i.e., 98.6% and 99.0%, respectively. The latter proves that a calcination regime of 1 h is sufficient to produce a high MgO yield.

Further features revealing the calcination performance of $\mathrm{Mg}{(\mathrm{OH})}_2$ compared with the commercial MgO are presented in Table 4. The results clearly indicate that the loss on ignition (LOI) measured for the calcined $\mathrm{Mg}{(\mathrm{OH})}_2$ samples were at least 97% lower than that of the commercial MgO. The latter is indicative of the presence of carbonate in the commercial MgO due to incomplete combustion of the ${\mathrm{MgCO}}_3$. It also confirms that the calcination conditions applied in the preparation of the MgO from $\mathrm{Mg}{(\mathrm{OH})}_2$ investigated in this study were sufficient because LOI was measured to be $<0.2\%$. It was also evident that the LOI decreased more so due to the increase in calcination temperature than calcination time. The specific surface of the MgH-650/1 was measured to be 35% higher than of the C-MgO, which gives an indication of the increased reactivity of the MgO produced in this study. The calcination time did not significantly affect the specific surface area and, thus, temperature remained as the primary process variable. The activity of the MgO increased with calcination temperature and thus, reactivity (inversely proportional to activity) decreased.

The variation of reactivity with temperature correlates well with that observed for the measured specific surface area. The lowest activity value and the highest specific surface area, which indicates the most reactive MgO, was obtained through calcination at 650°C and 1 h. Previous studies related to examining calcinations conditions of ${\mathrm{MgCO}}_3$ recommended a temperature of 900°C and time of 4 h (Birchal et al. 2001). Also, in the study of Zhu et al. (2013), MgO with the highest hydration conversion ratio was obtained with an activity of 40 s by calcinating ${\mathrm{MgCO}}_3$ at 600°C for 1.5 h. However, this was accomplished with a trade-off for a very low decomposition ratio of ${\mathrm{MgCO}}_3$.

The hypothesis that the calcination temperature is the more dominant influence in controlling the MgO content as opposed to the time of calcination is further supported by the XRD spectra presented in Fig. 1. It illustrates that $\mathrm{Mg}{(\mathrm{OH})}_2$ calcined at 650°C for both 1 and 2 h durations exhibited almost equivalent peak characteristics, i.e., peak intensities and relative peak positions.

Furthermore, almost identical peak characteristics were found to exist between the commercial MgO (C-MgO) and MgO obtained from calcination of $\mathrm{Mg}{(\mathrm{OH})}_2$ at 650°C for 1 h (MgH-650/1), as shown in Fig. 2. These findings provide beneficial implications for future practical usage purposes because they demonstrate that it is highly possible to use $\mathrm{Mg}{(\mathrm{OH})}_2$ to produce a high quality (chemically pure) MgO comparable with that of commercial MgO.

Further XRD spectra presented in Fig. 3 of two samples of MgO obtained from $\mathrm{Mg}{(\mathrm{OH})}_2$ and ${\mathrm{MgCO}}_3$ via the same calcination conditions (calcination temperature and duration) illustrate a vast difference in the identified peak intensities of MgO, as well as a varied chemical composition. More specifically, the identified MgO peaks belonging to the MgO sample (MgH-650/2) obtained from $\mathrm{Mg}{(\mathrm{OH})}_2$ have a significantly higher intensity than the MgO sample (MgC-650/2) obtained from ${\mathrm{MgCO}}_3$. It is also observable that contamination is present in the chemical composition of the sample MgC-650/2 in the form of silicon dioxide (${\mathrm{SiO}}_2$), indicating its reduced purity.

The energy requirements involved in the calcination of $\mathrm{Mg}{(\mathrm{OH})}_2$ and ${\mathrm{MgCO}}_3$ is another controlling factor in the production of MgO. During the calcination of ${\mathrm{MgCO}}_3$, the decomposition reaction involves the undesirable emittance of ${\mathrm{CO}}_2$, as expressed in the following thermal decomposition reaction:

At 650°C, 1 kg of ${\mathrm{MgCO}}_3$ was found to yield 0.501 kg of MgO and 0.499 kg of ${\mathrm{CO}}_2$. The latter calculation is based on the assumption that all MgO powders were dry before the calcination process, and residual water possibly adsorbed onto the magnesium oxide surfaces in a monolayer was ignored. Therefore, it can be calculated that for manufacturing purposes, 2.01 kg of ${\mathrm{MgCO}}_3$ is required to produce just 1.00 kg of MgO.

In comparison, $\mathrm{Mg}{(\mathrm{OH})}_2$ can be considered a much more energy-efficient precursor for obtaining MgO because it only emits ${\mathrm{H}}_2\mathrm{O}$, as expressed in the following thermal decomposition reaction:

Also at 650°C, 1 kg of $\mathrm{Mg}{(\mathrm{OH})}_2$ when fully decomposed yielded 0.685 kg of MgO and 0.315 kg of ${\mathrm{H}}_2\mathrm{O}$. More importantly, no ${\mathrm{CO}}_2$ originating from decomposition was emitted during calcination.

The energy required to raise the temperature of the hydroxide solids from ambient temperature (298 K) to the decomposition temperature of 350°C (623 K) can be calculated using the specific heat capacity of magnesium hydroxide at 600 K (1.78 kJ/kg K) as follows:

Once the solids are at decomposition temperature, the energy required to effect the decomposition can be calculated from the enthalpy of decomposition, which at 600 K is $\mathrm{1,304}\mathrm{kJ}/\mathrm{kg}$

The total energy required to decompose 1 kg $\mathrm{Mg}{(\mathrm{OH})}_2$, which will lead to 0.685 kg of MgO and water vapor, is the sum of $893+396=\mathrm{1,289}\mathrm{kJ}$ (1,221 Btu). The energy needed to be expended to produce 1 t of MgO will, therefore, be 1,881 MJ (1.78 MBtu).

Performing the same latter calculations for ${\mathrm{MgCO}}_3$ using a specific heat capacity of $116.2\mathrm{J}/\mathrm{mol}·\mathrm{K}$ yields an energy requirement of $999\mathrm{kJ}/\mathrm{kg}$. The enthalpy of decomposition of ${\mathrm{MgCO}}_3$ at 650°C is $\mathrm{1,284}\mathrm{kJ}/\mathrm{kg}$, which is the energy required for the second step. This gives a total energy requirement of $999+\mathrm{1,284}=\mathrm{2,283}\mathrm{kJ}$ (2,163 Btu) to decompose 1.0 kg of ${\mathrm{MgCO}}_3$, or $2.28\times {10}^6\mathrm{kJ}$ per 1 t [2,283 MJ (2.16 MBtu)].

This proves that as well as the significant benefit of not emitting ${\mathrm{CO}}_2$, the calcination process of $\mathrm{Mg}{(\mathrm{OH})}_2$ is 17.6% less energy intensive than that of ${\mathrm{MgCO}}_3$ and yields 26.8% more MgO.

The morphological characteristics of the MgO samples Mg-H-650/1, Mg-H-850/1, and C-MgO were also observed using the electron microscope and are presented in Fig. 4. Fig. 4(a) of MgH-650/1 shows a largely balanced particle-size distribution with the mean particle size being measured as 6.54 μm. Most of the MgH-650/1 particles are distinctly separate from each other and did not agglomerate during calcination. Avoiding agglomeration would essentially provide for more contact area, thus leading to a higher rate of reaction. The morphology of sample MgH-850/1 seen in Fig. 4(b), however, is vastly different; MgH-850/1 particles have agglomerated due to a higher calcination temperature, resulting in an increased mean particle size of 8.056 μm. Moreover, it can be seen from Fig. 4(c) that the particle distribution of C-MgO is largely irregular, with particle sizes ranging from small (approximately 2–3 μm) to large (approximately 15 μm). Also, agglomeration of C-MgO particles is similar to that observed for MgH-850/1. As discussed previously, this would result in a larger mean particle size and lower surface area because sample C-MgO had both the smallest specific surface area of 1.07 m²/g and largest mean particle size of 14.77 μm.

The morphologies of the MgO powders illustrated through observations gathered from the SEM images corresponded well with the particle characterization mentioned in Table 4. The surface structure of the MgH-650/1 particles provides an ample contact area for a rapid reaction. Therefore, it is expected that a MgO such as MgH-650/1 possessing a large surface area and low particle size, in combination with appropriate molar ratio formulation, should ultimately produce a MOC binder with the highest compressive strengths.

Based on the aforementioned theory, because samples MgH-650/1 and MgH-850/1 have significantly higher surface areas compared with sample C-MgO, they are believed to be more reactive and thus have the potential to produce a MOC binder with a higher early strength.

The mixtures of produced MOC binder, made with different molar ratios of $\mathrm{MgO}/{\mathrm{MgCl}}_2$ and different variants of MgO, can be seen in Table 5. In the MOC sample name, the number following M denotes the molar ratio of $\mathrm{MgO}/{\mathrm{MgCl}}_2$ and the number following H denotes the molar ratio of ${\mathrm{H}}_2\mathrm{O}/{\mathrm{MgCl}}_2$.

The flexural strengths of the C-MgO, MgH-650/1, and MgH-850/1 samples are summarized in Fig. 5. The error bars for the flexural strength correspond to measured standard deviation in testing three prisms of each sample. The results show that as the molar ratio of $\mathrm{MgO}/{\mathrm{MgCl}}_2$ is increased, the flexural strength of the binder decreases. Sample MgH-650/1-M3H10 had the highest flexural strength of 38.6 MPa, whereas mixture C-MgO-M9H10 had the lowest flexural strength of 0.7 MPa. It was also found that a significant decrease of 19.0 MPa in flexural strength occurred for sample MgH-650/1 as the molar ratio of $\mathrm{MgO}/{\mathrm{MgCl}}_2$ increased from 3 to 5. However, ultimately, the highest flexural strength was still attained by sample MgH-650/1-M9H10 at 14.2 MPa. Moreover, as the molar ratio of $\mathrm{MgO}/{\mathrm{MgCl}}_2$ increased from 3 to 5 for samples C-MgO and MgH-850/1, the flexural strength exhibited only a small decrease of 2.5 and 0.9 MPa, respectively. However, for the same samples, a much more significant decrease in flexural strength takes place upon increasing the molar ratio of $\mathrm{MgO}/{\mathrm{MgCl}}_2$ from 5 to 9. This can be equated to an average reduction of 8.1 and 5.1 MPa per additional 2 moles of MgO for samples C-MgO and MgH-850/1, respectively.

As mentioned previously, evidence suggests that MgO samples MgH-650/1 and MgH-850/1 are more reactive than sample C-MgO and should therefore theoretically produce a MOC cement with a high early strength. This was found to be true throughout all molar ratios of $\mathrm{MgO}/{\mathrm{MgCl}}_2$. However, it was also found that samples with higher molar ratios of $\mathrm{MgO}/{\mathrm{MgCl}}_2$ exhibited more surface cracking, which would also inevitably result in a lower flexural strength because large defects such as cracks make the sample more susceptible to failure. The differences between cracking patterns in the MOC cement due to a varying molar ratio of $\mathrm{MgO}/{\mathrm{MgCl}}_2$ is demonstrated through the comparison of MOC samples C-MgO-M3H10 and C-MgO-M9H10 shown in Fig. 6. It is evident that the sample C-MgO-M9H10 has significant defects and cracks [Points a and b in Fig. 6(a)], whereas C-MgO-M3H10 has only minor defects [Points a and b in Fig. 6(b)].

The compressive strengths of the C-MgO, MgH-650/1 and MgH-850/1 samples after 7 days of air curing are presented in Fig. 7. The error bars for the compressive strength correspond to measured standard deviation in testing three cubes of each sample. The results show that as the molar ratio of $\mathrm{MgO}/{\mathrm{MgCl}}_2$ was varied between 3 and 9, the compressive strengths of the C-MgO and MgH-650/1 samples increased. Sample MgH-650/1 attained the highest compressive strengths of 71.8, 93.0, and 111 MPa at molar ratios of 3, 5, and 9, respectively. However, the sample of MgH-850/1, although showing a dramatic increase in compressive strength with the transition of the $\mathrm{MgO}/{\mathrm{MgCl}}_2$ molar ratio from 3 to 5, exhibited a sharp fall in compressive strength at a molar ratio of 9. It is believed that the compressive strengths exhibited are the result of the particular phase assemblages produced, and the subsequent microstructure of the MOC binder. It is highly likely that the high specific surface area and thus reactivity of the MgO obtained from the calcination at 650 for 1 h, i.e., MgH-650/1, permitted for such high compressive strength to be obtained.

Moreover, it is thought that specimens with a higher compressive strength have a phase composition consisting dominantly of phase-5 hydration products, which is the most significant source of strength in the MOC system.

The influence of different MgO powder properties, namely the chemical composition (chemical purity), particle-size properties, and reactivity, on the microstructure was investigated and hence linked to the mechanical properties of MOC cement. The XRD spectra and SEM images of MOC mixtures formed from C-MgO are presented in Figs. 8 and 9, respectively. The XRD results illustrate that mixtures of C-MgO-M5H10 and C-MgO-M9H10 largely contain the favorable phase-5 reaction product. However, sample C-MgO-M9H10 had a much higher magnitude of unreacted MgO present than C-MgO-M5H10. This is due to C-MgO-M9H10 containing almost double the molar ratio of $\mathrm{MgO}/{\mathrm{MgCl}}_2$, leaving a significant amount of unconsumed MgO. Also, the mixture with the lowest $\mathrm{MgO}/{\mathrm{MgCl}}_2$ molar ratio of 3, i.e., C-MgO-M3H10, was detected to have the highest content of phase-3 crystals, fewer phase-5 crystals, and no unconsumed MgO.

The XRD spectra are shown to be in strong agreement with the acquired SEM images, which can be seen in Fig. 9. It can be observed from Fig. 9(a) that the mixture C-MgO-M3H10, consisted mostly of a homogenous distribution of phase-3 crystals without any unconsumed MgO. Fig. 9(b) of mixture C-MgO-M5H10 reveals that a significant amount of unconsumed MgO was detected. However, the denser microstructure also was observed to contain an abundance of phase-5 crystals appearing as scroll-tubular whiskers, explaining why the C-MgO-M5H10 sample produced a MOC binder with higher compressive strength than the C-MgO-M3H10 sample, namely because phase-5 crystals are known to be the primary contributors of mechanical strength (Chau and Li 2008b).

The XRD spectra and SEM images of MOC samples formed from MgH-850/1 are presented in Figs. 10 and 11, respectively. Again, the results show good agreement with each other, with the phase assemblages identified in the XRD, reflected accurately in the SEM images. Notably, it can be observed from the XRD spectra that a substantial content of $\mathrm{Mg}{(\mathrm{OH})}_2$ is detected in the sample with the highest molar ratio of $\mathrm{MgO}/{\mathrm{MgCl}}_2$, i.e., MgH-850/1-M9H10. This is due to the large molar ratio of ${\mathrm{H}}_2\mathrm{O}/{\mathrm{MgCl}}_2$ that was used (molar ratio of $10+2.78$ moles of additional water to correct the workability of the paste).

In the SEM images in Fig. 11, sample MgH-850/1-M3H10 is observed to contain mostly phase-3 crystals [shown in Fig. 11(a)], which then go on to disappear with the increase of the $\mathrm{MgO}/{\mathrm{MgCl}}_2$ molar ratio from 3 to 5 and get replaced with phase-5 crystals, as shown in Fig. 11(b). Fig. 11(c) shows an abundant presence of sheetlike $\mathrm{Mg}{(\mathrm{OH})}_2$ crystals and amorphous phase-5 crystals of a significantly less compact nature, appearing to be poorly formed with no evidence of intergrowth as observed in the MgH-850/1-M5H10 sample. The formation of $\mathrm{Mg}{(\mathrm{OH})}_2$ is shown to be undesirable in the MOC system as the phase-5 intergrowth process is hindered and a less homogenous binder matrix is formed. The detection of the $\mathrm{Mg}{(\mathrm{OH})}_2$ fits with the reduced compressive strength measured for the MgH-850/1-M9H10 sample presented in Fig. 7.

Furthermore, the XRD spectra and SEM images of MOC samples formed from MgH-650/1 using the $\mathrm{MgO}/{\mathrm{MgCl}}_2$ molar ratios of 3 and 5 are shown in Figs. 12 and 13, respectively. The XRD results reveal that unlike the MgH-850/1-M3H10 sample, which contained phase-3 crystals, for the equivalent molar ratio sample made with MgH-650/1, phase-3 crystals were not detected. Phase-5 crystals appeared to be the most prevalent in both MgH-850/1-M3H10 and MgH-850/1-M5H10 samples.

The SEM image in Fig. 13(a) reveals that the phase-5 crystals in sample MgH-850/1-M3H10, although still broken-tipped and interlocked, possess some characteristics of phase-3 crystals such as bulkiness and an irregular nature. It is understood that the transformation from phase-3 to phase-5 for a $\mathrm{Mg}{(\mathrm{OH})}_2$ calcined at 650°C for 1 h occurs at the $\mathrm{MgO}/{\mathrm{MgCl}}_2$ molar ratio of 3, rather than 5 as is the case for the MgO calcined at 850°C. The microstructure observed in Fig. 13(b) of sample MgH-850/1-M5H10, however, followed the corresponding XRD results in Fig. 12, revealing a tightly packed microstructure of interlaced phase-5 crystals coexisting with unconsumed MgO particles. There is also an evident intergrowth mechanism of phase-5 crystals taking place within the microstructure that makes it more compact, corroborating the compressive strength results of sample MgH-650/1 presented in Fig. 7. The microstructure may also be described as largely homogeneous if the unconsumed MgO particles are disregarded; the neatly broken-tipped phase-5 crystals are in abundance throughout the entirety of the structure.