Production and Characterization of Magnesium Cement Using Kaolinite Clay

Magnesium silicate cement is produced by mixing reactive magnesium oxide with a source of reactive silicon oxide. This cement is an interesting alternative to Portland cement due to the potential for low energy consumption, reduced greenhouse gas emissions, and use of renewable resources. Aluminosilicate-based raw materials can also be utilized to produce this type of cement. Thus, this work aims to study the use of kaolinite clay to produce magnesium aluminosilicate cement. The cement was produced by calcination of magnesium carbonate (MgCO 3 ) and kaolinite clay at a temperature of 800 °C for 45 minutes with MgCO 3 /kaolin mass ratios of 90/10, 80/20, and 70/30. Mortars and pastes samples were cured at 60 °C for 1, 3, and 7 days. The results showed that the maximum compressive strength (32.7 MPa) was yielded for the 70/30 mortar mix after 3 days of curing. Microstructural studies of pastes indicated the incorporation of aluminum for the formation of magnesium aluminosilicate hydrated products, in addition to the formation of brucite.


Introduction
Portland cement (PC) production emits a large amount of carbon dioxide (0.7-1.0 ton of carbon dioxide per ton of PC), consumes a high amount of non-renewable mineral resources (limestone and clay), and requires calcination at high temperature (1450 °C), reaching an energy consumption for its production of 109 kWh/t 1 .In addition to environmental issues, PC has a high pH (around 13), which is harmful to some applications.As an alternative, researchers have studied a new type of cement: magnesium silicate cement.This new cement is not usually produced by traditional clinker (calcined limestone and clay).Instead, the magnesiumbased cement can be obtained by magnesite (MgCO 3 ) calcination at an average temperature of 700-900 °C, obtaining magnesium oxide (MgO), and then mixing it with a source of reactive silica (SiO 2 ).When hydrated, the reaction products formed are Mg(OH) 2 and magnesium silicate hydrate (M-S-H).This type of cement yields very satisfactory mechanical results and has a considerably less alkaline pH, which reaches values between 9 and 10 2-4 .Among its advantages, in addition to using magnesium carbonate calcined at low temperatures (700-900 °C) and its lower pH values (around 9-10), its use is beneficial in several applications, such as waste encapsulation radioactive materials, refractory castables, cement-based matrices for natural fibers, and aggressive environments involving MgCl 2 and MgSO 4 solutions [5][6][7] .
Normally, silica fume is used as a source of silicon in MgO-SiO 2 systems.However, it is an expensive and poorly available material.In this context, calcined clays, which are rich in silica and alumina, prove to be an interesting alternative to produce this type of cement; nevertheless, only a few works have studied this type of cement.Bernard et.al 8 have investigated the aluminum influence on magnesium silicate hydrate, and authors confirmed that aluminum can be incorporated into its structure to form M-A-S-H type products.Dhakal et.al 9 produced the magnesium-silicate binder system using industrial MgO (Mg) and metakaolin (Mk) as a silica source, and the samples yielded a compressive strength of 50 MPa with a 1.25 Mg/Mk mass ratio.
The aim of this study is to present a new production methodology with the calcination of the raw materials in a furnace at 800 °C for 45 minutes to produce magnesium silicate cement using magnesium carbonate and kaolin, a material with high availability at low cost as a source of silicon and aluminum oxides.

Materials and Methods
The raw materials, magnesium carbonate (MgCO 3 ) and kaolin, were obtained from Brasilminas.Siliceous sand was used as fine aggregate (fineness modulus of 2.50 and apparent specific gravity of 2.42).Superplasticizer ViscoCrete HE-20 was supplied by Sika.The particle size distribution (Mastersizer 3000 Malvern equipment) of the raw materials was assessed and its chemical composition was sourced by Brasilminas.
In the production of magnesium aluminosilicate cement, the MgCO 3 and kaolin were homogenized in a ball mill for 30 minutes.The mixture was calcined in a laboratory furnace (18 kW, 200 L) for 45 minutes at a temperature of 800 ± 15 °C.*e-mail: jpedrobb92@gmail.comBatista JPB et al.
After, the generated cement was cooled in the open air, and then it was homogenized in a ball mill for 30 minutes.The MgCO 3 /kaolin mass proportions used were 90/10, 80/20, and 70/30.For characterization of magnesium aluminosilicate cements, the samples were assessed by X-ray diffraction (XRD, PANalytical Empyrean diffractometer quipped with an X'Celerator detector, Bragg Angle interval of 10-70°, Cu-Ka radiation, Ni filter, step of 0.01° and 10 s/step), Fourier transform infrared-attenuated total reflectance spectroscopy (FTIR-ATR, PerkinElmer spectrophotometer, spectrum range of 4000-400 cm -1 and spectral resolution of 1 cm -1 ) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS, TESCAN VEGA 3 XMU equipment, Oxford EDS 133 eV detector).
Mortars and pastes were produced to assess the compressive strength and microstructure, respectively.The water/binder mass ratio was 0.55, the fine aggregate/binder mass ratio (for mortars) was 2.00, and the mass of superplasticizer addition was 4% by mass of binder.The compressive strength of 40-mm cubic specimens was assessed after 1, 3, and 7 days of curing at 60 ºC in a universal testing machine (600 kN-load limit and load rate of 0.5 kN/s).Microstructural studies of pastes were analyzed by means of XRD, FTIR, and SEM/ EDS tests with the same parameters as the raw materials and anhydrous cement characterization.In the case of XRD and FTIR, the tests were performed for powdered samples; whereas, in the case of SEM/EDS, the test was performed for fractured samples with flat surface.
The nomenclature of the cements, pastes, and mortars are presented, respectively, in the "C-x/y", "P-x/y", and "M-x/y" formats, in which "x" is the value of the mass proportion of MgCO 3 and "y" is related to the mass proportion of kaolin.
Table 1 summarizes the information on the raw materials, magnesium aluminosilicate cements, pastes, and mortars.

Results and Discussion
Table 2 shows the chemical composition, bulk density, and mean particle size of MgCO 3 and kaolin.The magnesium carbonate is mainly composed by MgO (47.68 wt.%), has a bulk density of 2.9 g/cm 3 , and presents a mean particle size of 21.0 µm.The kaolin is mainly composed of SiO 2 (45.50 wt.%) and Al 2 O 3 (34.50wt.%).Its bulk density is 2.5 g/cm 3 and the mean particle size is 10.3 µm.
Figure 4 shows compressive strengths of mortars M-90/10, M-80/20, and M-70/30 after 1, 3, and 7 days of curing.Results showed that the optimum magnesium aluminosilicate cement was 70/30 after 3 days of curing, which yielded a compressive strength of 32.7 MPa.In addition, it could be noted that the values of compressive strength decreased at 7 days.This result agrees with Sonat et.al 11 , which observed a rapid gain in compressive strength values at early ages due to the acceleration of the dissolution of MgO and SiO 2 ,increasing M-S-H formation.However, the decrease in strength at ages higher than 3 days was due to instability and the potential disintegration of M-S-H after losing its interlayer water at high temperatures.Figure 5 shows the XRD patterns of pastes P-90/10, P-80/20, and P-70/30 after 3 days of curing at 60 °C.The identified crystalline phases attributed to reaction products were brucite (Mg(OH) 2 , PDF Card 0001637), hydrotalcite (Mg 6 Al 2 (OH) 2 (OH) 6 .3H 2 O, PDF Card 0014738), and magnesium silicate hydrate-like reaction product (M-S-H).In addition, Figure 6 shows the FTIR spectra of pastes P-90/10, P-80/20, and P-70/30 after 3 days of curing.The detected FTIR spectra bands for all pastes were 1012 cm -1 (Si-O-Si/Al  stretching mode), 1388 cm -1 (O-C-O asymmetric stretching mode), and 1637 cm -1 (O-H bending mode).Moreover, a peak at 3690 cm -1 (O-H stretching mode) was also identified.Brucite was identified in magnesium silicate cement due to the hydration of MgO, as shown in the XRD pattern and FTIR spectra (band at 1637 cm -1 and peak at 3690 cm -1 ).The presence of Al and Mg in the composition of cement leads to the formation of hydrotalcite, as identified in the XRD pattern and FTIR spectra (band at 1388 cm -1 ).M-S-H-like reaction product (probably M-A-S-H) presents a great tendency to form with the increase of SiO 2 /Al 2 O 3 content, and it was also identified in XRD pattern and FTIR spectra (shifting of the band at 1076 cm -1 from the anhydrous cement to 1012 cm -1 in pastes).Formation of M-A-S-H hydration products can result in densified microstructure, leading to suitable compressive strength.However, in a previous study with MgO-SiO 2 , an increase in M-S-H products not necessarily increased compressive strength 12,13 .Figure 7 shows the SEM images and EDS results for 90/10 (P-90/10), 80/20 (P-80/20), and 70/30 (P-70/30) pastes after 3 days of curing.It can be observed in the images with higher magnification (Figure 7d-f), that the P-80/20 and P-70/30 samples presented denser microstructure.In general, two types of structure can be seen, one with a more defined shape and smooth surfaces, attributed to brucite (identified as C, E, and H in Figure 7) and kaolinite (identified as B, F, and I in Figure 7) particles.In the another structure, a denser and more continuous shape was attributed to M-A-S-H product (identified as A, D, and G in Figure 7).The identification of the structures was based on the mean atomic content and atomic ratios by the semi-quantitative results of EDS test, which is shown in Table 3.The structure identified as brucite presented the highest values for the Mg/(Si+Al) ratio (7.4-13.1).On the other hand, the M-A-S-H product showed a variation of 0.7-1.6 for the Mg/(Si+Al) ratio.Finally, for the kaolinite particles, the ratio Mg/(Si+Al) was 0.2.It is important to highlight that the lower the amount of MgO in the cement, the greater the amount of silicon and aluminum in the M-A-S-H product.

Conclusions
The calcination process utilized for MgCO 3 /kaolin mass ratios of 90/10, 80/20, and 70/30 resulted in magnesium  aluminosilicate cements with suitable properties.Compressive strength results indicated that the optimum MgCO 3 /kaolin mass ratio was 70/30, yielding 32.7 MPa after 3 days of curing at 60 °C.The XRD test indicated that the calcination process was effective due to the absence of kaolinite and the formation of periclase in the cements.The EDS test confirmed that the aluminum atoms were incorporated to form M-A-S-H gels.Microstructural studies of pastes showed the formation of the following reaction products: M-A-S-H, Mg(OH) 2 , and hydrotalcite.Finally, this work enables the validation of low-energy cement using kaolin as a source of aluminosilicate.Table 3. Semi-quantitative mean atomic content (in %) and atomic ratios of pastes P-90/10, P-80/20, and P-70/30 after 3 days of curing at 60 °C (spots indicated in the Figure 7).

Table 1 .
Information on the sample types raw materials, anhydrous cement, paste and mortar: sample name, raw materials mass proportions, water/binder and fine aggregate/binder ratios, curing time and temperature, and tests performed.

Table 2 .
Chemical composition, bulk density, and mean particle size of MgCO 3 and kaolin.