Considerable research has been conducted investigating the potential use of MgO-SiO₂ binder systems (Zhang et al., 2011, Zhang et al., 2014; Tran and Scott, 2017; Shah and Scott, 2019; Bhagath Singh et al., 2020). The poorly crystalline magnesium silicate hydrates (M-S-H) formed by the reaction between MgO and reactive silica sources provides binding characteristics similar to calcium silicate hydrates (C-S-H) formed in the pozzolanic reaction of supplementary cementitious materials with calcium hydroxide. The structural form of the two are somewhat different with C-S-H typically having a dreierketten structure whereas M-S-H has a layered silicate structure similar to clay minerals (Bernard et al., 2017). The specific characteristics of the M-S-H is affected by the physical and chemical properties of the constituent raw materials (MgO and SiO₂), mix design and curing conditions (Jia et al., 2017; Tran and Scott, 2017; Abdel-Gawwad et al., 2018; Li et al., 2018; Sonat and Unluer, 2019; Dhakal et al., 2021; Shah and Scott, 2021b). MgO-SiO₂ binders have shown potential for applications in: waste immobilization/encapsulation where a lower pH (< 10.5) environment is required; as refractory castables because of high melting temperature; and as building materials due to excellent mechanical properties (Zhang et al., 2012, Zhang et al., 2014; Walling et al., 2015; Walling and Provis, 2016; Tran and Scott, 2017).

The MgO required for such a binder system is typically obtained from the calcination of magnesite (MgCO₃) in a temperature range of 700–1000°C. The reserves of magnesite are sporadically distributed around the world with the majority of the deposits located in a small numbers of countries. The identified total reserves of magnesite is around 13 billion tonnes while the annual global production of Portland cement (PC) is ∼3 billion tonnes (Shand, 2006; UN Environment et al., 2018). If MgO-SiO₂ binder are to be used on a large scale and as a potential substitute for PC, it is essential to look for alternate sources of magnesium oxide. In addition, most of the studies investigated MgO-SiO₂ binders, use either silica fume or micro-silica as the reactive SiO₂ source (Jin and Al-Tabbaa, 2014; Zhang et al., 2014; Tran and Scott, 2017). The quantities of these SiO₂ sources are limited and most are already being used in the conventional cement industry. The use of fly ash and slag alongside MgO has also been reported. However, the low reactivity of fly ash and the presence of CaO in slag limits the effectiveness of the binder (Vandeperre et al., 2008; Jia et al., 2017; Tran, 2019). Alternatively, the use of calcined clay and rice husk ash as SiO₂ source has also been reported (Sonat and Unluer, 2019; Shah and Scott, 2021b, 2021a).

Ultramafic magnesium silicate minerals are abundantly available worldwide with reserves of over 10 trillion tonnes (Pacheco-Torgal et al., 2018). The use of magnesium silicate minerals has been proposed as one potential option for carbon sequestration. The sequestration of CO₂ using magnesium silicate minerals and production of MgO, as a potential commercial binder, is currently limited by the availability of an energy efficient industrial process (Matter and Kelemen, 2009; Sanna et al., 2013, 2014; Vlasopoulos and Cheeseman, 2013; Walling and Provis, 2016; Gartner and Sui, 2018; UN Environment et al., 2018). Magnesium silicate minerals could however serve as an ideal feedstock to produce MgO-SiO₂ binders, if the MgO and SiO₂ could be extracted efficiently. Unlike magnesite for MgO or limestone for PC that releases chemical CO₂ in raw materials during calcination, magnesium silicate minerals do not have any inherent CO₂ associated with them. Therefore, it possesses the potential to be a truly carbon negative binder. The cement industry accounts for approximately 8% of global CO₂ emissions (Kajaste and Hurme, 2016; Gartner and Sui, 2018), and the successful development of a method to extract oxides from magnesium silicate minerals would bring significant benefits.

The authors have recently demonstrated an energy-efficient method that could be used to separate MgO and SiO₂ from olivine, a form of magnesium silicate minerals (Scott et al., 2021). The process uses a combination of acid digestion and electrolysis techniques for the successful extraction of oxides. The binding characteristics of the extracted oxides however have not been investigated yet. In this proof of concept study, the efficacy of the oxides extracted from olivine used to produce a MgO-SiO₂ binder system are investigated. The MgO-SiO₂ binder characteristics are determined using a range of techniques including isothermal calorimetry, XRD, FTIR, and compressive strength.

Ground olivine with an average particle size of 30 um was used in the study. The olivine was mixed with 2 M HCl in a ratio of 1:10 (% W/V) in a beaker and placed on a hot plate at a temperature of 60°C and stirred continuously. After 2 h of digestion, the solution was left to stand for one hour and subsequently decanted to remove any unreacted olivine. The decanted solution primarily consists of silica, magnesium, iron, and chloride ions as confirmed by ICP-MS testing. The components were individually separated by increasing the pH in steps. Firstly, magnesium hydroxide (0.2% w/v) was added in the decanted solution to raise the pH to >3.1. The increase in pH results in condensation and polymerization of silicic acid (Si(OH)₄) forming silica gel (Besbes et al., 2009). The silica formed through this process is analogues to the sol-gel method used to extract nano-silica from sodium metasilicate (Gorrepati et al., 2010; Katoueizadeh et al., 2020). The polymerized silica was filtered out and repeatedly washed with water to remove any excess acid or chloride salts. Thereafter, 2 M NaOH was added in the filtered solution to raise the pH to 6.5 to 7. The increase in pH leads to precipitation of iron hydroxide, which was subsequently removed through either filtration or centrifugation. The filtered solution was electrolyzed to form magnesium hydroxide at the cathode. Approximately 35 g of Mg(OH)₂ was obtained from 100 g of olivine, of which 1 g was added during the silica precipitation stage. Hydrogen gas (cathode) and chlorine gas (anode) could be recovered to produce HCl for reuse in the digestion process. The developed process could result in significant CO₂ savings, even if fossil fuels were used as the energy source, as no chemical CO₂ is released in the process. In the final stage of the binder component production, the Mg(OH)₂ obtained from electrolysis was calcined at 500°C for 1 h to produce MgO.

The silica (RS) and MgO (RM) recovered from olivine using the extraction procedure was assessed for its binder characteristics. The performance of MgO-SiO₂ binder prepared from the recovered material was compared with commercially available MgO and silica fume using: isothermal calorimetry, XRD, FTIR, and SEM.

Olivine used for extraction of MgO and SiO₂ was sourced from Red Hills, New Zealand. Commercial MgO (CM) and Silica fume (SF) were obtained from Sibelco Australia and Sika New Zealand, respectively. The chemical composition of the olivine, recovered and commercial MgO and SiO₂ are given in Table 1. The high LOI of recovered silica is associated with the loss of water molecules trapped in polymerized silica network (Raza et al., 2018). Figure 1 shows the XRD patterns of the raw materials. The recovered and commercial MgO show similar characteristics with minor peaks of uncalcined brucite. The recovered silica has an amorphous hump analogous to the commercial silica fume with minor impurities of unreacted olivine (lizardite and forsterite).

The MgO-SiO₂ binders were prepared by mixing MgO and SiO₂ at 1:1 ratio by mass. The composition of the mixes investigated are presented in Table 2. Paste samples were prepared at a water to binder (w/b) ratio of one. This produced mixes with consistent workability and allowed the intrinsic hydration behaviour of the materials to be determined without the influence of any external agents such as superplasticiser.

Approximately 20 g of paste sample was placed in an isothermal calorimeter maintained at 20°C (Calmetrix I-Cal Flex) to measure the heat evolved during hydration in different mixes. The heat values were recorded every 5 min for 72 h. The remaining paste was poured into PVC vials (Diameter 20 mm, Height 100 mm) and cured until the age of testing in a room maintained at 20°C.

Paste samples were taken from the PVC vials after 3, 7, and 28 days for hydration study. The samples were subsequently stored in isopropanol for 7 days to arrest hydration followed by 3 days vacuum drying. The dried samples were ground with a mortar and pestle and used for further analysis. XRD scans were carried out in a range of 5 to 70° using Rigaku SmartLab Diffractometer (40 kV, 30 mA) installed with CuKα radiation. The FTIR spectra were obtained in the range of 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹ using Bruker Spectrometer Alpha II. The spectrum obtained was based on an average of 25 scans. The morphology of hydration products of the different mixes at 28 days were characterized using scanning electron microscopy. The vacuum dried fractured samples were coated with carbon prior to imagining using a JEOL 6400 in secondary electron mode with an accelerating voltage of 15 kV.

Mortar samples were also prepared for the mixes to determine compressive strength. A binder to sand ratio of 1:3 was kept constant for all the mixes. The recovered materials were found to have a very high water demand similar to silica fume. The water to binder ratio and superplasticizer addition therefore was varied to obtain consistent spread, using the drop table, of approximately 110 ± 10 mm between the mixes, similar to ASTM C109, (2010). 50 × 50 × 50 mm cube samples were cast and placed in an environmental controlled room maintained at a temperature of 20°C and 60% relative humidity for 24 h. The samples were then wrapped in plastic and stored at 20^oC until the age of testing.

This study investigated the hydration and microstructure features of MgO-SiO₂ binders produced using materials recovered from olivine. The characteristics of the binder produced were compared with binders made using commercially available materials. The major findings from the study can be summarized as follows:

• The recovered MgO and SiO₂ both showed a greater reactivity compared to commercial counterparts. The use of recovered silica in particular helped in enhancing the rate of hydration.

The results confirm that is possible to produce MgO-SiO₂ binders from magnesium silicate minerals with comparable results to more commonly available commercial materials.