applied sciences

Article The Conversion of to CaCO3 Considering Its Use for CCS Application as Cementitious Material

Kristoff Svensson *, Andreas Neumann, Flora Feitosa Menezes, Christof Lempp and Herbert Pöllmann Institute for Geosciences and Geography, Martin-Luther-University of Halle-Wittenberg, Von-Seckendorff-Platz 3, 06120 Halle (Saale), Germany; [email protected] (A.N.); fl[email protected] (F.F.M.); [email protected] (C.L.); [email protected] (H.P.) * Correspondence: [email protected]; Tel.: +49-345-55-26138

Received: 21 December 2017; Accepted: 8 February 2018; Published: 20 February 2018 Featured Application: Building materials and CCS.

Abstract: The reaction of wollastonite (CaSiO3) with CO2 in the presence of aqueous solutions (H2O) and varied temperature conditions (296 K, 323 K, and 333 K) was investigated. The educts (CaSiO3) and the products (CaCO3 and SiO2) were analyzed by scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), and differential scanning calorimetry with thermogravimetry coupled with a mass spectrometer and infrared spectrometer (DSC-TG/MS/IR). The reaction rate increased significantly at higher temperatures and seemed less dependent on applied pressure. It could be shown that under the defined conditions wollastonite can be applied as a cementitious material for sealing wells considering CCS applications, because after 24 h the degree of conversion from CaSiO3 to CaCO3 at 333 K was very high (>90%). As anticipated, the most likely application of wollastonite as a cementitious material in CCS would be for sealing the well after injection of CO2 in the reservoir.

Keywords: CCS; wollastonite; carbonation; CLUSTER; CS-; sealing

1. Introduction

To reduce the amount of CO2 in the atmosphere (currently 403 ppm [1]) several possibilities need to be considered. One possible technology being investigated and discussed is carbon capture and storage (CCS). The goal of CCS is reducing CO2 emissions from burning of fossil fuel by storing considerable amounts in suitable deep geologic reservoirs. The reservoirs considered for injection could be on- or offshore formations which could be in part already former exploited gas fields [2]. After injection, the sealing of wells in deep geologic formations being considered for CCS must be sustainable. Therefore, the choice of a tight and durable cement, used for sealing, is critical. The material has to resist the extreme conditions at the point of injection considering pressure, temperature, pH, and the composition of aggressive fluids (chloride rich brines, supercritical (sc) CO2 accompanied with NO2 and SO2). The high amount of CO2 and the fluids will have a considerable impact on cement properties. Hence, the term carbonation is occupied in the literature as a post hydration reaction of cementitious materials [3,4]. Ordinary Portland (OPC) cannot be used under these conditions of well boring due to their instability. The current study describes carbonation as a curing method to obtain carbonates as reaction products. In the present study, a literature review describing the influence of CO2 on conventional hydrated cements as an alteration reaction is presented. Studies on the reaction of natural wollastonite with CO2 as curing method under different pressures and temperatures were investigated.

Appl. Sci. 2018, 8, 304; doi:10.3390/app8020304 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 304 2 of 18

Although natural deposits are limited, wollastonite can be fabricated using a conventional cement rotary kiln production process. Wollastonite has a Ca:Si ratio of 1:1 and therefore during production a better CO2 balance than conventional cements with Ca:Si ratio 2:1 and 3:1 (Table1,[5]).

Table 1. Raw meal CO2-output of selected phases. [5].

Phase g of CO2 per g Phase Raw Materials

C3S 0.58 3 CaCO3/SiO2–OPC C2S Belite 0.51 2 CaCO3/SiO2 CS Wollastonite 0.38 CaCO3/SiO2 C3A 0.49 CaCO3/Al2O3 CA 0.28 CaCO3/Al2O3 CA2 0.17 CaCO3/Al2O3 C2F 0.32 CaCO3/Fe2O3 C2A 0.41 CaCO3/Al2O3 C4AF 0.36 CaCO3/Al2O3/Fe2O3 C3A3Cs Sulfoaluminate 0.22 CaCO3/Al2O3/CaSO4

Additionally, the CO2 will be consumed during the carbonation reaction thereby improving the CO2 balance by forming carbonates. The interaction of wollastonite and CO2 [6] was investigated within the joint BMWi (German Federal Ministry for Economic Affairs and Energy) research project CLUSTER [7]. The project CLUSTER focusses on two main topics:

(I) Potential impacts of temporal variations in CO2 stream composition and mass flow that occur in larger pipeline networks on transport, injection, and geological storage of CO2. (II) Flexibility of individual steps and the whole CCS process chain in response to this temporal variability [8].

In nature, wollastonite is formed during metamorphism. The present study considers the back reaction by carbonation of wollastonite forming CaCO3 and SiO2 as a base [6]. The carbonation reaction of wollastonite is described as extremely slow in absence of at low temperatures [9]. Yet at 333 K and ambient pressure the CCS reaction is fast (several hours up to one day) in presence of water [9]. Microstructure aspects of the carbonation were investigated by Villani et al. [10] and Sahu and DeCristofaro [11]. Both studies showed a carbonation from the surface to the center of the wollastonite grains, similar to the shrinking core model [12]. Wollastonite reacts in the presence of CO2 in aqueous conditions to and amorphous oxide:

H2O CaSiO3 + CO2 −−→ CaCO3 + SiO2(amorphous) (1) The chosen temperature (333 K) defined by the project CLUSTER [8] equates to the conditions in deep geological formations envisaged for CCS. Gartner and Hirao [9] and Longo et al. [13] found that the carbonation reaction strongly depends on the presence of water. Our experiments were performed under aqueous conditions, because aqueous solutions are expected to be present at the point of injection in wellbores.

1.1. Interaction of CO2 and Conventional Hydrated Cements

Carbonation (i.e., formation of CaCO3) in hydrated cements causes a decrease of the pH-value from approximately 13 to approximately 8. The carbonate (CC) mostly appears in form of calcite but also as vaterite and aragonite [3,4]. Carbonation is responsible for volume changes and formation of carbonates, hemicarbonates and monocarbonates, which leads to changes in porosity and permeability, and the formation of a network. Dissolution of phases also may occur [3,4]. Appl. Sci. 2018, 8, 304 3 of 18

Considering the chemical and mechanical stability for construction due to changes of the porosity, this process is very critical [3,4]. Therefore, extensive research on the influence of CO2 on OPC and wellbore cement was performed. Shi et al. [14] investigated the carbonation of white (wPc, CEM I 52,5 N) from Aalborg Portland A/S, Denmark, metakaolin (MK) and (LS) mortars. Their mortars were hydrated for 91 days and exposed to a controlled atmosphere with one percent CO2 at 293 K and 57% relative (RH) for 280 days. They found that the Portland cement had the highest CO2 binding capacity followed by limestone and metakaolin. Abdoulghafour et al. [15] and Luquot et al. [16] performed flow-through experiments with CO2 and brine on fractured Portland cement. Their experiments were carried out at 333 K and 10 MPa for more than 100 h. They found that the leakage potential is controlled by the aperture of the fracture. Ozyurtkan and Radonjic [17] investigated the effect of CO2 rich brine (two percent solution) on artificially fractured well-cement (class H) in a flow-through experiment over 30 and 100 days. After 100 days, a secondary fracture network was developed which was not present after 30 days. The secondary fractures caused higher leaching rates. The obtained data by Ozyurtkan and Radonjic [17] indicates a negative effect on porosity of cement under dynamic conditions which can be dependent on time. Yalcinkaya et al. [18], performed an experimental study on the dynamic alteration process of class-H wellbore cement caused by acidic brine. Their experiments confirmed leaching of Ca2+ from the reacted cement. Carey et al. [19] investigated the wellbore integrity and CO2-brine flow along the casing-cement. They performed flow-through experiments at 313 K and 14 MPa pore pressure for 394 h. The brine was a 50:50 mixture of scCO2 and 30,000 ppm NaCl-rich brine. They were able to confirm that degradation of Portland cement is slow. Barlet-Gouédard et al. [20] compared the behavior of a Portland cement with a Schlumberger CO2 resistant cement (SCRC) with expanding agent under CO2 geological storage environment (363 K, 28 MPa). They used brine containing 220 g/L NaCl. They focused on the expansion of the cement because fractures caused by the expansion are one possible occurrence of leakages. The simulated injection scenario presented by Barlet-Gouédard et al. [20] showed, that conventional Portland cement does not fulfill the requirements for the well integrity. Wigand et al. [21] performed flow-through experiments which simulated the of brine (1.65 M) and scCO2 from the interface between wellbore cement and caprock into a fracture-bearing Portland cement. The experiments were performed at 327 K with a pore pressure of 19.9 MPa and a confining pressure of 26.2 MPa for 174 h. They showed that the carbonation of the wellbore cement at first caused closing and blocking of pores and fractures but opened other fractures in the following process, which increased the permeability. The chemical interaction along the cement– interface with scCO2 was investigated by Nakano et al. [22]. They prepared a well composite sample consisting of casing, Portland cement and sandstone. The experiments were performed under 10 MPa and 323 K with a 0.5 M NaCl-brine. Their results indicate that the formation of carbonation layers act as a barrier to further CO2-attack. Zhang et al. [23,24] investigated the reaction of two different pozzolan-amended (Class F fly ash, with ~70% silica, alumina and Fe2O3, ~5% sulfate, and less than 20% CaO) wellbore cements (API RP 10B; 35 vol % pozzolan and 65 vol % pozzolan) being exposed to CO2 and H2S gas mixtures and brine (one percent NaCl) at 323 K and 15.1 MPa for 2.5, 9, 28 and 90 days. The samples of the pozzolan-amended wellbore cement with 35 vol % pozzolan showed higher resistance against H2S. Mito et al. [25] and Wolterbeek et al. [26] investigated the influence of CO2-induced reactions on the mechanical behavior of fractured wellbore cement (class-G Portland cement). They found that the reaction with CO2 did not produce further geomechanical weakening. In fact, samples with six weeks reaction time recovered peak-strength (83%) and increased frictional strength (15–40%). Kutchko et al. [27] investigated the reaction of hydrated well cement (class-H) with H2S-CO2 (21 mol % H2S) acid-gas for 28 days at 323 K and 15 MPa. The H2S added to the experimental setup resulted in significant formation of ettringite (interior region of the cement) and pyrite (carbonated rim of the cement). Although secondarily formed ettringite is known to cause cracking and strength loss, a mechanical damage caused by formation of ettringite was not found. Even the long-term Appl. Sci. 2018, 8, 304 4 of 18 cement corrosion in chloride-rich solutions was investigated by Bube et al. [27]. They performed leaching experiments and thermodynamic simulations on cement (OPC; CEM I 32,5 R; w/c ratio 2+ + + − 2− of 0.4) with brine (4.3 mol Mg , 0.41 mol Na , 0.54 mol K , 8.9 mol Cl , and 0.23 mol SO4 ) for 200 days. Comparing the results of laboratory and full-scale experiments, they were able to establish a link between the results of short-term laboratory experiments to long-term full-scale experiments, using thermodynamic equilibrium calculations. The above research shows that carbonation can negatively impact the sealing qualities of OPC wellbore cements. Therefore, alternative materials are required to ensure wellbore integrity during the subsurface storage of CO2. One such alternative is a cement based on wollastonite (CaSiO3), which will create a seal through a carbonation reaction. In this study, experimental work addressing wollastonite carbonation kinetics under in-situ conditions relevant for subsurface CO2 storage in aquifers or depleted reservoirs is presented.

1.2. Interaction of Wollastonite and CO2

CO2 has an impact on the stability of conventional hydrated cements. Therefore, the use of a cement, based on (CS) curing in the presence of CO2 (C), seems very promising [28–30]. Yet knowledge considering this approach is rather scarce. Ashraf et al. [31] investigated the effects of high temperature (723–783 K) on calcium silicate-based cement (CSC), OPCs and OPCs with 20% fly ash. During the experiments, the OPCs showed a reduction of stiffness (60%), an increase of porosity (30%) and a loss of mass (13–15%) higher than the CSC (stiffness: 6%; porosity: 20%; mass loss: 3%). Huijgen et al. [32] investigated the mechanisms of aqueous wollastonite carbonation, using three different grain sizes (<38 µm, <106 µm and <500 µm) at short time intervals (5–60 min), a temperature range from 298 to 498 K and a pressure variation up to 4 MPa. A maximum conversion of 70% in 15 min at 473 K, 2 MPa CO2 partial pressure and with a particle size of <38 µm was achieved. They were able to show that the aqueous carbonation mechanisms of wollastonite, steel and olivine are generally similar, although they showed that wollastonite carbonation was rapid, relative to the carbonation of Mg-silicates. Longo et al. [13] showed that water has a huge impact on the carbonation reaction of wollastonite. Without water the wollastonite sample carbonated on the surface rather fast but after a carbonate monolayer was formed, the reaction comes to a standstill. The presence of water completely changes the reaction and caused ongoing carbonation. Daval et al. [33] investigated the carbonation reactions of wollastonite at conditions relevant to geologic CO2 sequestration in subsurface environments. They performed experiments in batch reactors at 363 K and 25 MPa. After carbonation, they found two reaction products: Calcite and a silica-rich phase. No other carbonate polymorph was found. Min et al. [34] performed carbonation experiments on wollastonite at typical geological storage conditions (333 K and 10 MPa). They investigated the microstructure of the carbonated wollastonite and found that after 20 h the reaction was controlled by the diffusion of water-bearing scCO2 across the product layer on wollastonite surfaces. A layer of amorphous SiO2 covered the surface of the wollastonite and functioned as a barrier.

2. Materials and Methods To determine carbonation rates, several series of experiments were performed and different analytic techniques were applied. Educts and products were analyzed with powder X-ray diffraction (PXRD) and differential scanning calorimetry with thermogravimetry (DSC-TG) coupled with mass spectrometer (MS) and infrared spectrometer (IR). The chemical composition of the educts was analyzed with wave length dispersive X-ray fluorescence (XRF).

2.1. Materials For the experiments, steel reactors with teflon liners from the company Büchi AG were used. According to the investigations of Huijgens et al. [32] (best conversion rate of 70% after 15 min at Appl. Sci. 2018, 8, 304 5 of 18

473 K and 2 MPa) a sample material with a grain size <38 µm was chosen and the experiments were performed with 2 MPa. In this work, experiments at lower temperatures were performed because this work is connected to a specific CCS process, in which temperatures do not exceed 333 K. The used raw material (natural CaSiO3, calcium silicate, meta, reagent grade, <20 µm powder) was supplied by Alfa Aeser. The chemical composition of the used raw material was analyzed with an XRF spectrometer (Siemens SRS 3000, Table1). As expected the main elements were Si and Ca, but in addition the raw material contained small amounts of Fe, Al, Mg, Mn, Ti, K, and Sr (Table2), which is due to the natural origin of this material.

Table 2. Chemical composition of the used raw material determined by Siemens SRS 3000.

Element (Oxides) Content (wt %; Normalized)

SiO2 50.55 CaO 45.95 Fe2O3 0.52 Al2O3 0.52 MgO 0.17 MnO 0.13 TiO2 0.05 K2O 0.03 SrO 0.01 LOI 1 2.09 Total 100.02 1 LOI: loss of ignition at 1323 K.

The pressure in the reactors was adjusted using an ISCO-pump (Teledyne ISCO, Model 500D Syring pump). CO2 was supplied by Westfalen AG and had a purity of 4.5 (99.995%).

2.2. Analytical Methods The mineralogical composition of the raw materials was measured by PXRD (Panalytical X’PERT3 Powder with Pixcel 1D detector). The PXRD measurements were carried out with 45 kV and 40 mA. As apertures 0.04 rad soller slit and fixed slits (0.125◦ and 0.25◦) were used. Diffractograms were recorded from 5◦ to 100◦ 2Θ with 49.725 s per step and stepsize: 0.013◦ 2Θ. For qualitative investigation the Highscore Plus Suite [35] and for quantitative phase analysis the Rietveld method [36] with Profex-BGMN [37,38] was used. To determine the amorphous content, 10 wt % rutile (Kronos 2900–TiO2; Rutile, supplied by KRONOS TITAN GmbH, Leverkusen, Germany) was added as internal standard. Chemical composition was analyzed with wave length dispersive X-ray fluorescence (SRS 3000; Siemens, Karlsruhe, Germany; Rh-tube; vacuum: 200 mbar; measurement, 30 s for each line). Therefore, melting tablets with 1 g sample material and 8 g Di-Lithium Tetraborate (FX-X100-2) supplied by FLUXANA (Bedburg-Hau, Germany) were prepared. To investigate the degree of carbonation depending on pressure (p) and temperature (T), the contents of wollastonite accompanied by the increase of and increase of the amorphous phase contents different approaches were applied to identify and quantify educts and products. For quantitative phase analysis, PXRD with the Rietveld method [36] and differential scanning calorimetry with thermogravimetry (DSC-TG; STA 449 F3 Jupiter, Netzsch, Selb, Germany) coupled with mass spectrometer (MS; QMS 403D Aeolos Netzsch; atom mass units 1 to 50 u were recorded) and infrared spectroscopy (IR; Tensor II with TGA-IR extension compartment from Bruker, Karlsruhe, Germany) were used. The thermoanalytical measurements were carried out from ambient temperature (296 K) to 1273 K (1000 ◦C) with a heating rate of 10 K per minute. For the DSC-TG measurements ~15 to ~25 mg sample material per measurement were used. With DSC-TG/IR/MS the amount of X-ray amorphous content of CaCO3 in the product could be analyzed as well. Appl. Sci. 2018, 8, 304 6 of 18

The content of carbonate was calculated from the mass loss associated with CO2:

mass loss CO2 [wt %] CaCO3[wt %] = (2) ( mass CO2 [g/mol] ) mass CaCO3 [g/mol]

Microstructure of educts and products were characterized by SEM (JEOL JSM 6300 combined with X-Flash 5010 detector from Bruker; secondary electron pictures were taken with 15 keV; samples were sputtered with gold).

2.3. Experimental Setup Carbonation of wollastonite at three different temperatures (296 K, 323 K, and 333 K) was investigated. As starting conditions 1 g sample material was mixed with 5 mL H2O in the steel reactor. 20 mL CO2 (5.5 MPa) were pumped by the ISCO-pump into the steel reactor, to obtain a pressure of 2 MPa inside the reactor. The temperature in steel reactors was adjusted by placing them in a drying chamber. For each temperature (296 K, 323 K, and 333 K) several experiments with different time intervals and constant pressure (2 MPa) were performed (Table3).

Table 3. Overview of performed series of experiments at constant pressure and varied time and temperature.

Temperature (K) Pressure (MPa) Reaction Time (h) 296 2 0, 24, 48, 72, 96, 120, 144 323 2 0, 6, 18, 24, 48 333 2 0, 3, 6, 12, 18, 24

In addition, a series of experiments with constant time and temperature but varied pressure were performed. The applied pressure ranged from 0.2 MPa to 5 MPa (Table4).

Table 4. Overview of performed series of experiments at varied pressure and constant time and temperature.

Temperature (K) Pressure (MPa) Reaction Time (h) 333 0.2, 0.5, 1.0, 1.5, 2.0, 2.6, 4.0, 5.0 6

To determine carbonation and reactions rates educts and products were analyzed by PXRD, DSC-TG/MS/IR and SEM.

3. Results and Discussion The results of the quantification of the educts and products at different temperatures and pressures showed that temperature has a higher impact on the conversion of wollastonite to CaCO3 than applied pressure.

3.1. Microstructure (SEM) For the microscopic analysis of the morphology of educts and products SEM was used. The raw material exhibited wollastonite grains, showing elongated crystals of 5–6 µm × 10–12 µm (Figure1, white arrows). The carbonated wollastonite still showed small and elongated (2.5 × 5 µm) wollastonite (Wo) crystals, covered with carbonate crystals (Figure2a, white arrow). Yet aggregates of carbonate crystals (Cb, Figure2b, white arrow) were observed in the micrograph shown in Figure2 (white arrow). Appl. Sci. 2018, 8, 304 7 of 18 Appl. Sci. 2018, 8, 304 7 of 19 Appl. Sci. 2018, 8, 304 7 of 19

Figure 1. Secondary electron (SE) images of two elongated wollastonite grains (arrows, Wo). The grain Figure 1. SecondarySecondary electron electron (SE) images images of of two two elongated elongated wollastonite wollastonite grains (arrows, Wo). The grain on the left (a)) isis 66 × 1212 µm,µm, the the grain grain on on the the right right ( (bb)) 5 5 ×× 1010 µm.µm. on the left (a) is 6 × 12 µm, the grain on the right (b) 5 × 10 µm.

Figure 2. Secondary electron (SE) images of carbonated wollastonite. A wollstonite grain with small Figure 2. Secondary electron (SE) images of carbonated wollastonite. A wollstonite grain with small Figurecrystals 2. ofSecondary carbonate electronon the left (SE) (a, images Wo). An of carbonated ofwollastonite. carbonate crystals A wollstonite on the right grain (b, with Cb). small crystals of carbonate on the left (a,, Wo).Wo). AnAn aggregateaggregate ofof carbonatecarbonate crystalscrystals onon thethe rightright ((bb,, Cb).Cb). 3.2. PXRD Analysis 3.2. PXRD Analysis 3.2. PXRDTo quantify Analysis the carbonation reaction of natural wollastonite (Table 5), samples were measured To quantify the carbonation reaction of natural wollastonite (Table 5), samples were measured with To PXRD quantify before the and carbonation after the reaction CO2 treatment. of natural The wollastonite results of (Table the PXRD5), samples-analysis were showed measured three with PXRD before and after the CO2 treatment. The results of the PXRD-analysis showed three withcrystalline PXRD phases before (wollastonite, and after the calcite, CO treatment. and aragonite) The resultsand an of amorphous the PXRD-analysis content, determined showed three by crystalline phases (wollastonite, calcite,2 and aragonite) and an amorphous content, determined by crystallineadding 10 wt phases % of (wollastonite,rutile as internal calcite, standard and aragonite) (Table 5 and and Figures an amorphous 3 and 4) content,in the starting determined material by adding 10 wt % of rutile as internal standard (Table 5 and Figures 3 and 4) in the starting material addingand the 10reaction wt % of products rutile as as internal well. standard (Table5 and Figures3 and4) in the starting material and and the reaction products as well. the reaction products as well. Table 5. Composition of the used raw material and carbonated wollastonite (24 h, 333 K), determined Table 5. Composition of the used raw material and carbonated wollastonite (24 h, 333 K), determined Tableby powder 5. Composition X-ray diffraction of the used ( rawPXRD material) and andRietveld carbonated refinement wollastonite [38] (24 (Rwp h, 333Raw K), material determined: 7%, Rwp by powdercarbonated by powder X-ray diffraction (PXRD) and Rietveld refinement [38] (RwpRaw material: 7%, Rwpcarbonated X-raywollastonite diffraction: 7%). (PXRD) and Rietveld refinement [38] (RwpRaw material: 7%, Rwpcarbonated wollastonite: 7%). wollastonite: 7%). Raw Material Carbonated Wollastonite Phase RawRaw Material Material Carbonated Wollastonite Wollastonite PhasePhase Content (wt %) Content (wt %) ContentContent (wt (wt % %)) ContentContent (wt (wt %) %) Wollastonite 90 ± 1 4 ± 1 Wollastonite 90 ± 1 4 ± 1 CalciteWollastonite 5 90± 1± 1 429± 1± 1 Calcite 5 ± 1 29 ± 1 AragoniteCalcite 51 ± 1 2921 ± 1 AragoniteAragonite 1 ± 1 1± 1 2121± ±1 1 AmorphousAmorphous 4 ± 4 ±1 1 4545± ±1 1 Amorphous 4 ± 1 45 ± 1 Appl. Sci. 2018, 8, 304 8 of 19

Appl. Sci. 2018, 8, 304 8 of 19 Appl. Sci. 2018, 8, 304 8 of 18

Figure 3. PXRD of Rietveld analysis [36–38] of the used raw material. Three crystalline phases and an

Figureamorphous 3. PXRD content of Rietveld were determined analysis [36 – by38 ] adding of the used an internal raw material. TiO2-standard Three crystalline of known phases weight: wollastoniteFigure 3. PXRD (cyan), of Rietveld calcite (magenta) analysis [36 and–38] aragonite of the used (purple). raw material. Three crystalline phases and an and an amorphous content were determined by adding an internal TiO2-standard of known weight: amorphous content were determined by adding an internal TiO2-standard of known weight: wollastonite (cyan), calcite (magenta) and aragonite (purple). Inwollastonite Figure 4, (cyan),the wollastonite calcite (magenta) with the and fastest aragonite and (purple). most advanced conversion to carbonates (24 h, 333 K, 2 MPa) is shown. The results at the quantification of all carbonation reactions of wollastonite In Figure4, the wollastonite with the fastest and most advanced conversion to carbonates (24 h, at 2 MPaIn F igureis shown 4, the in wollastonite part 3.4 of th withis study. the fastest and most advanced conversion to carbonates (24 h, 333333 K, K, 2 MPa)2 MPa) is shown.is shown. The The results results at theat the quantification quantification of all of carbonationall carbonation reactions reactions of wollastonite of wollastonite at 2 MPaat 2 MPa is shown is shown in part in part 3.4 of 3.4 this of study.this study.

Figure 4. PXRD of Rietveld analysis [36–38] of carbonated wollastonite (carbonated at 333 K, 2 MPa, Figure 4. PXRD of Rietveld analysis [36–38] of carbonated wollastonite (carbonated at 333 K, 2 MPa, with 24 h reaction time). Three crystalline phases and an amorphous content were determined by adding with 24 h reaction time). Three crystalline phases and an amorphous content were determined by an internal TiO2-standard of known weight: wollastonite (cyan), calcite (magenta) and aragonite (purple). addingFigure 4. an PXR internalD of Rietveld TiO2-standard analysis of[36 known–38] of carbonated weight: wollastonite wollastonite (cyan), (carbonated calcite at (magenta) 333 K, 2 MPa, and aragonitewith 24 h (purple).reaction time). Three crystalline phases and an amorphous content were determined by 3.3. DSC-TG/MS/IR Measurements adding an internal TiO2-standard of known weight: wollastonite (cyan), calcite (magenta) and 3.3.DSC-TG/MS/IR DSCaragonite-TG/MS/IR (purple). Measurements measurement of the raw material and the carbonated wollastonite were carried out to determine the entire content of water and carbonate in the material because X-ray amorphous DSC-TG/MS/IR measurement of the raw material and the carbonated wollastonite were carried carbonate3.3. DSC could-TG/MS/IR not be Measurements determined with PXRD, when additional amorphous phases (SiO ) are also out to determine the entire content of water and carbonate in the material because X-ray 2amorphous presentDSC in the-TG/MS/IR mixtures. measurement of the raw material and the carbonated wollastonite were carried outDSC-TG/MS/IR to determine the measurementsentire content of as water shown and in carbonate Figure5 were in the performed material because with the X raw-ray amorpho material.us During the measurement, the sample lost 2.1% of its weight. 1.7 wt % were associated with CO2, Appl. Sci. 2018, 8, 304 9 of 19 carbonate could not be determined with PXRD, when additional amorphous phases (SiO2) are also present in the mixtures. DSC-TG/MS/IR measurements as shown in Figure 5 were performed with the raw material. DuringAppl. Sci. the2018 measurement, 8, 304 , the sample lost 2.1% of its weight. 1.7 wt % were associated with CO9 of2 18, which results in original content of 3.9 wt % carbonate in the sample (Figure 5, Equation (2)). At the beginning of the experiment only H2O were detected by mass spectrometry. The detected mass flow which results in original content of 3.9 wt % carbonate in the sample (Figure5, Equation (2)). At the of H2O (Figure 5, green line) decreased from 400 K to 800 K. From 800 K to 1050 K only CO2 were beginning of the experiment only H2O were detected by mass spectrometry. The detected mass flow measured by mass spectrometry. The mass flow of CO2 reached its maximum at approximately of H2O (Figure5, green line) decreased from 400 K to 800 K. From 800 K to 1050 K only CO 2 were 1000 K (Figure 5, magenta line). The measured weight loss (Figure 5, black line) decreased slowly measured by mass spectrometry. The mass flow of CO2 reached its maximum at approximately 1000 K from 400 K to 850 K. Starting with the increased mass flow of CO2 the weight loss increased (Figure5, magenta line). The measured weight loss (Figure5, black line) decreased slowly from 400 K significantly (850 K to 1050 K). At temperatures higher than 1050 K, the mass of the sample did not to 850 K. Starting with the increased mass flow of CO2 the weight loss increased significantly (850 K to change. The DSC-signal (Figure 5, blue line) showed amplitudes at 400 K and 1000 K, which were 1050 K). At temperatures higher than 1050 K, the mass of the sample did not change. The DSC-signal associated with the detected peaks of the mass flows of H2O (400 K) and CO2 (1000 K). (Figure5, blue line) showed amplitudes at 400 K and 1000 K, which were associated with the detected peaks of the mass flows of H2O (400 K) and CO2 (1000 K).

FigureFigure 5.5. ResultResult of the of thedifferential differential scanning scanning calorimetry calorimetry with withthermogravimetry thermogravimetry coupled coupled with a withmass a spectrometermass spectrometer (DSC-TG/MS (DSC-TG/MS)) measurement measurement of wollastonite of wollastonite raw raw material material (initial (initial weight weight 20.3 20.3 mg). mg). MeasurementMeasurement from from 296 296 K K to to 1273 K with heatingheating raterate ofof 1010 K/min. K/min. TheThe weightweight loss loss (wt (wt %) % is) is shown shown by bythe the black black line line and and the the DSC-signal DSC-signal (mW) (mW by) by the the blue blue line. line. The The green green line line shows shows the the mass mass flow flow (A) (A for)

forH 2HO,2O, and and the the magenta magenta line line the the mass mass flow flow (A) (A for) for CO CO2. 2.

TheThe results results of of the the Rietveld Rietveld analysis analysis and and the the DSC DSC-TG-TG measurements measurements of of the the raw raw material material for for the the contentcontent of of carbonate carbonate were were in in good good agreement agreement (Rietveld: (Rietveld: 5 5%;%; DSC DSC-TG:-TG: 4 4%).%). DSCDSC-TG/MS/IR-TG/MS/IR meas measurementsurements on on the the carbonated carbonated wollastonite, wollastonite, shown shown exemplarily exemplarily in in Figure Figure 6,6 , werewere performed. performed. During During the the measurement measurement,, the the sample sample lost lost 25.4 25.4%% of of its its weight. weight. 23.9 23.9%% were were associated associated with CO2, which resulted in original content of 54.8 wt % carbonate in the sample (Figure 6, Equation with CO2, which resulted in original content of 54.8 wt % carbonate in the sample (Figure6, Equation (2)). (2)). At approximately 400 K the mass spectrometer detected a peak of the mass flow of H2O (Figure 6, At approximately 400 K the mass spectrometer detected a peak of the mass flow of H2O (Figure6, greengreen line). line). The The mass mass flow flow decreased decreased rapidly rapidly until until 500 500 K K was was reached. reached. From From 500 500 K K to to 750 750 K K the the mass mass flow of H2O rose again and held a plateau until 950 K. The IR analysis did also confirm the flow of H2O rose again and held a plateau until 950 K. The IR analysis did also confirm the investigation investigation of the releasing behavior of H2O from CaCO3. The mass flow of H2O decreased of the releasing behavior of H2O from CaCO3. The mass flow of H2O decreased significantly at significantly at temperatures higher than 950 K. The detection of the mass flow of CO2 (Figure 6, temperatures higher than 950 K. The detection of the mass flow of CO2 (Figure6, magenta line) started magentaat 800 K line) and increasedstarted at slightly800 K and until increased 950 K was slightly reached, until then 950 itK formedwas reached, a sharp then peak it untilformed 1100 a sharp K was peak until 1100 K was reached. At temperatures higher than 1100 K, the mass flow of CO2 was nearly reached. At temperatures higher than 1100 K, the mass flow of CO2 was nearly zero. The measured zero.weight The loss measured (Figure 6weight, black loss line) (Figure decreased 6, black slowly line) until decreased 900 K. Starting slowly until with 900 the K. increase Starting of with the mass the flow of CO2 the weight loss increased significantly (900 K to 1100 K). At temperatures higher than 1100 K, the mass of the sample did not change. The DSC-signal (Figure6, blue line) showed a negative (endothermic) amplitude at 1100 K which was associated with the detected peak of the mass flow of CO2. At 1150 K the DSC-signal showed a positive amplitude, which could be ascribed to the formation Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 19 increase of the mass flow of CO2 the weight loss increased significantly (900 K to 1100 K). At temperatures higher than 1100 K, the mass of the sample did not change. The DSC-signal (Figure 6, blue line) showed a negative (endothermic) amplitude at 1100 K which was associated with the detected peak of the mass flow of CO2. At 1150 K the DSC-signal showed a positive amplitude, which couldAppl. be Sci. ascribed2018, 8, 304 to the formation of Ca2SiO4. PXRD-measurements were performed on the same10 of 18 sample which showed small contents of C2S (, i.e., Ca2SiO4). The results of the Rietveld method and the DSC-TG measurements of the carbonated of Ca2SiO4. PXRD-measurements were performed on the same sample which showed small contents wollastonite for the content of carbonate were again in good agreement (Rietveld: 50%; DSC-TG: of C2S (larnite, i.e., Ca2SiO4). 55%). The results of the Rietveld method and the DSC-TG measurements of the carbonated wollastonite for the content of carbonate were again in good agreement (Rietveld: 50%; DSC-TG: 55%).

FigureFigure 6. 6.ResultResult of of the the DSC DSC-TG/MS-TG/MS measurement measurement of carbonatedof carbonated (at (at 2 MPa 2 MPa and and 333 333 K forK for 24 24h) h) wollastonitewollastonite (initial (initial weight weight 15.2 15.2 mg). Measurement from from 296 296 K K to to1273 1273 K withK with heating heating rate ofrate 10 of K/min. 10 K/min.The weight The weight loss (wt loss %) (wt isrepresented %) is represented by the blackby the line, black the line, DSC-signal the DSC (mW)-signal by (mW the) blue by the line. blue The line. green

Theline green shows line the shows mass the flow mass (A) flow for H (2AO,) for and H the2O, magentaand the magenta line the massline the flow mass (A) flow for CO (A2). for CO2.

3.43.4.. Discussion Discussion of ofthe the PXRD PXRD and and DSC DSC-TG/MS-TG/MS Results Results TheThe results results of of the the PXRD-measurements PXRD-measurements with withthe Rietveld the Rietveld method and method the DSC-TG/MS/ and the DSCIR-measurements-TG/MS/IR-measurements were collected were and collected plotted and in three plotted diagrams in three (Figures diagrams7–9). (Figures The quantity 7–9). of The the quantityphases wollastonite of the phases (black wollastonite squares), (blackamorphous squares), (blue amorphous triangles), calcite (blue (magenta triangles), circles) calcite and (magenta aragonite circles)(green and circles) aragonite were determined (green circles) by the were Rietveld determined method. by The the red Rietveld circles symbolized method. The the sum red of circles calcite symboland aragoniteized the andsum thereforeof calcite the and content aragonite of carbonate and therefore in the samplethe content detected of carbonate by PXRD. in The the amount sample of detectedCaCO3 bydetermined PXRD. The by amount TG measurements of CaCO3 weredetermined shown by cyanTG measureme circles. nts were shown by cyan circles.In Figure7, the carbonation of wollastonite at 296 K is shown with seven different reaction times. ToIn show Figure the 7 good, the reproducibilitycarbonation of wollastonite several experiments at 296 K wereis shown performed with seven two different times (Figure reaction7). At times. 296 K, Tothe show reaction the good was reproducibility not completed several after 144 experiments h. The content were of performed calcite (magenta two times circles), (Figure the 7). amount At 296 of K,CaCO the reaction3 determined was not by completed TG measurements after 144 (cyanh. The circles) content and of calcite of the amorphous(magenta circles), content the (blue amount triangles) of CaCOin the3 determined sample increased by TG m exponentiallyeasurements (cyan while circles) the content and of of the wollastonite amorphous (black content squares) (blue triangles) decreased in similarly.the sample No increased further aragonite exponentially (green while circles) the was content formed of(Figure wollastonite7). (black squares) decreased similarly.At No 323 further K, a high aragonite degree (green of conversion circles) was (>90%) formed was (Figure already 7). achieved after 48 h (Figure8). TheAt content 323 K, of a calcitehigh degree (magenta of conversion circles), aragonite (>90%)(green was already circles), achieved the amount after of 48 CaCO h (Figure3 determined 8). The by contentTG measurements of calcite (magenta (cyan circles)circles), and aragonite the amorphous (green circles), content the (blue amount triangles) of CaCO increased3 determined exponentially, by TG measurementswhile the content (cyan of wollastonitecircles) and (blackthe amorphous squares) decreased content (blue exponentially triangles) in incr an analogeased exponentially, manner. At 24 h, the contents of calcite and aragonite almost reached the same amount (~20 wt %), while at the beginning of the experiment the content of calcite was higher (Figure8, grey circles marked by arrows). Appl. Sci. 2018, 8, 304 11 of 19

while the content of wollastonite (black squares) decreased exponentially in an analog manner. At 24 h, the contents of calcite and aragonite almost reached the same amount (~20 wt %), while at the beginning of the experiment the content of calcite was higher (Figure 8, grey circles marked by arrows). Appl. Sci. 2018, 8, 304 11 of 18

◦ Figure 7. Carbonation of wollastonite at 296 K (23 C, ambient), 2 MPa and aqueous conditions (H2O). Figure 7. Carbonation of wollastonite at 296 K (23 °C, ambient), 2 MPa and aqueous conditions (H2O). The quantity of the phases wollastonite (black squares), amorphous (blue triangles), calcite (magenta The quantity of the phases wollastonite (black squares), amorphous (blue triangles), calcite (magenta circles)Appl. and Sci.circles) aragonite 2018, 8 , and304 (green aragonite circles) (green were circles) determined were determined by the Rietveld by the Rietveld method. method. The red The circles red circles symbolize 12 of 19 the sum of calcitesymbolize and the aragonite sum of calcite and therefore and aragonite the contentand therefore of carbonate the content in of the carbona samplete in detected the sample by PXRD. The amountdetected of CaCO by 3PXRD.determined The amount by of TG CaCO measurements3 determined by are TG symbolized measurements by are cyan symbolized circles. by cyan circles.

◦ Figure 8. Carbonation of wollastonite at 323 K (50 C), 2 MPa and aqueous conditions (H2O). The quantityFigure of the8. Carbonation phases wollastonite of wollastonite (black at 323 squares), K (50 °C), amorphous 2 MPa and (blueaqueous triangles), conditions calcite (H2O). (magenta The quantity of the phases wollastonite (black squares), amorphous (blue triangles), calcite (magenta circles) and aragonite (green circles) were determined by the Rietveld method. The red circles symbolize circles) and aragonite (green circles) were determined by the Rietveld method. The red circles the sum ofsymbolize calcite and the aragonite sum of calcite and thereforeand aragonite the andcontent therefore of carbonate the content in the of carbonate sample detected in the sample by PXRD.

The amountdetected of CaCO by PXRD.3 determined The amount by of TG CaCO measurements3 determined areby TG symbolized measurements by cyanare symbolized circles. The by ellipsoidscyan with arrowscircles. depict The theellipsoids symbols, with atarrows which depict aragonite the symbols, and calciteat which exhibite aragonite nearly and calcite thesame exhibite amount. nearly the same amount.

At 333 K, the conversion was nearly completed already after 24 h (Figure 9). The contents of calcite (magenta circles), aragonite (green circles) and the total amount of CaCO3 determined by TG measurements (cyan circles) and the amorphous content (blue triangles) increased exponentially, while the content of wollastonite (black squares) decreased exponentially. At 12 h the contents of calcite and aragonite almost reached the same amount (~20 wt %) whereas at the beginning (6 h and 18 h) of the experiment the content of calcite was higher (Figure 9, grey circles marked by arrows). Appl. Sci. 2018, 8, 304 13 of 19 Appl. Sci. 2018, 8, 304 12 of 18

◦ Figure 9. Carbonation of wollastonite at 333 K (60 C), 2 MPa and aqueous conditions (H2O). The quantity of the phasesFigure wollastonite 9. Carbonation (black of wollastonite squares), at amorphous 333 K (60 °C), (blue 2 MPa triangles), and aqueous calcite conditions (magenta (H2O). circles) The and quantity of the phases wollastonite (black squares), amorphous (blue triangles), calcite (magenta aragonite (green circles) were determined by the Rietveld method. The red circles symbolize the sum of circles) and aragonite (green circles) were determined by the Rietveld method. The red circles calcite andsymbolize aragonite the and sum therefore of calcite the and content aragonite of carbonate and therefore in the the sample content detected of carbonate by PXRD. in the The sample amount

of CaCO3 detecteddetermined by PXRD. by TG The measurements amount of CaCO are3 determined symbolized by byTG cyanmeasurements circles. The are symbolized ellipsoids withby cyan arrows depict thecircl symbols,es. The at ellipsoids whicharagonite with arrows and depict calcite the exhibitsymbols, nearly at which the aragonite same amount. and calcite exhibit nearly the same amount.

At 333 K,At the each conversion investigated wastemperature nearly (296 completed K, 323 K, 333 already K), an afterexponentially 24 h (Figure decrease9). of The wollastonite contents of calcite (magentawas observed. circles), This aragonitewas also true (green for the circles)increase of and calcite the and total the amount amorphous of content CaCO 3vicedetermined versa. At by TG measurementshigher temperatures (cyan circles) (323 K and and the333 amorphousK), aragonite exhibited content (bluealso exponential triangles) increase increased (Figures exponentially, 8 and while the9). content of wollastonite (black squares) decreased exponentially. At 12 h the contents of calcite and aragoniteAn additional almost experiment reached was the performed, same amount in which (~20 the wt temperature %) whereas (333 at K) the and beginning time (6 h) were (6 h and held constant and only the pressure varied. The results showed that the reaction was mostly 18 h) of theindependent experiment of pressure the content (Figure of 10). calcite was higher (Figure9, grey circles marked by arrows). At each investigatedAt low pressure temperature (0.2 MPa) the (296 decrease K, 323 K,of wollastonite 333 K), an exponentially was slower and decrease newly formed of wollastonite aragonite was observed.was This not was observed. also true At forhigher the pressu increaseres (>1 of calciteMPa) the and carbonation the amorphous of natural content wollastonite vice versa. showed At no higher temperaturesfurther (323 dependence K and 333 of K), the aragonite applied exhibited pressure (Figure also exponential 10). From 1 increase MPa to (Figures5 MPa no8 and differences9). An additionalconsidering experiment the rate of wollastonite was performed, transformation in which were the observed. temperature However (333 the K) formation and time of (6 h) aragonite was dependent on pressure. From 0.2 MPa to 2 MPa the formation of aragonite increased, were held constant and only the pressure varied. The results showed that the reaction was mostly from 2 MPa to 4 MPa it was constant and from 4 MPa to 5 MPa the formation of aragonite decreased. independentFigure of 11 pressure shows this (Figure observation 10). as normalized amount of the wollastonite conversion. At low pressure (0.2 MPa) the decrease of wollastonite was slower and newly formed aragonite was not observed. At higher pressures (>1 MPa) the carbonation of natural wollastonite showed no further dependence of the applied pressure (Figure 10). From 1 MPa to 5 MPa no differences considering the rate of wollastonite transformation were observed. However the formation of aragonite was dependent on pressure. From 0.2 MPa to 2 MPa the formation of aragonite increased, from 2 MPa to 4 MPa it was constant and from 4 MPa to 5 MPa the formation of aragonite decreased. Figure 11 shows this observation as normalized amount of the wollastonite conversion. Appl. Sci. 2018, 8, 304 14 of 19

Appl. Sci. 2018, 8, 304 14 of 19

Appl. Sci. 2018, 8, 304 13 of 18

Figure 10. Carbonation of wollastonite at 333 K (60◦ °C), aqueous conditions (H2O) and reaction time Figure 10. Carbonation of wollastonite at 333 K (60 C), aqueous conditions (H2O) and reaction time of 6 h. Theof 6 pressureh. TheFigure pressure was10. Carbonation varied was fromvaried of 0.2 wollastonite from MPa 0.2 to 5MPa MPa.at 333 to The 5K MPa. (60 quantity °C), The aqueous quantity of the conditions phases of the wollastonite phases (H2O) andwollastonite reaction (black squares), time (black amorphoussquares),of 6 amorphoush. (blue The pressure triangles), (blue was calcite triangles),varied (magenta from calcite0.2 MPa circles) (magenta to 5 and MPa. aragonite The circles) quantity (green and of aragonite circles)the phases were (greenwollastonite determined circles) (black by were the squares), amorphous (blue triangles), calcite (magenta circles) and aragonite (green circles) were Rietvelddetermined method. by the The Rietveld red circles method. symbolized The red the circles sum ofsymbolized calcite and the aragonite sum of and calcite therefore and aragonite the content and of determined by the Rietveld method. The red circles symbolized the sum of calcite and aragonite and therefore the content of carbonate in the sample detected by PXRD. The amount of CaCO3 determined carbonate in the sample detected by PXRD. The amount of CaCO3 determined by TG measurements were therefore the content of carbonate in the sample detected by PXRD. The amount of CaCO3 determined by TG measurements were symbolized by cyan circles. symbolizedby TG by measurements cyan circles. were symbolized by cyan circles.

Figure 11. Conversion rate of wollastonite at different pressures and constant temperature (333 K) and time (six hours). Appl. Sci. 2018, 8, 304 15 of 19 Appl. Sci. 2018, 8, 304 15 of 19 Figure 11. Conversion rate of wollastonite at different pressures and constant temperature (333 K) andFigure time 11. (six Conversion hours). rate of wollastonite at different pressures and constant temperature (333 K) and time (six hours).

Appl. Sci.Considering2018, 8, 304 the conversion of wollastonite to CaCO3 in the presence of water the reaction14 of rate 18 of theConsidering carbonation the depended conversion strongly of wollastonite on the temperature, to CaCO3 in at thehigher presence temperatures of water thethe reactionreaction rate wasof the significantly carbonation higher depended (Figures strongly 7–9). on the temperature, at higher temperatures the reaction rate Considering the conversion of wollastonite to CaCO in the presence of water the reaction rate of was significantlyA first order higherreaction (Figures was assumed 7–9). because the reaction3 was only dependent on wollastonite. the carbonationA first order depended reaction strongly was assumed on the temperature,because the reaction at higher was temperatures only dependen the reactiont on wollastonite. rate was The amount of CO2 was one order of magnitude higher than the amount of wollastonite. This significantlyassumptionThe amount highercould of CO be (Figures2 supportedwas 7 one–9). orderby plotting of magnitude ln (C) vs time higher (Figure than 12) the from amount which of a linear wollastonite. dependency This afterassumptionA a firstconversion order could reaction beof ~90supported% was could assumed by be plotting derived. because ln (C) thevs time reaction (Figure was 12) only from dependent which a linear on wollastonite. dependency Theafter amount a conversion of CO2 was of ~90 one% order could of be magnitude derived. higher than the amount of wollastonite. This assumption could be supported by plotting ln (C) vs time (Figure 12) from which a linear dependency after a conversion of ~90% could be derived.

FigureFigure 12. 12.Conversion Conversion rate rate of of wollastonite wollastonite plotted plotted as as ln ln (C) (C vs) vs time. time. Figure 12. Conversion rate of wollastonite plotted as ln (C) vs time.

In Figure 13 the wollastonite hkl reflection (100) at 12◦ ° 2Θ decreased from 296 K to 333 K. InIn Figure Figure 13 13 the the wollastonite wollastonite hkl hkl reflection reflection (100) (100) at at 12 12 2° Θ2Θdecreased decreased from from 296 296 K K to to 333 333 K. K.

Figure 13. X-ray powder diffractograms of carbonated wollastonite, recorded at different temperatures (296 K, 323 K, 333 K). Pressure (2 MPa) and time (24 h) were constant. The carbonation rate increases at higher temperatures. Appl. Sci. 2018, 8, 304 16 of 19

Figure 13. X-ray powder diffractograms of carbonated wollastonite, recorded at different Appl. Sci.temperatures2018, 8, 304 (296 K, 323 K, 333 K). Pressure (2 MPa) and time (24 h) were constant. The carbonation15 of 18 rate increases at higher temperatures.

Considering the conversion rates from wollastonite to calcite the rate constants and activation Considering the conversion rates from wollastonite to calcite the rate constants and activation energies could be estimated (Table6). energies could be estimated (Table 6).

Table 6. Rate constants k and activation energies EA of CaSiO3/CaCO3 conversion. Table 6. Rate constants k and activation energies EA of CaSiO3/CaCO3 conversion.

ActivationActivation Energies Energies Rate ConstantsConstants kJ/mol kJ/mol −6-06 −7 EA (323–296EA (323 K)–296 K) −3.79−3.79× ×10 10 ± ±2.36 2.36E× 10-07 63.8863.88 ± 0.2± 0.2 −5 −6 EA (333–296EA (333 K)–296 K) −3.32−3.32× ×10 10-05± ±1.87 1.87E× 10-06 58.8858.88 ± 0.6± 0.6 − × −5 ± × −6 ± EA (333–323EA (333 K)–323 K) 5.42−5.42 ×10 10-05 ±1.95 1.95E10-06 43.7043.70 ± 2.0 2.0

The carbonation reaction of wollastonite starts starts at at the surface of a grain and moves towards the center [10,11,34 [10,11,34]. The The reaction reaction rate rate will will decrease because of the growing layer thicknessthickness of CaCOCaCO33 between un unreactedreacted CaSiO3 andand CO CO22.. Consequently Consequently,, low low amounts amounts (5 (5%% to to 10 10%)%) of of material material will will remain remain unreacted within experimental periods. periods. At At 323 323 K K,, the the reaction reaction rate rate is is three three times times faster faster,, whereas at 333 K the reaction rate increased by a factor of six (Figure 1414).). The values of the activation energy EEA for the carbonation reaction ranged fromfrom 4444 toto 6464 kJkJ·mol·mol−−11 (Table(Table 66)) depending onon ∆T,∆T, with with the the latter being in a good agreement with the results reported in Brady [[39]39] (~72(~72 kJkJ·mol·mol−1).1). The carbonation ra ratete of wollastonite at different temperatures (when ~10%~10% of wollastonite were still present) showed a non-linearnon-linear behavior (exponential(exponential decrease) of reaction timetime (Figure(Figure 1414).).

Figure 14. CarbonationCarbonation rate rate of of wollastonite wollastonite at at different different temperatures temperatures at varying varying reaction reaction times. Higher Higher temperature resultedresulted inin a a faster faster carbonation carbonation rate. rate. The The reaction reaction time time showed showed a non-linear a non-linear behavior behavior (data (datapoints points were fittedwere fitted with anwith exponential an exponential function). function).

The p-T p-T conditions conditions in the in thesteel steel reactor reactor were thermodynamically were thermodynamically not suitable not [ 40 suitable] for the [ formation40] for the of formationaragonite. Althoughof aragonite. metastable Although aragonite metastable formation aragonite can be formation stabilized can in presence be stabilized of Sr[ 41in ],presence the content of Sr of [Sr41 in], the materialcontent of was Sr very in the low material (0.01 wt was %). Despitevery low in (0.01 preliminary wt %). performedDespite in experiments preliminary no performed aragonite experimenwas formedts whenno aragonite the carbonate was formed was previously when the removedcarbonate from was thepreviously raw material. removed Therefore, from the it raw was assumed that the small content of aragonite in the raw material functioned as a seed crystal for further metastable aragonite growth. Appl. Sci. 2018, 8, 304 16 of 18

4. Conclusions

Experiments considering the conversion of CaSiO3 to CaCO3 in aqueous conditions were performed at different temperatures and pressures. The kinetics of this reaction was strongly dependent on temperature (Figure 13). Pressure seemed not to be critical for reaction kinetics (Figures 10 and 11). The degree of conversion, i.e., the amount of reaction products (amorphous and crystalline), could be determined by XRD and coupled DSC-TG/MS/IR analysis. Calcite, aragonite and amorphous CaCO3 and amorphous SiO2 were observed. Depending on the temperature a nearly complete reaction was achieved within the experimental periods of 1–144 h. Activation energies EA could be determined and were in better agreement with the values reported by Brady [39] compared to those given by Huijgen (22 kJ·mol−1 at <38 µm, 20 kJ·mol−1 at <106 µm, 16 kJ·mol−1 at <500 µm) [32]. At the defined conditions (333 K, 2 MPa) relevant for the point of injection of CO2, the carbonation reaction of wollastonite was already completed after 24 h (Figure9). At lower temperatures, the carbonation reaction was rather slow (144 h at 296 K, Figure7 and 48 h at 323 K, Figure8). Hence, it could be proven that the use of wollastonite as a cementitious material suitable for sealing the borehole in CCS is applicable. To characterize association and their stability as well, the reaction at the interfaces between rocks and casing materials, more experiments at selected pressure, temperature and composition of formation will be performed. The long-term stability is still under investigation. As anticipated, the most likely application of wollastonite as cement in CCS would be for sealing the well after injection of CO2 in the reservoir. In that case, the wollastonite will be mixed with water and pumped into the well like conventional cement, mortar or . The CO2 stored in the reservoir will cause the hardening and therefore the sealing of the borehole.

Acknowledgments: We want to thank the German Federal Ministry for Economic Affairs and Energy (BMWi; FKZ: 03ET7031D) for financial support and Sabine Walther for sample preparation and support on the SEM. The constructive comments and helpful suggestions of two anonymous reviewers are gratefully acknowledged. We acknowledge the financial support within the funding program Open Access Publishing by the German Research Foundation (DFG). Author Contributions: The authors worked together in a research project. Herbert Pöllmann, Andreas Neumann, Kristoff Svensson and Christof Lempp conceived and designed the experiments. Kristoff Svensson and Andreas Neumann performed the experiments. Kristoff Svensson and Andreas Neumann analyzed the data. Kristoff Svensson, Andreas Neumann, Flora Feitosa Menezes, Herbert Pöllmann and Christof Lempp contributed ideas, knowledge and discussion. Kristoff Svensson, Andreas Neumann and Herbert Pöllmann wrote the Paper. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References

1. NOAA (National Oceanic and Atmospheric Administration); Earth System Research Laboratory. Trends in Atmospheric . Available online: https://www.esrl.noaa.gov/gmd/ccgg/trends/ (accessed on 18 December 2017). 2. Benson, S.M.; Orr, F.M., Jr. Carbon Dioxide Capture and Storage. MRS Bull. 2008, 33, 303–305. [CrossRef] 3. Locher, F. Zement–Grundlagen der Herstellung und Verwendung, 1st ed.; Verlag Bau+Technik GmbH: Düsseldorf, Germany, 2000; p. 522, ISBN 978-3-7640-0400-2. 4. Taylor, H.F.W. Cement Chemistry, 2nd ed.; Thomas Telford Publishing, Thomas Telford Services Ltd.: London, UK, 1997; p. 459, ISBN 0-7277-2592-0.

5. Pöllmann, H. Mineralogical strategies to reduce CO2 in the fabrication of alternative cements. Ibausil Conf. Rep. 2015, 1, 111–129. 6. Rice, J.; Ferry, J. Buffering, Infiltration, and the Control of Intensive Variables during Metamorphism. Rev. Miner. Geochem. 1982, 10, 263–326. 7. Svensson, K.; Neumann, A.; Pöllmann, H.; Menezes, F.; Lempp, C. Curing by carbonatisation of Wollastonite. GDCh–Tag. Bauchem. 2017, 52, 80–83. 8. CLUSTER. Joint Research Project CLUSTER. Available online: www.bgr.bund.de/CLUSTER/ (accessed on 18 December 2017). Appl. Sci. 2018, 8, 304 17 of 18

9. Gartner, E.; Hirao, H. A review of alternative approaches to the reduction of CO2 emissions associated with the manufacture of the binder phase in concrete. Cem. Concr. Res. 2015, 78, 126–142. [CrossRef] 10. Vilani, C.; Spragg, R.; Tokpatayeva, R.; Olek, J.; Weiss, W. Characterizing the Pore Structure of Carbonated Natural Wollastonite. In Proceedings of the 4th International Conference on the Durability of Concrete Structures, West Lafayette, IN, USA, 24–26 July 2014. 11. Sahu, S.; DeCristofaro, N. Solidia CementTM–Part One of a Two-Part Series Exploring the Chemical Properties and Performance Results of Sustainable Solidia CementTM and Solidia ConcreteTM; Solidia Technologies: Piscataway, NJ, USA, 2013; pp. 1–12.

12. Abbasi, E.; Hassanzadeh, A.; Abbasian, J. Regenerable MgO-based surbent for high temperature CO2 removal from syngas: 2. Two-zone variable diffusivity shrinking core model with expanding product layer. Fuel 2013, 105, 128–134. [CrossRef] 13. Longo, R.C.; Cho, K.; Brüner, P.; Welle, A.; Gerdes, A.; Thissen, P. Carbonation of Wollastonite(001) Competing Hydration: Microscopic Insights from Ion Spectroscopy and Dunctional Theory. Am. Chem. Soc. Appl. Mater. Interfaces 2015, 7, 4706–4712. [CrossRef][PubMed] 14. Shi, Z.; Lothenbach, B.; Geiker, M.R.; Kaufmann, J.; Leemann, A.; Ferreiro, S.; Skibsted, J. Experimental studies and thermodynamic modeling of the carbonation of Portland cement, metakaolin and limestone mortars. Cem. Concr. Res. 2016, 88, 60–72. [CrossRef] 15. Abdoulghafour, H.; Gouze, P.; Luquot, L.; Leprovost, R. Characterization and modeling of the alteration of

fractured class-G Portland cement during flow of CO2-rich brine. Int. J. Greenh. Gas Control 2016, 48, 155–170. [CrossRef] 16. Luquot, L.; Abdoulghafour, H.; Gouze, P. Hydro-dynamically controlled alteration of fractured Portland

cements flowed by CO2-rich brine. Int. J. Greenh. Gas Control 2013, 16, 167–179. [CrossRef] 17. Ozyurtkan, M.H.; Radonjic, M. An experimental study of the effect of CO2 rich brine on artificially fractured well-cement. Cem. Concr. Compos. 2014, 45, 201–208. [CrossRef] 18. Yalcinkaya, T.; Radonjic, M.; Willson, C.; Bachu, S. Experimental Study on a Single Cement-Fracture Using

CO2 Rich Brine. Energy Procedia 2011, 4, 5225–5342. [CrossRef] 19. Carey, W.J.; Svec, R.; Grigg, R.; Zhang, J.; Crow, W. Experimental investigation of wellbore integrity and

CO2-brine flow along casing-cement microannulus. Int. J. Greenh. Gas Control 2010, 4, 272–282. [CrossRef] 20. Barlet-Gouédard, V.; Rimmelé, G.; Porcherie, O.; Quisel, N.; Desroches, J. A solution against well cement

degradation under CO2 geological storage environment. Int. J. Greenh. Gas Control 2009, 3, 206–216. [CrossRef] 21. Wigand, M.; Kaszuba, J.P.; Carey, W.J.; Hollis, K.W. Geochemical effects of CO2 sequestration on fractured wellbore cement at the cement/caprock interface. Chem. Geol. 2009, 265, 122–133. [CrossRef] 22. Nakano, K.; Ohbuchi, A.; Mito, S.; Xue, Z. Chemical interactionof well composite samples with supercritical

CO2 along the cement–sandstone interface. Energy Procedia 2014, 63, 5754–5761. [CrossRef] 23. Zhang, L.; Dzombak, D.A.; Nakles, D.V.; Hawthorne, S.B.; Miller, D.J.; Kutchko, B.G.; Lopano, C.L.;

Strazisar, B.R. Characterization of pozzolan-amended wellbore cement exposed to CO2 and H2S gas mixtures under geologic carbon storage conditions. Int. J. Greenh. Gas Control 2013, 19, 358–368. [CrossRef] 24. Zhang, L.; Dzombak, D.A.; Nakles, D.V.; Hawthorne, S.B.; Miller, D.J.; Kutchko, B.G.; Lopano, C.L.;

Strazisar, B.R. Rate of H2S and CO2 attack on pozzolan-amended Class H well cement under geologic squestration conditions. Int. J. Greenh. Gas Control 2014, 27, 299–308. [CrossRef]

25. Mito, S.; Xue, Z.; Satoh, H. Experimental assessment of well integrity for CO2 geological storage: Batch experimental results on geochemical interactions between a CO2-brine mixture and a sandstone-cement-steel sample. Int. J. Greenh. Gas Control 2015, 39, 420–431. [CrossRef]

26. Wolterbeek, K.T.; Hangx, S.J.T.; Spiers, C.J. Effect of CO2-induced reactions on the mechanical behaviour of fractured wellbore cement. Geomech. Energy Environ. 2016, 7, 26–46. [CrossRef] 27. Kutchko, B.G.; Strazisar, B.R.; Hawthorne, S.B.; Lopano, C.L.; Miller, D.J.; Hakala, A.J.; Guthrie, G.D.

H2S-CO2 reaction with hydrated Class H well cement: Acid-gas injection and CO2 Co-sequestration. Int. J. Greenh. Gas Control 2011, 5, 880–888. [CrossRef] 28. Bube, C.; Metz, V.; Bohnert, E.; Garbev, K.; Schild, D.; Kienzler, B. Long-Term cement corrosion in chloride-rich solutions relevant to radioactive waste disposal in rock salt–Leaching experiments and thermodynamic simulations. Phys. Chem. Earth 2013, 64, 87–94. [CrossRef] Appl. Sci. 2018, 8, 304 18 of 18

29. Jain, J.; Deo, O.; Sahu, S.; DeCristofaro, N. Solidia CementTM–Part Two of a Series Exploring the Chemical Properties and Performance Results of Sustainable Solidia CementTM and Solidia ConcreteTM; Solidia Technologies: Piscataway, NJ, USA, 2014; pp. 1–17. 30. Jain, J.; Atakan, V.; DeChristofaro, N.; Jeong, H.; Olek, J. Performance of Calcium Silicate-based Carbonated vs. Hydrated Concretes under Freeze-thaw Environments; Solidia Technologies: Piscataway, NJ, USA, 2015; pp. 1–8. 31. Ashraf, W.; Olek, J.; Atakan, V.; Hyungu, J. Effects of High Temperature on Carbonated Calcium Silicate Cement (CSC) and Ordinary Portland Cement (OPC) Paste. In Proceedings of the 5th International Conference on Durability of Concrete Structures, Shenzhen, , 30 June–1 July 2016. 32. Huijgen, W.J.J.; Witkamp, G.-J.; Comans, R.N.J. Mechanisms of aqueous wollastonite carbonation as a

possible CO2 sequestration process. Chem. Eng. Sci. 2006, 61, 4242–4251. [CrossRef] 33. Daval, D.; Martinez, I.; Corvisier, J.; Findling, N.; Goffé, B.; Guyot, F. Carbonation of Ca-bearing silicates, the case of wollastonite: Experimental investigations and kinetic modeling. Chem. Geol. 2009, 262, 262–277. [CrossRef] 34. Min, Y.; Li, Q.; Voltolini, M.; Kneafsey, T.; Jun, Y.-S. Wollastonite Carbonation in Water-Bearing Supercritical

CO2: Effects of Particle Size. Environ. Sci. Technol. 2017, 51, 13044–13053. [CrossRef][PubMed] 35. Degen, T.; Sadki, M.; Bron, E.; König, U.; Nénert, G. The Highscore suite. Powder Diffr. 2014, 29, 13–18. [CrossRef] 36. Rietveld, H.M. A profile refinement method for nuclear and magnetic structures. J. Appl. Crystallogr. 1969, 2, 65–71. [CrossRef] 37. Döbelin, N.; Kleeberg, R. Profex: A graphical user interface for the Rietveld refinement program BGMN. J. Appl. Crystallogr. 2015, 48, 1573–1580. [CrossRef][PubMed] 38. Bergmann, J.; Friedel, P.; Kleeberg, R. BGMN–A new fundamental parameters based Rietveld program for laboratory X-ray sources, it’s use in qualitative analysis and structure investigations. Comm. Powder Diffr. Int. Union Crystallogr. CPD Newsl. 1998, 20, 5–8.

39. Brady, P.V. The Effect of Silicate Weathering on Global Temperature and Atmospheric CO2. J. Geophys. Res. 1991, 96, 18101–18106. [CrossRef] 40. Salje, E.; Viswanathan, K. The Phase Diagram Calcite-Aragonite as Derived from the Crystallographic Properties. Contrib. Mineral. Petrol. 1976, 55, 55–67. [CrossRef] 41. Sunagawa, I.; Takahashi, Y.; Imai, H. Strontium and aragonite-calcite precipitation. J. Mineral. Petrol. Sci. 2007, 102, 174–181. [CrossRef]

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