An Innovative Approach to Integrated Carbon Mineralization and Waste Utilization: A Review

Carbon dioxide (CO2) emission reduction in industry should be a portfolio option; for example, the carbon capture and utilization by mineralization (CCUM) process is a feasible and proven technology where both CO2 capture and alkaline waste treatment occur simultaneously through an integrated approach. In this study, the challengeable barriers and significant breakthroughs of CCUM via ex-situ carbonation were reviewed from both theoretical and practical perspectives. Recent progress on the performance of various carbonation processes for different types of alkaline solid wastes was also evaluated based on CO2 capture capacity and carbonation efficiency. Moreover, several process intensification concepts such as reactor integration and co-utilization with wastewater or brine solution were reviewed and summarized. In addition, the utilization of carbonated products from CCUM as green materials including cement, aggregate and precipitate calcium carbonate were investigated. Furthermore, the current status of worldwide CCUM demonstration projects within the ironand steelmaking industries was illustrated. The energy consumption and cost analyses of CCUM were also evaluated.


Carbon Capture and Utilization by Mineralization (CCUM)
Carbon capture and utilization by mineralization (CCUM) is a promising and feasible technology because the CO 2 generated from combustion of fossil fuels or carbon-related chemicals can be directly fixed as mineral carbonates.As shown in Fig. 1, since the carbonates are naturally occurring minerals and possess the lowest free energy of formation, the carbonation product is safe and stable over geologic periods of time, resulting in negligible release of CO 2 to the environment.The theoretical consideration of CCUM is based on the so-called "accelerated carbonation" process, whereby the gaseous CO 2 is reacted with alkalineearth metal-oxide (e.g., CaO and MgO) and converted into carbonate precipitates in the presence of aqueous environments.Appropriate feedstock sources for carbonation reaction include (1) natural ores such as serpentine and wollastonite and (2) alkaline solid wastes such as iron-and steelmaking slag and municipal solid waste incineration fly ash (MSWI-FA) (Eloneva et al., 2012;Teir et al., 2009;Huntzinger et al., 2009;Renforth et al., 2011;Sanna et al., 2012a;Nduagu et al., 2013;Olajire, 2013;Said et al., 2013;Pan et al., 2014).However, accelerated carbonation using natural ores creates its own environmental legacy because of the massive mineral requirements and associated scale of mining (Gerdemann et al., 2007).Therefore, the size of the mining operation is believed to comprise the most significant economic and environmental barrier to be overcome if any large-scale process is implemented.
Accelerated carbonation can be accomplished either in-situ (underground in geologic formation) or ex-situ (aboveground in a chemical processing plant) (Pan et al., 2012;International Energy Agency (IEA), 2013a;Sanna et al., 2014).In the case of in-situ carbonation, CO 2 is transported to underground igneous rocks, typically basalt, and is permanently fixed within the hosting rocks as solid carbonates (Kelemen and Matter, 2008;Muriithi et al., 2013).In the case of ex-situ carbonation, a source of calcium-silicate feedstock (e.g., natural ores and alkaline wastes) is carbonated aboveground; for instance, carbonation at industrial sites, biologically mediated carbonation, and carbon mineralization in industrial reactors (Gerdemann et al., 2007;Pan et al., 2013a).

Integrated Ex-situ Carbonation for Alkaline Wastes
In the ex-situ carbonation approach, two branches have been developed: direct carbonation and indirect carbonation.Basically, direct carbonation occurs in one single step, while for indirect carbonation the mineral has to first be refined, and then the refined mineral is carbonated.Although ex-situ carbonation is not economically viable so far, relevant research is still active and attractive, since the raw materials required for carbonation are globally abundant.
In this study, an innovative approach to ex-situ carbonation using industrial wastes including waste CO 2 gas, alkaline solid wastes and wastewater was proposed, as shown in Fig. 2. Through the carbonation process, the gaseous CO 2 in flue gas was fixed as solid carbonates, while the wastewater was neutralized to a pH value of 6-7.In addition, the physicochemical properties of carbonated solid waste can be upgraded, which makes it suitable to be utilized as green cementitious materials (Fernandez Bertos et al., 2004a).Carbonation could also act positively in the immobilization of heavy metals, such as Cd, Pb, and Cr, leaching from the alkaline solid waste by sorption in the newly formed products.The leaching of Pb, Zn, Cr, Cu, and Mo is markedly reduced upon carbonation for both APC and BA (Fernandez Bertos et al., 2004a;Arickx et al., 2006;Cappai et al., 2012;Santos et al., 2013b).Cd and Pb have a strong affinity with calcium carbonate and also form complexes with Fe and Al (hydr-)oxides (Rendek et al., 2006).Immobilization of Sb could also be achieved by combining with other processes (e.g., sorbent adding) during carbonation reaction (Cappai et al., 2012).Furthermore, carbonation has recently proved to be an effective way to improve the durability of concrete because relatively insoluble CaCO 3 is formed from the soluble Ca(OH) 2 in the concrete (Tsuyoshi et al., 2010).Therefore, improvements in the chemical and physical characteristics of treated residues after carbonation can facilitate its reuse in a variety of applications, such as construction materials.

Objectives
In this paper, the challenging barriers and significant breakthroughs of CCUM were reviewed from the perspective of both theoretical and practical considerations.Recent progress on the performance of various carbonation processes for different types of alkaline solid wastes was evaluated based on CO 2 capture capacity and carbonation efficiency.Moreover, several process intensification concepts such as physicochemical activations, co-utilization with wastewater or brine solution, and integration of novel reactors were discussed and summarized.In addition, the utilization of carbonated products as green materials including cement, aggregate, and precipitate calcium carbonate (PCC) were reviewed.Furthermore, the current status of CCUM demonstration projects within the iron-and steelmaking industries in the U.S., France, Australia and Taiwan were illustrated.The energy consumption and cost analyses of CCUM were also evaluated.

Properties of Alkaline Wastes
Alkaline wastes are suitable and can be used as alternative reactants in carbonation to natural ores because Fig. 2. Integrated carbon mineralization and waste utilization through accelerated carbonation: an innovative CCUM process.they are abundant, cheap and usually generated from nearby CO 2 emission points in many industries.However, it is difficult to directly compare different wastes for mineral CO 2 sequestration because each waste has its own unique set of advantages and disadvantages, and different capacity results are proposed and published in a variety of forms.Fig. 3 shows the relationship of CO 2 capture capacity (in terms of CaO and MgO contents) and hardness (in terms of Fe 2 O 3 and Al 2 O 3 contents) for different types of solid wastes.In general, the contents of CaO and MgO in ironand steelmaking slags were relatively higher than those in fly ash (FA) or bottom ash (BA).However, some steelmaking slag such as basic oxygen furnace slag (BOFS) is hard (due to high contents of Fe 2 O 3 and Al 2 O 3 ) and requires energy-intensive processing, which makes it challenging as viable sinks of CO 2 .Conversely, FA is relatively more promising, since it is a fine powder and already close at hand, which means the costs for transportation, extraction and crushing are minimal.The amounts of CaO and SiO 2 contents in BOFS increase remarkably with the decrease of particle size, while the Fe 2 O 3 fraction decreases significantly (Zhang et al., 2011;Pan et al., 2013b).
In addition, wastewater or brine, which is a saline-based waste solution (total dissolved solid is generally more than 10,000 mg/L) produced from some industrial procedures such as oil and natural gas extraction (known as oil-field brines), could be used as the liquid agents in the carbonation reaction (Druckenmiller and Maroto-Valer, 2005;Uibu and Kuusik, 2009;Liu andMercedes Maroto-Valer, 2011, 2012).Most wastewater treatments use chemicals as a neutralizing agent to adjust pH and enhance metal precipitation.However, the use of chemicals carries with it a high economic and environmental cost because it is a "resource" and not a "residue."Under the appropriate conditions, CO 2 would dissolve in the brine solution to initiate a series of reactions that ultimately lead to the bonding of carbonate ions to various metal cations inherent in brine to form carbonate precipitates such as calcite (CaCO 3 ), magnesite (MgCO 3 ), and dolomite (CaMg(CO 3 ) 2 ).

Types of Carbonation Processes
There are two approaches to ex-situ carbonation: direct and indirect carbonation.Direct carbonation can be conducted in two ways, i.e., a dry gas-solid reaction or an aqueous reaction (gas-liquid-solid).The gas-solid carbonation, as described in Eq. ( 1), is the simplest method to mineralization.Ca/Mg-silicate (s) + CO 2 (g) → (Ca/Mg)CO 3(s) + SiO 2(s) (1) However, the reaction kinetics of the dry gas-solid reaction at ambient pressure and temperature is far too slow to be effective in CCUM on a wide-scale basis.The reaction can be slightly accelerated by either pre-treating feedstock (such as grinding process and thermal activation) to increase the reactive surface area, or possessing carbonation up to 500°C (Pan et al., 2012).Nevertheless, those treatments and processes are very energy intensive; therefore, the environmental benefits of carbonation might be easily offset.Moreover, the low capture efficiency is not currently viable on the industrial scale.
The addition of water to the direct carbonation process (i.e., aqueous carbonation) can significantly increase the reaction kinetics because of the mobilization of ions in the reaction of carbonic acid with alkaline materials.Fig. 4(a) shows a schematic diagram of direct aqueous carbonation for CO 2 fixation and construction material production.The carbonation of alkaline solid waste was carried out with the direct contact of flue gas from a stack, in the presence of a liquid phase, typically using tap water.The wastewater generated in the same industries also can be introduced for the carbonation process to avoid the use of freshwater resources.After carbonation, the slurry is separated into  (Teir et al., 2007;Reddy et al., 2010;Sanna et al., 2012a;Abo-El-Enein et al., 2013;Dri et al., 2013;Hekal et al., 2013;Muriithi et al., 2013;Santos et al., 2013b, c;Dri et al., 2014;Jo et al., 2014;Reynolds et al., 2014;Salman et al., 2014;Santos et al., 2014;Ukwattage et al., 2014).carbonated solid wastes and liquid solution.The separated liquid solution can be heated by a heat exchanger with relatively hotter flue gas, and recirculated into the reactor for the next carbonation.It was noted that no excessive heat was required in aqueous carbonation for enhancing the reaction; however, the solution can be moderately heated to 60-80°C to achieve a higher carbonation conversion of solid wastes compared to that at ambient temperature (Chang et al., 2012a(Chang et al., , 2013a)).
On the other hand, indirect carbonation, as shown in Fig. 4(b), involves several steps: (1) extraction of metal ions from alkaline solid wastes, (2) liquid-solid separation, and (3) carbonation of the filter solution.Eq. ( 5) shows the first step of indirect carbonation (i.e., so-called extraction), where calcium ions are extracted for example using acetic acid (CH 3 COOH) from mineral crystals of CaSiO 3 .The extracted solution is filtered through a fiber membrane to separate the mother solution (i.e., rich in calcium ion) and extracted solids (i.e., calcium-depleted SiO 2 particles).After that, the filtered solution is carbonated with a CO 2 source where the precipitation reaction occurs, as shown in Eq. (6).Several effective extractants such as acetic acid and ammonium salts (e.g., CH 3 COONH 4 , NH 4 NO 3 , NH 4 SO 4 and NH 4 HSO 4 ) are commonly used in the extraction stage (Kakizawa et al., 2001;Dri et al., 2013;Jo et al., 2014;Santos et al., 2014).After carbonation, the end product is usually a pure carbonate (i.e., CaCO 3 and MgCO 3 ) because most of the oxides and hydroxides from the material have been extracted, and followed by direct carbonation of the oxides and hydroxides with the CO 2 .(5) Multistep indirect carbonation in the presence of additives can reach high carbonation efficiency under mild operating conditions within a short residence time (Jung et al., 2014a;Sanna et al., 2014).It was observed that high-purity spherical carbonates (e.g., vaterite) can be obtained by indirect carbonation (Jo et al., 2014).However, the requirement of an energy-intensive process for chemical regeneration is still a limiting factor for large-scale deployment.Research has indicated that energy and chemical costs could be reduced by carrying out the reaction between hydroxide and CO 2 at high pressure and temperature (i.e., 25 bar and 450°C), potentially making the hydroxide route technically achievable on an industrial scale (Nduagu et al., 2012).
Table 1 presents the various approaches to ex-situ direct or indirect carbonation using alkaline solid wastes as reported in the literature.The overall CO 2 capture capacity of steelmaking slag such as BOFS, CODS and CCS was relatively higher than that of other solid wastes such as FA.However, it was noted that the energy requirement for crushing, heating, and stirring needs to be offset by the carbonation process exotherm to make the process economically viable in an industrial context (Huijgen et al., 2006, 2007, International Energy Agency (IEA), 2013a; Pan et al., 2013b).Since the reaction temperature would affect not only the leaching rate of calcium ions from solid wastes but also the rates of CO 2 dissolution and carbonate precipitation, it was observed that the appropriate temperature for overall CO 2 capture performance should be set at around 60-80°C in the cases of aqueous carbonation using SS and FA (Chang et al., 2012a, b;Ukwattage et al., 2014).
In addition, utilization of the reacted materials should be implemented to couple the CO 2 reduction and waste utilization in industries (Pan et al., 2012).Although the CO 2 capture efficiency using direct carbonation was found Table 1.Performance evaluation of ex-situ carbonation using alkaline solid wastes in the literature.to be superior to that using indirect carbonation, the purity of the produced CaCO 3 precipitate using indirect carbonation was higher than that using direct carbonation.

Thermal Heat Activation (Pre-treatment)
Thermal heat activation is commonly used as a pretreatment for solid wastes to remove chemically bound water.Especially for serpentines containing up to 13% of chemically bound water, the crystalline features are changed to amorphous ones following the decomposition of hydroxyl groups after heating to 600-650°C (Park and Fan, 2004;Li et al., 2009).It was also observed that the porosity and surface area of solid wastes increase, and the structural instability was created after thermal heat treatment, thereby promoting the rate of carbonation afterward (Park and Fan, 2004;Li et al., 2009).The drawback is that huge amounts of energy are required to achieve the high temperatures, which makes it impractical for use as a largescale treatment.Another new approach is utilizing the exothermic reaction that comes from mineral carbonation, which has been proved to be self-sufficient in terms of energy (Moazzem et al., 2013).

Chemical Activation Using Acids and Bases
Chemical activation utilizes the solvent, acid or base agents to weaken the Mg-bonds in an Mg-silicate structure.This allows the improvement of dissolution kinetics, thereby increasing the carbonation efficiency.Many chemical solutions such as ammonia, acetone and HCl have been evaluated in the literature (Maroto-Valer et al., 2005;Power et al., 2013;Jung et al., 2014b).Another approach is the "pH-swing" process used in indirect carbonation (Park and Fan, 2004).This process allows silicates to be dissolved at a low pH and precipitates carbonates at a higher pH, resulting in more control of the carbonation process.However, it is much more expensive than traditional carbonation.

Co-utilization with Wastewater or Brine Solution
Another approach is utilizing the wastewater (or brine solution) to enhance the rate of calcium ion leaching from solid wastes or gaseous CO 2 dissolution into the liquid phase.The carbonation conversion was found to be higher in the wastewater (brine)/solid residue system than in the water/solid (Nyambura et al., 2011;Chang et al., 2013b;Pan et al., 2013b).The leaching rate and capacity of calcium ions from steelmaking slag in metal-working wastewater was greater than that in tap water.Both organic and inorganic (non-acid) ionic species in wastewater (e.g., sodium and/or chloride ions) can promote the dissolution of silicate minerals (Beard et al., 2004;O' Connor et al., 2005;Krevor and Lackner, 2011;Jo et al., 2012;Chang et al., 2013a;Pan et al., 2013b).The findings reported by Jo et al. (2012) show that a greater reactivity of the calcium-bearing complex towards chloride than other ions could thereby result in greater rates of mineral dissolution in the presence of chloride.The selection criteria of wastewater (or brine) for Fig. 5. Different approaches to process enhancement of ex-situ direct carbonation for alkaline solid wastes.carbonation were as follows: first, the pH of the solution should be over 9.0 where CO 3 2-dominates because the precipitation of carbonate is favored under a basic condition.Second, the selected solution should contain neither bicarbonate nor carbonate ions (Druckenmiller and Maroto-Valer, 2005;Liu and Maroto-Valer, 2012).

Biological Enhancement (Bio-Carbonation)
Microorganisms, e.g., anaerobic and aerobic bacteria, can positively affect carbonation by directly or indirectly enhancing the solubility and dissolution of minerals (Huang and Tan, 2014).Another approach is to utilize algae and cyanobacteria to perform photosynthesis in the presence of sunlight as energy to convert gaseous CO 2 (Huang andTan, 2014, McCutcheon et al., 2014).The absorption of cations by a net negative cell wall increases the cation concentration within the cell, which can be used to facilitate mineral precipitation.Specially designed "carbonation ponds" or "basins" with a high alkalinity would have to be made using a natural cyanobacteria-dominated consortium in order for the photoautotrophs to thrive and precipitate carbonates (McCutcheon et al., 2014).In addition, this process can serve as silicon sinks by the formation of mineralized cell walls (i.e., frustule) from [SiO 2 nH 2 O] in the bioreactor.
Furthermore, "enzymatic carbonation" relying on carbonic anhydrase (CA) can be effective, especially when the supply of CO 2 is limiting the rate of carbonation, even in an industrial environment, or utilized in an open-air environment.In addition, the CA enzyme enhances the carbonation of Ca-and Mg-bearing materials (Li et al., 2013a, b).A CA enzyme-based system has been developed inside a membrane that was able to remove 90% of the CO 2 supplied (Figueroa et al., 2008).However, the instability and very high costs of CA may be the challenges for practical applications (Sanna et al., 2014).

Combination with Novel Processes
Since the aqueous carbonation of alkaline solid wastes was believed to be a diffusion-controlled reaction, intensification of mass transfer efficiency among phases was essential to improve CO 2 capture capacity and reduce energy consumption and operating costs.A slurry reactor incorporated with ultrasound vibration has been introduced to accelerate the precipitation rate of calcium carbonate via ultrasonic irradiation (Rao et al., 2007(Rao et al., , 2008;;Santos et al., 2012, Santos et al., 2013a).The results indicate that the efficiency of physical mixing, particle breakdown and removal of passivation layers increased with sound waves with frequencies in the range of 16-100 kHz.Therefore, a better conversion can be achieved in a shorter time compared to that without ultrasound; for instance, the carbonation conversion of combustion ashes increased from 27% to 83% with ultrasound for 40 min (Rao et al., 2007).
Another approach is using a rotating packed bed (RPB) reactor, which has been successfully introduced for carbonation of solid wastes (Chang et al., 2012a(Chang et al., , 2013a;;Pan et al., 2013aPan et al., , b, 2014)), as a so-called "high-gravity carbonation process."RPB can provide a mean acceleration of up to 1,000 times greater than the force of gravity, thereby leading to the formation of thin liquid films and micro-or nano-droplets.(Chen et al., 2004, Chen et al., 2010, Cheng and Tan, 2011, Kelleher and Fair, 1996, Lin and Chen, 2008, Wang, 2004, Yu et al., 2012).Both the CO 2 removal efficiency and carbonation conversion of steelmaking slag in an RPB were observed to be greater than those in an autoclave or a slurry reactor (Chang et al., 2012b;Pan et al., 2013a, b).High CO 2 removal efficiency can be achieved with a retention time of less than 1 min under ambient temperature and pressure conditions (Pan et al., 2013a).

Summary
In this section, the principles and applications of several novel processes were illustrated, as summarized in Table 2. Alkaline wastes including wastewater (e.g., cold-rolling mill wastewater) and solid wastes (e.g., steelmaking slag) from steel manufacturing plants could be used to sequester meaningful quantities of CO 2 , especially if the wastes are generated near a point source of CO 2 emission, although the CO 2 sequestration potential remains marginal on a global scale of CO 2 emission.In other words, the integrated waste treatment, i.e., CO 2 , wastewater, and steelmaking slag, for sustainable development should be promoted and implemented in industries.It was observed that mineralization of CO 2 by accelerated carbonation of alkaline wastes has the potential to not only sequester CO 2 but also upgrade the physicochemical properties of waste streams.

Cement in Concrete
Concrete is made of roughly 80% aggregate (sand and gravel), 10-15% cement, and 5-10% additives, water, and air.The worldwide cement industry is increasingly turning to the use of alkaline solid wastes, such as blast furnace slag (BFS) and FA, as supplementary cementitious materials (SCM) because of increasing petroleum prices and government regulations, as well as a lack of raw materials and increasing demand for concrete and cement.Processes using fresh fine BFS or fly ash as alternative binders in place of Portland cement in concrete have been developed and widely used, especially in the U.S., to reduce the energy consumption and CO 2 emission associated with concrete production.However, the challenges resulting from the negative effects of this substitution, which are mainly related to early strength development, remain.
Engineering experience shows that with 50% clinker replacement with fresh FA, the early strength of concrete is reduced dramatically (Crow, 2008).Despite the fact that FA usually replaces no more than 25% of the Portland cement in concrete, research conducted at Montana State University successfully demonstrated the use of 100% fly ash concrete with glass aggregate to construct a building (Hasanbeigi et al., 2012).Furthermore, not every alkaline solid waste can be successfully utilized in concrete directly.Several types of alkaline solid wastes like BOFS typically containing 3-10% free-CaO and 1-5% free-MgO would lead to fatal expansion of hardened cement-BOFS paste (Zhang et al., 2011), which has limited its application as aggregates or cements during the past.The carbonation process is capable of permanently mineralizing carbon in the form of either fine or coarse aggregates or an SCM to meet the growing green product market as a carbon-negative material.In other words, when alkaline solid wastes are carbonated via direct carbonation, the products can also be used for a broad range of applications such as construction aggregate (large particles) and cement (fine particles), without the potential presence of free-CaO and free-MgO.Accelerated carbonation of alkaline wastes could be a potential commercialization route where the produced carbonates can be used as a substitute for components of cement (International Energy Agency (IEA), 2013b).The suitability of the calcareous material as a partial replacement for cement clinker in cement has been documented in some non-structural applications in the U.S., but the suitability of the calcareous material as a cement ingredient in concrete applications (Zaelke et al., 2011) has not yet been demonstrated publicly.It was also reported that CKD and FA have been successfully used to produce a green Portland ash (Shah, 2004;Zaelke et al., 2011).Currently, the challenges in the application of this process include (1) the effect of impurities on performance, (2) acceptance of the product by the cement industry, (3) the ability to capture large amounts of CO 2 , (4) energy requirements, (5) finding an appropriate water source, (6) production of alkalinity, and (7) having sufficient demand for the end product.

Aggregate in Concrete
In addition to the "green" cement, the carbonated alkaline solid waste can function as construction aggregate to partially replace sand, gravel, and crushed stone.Many industrial waste materials can potentially be used as economical and environmentally friendly sand substitutes for cementitious building products.There are two types of aggregate: (1) coarse aggregate (generally ranging from 9.5 mm to 37.5 mm) including gravel and crushed stone, and (2) fine aggregate (usually smaller than 9.5 mm) including sand and crushed stone.Most construction aggregate is used to strengthen composite materials, such as concrete and asphalt concrete, for a myriad of uses ranging from railroad bases to housing foundations.Monkman et al. (2009) evaluated the use of carbonated ladle slag as a fine aggregate in zero-slump press-formed compact mortar samples and compared them to similar samples containing control river sand.The 28-day strengths of the mortars made with the carbonated slag sand were comparable to the strengths of the normal river sand mortars (Monkman et al., 2009), which clearly indicates the successful use of carbonated ladle slag as fine aggregates to prepare mortar samples simulating its applications in precast products, such as masonry units, paving stones, and hollow core slabs, which could be further treated by carbonation curing.Moreover, the carbonated particles became coarser due to agglomeration, which should be beneficial for use in aggregate manufacturing (Fernandez Bertos et al., 2004b).
In addition, recycled concrete aggregate (RCA), collected from old roads and buildings, has shown promise as an alternative to natural aggregate (NA).Table 3 presents the various properties of RCA for replacement of NA in construction concrete.While RCA and NA have similar gradation, RCA particles are more rounded in shape and have more fine particles broken off in L.A. abrasion and crushing tests (McNeil and Kang, 2013).This suggests that the use of RCA as a structural concrete should be viable because the performance of RCA concrete beams was still within standard specifications (Sagoe-Crensil et al., 2001;Shayan and Xu, 2004;McNeil and Kang, 2013;Behera et al., 2014).Furthermore, creep can be minimized by incorporating FA as either an additive or a replacement in concrete in the case of RCA utilization (Behera et al., 2014).

Precipitate Calcium Carbonate (PCC)
Precipitate calcium carbonate (PCC) is a product from indirect carbonation.Different crystal morphologies and shapes of PCC can be produced and utilized in various applications in the construction, oil, plastics, paper, and pharmaceutical industries (Teir, 2008;Eloneva et al., 2010Eloneva et al., , 2012)).Approximately 75% of the produced PCC is expected to be used in the paper industry (Teir et al., 2005, Zevenhoven et al., 2008), where PCC can serve as a replacement for more expensive pulp fiber and optical brightening agents to improve the quality and printing characteristics of paper (e.g., smoothness, gloss, whiteness, opacity, brightness, and color).
PCC also can be used to replace some of the cellulose as fillers and coating pigments in plastics, rubbers, paints, and papers (Eloneva et al., 2009(Eloneva et al., , 2010)).For these applications, several physicochemical properties of PCC play an important role, including particle size distribution, specific surface area, morphology, polymorphism and purity (Sanna et al., 2012b).In addition, PCC can be utilized as an additive and filler in construction materials.Companies in North America, the EU, and Australia are working on developing a similar process, which involves CO 2 capture by bubbling flue gases through saline water to produce solid carbonates as an aggregate material in cement (International Energy Agency (IEA), 2013b).

Summary
Application of fresh steelmaking slag as an alternative to standard materials has been known for a number of years around the world.It was used most often in asphalt mixtures, other layers of pavement structure, unbound base courses and embankments.Nonetheless, the use of alkaline solid wastes must comply with strict regulations, consisting of civil-technical and environmental requirements.Several barriers including volume expansion of blended materials (e.g., in the case of fresh BOFS) and concerns about environmental impacts and social acceptance have been encountered.It has been proved that the CaO f and Ca(OH) 2 in steelmaking slags can be eliminated after carbonation, thereby preventing the expansion problem of the blended materials.In addition, several studies concluded that the mechanical properties of mortar blended with carbonated  (Sagoe-Crensil et al., 2001;Shayan and Xu, 2004;McNeil and Kang, 2013;Behera et al., 2014).solid wastes were superior to those with fresh solid wastes.At the same time, the carbonated materials can meet the standards of construction engineering, providing positive benefits for practical applications.

CASE STUDIES
Pilot Study/Demonstration Project Scale-up of the CO 2 post-combustion process is possible without significant developments or costs (International Energy Agency (IEA), 2013b).However, process performance should be further improved by process integration, thereby reducing the energy requirement for the capture process.An integrated steelmaking process is composed of numerous facilities from the entire life-cycle of iron ore to steel products including raw material preparation (e.g., coke production, ore agglomerating plant and lime production), ironmaking (e.g., blast furnace, hot metal desulphurization), steelmaking (e.g., basic oxygen furnace, ladle metallurgy), casting and finishing mills.Therefore, deployment of the CCS process in steelmaking mills is challenging, since the CO 2 emissions come from multiple sources.The largest part of direct CO 2 emissions in steelmaking mills is from power plants which are around 48% in total CO 2 emissions, followed by blast furnaces at around 30% in total CO 2 emissions (Santos, 2013).In addition, the source of the CO 2 may not be the emitter of the CO 2 .In other words, the emissions also strongly depend on management of the use of byproduct gases, as well as on the definition of boundary limits.
Table 4 summarizes the pilot studies and demonstration projects of CCUM using alkaline wastes.At Rocks, Wyoming, in the U.S., accelerated carbonation has been demonstrated in a 2,120 MW coal-fired power plant using FA since 2007 (Reynolds et al., 2014).Another pilot study in the U.S. has been developed by Calera Corp.The Calera technology can capture CO 2 (approximately 30,000 t/y) from fossil fuel power plants and other industrial sources and sequester it in geologically stable substances suitable for disposal, storage, and/or use as building materials.In the summer of 2009, Calera identified what was believed at the time to be an ideal demonstration site at a browncoal power plant in the Latrobe Valley, Victoria, Australia (Zaelke et al., 2011).
In France, the Carmex project was initiated in 2007 to carbonate various materials such as harzburgite, wehrlite, iherzolite, slags, and olivine through direct carbonation with and without organic ligand or mechanical exfoliation.The highlight of this project is that the accessible alkaline wastes are matched to large CO 2 emitters through a dedicated geographical information system (GIS).A high carbonation conversion, 70-90%, can be achieved without heat activation of feedstock.The Carmex experiences indicate that the use of mineral carbonation is feasible for industries (Bodénan et al., 2014).
Recently in Australia, the MCi project has been carried out to transform CO 2 into carbonates for use in future building products like bricks, pavers and plasterboard replacements that are non-fired products (Mineral Carbonation International, 2013).Similarly, the China Steel Corporation (CSC) project in Taiwan was launched in 2013 by introducing the highgravity carbonation process (i.e., RPB reactor) for smallscale carbonation of BOFS and alkaline wastewater.The CO 2 removal efficiency of hot-stove gas from the blast furnace was greater than 95%, with total elimination of CaO f and Ca(OH) 2 content in the BOFS.The carbonated BOFS was further used as green cement with different substation ratios in mortar.As presented in Fig. 6, an integrated approach to applying the high-gravity carbonation process (so-called HiGCarb process) was proposed for CO 2 capture in flue gas and solid product utilization within a steelmaking plant.It was noted that the CO 2 removal rate by the high-gravity process can meet the time scale in industrial plants.
The world's first commercialized carbon mineralization plant (Capitol SkyMine®) is under construction by Skyonic at the Capitol Aggregates, Ltd., cement plant in San Antonio, Texas, U.S. and scheduled for completion by 2014.This plant is expected to directly remove CO 2 (~75,000 t/y) from industrial waste streams through co-generation of carbonate and/or bicarbonate materials (~143,000 t/y) for use in bioalgae applications to become a profitable process.In addition to capturing and mineralizing CO 2 , the SkyMine® process can remove SO x , NO 2 and heavy metals such as mercury from existing power plants and industrial plants that can be retrofitted with SkyMine®.

Cost Evaluation and Energy Consumption
CCUM is definitely an important part to the reduction of CO 2 from the industrial sector and the use of industrial wastes as cement replacement.Technology may not be the only barrier to the deployment of CCS in the industrial sector.Market competitiveness and the global nature of some of these industries are important issues that should be addressed.Energy and cost penalties are related to plant scale, operation conditions, and operation modulus such as pre-treatment (e.g., grinding and thermal activation) and/or post-treatment processes (e.g., product separation and disposal) (Pan et al., 2012;Sanna et al., 2012a).However, due to lack of commercialized plant studies, cost estimations of accelerated carbonation are based roughly on pilot-or laboratory-scale operations.
In the case of indirect carbonation using chemical extraction (e.g., HCl, HNO 3 , CH 3 COOH and NaOH) without regeneration of chemicals, a fairly high cost, US$600-4,500, would be required for capturing one ton of CO 2 (Sanna et al., 2014).In addition, the regeneration of the chemicals would generate more than 2.5 times the amount of CO 2 fixation in the carbonation process (Teir et al., 2009).The operating costs would also depend largely on the purity of the PCC product.An average cost of US$80 was required per ton of the PCC production from two-stage carbonation using cement wastes at 50°C and 30 bar, where the energy consumption including pulverization, carbonation, CO 2 separation, CO 2 pressurization, and stirring process for both extraction and carbonation were considered at a total of 52.8 MW (Katsuyama et al., 2005).On the other hand, as presented in Table 5, the energy consumption and cost evaluation of direct carbonation were found to be relatively Fig. 6.An integrated approach to applying high-gravity carbonation (HiGCarb) process for CO 2 capture in flue gas and solid product utilization within steelmaking plant.
lower than that of indirect carbonation.In the case of direct carbonation, the energy requirement of the grinding process was the major cost in the overall process (Gerdemann et al., 2007, Pan et al., 2013b).It was observed that the cost of ex situ direct carbonation was in the range of US$54-133 per t-CO 2 , depending on the types of feedstock and operating modulus.Moreover, the handling of solids in the process has the potential to raise the O&M costs when compared to CO 2 absorption using ammonia and amine technologies (Yu et al., 2012).
In contrast, the total cost of in-situ carbonation was estimated at US$72-129 per t-CO 2 (considering an estimated transportation and storage cost of ~US$17 per t-CO 2 in basaltic rocks (Gislason and Oelkers, 2014)), without taking into account long-term monitoring costs.However, all of those costs are by far greater than the recent European carbon market price of ~US$7 per t-CO 2 in 2014 (Sanna et al., 2014).It was noted that the CO 2 price may increase to US$35-90 per t-CO 2 by 2040 (Wilson et al., 2012).In another scenario estimated by the International Panel on Climate Change, the price of carbon credit gives ~US$55 per t-CO 2 as a lower bound estimate, necessary to meet Kyoto protocol targets.
It has been observed that the carbonated SS can potentially be used as partial cement replacement materials (Liang et al., 2012;Salman et al., 2014).However, in order to make exsitu carbonation more economically feasible, a breakthrough on the use of carbonated solid wastes or products should be sought in the areas of technology, regulation, institution and finance.The global cement market is quite large, with roughly 3.5 billion metric tons used in 2011, and a processing cost of nearly US$100 per ton (International Energy Agency (IEA), 2013b).The benefits returned from product utilization should be taken into consideration in the fiscal evaluation of the overall carbonation process.From the viewpoint of energy consumption, fine FA is a good candidate for lowcost carbonation because no grinding process is required in advance.In addition, waste-heat integration from manufacturing processes could be implemented instead of electrical heating to reduce the overall energy requirement and operating cost (Balucan et al., 2013).

Life Cycle Assessment (LCA)
Although accelerated carbonation can potentially fix and store CO 2 over a geological time scale, the environmental effects of the process involved need to be considered from the perspective of life cycle assessment (LCA).The LCA of the CCUM process is particularly important, since energy is used in transporting slag, grinding, sieving, pressuring, heating, and operating the reactor (Xiao et al., 2014).CCUM may increase other environmental impacts such as eutrophication or acidification due to the increase in the concentrations of other pollutants.Hence, the effects of CCUM should be weighed and compared carefully according to changes in the environmental impacts.
Steelmaking slag is a solid waste of the steel manufacturing industry characterized by its strongly alkaline nature and significant levels of metal ions, especially calcium.After carbonation, the physicochemical properties of slag can be improved and suitably used as SCM to replace portions of the Portland cement in concrete.Typically, 1.5-1.7 tons of natural resource (e.g., limestone and clay) and 0.11-0.13ton of coal are used per ton of cement clinker production (Kumar et al., 2006).Moreover, concrete made from Portland cement carries a carbon footprint of approximately 0.73-0.99ton CO 2 per ton cement, corresponding to roughly 537 pounds of CO 2 per cubic yard of concrete (Hasanbeigi et al., 2012).Therefore, both the natural resource consumption and carbon footprint of concrete can be mitigated through the use of carbonated solid wastes as construction aggregate (large particles) or SCM (fine particles).
The annual world cement production is expected to grow by ~60% and reach 3.7-4.4Gt by 2050, compared to approximately 2.5 Gt in 2006 (Hasanbeigi et al., 2012).According to Zhang et al. (2011), blended cements with 30-60% residual slag product have properties comparable to those of Portland cement (Kumar et al., 2006).Therefore, it can be expected that huge environmental benefits can be obtained, e.g., approximately 1.45-3.89Gt of limestone,  0.22-0.60Gt of clay, and 0.12-0.34Gt of coal would be annually avoided by 2050, with the use of BOFS in blended cement.Furthermore, to achieve an acceptable reaction rate for industrial applications, alkaline wastes are usually ground down to fine particles (~ 100 μm) prior to use.The effect and toxicity of finely ground particles seems to cause significant health issues related to particulate formation, which is often believed to be a leading cause of respiratory disease (Koornneed and Nieuwlaar, 2009;International Energy Agency (IEA), 2013a;Giannoulakis et al., 2014).

Summary
Despite the great amount of alkaline waste available for CO 2 capture, the costs of both direct and indirect carbonation are too high for large-scale industrial deployment, suggesting that it may not be a complete solution to carbon capture issues.Ex-situ carbonation of alkaline wastes, which combines the treatment of industrial wastes that are readily available near a CO 2 emission point, could be part of an integrated approach to CO 2 mitigation issues for industrial plants.Another interesting issue is the problem of costallocation in the case of multiple emission reductions.Although CO 2 emission in an industrial plant was lower than in a power generation plant, the management of a variety of CO 2 sources within a single industrial plant and the selection of appropriate process and technology for CO 2 capture were the main barriers to lowering the industrial capture costs.To promote industrialization of CCUM, future work should be concentrated on the selection of appropriate processes, design and build-up of full-scale equipment, and management of material recycle and residue treatment.

Conclusions
Since reducing CO 2 emissions is critical to limit global warming to 2°C, CCUS technologies need to be deployed in steelmaking industries.Currently, the technology development is on track; however, the financial benefits are not great, since CCS projects need to demonstrate long-term viability and market support in the form of direct incentives.The proposed integrated CCUM process provides a solution for multiple waste treatments, i.e., reduction of CO 2 in flue gas, neutralization of alkaline wastewater, and stabilization of alkaline solid waste such as steelmaking slag.This could both lower the cost for wastewater treatment and CO 2 capture and enhance the utilization potential for steelmaking slag.Since the CO 2 emitted from the stacks in industries was generally pressurized and purified, it could be applied for accelerated carbonation directly at the source point, thereby reducing the cost for CO 2 capture.Although the use of waste steelmaking slag and metalworking wastewater for CO 2 capture will not lead to significant carbon credits, these wastes are available in large amounts and are near the emission sites, hence eliminating the need for transportation of raw material.The integrated CCUM process seems to be a viable option for industries because it is capable of the production of a saleable product, multi-pollutant treatments for flue gas and wastewater, and potential process integration for lowering the costs.

Recommendations
Since industrial waste products are generally produced near places of CO 2 emissions, using the CO 2 that is emitted to carbonate industrial waste offers an improvement over existing methods because it does not require the CO 2 or the industrial waste to be transported, and it allows better monitoring of total emissions.Several recommendations of CCUM were proposed for further investigation: 1.The reduction of CO 2 emissions in industry can be integrated with the waste treatment including alkaline solid wastes and wastewater, and their utilization such as green cement and aggregate in the construction engineering industry.2. A cost benefit analysis should be systematically carried out based on the consideration of (a) capital and operating costs for carbonation process and (b) profits from CO 2 reduction (e.g., carbon tax or carbon credit) and waste utilization (i.e., high-value products).3. Carbon footprint and energy consumption of the developed novel processes should be calculated using the LCA to determine the optimal particle size of solid wastes from the viewpoint of both CO 2 capture efficiency and product utilization.

Fig. 1 .
Fig. 1.Standard molar free energy of formation for several carbon-related substance at 298 K.

Table 2 .
Process intensification of ex-situ carbonation using alkaline solid wastes in the literature.

Table 3 .
Various properties of recycled concrete aggregate for natural aggregate replacement

Table 4 .
Pilot Studies and Demonstration Projects of accelerated carbonation around the world.

Table 5 .
Cost evaluation and energy consumption of ex-situ direct carbonation using various processes.