Effectiveness of novel and traditional methods to incorporate industrial wastes in cementitious materials—An overview

Resources, Conservation and Recycling 74 (2013) 134–143

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Review

Effectiveness of novel and traditional methods to incorporate industrial wastes in cementitious materials—An overview Tongsheng Zhang, Peng Gao, Pinhai Gao, Jiangxiong Wei, Qijun Yu ∗ School of Materials Science and Engineering, South China University of Technology, 510640 Guangzhou, China

a r t i c l e

i n f o

Article history: Received 12 February 2012 Received in revised form 2 March 2013 Accepted 6 March 2013 Keywords: Industrial waste Pozzolanic activity Blended cement Effectiveness Environmental benefit

a b s t r a c t Sustainable development and eco-efficiency are urgent and imperative demands for the well-being of our planet, continued growth of a society, and human development. Traditional Portland cement production seems unsustainable due to consumption of huge natural resources and energy and significant CO2 emissions. The volume of industrial wastes is increasing significantly, leading to a number of economical and ecological problems. Although industrial wastes can be incorporated in cementitious materials by various traditional methods, the substitution ratio of industrial wastes in cementitious materials is relatively low to avoid unacceptable performance loss. Novel methods, such as improving hydraulic activities of metallurgical slags by adding composition adjusting material at high temperature, improving surface cementitious properties of fly ashes by dehydration and rehydration treatment, and arranging cement clinker and industrial wastes in the particle size distribution of blended cements according to their hydraulic activities, are reviewed. These methods provide more effective approach to prepare high performance blended cements with larger amount of industrial wastes, leading to a very significant role in CO2 emissions reducing, resources and energy conservation of the cement industry. © 2013 Published by Elsevier B.V.

Contents 1. 2.

3. 4.

5.

Urgent demand of the industry: Sustainable development and eco-efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Traditional methods of incorporating industrial wastes in cementitious materials manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. As raw material to produce alkali-activated cementitious materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. As raw material to produce cement clinker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. As supplementary cementitious material to produce blended cement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. As gypsum in cement manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Barriers to incorporate industrial wastes in cementitious materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Novel methods of incorporating industrial wastes in blended cement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Improving hydraulic (or pozzolanic) activity by adding composition adjusting material into molten industrial wastes . . . . . . . . . . . . . . . . . . . . 4.1.1. Basic oxygen furnace slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. Coal gangue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3. Municipal solid waste incineration fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Improving surface cementitious properties of industrial wastes by dehydration and rehydration process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Coal fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Coal gangue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Optimizing the particle size distribution of clinker and industrial wastes in blended cements according to their hydraulic activities . . . . . Benefits of incorporating industrial wastes in cement production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Reducing natural resources consumption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Energy conservation and CO2 emissions reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Engineering and economic benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +86 020 87114233; fax: +86 020 87114233. E-mail addresses: [email protected], [email protected] (Q. Yu). 0921-3449/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.resconrec.2013.03.003

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Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

1. Urgent demand of the industry: Sustainable development and eco-efficiency Concrete is the most widely used manufactured construction material in the world. However, the production of Portland cement, an essential constituent of concrete, is not only energy-intensive, but is also responsible for significant emissions of CO2 – a greenhouse gas (GHG) (Malhotra, 2004). Typically, 1.5–1.7 tons of natural resources (about 1.4–1.5 tons of limestone (or similar rock) and 0.2–0.3 tons of other materials such as clay, silica sand, bauxite, etc.) and 0.11–0.15 tons of coal (or alternative fuels, such as petroleum coke, fuel oil, and natural gas) are used per ton of cement clinker production, and 0.7–1.0 tons of CO2 is also generated (Kumar et al., 2006). The total output of cement in the world was about 3.6 billion tones in 2012, of which China accounted for about 60%. Therefore, Chinese cement industry consumes about 1.5 billion tones of natural resources (mainly limestone and clay) per year, the energy consumption and CO2 emissions account for about 7% and 15% of the total energy consumption and CO2 emissions of China, respectively (Zhang et al., 2011a). The sustainability of the cement and concrete industries is essential and imperative to the well-being of our society and mankind’s development. The environmental issues associated with GHG and natural resource conservation will play a leading role in the sustainable development of the cement and concrete industries during this century (Naik, 2005). As a result of industrial development and urbanization, billion tons of industrial wastes (mainly referred to solid wastes as listed in Table 1) are generated annually and the amount of landfilled wastes is dramatically increasing. For instance, about 10 billion tons per year of industrial wastes are landfilled in China, and the annual output of industrial wastes that can be used in cement production (mainly combustion ashes, metallurgical slags, coal gangue and industrial synthetic gypsum) in China exceeded 2 billion tons in 2010 (Xu, 2010). In many countries, the landfilled wastes occupy a lot of farmland and cause serious environmental problems (such as water, air, and soil pollution). In addition, the lack of ample space for landfill is a common problem for municipal areas which have dense population in developing and developed countries (Naik, 2005). How can we reduce the environmental impact, especially the volumes being landfilled, of the industrial wastes? The simplest and most efficient way is recycling the industrial wastes by substituting them for the virgin raw materials, supplementary cementitious material (SCM) and set-controlling material (gypsum) for cement production, thereby reducing the environmental impact of cement industry and other related industries (Methta, 2001). Due to largest amounts of cement production and industrial wastes, more novel methods in utilizing industrial wastes in cement and concrete production are been developed in China than other countries, thus data specific to China was analyzed detailedly. Portland cement clinker mainly consists of tricalcium silicate (C3 S), dicalciume silicate (C2 S), tricalciume aluminate (C3 A) and tetracalcium aluminoferrite (C4 AF). During the hydration of clinker, C S H gel, which is the strength source of Portland-cementbased materials, is generated. Calcium hydroxide (CH) is also released during hydration and this CH is a traditional “activator” to increase the hydraulic activity of vitreous phases. Certain amount of hydraulic minerals (such as C3 S, C2 S, C3 A) and/or vitreous phases are formed in some industrial wastes during their high temperature formation and/or subsequent cooling treatment, therefore these industrial wastes present hydraulic or pozzolanic activity and

can be used as SCM. In addition, synthetic gypsum (such as phosphogypsum and flue gas desulfurization gypsum) can be used as set-controlling material in cement production partly or fully. Significant portion of industrial wastes can be used as raw material and/or SCM or set-controlling material, which contributes not only to the reduction of the cost of cement and concrete, but also to the improvement of microstructure, mechanical and durability properties of concrete, which are difficult to achieve by the use of only Portland cement (Siddique, 2008). Consequently, there is an urgent demand for the development and application of technologies to utilize industrial wastes in the production of cementitious materials from the standpoint of resources and energy conservation, cost saving and CO2 emissions reducing. Combustion ashes and metallurgical wastes, such as coal fly ash, coal bottom ash MSWI ash, blast furnace slag and steel slag, generally contain a number of heavy metals, such as lead, cadmium, chromium, arsenic, mercury, zinc and perhaps radioactive elements (Batchelor, 2006; Chen et al., 2011; Izquierdo et al., 2009; Polettini et al., 2001; Van Zomeren et al., 2011; Windt et al., 2011), although most of which are in low concentrations. These heavy metals can be hazardous to human health if not disposed in a safe manner. Thus dumping into lakes, streams or landfills and use as a road-based material is not a safe practice because the heavy metal ions can find their way into groundwater. Portland cement-based materials provide a preferred method for heavy metals immobilization (Van Herck et al., 2000; Astrup et al., 2006; Stephan et al., 2003; Watanabe et al., 2006). Significant role in the immobilization process is attributed to the hydration products (calcium silicate hydrates, calcium aluminate hydrates, and sulfoaluminate ´ hydrates) (Nocun-Wczelik and Małolepszy, 1997; Taylor, 1990). Yu et al. (2005) proved that blended cement with GBFS showed better immobilization of Cr(VI) than Portland cement. Thus, utilization of industrial wastes in cement production is also beneficial to immobilize heavy metal ions, which is very important for environmental ˇ et al., 2009). protection (Sturm 2. Traditional methods of incorporating industrial wastes in cementitious materials manufacture Industrial wastes that can be used as raw materials and/or SCM or set-controlling material in cement production are listed in Table 1. These wastes, accounting for approximately 80% (by mass) of total industrial wastes in China, are mainly come from metallurgical industry, coal-fired power plant, municipal solid waste incineration (MSWI) process, and coal mining industry (Xu, 2010). Traditional methods of incorporating these industrial wastes in cementitious materials are summarized as follows: 2.1. As raw material to produce alkali-activated cementitious materials Alkali-activated cementitious materials are receiving increasing attention as an alternative to Portland cement due to their high early strength and low environmental impact (Shi et al., 2011; Shi and Qian, 2000; Palomo et al., 1999). Alkali-activated cementitious materials are generally made by mixing solid aluminosilicate powder with an alkaline activator. The reaction products, or gel, can have a network structure similar to those of organic thermoset polymers, thus alkali-activated cementitious materials are sometimes called “inorganic polymer” or “geopolymer”. The raw

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Table 1 Sources and estimated amounts produced and landfilled for various industrial wasteas that are usable in cement and clinker production (for China). Industrial waste

Source

Annual output (ton)

Amount landfilled (ton)

Utilization ratio (%)

Blast furnace slag Steel slag Fly ash Coal bottom ash Coal gangue Phosphogypsum Phosphorous slag Red mud Flue gas desulfurization gypsum MSWI ash Total industrial wastes

Iron ore-based iron making industry Steel making industry Coal fire power plant Coal fire power plant Coal mining industry Phosphate industry Phosphate industry Aluminum industry Flue gas desulfurization Municipal solid waste incineration

0.22 billion 0.1 billion 0.6 billion About 0.1 billion 0.5 billion 50 million 10 million 30 million 20 million About 10 million More than 2 billion

Nearly zero About 0.5 billion About 4 billion No record About 8 billion More than 0.2 billion No record More than 0.4 billion About 40 million No record About 12.5 billion

Near 100 About 15 Lower than 40 About 50 About 45 Near 20 About 30 About 4 About 50 Lower than 5 About 50

materials of alkali-activated cementitious materials can be entirely waste-stream materials, particularly granulated blast furnace slag (GBFS), fly ash, low-reactive circulating fluidized bed combustion fly ashes, steel slag and phosphorus slag (Li et al., 2010a; Xu et al., 2010). Shi and Day (Shi et al., 1992; Shi and Day, 1996a) examined the selectivity of activators for the activation of GBFS from microstructure and compressive strengths development aspects. Na2 SiO3 -activated GBFS mortar exhibited the highest strength, much higher than Portland cement mortar at all testing ages. Na2 CO3 -activated GBFS mortar did not present a strength as high as Portland cement mortar at early ages, but an equivalent strength at later ages. NaOH-activated GBFS mortar showed strengths much lower than the Portland cement mortar at all ages (Shi et al., 1992). There was no porous interfacial transitional zone between reaction products and quartz sand aggregate in alkali-activated GBFS mortar, therefore alkali-activated GBFS mortar presented a better impermeability and durability than Portland cement mortar (Shi and Xie, 1998; Shi and Day, 1996b). The effects of alkaline activators on the reaction mechanism, composition, microstructure and mechanical properties of alkaliactivated fly ash (class C fly ash according to ASTM C 618) cementitious materials were closely investigated (Shi and Day, 1996b, 1999; Shi et al., 1992). Together with the increase of SiO2 /Na2 O molar ratio of sodium silicate, the Si/Al molar ratio of the reaction products of alkali-activated fly ash cementitious material was also increased, leading to a high compressive strength (Shi et al., 1989; Shi and Li, 1989). Other industrial wastes, such as class F fly ash (Pan et al., 2002; Douglas and Mainwaring, 1985), steel slag (Criado et al., 2007), granulated phosphorus slag (Puertas and Fernández-Jiménez, 2003; Fernández-Jiménez and Palomo, 2005), ˇ et al., 2009), sewage sludge (Hong and Li, 2011) red mud (Skvára and copper slag (Komljenovic´ et al., 2010), can also be used as raw materials of alkali-activated cementitious materials, and the alkali activator should be chosen according to the characteristics of raw materials.

2.2. As raw material to produce cement clinker Portland cement clinker is generally manufactured by sintering the mixture of limestone, quartz sand, iron ore, clay or bauxite. Some of industrial wastes can be used as raw materials to produce cement clinker, which helps not only to conserve natural resources, but also to additional benefits, such as lowering the sintering temperature and widening sintering range of clinker-making process (Ma et al., 2005). For instance, industrial wastes with high CaO, SiO2 and Fe2 O3 content can be used as calcium, silicate and iron sources respectively in cement clinker production. Numerous studies proved that utilization of basic oxygen furnace (BOF) slag and electric arc furnace steel slag (EAFS) (Tsakiridis et al., 2008), pyrite cinders (Alp et al., 2009), red mud (Tsakiridis et al., 2004),

ferroalumina waste (Vangelatosa et al., 2009), MSWI ash (Pan et al., 2008; Saikia et al., 2007; Singh et al., 2008), and copper slag (Alp et al., 2008) as raw materials didn’t affect the mineralogical characteristics and mechanical properties of the produced cement clinker, that is to say, these alternative materials had no adverse effects on the quality of the clinker. Al-Dhamri and Melghit (2010) investigated the use of spent alumina catalyst and fluidized cracking catalyst wastes from petroleum refinery to substitute for bauxite in the preparation of cement clinker. The results showed that substitution of bauxite by spent catalysts had nearly no influence on the chemical composition, physical and mechanical properties of the cement clinker. Special cement clinkers, in terms of sulphaluminate cement clinker (Shi et al., 2009; Wu et al., 2011) and aluminoferrite cement clinker (Singh et al., 1996), were also successfully made using MSWI fly ash and red mud as major raw materials, respectively.

2.3. As supplementary cementitious material to produce blended cement A large proportion of industrial wastes, such as coal fly ash, MSWI fly ash, GBFS and steel slag, present pozzolanic or hydraulic activity (Sinthaworn and Nimityongskul, 2009), therefore these wastes can be used as SCM to produce blended cement or as direct additions to concrete. However, the addition of SCM may decreases the early strengths of blended cement, but increases the later strengths and improves microstructure and durability of hardened blended cement paste significantly (Shi and Qian, 2000). Tüfekqi et al. (1997) proved steel slag could be used as partial replacement of cement (10%), without decreasing the compressive strengths significantly. Zhang et al. (2008) reported that when a compound alkaline activator was added into steel slag blended cement, both early and late compressive strengths of blended cement with 30% steel slag were increased significantly, indicating steel slag that has not traditionally been viewed as a useful SCM can be effectively used. Due to the high carbon content and large particle diameter, a significant portion of the fly ashes produced from coal-fired power plants cannot be used as SCM to produce cement. Antiohos and Tsimas (2007) classified rejected fly ash into two fractions (>45 ␮m and <45 ␮m), and then both ground into same fineness (about 430 m2 /kg Blaine). The results proved that blended cements with less than 20% of ground coarse-fraction fly ash gave no performance loss, while blended cements with ground fine-fraction fly ash showed relative lower strengths, which can be attributed to higher carbon content in fine fly ash fractions. Aubert et al. (2006, 2007) reported that blended cement with 25% MSWI fly ash showed a slight compressive strengths loss relative to Portland cement at 28 days, and the strengths decreased remarkably when the addition of MSWI fly ash exceeded 25%. Lin et al. (2004) investigated the hydration properties of blended cements prepared by mixing

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Type I, III and V Portland cements and MSWI fly ash, respectively. The results showed that the hydration degree increased slowly at early ages, but increased rapidly with curing time at late ages. The compressive strengths of blended cements with up to 20% MSWI fly ash varied from 95 to 110% of that developed by the plain Portland cement pastes at late ages. Similar conclusion was also drawn form Rémond et al.’s (2002a,b) researches. Rajamma et al. (2009) examined the feasibility of utilization of biomass combustion ash in blended cements. The results indicated that blended cement with 10% biomass combustion ash gave no obvious strength loss compared with Portland cement at 28 days, however, when 20% biomass combustion ash was added the compressive strength of blended cement decreased to about 75% of that of the Portland cement. Abali et al. (2006) evaluated the utilization of borax waste and fly ash as SCM in blended cement production. The results showed that blended cements gave slightly strength loss both at 3 and 28 days when the substitution ratio of waste blends was limited in 20%, while water requirement and setting time increased slightly. Monzó et al. (2003) and Garcés et al. (2008) reported that the workability, mechanical and physical properties of cement blended with 10% sewage sludge ash can meet the requirements of CEM II 42.5 blended cement. 2.4. As gypsum in cement manufacture A large quantity of natural gypsum is used by the cement industry as a set-controlling material. It is added to the mixture of cement clinker and SCMs at the grinding stage, usually at a level of 2–7%, depending on its purity (Taylor, 1990). About 70 million tones of industrial synthetic gypsums (mainly phosphogypsum and flue gas desulfurization gypsum) are produced annually in China. It would therefore be advantageous if industrial synthetic gypsums could be utilized instead of natural gypsum. Potgieter et al. (2004) proved that phosphogypsum can be used successfully as set-controlling material for cement and can act as replacement for natural gypsum without compromising the performance of Portland cement. However, some treatment of the phosphogypsum would have to be performed before it can be used in the cement production (Singh, 2002; Erdo˘gan et al., 1994). Özkül (2000) reported that reduction or delaying of setting time of Portland cement mainly depended on the type of industrial synthetic gypsums used, and the addition of industrial synthetic gypsums did not change the strengths of Portland cement significantly at all ages. Tzouvalas et al. (2004) proved that the addition of flue gas desulfurization (FGD) gypsum delayed the setting time of blended cements without affecting their compressive strength. Other industrial waste sulfates, such as jarosite/alunite precipitate (Katsioti et al., 2005) and borogypsum (Boncukcuo˘glu et al., 2002) can also be used as set-controlling material of Portland cement or blended cement. 3. Barriers to incorporate industrial wastes in cementitious materials Although numerous methods summarized in Section 2 can be applied to incorporating industrial wastes in cementitious materials, the utilization ratios of industrial wastes were still very low due to barriers of these traditional methods. For instance, the average utilization ratio of industrial wastes only reaches about 50% in China, that is to say, there is still a lot more wastes that could be used. More than 95% raw materials of alkali-activated cementitious materials or geopolymers are industrial wastes, however, the alkali activators (mainly water glass and alkali metal hydroxides) are much more expensive relative to conventional cementitious

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materials. Both initial and final setting times of alkali-activated cementitious materials are very short, leading to difficulty in practical use. In addition, the durability of alkali-activated cementitious materials is relatively poor due to high heat evolution, large volumetric shrinkage, and alkali-aggregate reaction. Therefore the application of alkali-activated cementitious materials is much limited. Only about 5–15% of raw materials in cement clinker production can be replaced by industrial wastes (mainly industrial wastes with high CaO, SiO2 and Fe2 O3 contents), and industrial synthetic gypsums used as set-controlling material account for about 3–5% of the produced cement. These tow methods make only a small contribution to the utilization of industrial wastes and are limited in actual practice mainly due to the distribution, varying moisture content and impurities of industrial wastes, which can interfere with the workability of the concrete. Therefore, utilization of industrial wastes to produce blended cement is regarded as one of the main approaches to utilize industrial wastes in cement production, and the substitution of industrial wastes in blended cements lies in the range of 5–70%. However, one shortcoming of blended cements with high industrial wastes content that has not been solved perfectly is the relatively low early strength, which also limits the substitution level of industrial wastes in blended cements. 4. Novel methods of incorporating industrial wastes in blended cement To increase the substitution level of industrial wastes and improve the performance of blended cements, there is a need to investigate other approaches to incorporate industrial wastes in cementitious materials. Therefore, novel methods of utilizing industrial wastes in blended cement production are reviewed. 4.1. Improving hydraulic (or pozzolanic) activity by adding composition adjusting material into molten industrial wastes Compared with cement clinker, industrial wastes generally contain lower amount of CaO, which is regarded as one of the main reasons resulted in poor hydraulic or pozzolanic activity. By adding calcium enriched adjusting materials into molten industrial wastes, the location of industrial wastes in (CaO + MgO) SiO2 (Al2 O3 + Fe2 O3 ) ternary phase diagram may shift toward the location of cement clinker or GBFS as shown in Fig. 1. If sufficient heat is available, it is possible to increase the content of cementitious minerals (or vitreous phase) in modified industrial wastes (depending on the calcination temperature and cooling process), which will result in improved hydraulic (or pozzolanic) activity. 4.1.1. Basic oxygen furnace slag Molten slag in the BOF contains much heat and has good fluidity. By adding high calcium adjusting material into the molten BOF slag during the high temperature discharging process and cooling to solid form rapidly, the C3 S, C2 S and calcium alumino-ferrite content in modified BOF slag should be increase, resulting in improved hydraulic activity (Li, 2011; Wang et al., 2011). The schematic diagram of the modification process in steel plant is shown in Fig. 2 (Li, 2011). Yu and his colleagues simulated the modification process in the laboratory, in which EAFS, lime (CaO) and GBFS were used as composition adjusting material individually (Gong et al., 2010; Li et al., 2010, 2011). Compared with original BOF slag, C3 S in BOF slag modified by adding lime increased by 7.3–12.7% (Table 2) and decreased in crystal size, both of which increased the hydraulic activity of modified BOF slag (Fig. 3), meanwhile the formation of calcium alumino-ferrite (such as C6 AF2 ) was proved (Li et al., 2011). Modified BOF slag gave much more similar calorimetric curves to

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Table 2 The contents of alite and belite in original and modified BOF slag (Li et al., 2011). Sample

Modification parameters

Content (mass%)

OSS S15–1250 S15–1300 S15–1350

Original BOF slag 85% BOF slag-15% lime calcined at 1250 ◦ C 85% BOF slag-15% lime calcined at 1300 ◦ C 85% BOF slag-15% lime calcined at 1350 ◦ C

Alite 8.2 15.5 17.9 20.9

Belite ± ± ± ±

0.3 0.7 0.9 1.0

38.2 36.3 34.2 33.5

± ± ± ±

1.3 1.1 1.2 1.1

Note: Alite is a solid solution of C3 S and minor other elements, and belite is a solid solution of C2 S and minor other elements.

cement without SCM) higher than 100%, indicating blended cement obtained by mixing 70% Portland cement and 30% modified BOF slag had higher 28 days compressive strength than the Portland cement (Li et al., 2011).

Fig. 1. Location of Portland cement clinker and industrial (CaO + MgO) SiO2 (Al2 O3 + Fe2 O3 ) ternary phase diagram.

wastes

in

ordinary Portland cement (OPC), and the cumulative hydration heat of which was much higher than that of original BOF slag (Li, 2011). As a result, modified BOF slag showed a compressive strength index (denoted as compressive strength ratio of cement with SCM and

4.1.2. Coal gangue To improve the pozzolanic activity of coal gangue, the coal gangue was mixed with limestone, and then heated at 700–1000 ◦ C in Li’s research (Li et al., 2006). The results showed that Ca incorporated in vitreous phase of modified coal gangue, resulting in dramatic increase of the pozzolanic activity, and blended cement with 30% modified coal gangue presented no compressive strength loss at 28 days. Red mud was used as composition adjusting material of coal gangue in Zhang’s researches (Zhang et al., 2011g). Coal gangue and red mud were blended into a mixture at a mass ratio of 3:2, the mixture was pressed into sample with size ϕ100 mm × 10 mm and heated at 600 ◦ C for 2 h. A silica–alumina based cementitious material was prepared by blending 50% ground calcined red mud–coal gangue mixture, 24% GBFS, 20% clinker and 6% gypsum. The results proved that all the strengths of the cementitious material at 3, 7, 28 and 90 days were significantly higher than those of ordinary Portland cement (Liu et al., 2011; Zhang et al., 2009). 4.1.3. Municipal solid waste incineration fly ash Lee et al. (2009) investigated the effects of basicity adjustment, modified by CaCO3 as an additive to the molten MSWI fly ash, on the compressive strengths of cement with MSWI fly ash. The results showed that the compressive strength index of MSWI fly ash increased significantly after modification, the 14, 28, and 90 days compressive strengths of blended cement with 20% modified MSWI fly ash were 100%, 99%, and 106% of that of Portland cement, respectively. 4.2. Improving surface cementitious properties of industrial wastes by dehydration and rehydration process It is proved that after been heating at 750 ◦ C, C S H gel transformed into a nesosilicate form (in which the silica tetrahedrons are connected by the metal ions) with a C2 S stoichiometry close to larnite (␤-C2 S), but less crystalline, and rehydration of the new generated nesosilicate was observed following water addition (Alonso and Fernandez, 2004; Pimraksa et al., 2009; Pan et al., 1997). With such a possibility, surface cementitious properties of some industrial wastes (mainly coal combustion ashes) can be improved by dehydration and rehydration process, meanwhile the un-burnt carbon in coal combustion ashes is ignited during the heating process, which will leads to improvement of workability, mechanical properties and durability of concrete with these ashes.

Fig. 2. Schematic illustration of the modification process in steel plant (Li, 2011): (a) basic oxygen furnace; (b) melting steel slag; (c) adjusting materials; (d) conveyer; (e) modified steel slag and (f) steel slag ladle.

4.2.1. Coal fly ash Zhang et al. (2012) blended 90% coarse fly ash (Class F) and 10% lime (CaO) into a paste at a solid to solution ratio of 1.0, the mixture was cured for different ages for hydration and then dehydrated at various temperatures in the range of 650–900 ◦ C. The

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Fig. 3. Reflected optical microcopy photos of original and modified BOFS (Li et al., 2011): (a and b) original BOFS; (c, d and e) modified BOFS with 10%, 15% and 20% lime, respectively, at 1250 ◦ C; (f, g and h) modified BOFS with 10%, 15% and 20% lime, respectively, at 1350 ◦ C.

results showed that the C S H gel mainly transformed into a nesosilicate (similar to a less crystalline C2 S) during the dehydration at 750 ◦ C. Rehydration of new generated nesosilicate on the surface of coarse fly ash led to a better bonding between coarse fly ash particles and hydration products as shown in Fig. 4. Blended cement with 39% treated fly ash presented higher compressive strengths than Portland cement at all tested ages. ˜ et al. (2000, This method can also be further developed. Goni 2003) synthesized a new kind of low-energy cement called fly

ash belite cement (FABC) from coal fly ash by alkaline hydrationdehydration method. And the synthesis parameters, hydration properties, mechanical characteristics and durability of FABC were ˜ and Guerrero, 2007, 2008; Guerrero et al., closely investigated (Goni 1999a,b, 2000, 2004, 2009a,b, 2009c,d). In addition, these materials had potential properties to intercalate heavy metal ions and there˜ et al., fore can be used as immobilization systems of these ions (Goni ˜ and Guerrero, 2010). 2006; Goni 4.2.2. Coal gangue To improve the pozzolanic activity of coal gangue, a mixture of 92% ground coal gangue and 8% CaO was heated at 800 ◦ C for 2 h in Li’s researches (Li et al., 2010b). The results show that the mineral phases such as feldspar and muscovite in raw coal gangue were partially decomposed, and the crystallinity of quartz decreased, due to the effect of adding CaO and the heating process, resulting in significant increase of pozzolanic activity of coal gangue. 4.3. Optimizing the particle size distribution of clinker and industrial wastes in blended cements according to their hydraulic activities

Fig. 4. SEM images of hardened blended cement pastes (Zhang et al., 2012): (a) blended cement with original fly ash cured for 3 days; (b) blended cement with original fly ash cured for 28 days; (c) blended cement with treated fly ash cured for 3 days; (d) blended cement with treated fly ash cured for 28 days.

Portland cement clinker and industrial wastes particles with different diameters have varying characteristics, including water requirement, hydration and mechanical performance (Frigioine and Marra, 1976; Xu, 1986). It is generally accepted that cement clinker particles larger than 45 ␮m are difficult to hydrate, particles larger than 60 ␮m have only a “filling effect” and make little contribution to strength development, and those larger than 75 ␮m ˇ hardly hydrate at all (Skvára et al., 1981). Very fine particles (<3 ␮m) may results in high early strength, but also in high water requirement, undesirable volume changes and deterioration in rheological properties (Zhang and Napier-Munn, 1995). Therefore, Tsivilis et al. (1990) concluded that cement clinker particles in the range of

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Table 3 The target parameters of the gap-graded particle size distribution (Zhang et al., 2011e). Filling grade

First

Second

Third

Mean size (␮m) Size range (␮m) Incremental volume (%) Cumulative volume (%)

45 32–80 39 100

16 8–32 25 61

6 <8 36 36

3–32 ␮m contribute most to the strength development of Portland cement and blended cement. Bentz and Haecker (1999a) and Bentz et al. (1999, 2008) proposed using coarse cements and fine SCMs in high-performance concretes. The results indicated that for high-performance concretes, coarser cements were suffice, which could result in less electricity consumption in grinding process. In consideration of coarse clinker particles gave low hydration efficiency, the possibility of replacing the coarser fraction of cement by industrial wastes or inert fillers was also investigated (Bentz et al., 1999; Bentz, 2010). The results showed that by appropriately selecting the particle size distributions of cement and fly ash, equivalent 1 day and 28 days strengths (relative to Portland cement) were achieved with 35% volumetric replacement of cement clinker by class F fly ash (Bentz et al., 2011). Replacement of coarse cement particles in an appropriate ratio (15–20%) by similarly sized limestone particles provided equivalent economic incentives with little or no reduction in long-term performance of blended cement (Bentz and Conway, 2001; Bentz, 2005). To specify how to utilize cement clinker and SCMs (industrial wastes) more efficiently, the cementitious efficiency, in terms of water requirement, hydration process and strength contribution, of cement clinker, GBFS, Class F fly ash and BOF slag was characterized in Zhang’ researches (Zhang et al., 2011a,b,c,d). The results showed that both fine and coarse fractions of Portland cement had undesirable strength contribution relative to the middle size fraction. Cement clinker fractions in the range of 8–24 ␮m had proper hydration process and were anticipated to play dominant contribution to the properties of Portland cement and blended cement. Fine GBFS and BOF slag fractions also gave comparable or higher strength contribution ratio than corresponding clinker fractions, therefore both fine and coarse clinker fractions were suggested to be replaced by SCMs with high activity and SCMs with low activity (or inert fillers), respectively (Yu et al., 2011). Zhang et al. (2011e) proposed a gap-graded particle size distribution based on close packing theory to achieve high initial packing density of fresh blended cement paste. The gap-graded particle size distribution was simplified into three fractions as shown in Table 3 and Fig. 5, in which cement particles were classified into fine (<8 ␮m), middle (8–32 ␮m) and coarse (32–80 ␮m) fractions. By arranging GBFS, clinker, and low activity industrial wastes (or inert fillers) in the fine, middle and coarse fractions respectively, both early and late properties of the gap-graded blended cements with only 25% (volume percentage) clinker content were comparable with those of Portland cement (Zhang et al., 2011f).

5. Benefits of incorporating industrial wastes in cement production Benefits that can be drawn from efficient utilization of industrial wastes in cement and concrete production are summarized as follows.

Fig. 5. Schematic outline of the gap-graded particle size distribution (Zhang et al., 2011e).

5.1. Reducing natural resources consumption Some of industrial wastes can be used as raw materials for clinker production, therefore natural resources consumption in clinker-making process can be significantly reduced. For example, steel slag can be used as an iron and calcium source material instead of iron ore and some of the limestone, silica sand and limestone can also be partly or fully replaced by Class F fly ash, calcium enriched industrial wastes respectively. Utilization of SCMs either in the form of blended cements or as direct additions to concrete may lead to a reduction in clinker requirement (hence reduced natural resources consumption and CO2 emissions), but the reality is that the reduction would more likely be to the clinker fraction, and hence the emissions, per ton of finished product. Use of SCMs will likely not result in less clinker being made by a given plant, but that more cement will be made from the same amount of clinker. However, it could reduce the need for additional cement plants, thereby reducing natural resources consumption significantly. 5.2. Energy conservation and CO2 emissions reduction Industrial wastes used as raw materials can reduce the energy requirements and CO2 emissions of the clinker-making process: (a) some materials require little or no grinding; (b) some are high enough in CaO to significantly reduce (up to perhaps 10%) the limestone consumption, which saves: (1) heat for calcination (industrial wastes contains less CO2 (not 40 + %)); and (2) releases less CO2 (from both fuel combustion and calcination); (c) some of these materials (slags, ashes) are considered to be much easier burning than the natural raw materials, for instance, fly ash is an easierburning silica source than silica sand. So anything that can be done to reduce the amount of limestone helps a lot to energy conservation and CO2 emissions reducing. In addition, large proportion of industrial wastes (except for steel slag and GBFS) used as SCM usually reduces the energy consumption of grinding stage. 5.3. Engineering and economic benefits Utilization of SCMs either in the form of blended cements or as direct additions to concrete tends to improve the workability, and reduce the water requirement at a given consistency (except for SCMs with a very high surface area, which usually lead to high water requirement and undesirable volume changes and deterioration in rheological properties). Secondly, there is an enhancement of ultimate strength, impermeability, and resistance to chemical attack.

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Thirdly, an improved resistance to thermal cracking is obtained due to low hydration heat and increased tensile strain capacity, especially for blended cements produced by method specified in Section 4.3 (Zhang et al., 2011f). As industrial wastes are much cheaper than cement clinker, the cost of blended cements can be reduced up to 50–80% of that of Portland cement depending on the substitution ratio of industrial wastes and the utilization method. 6. Conclusions The main conclusions that can be drawn from the present review are summarized as follows: (a) Sustainable development and eco-efficiency are urgent and imperative demands for cement industry, metallurgical industry, coal-fired power industry, and coal mining industry. The simplest and most efficient way is incorporating industrial wastes in cementitious materials production, thereby reducing the environmental impact of cement industry and other related industries. (b) Although industrial wastes can be incorporated in cementitious materials by various traditional methods, the substitution ratio of industrial wastes is relatively low due to barriers of these traditional methods (mainly low early strengths of blended cements with high industrial wastes content). (c) By adding composition adjusting material into molten industrial wastes, the cementitious minerals (and/or vitreous phase) content is increased significantly, therefore modified industrial wastes show a dramatically increase in hydraulic or pozzolanic activity, although the treatment process may be complex and the cost may increase. The method is particularly suitable to be applied while the slag is still molten, such as during the BOF slag discharging process. (d) Surface cementitious properties of fly ashes can be improved by dehydration and rehydration treatment, which leads to a better bounding between fly ash particles and hydration products, resulting in improved workability, mechanical properties and durability of blended cement. (e) Arranging cement clinker and industrial wastes in the particle size distribution of blended cement according to their hydraulic activities is much more practicable in blended cement production. This novel method provides effective approach to prepare high performance blended cements with larger amount of industrial wastes. (f) Efficient utilization of industrial wastes in cementitious materials plays a very significant role in CO2 emissions reducing, resources and energy conservation of the cement industry. Acknowledgments This work was funded by 973 National Foundational Research of China (No. 2009CB623104), National Natural Science Foundation of China (No. 51072058), Joint Foundation of the National Natural Science Foundation of China (No. U1134008), and the China Postdoctoral Science Foundation (No. 2012M521599), their financial support is gratefully acknowledged. References Abali Y, Bayca SU, Targan S. Evaluation of blends tincal waste, volcanic tuff, bentonite and fly ash for use as a cement admixture. Journal of Hazardous Materials 2006;131(1–3):126–30. Al-Dhamri H, Melghit K. Use of alumina spent catalyst and RFCC wastes from petroleum refinery to substitute bauxite in the preparation of Portland clinker. Journal of Hazardous Materials 2010;179(1–3):852–9. Alp I˙ , Deveci H, Süngün YH. Utilization of flotation wastes of copper slag as raw material in cement production. Journal of Hazardous Materials 2008;159(2–3):390–5.

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