5
The properties and durability of clay fly ash-based fired masonry bricks
Abstract
In the thermal power stations, huge amounts of coal combustion fly ashes are discarded. The fly ashes have very different chemical, mineralogical, and physical compositions, depending on factors such as source and type of coal and the combustion process. They can contain appreciable amounts of highly toxic trace elements. Because of this, the fly ashes are recognized as an environmental pollutant. A significant amount of coal combustion fly ashes produced worldwide is still disposed of in landfills and ash lagoons, costing money and causing environmental impact. Thus, disposal of fly ashes in an environmentally safe manner is a major challenge for thermal power stations using coal as a combustible. The reuse of fly ash as a partial replacement for common clay into fired clay masonry bricks appears to be a viable economic, safe, and sustainable option. This chapter focuses on the reuse of coal combustion fly ash as an alternative raw material to produce clay-based fired masonry bricks. Emphasis is given to the fly ash characteristics and its influence on the physical and mechanical properties. It also covers the durability of the clay-based fired masonry bricks.
Keywords
Clay bricks; Durability; Fly ash; Properties; Reuse5.1. Introduction
The thermal power stations worldwide produce huge amounts of solid waste material in the form of a fine powder, namely fly ash (Joshi & Lothia, 1997). It is noteworthy that there are other types of fly ashes originated from processes such as agricultural biomass and incineration (Barbieri, Andreola, Lancellotti, & Taurino, 2013; Cheng, Tu, Ko, & Ueng, 2011; Faria, Gurgel, & Holanda, 2012; Quaranta et al., 2011; Thy, Jenkins, Grundvig, Shiraki, & Lesher, 2006). In this chapter, however, emphasis is given mainly to the coal combustion fly ashes produced in the thermal power stations.
Fly ash is a coal combustion by-product with large chemical, mineralogical, and physical variability, depending on the coal origin and kind, and also on the combustion process. According to the ASTM C618-92a standard (ASTM, 2005) two classes of coal fly ash are defined based on the chemical composition: (1) F Class: the fly ashes of this group usually present SiO2 + Al2O3 + Fe2O3 > 70%, SO3 < 5%, moisture content <3%, and loss on ignition <6%; and (2) C Class: bothfly ashes of this group usually present SiO2 + Al2O3 + Fe2O3 < 70%. In addition, the fly ashes contain toxic trace elements and are considered to be highly polluting (Ahmaruzzaman, 2010). For this reason, the thermal power stations that use coal as a combustible are usually confronted worldwide with the environmental pollution problems. A significant amount of the coal combustion fly ashes produced worldwide have been primarily disposed of in landfills and ash lagoons (Alam & Akhtar, 2011; Sarkar, Singh, & Das, 2007), causing significant environmental impacts. In addition, such methods are very expensive. Significant impacts of improper disposal of fly ashes include land, water, and air pollution. This means that the use of safe methods for fly ash recycling that are consistent with current needs is of high economical, environmental, and social interest.
In recent years, the fired clay masonry bricks have become a highly promising technological approach for the reuse of solid wastes (Dondi, Guarini, Raimondo, & Venturi, 2002a; Faria et al., 2012; Kadir & Sarani, 2012; Quaranta et al., 2011). The waste reuse is possible if it is compatible from the technical, environmental, and economical viewpoints. Some reasons in favor of reuse of solid wastes as alternative raw materials to produce fired clay masonry bricks are: (1) availability of the clay brick industry that utilizes large volumes of natural raw materials; (2) less use of virgin raw materials; (3) the use of low-cost raw material; (4) use of common clays with large chemical and mineralogical variability; (5) the ceramic processing is not greatly modified; (6) inertization of toxic substances in the fired clayey matrix; and (7) destroying any pathogens during the firing step at high temperature.
The fired clay masonry bricks are considered one of the oldest building materials, whose use began about 5000 BC (Kadir & Sarani, 2012). Since that time, the fired clay masonry bricks are being widely used for building due to the following factors (Christine, 2004; Lynch, 1994): (1) good physical and mechanical properties; (2) durability; (3) beauty; (4) little maintenance; and (5) low cost. The industrial processing of fired clay masonry bricks consists of three main steps: (1) preparation of the clayey body; (2) conformation; and (3) thermal treatment. The clay bricks are produced in a wide firing temperature range (∼600–1100 °C), using a slow-firing cycle. The raw materials for clay bricks are mainly common clays or shales with varied colors after firing, and they also have large compositional variation. For this reason, the clay body formulations can tolerate the incorporation of different solid wastes in significant amounts.
A considerable effort is being made worldwide on the reuse of coal combustion fly ashes as a source of alternative raw materials to produce new materials such as cement, concrete, zeolites, glass-ceramics, adsorbents for cleaning of flue gas, lightweight aggregate, road subbase, and clay bricks (Ahmaruzzaman, 2010; Alam and Akhtar, 2011; Moreno et al., 2005). However, in spite of the increasing interest in using fly ash as a ceramic raw material to produce fired clay masonry bricks, more research is still needed.
This chapter is to give an overview of the current literature on the reuse of fly ash from thermal power stations to produce fired clay masonry bricks. Special emphasis is given to the fly ash characteristics and its effect on the technical properties and durability of the fired clay masonry bricks.
5.2. Fly ash characterization
The reuse of fly ash into a clay brick body requires its prior characterization (John & Zordan, 2001). In particular, the characterization of the fly ash helps to define the composition of the clayey formulation, and also helps to preview the effects of its addition during the ceramic processing and final properties. The following characteristics of the fly ash should be evaluated: chemical composition, mineral phases, physical characteristics, thermal behavior, and environmental analysis.
5.2.1. Chemical and mineralogical compositions
Table 5.1 gives the chemical composition of the fly ashes produced in several countries. As can be observed, the fly ashes present complex chemical composition. The very different chemical composition they present is related to various factors such as coal composition, pulverization degree, design of the furnace, combustion process conditions, and nature of ash collection (Reddy, Abhishek, & Babu, 2012; Sukala, Singh, Solanki, Sutariya, & Thakur, 2012). The fly ashes are essentially composed of silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3) with lesser amounts of Ca, Mg, Na, K, P, Ti, and Mn oxides. This makes the fly ashes potentially valuable sources of oxides for the manufacture of fired clay masonry bricks. In fact, the common clays and shales used in the production of fired clay bricks also are rich in SiO2, Al2O3, and Fe2O3 (Gomes, 1988).
Table 5.1
Chemical compositions of several fly ashes
| Oxides | Fly ashes (wt%) | |||||
| European fly ash | Indian fly ash | American fly ash | Brazilian fly ash | Chinese fly ash | Malaysia fly ash | |
| SiO2 | 28.50–59.60 | 45.00–65.25 | 35.00–52.00 | 58.80–71.00 | 56.58 | 56.58 |
| Al2O3 | 17.60–35.60 | 14.00–31.10 | 15.00–32.00 | 19.00–26.30 | 27.83 | 27.83 |
| Fe2O3 | 2.60–16.00 | 3.00–15.00 | 8.00–25.00 | 2.34–8.00 | 4.05 | 4.00 |
| CaO | 0.50–27.30 | 0.10–6.50 | 0.70–8.00 | 0.20–3.78 | 4.31 | 4.30 |
| MgO | 0.60–3.80 | 0.20–3.90 | 0.30–1.50 | 0.20–180 | 1.49 | 1.40 |
| Na2O | 0.10–1.80 | – | – | 0.10–0.70 | – | – |
| K2O | 0.40–4.50 | – | – | 0.40–1.76 | – | – |
| P2O5 | 0.10–1.70 | – | – | 0.04–0.06 | – | – |
| TiO2 | 0.50–2.60 | – | – | 1.18–1.21 | – | – |
| MnO | 0.02–0.10 | – | – | 0.02–0.04 | – | – |
| SO3 | 0.10–8.60 | 0.40–1.80 | 0.10–2.80 | 0.10–0.70 | – | – |
| LOI | 1.10–8.10 | 1.00–11.30 | 1.30–13.00 | 0.20–2.71 | 2.82 | 2.53 |
Sources: Silva et al., 1999; Lingling et al., 2005; Moreno et al., 2005; Sabedot et al., 2011; Shakir et al., 2013.
The fly ashes present very complex mineralogical composition with both amorphous and crystalline phases (Moreno et al., 2005; Queralt, Querol, Soler, & Plans, 1997; Reddy et al., 2012; Silva, Calarge, Chies, Mallmann, & Zwonok, 1999). The fly ashes are, from a mineralogical viewpoint, compounds of (1) a main amorphous phase (48–90% depending on the type of fly ash) in the form of alumino-silicate glass, and (2) mineral phases such as quartz (SiO2), cristobalite (SiO2), mullite (Al6Si2O13), hematite (Fe2O3), magnetite (Fe3O4), anhydrite (CaSO4), ettringite (3CaOAl2O33CaSO432H2O), feldspars ((Ca,Na)(Al,Si)4O8), calcite (CaCO3), and calcium oxide (CaO). In addition, the fly ashes also contain unburned carbon (coal).
5.2.2. Physical characterization
The fly ash is a fine powdery waste material with color varying from gray to black, depending on the amount of unburned carbon. The fly ash particles exhibit distinct morphologies of various sizes (Chusid, Miler, & Rapoport, 2009; Reddy et al., 2012) such as spherical (cenosphere or plerosphere) and irregular, with a predominance of spherical particle morphology. The specific gravity of fly ash usually ranges from 2.1 to 3.0 (Ahmaruzzaman, 2010), depending on the iron oxide content (hematite and magnetite). The specific surface area of fly ash usually ranges from 0.17 to 12.40 m2/g (Cultrone & Sebastián, 2009; Dondi et al., 2002a; Moreno et al., 2005; Silva et al., 1999), which is within the specific surface area range of the kaolinitic clays (Gomes, 1988). The real density of fly ash may vary from 1.3 to 2.7 g/cm3 (Cultrone & Sebastián, 2009; Moreno et al., 2005; Silva et al., 1999), depending on the mineral phase composition. The fly ashes exhibit a wide range of particle size distribution (0.10–250 μm) (Cultrone & Sebastián, 2009; Moreno et al., 2005; Queralt et al., 1997; Silva et al., 1999). This particle size range is compatible with the size range of common clays used to produce fired clay masonry bricks. In terms of soil mechanics, the fly ash can be classified as a nonplastic material (Silva et al., 1999). Thus, when added to a clay or shale, it influences the plastic behavior, conformation, and drying of the clayey body. This means that a clay or shale of high plasticity should be selected for use in the clay/fly ash mixes.
5.2.3. Thermal behavior
The thermal behavior of the coal combustion fly ashes strongly reflects their chemical and mineralogical compositions. In addition, the thermal behavior of fly ashes exhibits complex exothermic and endothermic events at different temperature ranges, which can influence the sintering behavior of a clay–fly ash mix. The main thermal event observed in the DTA curve of Brazilian fly ash was an exothermic peak at 530 °C associated with the presence of carbonaceous material (Silva et al., 1999). Orimulsion fly ashes (Dondi et al., 2002a) show complex thermal behavior, whose main exothermic and endothermic reactions are: (1) high weight loss at 1000 °C (41.8% ash B-Apulia; 39.6% ash F-Sardinia); (2) wide exothermic trend of the DTA curve up to about 500 °C; and (3) an endothermic event between 500 and 1000 °C. The DTA curve of coal gasification fly ash (Aineto, Acosta, & Iglesias, 2006) shows two exothermic peaks at 450 and 700 °C due to oxidation processes.
5.2.4. Environmental characterization
It is well known that the fly ashes contain heavy metals. However, their classification as a hazardous material is controversial. Some European fly ashes showed high levels of leachable trace elements (As, B, Ba, Cr, and Sr) according to the DIN-38414 standard (Moreno et al., 2005). Leaching tests in municipal solid waste incineration (MSWI) fly ash showed that the leaching concentrations of As, Hg, Pb, Zn, and Cd exceeded the Chinese standard limits (Haiying, Youcai, & Jingyu, 2011). Hence, this type of fly ash presents a high potential for environmental pollution. It was found that Brazilian fly ashes presented leaching concentrations of As, Pb, and total Cr above Brazilian leaching limits (Sabedot et al., 2011). This means that the fly ash sample can be classified as a hazardous material according to the NBR 10004 standard. On the other hand, the vast majority of the American fly ashes present low concentrations of heavy metals (Chusid et al., 2009), being considered nonhazardous materials.
5.3. Fly ash-based fired clay masonry brick processing
The processing flow diagram in Figure 5.1 summarizes the main steps used to produce fired clay masonry bricks incorporated with different fly ashes. As can be observed, all research works have been conducted to follow the conceptual flowchart for the industrial production of fired clay masonry bricks (Gomes, 1988; Santos, 1989): common clays, preparation of the clayey body, shaping, drying, and firing. However, the researchers have used different fly ashes and common clays and also adopted different processing conditions to produce fired clay masonry bricks.
The common clays and fly ashes used to produce fired clay masonry bricks present a very broad range of chemical, mineralogical, and physical characteristics. The coal combustion fly ashes are chemically very similar to the natural common clays. Thus, a good chemical compatibility between fly ash and common clay should be expected.
Table 5.2 gives the different processing conditions used to produce fired clay masonry bricks containing fly ash. The clay bodies could tolerate the incorporation of a variety of coal combustion fly ashes in the form of fine powder.
The common clay was replaced by up to 90 wt% of fly ash. It can be seen that different mixing methods (mechanical and manual), different shaping methods (pressing and extrusion), different drying methods, and different firing process (different heating/coaling rates and firing temperatures) have been used.
Table 5.2
Processing conditions used in the production of fly ash-based fired clay masonry bricks
| Fly ash | Fly ash amount (wt%) | Mixing method | Forming method | Drying | Firing |
| Teruel fly ash | 40 | Mechanical/ 5 wt% Water | Pressing/100 MPa | Not defined | Fired between 900–1200 °C for 3 h |
| Orimulsion fly ash | Up to 6 | Manual | Extrusion | Dried at room temperature for 48 h, and then dried at 100 °C for 24 h in an oven | Fired in an electrical kiln at 900 °C |
| Nanjing fly ash | Up to 80a | Not defined | Not defined | Dried at room temperature for 48 h, and then at 60 °C for 4 h and 100 °C for 6 h | Fired in an electrical kiln between 1000 and 1100 °C for 8 h |
| IGCC fly ash | 20 | Not defined/5 wt% Water | Pressing/20 MPa | Dried at 110 °C | Fired at 900 °C for 5 h |
| ESP fly ash | Up to 90 | Mechanical/7 wt% Water | Pressing/40 MPa | Dried at room temperature for 24 h, and then dried at 110 °C for 24 h in an oven | Fired in an electrical kiln at 1000 °C for 2 h |
| High-sulfate-containing fly ash | 85–90 | Mechanical | Pressing/10 MPa | Dried at room temperature for 72 h, and then dried at 110 °C in an oven | Fired in an electrical kiln between 800–1000 °C |
Sources: Aineto et al., 2006; Baspinar et al., 2010; Dondi et al., 2002b; Lingling et al., 2005; Queralt et al., 1997; Sarkar et al., 2007.
5.4. Effects of fly ash on the technological properties
The quality of the fly ash-based clay masonry bricks after firing has been evaluated in terms of the main technological properties (linear shrinkage, water absorption, apparent density, apparent porosity, and mechanical strength). Table 5.3 summarizes the technological properties of fly ash-based clay masonry bricks incorporated with different coal combustion fly ashes.
Queralt et al. (1997) reported on the firing behavior of fly ash and plastic clay (60 wt%)–fly ash (40 wt%) mixture between 900 and 1200 °C. Substantial changes to the mineralogical evolution of fly ash with increasing firing temperature (Figure 5.2) were found. The main changes suffered by the fly ash during the firing process at high temperature were: (1) decreasing the amount of the glassy phase (alumino-silicate glass); (2) increasing the amount of mullite; (3) the iron-based minerals are converted to hematite; and (4) the development of feldspar minerals and cristobalite. By increasing the firing temperature, important changes to the physical properties (linear shrinkage, water absorption, and apparent density) of plastic clay (60 wt%)–fly ash (40 wt%) mixture were observed. The linear shrinkage and the apparent density increase, while the water absorption (open porosity) decreases, mainly above 1000 °C. In addition, all these changes were favored by the mineralogical evolution of the fly ash on firing. The ceramic pieces with plastic clay (60 wt%)–fly ash (40 wt%) could be used in the production of ceramic building materials, including fired clay masonry bricks. It was also found the fired clay pieces change color from pale brown to dark brown.
Dondi, Guarini, Raimondo, and Venturi (2002b) investigated the use of up to 6 wt% of two Orimulsion fly ashes (ash B–Apalua and ash F–Sardinia) in the manufacture of fired clay masonry bricks. The use of Orimulsion fly ashes has caused several changes to the technical properties of both unfired and fired clay bricks. The preparation of a homogeneous clay–Orimulsion fly ash mixture is very difficult due to the fly ash being highly hygroscopic. Orimulsion fly ashes have caused detrimental effects on the plasticity, drying rate, and drying sensibility of the plastic clay–fly ash mixtures. Increasing the amount of fly ash tends to change the firing color, resulting in a more yellow color of clay masonry bricks. The technological properties (linear shrinkage, water absorption, bulk density, total porosity, and flexural strength) of clay masonry bricks fired at 900 °C depend to a large extent on the added Orimulsion fly ash amount. It was found that the Orimulsion fly ash could be used in fired clay masonry bricks, in the range of 1 to 2 wt%, as a partial replacement for common clay.
Lingling, Wei, Tao, and Nanru (2005) reported on the influence of fly ash with a high replacing ratio (up to 80% vol) of clay on the physical properties of fired clay masonry bricks. The plasticity index of clay–fly ash mixtures was found to be strongly decreased by the fly ash addition. This substantial decrease in plasticity is in line with the nonplastic nature of the fly ash. When the proportion of fly ash in clayey formulation is 80% by volume, the plasticity index reaches 3.8%. The improvement of the plasticity characteristics of clay–fly ash mixtures with a high volume ratio of fly ash can be done with the use of additives. The results showed that significant changes to the physical and mechanical properties (Figures 5.3–5.5) of clay–fly ash fired masonry bricks with a high volume ratio of fly ash occurred. It was observed that the water absorption increases (Figure 5.3), apparent density decreases (Figure 5.4), and compressive strength (Figure 5.5) decreases with increasing the ratio of fly ash in fired clay bricks. Despite this, the clay–fly ash fired masonry bricks with a high ratio of fly ash met the requirements in the Chinese standard specifications.
Table 5.3
Physical and mechanical properties of fly ash-based fired clay masonry bricks
| Fly ash | LS (%) | AD (g/cm3) | AP (%) | WA (%) | CS (MPa) |
| Teruel fly ash | 2.00–3.00 | 2.00–2.40 | – | 0.50–9.25 | – |
| Orimulsion fly ash (B) | 0.10–1.30 | 1.79–1.81 | 26.00–34.00 | 14.50–19.00 | 19.00–25.00a |
| Orimulsion fly ash (F) | 0.00–2.10 | 1.91–2.04 | 26.00–30.50 | 12.50–16.00 | 11.00–14.00a |
| Nanjing fly ash | – | 1.35–2.20 | 1.34–42.12 | 0.61–31.26 | 14.70–98.50 |
| IGCC fly ash | Negligible | – | – | 10.38–14.13 | 0.71–2.20 |
| ESP fly ash | – | 1.426–1.430 | 35.6–36.2 | 24.4–25.2 | 18.6–19.1 |
| High-sulfate-containing fly ash | – | 1.02–1.27 | 32.31–48.90 | 25.46–48.00 | 1.90–8.65 |
| Jalna fly ash | – | – | – | 3.74–14.23 | 2.18–953 |
| Indian fly ash | – | 1.20–1.85 | – | 10.00–21.00 | 6.50–30.00 |
LS–linear shrinkage; AD–apparent density; AP–apparent porosity; WA–water absorption; CS–compressive strength.
Sources: Queralt et al., 1997; Dondi et al., 2002b; Lingling et al., 2005; Aineto et al., 2006; Sarkar et al., 2007; Baspinar et al., 2010; Bansode, 2012.
Aineto et al. (2006) reported on the use of coal gasification fly ash in the production of fired clay-based ceramics. The results showed that the addition of coal gasification fly ash has caused only a slight reduction of the plasticity of the clay–fly ash mixes. It was found that the coal gasification fly ash (added up to 20 wt%) acts as an active additive that improves the densification behavior and technical properties of the clay masonry bricks fired at 900 °C. This behavior can be explained by the chemical and mineralogical compositions that tend to favor the formation of a liquid phase at lower firing temperatures than the fly ash-free clay bodies. The results also showed that the introduction of coal gasification fly ash had a beneficial effect on the technical properties (lower absorption, lower saturation coefficient, and higher mechanical strength) of the fired bricks. In addition, the coal gasification fly ash containing clay masonry bricks showed only a slightly darker reddish coloration, as well as no efflorescence observed.
Sarkar et al. (2007) investigated the effect of the electro-static precipitator (ESP) fly ash addition on the technological properties of clay masonry bricks fired at 1000 °C. Addition of EPS fly ash to common clay was found to reduce the apparent density and increase the water absorption. During the firing process, the EPS fly ash remains inert and negatively influences the densification behavior of the EPS fly ash containing fired clay masonry bricks. Despite this, up to 80 wt.% of EPS fly ash could be incorporated to clay to produce fired clay masonry bricks with good technical properties (apparent density = 1.426–1.430 g/cm3, water absorption = 24.4–25.2%, apparent porosity = 35.6–36.2%, and mechanical strength = 18.6–19.1 MPa).
Baspinar, Kahraman, Gorhan, and Demir (2010) reported on the use of high-sulfate-containing fly ash with boric acid (H3BO3) addition in the production of fired clay bricks. The test pieces were fired between 800 and 1000 °C, which is the industrial clay brick firing temperature range. Increasing the firing temperature improved the technological properties (water absorption, apparent density, apparent porosity, and compressive strength) of the fired high-sulfate-containing fly ash–clay bricks. In addition, an increase in the firing temperature also contributed to improving the sintering behavior of the fly ash particles. The experimental results indicated that it was possible to produce fired bricks with good technological properties (Figures 5.6 and 5.7) from high-sulfate-containing fly ash by adding clay and boric acid simultaneously. The better clayey formulation was that composed of high-sulfate-containing fly ash with the addition of 10% clay and 5% boric acid. Such behavior was mainly due to the increase of alkali and earth alkali oxides present in the fly ash and clay with B2O5, which tends to accelerate the vitrification process. This formulation also had a denser well-sintered microstructure.
Haiying et al. (2011) investigated the use of MSWI fly ash as an alternative raw material to produce fired ceramic bricks. The leaching characteristics of the fired bricks also were determined. The fly ash is rich in SiO2, Al2O3, and CaO, as well as having a low plasticity index (3.7%). The orthogonal test and performance analysis indicated that the optimal clayey formulation (named R20) was fly ash: 20%, red ceramic clay: 60%, feldspar: 10%, and gang sand: 10%. The results showed that the technological properties (shrinkage, water absorption, and compressive strength) of the R20 formulation are strongly influenced by the firing temperature. It was found that the firing shrinkage decreased with increase of firing temperature (900–1050 °C). The compressive strength of the ceramic bricks tends to increase with the increase of firing temperature. However, the water absorption presented different behaviors, depending on the firing temperature. Between 900 and 1000 °C, the water absorption decreased, and then rose with an increase of firing temperature from 1000 to 1050 °C. This behavior was well correlated with the appearance quality. The results also showed that the sintered microstructure was strengthened with the increase of firing temperature. The open porosity was strongly reduced. In terms of mineralogical evolution during the firing process the following trends were found: (1) the amount of crystalline phases of the sintered ceramic matrix first increased and then decreased; and (2) the glassy phase amount presented a reverse trend. The following crystalline phases were found: quartz, cordierite, enstatite, mullite, and andradite. Leaching results of heavy metals (As, Cd, Cr, Cu, Ni, Pb, Hg, and Zn) from sintered ceramic matrix were reduced considerably in comparison with those from unfired ceramic matrix.
Bansode (2012) investigated the influence of fly ash and steel slag on the technological property behavior of solid bricks. The fly ash sample used met specification ASTM C618 for Class C fly ash. The results showed that the addition of fly ash and steel slag have strongly influenced the technological properties of clay bricks, particularly for water absorption (3.74–14.23%) and compressive strength (2.18–9.53 MPa). However, the use of a high amount of fly ash in the clayey formulations is very problematic due to the less adhesive effect of clay. Despite this, a clayey formulation containing 40% fly ash with 60% clay could be a good formulation to produce clay–fly ash masonry bricks.
Resourceefficientbricks (2013) has reported on the processing and technological properties of clay–fly ash masonry bricks fired at 1000 °C. Indian fly ashes and alluvial, black, and red soils in different proportions were used. It was found that the fly ash reduces the plasticity of the clay–fly ash mixes, as well as reducing the drying time and shrinkage cracks. The technological properties (water absorption, bulk density, and compressive strength) of the fired bricks are strongly influenced depending on the fly ash amount and soil type used. In this report, the following advantages of clay–fly ash fired masonry bricks were found: (1) bricks conforming to IS:3102 can be produced; (2) fuel saving in the range of 15–35% (coal composition) or coal savings up to 3–7 tons per lakh bricks; (3) drying losses are checked in the case of plastic black and red soils. Excessive linear drying shrinkage is reduced; (4) brick strength in the case of black and red soil is increased by almost one and a half times (30–50%); (5) waste material is used 30–40 tons per lakh in the case of alluvial soils and 100–125 tons per lakh bricks in the case of red and black soils; and (6) clay savings in brick manufacture is 10–40 wt%.
As described previously, different results have been obtained. However, a direct comparison between the several results is very complex and difficult. This is related primarily to three main factors: (1) the use of various types of fly ashes in different proportions; (2) the use of different common clays; and (3) the use of different processing conditions. Despite this, some observations are given below:
• In general, the introduction of fly ash into a clay body has a positive effect on the ceramic processing and technological properties. The clay–fly ash masonry bricks present good dimensional stability (low firing shrinkage), lower open porosity (lower water absorption), and higher mechanical strength (higher compressive strength).
• The amount of fly ash that could be added to a clay depends on two factors: common clay nature and fly ash chemical, mineralogical, and physical characteristics.
• The common clays and fly ashes present large chemical and mineralogical variability from one source to another. This means that no general rule can be applied for their use in fired clay masonry bricks.
• The fly ash in fine powder form (<250 μm) tends to favor good mixture and packaging of the clay–fly ash mixes in the forming step.
• The firing temperature of fly ash-containing fired clay masonry bricks is usually higher than that of fly ash-free fired clay bricks. This indicates that the addition of fly ash into fired clay masonry bricks could be problematic in many countries because it tends to increase the manufacturing cost.
• The firing color is an important criterion used to qualify fired clay masonry bricks. The fly ash addition tends to cause a gradual shift from light to dark or vice versa on the firing color. The greater the amount of fly ash added, the greater the shift in color.
5.5. Durability
Durability is one of the major requirements to be considered in the use of building materials. The durability of fired clay masonry bricks is strongly dependent on the common clays and processing conditions (Koroth, Fazio, & Feldman, 1998). In particular, this property is considered to be closely related to the densification degree (lower water absorption and higher density) of the clay bricks. The main factors that influence the durability of fired clay masonry bricks are: (1) cycles of heat and cold; (2) cycles of wetting and drying; and (3) soluble salt crystallization.
Despite its practical importance, the durability of clay–fly ash fired masonry bricks has been little investigated. However, Cultrone and Sebastián (2009) have shown that the incorporation of fly ash into clay tends to improve the durability of fired clay masonry bricks. This behavior is due to a reduction of the volume of the smallest pores in the sintered microstructure of clay bricks. In particular, the smallest pores are primarily responsible for most damage caused to the fired clay bricks due to soluble salt crystallization.
5.6. Future trends
In this chapter, it has been shown that coal combustion fly ash can be a good alternative raw material as a partial replacement of common clay in the manufacture of fired clay masonry bricks. However, more research should be conducted in this area in order to clarify several aspects of the incorporation of fly ash into the clay body, such as:
1. A key issue in the manufacture of clay–fly ash based fired masonry bricks is the clayey body formulation. In general, an empirical approach has been used. Thus, the use of experimental design to optimize the clayey formulation must be studied.
2. The phase composition and sintered microstructure of the fired masonry bricks can change substantially with the incorporation of the fly ash. Thus, more research must be done in order to understand the complex interactions between clay and fly ash during firing.
3. The coal combustion fly ashes have toxic components, such as heavy metals, which are hazardous to the environment and health. Thus, the environmental assessment (leaching and solubilization tests) of the fired bricks and gaseous emissions during the thermal treatment step (drying and firing) must be studied.
4. It has been observed that there is a lack of studies about the evaluation of the life cycle of the fly ash-based fired clay masonry bricks.
Acknowledgments
This work has been supported by CNPq and FAPERJ.
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