# Influence of Unburned Carbon on Environmental-Technical Behaviour of Coal Fly Ash Fired Clay Bricks
## 3 Results and Discussion  
### Characterisation of Raw Materials  
Table 2 shows the results of the main oxides and elements in all of the raw materials used to form bricks. Clays and FAA had three major oxides (silicon, aluminium, and iron), which represent over 75% of raw materials, whereas FAB represents 61%. The minor fraction consists of eight oxides: manganese, magnesium, calcium, sodium, potassium, titanium, and phosphorus. LOI values obtained for each type of clay were related to the content of unburned carbon. Furthermore, FAA and FAB had a higher content of alkaline, earth alkaline, and iron oxides, which make them suitable for their valorisation in fired clay ceramic bricks due to its melting character. Particle size of quarry clays was less than 0.5 mm in clay A, and 1 mm in clay B, with average particle sizes of 180 \(\upmu\)m and 220 \(\upmu\)m, respectively. The particle size distribution of fly ashes is given in Figure 2. FAA had a lower particle size than FAB (average particle size of 65 \(\upmu\)m of FAA versus 111 \(\upmu\)m of FAB). FAA showed 4% ash particles above 100 microns; 23% of particles between 63 and 100 microns; and 73% of fly ash with a particle size below 63 microns. Instead, FAB had 25% of particles with sizes greater than 100 microns; 23% with sizes between 63 and 100 microns; and 52% of particles with sizes lower than 63 microns. Figure 2: Particle size distribution of coal fly ashes A and B.  
Mineralogical analysis of raw materials is shown in Figure 3. The mineralogical analysis of clays A and B showed an amorphous fraction in the form of maximum width between 15 and 30 degrees. On the other hand, the crystal phases present were mainly quartz (SiO\({}_{2}\)), muscovite (KAl\({}_{3}\)Si\({}_{3}\)O\({}_{10}\)(OH)\({}_{1.8}\)F\({}_{0.2}\)), dickite (Al\({}_{2}\)Si\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\)), and kaolinite (Al\({}_{2}\)Si\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\)). These phases can constitute approximately 90% of the total clay, respectively. Furthermore, minority phases are iron oxides (Fe\({}_{3}\)O\({}_{4}\)). FAA and FAB showed an amorphous fraction in the form of maximum width, much larger than clays, between 15 and 30 degrees. On the other hand, the crystal phases present were mainly quartz (SiO\({}_{2}\)), mulitle (Al\({}_{4+2}\)Si\({}_{2-2}\)O\({}_{10-x}\) being x\(\approx\)0.4), and anhydrous aluminium silicate (Al\({}_{2}\)O\({}_{7}\)Si\({}_{2}\)). These phases can constitute approximately 70 and 50% of FAA and FAB, respectively. Furthermore, minority phases were dolomite CaMg(CO\({}_{3}\))\({}_{2}\), calcium aluminate (xCaO yAl\({}_{2}\)O\({}_{3}\) being x and y the number of molecules), calcium silicate (2 CaO.SiO\({}_{2}\)), and iron oxides (Fe\({}_{3}\)O\({}_{4}\)). Figure 4a shows the thermal behaviour during firing (10 \({}^{\circ}\)C/min) of clays A and B up to 1200 \({}^{\circ}\)C. Thermogravimetric analysis showed the typical clay dehydroxylation process (water of hydration) with a mass loss of 2.33% and 3.57% up to 100 \({}^{\circ}\)C, respectively. The second step was around 500-550 \({}^{\circ}\)C, which corresponds with the dehydroxylation of clays (aluminosilicate decomposition) with a mass loss of 5.70% and 5.84%, respectively. Figure 4b represents the thermogravimetry of both FA. The mass loss of 1.55% for FAA and 2.93% for FAB between 100 and 400 \({}^{\circ}\)C corresponded to the thermal decomposition of water present in ashes. Between 500 and 700 \({}^{\circ}\)C, where the mass loss for FAA was 5.69% and for FAB was 27.91%, this corresponded to the thermal decomposition of carbonates and unburned carbon. In this case, the decomposition of carbonates was due to the presence of dolomite MgCa(CO\({}_{3}\))\({}_{2}\), which is decomposed into calcium and magnesium oxides and carbon dioxide. The total mass loss due to thermal decomposition of water, carbonates, and unburned carbon for FAA was 7.95% and 30.84% for FAB, much higher due to the high contents of unburned carbon of FAB.