# Influence of Unburned Carbon on Environmental-Technical Behaviour of Coal Fly Ash Fired Clay Bricks
## 2 Materials and Methods
### Characterisation of Fixed Samples  
Fired ceramic products of all replicates were characterised in terms of linear firing shrinkage (using a precision calliper), weight loss during firing (UNE 772-13), water absorption (UNE 67027), bulk density (UNE-EN 772-13:2001), and 3-point flexural strength (UNE-EN 843-1).  
Leaching behaviour of potentially hazardous pollutants was determined by applying the equilibrium leaching test EN12457-2 proposed by European regulation. This test was realised for raw materials and fired specimens (three replicates). Samples were milled to below 4 mm and contacted with deionised water, used as the leachant, at a liquid/solid ratio of 10 and 24 h stirring. At the end of the test, samples were filtered, and the leachates analysed. The pH was measured and the concentrations of As, Ba, Cd, Cr, Cu, Mo, Ni, Pb, and Zn in the leachates were determined by inductively coupled plasma mass spectrometry (ICP-MS). These concentrations were compared with the limits established for open landfill conditions (landfill Directive 2003/33/EC) to assess the environmental performance of the construction products at the end of life when they become demolition waste.  
The mineralogical characterisation was obtained, as described earlier for the raw materials, by XRD. Microstructural analysis of the clays, coal fly ashes, and fired bricks was carried out by field emission scanning electron microscopy (FESEM) using an accelerating voltage of 20 kV. The microstructural porosity study was performed on polished fracture surfaces, while phase distribution observations were carried out on polished samples, which were subsequently etched for 10 s in a 5% HF solution. Before observation, all samples were coated with a layer of C. The ion distribution between the different crystalline phases was determined by digital X-ray mapping. The mineralogical characterisation was obtained, as described earlier for the raw materials, by XRD.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Coal Fly Ash (A/B)** & **Clay (A/B)** \\ \hline P1 & 100 & 0 \\ P2 & 90 & 10 \\ P3 & 80 & 20 \\ P4 & 70 & 30 \\ P5 & 60 & 40 \\ P6 & 50 & 50 \\ P7 & 40 & 60 \\ P8 & 30 & 70 \\ P9 & 20 & 80 \\ P10 & 10 & 90 \\ P11 & 0 & 100 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Coal fly ash-clay brick formulation.  
Finally, the emission of potentially harmful species (HCl, HF, SO\({}_{x}\), NO\({}_{x}\), CO\({}_{2}\)) into the atmosphere during the firing of ceramic bricks were determined according to the standard methodology described in the BREF Document on Ceramic Manufacturing Industry. The method involves analysing the raw materials and the fired specimens for potentially polluting elements. The concentration of carbon, sulphur, and nitrogen was determined using a FISONS EA 1108 elemental analyser, and the concentration of halogens (Cl and F) was determined by neutron activation (INAA) and an ion-selective electrode (ISE), respectively, at Activations Laboratories (Canada). Emission values (Y) for each element were determined by simple mass balance according to Equation (1). This equation assumes that elemental Cl, F, S, N, and C are released into the atmosphere during the firing process in the form of HCl, HF, SO\({}_{2}\), NO\({}_{2}\), and CO\({}_{2}\), respectively:  
\[\psi_{\rm i}=10,000\cdot\frac{\rm M_{wi}}{\rm A_{mj}}\left[\left(\frac{100}{100 -\rm LOI}\right)C_{\rm j}^{\rm r}-C_{\rm j}^{\rm f}\right] \tag{1}\]  
where i is the polluting compounds (HCl, HF, SO\({}_{2}\), NO\({}_{2}\), CO\({}_{2}\)) and j is the constituting elements (Cl, F, S, N, and C) of the polluting compounds. \(\rm\Psi_{\rm i}\) = emission (in mg of compound i per kg of brick produced). \(\rm LOI\) = loss on ignition (%). \(\rm M_{wi}\) = molecular weight of the polluting compounds. \(\rm A_{mj}\) = atomic mass of the element. \(\rm C_{\rm j}^{\rm f}\) = concentration of element (j) in the raw material and \(\rm C_{\rm j}^{\rm f}\) = concentration of element (j) in the fired product.