## 3 Results and Discussion
Figure 18 compares two different recipes having combustion chamber dust and oxy-cutting fines as base raw materials. Between the recipes, a number of parameters were altered at the same time, so it is not so easy to draw clear conclusions. However, it is remarkable that, again, the samples using fibres as a reinforcement showed an increase in compression strength. The increase was up to 150%, even though more than double the amount of starch compensated for the omission of the fibres in recipe CC03. At the same time, the results indicate that the use of BS (500 \(\upmu\)m clearance) did not lead to reduced compression strength in comparison to the use of SC (50 \(\upmu\)m clearance), at least for the higher MP. The results obtained with the RWTH stamp press were verified with the bricks produced by the UOulu vibrating press. Compression strength tests at UOulu were performed after 2 and 7 days of curing at room temperature. The results are shown in Table 14 below. After 28 days of curing, cold-bonded briquettes having around 11% Portland cement (the same as the starch content used  
Figure 17: Compression strength (average of four tests) of recipes based on grinding sludge and oxy-cutting fines with and without fibres. Figure 18: Compression strength (average of four tests) of recipes based on combustion chamber dust and oxy-cutting fines. in our biquettes) are reported to have a compression strength of 54 kN for fine raw material and 25 kN for coarser raw material [14]. The compression strength of UO recipe was comparable to CC03 briquettes. However, it was lower than that of MA604B BC. This could have been due to the shorter curing time but could also indicate that increasing the paper fibre content over 0.9% may have a negative influence on the compression strength. The geometry of produced briquettes might also be a factor that influences that final strength [15]. A drop test was performed after 2 days of curing from a height of 1 m and after 7 days of curing from a height of 5 m. Briquettes were able to survive 50 drops without falling below 50% of their original mass, which means the bricks had adequate mechanical strength for material handling (Figure 19). Furthermore, the 7-day drop test from 5 m showed that the bricks can withstand the high drop distance associated with charging the bricks to raw material silos. Mass measurements and mass loss percentage are shown in Table 15.  
SEM observations were carried for the fracture surface of the briquettes on a Zeiss ULTRA plus field emission scanning electron microscope (FESEM), using a secondary electron detector at 5-15 kV. Figure 20 shows clearly how the starch formed a film that surrounded smaller spherical particles originating from oxy-cutting fines and interconnected smaller and bigger particles together. Figure 20b \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{2}{c}{**BRIQUETTE**} & \multicolumn{2}{c}{**TEST**} & \multicolumn{2}{c}{**COMPRESSION**} & \multicolumn{2}{c}{**STRENGTH**} \\  & & & **TEST RESULT (N)** & \multicolumn{2}{c}{**(N/mm\({}^{2}\))**} \\ \hline
21 & S11 & W0 & 2-day compression test & 34,156.20 & 17.05 \\
23 & S11 & W0 & 7-day compression test & 45,476.59 & 22.70 \\ \hline \hline \end{tabular}
\end{table} 
Table 14: Compression test results after 2 and 7 days. Figure 19: Mass loss in drop tests conducted after 2 and 7 days of curing in ambient air. captures the local influence of the fibrous reinforcement on the briquette. Fibres appeared to be well dispersed, while starch and smaller particles appeared to be adhering to the fibre body, indicating that the fibres and the starch matrix were well bonded. The fibre bridging action was likely one of the factors contributing to the enhanced mechanical properties of the produced briquettes. LECO analysis was performed to obtain the total amount of carbon and sulphur in the bricks. The results of the analysis are presented in Table 16. The analysis showed some variance between different bricks of the same batch, but the average carbon content was on average 15.0 wt %.