# Evaluation of Rice Straw Ash as a Pozzolanic Addition in Cementitious Mixtures
## 3 Results  
### Physical and Chemical Characterization of RSA  
The particle size distribution of RSA after two grinding procedures is seen in Figure 3. The mean particle diameter was 13.3 um after the second grinding, which is appropriate to obtain good performance pozzolan [29]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mortar** & **PC (g)** & **RSA (g)** & **Water (g)** & **Sand (g)** \\ \hline
**Control** & 450.0 & - & 225.0 & 1350.0 \\
**RSA 15\%** & 382.5 & 67.5 & 225.0 & 1350.0 \\
**RSA 30\%** & 315.0 & 135.0 & 225.0 & 1350.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Doses of the control and PC/RSA mortars. The chemical composition of RSA is summarized in Table 2. The main oxides in the ash were SiO\({}_{2}\) and K\({}_{2}\)O. The material had a high loss of ignition percentage (14.6%), which is possibly attributed to the presence of unburned particles and carbonates. Chloride content was high (3.5%), which should be taken into account for certain uses as a PC substitute (not recommended for steel-reinforced concrete because of the potential activation of metal corrosion). The high quantity of alkali compounds should be taken into account when reactive aggregates are used to avoid dangerous aggregate-alkali reactions; however, the studied residue (RS) was locally produced from the Albufera (Valencia, Spain) for its near geographical zones application and this type of aggregate is not usually used in this area. The obtained insoluble waste was 63.1% and total silica was 56.8%. This last value differed from that obtained by the XRF analysis, which can be attributed to differences in the analytical method because of its distinct filtration and calcination processes. The percentage of amorphous silica was 51.8%, which is very high compared to that of rice husk ash (RHA) analyzed by [PERSON] et al. [25] (85% of total SiO\({}_{2}\) and 31.5% amorphous silica). This means that more than 90% of the silica in RSA is in an amorphous state, which is a good indicator of its potential pozzolanic activity. In Figure 4, the XRD diffractograms of RSA and RSAW are represented. For RSA, the main phase was silvine (KCI, PDFcard #411476) according to the marked presence of chloride. There were also two crystalline compounds associated with silica: quartz (SiO\({}_{2}\), PDFCard #331161) and cristobalite (SiO\({}_{2}\), PDFCard #391425). Some minor peaks associated with the crystalline phases of arkanite (K\({}_{2}\)SO\({}_{4}\), PDFCard #050613) and calcite (CaCO\({}_{3}\), PDFCard #050586) were observed. The baseline deviation fell within the 2\(\Theta\) =18\({}^{\circ}\)-37\({}^{\circ}\) range, which was attributed to the amorphous phase [25]. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \% & SiO\({}_{2}\) & Al\({}_{2}\)O\({}_{3}\) & Fe\({}_{2}\)O\({}_{3}\) & CaO & MgO & SO\({}_{3}\) & K\({}_{2}\)O & Na\({}_{2}\)O & P\({}_{2}\)O\({}_{5}\) & Cl\({}^{-}\) & Other & LO.I \\ \hline
**RSA** & 52.44 & 0.47 & 0.17 & 8.01 & 2.71 & 2.26 & 12.05 & 0.89 & 2.58 & 3.52 & 0.29 & 14.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition (wt %) of RSA (X-ray fluorescence (XRF) analysis). Figure 3: Particle size distribution of RSA. For the diffractogram of RSAW (Figure 4b), compared to RSA (Figure 4a), the silvine peak decreased and the baseline deviation significantly increased. A new peak also corresponded to hydroxyl chlorinated apatite (Ca\({}_{5}\)(PO\({}_{4}\))\({}_{3}\)(OH,Cl), PDFCard #250166). This peak was masked before in the unwashed ash because of the high-intensity silvine peak. The low height value of the peaks vs. the baseline deviation confirmed the main amorphous nature of ash, which agrees with 51% amorphous silica. To analyze the effectiveness of the washing process, the soluble chloride contents in the original and washed samples were determined: 4.7% Cl appeared in the original RSA, which is far removed from the required value according to Standard UNE-EN 450-1 [30] (0.1% Cl) for using fly ash pozzolan in cement.