# Evaluation of Rice Straw Ash as a Pozzolanic Addition in Cementitious Mixtures
## 3 Results
### Analysis of RSA Reactivity
#### 3.2.4 Mechanical Studies about Mortars  
The mechanical behavior values, obtained for the different analyzed PC/RSA mortars, are represented in Figure 12. \begin{table}
\begin{tabular}{c c c c} \hline
**Key** & **Mineral Phase** & **Chemical Formula** & **PDFcard** \\ \hline Q & Quartz & SiO\({}_{2}\) & 331161 \\ \hline R & Cristobalite & SiO\({}_{2}\) & 391425 \\ \hline C & Calcite & CaCO\({}_{3}\) & 050586 \\ \hline A & Arkanite & K\({}_{2}\)SO\({}_{4}\) & 050613 \\ \hline P & Portlandite & Ca(OH)\({}_{2}\) & 040733 \\ \hline H & Hydrotalcite & Mg\({}_{6}\)Al\({}_{2}\)CO\({}_{3}\)(OH)\({}_{16}\).4H\({}_{2}\)O & 140191 \\ \hline S & Hydrated potassium carbonate & K\({}_{2}\)CO\({}_{3}\).1.5H\({}_{2}\)O & 110655 \\ \hline \end{tabular}
\end{table} 
Table 4: XRD characterization: name of minerals, PDF card of phases and chemical formula of phases presented in CH/RSA and CH/RSAW pastes. \begin{table}
\begin{tabular}{c c c c c} \hline  & **3 Days** & **7 Days** & **28 Days** & **90 Days** \\ \hline
**PC/RSA 7/3** & 60.9\% & 64.8\% & 81.5\% & 90.7\% \\
**PC/RSA 7.5/2.5** & n.d * & 62.1\% & 69.5\% & n.d \\
**PC/RSA 8/2** & n.d & 59.2\% & 61.1\% & n.d \\
**PC/RSA 8.5/1.5** & 37.8\% & 45.1\% & 49.8\% & 55.7\% \\ \hline \end{tabular}
\end{table} 
Table 5: Fixed CH percentages for pastes PC/RSA. For flexural strength (Figure 12), the mortars with 15% RSA yielded similar strengths to those of the control mortar, while the mortars with 30% RSA slightly decreased. Usually, the flexural strength values of mortars show more dispersion and relatively high standard deviation values. As expected, by substituting part of PC with RSA, similar or lower strengths are obtained, because the reduction in the amount of hydrates, formed by hydration of PC, is not completely compensated by the pozzolanic reaction products. This is why the flexural strengths for the 30% RSA replacement mortars were slightly lower than those for the 15% RSA replacement mortars  
To assess the contribution of pozzolanic material to developed strength, the strength activity index (SAI) was calculated following UNE-EN-450-1 [30]. The obtained results are shown in Table 6. With both the 15% and 30% replacements, the SAI yielded values close to 100% after 3 curing days, which indicates good pozzolanic behavior for RSA. After 90 days, values exceeded 100%. Similar behavior appeared after analyzing other ashes with good resistant activity, such as FA [39]. The compressive strength values are shown in Figure 13. It is noteworthy that the 15% and 30% replacements did not bring about a marked reduction in strength: for the 15% replacement series, compressive strength after 28 days was greater than that for the control mortar, whereas this behavior appeared after 90 curing days for the 30% series. In the same way that has been explained for the flexural strength, in the compressive strength we find lower values for the mixture with 30% RSA with respect to the mixture with 15% RSA. To assess the contribution of RSA to compressive strength evolution, fitting strength vs. curing time was carried out. The curve followed a logarithmic trend (Equation (5)):  
\[\text{Rc}=\alpha+\beta\text{ln(t)} \tag{5}\] \begin{table}
\begin{tabular}{c c c} \hline \hline
**AGE (Days)** & **RSA 15\%** & **RSA 30\%** \\ \hline
**3** & 97.45 & 80.79 \\
**7** & 98.16 & 83.41 \\
**28** & 105.97 & 93.15 \\
**90** & 108.95 & 107.54 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Strength activity index of the mortar with RSA. Figure 12: Flexural strength of the analyzed PC/RSA mortars after 3, 7, 28 and 90 days. Figure 13 represents compressive strength evolution versus curing time. The adjustment parameters are shown in Table 7.  
As seen in Table 7, the \(\alpha\) values are lower for the mortar control, which causes the curve to move vertically downward, which indicates lower compression strength values at an early age. Thus, for short curing ages, the contribution of pozzolan to strength did not compensate the PC replacement with RSA.