# Evaluation of Rice Straw Ash as a Pozzolanic Addition in Cementitious Mixtures
## 3 Results
### Analysis of RSA Reactivity
#### 3.2.3 Thermogravimetry Analysis  
Figure 9 represents the derivatives curves (DTG) for the 1/3 and 1/1 CH/RSA pastes after 3, 7, 28 and 90 curing days. The presence of a wide peak within the 100-250 \({}^{\circ}\)C range is observed, mainly due to the formation of hydrated calcium silicates (CSH) by the pozzolanic reaction. A peak within the 520-580 \({}^{\circ}\)C temperature range can be attributed to  
Figure 8: Comparison results of Frattini test for RSA, rice husk ash (RHA) and SF at 10% and 25% after 8 days. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Percentage of Replacement (\%)** & **8 Days** & **15 Days** \\ \hline \multirow{3}{*}{**PC/RSA**} & 10\% & Negative & Negative \\  & 20\% & Positive & Positive \\  & 25\% & Positive & Positive \\ \hline \multirow{3}{*}{**PC/RSAW**} & 10\% & Negative & Negative \\  & 20\% & Positive & Positive \\ \cline{1-1}  & 25\% & Positive & Positive \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results obtained in the Frattini test of Portland cement/rich straw ash (PC/RSA) and PC/RSAW after 8 and 15 days. the dehydroxylation reaction of the unreacted portlandite (CH). The fixed CH percentage was calculated with this mass loss according to the following equation (Equation (3)) [35]:  
\[Fixed\;CH\;(\%)=\frac{CHo-CHp,t}{CHo}\times 100 \tag{3}\]  
where:  
CH\({}_{o}\) is the initial quantity of CH in paste CH/RSA  
CH\({}_{p,t}\) is the quantity of CH present in paste CH/RSA after a given reaction time (\(t\))  
In the mixtures with the highest RSA proportion (1/3, Figure 9a) after 7 curing days, all the CH present in the paste reacted. The pozzolanic reaction was fast and most reaction products occurred on the first 7 hydration days. This has already been reported for other pozzolanic materials, such as the spent fluid catalytic cracking residue (FCC) [35]. For the paste with the lowest RSA proportion (1/1, Figure 9b), a large part of CH remained unreacted after 7 curing days, and CH was completely consumed only after 90 days. At this age, an intense peak within the 100-250 \({}^{\circ}\)C range was observed, which indicated the presence of many pozzolanic CSH-type products. This fact has also been found for other pastes made with RSA: in order to quantify the RSA reactivity of the studied pastes CH/RSA at a proportion 1/1, [PERSON] et al. [18] determined the mass loss attributed to portlandite decomposition, and obtained values of 2.15% and 1.63% for 7 and 28 days, respectively. In the present research, mass losses were similar, 3.95% and 0.42%, despite the biomass being burned under uncontrolled conditions. The CH/RSAW pastes were manufactured in the same proportions (1/1 and 1/3) and TG/DTG analyses were carried out at 7 and 28 days. In this case, the percentage of fixed CH was lower, mainly at an early age (Figure 10). One possible explanation could be that soluble salts (sulfates and chlorides) activated the dissolution of portlandite and enhance the pozzolanic reaction rate. [PERSON] et al. [35] stated that, in the presence of compounds such as calcium chloride or sodium sulfate, the pozzolanic reaction would be enhanced. To clarify the behavior of the washed ash, an XRD analysis was carried out by comparing pastes 1/1 at 28 days with RSA and RSAW. The obtained diffractograms are shown in Figure 11 and the identified mineral phases are summarized in Table 4. The fundamental difference lies in the paste with untreated RSA, where the minerals related to sulfates (arkanite) and chlorides (silvine) were detected, while these minerals were not identified in the sample with the washed ash. Figure 9: DTG curves for pastes CH/RSA: (**a**) 1/3 paste; (**b**) 1/1 paste. Figure 11: X-ray diffractograms of pastes 1/1 after 28 curing days: (**a**) CH/RSA; (**b**) CH/RSAW. Figure 10: Percentages of fixed CH for pastes CH/RSA and CH/RSAW. A similar thermogravimetric study was carried out on the cement pastes. In order to calculate the percentage of fixed CH in these pastes, a control cement paste was characterized for each curing age.