# Evaluation of Rice Straw Ash as a Pozzolanic Addition in Cementitious Mixtures
## 3 Results
### Analysis of RSA Reactivity  
3.2.1. pH and Electrical Conductivity Studies  
Figure 6 depicts the obtained pH results for both ash types, RSA and RSAW. For the pozzolanic activity assessment, pH values are represented as the hydroxyl ions OH\({}^{-}\) concentration. The relative terms ([OH\({}^{\circ}\)]\({}_{i}\)/[OH\({}^{-}\)]\({}_{o}\)) were represented as follows (Equation (1)):  
\[\begin{split}\text{pH}+\text{pOH}=14\\ \text{pOH}=-\text{log [OH}^{-}\end{split} \tag{1}\]  
where:  
[OH\({}^{-}\)]\({}_{i}\): hydroxyl ion at a given time i  
[OH\({}^{-}\)]\({}_{o}\): initial hydroxyl ion at time 0 immediately after homogenisation  
To compare the behavior of ashes, the selected CH/RSA suspensions are the extremes of 1/9 and 5/5. As we can observe, both ash types (RSA and RSAW) display the same pH evolution. For the 1/9 solutions, the behavior is characterized by CH barely being present. However for the 5/5 solution, the hydroxyl ion significantly increments 24 h before it stabilizes. This initial increment can be associated with ash soluble salts. The OH\({}^{-}\) consumption started after 48 h.  
The methodology proposed by [PERSON] et al. [26] establishes that conductivity loss greater than 30% was a criterion (Lc > 30%), and indicates that aqueous solutions are unsaturated in relation to calcium hydroxide. When assessing the results obtained from the loss of conductivity, modifying the method was proposed.  
Figure 5: FESEM micrographs of RSA: (**a**) original; (**b**) milled.  
Figure 6: Hydroxyl ion consumption in relative RSA and RSAW terms.  
The RSA employed in the present research contains many soluble chlorides among other elements, which possibly influenced measurements. For this reason, we decided to modify the definitions and criterion proposed by [PERSON] et al. for pH and the conductivity analysis [26]. Herein, the proposed definition of the conductivity value for the CH/RSA mix for time (t) is as follows (Equation (2)):  
\[\mathrm{C_{i}}=((\mathrm{C_{t}}-\mathrm{C_{RSA}}*\%\mathrm{RSA})/\mathrm{C_{t} })*100) \tag{2}\]  
where:  
\(\mathrm{C_{i}}\) is the corrected electrical conductivity value for a given time (t)  
\(\mathrm{C_{t}}\) is the electrical conductivity in the CH/RSA aqueous suspension measured for t hours \(\mathrm{C_{RSA}}\) is the electrical conductivity of RSA in water for a given time (t)  
% RSA is the percentage of RSA in CH/RSA  
In Figure 7, losses of the electrical conductivity curves for all the mixtures prepared with RSA and RSAW are found.  
As seen in Figure 7a for RSA, the loss of conductivity criterion exceeding 30% is met by mixtures CH/RSA of 1/9, 2/8 and 3/7. According to the classification proposed by [PERSON] et al. [26], the pozzolanic reactivity shown by RSA was low.  
The washing process enhanced RSA reactivity (Figure 7b) and mixture CH/RSAW 4/6 met the loss of conductivity criterion. In this case, RSAW can be classified as a pozzolan with medium reactivity.  
Figure 7: Loss of conductivity versus time tested at 60 \({}^{\circ}\)C: (**a**) lime/rice straw ash (CH/RSA) mixtures; (**b**) CH/RSAW mixtures.