# Steel Corrosion Behavior in Light Weight Fly-Ash Based Alkali Activated Mortars
## 3 Results and Discussion  
### Physical Properties and Microstructure  
MIP results reported in Figure 3 showed a strong difference in pore size distribution and total open porosity between M and LWM: while M sample exhibited a total cumulative intruded Hg volume of 52 mm\({}^{3}\)/g and a total open porosity of 11%, LWM sample showed values of 367 mm\({}^{3}\)/g and 48%, respectively. These results confirm the efficiency of H\({}_{2}\)O\({}_{2}\) as foaming agent together with the use of expanded perlite as light weight aggregate, as previously observed [24; 34]. Furthermore, a strong difference between the two samples was observed in terms of pore size distribution as reported in Figure 3b. M samples exhibited a bimodal pore distribution mainly in the ranges of 0.020-0.35 \(\upmu\)m and 1.50-2.0 \(\upmu\)m. Conversely, LWM were characterized by pores in the ranges of 0.020-0.040 \(\upmu\)m, 0.080-0.20 \(\upmu\)m and bigger pores with dimensions of 1.5-2.0 \(\upmu\)m and 3.0-6.0 \(\upmu\)m, showing both the contribution of the alkali activated binder and expanded perlite aggregates. The different pore size distributions between M and LWM, depicted in Figure 3, clearly showed that, while small capillary pores are present in M samples (that usually slow down the mortar saturation rate but increase the height of penetration), an increased porosity with also larger pores characterized LWM microstructure. Thus, this induced a faster water absorption but lower penetration height [36]. However, for both the samples, the water penetration surely reached 20 mm that represents the thickness of the mortar coating during the 60-days exposure performed for durability tests.  
\begin{table}
\begin{tabular}{c c} \hline
**Element** & **Concentration (mg/L)** \\ \hline Si & 2000 \\ Al & 6.1 \\ Ti & 0.1 \\ Fe & 0.6 \\ Ca & 4.4 \\ Mg & 0.7 \\ K & 166 \\ Na & 10,300 \\ S & 3800 \\ \hline \end{tabular}
\end{table}
Table 2: Chemical composition of the leachate pore solution (LPS). The measurements of the most concentrated elements were carried out using a 1:100 diluted solution.  
To better characterize the pore distribution and morphology, SEM observation was carried out on fracture surfaces of M and LWM and the results are reported in Figure 4. In particular, Figure 4a,c (at higher magnification) showed the typical microstructure of the fly-ash based alkali activated mortars (M sample): some unreacted fly ashes can be detected within a homogenous and dense gel. Only few pores in the range of 10 to 50 \(\upmu\)m were observed at these magnifications. On the contrary, LWM microstructure (reported in Figure 4b,d) showed a more porous matrix, induced by the effect of the foaming agent and the addition of expanded perlite (characterized by a developed open cell porosity). This morphological observation confirms the results on pore size distribution previously discussed.  
Figure 4: SEM morphological observation of (**a,c**) M and (**b,d**) LWM samples at different magnifications: (**a,b**) at 200\(\times\) and (**c,d**) at 1000\(\times\).  
Figure 3: Pore size distribution of LWM and M samples measured by mercury intrusion porosimetry: (**a**) cumulative specific Hg intruded volume versus pore size; (**b**) histograms representing the specific intruded volume in the pore size ranges.