# Early Age Assessment of a New Course of Irish Fly Ash as a Cement Replacement
## 4 Results
### Thermodynamic Modelling Analysis  
The change in the solid phase assemblage predicted by PHREEQC over time is shown in Figure 13. The default output from PHREEQC is in moles. By multiplying this by the molar volume of each phase (using the SELECTED_OUTPUT and USER_PUNCH data blocks), the change in volume can be observed. The results show that the formation of monocarbonate with a stable ettrinigte phase is predicted, which agrees with the experimental observations. The formation of monocarbonate occurs at approximately 3 days, which is evident in the XRD patterns above and seen in the work by [PERSON] et al. (2018). Furthermore, with increasing fly ash, the volume of monocarbonate increases, which is due to additional calcium and aluminates being released into the solution and precipitating. [PERSON] et al. (2018) showed that increasing fly ash in blended cements results in a faster OPC/CEM I reaction. Also confirmed is the reduction in portlandite as it reacts with silicates, along with additional alumina, to form additional C-A-S-H. Also, part of the additional aluminates will contribute additional ettrinigte and monocarbonate phases, as the sulphate content is lower in high-fly ash blended cement (2018).  
Figure 13: Phase assemblage changes over time for 0, 10, 20 and 35% FA replacement levels.  
[PERSON] et al. [25] found that above 40% fly ash, monocarbonate and eventually ettringite both become unstable with the formation of stratlingite, depending on the reactivity of the fly ash, the Al\({}_{2}\)O\({}_{3}\) content and Al uptake into the C-A-S-H. It was also found that, above 20% fly ash, portlandite was completely depleted. Stratlingite formation has not been observed in the experimental data but is evident in the thermodynamic simulations within the timeframe shown, at replacement levels at and above 20%, which agrees with previous research. Indeed, as seen in Figure 14, which depicts phase assemblages assuming full hydration at an infinite time in the future, stratlingite is thermodynamically predicted to form for fly ash blended cements ranging from 5 to 35%. Note that, although automatic peak identification assigns the reflection at 11.8\({}^{\circ}\) 20 to the ferrite phase C4 AF, we believe this to be erroneous. Our thermodynamic predictions suggest that this peak is due to the precipitation of gypsum, as shown in Figure 14, below.  
As shown in Table 2, the SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\) contents are high, whereas CaO is low. According to ASTM C618 [6], with a combined SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\) of over 70% by mass, the ash belongs to the Class F family. It can be further classified as pozzolanic, and its reactivity is a function of aluminosilicate glass characteristics and particle fineness. The thermodynamic predictions here show portlandite completely depleted before 1000 days for fly ash blends over 10%, indicating that this source of ash is very reactive. This is observed in the XRD data, with the intensity of portlandite lowering over time for all replacement levels. [PERSON] et al. [16] found that the ash is most reactive after approximately 20 h of hydration, using the method developed by [PERSON] et al. [26] and [PERSON] et al. [27], where the capacity of ash to combine with portlandite is a measure of its reactivity.