# Reactivity Assessment of Modified Ferro Silicate Slag by R\({}^{3}\) Method
## 3 Results and Discussion
### Reactivity Assessment of \(R^{3}\) Model Paste
#### 3.2.5 XRD Analysis
Al\({}^{3+}\) in presence of CO\({}_{3}\)\({}^{2-}\) and Ca\({}^{2+}\) forms Al-Mc and/or Hc. However, the role of CaCO\({}_{3}\) in the hydration mechanism is still an open discussion. Different authors have already published their hypotheses, but there is no general agreement on the role of CaCO\({}_{3}\) during cement hydration [54; 55; 56]. An interesting study [57] proposed two possible hypotheses by modelling and calculating the role of CaCO\({}_{3}\) in cement hydration. They proposed that the role of CaCO\({}_{3}\) has two functions: (1) Active participant, and (2) inert participant. With respect to their findings, XRD results of the presented work can be interpreted as follows. As an active participant, the CaCO\({}_{3}\) present in the R\({}^{3}\) mix tends to dissolve and releases CO\({}_{3}\)\({}^{2-}\) into pore solution forming AFm phases, as is the case for GGBFS and MFS slag. As an inert participant, the CaCO\({}_{3}\) tends to just act as an inert filler or reacts slowly after 7 days without participating in the short term reaction mechanisms as in the case for FA. Implementation of MFS slag as Sustainable Binder in Mortar and Concrete--Strength and Ecological Benefits  
Mortar with replacement levels of 15, 30, and 50 wt% of PC by MFS-slag were prepared to evaluate the compressive strength in function of time (Table 8). SCM mortar synthesized from MFS slags showed a slow increase in compressive strength from 2 days to 7 days, and a high increase in the period from 7 to 91 days compared to the reference PC value. Especially from the age of 28 days, the MFS slag clearly contributes to the strength, since the mixes with 15%, 30%, and 50% MFS slag reach higher compressive strength than the nominal reference values corresponding to 85%, 70%, and 50% of the PC compressive strength. This foreseen strength development is due to the possible mechanism of slow dissolution of amorphous silica from MFS slag in the alkaline pore solution and the formation of strength providing phases such as C-S-H and C-A-S-H as mentioned in Sections 3.2.1 and 3.2.4. Moreover, mortar with MFS slag with a replacement level of 70 wt% of MFS slag in PC with presence of solid activators (Na\({}_{2}\)SO\({}_{4}\)) were prepared to study their compressive strength. Strength development of hybrid mortar (HM) also showed similar trends to the SCM mortars. However, HM mortars showed improved strength in comparison with 50 wt% MFS-slag SCM mortars after 28 days. This is due to the combined effect of pozzolanic and (geo)polymeric reaction. Nevertheless, HM mortars showed lower strength after 91 days with respect to 50 wt% MFS-slag SCM mortars. With respect to the increase in the global annual production of PC, global CO\({}_{2}\) emissions are also expected to increase to approximately 6 billion tons by the end of 2030. These MFS slags, if used in the concrete, can make the material much 'greener'. MFS slag can be incorporated in concrete as SCM or with additional activation, depending on the application. Partial replacement of PC up to certain replacement levels can be achieved without comprising the mechanical properties of the concrete (or at least with no larger effect than expected for traditional SCMs). However, a larger replacement level of around 70 wt% of MFS slag in PC can also be achieved with activation. At the same time, a by-product from Cu industry finds a high-level application. With less cement used to make concrete and the possibility of using MFS slag as SCM, sustainability and circular economy can be fairly well achieved.