I am investigating the pore structure development in slags of different chemical compositions.
Many thanks @Nasser, your response is very much in line with the literature, where the researchers have only reported about increase in MgO leading to reduced porosity, without any explanation about the processes leading to this outcome, which was the basis for my question above.
MgO in slags is different from free MgO in cement. In case the cement contains too much free MgO, this will cause long-termexpansion due to hydration as the MgO is present locally as grains inside the matrix.
In slags the situation is different. The majority of MgO is present in the glass, and some might be present in crystalline phases such as merwinite, which are not hydraulically active. Thus no local areas of free MgO are present which would induce long-term expansion. When the slags are dissolving, also the MgO is set free and generally forms hydrotalcite-like phases. In the case of alkali-activated slags we found increasing compressive strength and decreasing porosity with increasing MgO content in the slag, see attachment. Other authors found similar trends. The main reason is that the volume of hydration products as derived by thermodynamic modelling increases with increasing MgO content. Probably the situation is similar in slag-blended PC.
Article Influence of Slag Chemistry on the Hydration of Alkali-Activ...
Many thanks @Frank Winnefeld for your detailed discussion. I have earlier read some of your papers relating to this topic. It is interesting to note that our on-going investigations of the pore systems of slag-blended PCs using MIP, gas permeability, and sorptivity have generally agreed with your findings for alkali-activated slags, and your explanations here are quite helpful. Slag with higher MgO content have generally exhibited lower porosity in spite of lower basicity/reactivity index compared with the other slag.
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