Magnesium silicate hydrates (M-S-H) may form in the interaction zone between “low-pH” cement and clays. Low-pH cements contain mainly calcium silicate hydrate gels (C-S-H), with a Ca/Si between 0.7-1.2. “Low-pH” cements have been observed to deteriorate in contact with clays rock, i.e. decalcification of C-S-H, calcium’s leaching, carbonation and the formation of M-S-H. C-S-H and M-S-H are structurally different. In addition, C-S-H is observed to be stable at pH &gt;10.5, while M-S-H is formed at lower pH values (8.5-10.5). Batch experiments with magnesium oxide and magnesium chloride additions to synthetic C-S-H were carried out to investigate the effects of magnesium and pH on the stability of C-S-H. At low pH values (8 in the presence of MgCl2) the decalcification of C-S-H and formation of M-S-H was observed by 29Si MAS NMR and XRD, while at high pH values (10.5 in the presence of MgO) C-S-H remained stable and magnesium precipitated partially as brucite (Mg(OH)2) and M-S-H is predicted by thermodynamic modelling. In addition the destabilisation of C-S-H was also studied in interface experiments, where synthetic C-S-H and M-S-H samples, prepared from CaO, MgO and SiO2, were pressed into disks to create a well-defined interface. C-S-H was in direct contact with a magnesium-rich environment which enabled us to reproduce the condition in field experiments and to better understand the processes during M-S-H formation. SEM/EDS data indicated a decalcification of the C-S-H disk coupled with the opening of cracks while a front of magnesium precipitated at the surface. This interface experiment showed the same phenomena as observed in field studies and thus allowed studying the actual deterioration phenomena under well controlled conditions. The formation of M-S-H have been observed in locations where originally C-S-H have been at pH values lower than 10.5. A good correlation between the experiments and reactive transport modelling part has been obtained.

Effect of magnesium on the stability of C-S-H

253rd National Meeting (2017)

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