Other Magnesium Minerals in Concretes
Magnesium occurs in many different forms in cements. The only one that is dangerous is Periclase which is a high temperature mineral formed if magnesium is present during the clinkering process. This page describes some of these other forms.
Magnesium is found in fly ash and ground granulated blast furnace slag high temperature glasses and in microcrystallites that form in those glasses as they cool. As more dolomite is being used in fluxes to make steel and many coals have a high level of magnesium the proportain appears to be rising and can typically be in the range of 5-10%
The composition of a fly ash, particularly the calcium aluminium ratio and the amount of calcium controls what forms should the glass break down and crystallize the composition. This always occurs to some extent and crystallites (small crystals) are usually always present.
If there is some tendency to crystallinity in slag or fly ash it is rare that magnesium turns up as periclase and in most cases impossible because the composition would not allow it. There is just never enough magnesium and generally more calcium and much more silicon.
The following phase diagram from the Forschung institute (FEhS) showing isotherms gives the minerals present in the CaO-SiO2-MgO system. The figure shows the area for a periclase formation depends on the basicity (CaO/SiO2) with Al2O3-content of 10 %. Increasing the Al2O3 content does not have a significant influence on whether periclase will or will not form. FEhS has never found samples of ground granulated blast furnace slag (gbfs) with periclase in them and the numbers 1 to 7 show some real gbfs with MgO up to 15 %. Note that they are still very far from the periclase area.
High Temperature Phase Diagram in the System CaO-MgO-SiO2 (105 Al2O3) (Provided 2004 by Denis Higgins, Director General of the UK based Cementitious Slag Makers Association in a personal communication)
The diagram was constructed by the Forschung institute (FEhS) in relation to gbfs which like fly ash varies in composition but generally contains SiO2, Al2O3, Fe2O3, MgO, and CaO with at least 10% Al2O3, usually more like 20-30%. It can be seen from the diagram that much more likely in blast furnace slags are minerals containing calcium and magnesium and also silica such as merwinite and melilite. Given the similar compositions but lack of further experimental evidence available to us it would be reasonable to conclude that magnesium would express as these minerals in fly ash that was not entirely glassy. We also know that some magnesium is taken up in mullite in fly ash, which together with quartz is the main mineral found.
Note however that even though phase diagrams are useful they do not actually show minerals present in a melt, only what could form at a temperature given the composition as a melt crystallises. From the above it can be concluded that the composition of fly ash or slag is important. It is also essential that both fly ash and iron slag are cooled quickly so they remain glassy and thus more reactive although we believe some minerals like melilite will hydrate without causing dimensional problems.
The presence of magnesium dramatically improved the durability of concrete as used by the Romans, Vicat and more recently those examining more durable high Mg concretes of yesteryear such as those containing Rosendale cement. As pointed out in ACI Education Bulletin E3-01 and by Neville on page 52 magnesium in a glass derived from high temperatures melts is harmless in relation to delayed expansion.
All magnesia that has been through a high temperature kiln such as used for the manufacture of Portland cement is crystalline as generally it is cooled at a rate sufficiently slow to allow their growth. According to Taylor at page 2 MgO up to 2% MgO can also be taken up by CSH. Furthemore on pages 9-11 of Taylor  it is made clear that small quantities of magnesium are essential to stabilise the higher temperature forms of alite and other minerals. Accordingly in the various standards a proportion of MgO on oxide analysis of between 4 and 6% is tolerated.
In summary, magnesium in other minerals of glasses is substantially harmless as is the low temperature calcined essentially amorphous highly reactive magnesia used in Tec-Cements in particular.
 Hulett, L. D et. al. "Chemical Species in Fly Ash from Coal-Burning Power Plants," Science, New Series, Vol 210. No. 4476 (Dec. 19, 1980, pp 1356 - 358
 ACI (2001). ACI Education Bulletin E3-01 "Cementitious Materials for Concrete". C. E-701.
 Neville, A. M. (1995). Properties of Concrete. England, Pearson Education Limited.
 Taylor, H. F. W. (1997). Cement Chemistry, Thomas Telford
 Wikipeida accessed 4 Feb 2007 "Reactive Magnesia"