Reactive Magnesia

The Importance of the Temperature of Calcination

TecEco prefer as low temperatures as technically possible for the production of reactive magnesia for use in Tec-Cement. The reason is that if magnesia does not hydrate rapidly enough there is a risk of dimensional distress which must be controlled by carefully regulating the amount of magnesia added (See The Use of MgO by the Chinese). Whether or not there is expansion depends mainly on whether the water required comes from mix water or outside the system after concrete has set. If excessively from outside the system (cement + aggregates + water) expansion will ensue. See Rheological and Shrinkage Reduction Affects of Adding Reactive Magnesia to Concretes. If this expansion is controlled as by the Chinese (See The Use of MgO by the Chinese), it can usefully compensate for shrinkage, if not it can cause dimensional distress.

Generally the lower the temperature of calcination and finer the grind, the more reactive the magnesia is and the faster it hydrates. Magnesia calcined at less than 750 degrees C passing 45 micron and more preferably at 650 degrees C or less is preferred by TecEco as there is negligable risk of dimensional distress as a result of delayed hydration and shrinkage is controlled in a safer way.

Magnesia that is finely ground but generally calcined at at least 100 degC (and usually more see Du [5] and Li [1]) and thus much less reactive than that used by TecEco has been used for controlled delayed expansive hydration in dam construction by the Chinese (See The Use of MgO by the Chinese) so that hydration with associated stoichiometric expansion "closely matches the shrinkage of mass concrete as it cools" (See Du [5]) The purpose of TecEco's addition of much more reactive material is so that it goes into solution more quickly and influences fresh concrete properties as well such as rheology and plastic shrinakge. There are also many other beneficial side affects and the mechanism for controlling shrinkage quite different. (See The Use of MgO by the Chinese and Rheological and Shrinkage Reduction Affects of Adding Reactive Magnesia to Concretes.)

Du [5] shows on page 47 a table reproduced below that serves to indicate the enormous effect the calcination temperature of MgO has on the hydration rate expressed as the time taken for full hydration which is the important outcome of greater reactivity and this is commented on by numerous other authors including Birchal et. Al. in his conclusions on p1632[2]. Blaha et. Al in parts 2[3] and 3[4] also makes it clear that the temperature of firing is all important explaining that the lower the temperature of calcination, the more reactive the magnesium hydroxide will be and the faster the rate of hydration.

Hydration of MgO Powder from Du[5]

The references cited above talk about specific surface area (SSA) being the relevant factor for reactivity. It is true that the temperature of calcination is strongly correlated to reactivity and this is because SSA is a function of the nanostructure and porosity of the particles encompassing for example the degree of crystallinity (lattice energy), surface imperfections and fracturing affects, all of which are relevant factors.

Reactivity also increases with grind size which in turn affects the specific surface area. We therefore see specific surface area as a proxy measure of the reactivity of magnesium oxide because it is not the only determinant.

We believe lattice energy to be the most important determinant and prefer to describe reactivity in terms of the kinetic barrier being the lattice energy of the magnesia. Crystalline magnesium oxide, or periclase, has a calculated lattice energy of 3795 Kj mol-1 which must be overcome for it to go into solution or for reaction to occur. Unfortunately lattice energy cannot be directly measured and is not easily calculated which is probably why the term is not generally used to describe the reactivity of magnesia and specific surface area or the result of a reactivity test such as that with citric acid is used..

With corrections to the English shown the text of "Gelling Materials Science"[6] states as follows "Figure 3-3 (reproduced below) shows the inner surface area (specific surface area) for MgO made at different temperatures made using Mg(OH)2 as (a) raw material. At 4000C, the (specific surface area) distribution of MgO is (at a ) reached to maximum, S= 180m2/g. It will be decreased (decreases) when the temperature increases. At about 1000 deg C, the (specific) surface area is only ~10m2/g. This fact (is) also shown in Table 3-2 (also reproduced below.)"

Effect of Temperature of Calcination on Hydration Rate[6]

Table 3-2 from "Gelling Materials Science"[6] reproduced above is not in conflict with Du's table shown earlier. It merely extends it to lower temperatures. Notice how steep the curve is - this means that our specification of 750 degrees C max with 650 degrees C preferred results in vastly different properties to those that result if calcination occurs at 850 - 1200 degrees C (Light burned according to Li [1]) Th reader should also refer to The Use of MgO by the Chinese where the differences between the two technologies are addressed ).

The importance of calcination in relation to reactivity and thus hydration rate is confirmed independently by Blaha.[3] who examined in detail the affect of conditions of calcination on hydration rate. Blaha states in the abstract to the above paper "The specific surface area of the oxide decreases exponentially with increasing firing temperature." He produces the graph below at page 22.

Specific surface area of MgO vs temperature of firing. Time of firing is 60 minutes [4]

Because the relationship of specific surface area (as before a proxy measure of lattice energy) which in turn controls reactivity in relation to temperature is exponential as reported by Blaha, the difference in reactivity between calcining at a minimum of 800 degrees C compared to around 650 degrees C is almost double as can be seen in the above graph.

We conclude by making it clear that calcination temperature is very important and that without fluxes, no other citation is within the low range specified by our patents. Furthermore the temperature of calcination of 750 degrees C is set by us as a maximum and as can be see from the tables and graphs of Du, Gelling Materials Science and Blaha, calcination temperature is very important and the properties of higher temperature calcined magnesia are very different.

The Importance of Grind Size

An important part of our teaching is that grind size makes nowhere near the difference to reactivity that temperature of firing does. It is however important to properly pack particles to improve rheology and many other properties.

That is why TecEco use magnesia that has been calcined at much lower temperatures than has previously been used to make magnesium cements. In dense concretes in particular it is essential to make sure there is no risk of delayed hydration and consequential dimensional distress that is not precisely controlled as is the case with the Chinese addition of less reactive magnesia (See The Use of MgO by the Chinese). To understand lattice energy and reactivity think about diamonds. Even if they could be ground, because diamond crystals have such a high lattice energy then it would not really matter how fine they were ground - they would still not be reactive.

The lattice energy of periclase (the crystalline dead burned or hard burned form of magnesia) is high compared to most other minerals at 3795 Kj mol-1 and that is why it is used as a refractory. The fact that grind size makes nowhere near the difference to reactivity that temperature of firing does was probably first recognised by the Chinese, however this was in relation to Bajun Stone, a Sorel type composition. Blaha's work cited above is confirmed by Birchal et al.[7] This is further confirmed by Rocha et. al. more recently than our work when they say at page 819 "No effect from different particle sizes on the degree of magnesia hydration has been found (in relation to magnesia made at the same temperatures)"[8]

Mangesia for TecEco cements does not necessarily require fine grinding, but must have low lattice energy. The ideal grinding size ranges depending on an analysis of the particle packing and the important thing is to not compete with space that could be taken by other cementitious minerals.

Because the water demand for larger particles is generally less in a hydraulic cement it may be preferable to use some coarser magnesia particles as long as they are reactive enough. Smaller particles (unless they fit between other particles) generally have a high water demand in a concrete mix and as excess water weakens concrete an ideal reactive magnesia particle sizing may be a blend of smaller and larger particles than other cement components. That is why the maximum particle size in our patents is around 120 micron whereas our temperature of calcination maximum is much lower. A more detailed discussion on the importance of particle packing is to be found under technical on the web site under the heading The Importance of Particle Packing

The notion that the lower water demand of a more reactive yet larger particle can be achieved with magnesia is foreign to many engineers but is technically possible. Lattice energy is related to the state of disorder of a crystal. The high the state of disorder, the more reactive a mineral such as magnesia is. Disorder can be achieved to a much lesser extent by grinding.


The peer reviewed on line encyclopedia Wikipedia [9] defines reactive magnesia (also variously known as caustic calcined magnesia, caustic magnesia or CCM) as essentially amorphous magnesia with low lattice energy and is made at low temperatures and finely ground. energy. We did not put this definition there but it is John Harrison's and it has been peer reviewed. In contrast Crystalline magnesium oxide or periclase has a calculated lattice energy of 3795 Kj mol-1 which must be overcome for it to go into solution or for reaction to occur.