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. 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) problematic expansion may ensue. See Rheological and Shrinkage Reduction Affects of Adding Reactive Magnesia to Concretes.

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 750degC passing 45 micron or less is preferred by TecEco as there is negligable risk of dimensional distress as a result of delayed hydration.

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 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 is for durability, shrinkage and rheological reasons. There are also many other beneficial side affects and the mechanism for controlling shrinkage different. (See 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, 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 only asit is not the only determinant.

We prefer to describe reactivity in terms of a 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. Unfortuantely 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 750degC max with 650degC preferred results in vastly different properties to those that result if calcination occurs at 850 - 1200degC (Light burned according to Li [1]).

The importance of calcination in relation toreactivity 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 (Blaha)

Time of firing is 60 minutes. 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 800degC compared to around 650degC 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 750degC is set by us as a maximum and as can be see from the tables and graphs of Du, Gelling Materials Science (provided by a third party) 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 in relation to magnesia. That is why TecEco use magnesia that has been calcined at much lower temperatures than has previously been used to make magnesium cements. This is because in dense concretes in particular it is essential to make sure there is no risk of delayed hydration and consequent dimensional distress. To understand lattice energy and reactivity think about diamonds. Even if they could be ground, because diamond molecules 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 also very 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]

TecEco cements do not necessarily require fine grinding, they must have low lattice energy. Because the water demand for larger particles is less in a hydraulic cement it may be preferable to use coarser magnesia particles as long as they are reactive enough. What we teach is that what is much more important for reactivity is low temperature calcination. It follows that a coarse low temperature calcined material calcined at say 650degC will be more reactive than a finely ground less reactive material calcined for arguments sake at 800degC. This is because the former will have a much lower lattice energy (excess energy goes into forming crystals) and is supported by independent work after the priority date of our patents by Rocha[8]. Smaller particles have a high water demand in a concrete mix and excess water weakens concrete so to us the ideal reactive magnesia particle is a larger particle with little or no lattice energy. That is why the maximum particle size in our patents is around 120 micron whereas our temperature of calcination maximum is much lower.

The notion that the lower water demand of a more reactive yet large particle can be achieved with magnesia is foreign to many engineers but is technically possible. Lattice energy is related to the state of disorder or a molecule. The high the state of disorder, the more reactive a mineral such as magnesia is. Disorder can be achieved to a much lesser extend by grinding. Although we cite Du[5] above in relation to temperature of calcination we wish to make it clear that in his text he over emphasises grinding relative to the teaching of the present invention and authors such as Rocha et. al. also cited above[8]. Our large particle size (greater than 95% passing 120pm) but relatively low temperature of calcination was chosen because the water demand for larger particles is less in a hydraulic cement and it is well known in the industry and expressed by Duff Abrahams "law" that strength is inversely proportion to the amount of mix water.

Summary

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.

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