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 in 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. In the latter case problematic expansion ensues

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.

Numerous authors including Birchal et. Al. in his conclusions on p1632[1] and Blaha et. Al in parts 2[2] and 3[3] also make 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. Du [4] 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.

Hydration of MgO Powder from Du[4]

The references cited above talk about specific surface area, related to the degree of crystallinity being the relevant factor. We prefer to describe reactivity purely 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. Lattice energy is not easily measured, in fact it cannot be directly which is probably why the papers refer to specific or internal surface area. It follows that specific surface area is a measure of the reactivity of magnesium oxide and is directly related to the lattice energy. The higher the specific surface area the lower the lattice energy. The lower the lattice energy barrier to be overcome, the greater the hydration rate. With corrections to the English shown we quote below the text of "Gelling Materials Science"[5] 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 1000degC, the (specific) surface area is only ~10m2/g. This fact (is) also shown in Table 3-2 (also reproduced."

Effect of Temperature of Calcination on Hydration Rate[5]

Table 3-2 from "gelling Materials Science"[5] 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 at 800 - 900degC.

The importance of calcination in relation to hydration rate is also confirmed independently by Blaha.[2] 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 which is a measure of lattice energy which in turn controls reactivity in relation to temperature is exponential as reported by Blaha, and as can be seen in the above graph, the difference in reactivity between calcining at a minimum of 800degC compared to around 650degC is almost double.

We conclude by making it clear that temperature is very important and that without fluxes, no other citation is within the low range specified by our patents. To our knowledge there is no reactive magnesia available on the market calcined at a temperature as low as around 750degC other than XLM from Causmag Ltd. supplied by us. 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, temperature is very important, particularly in relation to Tec-Cements..

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. In dense concretes in particular it is essential to make sure there is no risk of delayed hydration and consequent dimensional distress. To understand 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.[6] 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)"[7]

TecEco cements do not require fine grinding, they must have low lattice energy however. Because the water demand for larger particles is less in a hydraulic cement we actually prefer 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[7]. 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[4] 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[7]. 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. As demonstrated above reactivity is not affected.

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[1] Birchal, V. S. S., S. D. F. Rocha, et al. (2000). "Technical Note. The Effect of Magnesite Calcination Conditions on Magnesia Hydration." Minerals Engineering 13(14-15): 1629-1633

[2] Blaha, J. (1997). "Kinetics of Hydration of Magnesium Oxide in Aqueous Suspension, Part 2. The Effect of Conditions of Firing Basic Magnesium Carbonate on the Specific Surface area of Magnesium Oxide." Ceramics - Silikaty 41): 21-27.

[3] Blaha, J. (1997). "Hydration Kinetics of Magnesium Oxide, Part 3. Hydration Rate of MgO in terms of Temperature and Time of Its Firing." Ceramics - Silikaty 41(4): 121 - 123.

[4] Du, C. (2005). "A Review of Magnesium Oxide in Concrete - A serendipitous discovery leads to new concrete for dam construction." Concrete International (December 2005): 45 - 50.

[5] “Gelling Materials Science”, 1st Edition. chapter 3. Pages 43-50

[6] Birchal, V. S. S., S. D. F. Rocha, et al. (2000). "Technical Note. The Effect of Magnesite Calcination Conditions on Magnesia Hydration." Minerals Engineering 13(14-15): 1629-1633.