History of Magnesium Cements
A number of cements based on magnesia have previously been made. If a salt such as magnesium chloride or sulfate is added to reactive magnesia and the mixture is allowed to react and hydrate magnesium oxy chlorides and magnesium oxy sulfates are formed that can be very strong but are not sufficiently weatherproof and are corrosive. Although there are many patents describing improvements to overcome these deficiencies such as the use of phosphates or soluble silicates, they are not generally economic.
Magnesium oxy chlorides were first discovered and prepared by Sorel in 1867. Magnesium oxy sulfates were discovered by Olmer and Delyon in 1934 .
Magnesium oxy chlorides and oxy sulfates are commonly referred to as Sorel cements. A number of compounds are formed when magnesia reacts with magnesium chloride to form oxy chlorides The main bonding phases so far found in hardened cement pastes are Mg(OH)2, (Mg(OH)2)3 .MgCl2 .8H2O and (Mg(OH)2)5.MgCl2.8H2O. (Mg(OH)2)5.MgCl2.8H2O has superior mechanical properties and is formed using a molar ratio of MgO:MgCl2:H2O = 5:1:13
MgCl2 + 5MgO + 13H2O = (Mg(OH)2)5.MgCl2.8H2O
If magnesium sulfate is used instead four oxy sulfate phases are considered to form at temperatures between 30 and 120oC; (Mg(OH)2)5.MgSO4.3H2O,(Mg(OH)2)3.MgSO4.8H2O, Mg(OH)2.MgSO4.5H2O, and Mg(OH)2.2MgSO4.3H2O. Only (Mg(OH)2)3.MgSO4.8H2O is stable below 35 deg C.
3MgO + MgSO4 +11H2O = (Mg(OH)2)3.MgSO4.8H2O
Zinc, calcium, copper and other elements also form similar compounds.
Magnesium oxy chlorides achieve higher compressive strengths than magnesium oxy sulfates The main problem with Sorel cements is that both magnesium oxy chlorides and magnesium oxy sulfates tend to break down in water and particularly in acids. Corrosion of steel reinforcing also occurs.
The use of soluble silicates such as sodium silicate has been described as a means of improving the water resistance of Sorel type cements. These cements are of little practical use however because of the high cost of soluble silicates.
Magnesia also reacts with soluble phosphates to precipitate almost totally insoluble magnesium phosphate.
MgO + H2O = Mg(OH)2
3Mg(OH)2 + 2H3PO4 = Mg3((PO)4)2 + 6H2O
The use of phosphates has also been advocated as a means of improving the water resistance of Sorel type cements. Such cements, although described in the literature, are expensive due to the shortage of economic deposits of phosphate and as a result widespread use is limited.
A range of magnesium phosphate cements has been used including magnesium ammonium phosphate which is thought to be formed by an acid-base reaction between magnesia and di hydrogen ammonium phosphate. This results in an initial gel formation followed by crystallisation into an insoluble phosphate, mainly magnesium ammonium phosphate hexahydrate, [NH4MgPO4.6H2O]. The magnesium oxide used in this system is produced by calcining at higher temperatures and is referred to in the industry as being “dead burned” and is not as reactive as magnesia made at lower temperatures. A set retarder, typically either borax or boric acid is also used to give a workable set time.
MgO + NH4H2PO4 + 5H2O = NH4MgPO4.6H2O
High-lime magnesiochrome cement finds use in refractories. The cement is based upon magnesia plus calcium chromate - chromite, a complex mineral produced by the combination of lime with chrome oxide (Cr2O3) in an oxidising environment. Hydration is normally performed with a 30% aqueous solution of magnesium chloride hexahydrate (MgCl2.6H2O) solution at 8 per cent by weight of the cement. The products are complex. As well as hydrates they also consist of carbonates, which are formed by the effects of carbonation. Typical products formed can include brucite [Mg(OH)2], various magnesium oxy chlorides [(Mg(OH)2)X.MgCl2.YH2O.] calcium chromate dihydrate (CaCrO4.2H2O), calcium monochromite (CaCr2O4) portlandite [Ca(OH)2], secondary magnesium carbonate (MgCO3), secondary calcium carbonate (CaCO3) and mixed calcium magnesium carbonates [(Ca,Mg)CO3].
Other known cementitious magnesia compounds include hydroxychlorides and sulfates such as Mg(OH)2.MgCl2.8H2O, hydroxy carbonates [Mg5(OH)2(CO3)4.4H2O] and hydroxy chloro carbonates [e.g. Mg2OHClCO3.3H2O] as well as hydro magnesite and magnesite. Hydroxy chloro carbonates and sulfates are also formed as a result of atmospheric carbonation of magnesium oxy chloride and magnesium oxy sulphate, and these often ultimately revert to magnesite and hydromagnesite.
Brucite [Mg(OH)2] alone has not found much commercial use as a cement previously mainly because the setting rate is too slow. TecEco have used brucite to replace Portlandite in modern cements. In porous substrates carbonation to nesquehonite occurs.
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[1] Various sources including Shand, M. A. (2006). The Chemistry and Technology of Magnesia, Wiley Interscience and Hewlett, P. C. (1956). Lea's Chemistry of Cement and Concrete. London, Sydney, Auckland, Arnold.