Brucite cement

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Brucite [magnesium hydroxide Mg(OH)2] is isostructural with CH. It is formed in Portland cement concrete that has been attacked by magnesium salts, and on hydration of Portland cements high in MgO and possibly of Portland cements in general. It has a = 0.3147 nm, c = 0.4769 nm, Z = 1, >, = 2368kgm", oi = 1.561, e = 1.581 (S63). Three polytypes of aluminium hydroxide [Al(OH)3], gibbsite, bayerite and nordstrandite. contain layers essentially similar to those in brucite, but with an ordered pattern of... 

Sulphate attack has often been discussed in terms of reaction between solid phases in the cement paste and dissolved compounds, such as Na SO or MgSO, in the attacking solution. This obscures the fact that the reactions of the cations and anions in that solution are essentially independent for example, a solution of Na2S04 may cause both sulphate attack and ASR (T60,P53), and one of MgS04 causes sulphate attack and reactions forming brucite. 

The brucite forms a hard, dense skin on the mortar or concrete, which tends to hinder further attack (L6). The OH present in the pore solution would soon be consumed were it not replenished by dissolution of CH and decalcification of C-S-H, by the reactions shown in equations 12.3 and 12.4. In the reaction of a cement paste with an MgS04 solution, the CH and C-S-H thus serve as sources of both Ca " and OH". 

Under some circumstances, the Mg enters sparingly soluble phases other than brucite. Roy et al. (R70) observed the formation of a hydro-talcite-type phase in a slag cement paste that had been treated with MgS04 solution. Its formation was possibly favoured by the enhanced availability of A1(0H)4 provided by the slag. Cole (C70) reported the formation of a hydrated magnesium silicate in a deteriorated concrete seawall. 

Chlorite with Structural Defects. The mineral may be considered as a phase following the formation of a brucite layer in the series trioctahedral montmorillonite -4 trioctahedral chlorite. It is observed especially in the cement of the sandstones and in the siltstones of the Upper Lagoonal Complex. This chlorite with structural defects is characterized by a lower thermal stabilitiy, as on calcination to 600 °C the peak passes to 10.2 A. 

There are also the other reactive aggregates, namely gneiss and mica containing shales [41], In the interfacial transition zone, in the vicinity of aggregate surface— kaolinite and hydromicas, while from cement paste side—gel of sodium-calcium silicate hydrate, respectively are formed. However, in the case of serpentine concrete deterioration is due to the formation of brucite [75]. The clay minerals, such as chlorites, vermiculite, as well as micas and feldspars, are also included to reactive aggregate components. 

In the hydration of cement, Mg ions present in the crystalline lattices of clinker minerals are incorporated into the stmcture of the formed hydrate phases. Free MgO, present as periclase, also hydrates, yielding hexagonal magnesium hydroxide [Mg(OH)2], called brucite ... 

Cooling of the clinker with water, rather than in air, has been foimd to be particularly effective (Sharma et al, 1992). Under these conditions the average size of periclase crystals that crystallize from the melt in the comse of coohng is reduced, white their number is increased. In the hydration of such a cement the stresses caused by the conversion of periclase to brucite are more evenly distributed within the hardened paste, and the formation of cracks due to the presence of large MgO crystals may be prevented or reduced. Some reduction of the size of the formed periclase crystals may also be achieved by finer grinding of the raw meal. 

Sulfate crystals cannot be identified in the cement paste or concrete partially exposed to Na2S04 and MgS04 solutions. The chemical reaction products, ettiingite, gyp>sum and brucite, were the deterrnining factors for material damage. 

It reappeared when alkalis were leached out into the aqueous phase. More than one type of C-S-H appears to have been detected. Small amounts of MgO were present at three days of hydration and beyond. At one year, phases identified included C-S-H, monosulfate/C4AHi3, syngenite (only in the low alkali cement), brucite, and Ca(OH)2. 

Steefel Lichtner (1994) Lichtner et al. (1998) Model >500 a Marl tuff Cement leachate Mainly calcite brucite, C-S-H and C-A-H phases Permeability decreases perpendicular to fracture and increases parallel to it matrix sealed off from fracture... 

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