Magnesium Hydroxide Nano Composites and Brucite Hydrates
The structure of a layered double hydroxide nano or mechano composites based on brucite layers[1]
This page discusses the amazing ability of magnesium hydroxide to form complex layered double hydroxide (LDH) compounds with many other substances including water. This property is important because it is why magnesium hydroxide hydrates can prevent autogenous of concrete and why magnesia is so useful for locking up wastes. It is also related to how it can bond so easily with other substances.
Many magnesium compounds are characterised by the ability to form polar bonds at the surface and internally with other compounds. This is especially so of Brucite which consists of layers of magnesium ions coordinated octahedrally by OH groups with the hydrogen pointing in the direction of the next layer. Each layer is held to the layer above and below it by hydrogen bonds which are a special case of polar bonds. The outside layers also have projecting hydrogen's which is why so many magnesium cements are so good for “sticking” to other surfaces.
Compounds like brucite with a divalent cation and two hydroxides are known as layered double hydroxides or LDH compounds. They are are particular interest because of their ability to form nano composites with a wide range of added components including in the common case water. LDH nano or mechano composites have been known for many years and there are literally and endless number of them. There are also many patents in the area some of which are mentioned in this article and most do not realised they are just dealing with another nanocomposite.
Brucite type nano or mechano composites are of the general structure depicted in the above diagram from D’Souza et. al.[1] and can contain a vast array of molecules (e.g water) or ions (e.g sulfate or chloride) between the brucite like layers as long as the principle of electronic neutrality is observed. Carbonate, chloride, sulfate, silicates (as in talc, vermiculite and montmorillonite which are all smectite type clays) organic citrate and carboxylate, etc can all potentially be accommodated as long as balancing cations such as aluminium, magnesium iron, zinc, copper, sodium or potassium are included between the layers. This is what makes such nano composites useful for toxic and hazardous waste immobilization.
According to Zeng [2] Many cementitious formulations should properly be considered as a permutations of the above described structural group of layered double hydroxide (LDH) nano or mechano composites which are “material systems of layered nano composites comprising of brucite-like sheets and intercalated anions.”
Nano composites have properties that depend on the ions trapped in the layers, and can be very strong. One problem often encountered is that water, which also has a high hydrogen bonding potential, tends to reside in and break up the layers. Claims that this problem has been overcome are really only relevant for the shorter term. In our systems this property is used to advantage and brucite tends to crystallise as hydrated forms that can later provide polar bound water for the more complete hydration of PC.
“Layered solids have interesting physical properties, because of their structural anisotropy, and can be easily functionalised by intercalation of species with specific properties. It should be pointed out that these properties generally differ from those of the pure guest species, being affected by guest organisation in the interlayer region as well as by the host - guest interactions.”[3] quoted by Aloisi, G., U. Constantino, et al. in 2002 [4].
The magnesium ion has a high charge density on a small ion of only 86 pico metres compared to a large size for most other positive divalent ions. (See the table below)
Cation Radii (6-Co-Ordinate) (PM)[5]
Radii are quoted for common oxidation states up to +3 (4 for Hf, Th, Ti, U, and Zr).
Elem. |
Rad. |
Elem. |
Rad. |
Elem. |
Rad. |
Elem. |
Rad. |
Ag(+1) |
129 |
Er(+3) |
103.0 |
Mn(+3) |
72/78.5* |
Ta(+3) |
86 |
Al(+3) |
67.5 |
Eu(+2) |
131 |
Mo(+3) |
83 |
Tb(+3) |
106.3 |
Au(+1) |
151 |
Eu(+3) |
108.7 |
Na(+1) |
116 |
Th(+4) |
108 |
Au(+3) |
99 |
Fe(+2) |
75/92.0* |
Nb(+3) |
86 |
Ti(+2) |
100 |
Ba(+2) |
149 |
Fe(+3) |
69/78.5* |
Nd(+3) |
112.3 |
Ti(+3) |
81.0 |
Be(+2) |
59 |
Ga(+3) |
76.0 |
Ni(+2) |
83.0 |
Ti(+4) |
74.5 |
Bi(+3) |
117 |
Gd(+3) |
107.8 |
Pb(+2) |
133 |
Tl(+1) |
164 |
Ca(+2) |
114 |
Hf(+4) |
85 |
Pd(+2) |
100 |
Tl(+3) |
102.5 |
Cd(+2) |
109 |
Hg(+1) |
133 |
Pm(+3) |
111 |
Tm(+3) |
102.0 |
Ce(+3) |
115 |
Hg(+2) |
116 |
Pr(+3) |
113 |
U(+3) |
116.5 |
Ce(+4) |
101 |
Ho(+3) |
104.1 |
Pt(+2) |
94 |
U(+4) |
103 |
Co(+2) |
79/88.5* |
In(+3) |
94.0 |
Rb(+1) |
166 |
V(+2) |
93 |
Co(+3) |
68.5/75* |
Ir(+3) |
82 |
Rh(+3) |
80.5 |
V(+3) |
78.0 |
Cr(+2) |
87/94* |
K(+1) |
152 |
Ru(+3) |
82 |
Y(+3) |
104.0 |
Cr(+3) |
75.5 |
La(+3) |
117.2 |
Sb(+3) |
90 |
Yb(+2) |
116 |
Cs(+1) |
181 |
Li(+1) |
90 |
Sc(+3) |
88.5 |
Yb(+3) |
100.8 |
Cu(+1) |
91 |
Lu(+3) |
100.1 |
Sm(+3) |
109.8 |
Zn(+2) |
88.0 |
Cu(+2) |
87 |
Mg(+2) |
86.0 |
Sr(+2) |
132 |
Zr(+4) |
86 |
Dy(+3) |
105.2 |
Mn(+2) |
81/97.0* |
*Low spin and high spin values (section 8.2.3)
The key to understanding nano or mechno composites is to appreciate the strong polar bonding that magnesium induces due to it's high charge density. The high positive charge density of Mg++ drags electrons from other ions and for example causes magnesium to be strongly polar bonded to the oxygen end of water which has a strongly negative charge density. This strong polarity affects many compositions that it is involved in with the high charge density on magnesium causing strongly regionalised positive and negative charge densities in most compounds. In magnesium hydroxide for example this results in a strong ability to form hydrogen bonds with other atoms a little like wet newspaper can hold pressed flowers between and stuck to the surface.
In compounds the strong charge on magnesium is propagated through the molecule it is incorporated in and this is why the hydrogen bonding in brucite and nesquehonite, a hydrated magnesium carbonate is sufficient to hold the crystal together.
The strong polar or hydrogen bonding capability of brucite as it forms is also how brucite interacts with so many other compounds added whereby nano composites with different properties are formed. It is the basis of Sorel type cements such as JP 52‑138522a, EP 352096 A (Falk), US4760039, JP55-037469A2, US4572862, AU 55715/73 (Horley), US 4838941 (Hill), WO90/11976 (Magnacrete), GB938853, US5180429, CN1247177, RU2158718 C1, GB1160029, DE908837 C, JP57188439, US1456667, US6200381, US6200381, WO9854107, RU2089525 C1, WO0024688, JP 57056364 which use salts and JP 57056364, EP352096 A, WO97/20784, US4003752, AU 55715/73, WO90/11976, GB938853, CN1247177 wherein magnesia reacts as a base with chlorides or sulfates.
Other cements such as EP 0 650 940 A1, WO97/20784 (Rechichi), AU 55715/73 (Horley), GB1160029 (Mayer), US1456667 (Berry) and US6200381 (Rechichi), WO9854107, RU2089525 C1 and US5669968 include an agent that produces carbon dioxide which can also form part of a nano composite with brucite in combination with a suitable anion as in the graphic cited above from D’Souza et. al. or possibly even trapped in the layers.
Some organic groups can also be incorporated in nano composites and this could include citrate from citric acid, carboxylates form polycarboxylate or other organic acids etc. as in US 7070647B2, WO97/20784 and US 7070647B2
Nano composites are an important sub-group of magnesium compounds, not just because they are a vast array of compounds with cementitious properties or because they result in polar bound water important for preventing autogenous shrinkage, but because when heated the metal hydroxides become oxides intimately in juxtaposition with the components trapped between the layers and catalyse reactions that may not otherwise occur between layer components.
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[1] D’Souza, N. A., P. Braterman, et al. "Flame retardant nano composites with layer double hydroxides." Retrieved 15 October 2006, 2006.
[2] Zeng, H. C. (2006). "A World First: First Solid-State Synthesis of Carbon Nanotubes at NUS." 15 Oct 06, from http://www.eng.nus.edu.sg/EResnews/0102/hl/highlight.html.
[3] Scho¨llhorn, R. (1994). Progress in Intercalation Research. Kluwer Academic Publishers. W. Mu¨ller-Warmuth and R. Scho¨llhorn, Dordrecht.
[4] Aloisi, G., U. Constantino, et al. (2002). "Preparation and photo-physical characterisation of nanocomposites obtained by intercalation and co-intercalation of organic chromophores into hydrotalcite-like compounds." Journal of Materials Chemistry 12: 3316 - 3323.
[5] Shannon, R.D. (1976) `Revised effective ionic radii in halides and chalcogenides’, Acta Cryst. A32, 751. This includes further oxidation states and coordination numbers.