Rheological and Shrinkage Reduction Affects of Adding Reactive Magnesia to Concretes.

Highly reactive magnesia will go into solution reasonably rapidly and effect the early age properties of concrete. There are many consequences. In the short term a marked reduction in water related shrinkage and substantially different thixotropic rheology that results in easy to finish concretes. In the longer term durability, modulus of elasticity and creep are affected. This web page discusses early age affects and in particular rheology.

There are many in the world investigating the use of Robotics for construction and mixes that are as plastic as a result of the addition of reactive magnesia are very suitable for this purpose as they exhibit what is probably best described as a non Newtonian pseudoplastic fluid properties with viscosity decreasing as shear rate increases in accordance with what may turn out to be a Power Law or similar model. Simply put they are very thixotropic exhibiting strong shear thinning.

Rheological Models[1]

TecEco cements containing reactive magnesia[2] also have low sag and high stick. They do not bleed much, are easy to finish and have low shrinkage. There are chemical and physical reasons for these characteristics.

The small highly charged magnesium ion has a strong influence on the rheological and other properties of TecEco magnesium cements including the pseudoplastic thixotropic nature, low bleed, low sag, high stick and low shrinkage. Water, being a polar molecule with a differential charge density and one of the main ingredients of Portland cement concretes when freshly mixed, is strongly affected. Once set strong polar bonding results as the highly charged magnesium ion causes strong differential surface charge densities in the compounds it forms. This tendency for strong polar bonding expresses in the structure of brucite, nesquehonite and other magnesium carbonates and is the reason for their ability to form a wide range of nanocomposites as well as form strong bonds to other surfaces thus increasing the range of waste materials that can be added to concrete composites to advantage.

The viscosity of a liquid is determined by the ease with which molecules can move relative to each other. It depends on the forces holding the molecules together and in water this force is known as hydrogen bonding which is a special case of polar or dipole bonding. Hydrogen bonding describes the attraction between the hydrogen on one polarised water molecule and oxygen on another and is described more fully below because of its importance to rheology and other properties.

Ions in water are either kosmotropic or chaotropic based on their "water structuring" or "water disrupting" nature.
Ions, such as Mg2+, Na+, K+, can disorder or order water structure modifying deeply the properties. Mg2+ and Na+ are known as ionic kosmotropes (order-maker), exhibiting stronger interactions with water molecules than water itself and therefore capable breaking water-water hydrogen bonds. While K+ is known as ionic chaotropes (disorder-maker), exhibiting weaker interactions with water than water itself and thus interfering little in the hydrogen bonding of the surrounding water [3].

Chaotropic and kosmotropic Ions

Water has cohesivity due to a network of extensive three-dimensional hydrogen bonding and this property is strengthened both by Brucite surfaces and the strongly kosmotropic magnesium ion in solution.

During the hydration process, reactive magnesia, MgO, is converted to brucite, Mg(OH)2 and an equilibrium establishes between the solid and the liquid phases. Even though brucite is far less soluble than other compounds such as portlandite, magnesium ions are present in the water pore solution in accordance with that equilibrium.

MgO + H2O → Mg(OH)2

Mg(OH)2 ↔ Mg2+ + 2 OH-                       Ksp = [Mg2+]*[OH-]2 = 1,8*10-11

If [Mg2+] = s (solubility)
Ksp = 4s^3 = 1,8*10-11
s = 1,65*10-4 mol.L-1

Interactions occur between the magnesium ions and water molecules present in the pore solution and the surrounding mineral surfaces .

Brucite has a relatively simple layered structure containing single trioctahedral sites and fully tetrahedral sites. The octahedral sheet has no permanent structural charge, and the interlayer is normally unoccupied [4] however can contain many other charges and uncharges species. See the discussion on nanocomposites.

In a recent personal communication Prof Chris Cuff[5] said “Dynamic models of the transient structural effects of magnesium ions in solution, indicate that it and its hydration sheaths are relatively large and also contain extensive arrays of oxygen and hydrogen dipoles and associated networks of hydrogen bonding …..The relatively large size of a transient cluster of water molecules around a magnesium ion (the hydration sheath) imparts a degree of structure not only to itself but also, by conventional electrostatic considerations, to the associated anions in solution. This “structural ordering’ of the solution phase may also lead to drastic effects on the thermal vacancies present in the water. This in turn will lead to major effects on the internal transport processes of the fluid phase. Additionally, due to these constraints, these interactions may lead to the formation of thermodynamically metastable phases many of which may have very strong but metastable internal chemical bonding.”

Brucite surfaces also affect proximate water and Wang and others concluded that "The oxygen and hydrogen atomic densities and H2O dipole orientations vary significantly with distance from the surface and deviate from the corresponding structural properties of bulk water to distances as large as 15 A° (~5 molecular water layers) from the surface."[4]

According to Lippmann[6], and conventional crystallographic electrostatic theory, the ion dipole bond energy, decreases proportionally to the square of the bond length:

E = − e.µ/ r2 .cos(µ, r)

Where e is the ion charge, µ the dipole moment, r the distance between cation center of the negative charge of he dipole (the oxygen center for H2O), and (µ, r) the angle formed by the dipole direction µ and the join cation-oxygen r. The values obtained are 44.9 kcal/mol for Ca2+ and 57.7 kcal/mol for Mg2+. The dipole bond energy of Mg++ is 28.5 % greater than that of calcium and together with the affect of the surface chemistry of brucite is probably sufficient to explain the higher viscosity at low stress (when no "work" is applied as during placement).

According to Wikipedia “A hydrogen bond is a special type of attractive interaction ( a variation of a dipole-dipole bond) that exists between an electronegative atom and a hydrogen atom bonded to another electronegative atom. This type of bond always involves a hydrogen atom, thus the name. Hydrogen bonds can occur between molecules (intermolecularly), or within different parts of a single molecule (intramolecularly).Typical hydrogen bonds are stronger than van der Waals forces, but weaker than covalent, ionic and metallic bonds. [7]

Water is a polar molecule due to the difference of electronegativity between oxygen atom and hydrogen atom. This results in a slightly negatively charged oxygen atom and slightly positively charged hydrogen atom.

Water Molecule Electronegativity

In terms of atomic chemistry the oxygen in water has 2 non-bonding electron doublets resulting in a net regional negative charge which propagates to a positve charge in the region of the hydrogens. As a consequence of its doubly charges polarity one water molecule can provide 2 hydrogen bonds and can accept 2 hydrogen bonds. As a result of this strong hydrogen bonding propagating through water it develops a well organized quaternary structure and a high surface tension caused by the strong polar attraction between water molecules as in the diagram below.

Quaternary Structure Water [8] (Note that the average size of water molecule is 3 Å (0,3nm = 0,3*10-9 m).

Other properties of liquid water are also very different from that of other liquids due to the polarity of the water molecule and resulting hydrogen bonding. The specific heat capacity, heat of vaporization and the temperature of vaporization (100°C at 1 atm) are all abnormally high.

In the presence of magnesium ions another type of dipole or polar bond develops between strongly positively charged magnesium ions (Mg++) and the negatively charged oxygen atoms in water molecules which are attracted to them. In solution weakly bonded "hydration shells" many layers deep form around magnesium ions which have a much stronger influence in solution on water than calcium ions and for that matter many dispersants. These sheaths are in addition to the more common primary and secondary hydration shells so important in the kinetics of surface-controlled reactions.

Hydration Shells Around a Central Magnesium Ion

Mg-O dipole bonds are very strong and they distort the electron cloud on the water molecule further towards the oxygen and strongly enhancing hydrogen bonding. The increased strength of hydrogen bonds then propagates (all charges must be statistically equal and opposite) through water and between water and sand or cement as well as additives such as dispersants.

Another way of considering the above is that the strong positive charge density on magnesium ions distorts the electron cloud around water, increasing the density of electrons around an associated oxygen atom in a dipole pair, exposing the hydrogen atoms in water molecules with even less electrons and thus rendering them more positively charged to more strongly hydrogen bond with other oxygen atoms.

Although a relatively large ion in solution due to hydration effects, crystallochemically as Mg++ is a small ion with a diameter, although dependent on site co-ordination and symmetry, of around 86 picometres compared to calcium for example of around 114 picometres. It also has a high bond energy of 57.7 kcal/mol. Compared to other alkali earth metal ions Mg++ has a very high charge density and attaches to oxygen in a similar way to the hydrogen attach to the oxygen of water.

The development of hydration sheaths around magnesium coupled with the interactions with Brucite surfaces with water increase the thixotropic pseudo-plastic property of what amounts to a highly electrostatic fluid that is most importantly very slippery to the bull trowel and float and thus easy to finish.

Although very difficult to determine, the strength of the dipole bonding system described above could be, statistically double that in normal water as a result of the presence of magnesium ions and this strong molecular charge interaction between the magnesium ion and oxygen significantly affects the rheology of the mix.

Still concretes, freshly made with either Tec or Eco-Cements show first set quickly due to the propagation of stronger than normal dipole bonding and interactions with Brucite surfaces with water. When stress (such as during placing) is applied, the electrostatic influence of magnesium ions de-propagates and the weak dipole bonds break up leaving very small balls of water surrounding magnesium ions that are positively charged on the surface and thus repel each other, sand and cement sufficiently to increase fluidity. The better particle packing achieved by using smaller magnesia particles also starts to show influence.

Another important factor that affects early tensile strength gain is that the average surface charge of metal oxides changes with pH and in the case of magnesium oxide goes negative at around pH 12.2 becoming strongly attracted to both sand and cement particles rather than mutually repulsive. The affect is obviously dramatic and helps explain the high early (day 0 - 4) tensile and compresive strengths of TecEco cements. Early setting is important because finishers push this requirement in the market place mainly so they can go home earlier.

The Change in the Surface Charge of Metal Oxides with pH. [9]

Not all the affects are electrostatic as the physical size of the magnesium ion added also affects rheology. The mean particle size of the magnesia TecEco generally use is around 8 micron. Experimental work backed up by the mathematical theory of Larrard [9] indicates that this is not far off the right size for ideal packing with Portland cement which in Australia has a mean particle size of around 20 um[11]. Assuming spheres Larrard calculates a ratio of 1:2.41 [9].

Magnesia is finer than most grinds of Portland cement (PC) and therefore adds to the fineness demand for water. Operators using the material should be careful adding water however as this fineness demand is compensated for by a much greater response to work including pumping. A formulation that may have a nominal slump of around 60 in a barrel mixer characterised by relatively low shear will end up being more like 140 nominal slump after passing through a pump. Hi shear mixers such as ribbon or paddle mixers are more suitable for TecEco cements and the plasticity induced by work will last for at least 5 minutes to allow placement. If a barrel mixer is all that is available then make sure it is spun up regularly to high revs before checking slump.

A further advantage of adding reactive magnesia is a reduction in shrinkage. As concrete sets plastic shrinkage is caused by the losses of water through bleeding followed by evaporation or by seepage if the substrate is porous [12]. Neville [13] describes drying shrinkage as being the losses of adsorbed water. He stresses that "the loss of free water, which takes place first, causes little or no shrinkage. As drying continues, adsorbed water is removed and the change of volume . . . is in the order of 10000 × 10^6." Both have in common loss of water.

The Chinese have used less reactive magnesia than we do for some time to control shrinkage through delayed autogenous expansion. See technical.chinese_mgo.php

Whether this is so and to what extent in our concretes depends on where the water is coming from during setting and curing and we think that in dense concretes containing magnesium it comes from water that would most likely bleed or evaporate out of concrete if it were not for the presence of magnesium introducing a thixotropic pseudo plastic behaviour property to water reducing its mobility inside concrete (and thus reducing bleeding). i.e there is little or no autogenous expansion in contrast to the Chinese discoveries. See technical.chinese_mgo.php

When magnesia hydrates it may or may not be expansive depending on whether the water used comes from within or from outside the system cement + aggregates + water in accordance with the following equations:

MgO (s) + H2O (l) = Mg(OH)2 (s)
40.31 + 18.0 => 58.3 molar mass
11.2 + liquid => 24.3 molar volumes

 

Changes in Surface Charge Result in High Early Tensile and Compressive Strength

The explanation has now become more complex as there are a number of other factors to take into account including the strongly induced electrostatic effect of the magnesium ion discussed above. Apart from introducing a thixotropic behaviour property to water reducing its mobility inside concrete (and thus reducing bleeding), the presence of magnesium ions and Brucite have other effects that act together to reduce plastic and drying shrinkage.

Water has an abnormally high surface tension. Water molecules at the liquid-gas surface have less hydrogen bonds and more easily move into the gas phase (and thus evaporate) if they statistically achieve enough energy. Raising the temperature increases this energy and at the boiling point all water molecules have enough energy to escape at the surface. To evaporate water molecules must have sufficient energy to overcome the hydrogen bonds of water molecules further to the interior of a liquid and the addition of the strongly charged kosmotropic magnesium ion in sufficient concentration increases the strength of these this effect retentive force thus increasing the boiling point and reducing evaporation at lower temperatures (See end of next paragraph for a more technical explanation).

Bleed water does not so readily exit TecEco cement concretes and this must be explained. Water in fresh ordinary Portland cement concretes has a strong capillary action and shortly after it is placed "percolates" through it to the surface where it evaporates or forms bleed water. If concrete is not sufficiently covered or protected by surface coatings of for example aliphatic alcohol, to some extent, depending on the weather it is also pulled by transpiration tension (evaporation). In such circumstances strong cohesive properties hold the water column in the capillary pores together, preventing tension rupture. The addition of magnesium ions may sufficiently increase the viscosity of water reducing transport through such capillary pores, rendering the capillary water to exposed surface transport system in Tec-Cement concrete, less effective.

Water also has an anomalously high enthalpy (44.8 kJ mol -1) and entropy of vaporisation (109 J K-1 mol-1) due to its strong dipole bonding (in water known as hydrogen bonding). Due to the kosmotropic effect of magnesium in solution, water structure is more ordered (the intrinsic entropy of the solution diminishes)  and more “stable” as a liquid. Therefore, evaporation is limited,i.e the polar bonds between water and the less volatile magnesium atoms prevents loss of water from the setting mix, owing to the higher energy required for evaporation (heat of evaporation) and reducing also shrinkage.

As less water exits concrete containing magnesium ions and Brucite in the above manner the internal relative humidity is maintained at a higher level also reducing later autogenous shrinkage. Longer term drying and autogenous shrinkage may also be reduced because of the surface tightening affect as Brucite carbonates further reduce the loss of internal water and because water is "stored" in brucite hydrates (Mg(OH)2.nH2O species) and available for later hydration.

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