TecEco Cements[1]

The most common hydraulic cement is Portland cement (PC or OPC), which hydrates to form mainly calcium silicate hydrates (CSH), Portlandite and minor components. John Harrison concluded that Portlandite and excess water are responsible for most of the problems of pre-mix concrete. He points out that Portlandite is too soluble, mobile and reactive. It carbonates, reacts with Cl- (chlorine) and SO4-- (sulfate) and being partially soluble can act as an electrolyte. In TecEco cements Portlandite is generally removed with the pozzolanic reaction and replaced by a more stable alkali in the form of Brucite (Mg(OH)2).

Much more water is generally required to make concretes workable than can be chemically used in the hydration reaction. To form Brucite TecEco add reactive magnesia [2] in various proportions which as it hydrates internally consumes significant excess water forming brucite hydrates which hold water between layers of brucite in such a way that it is available for the later more complete hydration of PC possibly preventing autogenous shrinkage. As a result of these changes in the chemistry of OPC concretes have better rheology, shrink less, are possibly stronger in the longer term (probably mainly due to the more complete hydration of PC[3]) and are much more durable.

As the pH falls due to consumption of Portlandite an equilibria established between CSH, Brucite and water maintains the pH at a lower level than Portlandite would but sufficiently high to prevent the corrosion of steel. Durability is a function of entry and reaction. The entry of aggressive agents is prevented by much less cracking due to the low shirnkage and expansive and thus sealing surface carbonation of brucite. Lower internal pH over the long term is also critical for durability as reactivity with internal components and external aggressive substances is reduced. Brucite is a much more stable alkali than Portlandite, reducing reactivity and providing a lower Eh-pH regime reducing reactivity an immobilising heavy metals that occur in waste streams[4].

The current method of making the magnesia required for TecEco cements is from magnesite ore. Magnesite is a naturally occurring magnesium carbonate and when calcined is decarboxulated to produce MgO (magnesia or magnesium oxide) in the similar way limestone is calcined to make quicklime or limestone and clay are calcined to make OPC, but at a much lower temperature and therefore more efficiently. Magnesia also has to be ground, but is softer and easier to grind than OPC clinker. Because the process is so simple and efficient TecEco hope to be the first in the world to make it using non fossil fuel energy using our Tec-Kiln from seawater as part of the Gaia Engineering tececology. Given economies of scale we believe the price of reactive magnesia [2] will fall below that of OPC. The magnesia used should be as reactive as is commercially feasible to prevent any risk of delayed hydration and preferably not too finely ground to reduce fineness water demand. Magnesia calcined and the temperatures preferred by TecEco so it can be less finely ground is not currently available and will be produced by the company funds permitting.

TecEco have demonstrated that the hydration reactions of magnesia are not only essentially independent of other reactions in Portland cement but that they occur sufficiently rapidly not to cause dimensional distress and that they have a wide and important role blended with them. Unfortunately all magnesia is banned in many specification in spite of the fact that it is only high temperature calcined material with high lattice energy barriers to hydration that causes dimensional distress (unsoundness).

TecEco calls its formulations of magnesia with Portland cement Tec-Cements, Eco-Cements or Enviro-Cements according to the degree of replacement of PC by magnesia and the type of concrete produced. Readers should consult the product and technical areas on this web site and numerous newsletters, papers and presentations for voluminous details as only a brief summary is possible here.

One ramification of the technology that has received considerable publicity around the world is that the Brucite in Eco-Cements carbonates in permeable materials resulting in the sequestering of CO2. Combined with seawater extraction of magnesium carbonates also used as aggregates and our Tec-Kiln technology, because of the high volume of material used in the built environment, a solution to global warming is provided. We call this paradigm Gaia Engineering.

Portland cement concretes are already a relatively sustainable material. With low cost and high thermal capacity they supply essential thermal mass to buildings. With the advent of TecEco technology, concretes will become even more sustainable with greater durability, waste utilisation and sequestration in the case of Eco-Cements.

Two main formulation strategies have so far been defined:

Tec-Cements (5-20% MgO substitution)

Tec-Cements contain more Portland cement than reactive magnesia[2]. As noted, reactive magnesia[2] hydrates in the same rate order as Portland cement forming Brucite and Brucite hydrates which use up water reducing the voids: paste ratio. Durability is improved particularly if pozzolans are also added and there is a marked decrease in shrinkage and cracking. Suitable pozzolans include fly ash and ground granulated iron blast furnace slag as well as a large range of other material such as quarry wastes.

Eco-Cement and Enviro-Cement concretes (20-95% MgO substitution)

Higher proportions of magnesia are used in Eco-Cements and Enviro-Cements and neither are as strong as Tec-Cements. The difference between Eco-Cement and Enviro-Cement concretes is that Eco-Cement concretes carbonate in permeable concretes such as masonry blocks and tend not to have too much pozzolan added and depending on the formulation objectives, usually none at all as pozzolans reduce porosity and will slow down carbonation. Pozzolans also react with lime in the pozzolanic reaction preventing its carbonation.

Enviro-Cement concretes are not generally permeable and do not contain other than surface carbonates and the use of pozzolans is optional but encouraged to reduce free lime. Enviro-Cements contain similar percentages of MgO to Eco-Cements, but in non-permeable concretes, Brucite does not carbonate readily. Higher proportions of magnesia are more suited to toxic and hazardous waste immobilisation and when durability is required mainly because of the lower pH regime[8].

Tec-Cement Concretes

Tec-Cements are suitable for a wide range of uses including any purpose for which Portland cement is currently used. Although as high as 15-20% substitution of PC can be effective if shrinkage over time is to be eliminated altogether a much more suitable substitituion for PC is around 8% as shrinkage is at least half as much without compromising too much 28 day strength (See the figure below). The normal use of pozzolans is recommended in Tec-Cements.

Benefits include improvements in durability, density, strength, cohesion and workability, reduced bleeding, permeability and shrinkage, and the use of a wider range of aggregates, many of which are potentially wastes, without reaction problems. Greater long term strength, less shrinkage and cracking and greater durability, given adequate engineering back up, should result in widespread use.

There are obvious advantages of including more stable alkalis or carbonates in cements so in our view it is time to bury the dogma regarding magnesia and rewrite all cement standards so that they only contain a performance based test such as in ASTM C 150 and ASTM C 595M where autoclaving is required. No special comment should be necessary regarding reactive magnesia[2]. which would then be classed as a supplementary cementitious material. The water consumption stoichiometry of Tec Cement is variable but involves the formation of still to be characterised Brucite hydrates:

MgO (s) + H2O (l) => Mg(OH)2. nH2O (s)

Tec-Cement formulations have a thixotropic rheology and gel quickly allowing finishers to do their work earlier and have a characteristic 1-4 day strength peak. This comparatively high and fast early "gelling" is probably due the interaction of a number of factors. Most likely are:

• Electrostatic interactions - Tec-Cement concrete formulations gel up very strongly due to the high charge density of the magnesium ion and change in surface charge of magnesium oxide with increasing alkalinity. As a consequence early tensile strength is introduced resulting in rapid "gelling".
• A rapidly achieved lower voids : paste ratio as a result of improved rheology due to better particle packing, some surface charge effects and high consumption of water by reactive magnesia [2] as it hydrates.
  
• The possible formation of another compound such as magnesium aluminium hydrates analogous to the hydrogarnets sometimes formed in Portland cement concretes with insufficient gypsum.
• Possible more and stronger silicification reactions including a more effective pozzolanic reaction during the early plastic stage whilst the pH is possibly elevated due to water removal by brucite hydrates.

Long term strength (>56 or 90 days) is generally higher prvided too much MgO is not added (<10% is recommended) and this is thought to relate to the slow release of water by hydrated Brucite gels (Mg(OH)2. nH2O ? Mg(OH)2 + H2O) resulting in more complete hydration reactions of PC.

Characteristic Strength Development of Tec-Cement Concretes

Users must be careful not to add too much water because of the fineness demand of magnesia and not be too concerned with a reduced slump as this is compensated for by higher workability as the rheology is much more shear thinning or thixotropic. This is partly due to the lubricating affect of the smaller magnesia particles and their packing with other components but mainly as a result of the influence of the highly positively charged magnesium ion in solution on water (which is a highly polar molecule) with the result that weak hydration shells are formed that break up with the appication of energy (the effects of a pump or working the concrete with a vibrator).

Hydration Shells Around a Central Magnesium Ion

As a consequence of the removal of Portlandite using the pozzolanic reaction and replacement by Brucite, Tec-Cement concretes have a different pH curve to Portland cement concretes with or without added pozzolan. As the hydration of magnesia takes up a lot of water (Brucite is 44.65 mass% water; Brucite hydrate gels contain even more water) and because Tec-Cement concretes do not bleed as much whereby alkalis remain in concrete, it is thought that during the early plastic stage the pH may be higher. We suspect but have no evidence as to whether this increases the surface hydrolysis and quasi "geopolymetric" reactions that occur. In the longer term however the pH is controlled by Brucite which has an equilibrium pH of 10.52 and CSH which has an equilibrium pH of 11.2 and remains somewhere between and a level that is less reactive with aggressive agents..

The long term equilibrium pH is at a sufficiently high level for steel to remain passive and for the stability of calcium silicate hydrates. It is thought that dense concretes made using Tec-Cement formulations should maintain reducing (+ Eh) and ion free conditions at a pH over around 8.9 required for the long term survival of the passive protective black iorn coating that forms on steel[3].

The removal of excess water by magnesia as it hydrates has a number of other consequences. Bleeding and the introduction of associated problems such as efflorescence, freezing of bleed water and weaknesses such as interconnected pore structures and high permeability do not appear to occur as much.

Tec-Cement concretes tend to dry out from the inside due to the water demand of magnesia as it hydrates. As free water is required for delayed reactions they do not occur. Yet water appears to be available from the brucite hydrates for more complete hydration of PC [4].

Brucite does not react with salts because it is a least 5 orders of magnitude less soluble, mobile or reactive than Portlandite. Sulfates, chlorides and other aggressive salts also have no effect. The Ksp (solubility product) of Brucite = 1.8 X 10-11 is much less that that of Portlandite is at 5.5 X 10-6.

The advantages of using quick setting and convenient Portland cement such as ambient temperature setting, easy placement and strength are not overly compromised and the advantage of adding reactive magnesia is that shrinkage is reduced, if not eliminated, due to low water loss and compensating stoichiometric expansion of magnesium minerals which in appropriate proportions can balance the slight shrinkage of Portland cement concrete eliminating cracks and reducing porosity. With higher proportions of reactive magnesia (15-18%) concretes can be made that are dimensionally neutral over time however testing may indicate too much loss of particularly short term strength. Concretes that shrink less crack less on all scales and combined with a more appropriate Eh - pH regime durability is significantly improved..

As Brucite is a relatively weak mineral it can also be compressed thereby also densifying the microstructure of concrete. Brucite and its various carbonates are also well known as a fire retardants.

Eco-Cements

Eco-Cements have higher proportions of MgO than Tec-Cements and usually less pozzolan if any at all[5]. Eco-Cements mortars and concrete require substrates permeable to air to carbonate such as can be produced in properly formulated in bricks, blocks, pavers, mortars renders and concretes[5]. In such substrates, as there are no kinetic transport barriers, the magnesia not only hydrates, but carbonates completing the thermodynamic cycle by reabsorbing the carbon dioxide produced during calcining.

Eco-Cement concretes have low pH and can include a large proportion of local low impact materials or recycled industrial materials[6] and are therefore likely to become a building material of choice in the future due to rising transport costs. Important uses will include providing a sustainable, low cost building material with high thermal capacity, low embodied energy and good insulating properties for construction in products such as bricks, blocks, stabilised earth blocks (mud bricks), pavers and mortars, pervious pavements and in combination with wood waste and other waste for packaging and building components.

The large scale use of Eco-Cements for such products would result in sequestration of very significant quantities of CO2 if in conjunction with the TecEco Tec-Kiln.

When Brucite carbonates it forms an amorphous phase, lansfordite, and nesquehonite all of which are hydrated carbonates. Strength gain in Eco-Cements is mainly micro structural because of the more ideal particle packing (Brucite particles at 4-5 micron are under half the size of cement grains) and the natural fibrous and acicular shape of magnesium carbonate minerals which tend to lock together.

SEM image of nesquehonite sample [7]

Magnesium is a small lightweight atom and the carbonates that form contain proportionally a lot of CO2 and water. Total volumetric expansion from magnesium oxide to lansfordite, for example, is 811%, meaning that a little binder goes a long way.

In terms of mass, by far the larger proportion of nesquehonite is CO2 and water as can be seen from the formula

Mg(OH)2 + CO2 => MgCO3.5H2O

Magnesium is an ideal mineral for the capture of carbon dioxide because of its low molecular weight and the ease of manufacture of magnesia without releases.

Comparison of the Manufacture of Eco-Cement with and without Releases

The manufacture of reactive magnesia [2] for Eco-Cement in Gaia Engineering will result in massive sequestration sufficient to reverse global warming.

Magnesium carbonates and hydrated magnesium carbonates are also fire retardants, releasing CO2 or water vapour or both at relatively low temperatures.

Enviro-Cements

Enviro-Cements are essentially Eco-Cements in that they have higher ratios of magnesia to hydraulic cement. The difference is only that they are used in not very permeable materials[8] so little or no carbonation occurs. Chemically and physically they are potentially more suited to toxic and hazardous waste immobilisation because they are more durable than either lime, Portland cement or Portland cement lime mixes. Enviro-Cements do not bleed water, are not attacked by salts in ground or sea water and dimensionally more stable with less cracking. In a Portland cement-Brucite matrix OPC takes up lead, some zinc and germanium.

The Brucite in Enviro-Cements is an excellent host for toxic and hazardous wastes as it has a layered structure and traps neutral compounds between the layers by hydrogen bonding forming a large series of nano composites.

Heavy metals not taken up in the structure of Portland cement minerals or trapped within the Brucite layers end up as hydroxides. The pH, which is controlled in the long term by Brucite and CSH is between 10.4 and 11.2, an ideal long term value at which most heavy metal hydroxides are relatively insoluble.

Waste and On Site Excavation Waste Utilization by TecEco Cement Concretes

As the price of fuel rises, the use of on-site natural/low impact low embodied energy materials, rather than carted aggregates, will have to be considered. The new hydraulic calcium-magnesium binders invented by TecEco provide benign low pH environments[9] allowing the use of many local materials and wastes without problems associated with delayed reactions.

Using materials regardless of their chemical composition for the physical properties they impart to composites is fundamental to sustainability and Brucite and magnesium carbonates bond well to many different materials including wood and will hold a large proportion of waste. Many wastes such as fly ash, sawdust, shredded plastics etc. can improve a property or properties of the cementitious composite. If wastes of any kind are to be incorporated in a cementitious matrix, such as Portland cement, it is essential that the long term pH is regulated in the region of the minimum solubility of heavy metals[9], as is the case in TecEco Cement concretes. In a Portland cement and Brucite matrix the calcium silicate hydrates take up lead, some zinc and germanium. Heavy metals not taken up in the structure of Portland cement minerals or trapped within the Brucite layers end up as hydroxides with minimal solubility.

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