TecEco Cements

As transport costs rise sustainable cement that can utilise local, low impact low wastes and that use less CO2 or preferably sequester the gas will be required for Gaia Engineering.

The research development and deployment of TecEco cements are the major focus of TecEco's activities. We are proceeding slowly but surely to bring these exciting new technologies to the world.

More detail is throughout the web site. A good place to start is Definitions and Short Simple Exlanations under the main menu, followed by our newsletters. If you are at all technical try some of the presentations and papers.

The TecEco Cement System

TecEco cements include in their formulation reactive magnesia[1], a hydraulic cement such as Portland cement and usually a pozzolan. The Portlandite released during the curing of the Portland cement component is consumed by the pozzolan (if present) to produce more calcium silicate hydrate, a strength giving mineral. With Eco-Cements in porous substrates the magnesia also carbonates.

When magnesia is substituted for OPC the first noticeable affect is an improvement in the rheology; blocks go through block machines with fewer failures, mortars spread more easily and stick better, concretes are easier to place. There are several reasons for this. Principal amongst them are the fineness of the reactive magnesia[1] which affects particle packing and lubrication and the high surface charge density of the magnesium ion in solution which attracts layers of orientated water molecules.

Hydration Shelling (one layer only shown) around the Strongly Charged Magnesium Ion

Water is consumed by the hydrating brucite reducing shrinkage[2], and decreasing the voids paste ratio increasing strength. A higher short term pH may also contribute to more affective pozzolanic and other silicification reactions. More wastes can be included however mainly because of the lower long term pH and drier internal environment as significant amounts of water are converted to solid during the hydration of magnesia to form brucite hydrates.

Strength Development in Tec-Cement Concretes

It is thought that the brucite hydrates provide water later on for more complete hysration of other hydraulic cement components such as PC and this may be why the strength development is more staight line

TecEco Tec-cements generally contain around 10% MgO for maximum strength and a little more for zero shrinkage and are more sustainable because they require less cement for the same strength (measured at 180 not 28 days) and because they are more durable and because potentially magnesia will take less energy to make. In concretes made using them, as for ordinary PC concretes, carbonation only proceeds to a relatively shallow depth as the formation of magnesium carbonates also results in greater density and the blockage of pores, which impedes further absorption of CO2 into the cement.

The internal environment is also different as the equilibrium pH controlled by brucite and CSH is much lower, reducing internal alkali realted reaction problems but still sufficiently high to maintain the passive oxide layer around steel rebar deep in the substrate. Durability is improved mainly because of the pore filling affect, lower pH, lower solubility of Brucite compared to Portlandite and drier conditions the hydration of magnesia having consumed much of the water.

In the presence of carbon dioxide and moisture inside an Eco-Cement block, mortar or render that is sufficiently porous brucite (Mg(OH)2 carbonates forming hydrated magnesium carbonates such as nesquehonite and lansfordite and possibly an amorphous phase at room temperatures. Although theories abound it is thought that there is a gradual desiccation of lansfordite whereby nesquehonite and eventually magnesite may be formed. Significantly, both magnesium and calcium appear to carbonate more readily in porous concretes made using TecEco Eco-Cements containing magnesia than in concretes containing only Portland cement (PC) as the binder or PC and lime.

As stated by Fred Pearce in the article on Eco-Cements that was published in the New Scientist magazine (Pearce 2002) “There is a way to make our city streets as green as the Amazon Forest. Almost every aspect of the built environment from bridges to factories to tower blocks, and from roads to sea walls, could be turned into structures that soak up carbon dioxide – the main greenhouse gas behind global warming. All we need to do is change the way we make cement.”

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[1] Reactive magnesia (rMgO) is also variously known as caustic calcined magnesia, caustic magnesia or CCM. The temperature of firing has a greater influence on reactivity than grind size as excess energy goes into lattice energy.

[2] Shrinkage is actually a probably a lot more complex - see TecEco newsletter 58