Is concrete a sustainable building material?  The range of potential answers to that question indicates the complexity of assigning a general label of sustainability to any material, especially concrete.  Sustainability of a material can be evaluated in many ways.  A material can be green if it uses renewable resources, is recyclable, is non-toxic, is locally sourced, has a long life expectancy, uses little energy in it production and installation, etc.  No construction material perfectly satisfies all these criteria so judgment calls must be made to determine if the benefits in one category are strong enough to outweigh the disadvantages of another.  In certain categories concrete rates well as a green material.  It is durable and therefore lasts a long time allowing the possibility for buildings to be reused with different functions.  It can be made with low-skilled labor and readily available local sand and stone in most parts of the world.  It can be crushed and recycled into new concrete aggregate (to a percentage limit), or if discarded, it will not release a significant source of toxins into the earth.  Add to this it’s potential as a thermal mass to retain heating and cooling energy and you have what appears to be a strong candidate for a green material.  If we look in more detail at the LEED categories that apply to concrete we find confirmation of these benefits.  LEED credit can be received by using concrete to control storm water runoff with pervious concrete, to lessen heat island effect due to its high light reflectivity, to store heat energy as a thermal mass, as well as credits for reusing, recycling and low emissions when in its final cured state.  However, there are no credits that relate to the material composition of concrete other than use of local materials.¹ In the evaluation of concrete as a green material this is a glaring omission.

The main flaw of concrete that prevents it from being clearly defined as a sustainable material is one ingredient in its mix, Portland cement.  Because it is created by superheating and crushing limestone, the great amount of fossil fuels burned to reach these extreme temperatures consumes an immense amount of energy.  Add to this the amount of pollution released from burning coal and oil and the CO² released from the limestone (estimates vary but emissions of CO² from the cement industry range from 5% to 7% of all global emissions) and the use of Portland cement greatly lowers the green potential of conventional concrete.  To get around this problem, waste materials like fly ash and steel slag may be substituted for the cement.  This seems like the perfect solution as it solves 2 problems at once by using a material bound for a landfill to substitute for a polluting material. Plus it is readily available as fly ash is by far the largest by-product of the consumption of coal for the generation of electricity.²  Yet, these materials do not perform as well as Portland cement in terms of setting time, strength and appearance.  So in an industry where time is money, contractors, owners and architects are reluctant to specify 100% fly ash or steel slag cement.  Often a mixture of one of these cements and Portland is used but this combination results in only a partial solution to the problem. Only 30% of the 71 million tons of fly ash generated each year are recycled for use in structural fill, waste stabilization and additives to concrete so Portland cement still plays a majority role in concrete production.³

Alkali Ash Material Concrete

One potential way to eliminate all Portland cement is by the use of Alkali Ash Material (AAM) concrete, a material developed in part by Dr. Hossein Rostami at Philadelphia University. For this material, inexpensive activating chemicals (Type N sodium silicate and 50% sodium hydroxide) are added to 100% fly ash and cured in a dry heat oven (40˚C to 90˚C) for up to 12 hours) to create a very strong and chemical resistant concrete.  The advantage of AAM is that it exhibits the excellent chemical resistance of silicate cements along with the excellent mechanical properties of High Performance Concrete (HPC), but at an economical price.

Table: Mix Design for Alkali Ash Material (90 Mpa)

“AAM has advantages over other concrete products.  AAM can be used to create high performance concrete (AAM-HPC), providing rapid strength gain and development of high ultimate strengths of more than 110 MPa.   AAM has far better environmental resistance than Portland cement concrete, resisting attack from sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and organic acids.   AAM resists freeze-thaw attack and high abrasion, possesses low chloride permeability and does not exhibit alkali silica reactivity.”⁴  These qualities could be very useful in a variety of architectural applications, especially in harsh environmental conditions.

Properties of AAM

AAM concrete has been tested to a very high strength.  The compressive strength and the ultimate strength of AAM increases as curing time and temperature increase. Increasing the temperature results in more rapid strength development and a shorter time needed to reach maximum compressive strength.  There is a maximum curing temperature in the early stages of hardening of about 100°C. AAM concrete and mortars can reach compressive strengths as high as 12,000 psi when cured for 24 hours in a 90 degree Celsius oven.  As the percentage of activating chemical increases, the compressive strength of AAM also increases. ⁵

 Another reason AAM has shown good potential as a building material is because of its resistance to environmental attacks.  With standard concrete, air entraining agents are added to increase the freeze/thaw durability.  However tests on AAM have shown that due to its excellent packing and low porosity, it has good resistance to freeze/thaw in its normal composition without the addition of these admixtures.  AAM is also prized for its resistance to acidic chemicals.  Because of its high resistance to acid attack, AAM has already been successful in the production of concrete pipes and containment vessels that hold volatile liquids such as gasoline and diesel fuel.

“Chemical attack of concrete occurs through decomposition of the products of hydration leading to formation of new compounds.  The new compound either leaches out or disrupts the integrity of the concrete in-situ.  The most vulnerable component of cement hydrate is Ca(OH)2.  Calcareous aggregates are also susceptible to low pH.  Acid rain (with pH of 4 to 4.5) containing sulfuric and nitric acids can cause surface weathering of exposed concrete. AAM made with class F fly ash which contains only small amounts of CaO, is not vulnerable to low pH.” ⁶

Potential as a Construction Element

Because of its high compressive strength and environmental resistance, AAM concrete has a potential for architectural, infrastructure and landscape elements.  It would be more durable in harsh marine environments and resists the acids found in many plant roots such as vines that attach themselves to building structures.   Dr. Rostami has already suggested its potential use in bridge structures, sound and median barriers, sea walls, and sewage containers.  As with all green materials there is no perfect answer and the material has small disadvantages.  With AAM concrete there is still the question if the amount of energy saved by using 100% fly ash offsets the extra energy used to run the low-heat ovens.  Preliminary estimates by Dr. Rostami indicate they would be much less but further data could confirm that.  There is also the question of the toxicity of the additive chemicals which can be potentially damaging if they escaped into the environment before they were combined into the mixture.  Since this process requires the curing of the concrete in a dry oven, the best potential for its use would be as precast construction elements because of their smaller size.  While room-sized elements have been created, most large-scale poured-in-place concrete applications would not be feasible so smaller repeatable elements show the greatest potential.

The potential of precast AAM fly ash concrete was further explored in one of my fourth-year design studio projects.  While competing in the Building Element category of the ACSA Concrete Thinking Competition, my students explored the uses of pre-manufactured concrete elements that are stronger and more corrosion resistant than standard concrete.  The results showed creative applications for uses such as erosion control and living walls.

As mentioned earlier, the roots of many clinging vines produce a slight acid that breaks down the blocks and mortar in a masonry wall.  Since AAM concrete is far more resistant to acid breakdown than normal concrete, several students explored how it could be used in conjunction with vegetation.  One project by Milena Bica explored the use of AAM concrete units as a system to control hillside erosion control.  Meant to be placed on excavated slopes along the side of roads and paths, the “Y” shaped units (nicknamed Land Crabs) would form a net over the hillside to control soil run-off.  The weight and shape of the units would help retain and trap the soil while the openings between would allow for vegetation to take root and additionally anchor the slope.  The chemical resistance of the concrete would provide a longer lifespan for the units in touch with the earth and plant roots.  So this system of unit and vegetation provides the multiple functions of erosion control, reducing heat island effect and consuming CO².  Another student created a similar system to control stream bank erosion.

A proposal by Dustin Snyder was to use the material as a masonry block system for a living wall.  Instead of using a steel grid system to train vines to grow on the side of a building, he would instead use AAM concrete masonry units formed in a concave shape on the front face over which a net of coconut fiber would be placed.  The vines would grow on the net but if they attached to the block the AAM concrete would still be able to resist the acid produced by the roots.  The airspace between the net and the blocks would allow for ventilation of excess moisture.  One example he proposes for application of the system is on the expansive sides of box store buildings where there exist large amounts of windowless masonry.  The vine covered walls would shade the building to reduce building cooling loads while exchanging oxygen for CO². 

Proposals by other students in the studio included using AAM to filter toxic groundwater, to protect against wetland erosion by the sea and to create tsunami resistant housing.

Conclusion

If the economical feasibility of the production of AAM concrete is achieved, significant improvements could be made to the sustainable issues associated with concrete.  The energy and pollution reductions by the use of fly ash are huge in their own right but because it has higher strength and greater resistance than normal concrete, AAM can be used in smaller quantities and can be expected to last longer.  Slender, lighter structural beams with longer spans and/or wider member spacing can be used thereby saving more material as well as the associated installation and transportation costs.  Since AAM must be a precast product, it would still not solve the problem of cement used in cast-in-place concrete applications, but because of the extent of the pre-cast concrete industry, it could have a significant positive effect on the sustainability of this prevalent construction material.

Notes

1          LEED Manual, NC v2.2, US Green Building Council

2-6       High Performance Alkali Ash Material (Paper) Dr. Hossein Rostami, Philadelphia University, 2005

Fig. 1 Dr. Rostami examining concrete pipe  Fig. 2 Erosion control project by Milena Bica    Fig. 3 Living wall blocks by Dustin Snyder