Sustainability Issues
Sustainability Advantages
The application of geopolymer technology in the mining, building and construction and waste containment industries offers the opportunity to address a variety of current sustainability issues within these industries.
These include:-
- Conversion of by-products currently treated as wastes to useful and valuable products,
- The ability to reduce Australian and Global greenhouse emissions by significant amounts,
- In particular, the ability to address the challenge facing the Concrete Industry over the necessity to reduce carbon dioxide emissions during the manufacture of cement,
- Other building related issues such as material chemical resistance, material fire resistance and behaviour in a fire, material life cycles, the use of petroleum based materials (eg many "plastics" or organic polymers) and in the waste containment industry, a better option for containing low level toxic wastes than those currently used.
Focus on Opportunities
- Opportunity 1: Waste Consumption
- Opportunity 2: Greenhouse Gases
- Opportunity 3: Concrete Industry Challenge
- Opportunity 4: Other Issues
Waste Consumption
Sustainability Oppportunity 1: Use of Wastes (Mining and Processing By-products)
Geopolymer cements can be made from binders which are basically waste products;-
- flyash and bottom ash produced in black coal fired power stations. Some 10% of these wastes are used as supplementary cementitious materials (SCM’s) in blended cements and in “fill” type applications. The remainder is dumped in tailing ponds constructed adjacent to power stations throughout Australia,
- ground granulated blast-furnace slags,
- bauxite processing residues,
- kaolinitic clays,
- certain mine wastes,
- naturally occurring pozzolans,
or any fine materials that contain significant amounts of silicon and aluminium in an amorphous form.
Most current formulations use a mixture of sodium hydroxide and/or sodium silicate (or the potassium alternatives) as the alkaline activator, but any strongly alkaline waste liquor can be used as a partial or full substitute.
Greenhouse Gases
Sustainability Opportunity 2: Greenhouse Gases
Wide-scale acceptance of Geopolymer Cements (GC) and the concretes they form could reduce the requirement for Ordinary Portland Cement (OPC). This represents a significant opportunity to reduce global carbon dioxide emissions as;-
- given that the production of OPC requires the calcining of limestone to form the calcium components of OPC, the production of 1 tonne of OPC (by milling OPC clinker) liberates approximately 1 tonne of carbon dioxide to the atmosphere (ref: 1).
- global OPC production accounts for about 5 to 10% of worldwide CO2 emissions (ref :1 ).
- assuming the use of a waste binder such as flyash and standard chemical activators, the production of 1 tonne of geopolymer cement liberates just 0.16 tonnes of CO2 (ref: 1). The use of waste alkalis would clearly reduce this further.
The conclusion is that substituting GC for OPC would reduce cement generated CO2 emissions by some 80% or more. For total replacement of OPC by GC, this potential saving represents some 4 to 8% of current world CO2 emissions.
Clearly collaborative efforts, by for example the coal industry and the building and construction industry could collectively generate significant greenhouse gas reductions for Australia.
Ref 1. Davidovits, J., “Global warming impacts on the cement and aggregate industries”, World Resource review, 1994, Vol 6, p263
Sustainability Opportunity 3
Sustainability Challenges in the Global Concrete Industry
In a recent presentation to the Concrete Summit on Sustainable Development, held by The American Concrete Institute’s Strategic Development Council in Washington DC on March 29-30, 2007, Mehta (University of California, Berkley) discussed the challenge facing the world of concrete with respect to anticipated growth of concrete volumes out to 2020 (ref: Mehta, P K, Sustainability of the Concrete Industry – Critical Issues, ACI Strategic Development Committee's "Concrete Summit on Sustainable Development", March 29, 2007, Washington DC ) and the conflicting global requirements to reduce greenhouse gas emissions.
The following figure summarizes his predictions:
1990 and 2005 values are the actual global quantities used (cement and supplementary cementitious materials) and the CO2 masses generated from the production of these binder masses.
2020 (1) is his prediction for “business-as-usual” but with some attempt to increase SCM proportions to 22% by mass of binder (the current hopes of the cement and concrete industry ; Ref: ACI Strategic Development Committee, "Concrete Sustainability. A Vision for Sustainable Construction with Concrete in North America", draft v7.3, appendix C, November 14, 2008.
2020 (2) is his prediction if the CO2 levels are to be kept at 2005 levels.
2020 (3) is his prediction if they are to be reduced to 1990 levels.
The CO2 contributions are based on the following assumptions;-
-
the cement clinker contribution reduces from 1.0 t/t in 1990 to 0.8t/t in 2020 as the fuels used to fire the clinker kilns change,
-
the SCM contribution increases to 0.15 t/t in 2020 as milling of coarse flyash becomes necessary to provide sufficient feedstock at these high replacement levels (ref: www.mecasil.com)
Scenario (1), business-as-usual, is not an option if the construction industry is take up it share of the burden of addressing climate change.
Scenario (2) is just possible with OPC concrete as usually OPC (the primary binder component) must be in excess of SCM content for usable concrete strengths. It should be noted that very high flyash OPC’s are actually alkali activated cements or geopolymer cements with the OPC component providing the alkalis in a gradual fashion as the OPC hydrates, rather than acting as the continuous binding matrix.
Scenario (3) is not possible with OPC concrete and is only achievable with geopolymer concrete where the SCM (flyash, slag, etc) becomes the (dominant) binder fraction.
Other Environmental Issues
Sustainability Opportunity 4
Unlike OPC concrete where they predominantly reside in the pore solution and capillary network, many toxic heavy metals can be encapsulated in the geopolymer network as part of the polymer chains or entrapped in the geopolymer network. This may provide an intermediate level solution for toxic waste disposal, allowing the safe storage of solidified mining tailings, industrial chemical wastes and low level radioactive wastes.
Geopolymer building materials (concretes, foams, laminates, etc) offer superior alternatives to OPC concretes and organic polymers (plastics; PVC, epoxies, polyesters, polyurethanes, etc). Unlike these organic substances they do not burn or emit toxic fumes during a fire, nor do they explosively spall when subjected to intense heat (as occurs with conventional OPC concrete).
Another environmental advantage is that they are not derivatives of the waning petroleum industry.
They are acid resistant providing long term solutions to world infrastructure problem areas such as sewerage systems. OPC Concrete sewer systems have a design life of 50 years whereas GC concrete alternatives are predicted to have service lives in excess of 300 year, with similar initial capital costs. Hence infrastructure replacement costs and future green house gas emissions are greatly reduced.
