Sustainable Cement Using Fly Ash An examination of the net role of High Volume Fly Ash cement on carbon dioxide emissions. John Anderson IABSE Anton Tedesko Fellow M.Eng Struc. Eng, UC Berkeley
What reduction of carbon dioxide emissions can be achieved through the use of coal combustion products? Can High Volume Fly Ash cement provide the carbon dioxide savings required for long-term sustainability of the cement industry? Questions behind study
Main raw ingredients (85% by weight):
limestone (mainly calcium carbonate, CaCO 3 ) and
silica (silicon dioxide, SiO 2 )
Raw materials are crushed and heated in a kiln at 1450°C.
Gypsum is then added and the mixture is finely ground clinker
Cement clinker is composed of tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite, and gypsum.
Cement clinker is hydrated (addition of water) to form calcium silicate hydrate (C-S-H * ), calcium hydroxide (CH), and ettringite. Concrete is the mixture of hydrated cement paste and aggregates (gravel, crushed stone, or sand). Portland cement *Please note the use of cement chemistry: C = CaO, S =SiO 2 , H = H 2 O
The production of cement clinker requires the calcination of limestone (CaCO 3 ) to produce calcium oxide (CaO), an essential ingredient in cement clinker. The production of carbon dioxide results from this reaction. CaCO 3 + heat CaO + CO 2 The other major source of carbon dioxide from the cement industry is from the burning of fossil fuels to achieve high kiln temperatures. Portland cement
The main sources of carbon dioxide are chemical processing (50%) and burning of fossil fuels in kilns (40%). Source: WBCSD (2002) Portland cement
CO 2 emissions from cement  Gt – Gigatonnes (1 Gt = 10 9 tonnes = 1 billion tonnes); Mt - Megatonnes (1 Mt = 10 6 tonnes = 1 million tonnes) 5 – 8 0.81 – 1.25 RANGE 6.97 Not given 0.932 * Not given Not given 2002 IPCC (2005) 5 Not given 1.37 0.87 1.57 2000 CSI: Substudy 8 (WBCSD 2002) 6.5 21.6 1.4 1 1.4 1995 Malhotra (1999) 5 22.7 1.13 0.81 1.38 1994 IEA GHG (1999); Worrell et al. (2001) 8 Not given 1.45 1.25 1.13 1991 Wilson (1993) CO 2 from cement (%) Total CO 2 from all sources (Gt) CO 2 from cement (Gt) CO 2 /cement (tonne/tonne) Cement Production (Gt)  Year Author
Fly ash is a by-product of coal combustion. Impurities in coal bottom ash or fly ash Fly ash -high quantity of reactive silica -particle size 1-100 microns -Class C (high calcium), Class F (low calcium) most common -with calcium hydroxide forms cementitious products Other pozzolans are natural pozzolans (volcanic), slag, silica fume, rice hull ash, and metakaolin. Source: Sindhunata et al. (2006) Fly ash
Tricalcium Water Calcium Silicate Calcium Silicate Hydrate Hydroxide Portland cement: C 3 S + H C-S-H + CH Silica (fly ash) Portland cement + fly ash: S + CH C-S-H Chemical reaction of Portland cement with fly ash. C-S-H provides strength, CH weak, brittle crystals Fly ash and cement
Fresh concrete -reduced water demand, reduced bleed water, increased workability, continuing slump Plastic concrete -extended set times, reduced heat of hydration, reduced plastic shrinkage Hardened concrete - slower rate of strength gain , reduced permeability, reduced drying shrinkage, resistance to scaling from deicing salts Fly ash and cement
Future coal and cement production World coal and cement production, 1980-2035 (OECD-Organization for Economic Cooperation and Development) Historical Projected
Coal ash production and utilization in 1998 Sources: Malhotra (1999) 8 60 America, United States of 6 10 Great Britain, United Kingdom of 1 8 Spain, Kingdom of NA 38 South Africa, Republic of 5 62 Russian Federation 3 5 Japan 2 >80 India, Republic of 12 28 Germany, Federal Republic of 14 >100 China, People’s Republic of < 1 9 Australia, Commonwealth of Utilization (Mt) Production (Mt) Country
Estimated availability of fly ash and blast furnace slag in 2020 Sources: WBCSD (2002) 10 3,219 325 123 205 Total 3 188 5 1 3 Middle East 3 288 8 2 7 Africa 5 341 18 7 11 Latin America 18 79 14 4 11 E Europe 16 175 28 13 15 Russian Federation 9 215 20 4 16 India, Republic of 30 33 10 7 3 Korea, Republic of 7 294 20 3 17 SE Asia 7 1154 81 20 62 China, People’s Republic of 50 8 4 1 2 Aus & NZ 22 88 19 15 4 Japan 20 239 47 27 20 W Europe 64 11 7 3 5 Canada 42 106 44 16 29 America, United States of Potential for CO 2 Reduction in 2020 (%) Est. Cement Demand in 2020 (Mt) Total SCM (Mt) Blast Furnace Slag (Mt) Fly Ash (Mt)
Up to 60% of ordinary Portland cement (OPC) can be replaced by fly ash (Mehta 1999; Mehta 2002; Malhotra 1999).
1 tonne of fly ash used = 1 tonne of OPC saved = 1 tonne CO 2 saved
Industry experts estimate that global carbon dioxide emissions will be required to achieve reductions of 30% by 2020 and this level could increase to 50% by 2050 (WBCSD 2002).
OPC is responsible for 5–8% of global anthropogenic CO 2 .
Past data (2000) 18% of coal turns to useable ash
Differences stem from assumptions of how much coal ash would be usable in blended cement.
Technologies available to increase percentage of useable ash
Decreasing ash due to carbon limitations
Increase of low NO x burners reducing suitable ash
Rate of growth of coal and cement also influences results.
If coal growth rate greater than cement, then greater potential for CO 2 savings. Greater potential savings seen early on.
2000 to 2007
coal (+35%) > cement (+21%)
2007 to 2020
coal (+31%) < cement (+47%)
2020 to 2030
coal (+20%) < cement (+29%)
Best case scenario (Malhotra) allows for reductions in accordance in 2020 (30%) requirements.
Reduction of 50% by 2050 not likely in any scenario.
Most conservative assumptions (WBCSD) would allow for at most 10% savings in any given year.
Assuming every industry is responsible for its own CO 2 reductions (30% by 2020, 50% by 2050), HVFA cement alone is not a sufficient solution for the cement industry.
Alternative cementitious binder(s) required
(or reduced consumption of cement)
Current usage rate of fly ash dismally low. HVFA cement does allow for noticeable CO 2 reductions. Location of increased cement demand aligns with location of increasing coal production (developing countries). Further issues of sustainability (raw material demand, habitat destruction, water use, etc.) need to be addressed as well. Discussion
Alkali activated cements Calcium sulfo-aluminate cements Calcium sulfate based cements Magnesia cements Alternative binders
Alumino-silicate bonding phase (reaction between alumina rich source materials, fly ash, and an alkali silicate solution)
Source material 100% fly ash (100% reductions in CO 2 )
CO 2 associated with alkali solution production
Reduced global fly ash availability
Alkali activated cements
Product of numerous materials being calcinated at elevated temperatures
Early strength from ettringite, long-term strength from C-S-H
High early strength, reduced CO 2 emissions, low energy requirements, long-term durability
Rapid setting time
Absence of international standards
Calcium sulfo-aluminate cements
Gypsum based mortar
Rapid setting, controllable shrinkage, and hardening rate
Low processing energy, reduced CO 2 emissions
Calcium sulfates are by-products of coal and oil power plants
Natural calcium sulfates less widespread than limestone sources
Low durability, little protection for corrosion resistance of steel reinforcing
High volume fly ash must be fully utilized today (regulations?). Sustainability of cement industry requires shifting away from one cement type. Future of cement will be regionally based (engineering characteristics easily communicated). Conclusions
Thank you. Selected References: (EIA) Energy Information Administration, 2006, Internal Energy Outlook 2006 , Chapter 5: World coal markets, Report #:DOE.EIA-0484(2006) [online], June Available at: http://www.eia.doe.gov/oiaf/ieo/coal.html, [cited on 10 January 2008] Malhotra, V.M., 1999, Making Concrete Greener with Fly Ash, Concrete International , 21(5), May, pp. 61-66. Mehta, P.K., 1999, Concrete Technology for Sustainable Development, Concrete International , November, pp. 47-53. World Business Council for Sustainable Development. (2002) Substudy 8, Towards a Sustainable Cement Industry: Climate Change. [online] March, Available at: http://www.wbcsd.org/DocRoot/oSQWu2tWbWX7giNJAmwb/final_report8.pdf [cited 10 January 2008]