by: Michael Matthews
May 1st, 2017
SCI 6464
Thermal Tectonics
Prof. Salmaan Craig
Harvard University
Graduate School of Design
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How Recycling Pozzolans Can Create Regional Recipes for Mixing Cement
Concrete, the world's most common building material, contributes to approximately 7% of the world's anthropogenic atmospheric CO2 emissions. The majority of emissions from concrete is produced during the process of cement manufacturing -- the blasting involved in the quarrying of raw materials, the crushing of raw materials with machinery into a fine powder, the heating of cement kilns to 1,500 degrees Celsius -- all comprise the major carbon-producing processes in the creation of concrete (Huntzinger, 2009). Since the cement manufacturing process is so environmentally destructive, new innovations in the concrete industry have identified that transforming the process of cement production is the most critical step in making concrete more environmentally sustainable.
Interactive Map (Figure 1): Top 10 producers of cement in 2014, according to the U.S. Geological Survey. China was the top producer, with 2,500 million tons of cement. Most of the top producers are in Asia and the Middle East, with the United States and Brazil as geographic outliers.
The Potential of Pozzolans
Surprisingly, cement's most promising technological innovation in recent years has been found from turning to antiquity. Pozzolans, or types of ash derived from silica, constitute the main ingredient in the concrete found in the Greek and Roman eras. Emerging research on pozzolans demonstrates that the ash serves as a good substitute for modern day Portland cement, reducing the total amount of cement needed to produce concrete by up to 40% (Al-Chaar, 2011).
From left to right: Class C fly ash, Metakaolin, Silica Fume, Class F fly ash, Slag, Calcined Shale. (image source: Portland Cement Association)
Different type of pozzolans vary dramatically in their origin, composition, and properties. In ancient times, pozzolans primarily came from harvesting volcanic ash, but in fact pozzolans can come from many diverse natural and man-made sources: fly ash from burning coal, slag from steel manufacturing, palm oil fuel ash (POFA), bamboo leaf ash (BLA), and rice husk ash (RHA). These pozzolans are fine-grain, ash-like materials that contain silica and/or aluminum, which have cementitious properties. They are added as supplements in their natural, unprocessed form to traditional Portland cement mixtures. The fact that they can be added to cement mixes in their raw state, as opposed to the highly involved manufacturing process for Portland cement, greatly reduces the amount of carbon produced compared to traditional concrete types (Valderrama, 2012).
Beyond the reduction of carbon emissions, cement substitutes also have other potentially transformative effects on the concrete industry. Pozzolanic concrete is cheaper to produce and structurally stronger than conventional concrete, leading to significantly lower life-cycle costs from cradle to grave in concrete structures (Knoeri, 2012).
Other improvements to the properties of concrete associated with pozzolans include:
- improved workability at low replacement levels: Concrete is easier to place with less effort, responding better to vibration to fill forms more completely (Separation Technologies, 2016)
- reduced bleeding and segregation: Fewer bleed channels decreases porosity and chemical attack. Bleed streaking is reduced for architectural finishes. Improved paste to aggregate contact results in enhanced bond strengths.
- low heat of hydration: The presence of pozzolans in cement reduces the amount of heat produced in hydration, resulting in reduced thermal cracking
- lower permeability: pozzolans increase the density of concrete, resulting in fewer bleed channels and decreases permeability
- high resistance to chemical attack at later ages (Kartini, 2011)
Dry cement mix, left, and fly ash pozzolan, right. Fly Ash is a type of glassy by-product created during the coal-burning process. Instead of being transported to landfills, Fly Ash is increasingly being recycled into cement mix. (image source: Civil Engineers Forum)
Of all of the recent innovations focusing cement production, pozzolanic concrete is at the forefront of current research. Because it makes concrete both stronger and cheaper, it provides the most promising combination of market feasibility and environmental savings of all of the current "green" concrete types (Huntzinger, 2008). Other contenders include cement where 100% of kiln dust is recycled into the kiln process, as well as cement where carbon emissions are sequestered back into the kiln. Other green concrete types, such as self-healing bio-concrete, provide advantages solely in terms of improving durability and longevity. All of these types provide significant reduction in carbon emissions through various means, yet they do not provide the same magnitude of economic and structural benefits as found in pozzolanic concrete.
Embracing a Life-Cycle Approach & Regional Recipes for Cement Mixes
Since the cement manufacturing has such a negative impact on the environment, and yet comprises such a fundamental part of the building industry, it is time for a radical shift in thinking about the life-cycle applications of concrete. The development of cement substitutes using a wide-ranging source of waste by-products has the potential to significantly reduce carbon emissions and also implement a system of recycling waste products directly into new materials. Because the pozzolans are sourced from such diverse natural, industrial, and agricultural processes, the incorporation of different pozzolans into cement could vary from region to region.
The collection of rice husks in Ecuador (image source: Planet Drum and Eco-Ecuador)
Current research in the concrete industry has focused on the specific performative capabilities of new cement mixes, such as that of the Army Corps of Engineers (Al-Chaar, 2011) but this type of analysis falls short of a comprehensive look at the broader life-cycle assessment and the regionalized character of the cement manufacturing industry and the origins of various pozzolans. The widespread implementation of one of these green concrete types, however, is only possible through creating a balance between competing economic, structural, and environmental factors, arguing for a more comprehensive, Life Cycle-driven approach to innovation in cement mixes. Weighing the advantages and disadvantages of each type of pozzolanic concrete, this study serves as a broad survey of potential overlaps between the cement manufacturing industry and the industries that produce pozzolanic wastes: primarily agricultural or industrial in nature.
The following collection of maps will serve as a starting point for designers to identify feasible options for "green" concretes based on their geographic location. The use of pozzolans in cement, dating to Antiquity, once again resurfaced in the mid-20th Century, but still has not become a commonplace practice. The responsibility for the ethical sourcing of materials does not belong to the building material industry alone, but also to the building design industry. The following survey documents the most promising emerging cement substitutes, and the various relevant sustainability issues surrounding their collection and implementation from a Life-Cycle perspective.
Pozzolan Profiles
RICE HUSK ASH:
Interactive Map (Figure 2): Top 10 Countries with the highest production of Rice in 2016, according to the Rice Outlook Report by the U.S. Dept. of Agriculture. All of the top ten producers were in Asia, with the exception of Brazil.
What:
Rice husks are the hard outer shell of rice granules. They are an agricultural waste by-product created in the processing of rice crops. When the husks are burned, the Rice Husk Ash (RHA) is created. The ash is made up of approximately 95% silica, providing for a very effective cement substitute
How Much:
495 million tons of rice were produced in 2015, of which the waste from husks comprise 20% of the total weight, or 99 million tons (United Nations FAO, 2015)
Where:
The top two producers of cement, China and India, are also the top two producers of rice crop, respectively. All of the top ten producers of rice crop are in Asia, with the exception of Brazil.
Substitution Value:
Up to 30% of cement can be replaced with RHA in concrete without adverse effects (Kartini, 2011).
Why:
The relatively high silcate ratio found in rice husk ash, compared to other agricultural waste pozzolans, makes the rice husk a strong candidate for more widespread adaptation as a cement substitute.
Rice is one of the world's most commonly grown crops. The physical properties of the rice husk, with its tough outer shell, resists natural degradation. The accumulation of husks over time from dumping sites has become a common environmental issue in developing nations in Asia, and now their disposal is more carefully regulated (Habeeb, 2009). A more sustainable alternative would be to incentivize the inclusion of rice husk ash into cement, rather than into landfills.
BAMBOO LEAF ASH
Interactive Map (Figure 3): Top 10 Countries with the largest bamboo forest coverage, according to the 2010 Report by the U.S. FAO. The largest forests are in Asia, Africa, and South America. Of all pozzolan varieties, bamboo leaf ash poses the greatest opportunity for Africa to supplement cement production.
What:
Bamboo leaves are an agricultural waste product accumulated during the harvest of bamboo stalks. The leaves can be collected and burned to produce Bamboo Leaf Ash (BLA).
How much:
Bamboo is a renewable resource, and so its quantities are difficult to capture. The practice of farming bamboo (as opposed to deforestation) is still relatively uncommon, and offers little data to compare worldwide industries. Instead, this report focuses on distribution of bamboo forests across the globe, but more as a means to identify suitable growing conditions, than to suggest deforestation as a means for collection of pozzolans. As with other industries, the deforestation of bamboo is inevitable, and so the responsible collection of "waste" products (such as the leaves, which otherwise have little value), is an important first step for bamboo leaf ash as a cement substitute.
Where:
The greatest concentrations of Bamboo are in Brazil. Significant bamboo forests are also found in Central Africa: in Ethiopia, Nigeria, and Senegal. Of all renewable pozzolans, bamboo leaf ash provides the greatest opportunity for South America and Africa to supplement cement production.
Substitution Value:
Up to 20% of cement can be replaced with bamboo leaf ash without affecting concrete performance (Dwivedi, 2006). This is relatively lower than other pozzolanic substitutes, which average around 30% optimal substitution.
Why:
Bamboo has been touted as a miracle crop, by researchers such as Bryan Nelson of EcoWorldly, for its ability to grow fast, sink carbon, and have a wide range of applications ranging from textiles, food, and the building industry. It is a more sustainable material than timber, and its widespread cultivation could help preserve relatively slow-growing forests.
One of the dangers of bamboo, however, is that it needs to be cultivated responsibly. As a raw material, bamboo has overly rigid fibers, and chemical solvents are needed to prepare raw bamboo for applications in textiles and fabrics. These chemical solvents pose environmental risks as potential pollutants of water sources, according to the U.S. FAO. Additionally, most bamboo forests are located in developing nations, where responsible forestry practices must be further established to prevent depletion of current bamboo forests. In fact, their location in developing nations is also a great opportunity, and Nelson argues that the expansion of the bamboo industry could help create tens of thousands of jobs in countries such as Vietnam.
Another advantage of bamboo leaf ash is that only the leafs are collected for use as a pozzolan substitute, while the more lucrative woody stalks can be used for other purposes. This has the potential to give the cultivation of bamboo additional value.
FLY ASH:
Interactive Map (Figure 4): Top 10 Countries with the highest coal consumption in 2015, according to the IEA Key World Energy Statistics 2016. The top consumers are spread across the globe. China, as by far the largest consumer, accounts for over half of all total annual consumption. The United States is the second largest consumer. It produces a substantial portion of the world's coal, but much of it is exported.
What:
Fly ash is a product of burning coal, most commonly at power plants producing electricity. When coal is burned, the non-combustible components (such as quartz, calcite, gypsum, pyrite, feldspar and clay minerals) melt away and form glassy particles called fly ash. The fly ash is carried up the plume of coal furnaces and is separated and collected using filters. Typically the fly ash is disposed of as waste, but increasingly it is being collected for cement substitutes.
How Much:
Approximately 7,876 million tons of coal were consumed worldwide in 2013 (International Energy Agency). Of this total amount, approximately 130 million tons of fly ash were produced (US EPA).
The quality of the fly ash depends on the original quality of the coal: for use in cement, Class F (high grade) coal yields the best results. About 50% of coal burned in the US is high-grade.
Where:
The top consumers of coal are China, India, and the United States, who together account for over 70% of consumption worldwide. These countries are also the top three producers of concrete, suggesting good potential for more overlap between these two industries.
Substitution Value:
Class F Fly Ash can optimally replace between 20 - 30% of Portland Cement (Detwiler, 2002).
Why:
Far from a dying industry, the mining and consumption of coal has dramatically increased in the last decade. In China, consumption has increased 45.7% in the past ten years. Currently, most of fly ash is not recycled, instead being disposed of in surface impoundments or in landfills. Another common practice is to discharge it into waterways surrounding power plants, as per the plant's water discharge permit. Coal ash contains many contaminants, including mercury, cadmium, and arsenic, and so their careful disposal is especially important to avoid the contamination of waterways and groundwater.
STEEL SLAG:
Interactive Map (Figure 5): Top 10 Countries with the highest crude steel production in 2016, according to the World Steel Association. China, again, is the top producer. The European Union is the second-highest producer.
What:
Furnace slag is a by-product of the steel manufacturing process. Slag is a residual glass-like product that is created when a desired metal has been separated (or "smelted") from raw ore. Steel slag is mostly comprised of silicates and oxides, whose pozzolanic properties make for an effective cement substitute.
How much:
It is estimated that 115-180 million tons of steel-slag is produced worldwide each year (Global Cement).
Where:
Total worldwide crude steel production in 2016 was 1,620 million tons. China produced the most steel, with approximately 800 million tons, accounting for 50% of the world's total production. The European Union was the second-largest producer, with 162 million tons. Furnace Slag is the only variety of cementitious pozzolan found in abundance in the European Union, making a strong case for its adaptation for cement mixes for this particular region (World Steel Association).
Substitution Value:
The optimum percentage of replacement for fine aggregate cement is 40% and for coarse aggregate cement is 30% (Devi, 2014).
Why:
It's important to note that not all types of slags are created equal. Steel slag is one of the few varieties of manufacturing slag without high concentrations of toxic elements. The slag derived from copper, lead, and cadmium, for example, contain potentially hazardous levels of toxicity, whose incorporation into materials could lead to gradual contamination of the surrounding environment via natural weathering conditions (Piatak, 2015).
Steel Slag, meanwhile, is a good candidate for adoption as a widespread cement substitute.
Further Research
A complete Life-Cycle Assessment for pozzolan substitutes involves a cradle-to-grave analysis of the entire impact of substituting Portland Cement. A more thorough investigation is needed, for example, on the carbon produced during the pozzolan collection and transportation phases. These carbon footprints, however, are negligible compared to the carbon that would otherwise be produced in the manufacturing of Portland cement, and so these complications have been largely omitted within the scope of this survey.
If the case for pozzolanic cement substitutes rests on the actual amount of pozzolan that can replace cement, then much more investigation is needed concerning more accurate substitution values. In their 2014 report, ASTM for the first time recognized Portland-Pozzolan cement mixes, and allows up to 40% cement replacement with pozzolanic substances. Further investigation into recommended substitution levels, such as in the studies by Kartini and Dwivedi, give lower percentage recommendations, in the range of 20-30% substitution. Because these values are so important for the actual magnitude of carbon emissions reduction, further study is needed concerning the relative values for different types of pozzolans.
The diverse origin of various types of pozzolans (whether natural, agricultural, or industrial) comes with many potentially unintended consequences. The initial intent with cement substitutions was to reclaim the waste products of other industries, many of which simply end up in landfills, and recycle them in a way that would significantly decrease the amount cement production. An important question, however, is whether it is ethical to subsidize particular industries whose very existence in the first place is environmentally destructive. In this respect, pozzolans derived from renewable resources, particularly from agriculture, offer much more promise as a carbon sink.
The various maps illustrate a strong correlation between countries with a high production volume of cement with access to the leading pozzolanic cement substitutes. China and India, as the top cement producers, as well as robust agricultural and industrial exporters, can feasibly incorporate all of the aforementioned types of pozzolans into cement production. The United States and the European Union, also top cement producers, have more correlation with industrial pozzolans, perhaps due to their relatively cool climates. More investigation is needed for renewable sources of pozzolans for these regions. Initial studies have been conducted on the feasibility of using corn cob ash or peanut shell ash by Dauda et. al. (2016), but further verification is needed. The promising initial results, however, suggest that crops even outside of the tropical region are strong candidates as cement substitutes.
Ultimately, the widespread adoption of pozzolanic cement substitutes might rely on economic incentives. Pozzolanic materials are cheaper than Portland cement, offering a strong financial argument for cement substitutes. The popularization of particular pozzolans, such as bamboo leaf ash, has the potential to spur development in bamboo farming, as well as other similar industries. More direct and thorough economic analysis is needed to better communicate the multi-faceted economic advantages posed by cement substitutes, underscoring the many other their many other advantages, in terms of durability, longevity, and sustainability.
Sources
Al-Chaar, et. al. (2011). The Use of Natural Pozzolan in Concrete as an Additive or Substitute for Cement. Accessed May 10, 2017. Link.
Cucek, et. al. (2012). A Review of Footprint Analysis Tools for Monitoring Impacts on Sustainability. Accessed May 10, 2017. Link.
Dauda, et. al. (2016). Exploring the Potential of Corn Cob Ash and Alternative Pozzolona Cement for the Northern Savannah Ecological Zone in Ghana. American Journal of Civil Engineering, Volume 4, Issue 3, May 2016, Pages: 74-79. Link.
Devi, et. al. (2014). Properties of Concrete Manufactured Using Steel Slag. Procedia Engineering, Volume 97, 2014, Pages 95-104. Link.
Detwiler, Rachel. (2002). Substitution of Fly Ash for Cement or Aggregate in Concrete: Strength Development and Suppression of ASR. Portland Cement Association, Research & Development Bulletin RD127, 2002. Link.
Dwivedi, et. al. (2006). A new pozzolanic material for cement industry: Bamboo leaf ash. International Journal of Physical Sciences Vol. 1 (3), pp. 106-111, November, 2006. Link.
Food Outlook: Biennial Report on Global Food Markets. May 2015. Accessed May 9, 2017. Link.
Forman and Wu (2016). Where to Put the next Billion People. Nature News & Comment. Accessed March 14, 2017. Link.
Global Coal Consumption Rising Despite Deep U.S. Cuts Bloomberg BNA. Accessed May 9, 2017. Link.
Habeeb, et. al (2009). Rice Husk Ash Concrete: The Effect of RHA Average Particle Size on Mechanical Properties Accessed May 9, 2017. Link.
Huntzinger, Deborah N., and Thomas D. Eatmon. (2009). A Life-Cycle Assessment of Portland Cement Manufacturing: Comparing the Traditional Process with Alternative Technologies. Journal of Cleaner Production 17, no. 7 (May 2009): 668–75.
Jonkers, H. M. (2011). Bacteria-Based Self-Healing Concrete. Heron 56, no. 1-2.
Knoeri, et. al. (2012). Comparative LCA of recycled and conventional concrete for structural applications. The International Journal of Life Cycle Assessment 18(5):909-918. Link.
Piatak, Nadine M., Michael B. Parsons, Robert R, and I. I. Seal. (2015). Characteristics and Environmental Aspects of Slag: A Review. Applied Geochemistry 57 (2015).
O. K. Keskin, and M. Lachemi. (2016). Self-Healing of Cementitious Composites to Reduce High CO2 Emissions. Materials Journal 114, no. 01 (January 1, 2017). Accessed May 9, 2017. Link.
Steel-Slag: A Supplementary Cementious Material and Basis for Energy-Saving Cement. Global Cement Magazine. Accessed May 9, 2017. Link.
Thomas, M., et. al. (2007). Optimizing the Use of Fly Ash Concrete Accessed May 9, 2017. Link.
Valderrama, et. al. (2012). Implementation of Best Available Techniques in Cement Manufacturing: A Life-Cycle Assessment Study. ResearchGate. Accessed May 10, 2017. Link.
Valipour, Mahdi, Mina Yekkalar, Mohammad Shekarchi, and Somayeh Panahi. (2014). Environmental Assessment of Green Concrete Containing Natural Zeolite on the Global Warming Index in Marine Environments. Journal of Cleaner Production 65 (February 2014): 418–23.
Van den Heede, P., and N. De Belie. (2012). Environmental Impact and Life Cycle Assessment (LCA) of Traditional and green Concretes: Literature Review and Theoretical Calculations. Cement and Concrete Composites 34, no. 4 (April 2012): 431–42.
Why Use Fly Ash. Separation Technologies LLC. Accessed May 9, 2017. Link.
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