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At 08:33 AM 6/24/2007, Rikki Nitzkin wrote:
I know all the arguments, but I don«t have any
firm statistics from reliable sources to back up my arguments.
The text of a 1993 article from Environmental
Building News, without its charts and diagrams, follows below.
On the whole, the cement industry hasn't changed
a lot since the article. One thing to note is
that in the U.S., we import a lot of cement -
it's not always a local or even regional product.
We even bring it over from China.
We had a couple higher-ups from the Portland
Cement Association's sustainability task group -
yes, they actually have one - in our offices last
year. They didn't have horns and tails, and were
surprisingly forthright in acknowledging their
industry's shortcomings. Doesn't mean that we
don't use way too much concrete in the built
environment, though. For their spin, see www.concrete.org
I'd like to recommend Bruce King's book, Making
Better Concrete: Guidelines to Using Fly Ash for
Higher Quality, Eco-friendly Structures.
- - - - -
Cement and Concrete:
Feature - Environmental Building News March/April 1993
Cement and concrete are key components of both
commercial and residential construction in North
America. The cement and concrete industries are
huge. There are approximately 210 cement plants
in the U.S. and 4,000 to 5,000 ready mix plants
(where cement is mixed with aggregate and water
to produce concrete). The Portland Cement
Association estimates that U.S. cement
consumption has averaged between 75 and 90
million tons per year during the last decade, and
projects that consumption will exceed 100 million
tons per year by 1997. Worldwide, cement
production totaled 1.25 billion tons in 1991,
according to the U.S. Bureau of Mines.
What does this mean in terms of the environment?
Are these products good or bad? As builders and
designers, should we be looking for alternatives
or embracing concrete over competing materials?
As with most building issues, the answers are not
clear-cut. Concrete and other cementitious
materials have both environmental advantages and
disadvantages. This article takes a look at how
these materials are made, then reviews a number
of environmental considerations relating to their
production, use, and eventual disposal.
Cement and Concrete Production
Cement is the key ingredient in concrete
products. Comprising roughly 12% of the average
residential-grade ready mix concrete, cement is
the binding agent that holds sand and other
aggregates together in a hard, stone-like mass.
Portland cement accounts for about 95% of the
cement produced in North America. It was patented
in England by Joseph Aspdin in 1824 and named
after a quarried stone it resembled from the Isle of Portland.
Cement production requires a source of calcium
(usually limestone) and a source of silicon (such
as clay or sand). Small amounts of bauxite and
iron ore are added to provide specific
properties. These raw materials are finely ground
and mixed, then fed into a rotary cement kiln,
which is the largest piece of moving industrial
equipment in the world. The kiln is a long,
sloping cylinder with zones that get
progressively hotter up to about 2700¡F (1480¡C).
The kiln rotates slowly to mix the contents
moving through it. In the kiln, the raw materials
undergo complex chemical and physical changes
required to make them able to react together
through hydration. The most common type of cement
kiln today (accounting for 70% of plants in the
U.S.) is a dry process kiln, in which the
ingredients are mixed dry. Many older kilns use the wet process.
The first important reaction to occur is the
calcining of limestone (calcium carbonate) into
lime (calcium oxide) and carbon dioxide, which
occurs in the lower-temperature portions of the
kiln Ð up to about 1650¡F (900¡C). The second
reaction is the bonding of calcium oxide and
silicates to form dicalcium and tricalcium
silicates. Small amounts of tricalcium aluminate
and tetracalcium aluminoferrite are also formed.
The relative proportions of these four principal
compounds determine the key properties of the
resultant portland cement and the type
classification (Type I, Type II, etc.). These
reactions occur at very high temperatures with
the ingredients in molten form. As the new
compounds cool, they solidify into solid pellet
form called clinker. The clinker is then ground
to a fine powder, a small amount of gypsum is
added, and the finished cement is bagged or
shipped bulk to ready mix concrete plants.
Concrete is produced by mixing cement with fine
aggregate (sand), coarse aggregate (gravel or
crushed stone), water, and Ð often Ð small
amounts of various chemicals called admixtures
that control such properties as setting time and
plasticity. The process of hardening or setting
is actually a chemical reaction called hydration.
When water is added to the cement, it forms a
slurry or gel that coats the surfaces of the
aggregate and fills the voids to form the solid
concrete. The properties of concrete are
determined by the type of cement used, the
additives, and the overall proportions of cement, aggregate, and water.
Raw Material Use
The raw materials used in cement production are
widely available in great quantities. Limestone,
marl, and chalk are the most common sources of
calcium in cement (converted into lime through
calcination). Common sources of silicon include
clay, sand, and shale. Certain waste products,
such as fly ash, can also be used as a silicon
source. The iron and aluminum can be provided as
iron ore and bauxite, but recycled metals can
also be used. Finally, about 5% of cement by
weight is gypsum, a common calcium- and
sulfur-based mineral. It takes 3,200 to 3,500
pounds of raw materials to produce one ton (2,000
lbs.) of finished cement, according to the
Environmental Research Group at the University of British Colombia (UBC).
The water, sand, and gravel or crushed stone used
in concrete production in addition to cement are
also abundant. With all of these raw materials,
the distance and quality of the sources have a
big impact on transportation energy use, water
use for washing, and dust generation. Some
aggregates that have been used in concrete
production have turned out to be sources of radon
gas. The worst problems were when uranium mine
tailings were used as concrete aggregate, but
some natural stone also emits radon. If
concerned, you might want to have the aggregate tested for radon.
The use of fly ash from coal-fired power plants
is beneficial in two ways: it can help with our
solid waste problems, and it reduces overall
energy use. While fly ash is sometimes used as a
source of silica in cement production, a more
common use is in concrete mixture as a substitute
for some of the cement. Fly ash, or pozzolan, can
readily be substituted for 15% to 35% of the
cement in concrete mixes, according to the U.S.
EPA. For some applications fly ash content can be
up to 70%. Of the 51 million tons of fly ash
produced in 1991, 7.7 million tons were used in
cement and concrete products, according to
figures from the American Coal Ash Association.
Thus, fly ash today accounts for about 9% of the cement mix in concrete.
Fly ash reacts with any free lime left after the
hydration to form calcium silicate hydrate, which
is similar to the tricalcium and dicalcium
silicates formed in cement curing. Through this
process, fly ash increases concrete strength,
improves sulfate resistance, decreases
permeability, reduces the water ratio required,
and improves the pumpability and workability of
the concrete. Western coal-fired power plants
produce better fly ash for concrete than eastern
plants, because of lower sulfur and lower carbon
content in the ash. (Ash from incinerators cannot be used.)
There are at least a dozen companies providing
fly ash to concrete producers. Talk to your
concrete supplier and find out if they are
willing to add fly ash to the mix. Portland
cement with fly ash added is sometimes identified
with the letter P after the type number (Type
IP). The EPA requires fly ash content in concrete
used in buildings that receive federal funding
(for information call the EPA Procurement
Guidelines Hotline at 703-941-4452). Fly ash is
widely used in Europe as a major ingredient in
autoclaved cellular concrete (ACC); in the U.S.,
North American Cellular Concrete is developing this technology.
Other industrial waste products, including blast
furnace slag, cinders, and mill scale are
sometimes substituted for some of the aggregate
in concrete mixes. Even recycled concrete can be
crushed into aggregate that can be reused in the
concrete mix Ð though the irregular surface of
aggregate so produced is less effective than sand
or crushed stone because it takes more cement
slurry to fill all the nooks and crannies. In
fact, using crushed concrete as an aggregate
might be counterproductive by requiring extra
cement Ð by far the most energy - intensive component of concrete.
Energy consumption is the biggest environmental
concern with cement and concrete production.
Cement production is one of the most energy
intensive of all industrial manufacturing
processes. Including direct fuel use for mining
and transporting raw materials, cement production
takes about six million Btus for every ton of
cement. The industryâs heavy reliance on coal
leads to especially high emission levels of CO 2,
nitrous oxide, and sulphur, among other
pollutants. A sizeable portion of the electricity
used is also generated from coal.
The vast majority of the energy consumed in
cement production is used for operating the
rotary cement kilns. Newer dry-process kilns are
more energy efficient than older wet-process
kilns, because energy is not required for driving
off moisture. In a modern dry-process kiln, a
pre-heater is often used to heat the ingredients
using waste heat from the exhaust gases of the
kiln burners. A dry-process kiln so adapted can
use up to 50% less energy than a wet-process
kiln, according to UBC researchers. Some other
dry-process kilns use a separate combustion
vessel in which the calcining process begins
before the ingredients move into the rotary
kilnÐa technique that can have even higher
overall efficiency than a kiln with pre-heater.
In the United States, producing the roughly 80
million tons of cement used in 1992 required
about .5 quadrillion Btus or quads. This is
roughly .6% of total U.S. energy use, a
remarkable amount given the fact that in dollar
value, cement represents only about .06% of the
gross national product. Thus, cement production
is approximately ten times as energy intensive as
our economy in general. In some Third World
countries, cement production accounts for as much
as two-thirds of total energy use, according to the Worldwatch Institute.
While cement manufacturing is extremely energy
intensive, the very high temperatures used in a
cement kiln have at least one advantage: the
potential for burning hazardous waste as a fuel.
Waste fuels that can be used in cement kilns
include used motor oil, spent solvents, printing
inks, paint residues, cleaning fluids, and scrap
tires. These can be burned relatively safely
because the extremely high temperatures result in
very complete combustion with very low pollution
emissions. (Municipal solid waste incinerators
operate at considerably lower temperatures.)
Indeed, for some chemicals thermal destruction in
a cement kiln is the safest method of disposal. A
single cement kiln can burn more than a million
tires a year, according to the Portland Cement
Association. Pound for pound, these tires have a
higher fuel content than coal, and iron from the
steel belts can be used as an ingredient in the
cement manufacturing. Waste fuels comprise a
significant (and growing) part of the energy mix
for cement plants, and the Canadian Portland
Cement Association estimates that waste fuel
could eventually supply up to 50% of the energy.
Energy use for concrete production looks
considerably better than it does for cement.
Thatâs because the other components of concrete Ð
sand, crushed stone, and water Ð are much less
energy intensive. Including energy for hauling,
sand and crushed stone have embodied energy
values of about 40,000 and 100,000 Btus per ton,
respectively. The cement, representing about 12%
of concrete, accounts for 92% of the embodied
energy, with sand representing a little under 2%
and crushed stone just under 6%.
Use of fly ash in concrete already saves about 44
trillion Btus (.04 quads) of energy annually in
the U.S. Increasing the rate of fly ash
substitution from 9% to 25% would save an additional 75 trillion Btus.
There are two very different sources of carbon
dioxide emissions during cement production.
Combustion of fossil fuels to operate the rotary
kiln is the largest source: approximately 3/4
tons of CO2 per ton of cement. But the chemical
process of calcining limestone into lime in the cement kiln also produces CO2:
CaCO 3 â CaO + CO 2 limestone â lime + carbon dioxide
This chemical process is responsible for roughly
1/2 ton of CO2 per ton of cement, according to
researchers at Oak Ridge National Laboratory.
Combining these two sources, for every ton of
cement produced, 1.25 tons of CO2 is released
into the atmosphere. In the United States, cement
production accounts for approximately 100 million
tons of CO 2 emissions, or just under 2% of our
total human-generated CO 2. Worldwide, cement
production now accounts for more than 1.6 billion
tons of CO2 Ð over 8% of total CO2 emissions from all human activities.
The most significant way to reduce CO 2 emissions
is improving the energy efficiency of the cement
kiln operation. Indeed, dramatic reductions in
energy use have been realized in recent decades,
as discussed above. Switching to lower-CO 2 fuels
such as natural gas and agricultural waste
(peanut hulls, etc.) can also reduce emissions.
Another strategy, which addresses the CO 2
emissions from calcining limestone, is to use
waste lime from other industries in the kiln.
Substitution of fly ash for some of the cement in
concrete can have a very large effect.
Other Air Emissions
Besides CO 2, both cement and concrete production
generate considerable quantities of air-pollutant
emissions. Dust is usually the most visible of
these pollutants. The U.S. EPA (cited by UBC
researchers) estimates total particulate (dust)
emissions of 360 pounds per ton of cement
produced, the majority of which is from the
cement kiln. Other sources of dust from cement
production are handling raw materials, grinding
cement clinker, and packaging or loading finished
cement, which is ground to a very fine powder Ð
particles as small as 1/25,000 of an inch.
The best way to deal with the dust generated in
cement manufacturing would be to collect it and
put it back into the process. This is done to
some extent, using mechanical collectors,
electric precipitators, and fabric filters
(baghouses). But recycling the dust is difficult,
according to UBC researchers; it first has to be
treated to reduce its alkalinity. Some cement
kiln dust is used for agricultural soil
treatments, and the rest (of that collected) is
often landfilled on site. There was investigation
into the possibility of using cement kiln dust
for treatment of acidified lakes in eastern
Canada, but rather than simply buffering the low
pH of the water, the dust chemically created a potentially harmful salt.
In addition to dust produced in cement
manufacturing, dust is also generated in concrete
production and transport. Common sources are sand
and aggregate mining, material transfer, storage
(wind erosion from piles), mixer loading, and
concrete delivery (dust from unpaved roads). Dust
emissions can be controlled through water sprays,
enclosures, hoods, curtains, and covered chutes.
Other air pollution emissions from cement and
concrete production result from fossil fuel
burning for process and transportation uses. Air
pollutants commonly emitted from cement
manufacturing plants include sulfur dioxide (SO2)
and nitrous oxides (NOX). SO2 emissions (and to a
lesser extent SO3, sulfuric acid, and hydrogen
sulfide) result from sulfur content of both the
raw materials and the fuel (especially coal).
Strategies to reduce sulfur emissions include use
of low-sulfur raw materials, burning low-sulfur
coal or other fuels, and collecting the sulfur
emissions through state-of-the-art pollution
control equipment. Interestingly, lime in the
cement kiln acts as a scrubber and absorbs some sulfur.
Nitrous oxide emissions are influenced by fuel
type and combustion conditions (including flame
temperature, burner type, and material/exhaust
gas retention in the burning zone of the kiln).
Strategies to reduce nitrogen emissions include
altering the burner design, modifying kiln and
pre-calciner operation, using alternate fuels,
and adding ammonia or urea to the process. The
cement industry claims to have reduced overall
pollution emissions by 90% in the last 20 years.
Another environmental issue with cement and
concrete production is water pollution. The
concern is the greatest at the concrete
production phase. ãWash-out water with high pH is
the number one environmental issue for the ready
mix concrete industry,ä according to Richard
Morris of the National Ready Mix Concrete
Association. Water use varies greatly at
different plants, but Environment Canada
estimates water use at batching plants at about
500 gallons per truck per day, and the alkalinity
levels of washwater can be as high as pH 12.
Highly alkaline water is toxic to fish and other
aquatic life. Environment Canada has found that
rainbow trout exposed to portland cement
concentrations of 300, 500, and 1,000
milligrams/liter have 50% mortality times (the
time required for 50% of the population in test
samples to be killed) of 68, 45, and 29 minutes, respectively.
At the batch plant, washwater from equipment
cleaning is often discharged into settling ponds
where the solids can settle out. Most plants are
required to have process water discharge permits
from state, federal, or provincial environmental
agencies to dispose of wastewater from these
settling ponds. As long as the pH of this
wastewater is lower than 12.5, it is not
considered a hazardous material by U.S. law. Some
returned concrete also gets put into settling
ponds to wash off and recover the aggregate. On
the positive side, many newer ready mix plants
have greatly reduced water use in recent years
because of both wastewater disposal issues and
drought conditions in some parts of the country.
ãMore companies are going to completely
closed-loop systems,ä according to Terek Kahn of
the National Ready Mix Concrete Association.
Despite the apparent significance of the
wastewater concern, the National Ready Mix
Concrete Association to date has not developed
standards for member companies on wastewater
treatment, including rinsing of trucks and chutes
at the building site. John Mullarchy of the
association says that procedures are developed on
a company-by-company basis. In many areas,
environmental regulations dictate procedures
relative to wastewater treatment. In more urban
areas, the on-site rinse water (for chutes) often
has to be collected and treated or disposed of at the plant.
While the cement and concrete industries can help
reduce some of our solid waste problems (burning
hazardous waste as cement kiln fuel and using fly
ash in concrete mixtures, for example), one
cannot overlook the fact that concrete is the
largest and most visible component of
construction and demolition (C&D) waste.
According to estimates presented in the AIA
Environmental Resource Guide, concrete accounts
for up to 67% by weight of C&D waste (53% by
volume), with only 5% currently recycled. Of the
concrete that is recycled, most is used as a
highway substrate or as clean fill around
buildings. As more landfills close, including
specialized C&D facilities, concrete disposal
costs will increase and more concrete demolition
debris will be reprocessed into roadbed aggregate and other such uses.
Concrete waste is also created in new
construction. Partial truckloads of concrete have
long been a disposal problem. Ready mix plants
have come up with many innovative solutions
through the years to avoid creating waste Ð such
as using return loads to produce concrete
retaining wall blocks or highway dividers, or
washing the unset concrete to recover the coarse
aggregate for reuse. But recently, there have
been some dramatic advances in concrete
technology that are greatly reducing this waste.
Concrete admixtures are available that retard the
setting of concrete so effectively that a partial
load can be brought back to the ready mix plant
and held overnight or even over a weekend Ð then reactivated for use.
When it is possible to use pre-cast concrete
components instead of poured concrete, doing so
may offer advantages in terms of waste
generation. Material quantities can be estimated
more precisely and excess material can be
utilized. Plus, by carefully controlling
conditions during manufacture of pre-cast
concrete products, higher strengths can be
achieved using less material. The Superior Wall
foundation system, for example, uses only about a
third as much concrete as the typical poured
concrete wall it replaces. Waste water run-off
can also be more carefully controlled at
centralized pre-cast concrete facilities than on jobsites.
Another interesting trend that relates to waste
minimization is the idea of producing reuseable
concrete masonry units. The National Concrete
Masonry Association has been working on
interlocking blocks called Formwallú, designed
specifically so that they can be reused. While
these blocks are not yet on the market, this type
of thinking is a big step forward.
Working with wet concrete requires a number of
precautions, primarily to protect your skin from
the high alkalinity. Rubber gloves and boots are
typically all that is required to provide
protection. Cement dermatitis, though relatively
uncommon, occasionally occurs among workers in
the concrete industry who fail to wear the proper protective clothing.
Once it has hardened, concrete is generally very
safe. Traditionally, it has been one of the most
inert of our building materials and, thus, very
appropriate for chemically sensitive individuals.
As concrete production has become higher-tech,
however, that is changing. A number of chemicals
are now commonly added to concrete to control
setting time, plasticity, pumpability, water
content, freeze-thaw resistance, strength, and
color. Most concrete retarders are relatively
innocuous sucrose- (sugar-) based chemicals,
added in proportions of .03% to .15%. Workability
agents or superplasticizers can include such
chemicals as sulfonated melamine-formaldehyde and
sulphonated napthalene formaldehyde condensates.
Air-entraining admixtures function by
incorporating air into the concrete to provide
resistance to damage from freeze-thaw cycles and
to improve workability. These are usually added
to the cement and identified with the letter A
after the type (Type IA). These materials can
include various types of inorganic salts (salts
of wood resins and salts of sulphonated lignin,
for example), along with more questionable
chemicals such as alkyl benzene sulphonates and
methyl-ester-derived cocamide diethanolamine.
Fungicides, germicides, and insecticides are also added to some concrete.
Because of these chemical admixtures, todayâs
concrete could conceivably offgas small
quantities of formaldehydes and other chemicals
into the indoor air. Unfortunately, it is
difficult to find out from the manufacturers the
actual chemicals in these admixtures. For
chemically sensitive clients, it may be advisable
to specify concrete with a bare minimum of
admixtures, or use a sealer on the finished
concrete to minimize offgassing.
Asphalt-impregnated expansion joint filler,
curing agents that are sometimes applied to the
surface of concrete slabs to reduce water
evaporation, special oils used on concrete forms,
and certain sealants used for treating finished
concrete slabs and walls can also cause health
problems with some chemically sensitive individuals.
Finally, concrete floors and walls can cause
moisture problems and lead to mold and mildew
growth, which cause significant health problems
in certain individuals. There are two common
sources of moisture: moisture wicking through
concrete from the surrounding soil; and moisture
from the house that may condense on the cold
surface of concrete. To eliminate the former,
provide good drainage around a foundation,
dampproof or waterproof the outside of the
foundation walls before backfilling, provide a
layer of crushed stone beneath the slab, and
install a polyethylene moisture barrier under the
slab (protected from the concrete with a layer of
sand if possible). To reduce the likelihood of
condensation on concrete surfaces, they should be
insulated. In northern climates, installing a
layer of rigid foam on the outside of the
foundation wall and under the slab will generally
keep inner surface of the concrete warm enough
that condensation will not occur. With interior
foundation insulation, provide a vapor barrier to
keep moisture from reaching the concrete surface.
In southern climates, protecting against condensation may be more difficult.
Cement and concrete are vital components in
building construction today. Concrete has many
environmental advantages, including durability,
longevity, heat storage capability, and (in
general) chemical inertness. For passive solar
applications, concreteâs ability to function as a
structural element while also providing thermal
mass makes it a valuable material. In many
situations concrete is superior to other
materials such as wood and steel. But cement
production is very energy intensiveÐcement is
among the most energy-intensive materials used in
the construction industry and a major contributor
to CO 2 in the atmosphere. To minimize
environmental impact, therefore, we should try to
reduce the quantity of concrete used in
buildings, use alternative types of concrete
(with fly ash, for example), and use that concrete wisely.