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The human impact on natural rock reserves by using basalt, anorthosite
and carbonates as raw materials in insulation products
Tais W. Dahla
a Ph.D, M. Sc. Geophysics
tais@snm.ku.dk
Nordic Center for Earth Evolution and Natural History Museum of Denmark
University of Copenhagen
Øster Voldgade 5-7
DK-1350 Copenhagen K, Denmark
tel: +45 3532 2362
fax: +45 3532 2450
Anders U. Clausenb*
B.Sc. Chemical engineering
anders.ulf.clausen@rockwool.com
Peter B. Hansenb
Ph.D., M.Sc. Chemical engineering
peter.binderup.hansen@rockwool.com
bRockwool International
Hovedgaden 584
DK-2640 Hedehusene, Denmark
tel: +45 4656 1616
fax: +45 4656 3011
* Corresponding author
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Abstract
Typical crustal rocks such as basalt, limestone, and anorthosite are used in stone wool insulation products.
The raw materials for stone wool production are not specific to any rare mineral source, but depend upon the
mixture of materials having the correct chemical composition; exemplified by 40 wt% basalt, 20 wt%
anorthosite, and 40 wt% cement bonded renewable materials. This study provides an overview of the natural
cycle of these resources, their abundances in nature, and set the consumption by stone wool industry and
other human activities in perspective.
Basalt, anorthosite, and carbonates exist in high abundances on all continents. While basaltic rocks cover
most of the ocean floor, these reserves are hidden below kilometers of water and therefore regarded as
inaccessible. Instead, large igneous provinces on land constitute major basaltic reserves useful for human
rock exploration. Globally, anorthositic provinces comprise smaller volumes than does limestone or basalt,
but still in sufficient amounts to supply the production of insulation materials for more than 100 million years.
An evaluation of the modern consumption rates and reservoir sizes show that all the crustal inventories of
these rock types are so large that they could supply current human demands for millions of years.
The natural degradation of surface rocks occurs by physical and chemical weathering creating sediment that
is transported along rivers and deposited in the ocean. Sediments are either obducted with continental
lithosphere or subducted with oceanic crust and recycled through the mantle by plate tectonic motions. The
insulation products have a chemical composition similar to average crustal rocks and eventually join the
natural rock cycle. However, the products need not to accumulate in nature, since old insulation materials
serve as excellent source material in new products. Moreover, current production lines exploit more than 30
natural and 20-30 synthetic source materials that circumvent regional depletion of source materials and
contribute to the recycling of other industrial materials.
Keywords: insulation products, stone wool, rock cycle, rock reserves, natural resources, human impact, plate
tectonics.
1 Introduction
Exhaustion of natural resources by human activities conceives much attention as we begin to realize the
limitations of Earth’s reservoirs. Fossil fuel inventories are being consumed at a rate that may encounter
depletion of the known oil and gas reserves within this century. New hydrocarbon reservoirs are generated
so slowly in nature that these resources easily can be exhausted by human activities. Also, fresh water
reserves have drawn attention, as prodigious use of water in industrial societies affect the natural state of
balance, particular in dry areas where the demand for water irrigation is prevalent. It is certainly true that
human activities consume some natural resources faster than can be restored by nature and further drive
reservoirs to exhaustion. This study is aimed at the raw materials for stone wool production and evaluates
the size of global reservoirs in comparison to the human consumptions. We focus on the major raw materials
for stone wool production: basalt, anorthosite, and carbonates and investigate if this industry or other human
activities can deplete these rock reserves.
Human excavation of rocks is probably the largest industry in the world if measured by weight of material
extracted. The majority of material is used in the construction industry. In the United States the total
extraction of nonfuel mineral materials from 1994 through 2001 was rather constant at approximately 5,500
Mt/yr (Mt = megatons = 106 metric tons) (Horvath 2004) and ca. 3,500 Mt/yr in the EU (Bringezu 2002). The
combined consumption in OECD countries may comprise 11,000 Mt/yr if scaled linearly to population
(calculation in the caption of table 3), and may be doubled if China starts to operate at similar intensity. For
comparison the global stone wool production is estimated to 10-20 Mt/yr based on data from the largest
manufacturer, Rockwool International, who produces 2 Mt/yr and probably accounts for 10-20% of the global
market.
Stone wool industry is better developed in Denmark than in most other parts of the world and therefore it
accounts for a larger fraction of national rock consumption than in most countries. The total material
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requirement of nonfuel mineral materials in Denmark was 102 Mt/yr in 1997 (Pedersen 2002) including sand,
gravel and clay materials. This is 500 times larger than the national stone wool production of approximately
0.2 Mt/yr (Rockwool International). Clearly, construction industry is a substantial consumer of rock resources.
This industry is, however, not restricted to utilize basalt, anorthosite, or carbonates.
Danish stone wool production began in 1938 and the average Danish home is still only 1/3 insulated
compared to the present Danish building regulations level (Tommerup and Svendsen 2006). At the current
production rate the households will be well insulated in one century (assuming a steadily increasing
production rate and no destruction). The market in OECD countries is 150 times larger if scaled by
population and would require a total of 150,000-300,000 Mt stone wool material before all households
become insulated corresponding to ~15,000 years of continued production. At this point, the industry can in
principle be self-sustained by utilizing old insulation material in the new productions and further rock
excavations could stop. This estimate provides a relevant measure of expected market saturation which is
useful when we turn to the assessment of reservoir sizes and potential depletion of rock resources. We
should keep in mind that the consumption by other human activities exceeds stone wool production
thousand-fold, and sufficiency of resources is only met when all human activities are taken into account.
The human impact on environment has more aspects than potential reservoir depletion. An investigation of
the full material life cycle (energy, materials and emission impacts) in stone wool production is presented in
the Life Cycle Assessment by Schmidt et al. 2004a and Schmidt et al. 2004b. In present study we describe
the raw materials used in stone wool production (section 2), the cycling of these resources in nature (section
3), their abundances and generation in nature (section 4). These considerations facilitate an evaluation of
the human impact on resource inventories (section 5).
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2 Stone wool production
Stone wool insulation products are made from natural or synthetic materials with a chemical composition
similar to average crustal rocks (table 1). The source materials are not restricted to specific minerals of high
purity, but rather on a mixture of abundant and homogenous resources that gives a chemical composition
within the specified CAS range (registrated chemical composition according to Chemical Abstract Service).
Oxide Upper Crust
a
)
Stone wool
b
)
Basalt
c
)
Anorthosite
d)
Cement
e)
wt% wt% wt% wt% wt%
SiO
2
65.9 33-43 50.6 55.9 14.5
Al
2
O
3
15.2 18-24 15.6 25.3 1.5
TiO
2
0.5 0.5-3.0 1.3 0.3 -
Fe as FeO 4.5 3-9 9.6 1.6 1.6
CaO + MgO 6.4 23-33 19.6 9.7 81.2
Na
2
O + K
2
O 7.3 1-10 2.8 6.3 1.0
Total 99.8 100 99.4 99.1 99.8
a) Average composition of the upper crust (Taylor and McLennan 1995), b) CAS registrered composition of
high-alumina low-silica (HT) stone wool fibers (Guldberg et al. 2002) and average composition of c) seven
mid-ocean ridge basaltic occurrences (Klein et al. 2003), d) seven anorthositic provinces (82 samples)
(Selbekk et al. 2000), and e) Portland cement ‘white clinker’ (Borup 2009).
Table 1: Chemical composition of average crustal rocks, high-alumina low-silica (HT) stone wool fibers, and
typical source materials: basalt, anorthosite, and Portland cement.
Stone wool is produced by mixing the source materials with foundry coke. The mixture is poured into a
vertical furnace, where it melts at 1500°C. The melt flows over high speed spinning wheels where a binder is
added to the material, and a horizontal air flow blows, at a speed of 170 m/s, the magma into fine intertwined
fibers. The outcome is stone wool with a density ca. 100 times lower than the original rocks. A by-product of
the melting process is pig iron that accumulates in the bottom of the furnace. The metal is tapped and reused
in steel industry. The estimated output is 97% stone wool and 3 wt% metal according to Rockwool
International (figure 1). There are no other waste products of this production. Gaseous and liquid emissions
are described in life cycle assessment reports (Schmidt et al. 2004a; Schmidt et al. 2004b).
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Figure 1: Simplified budget for stone wool production . The typical sources can be expressed as 40 wt%
basalt, 20 wt% anorthosite, and 40 wt% cemented renewables. However, the production does not depend on
any specific rock type and secondary materials from own and other industries are implemented in the
cemented briquette. The output is stone wool and pig iron. The latter is reused in the steel industry.
A wide range of source materials are useful in the production scheme. Basically, the input needs to fulfill only
two requirements. First, the source material must come in large enough sizes to allow air ventilation in the
furnace that facilitates efficient melting. Accordingly, hand size specimens are used whereas crushed rocks
and sands do not readily melt. Crushed materials can be included in a cement bonded briquette which is
robust enough for loading into the furnace, but not strong enough as a building stone. This technology also
allows for incorporation of secondary materials in the production.
The melting of source materials relaxes traditional constraints to industrial source materials. For example,
there is no need for rocks of high purity, unusual strength, or exceptional quality as long as the material is
coherent and does not break upon loading into the furnace.
The second material requirement concerns its chemical composition. Stone wool has a composition
specified by the CAS range (table 1), which can be obtained from various starting materials. Rockwool
International exploits more than 30 different rock materials and 20-30 secondary material worldwide to meet
CAS specifications. Every factory exploits 1-2 rock sources, cement briquettes containing industrial waste
materials, and a range of other materials. Blast Furnace slag is the most common secondary material which
supplies CaO to the melt. Other materials include bauxit (or secondary Al2O3 sources) and
calcium/magnesium-rich components such as slag, dolomite, or olivine sand. The great variety of source
materials obscures precise assessments of the ability to deplete natural rock reservoirs. Instead, we
approximate the ingredients by the “old recipe” in order to overestimate the consumption of rock materials. In
this picture high-alumina low-silica (HT) stone wool fibers are resembled by a mixture of 40 wt% basalt, 20
wt% anorthosite, and 40 wt% cement bonded bricks, including carbonate minerals (typically limestone or
dolomite).
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Figure 2: The chemical composition in various source materials normalized to the CAS specified stone wool
composition. The acceptable CAS range for ‘HT’ stone wool is shown with red dashed boxes. The Earth’s
crust (blue bars) has an average chemical composition nearly acceptable for stone wool production, but with
higher silica content and lower concentrations of aluminum and alkaline earth metals (calcium and
magnesium). A mixture of ca. 40% basalt, 20% anorthosite and 40% cement-containing briquettes is a
typical example of how the specified CAS range is obtained. The composition of basalt is shown as Mid-
Ocean Ridge Basalt (MORB) in green bars (Klein et al. 2003) which is elevated in silica and iron, but low in
aluminum and calcium/magnesium. Anorthosite (orange) supplies aluminum and lowers the iron content in
the melt (Selbekk et al. 2000), while Portland cement (pink) reduce silica content and adds calcium and/or
magnesium (Borup 2009).
The chemical composition of stone wool is similar to average crustal rocks with slightly lower silica content,
and elevated aluminum and calcium and/or magnesium concentrations. The low abundance of silica relative
to average crustal rocks (blue) lowers melt viscosity and enhances fluid properties. The elevated aluminum
(Al2O3) and calcium+magnesium (CaO+MgO) content optimize the insulation properties of the fiber.
The chemical compositions of various raw materials normalized to the CAS specifications of ‘HT’ stone wool
are shown in figure 2. The red boxes display the range of acceptable compositions according to CAS
specifications, and columns represent chemical composition of various raw materials. The figure clarifies the
role of the source materials.
Basaltic rocks (e.g. basalt, gabbro, diabase) constitute a major component in the melt (40 wt%). This is
mixed with anorthositic rocks to compensate for the relatively low aluminum content in basalt. The mixture of
anorthosite and basalt produces a silica-rich melt depleted in calcium or magnesium relative to CAS
specifications. Therefore, a cement bonded briquette containing secondary materials provides CaO plus
MgO without adding silica. In the absence of secondary products one would have to add a carbonate source
rock such as limestone, dolomite, chalk, or marble.
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3. The natural rock cycle
The Earth is a dynamic planet. Its surface is constantly being altered by internal driving forces such as
volcanic and tectonic activities, and by external forces such as erosion and deposition. These processes
have been active throughout Earth’s 4.5 billion year long history and will continue to operate in the future.
Figure 3: The geological cycle of crustal rocks. Oceanic spreading ridges produce new basaltic crust from
molten magma that enters the surface as new seafloor at oceanic spreading ridges (A) or as volcanoes (E)
and large flood basalts (D) on land. Continental rocks are constantly eroded by weathering processes (C)
and physically broken down to sand and gravel that travel with water and wind into the ocean. The Earth’s
lithosphere comprises the hard and rigid outer layer of the planet which is fragmented into plates that moves
relative to one another. Oceanic lithospheric plates are heavier than continental plates and are ultimately
subducted (B) into the Earth’s mantle. The subducted crust melts and gives birth to a new generation of
rocks.
Plate tectonic motions are responsible for renewing the surface over geological time scales - much longer
than a human life time. The outermost shell of the planet is a rigid lithosphere broken into plates that move
relative to each other. At mid-oceanic ridges new oceanic crust of basalt is produced as hot fluid magma
rises from the Earth’s interior towards the colder surface (figure 3, A). This process causes the Atlantic
Ocean to open as the North American plate and Eurasian plate (Europe and Asia) drift apart.
Most of the oceanic basalt reservoir stays beneath deep water, unexposed, and inaccessible for industrial
purposes. This occurs because basaltic rocks are heavier than average continental rocks, and thus oceanic
plates are subducted under less dense continental plates (figure 3, B). In this view the oceanic crust moves
as a gigantic conveyor belt by which the oceanic surface is recycled into the mantle. The ocean floor is
hydrated and geochemically altered during this process. The oceanic ridge system is 84.000 km long and
average plate motions ranging from 1 to 12 cm/yr (Skinner 1987), which means the ocean floor is on
average renewed every 100 million years. The ocean crust is 7-10 km thick and, thus, oceanic basalt
production exceeds 100.000 Mt/yr which is an order of magnitude higher than the rock consumption in
OECD countries.
Continental rocks are constantly eroded from the Earth’s surface (figure 3, C) by chemical and physical
weathering processes. Wind and water drives solutes into ground water and sediment to the ocean. We see
sediment in the transported waters after a rain fall and in the wind of a dust storm. The mud on a lake floor,
sand on a beach, and even dust in a window sill are examples of sediment. Sediment piles up in rivers and
in shallow waters on the way from continents to oceans. As a result continents are continuously broken down
by weathering processes and transported to the sea.
The continental lithosphere is in isostatic balance with the underlying mantle which means that rocks are
uplifted and exposed as the surface erodes away. The sediments are either obducted by plate tectonic
motions on land or subducted with oceanic plates back into the mantle. The long-term consequence of
erosion is clear to the geologist. Plutonic rocks of mostly granitic composition formed below the surface (<45
km depth) are now exposed in abundance on all continents. Other major rock reservoirs on land include
marine sediments that have been exposed by tectonic forces, volcanic rocks from eruptions of both mafic
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(basaltic) and felsic (granitic) composition, and large igneous provinces resulting from dramatic volcanic
eruptions in the past. All rocks on Earth’s surface participate in the geological rock cycle.
4. Comparison of rock reservoirs and human consumption for stone wool production
In the following we compare natural resource inventories to human consumption rates. This comparison is
made on a global basis, since humans can deplete any local reservoir provided the area in concern is small
enough. For example, basaltic and anorthositic rock reserves are essentially absent in Denmark. Therefore,
Danish territory is therefore already exhausted. The solution for the stone wool industry in Denmark has
been to import anorthosite from Norway and basalt from Sweden, where reservoirs exist in sufficient
amounts.
In this evaluation we disregard hardly accessible reservoirs. For example, the surface of the Moon
constitutes a huge reservoir of anorthosite and the deep ocean is made almost entirely of basalt. Here, we
only consider reservoirs on the continents and continental shelves.
More than 2% of the continental area is covered by large igneous provinces continental flood basalts and
oceanic plateaus (Courtillot and Renne 2003) that provide major reservoirs for stone industry (figure 3, D).
These reservoirs are generated episodically in eruptions occurring over short geological time scales. Five
events have been recognized during the last 100 million years. The rock inventories are enormous. Twelve
Phanerozoic provinces (<542 mio years old) of continental flood basalts are known, and each one contains
more than 3 billion megatons basalt – enough to supply stone wool production for 1 billion years. The
reservoirs of continental basalt alone are sufficient to supply world’s total material requirements for the next
10 million years without recycling. Therefore, basalt is not a limited resource. However, the human
consumption is substantial in the overall budget as it would take only 2 million years for humans to consume
what nature produced in 100 million years. Basalt can only be regarded as a renewable resource if we take
into account the oceanic production of basalt at mid-oceanic spreading ridges, or utilize other rock sources
for the majority of rock consuming human activities.
Volcanic eruptions (figure 3, E) give birth to new basaltic rocks that can be useful for stone wool
manufacture. The production rate is irregular over short time scales and rate estimates for the near future
carry high uncertainty. However, a single eruption in 1783 at the Laki fissure, Iceland, spewed out ca. 50,000
Mt (Thordarson and Self 1993). Only 2-4% of this would have been used if stone wool industry had existed
and produced insulation materials at the current rate ever since.
Anorthositic plutons are globally significant and span areas from 100 to 20.000 km2 (Selbekk et al. 2000).
The Nain province in Labrador, Canada contain about 0.5 billion megatons anorthosite (Emslie et al. 1994;
Funck et al. 2000), enough to sustain global stone wool production for more than 100 million years (table 3).
As with basalt, the global rock reservoir of anorthosite is inexhaustible with respect to the stone wool
industry. The production rate of anorthositic crust in the future is unknown. The vast majority of anorthosite
bodies were emplaced early in Earth history during the Archean and Proterozoic eons (before 542 million
years ago), and it is unknown if nature will produce more of this resource in future. Therefore humans can
exhaust the world’s anorthosite reserves, but it would take 100,000 years and require all construction
industry to target this resource (for no obvious reason).
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Global inventory (Mt) Natural production rate (Mt/yr)
Basaltic crust (continental) 8.8
.
10
10 [a]
390
[b]
Basaltic crust (oceanic) 7.7
.
10
12
[c]
>50.000
[d]
Anorthositic crust >4.9
.
10
8
[
e
]
unknown
Carbonate in sediments and crust 1.3
.
10
11
[
f
]
2,000
[f]
a) Continental flood basalts inventories estimated by Courtillot and Renne 2003. b) The production from flood
basalt is estimated from the last 100 million years where reservoirs of 39 billion megatons basalt formed in
five eruptions (Courtillot and Renne 2003), c) Estimated basaltic reservoir in the ocean floor using an average
crustal thickness of 8.5 km (Skinner 1987). d) Crustal production at spreading ridges based on thickness,
length and speed of the oceanic ridge system of 8.5 km thick, 84.000 km long, and 6 cm/yr (Skinner 1987).
e) The value only accounts for the Nain Plutonic Suite in Labrador area, Canada. It is calculated using the
areal extent of 19,000 km2 (Emslie et al. 1994) and average thickness of 8 km (Funck et al. 2000). f) Estimate
based on the inorganic carbon reservoir (Sundquist 1993).
Table 2: Reservoir sizes and their natural production rates (Mt/yr)
Limestone and dolomite are common sedimentary rocks made of calcium-carbonate (CaCO3) and calcium-
magnesium carbonate CaMg(CO3)2, respectively. The vast majority of carbon on the Earth’s surface is
stored in such carbonate deposits. It is therefore intuitively clear that the calcium and magnesium inventories
in these rock reserves dramatically exceed exhaustible hydrocarbon reservoirs. Carbonates are common on
all continents and constitute inexhaustible reservoirs for stone wool industry (table 3). Limestone is also an
important building stone and the human consumption is dominated by the cement industry. The US
Geological Survey estimated a global consumption of 13,000-26,000 Mt in 2007 which means insulation
industry accounts for less than 0.1% (Bliss 2008) of the total annual excavation.
New carbonate is produced in the shallow ocean on the continental shelves, where carbonate-secreting
organisms thrive and skeletal debris is deposited on the sea floor. The natural production rate of carbonate,
ca. 2000 Mt/yr (Sundquist 1993), exceeds the global stone wool consumption hundred times, but cannot
keep up with the cement industry. Therefore, the cement industry, but not stone wool industry, can deplete
these reservoirs. This would occur in 15-30 million years.
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Resource
Consumer Global Consumption Time to exhaustion
Anorthosite Global stone wool production 10-20 Mt/yr
a)
>120 Myrs
Carbonate Consumption of limestone (CaCO3),
mainly for cement industry 13,000-26,000 Mt/yr b) 15-30 Myrs
Basalt Consumption in construction industry 11,000 Mt/yr
c)
10 Myrs
a) Stone wool production estimated from 2.0 Mt/yr produced by Rockwool International and a market share
estimated to be 10-20%. b) Estimated by USGS (Bliss 2008). c) Consumption of natural stones in the
European Union, rounded numbers 3,500 Mt/yr (Bringezu 2002) and US consumption of nonfuel mineral
materials of 5,500 Mt/yr (Horvath 2004) is scaled by population to OECD countries: (1000 million people in
OECD)/(800 million people in EU, Canada, and USA) = 11,300 Mt/yr.
Table 3: Industrial consumption rates of basalt, anorthosite, or carbonate and the characteristic time scale for
the industry to cause global exhaustion.
All raw materials used for stone wool production are common and their abundances in nature can be
regarded inexhaustible reservoirs over million year time scales even when the dominant rock consuming
activities are taken into account. If basalt and carbonate were exclusively used in stone wool industry, those
materials may even be regarded as renewable resources over geological time scales, since production in
nature exceeds stone wool consumption (table 2 and 3).
When we recall the relevant inventory needed to insulate all houses in the world OECD countries (150,000-
300,000 Mt), it appears inconceivable for human beings to deplete these rock reserves. Moreover, the
variety of applicable raw materials including secondary products from other industries further reduces the
total rock consumption and allows rock reservoirs to last even longer.
5. Environmental impact of rock excavation
The built-up of used insulation material on Earth, is not dangerous, but takes up accommodation space
which could be used in better ways. Neither the raw materials used in stone wool production nor the stone
wool fibers themselves contain free quartz (SiO2). There is no elevated carcinogenic risk upon inhalation of
dust from high-alumina low-silica HT fibers (IARC 2002; Kamstrup et al. 1998). Fortunately, old insulation
material can readily be incorporated in the cemented briquette and reused in the production of new stone
wool. Therefore, the waste material should not wait for erosive destruction returning it into sedimentary
deposits, but it should instead recycle as buildings are broken down.
Human activities accelerate the erosion rate on Earth that reshape the surface environment, and enhances
the nutrient supply in rivers, oceans and soils. Recent estimates suggest humans accelerate erosion rates to
75,000 Mt/yr, almost 4 times faster than in pre-agricultural times (Wilkinson and McElroy 2007). This is
driven by farmland denudation. Rock mining for stone wool industry contributes to this process, but plays an
insignificant role. The natural variations of the erosion rate throughout the last 500 million years of Earth
history (Wilkinson and McElroy 2007) exceed the influence of stone wool industry more than 1,000 times. In
this perspective the rock consumption for insulation products is expected to have an insignificant effect on
changing the natural rock cycle.
5. Conclusion
The primary source materials for stone wool insulation products have been described and their reservoirs
sizes and production rates have been evaluated. We conclude that
11
• Raw materials for stone wool production are both natural and synthetic materials with an average
chemical composition similar to average crustal rocks.
• The primary source rocks for stone wool production can be represented by basaltic, anorthositic, and
cemented briquettes including secondary materials.
• The accessible rock reserves of basalt, anorthosite and carbonates are large enough to supply
current human demands for millions of years and can be regarded as inexhaustible resources.
• The natural production of carbonate and basalt even overwhelms the consumption in stone wool
production. These resources are renewable in the geological sense and can even be regarded as
sustainable resources, unless other human activities target these resources.
• The production rate of anorthosite is irregular and unpredictable for the future. However, the
reservoirs are huge and can supply stone wool production for more than 100 million years.
• The variability of the source materials in stone wool production assists an environmentally friendly
decomposition of industrial waste products by implementing secondary materials in the products.
Acknowledgements
This case study was financially supported by Rockwool International A/S. The authors would like to thank
Ole Kamstrup, Jens Konnerup Madsen, and Minik Rosing for thoughts and fruitful discussions. Jan S.
Adolfssen, Frederik Berg Nygaard, and Mikkel Pagh are thanked for reviewing the manuscript.
12
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