Deicing Salts: An Overview

Abstract

The world production of salt (NaCl) was over two hundred million tons in 2015. The US is the second larger producer of salt after China, produced over four million tons of which 43 % were consumed in highway deicing. While NaCl is the most commonly used salt, other salts are added to it to improve its performance, such as CaCl2, MgCl2. To reduce the use of the deteriorating NaCl, other salts are also used, such as magnesium acetate, calcium magnesium acetate or potassium and magnesium formate. The addition of sand and other inorganic insoluble compounds to aid in making surfaces less slippery is discussed, as well as the recent use of organic deicers and the problems that these can induce.
The paper aims to present an overview of deicing salts, and the differences with anti-icing or antifreeze solutions. It also discusses the problems they induce to vehicles, buildings and constructions, while
also considering the negative aspect they have for the environment as well as their contribution to air pollution. Some examples are presented to illustrate the problem and less aggressive alternatives are discussed, especially with regard to the conservation of valuable architectural heritage.

Full text

16 17
Deicing Salts: An Overview
A. Elena Charola1*, Bénédicte Rousset2 and Christine Bläuer2
 Museum Conservation Institute, Smithsonian Institution, Washington, D.C., USA
2 CSC Sàrl, Conservation Science Consulting, Fribourg, Switzerland
* charolaa@si.edu
Abstract
The world production of salt (NaCl) was
over two hundred million tons in 2015.
The US is the second larger producer of
salt after China, produced over four mil-
lion tons of which 43 % were consumed
in highway deicing. While NaCl is the
most commonly used salt, other salts are
added to it to improve its performance,
such as CaCl2, MgCl2. To reduce the use
of the deteriorating NaCl, other salts are
also used, such as magnesium acetate,
calcium magnesium acetate or potassi-
um and magnesium formate. The additi-
on of sand and other inorganic insoluble
compounds to aid in making surfaces
less slippery is discussed, as well as the
recent use of organic deicers and the pro-
blems that these can induce.
The paper aims to present an overview
of deicing salts, and the differences with
anti-icing or antifreeze solutions. It also
discusses the problems they induce to ve-
hicles, buildings and constructions, whi-
le also considering the negative aspect
they have for the environment as well as
their contribution to air pollution. Some
examples are presented to illustrate the
problem and less aggressive alternatives
are discussed, especially with regard to
the conservation of valuable architectu-
ral heritage.
Keywords: deicing salts, building deterio-
ration, environmental pollution
1. Introduction
The world production of salt (NaCl) was
over two hundred million tons in 2015.
The US, the second largest producer of
salt after China, generated over four mil-
lion tons of which 43 % were consumed
in highway deicing. While NaCl is the
most commonly used salt, other salts are
added to it to improve its performance,
such as CaCl2, MgCl2. To reduce the use
of the metal corrosive NaCl, other salts
are also used, such as magnesium aceta-
te, calcium magnesium acetate or potas-
sium and magnesium formate, urea, and
even sugar containing solutions from eit-
her sugar processing or equivalent proce-
dures.1
While the use of deicing salts is neces-
sary, they do have a negative impact on
the environment, such as: soil conta-
mination, negative effect on plants and
trees near the highways/streets, conta-
mination of water courses and eventual
drinking water, air contamination by
powdered salts, corrosion of reinforced
concrete in bridges and structures, as
well as of cars and trucks. For example,
in Austria it has been estimated that half
the induced vehicle corrosion could be
attributed to de-icing salt.2 As deicing
salts are distributed, the finer particles
(<10 μm, usually referred to as PM10) can
remain suspended in the air, thus conta-
minating it.
Differentiation between deicing and
anti-icing or antifreeze should be made.
Deicing salts are applied after snow
events, their effectiveness being based
on lowering the freezing point of wa-
ter. Antifreeze solutions of glycerol or
various glycols are applied prior to the
event to prevent a strong bond between
the pavement surface and frost by ap-
plying a freezing point depressant. These
are mainly used on aircraft, machinery
and vehicles as they are non-corrosive,
however, most of them are toxic.3 Other
solutions have been developed based on
special coatings.4
2. Deicing salt varieties
Deicing salts can be roughly divided
into inorganic salts, such as sodium
chloride (NaCl), organic salts, such as
magnesium calcium acetate (CaMg(CH3
COO)4) and organic compounds such as
urea (CO(NH2)2). Salts can be used in va-
rious mixtures, and other substances
added, such as anticaking agents, e. g.,
potassium ferrocyanide5 or anticorrosion
agents such as ammonium phosphate or
sodium hypochlorite.6 Also, they can be
spread directly in granulated form, or as
a solution, i.e., brine. With the former, the
mixture with sand (e.g., 75 % sand-25 %
NaCl), or other equivalent materials such
as fine gravel or expanded clay pellets
contributes to decrease the slippery sur-
face of compacted snow2; however, they
do increase small particulates in air by
about 45 %.7 Recently, potassium carbo-
nate (K2CO3) has been studied in compa-
rison to NaCl, and it was found that whi-
le it was more adsorbed to soil colloids,
the pH was elevated more than for NaCl,
and the species composition of the area
where it had been applied changed sig-
nificantly.8
2.1. Chloride based deicers
Chloride ions from deicing salts will
mobilize and increase soil salinity near
the roadways where they are applied.
While magnesium and calcium ions in-
crease the stability and permeability of
the soil, sodium ions will decrease them.
Furthermore, sodium, magnesium and
calcium chlorides may contribute to the
mobilization of trace metals from the
soil to surface and groundwater. The so-
lid chloride deicers, i. e., NaCl, may cont-
ribute to air pollution through particula-
tes released into the air.2 ,7
2.2. Acetate based deicers and others
Soil microorganisms will break down
acetate ions resulting in oxygen depleti-
on of the soil, which can impact vegeta-
tion. A similar oxygen depletion is most
likely to occur in slow flowing streams
and small ponds into which these ions
migrate.7 While the toxicity of calcium
magnesium acetate (CMA) to fish and in-
vertebrates is low, when also containing
potassium, CMAK (50 % CMA-50 % KA),
they have higher toxicity. Acetate deicers
will result in the decrease of air pollution
as sand use can be reduced; however, the
solid deicers, CMA and sodium acetate,
NAAC, may contribute fine particulates
to the air increasing its pollution. These
deicers are mainly approved for use at
airfields and aircraft, as they are less cor-
rosive, as is the case for potassium forma-
te, in either liquid or solid form.
2.3 . Urea
Urea [CO(NH2)2] is used as a deicing
agent for airport runways9 though it has
been mostly discontinued in larger US
airports.10 The main reason is that as a
fertilizer (46 % by weight nitrogen con-
tent) it contributes to environmental
pollution, e. g., acute toxicity to aquatic
invertebrates and plants, as well as some
fish.11 Several soil bacteria contain the
urease enzyme that catalyzes the de-
composition of urea into NH4
+ and HCO3
-.
Furthermore, NH4
+ (or NH3) is oxidized
SWBSS 2017 | 20-22 September
4th International Conference on Salt Weathering of Buildings and Stone Sculptures Deicing Salts: An Overview

18 19
A. E. Charola et al.
by nitrifying bacteria, Nitrosomonas and
subsequently by Nitrobacter, in a two-
step process to NO3-, an ion that is regu-
larly found on building façades.
Urea forms an eutectic mixture with
water (at ~33 % by weight) with the eu-
tectic point at 11.5°C. Solubility is about
1Kg/L at 20°C, the dissolution being endo-
thermic, and the equilibrium RH is 76.5 %
at 25°C. In dilute solutions (not specified
but probably below 5 %), urea decompo-
ses to NH3 and CO2 (the formation of iso-
cyanic acid occurs upon heating, tempe-
rature not specified). The most common
impurity in synthetic urea results from
the condensation of two molecules to
form biuret (C2H5N3O2) or carbamylurea,
a compound that interferes with plant
growth. As a deicer, urea proves practi-
cally useful, i. e., deicing within 15-20
minutes, at temperatures below -9.4°C
taking into account that its dissoluti-
on is endothermic.12 Many studies have
addressed the decomposition of urea in
aqueous solutions13 , while others address
its use to decrease vehicular emissions of
NOx which contribute to the formation
of nitrates or nitrites in buildings along
the streets.14
2.4 . Glycols and other alcohols
Methanol was used as antifreeze in
windshield fluids, but because of health
concerns the amount added is restric-
ted. Ethylene glycol, commonly referred
to as “glycol” is used as engine cooling
antifreeze. The freezing point of ethy-
lene glycol is about −12°C, however, mi-
xed with water, this is depressed, e. g., a
mixture of 60 % EG-40 % water freezes
at −45°C. Propylene glycol has replaced
ethylene glycol in many uses because
of its lower toxicity. These products are
used for aircraft deicing fluids (heated
aqueous solution of ethylene glycol), and
as antifreeze, as undiluted, thickened
propylene glycol.
2.5. Other organic deicers
In the USA, the Minnesota Department
of Transportation claims to have pionee-
red the use of sugar beet juice based on
the huge sugar beet industry in the Red
River Valley of Minnesota/North Dakota,
and the massive need for re-use of sugar
beet waste helped create a market for it,
and the fact that these states get a lot of
snow and ice contributed to the testing.15
The sugar beet syrup is mixed in with
traditional salt, sand or chloride brines
to improve performance and reduce the
impact on the environment.16 Not only
sugar beet syrup is used, but other re-
sidues of distilled or fermented agricul-
tural products7 such as corn, barley and
even pickle brines. The addition of syrup
from sugar processing to brines has been
shown to improve their effectiveness and
has been approved in Switzerland since
2015.17
3. Impact of deicing salts on buildings
When considering the effect of deicing
salts on buildings and constructions the
immediate image that comes to mind is
the damage at the foot of walls, resulting
from the rising damp from the solution
of the melted snow and salts, as shown
in Figure 1.
Experience has shown that to this de-
terioration mechanism two other direct
contamination processes have to be ad-
ded. The first one occurs in damp winter
conditions and affects buildings located
along high traffic roads, where topogra-
phy contributes to accumulate the salt
containing melted snow and that vehi-
cular traffic and snow clearance vehicles
splash on to the building walls or disper-
se into the air so that they enter directly
at a certain height (Figure 2 left). The se-
cond process occurs during dryer winter
periods when the excess deicing salts ap-
plied recrystallize and accumulate at the
base of buildings (Figure 2 right).
In certain streets, where regular and in-
tense winds are prevalent, the salt grains
on the ground can be mobilized and
suspended in the air, as well as thrown
against the façades by whirl winds. If the
building surfaces are moist or rough, the
salts will be “attached” to them and dete-
rioration will eventually occur (Figure 3).
Following the above mentioned proces-
ses, it is logical to ask what will be the
eventual impact of the presence of these
deicing salts in the air on the conservati-
on of buildings and monuments. Recent
studies regarding the composition of fine
particular matter suspended in urban air
have shown the recurrent presence of
NaCl.7,18,19 In sea-side areas, it is obvious
that most of the salt present can be attri-
buted to marine spray and fogs, however,
this cannot be applicable to inland areas.
For example, in Putaud et al. 2 0 (see figure
3, p. 2584), the measured annual average
values of PM10 for NaCl (from January
1998 to March 1999), is referred to as “sea
salt”, in both rural areas, such as Chau-
mont, and urban areas, such as Basel and
Zurich, as well as at street level (kerbside)
Figure 1: Lausanne (canton de Vaud, CH), Pier re Viret
stairs (20.01.2010). Deicing salts are spread lavishly on
the practically non-porous gneiss, and the salt and mel-
ted snow mixture accumulates at the base of the porous
molasse-sandstone bridge wall that promptly powders
and disaggregates.
Figure 2: Left: Belfaux (canton de Fribourg, CH), nor th façade of the Lanthen-Heid manor (1526).The damages that affect
the render and the underlying molasse-sandstone are mainly the result of splashing traffic along this street (3. 09.2008).
Right: Lausanne (canton de Vaud, CH), rue Saint-Et ienne (20.01.2010). During dry winter cycles the residues of recrystal-
lized deicing salts can be seen at the foot of façades that can either be mobilized and suspended in the air or enter the
material when dissolved in water via hygroscopicity, rain, or more snow.
Deicing Salts: An Overview

20 21
cannot be used freely elsewhere, no such
proviso has been considered for our cul-
tural heritage.
Another problem is the introduction of
additional hygroscopic salts into salt-con-
taining structures, as the hygroscopic
salts will mobilize and activate those
present within the structure so that they
will migrate and eventually crystallize
in other areas. This has been observed
on monuments, where salt efflorescence
changes places after events introducing
moisture and new salts, as described by
Rolland et al.24 who called it the “trans-
porting brine hypothesis”. But the pro-
blem is now increased with the use of
products from the food industry25 that
are added to deicing salts. No one has
as yet raised questions regarding their
long term effect when retained together
with the deicing salts in structures. For
example, sugar beet syrup is hygroscopic,
with a DRH ~60 %
26, so supposing that
this, together with NaCl is taken up into
the structure, in theory, less salt should
crystallize out. Of course, other problems
could set in with the introduction of or-
ganic materials, such as biocolonization,
and deterioration of the stone matrix,
should it have some solubility in water.
But this is an area that still needs to be
studied formally.
Another topic that requires evaluation
is whether applying a polymer surface
overlay system, such as SafeLane® having
an epoxy bonding agent, a special aggre-
gate capable of storing a deicer that au-
tomatically releases before frost and ice
can adhere to it4. The question is how fast
is this overlay system worn down by the
traffic and how much epoxy is released
into the environment.
Conclusions
It is clear that to maintain the econo-
mic system that has been developed since
the industrial revolution, many changes
have been made in the environment and
its ecology. But these changes take time
to implement, so it is important that, for
those of us concerned with the conser-
vation of our architectural heritage, we
should point out the problems that thre-
aten it to raise the awareness of the gene-
ral public so eventually some actions can
be taken to protect them. As every buil-
ding and its situation are different, solu-
tions adapted to the individual case have
to be found. Often it is possible to use
gravel and no deicing salt at the proximi-
ty of the walls of historic buildings, or it
is possible to cover the basis of the walls,
e.g., with boards to keep salt containing
snow away from the walls, similar to the
covers used to protect statues during
winter. In some instances, a French-drain
could be installed by the wall where dei-
cing salts are applied, so that the melted
snow and salt will get trapped in it, or as
in Switzerland, many staircases are half
closed and only a small part kept free
of ice or snow. Ingenuity has been the
mark of humankind, and it is time for it
to come to the rescue should we want to
preserve our architectural heritage.
in Bern. The PM10 immission of this city
is strongly influenced by traffic and sho-
wed a value of about 1.5 μg/m3 while the
other sites range between 0.5 μg/m3 for
the urban sites to 0.2 μg/m3 for the rural
site of Chaumont
The chemical composition and the
quantity of the coarser fractions in air
pollution reflect the contribution of mi-
neral dusts put into suspension by ve-
hicular traffic. These include remaining
deicing salts and explain the relatively
high concentration of NaCl, as clearly sta-
ted by Gianini et al.21, p. 104
4. Discussion
There is no question as to the contribu-
tion of deicing salts to the deterioration
of our architectural heritage as well as
to the environment. But it is also clear
that deicing salts, as well as antifreezing
formulations for airports and aircraft are
necessary and have to be used to avoid
traffic accidents, disruptions in the eco-
nomy, and taking into account that the
negative impact of closing roads far
exceeds the cost of snow and ice remo-
val.22, 23 While airport and aircraft are al-
lowed the use of some formulations that
Figure 3: Lausanne (canton de Vaud, CH), rue de l a Barre, Château Saint-Maire (10.04. 2012). The sanding and disaggrega-
tion that affects the base of the building and up to 2 m in some areas is mainly due to the presence of NaCl.
Deicing Salts: An Overview
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Deicing Salts: An Overview A. E. Charola et al.