Technical Papers and Articles
»Environmental Effects of Lime
The environmental effects of calcium oxide and
calcium hydroxide are minimal when properly used. Calcium oxide
(quicklime) reacts promptly with water to form calcium hydroxide
(hydrated lime). Recarbonation by air or bicarbonate ion in surface
waters converts calcium hydroxide to calcium carbonate, the same
mineral in limestone. The high pH (12.45 @ 25°C) is naturally reduced
to the 7.8 to 8.5 range. Normal precautions of working with chemicals
must be taken to protect workers and the environment, but the reactions
of lime moderate its effects. The principal hazards are (1) high
temperatures of hydrating calcium oxide, (2) the irritating nature of
lime dust to the eyes and lungs, and (3) alkaline chemical burn to
mucous membranes and the eyes if splashed lime (dust or slurry) is not
promptly washed off.
oxide (CaO, quicklime) is an effective agent for soil stabilization and
aggregate modification as well as drying up muddy construction sites
and a wide variety of alkaline chemical neutralization applications.
Calcium hydroxide (Ca(OH) as hydrated lime) is effective as an additive to hot mix asphalt to
improve adhesion between the cement and the aggregate, to reduce
stripping, rutting, moisture damage and age hardening and to increase
the retained tensile strength. This review of the chemistry of
quicklime and hydrated lime explores the health and environmental
PRINCIPAL REACTIONS - THE LIME CYCLE
(CaO) is a product of high temperature calcination of pure limestone.
It is a white, low density, porous solid which reacts more or less
vigorously with water. Hydrated lime (Ca(OH)) is a fine white powder with limited solubility in water.
1) Calcination of limestone: (Typically ~2000°F for calcite)
CaCO = CaO + CO
Limestone or calcite (calcium carbonate) = Quicklime (calcium oxide) + Carbon Dioxide
2) Hydration of quicklime:
CaO + HO = Ca(OH) + Heat (490 BTU/lb. CaO)
quicklime + water = hydrated lime (calcium hydroxide) + 272.8 cal/gram CaO
3) Recarbonation of calcium hydroxide:
Ca(OH) + CO = CaCO + H2O
calcium hydroxide + carbon dioxide = calcium carbonate + water
most applications, excess calcium hydroxide more or less promptly
reacts with carbon dioxide from the air or bicarbonate ion in water to
form calcium carbonate.
Ca(OH) + HCO = CaCO + OH + HO
calcium hydroxide + bicarbonate ion = calcium carbonate + hydroxyl ion + water
4) Dissociation of calcium hydroxide
hydroxide has a very limited solubility in water (~1.5 g/L at 20°C)
which decreases at higher temperatures. This controls the alkalinity
and pH of water in contact with hydrated lime. The limited solubility
restricts the amount of material that can be dissociated in the water,
thus limiting the amount of hydroxyl ion available to raise pH.
Ca(OH) = Ca + 2 OH
Calcium hydroxide = Calcium ion + 2 Hydroxyl ions
LIME IN SOIL STABILIZATOIN
is commonly used to stabilize clays through the pozzolanic reactions.
Pozzolanic reactions convert the clays the cement-like compounds,
initially through ion exchange and subsequently, through fundamental
rearrangement of the alumino-silicate mineral structure. These
reactions start at pH 12 and above and consume water and lime in the
course of the reaction, resulting in a reduction both of moisture
content and pH. These reactions have been in use since Roman times in a
variety of construction applications. The quicklime can be applied dry
or as a slurry, but the pozzolanic reactions all involve hydrated lime.
SAFETY AND HEALTH CONSIDERATIONS
search of the scientific, health and safety literature on effects of
calcium oxide and calcium hydroxide has yielded the following
- Exposure to calcium oxide dust is regulated. The
OSHA PEL (Permissible Exposure Limit) for 8 hour TWA (Time Weighted
Average) is 2 mg/m. The ACGIH also recommends a TLV (Threshold Limit Value) of 2 mg/m as do fifteen nations.
Exposure to calcium hydroxide dust is regulated. The OSHA PEL
(Permissible Exposure Limit) for 8 hour TWA (Time Weighted Average) is
5 mg/m. The ACGIH also recommends a TLV (Threshold Limit Value) of 5 mg/m as do fifteen nations. For comparison, the TLV or TWA for nuisance
dusts or particulates not otherwise classified (PNOC) is 10 mg/m, only two times the hydrated lime standard.
Quicklime is not a hazardous substance under federal highway, rail,
water-borne transportation regulations. It is covered by DOT
regulations as an ORM-B compound for air transportation. This limits
the shipment to 25 lbs or less unless special provisions are taken.
- Hydrated lime is not a hazardous substance under federal highway, rail, water-borne or air transportation regulations.
- Neither quicklime nor hydrated lime is a hazardous waste according to the RCRA 40 CFR 261 criteria.
California regulations also regard materials under pH 12.5 as
non-hazardous. The Corrosivity Criteria, Section 66708 in the
California Administrative Code(CAC) , states:
“A waste, or a
material, is corrosive and hazardous if ...its mixture with an
equivalent weight of water produces a solution having a pH less than or
equal to 2 or greater than or equal to 12.5.”
quicklime and hydrated lime are hazardous substances as determined by
the CAL/OSHA Director’s List of Hazardous Substances.
Calcium hydroxide (as 100% dry powder) is classed as a “severe irritant
or corrosive” to rabbit eye tissue based on the Draize test.
Neither calcium oxide nor calcium hydroxide is corrosive according to
federally specified tests. Specifically, “No sign of dermal corrosion
was noted at any of the treated sites (rabbit skin). Based on these
results, the test material (concentrated calcium hydroxide slurry) is
considered non-corrosive to the skin when applied as received.”
Both calcium oxide and calcium hydroxide are on the Food and Drug
Administration’s (FDA) list of chemicals that are Generally Recognized
As Safe (GRAS) for human consumption in foodstuffs.
- Quicklime and hydrated lime are not carcinogenic, teratogenic or mutagenic according to federal government tests.
oxide reacts with water or water vapor to form hydrated lime. Hydrated
lime reacts with carbon dioxide or carbonate ions, forming sparingly
soluble calcium carbonate (calcite). Any excess hydrated lime in the
environment is naturally converted to harmless minerals. The normal pH
of lime slurry (pH = 12.4 @ 25°C) is reduced as recarbonation converts
the hydrated lime to calcite (pH ~ 7.8 - 8.3). The excess lime
concentration and the access to carbon dioxide determine how rapidly
the pH drops. Good exposure to air achieves near complete recarbonation
in a few days.
Air emissions of quicklime and hydrated lime can occur during dry transfers.
Transfer Operation: Unloading quicklime or hydrated lime from a bulk truck to a storage silo
Dust Control Measure: Baghouse on silo; Good housekeeping and unloading procedures by trucker.
Transfer Operation: Transferring quicklime or hydrated lime to the slurry mixer
Dust Control Measure: Covered screws or belts; enclosed mixer; water spray to knock down and entrain any dust.
Transfer Operation: Dust generated from aggregate handling of cured aggregate stockpile material
Dust Control Measure: Generally none except 1) Most of the lime has reacted or is tightly
adherent as a partially recarbonated film; 2) Residual moisture on the
aggregate; 3) Minimizing handling and drop heights.
SOIL CONTAMINATION EFFECTS
lime treated aggregate inevitably generates some mineral fines through
abrasion and crushing. This material will commonly contain calcium
carbonate and calcium hydroxide. The calcium carbonate behaves like
fine-grained limestone, a natural material that buffers groundwater pH
and has a low solubility, causing water hardness. Calcium hydroxide has
a higher solubility and pH. However, natural reactions in soils consume
this alkaline material.
Calcium hydroxide reacts with a wide
variety of finely divided siliceous materials (pozzolans) to form
cement compounds. These “pozzolanic reactions” are the basis for soil
stabilization construction practices that have been in use at least
since Roman times. Pozzolanic materials include volcanic ash, clays,
some glasses, fly ash, and many pulverized siliceous rocks.
a lime material with a soil can start the pozzolanic reactions. This
will convert the clays (and other pozzolanic materials) in the soil to
a cement type compound. These reactions consume water just as hydrating
and curing a portland cement concrete does. The typical product is a
poorly crystalline zeolite-type mineral found in cured concrete,
As the calcium hydroxide is consumed, the pH drops as the hydrous
calcium aluminosilicate minerals form. The stable pH is comparable to
concrete until the outer portion has developed an impermeable
In the absence of pozzolanic
materials, the calcium hydroxide undergoes recarbonation reactions as
discussed above. This lowers the pH from the high 12.4 range of
hydrated lime to the 7.8 to 8.4 range of limestone. This reaction is
generally limited by the availability of carbon dioxide. This mitigates
the long term effects of applying excess calcium hydroxide to a soil or
water exposed to air.
WATER POLLUTION CONCERNS
every effort should be made to prevent contact with water and control
runoff from the limed aggregate stockpiles. However, the reactions
involving lime, water, air and soil materials ameliorate the effects of
hydrated lime contamination.
Water that is saturated with
calcium hydroxide will have a pH of 12.4. However, the low solubility
means that rain water falling on a treated aggregate pile are unlikely
to dissolve enough lime to significantly raise the pH of commingled
run-on/runoff water from adjacent sites. Each ten-fold dilution by
water brings a decrease of one pH unit for the mixed waters.
hydroxide in runoff water will initiate pozzolanic reactions with clays
or muds in suspension or on the stream bed. This consumes the calcium
hydroxide and lowers the pH. Carbonate and bicarbonate ions in
rainwater and surface waters will also consume calcium hydroxide
through standard water softening reactions:
Ca(HCO) + Ca(OH) = 2 CaCO + HO
Calcium bicarbonate + Calcium hydroxide = Calcium carbonate + Water
Refer to a current MSDS for legally mandated information.
Principal hazards associated with lime slurry are:
- Personnel Safety: Dust exposure to the eyes or respiratory tract.
- Personnel Safety: Splashes of lime slurry to the eyes.
- Site Hazards: Slick surfaces from wet lime or lime slurry spills.
- Site Hazards: Lime dust from vehicles driving through dry spills.
- Transportation Hazards: Spills resulting from traffic or loading accidents.
- Environment Protection: Discharge to streams or bodies of water.
The principal hazards associated with quicklime, hydrated lime and lime slurry are:
- Thermal burns from contact with hydrating quicklime.
- Thermal burns from spills or splashes of hot lime slurry when made from quicklime.
- Chemical burns through extended contact with lime dust or concentrated lime slurry.
- Dust irritation from quicklime or hydrated lime dust exposure to eyes, mucous membranes or sweaty skin.
- Dust irritation from disturbed dried deposits of lime slurry.
Thermal burns can be minimized by proper handling. Avoid contacting hydrating
quicklime or equipment in which the hydration reaction occurs. Avoid
getting quicklime on your clothes or body. Lime slurry produced from
quicklime is in the 210° - 190°F (100° - 88°C) temperature range. The
temperature remains high in large tank storage, but drops with time and
as it is handled in pipes, pumps and smaller vessels. Lime slurry
produced from hydrated lime remains at the water temperature.
First Aid: If a person is exposed to lime dust, remove them from that situation,
remove the dusty clothes and wash off the lime with copious amounts of
water. Follow the provisions on the MSDS.
If a person is exposed
to large amounts of hot lime slurry, remove the wet clothing and wipe
or wash off the slurry. Be especially careful to remove and wash slurry
from the eyes first. Treat any thermal burns using normal burn
treatment. Wash lime from any burn area. Obtain medical care.
Chemical burns are rarely associated with lime slurry. Quicklime (calcium oxide) is
more commonly involved in chemical lime burns than calcium hydroxide or
lime slurry. Lime slurry has a pH of 12.4 and is not a hazardous waste
by the RCRA Corrosivity Criteria. Lime slurry is not corrosive to
intact skin, but should be washed off as a regular prevention.
Lime slurry splashes on the skin should be washed off. A skin lotion
may be applied to counteract the drying tendency of calcium hydroxide.
Wash lime out of any cut or laceration.
Lime dust is an
irritant to the eyes and mucous membranes. Control of lime dust during
handling of dry lime products should be an integral part of any plant
operations involving lime.
Lime dust is not generated in normal slurry handling operations. However, if a lime slurry spill is permitted to dry, calcium hydroxide
cake forms. Traffic and handling over dried slurry solids may generate
calcium hydroxide dust. This can be an irritant if breathed or if it
contacts the eyes. The OSHA PEL is 5 mg/cu.m. Spills should be cleaned
First Aid: Wash off all lime material. Flush the eyes with clean water or eye wash solution. Obtain prompt medical attention.
Standard safety equipment for each person working in the slurry area should include:
- Safety glasses with side shields or safety goggles,
- Dry lined gloves for those handling slurry pipes or hoses.
- Rubber boots with safety tread for work areas where slurry might be spilled.
Dust masks for those working in quicklime or dry hydrate handling areas
and on clean up duty. Dust masks of OSHA category, N95, are
appropriate, unless the dust concentration is very high.
Install eye wash stations convenient to the lime slurry handling and
operations areas. A safety shower is recommended, but not required.
SPILL PREVENTION AND CONTROL
best control is spill prevention. Training and qualifying trucking
firms and drivers minimizes accident and spill opportunities. Lime
products, including lime slurry, are not hazardous materials under US
DOT regulations for surface transport and placarding transport
containers is not required.
Should an accident or spill occur, follow these guidelines:
- Remove persons from the accident scene to a safe place.
- Render first aid as appropriate.
- Remove any dry lime material in contact with a person by brushing or wiping the material away.
After removal of most of the lime, wash the body parts exposed to the
lime. Especially flood the eyes with water to dilute and remove the
- Notify emergency personnel.
Reroute or stop traffic from driving through spilled lime slurry. Lime
slurry is slick and will reduce traction, possibly causing accidents.
Contain the spill. Prevent people from contacting the lime. For
quicklime, do not dispose large quantities to landfills without first
fully slaking the lime (hydrating it to calcium hydroxide). Enough heat
can be generated in special conditions to ignite flammable materials.
Follow the MSDS provisions.
- Contain the spill. Block storm
drains and access to streams using materials at hand, such as dirt, hay
bales, plastic, etc. High solids slurries are viscous, assisting leak
- Clean up the spilled material with a front-end
loader, vacuum truck or shovels, being careful to avoid excessive dust
- The small amount of solid material remaining at the
spill site after clean up may, with permission, be flushed to sewer
with sufficient water to reduce the pH below 9.5. or
- The solids may be swept up and disposed of at a landfill. or
- It may be permissible to dry and recarbonate to limestone dust, CaCO3.
FOR QUICKLIME SPILLS
Do not add water to large quantities of quicklime. Clean up the bulk of
the material in a dry form. Do not dispose of quicklime with flammable
- Appropriate disposal of spilled quicklime depends
on the local situation. It may be used in applications like soil
stabilization, drying up construction sites, or waste acid
neutralization. It may be necessary to return it to the production
facility or terminal.