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No. 434, May 1994

An "Insulating" Shotcrete for Heat
Abatement in Deep Mines

Objective

Develop and demonstrate a lightweight material that
has low thermal conductivity and that would be suitable
for placement using shotcrete methods, This material
could be applied in deep, hot mines where mine drifts
must be insulated. Its use would decrease mine refrigera-
tion needs and reduce energy costs.

Background

As mining goes deeper, rock becomes hotter, The min-
ing process itself exposes large surface areas that release
geothermic heat from the rock into a mine’s ventilation
system. Operating mining equipment also releases quanti-
ties of heat. Ores at many mines contain minerals, pri-
marily sulfides, that exhibit exothermic behavior when
exposed (0 air. Past mining practice has been to use un-
derground refrigeration units to cool mine air. However,
energy costs limit mechanical refrigeration, A solution
that could lower energy costs is to insulate mine drifts.
U.S. Bureau of Mines (USBM) researchers are developing
an ‘insulating’ material for this need.

Approach

In cooperation with a mining company, a manufacturer
speciatizing in shotcrete equipment, and a cement ma-
terials company, the USBM has conducted laboratory and
field research to develop such a specialized material.
Cement-based products are well known for their fire-
resistant qualities. Perlite has long been used for its

insulation capabilities. Shotcrete technology has now be-
come well developed throughout the world. This project
blends these well-known technologies.

How It Works

The cement materials company developed a cement-
based mix design that contained enough silica fume to let
the mix stick to the mine walls. Perlite was added to the
mix as the insulating component, along with ait-entraining
agents and other additives. Low-pressure, wet-mix shot-
crete equipment was used because it is inherently safer
than other types of shotcrete equipment and lessens re-
bound (only a few percent versus up to 30 percent). Al-
though the shotcrete mix was applied manually in the
demonstration, it is suggested that robotic shotcrete equip-
meat be used to allow the operator to be removed from
potentially hazardous situations. The material had been
packaged in small-volume bags before the demonstration.
Bulk packaging could be used to reduce costs.

Test Results

The field demonstration at the cooperating mine
showed that the mix designs could be successfully applied
with wet-mix shotcrete equipment. The unit weight of
oven-dried shotcrete material averaged about
1,120 kilograms per cubic meter (kg/m?) (70 lb/ft), with
a therma! conductivity of 0.36 watt per meter kelvin
(W/m K) (2.5 Btu in/h ft °F), Oven-dried, unshotereted
material averaged about 640 kg/m’ (40 lb/ft), with a
thermal conductivity of about 0.24 W/m K (1.66 Btu in/h
ft? °F), All values were reached at zero moisture. The

‘This document was prepared by the U.S, Bureau of Mines. Neither the United States Government nor any person acting on dehalf of the United
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shotcreted material exceeded a compressive strength of
21 megapascals (3,000 psi) at 90 days. These strengths are
considered to be adequate for ordinary mine support, al-
though the insulating shotcrele has not been designed as
a ground support material.

A preliminary computer analysis of insulating shoterete
placed in a typical mine drift indicated that use of in-
sulating shotcrete might result in up to a 25-percent de-
crease in heat transfer between an insulated mine drift and
an uninsulated drift. Each mine will have site-specific con-
ditions, however, and its own set of circumstances.

Guidelines for Use

Suggested application in mines is with the wet-mix shot-
crete process, Neither high pumping pressures (which
cause rebound) nor repeated passes with the shotcrete
nozzle (which cause increased compaction of the shot-
crete) are recommended. Rebound can also be decreased
by the generous use of silica fume. Regular concrete-
forming methods may also be used, in which the material
is pumped into place. Since thermal conductivity is di-
rectly related to unit weight, mix designs in the range of
640 to 960 kg/m? (40 to 60 lb/ft?) are preferred to attain
the desired thermal values.

Patent Status

The USBM docs not intend to pursue patents. Shot-
crete equipment, including robotic shotcrete equipment, is
presently available from commercial vendors. The insu-
lating mix design was formulated by a commercial vendor
of conerete supplies,

For More Information

This research has been conducted by the USBM and its
cooperators. Additional information may be obtained by

contacting the principal investigator: Eugene H. Skinner,
U.S: Bureau of Mines, Spokane Research Center, East 315
Montgomery, Spokane, WA 99207-2291, (509-484-1610),
FAX: 509-353-2652.

Flold application of an ‘insulating’ shotcrete.

CAUTION: Under certain conditions, cement can be haz-
ardous. See NIOSH Pocket Gulde to Chemical Hazards, p. 198.
Protective clothing Is recommended when shotereting,

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