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EFFECT OF ADMIXTURES ON
CEMENT MORTARS AS SHOWN
BY PHOTO-MICROGRAPHY

Thai. for the Degroo of B. S.
WCHIGAN STATE COLLEGE

Jack C. Mackie
I942

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Effect of Admixturoa on Cement‘Mortars

as Shown by Photo-Micrography

A Thesis Submitted to

The Faculty of
MICHIGAN STATE COLLEGE
of
AGRICULTURE AND APPLIED SCIENCE

by

JACK C. MKIE
Candidate for the Degree of

Bachelor of Science

June 1942

THESIS

Acknowledgment

To Professor Mallman in the Bacter-
iology Department and Doctor Swanson of
the Botany Department for tneir kind
cooperation and the use of their photo-
miorographio equipment and to Professor

Allen for his advice and consultation.

143087

Introduction

The problem of the weakness of cement in tension
has been known to exist for some time. It is hoped
that this thesis will be a step in the direction of
strength analysis of concrete by photo-micrography,
as it is now being widely used in other fields of
stress analysis. The scope of this thesis is necess-
arily limited due to lack of equipment and time, but
since it is herein shown the reason for the failure of
cement under stress, logical steps may be taken to
prevent such failure.

An outline of the general procedure follows:
I A suspension of cement in water is photo-micrographed.
II To the above mixture of cement in water is added the
the following admixtures and each one photo-micrographed
separately,- Calcium chloride, red mineral oxide (pigment),
pozzolith, and sugar.
III Four briquets are molded of each type (admixture) and
two of each type broken at the end of seven days curing in
a moist closet, and a photo-micrograph made of each break.
IV The remaining two briquete of each type are subjected

to a freezing and thawing cycle and then broken in tension.

. A mortar was chosen merely as a convenience, assuming

that the same general theories evolved in this thesis will
be applicable to concrete as well. A fine lake sand passing
the number 30 sieve but retained on the number 50 sieve was
used to insure uniformity in photographing. The mix used
was 3 parts sand to one part cement with water added to
produce a standard workable mix. Approximately 15% by
volume of admixture was added in each case to insure posi-
tive results, the exact amount not being determined, as it
is not within the scope of this thesis to determine or
corroborate already established facts.

The calcium chloride was added by dissolving it in
the mixing water, but the other admixtures were added in
the dry state. In the case of the pozzolith, care was
taken in regards to the amount of mixing water as a very
plastic mix of standard quality was obtained with about
half the quantity of mixing water used in the other samples.
The sample containing sugar also produced an amazingly

workable mix with small amounts of water.

PLAIN CEMENT

 

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This is a 450 diameter magnification of cement
thoroughly mixed with water. The actual cement particle
size is shown by the smallest particles floating in free
suspension. The large masses or clusters are made up of
hundreds of smaller individual particles, clinging together
in such fashion by cohesive forces so that the water
cannot penetrate or break up the cluster. This sample
was thoroughly mixed and shaken, so on the average Job
with rapid or poor mixing, this condition is probably much
worse. Of course each large cluster will act as a smaller
particle and hydrate to a certain extent upon setting, but
it is easily seen that the strength of the resulting concrete

will be somewhat inferior to a properly hydrated cement.

 

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The above photograph is of the same subject, but of
lower power magnification,-lOO diameters. It covers a
much broader field, showing that the tendency for cement
particles to cluster is a general rule rather than the
exception, each spot on the photograph being a separate
cluster at low power magnification, except those in free
suspension which are protected by a surrounding film of
water. When these clusters are broken up or dispersed,

the resulting concrete will be much superior in strength.

 

 

 

 

 

 

The above is a photo-micrograph taken at the break
of a plain cement mortar briquet. Due to the extremely
small depth of focus on the high powered lens, the focus
is not too sharp, so it is slightly clearer at arms length.
The general structure has a tendency to be rather coarse
and honeycombed as shown by the black portions which are
deep shadows. There were no cracks caused by breaking,
but the effect is that it is splotchy and non-homogeneous,

indicating a loose structure.

CALC IUM C HIDRIDE

 

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The above is a 450 diameter photo-micrograph of a
solution containing water, cement, and calcium chloride.
The general tendency is the same as that of cement,-to
form clusters, but on careful scrutinization it will be
seen that each particle is protected from becoming united
with any other by a layer of calcium chloride. Although
they still tend to group together by cohesive forces, each
particle is permitted to hydrate thoroughly since it cannot
combine with other particles in one large lump, therefore
its ability to produce high early strength. The clusters
in general seem to be smaller than those in the plain cement,

exhibiting a mild tendency toward diffusion.

Perhaps the main reason for all this high early strength
is the fact that since each particle is surrounded with a
layer of calcium chloride, upon drying of the cement, the
calcium chloride will effloresce, supplying more water for
the cement particle to hydrate, thusly producing the high
early strength. '

 

 

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The above is also calcium chloride, but magnified only
100 diameters to cover a broader field of which the preceding
photo-micrograph shows the detail.

 

 

 

 

 

 

This is a photo-micrograph of a mortar briquet con—
taining calcium chloride, broken at the end of seven days
curing in a moist closet. There are several cleavage
cracks, but the mix appears to be quite smooth comparatively,-
more homogeneous, anyway, than the plain cement mortar. The
black portion is shadow, indicating a depression at that
point. When the photograph is splotchy, it indicates that
large clumps, or clusters of the material have pulled out
under tension instead of failing on a single plane as should

a true homogeneous mix.

PIGMEN '1‘

 

 

 

 

 

The above is a photo-micrograph of pigment (mineral
oxide) mixed with cement and water. A panchromatic film
was used in photographing, so the colors are all equally
represented although the pigment particles cannot be
distinguished from the cement particles. This photo is
characterized not by the dispersion effect, but by the
density of the clusters. These smaller clusters are more
compact than those of plain cement and do not string out
in chains nearly so much showing the condensing effect of
the pigment producing a higher strength concrete as shown
by the graph drawn upon testing the briquets in tension.

 

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This 100 diameter photo-micrograph of the pigment
is again reduced in power to cover a much broader field.
Again the small clusters are shown to be concise and
compact and not nearly as chain-like or stringy as the
plain cement particles. The smallest specks on the photo
are free pigment particles which when upon subsequent
mixing with aggregate will condense to fill in the voids,
producing a more dense mix resulting in a higher strength

concrete.

 

 

 

 

 

This is a photo-micrograph of 100 diameters, of cement
mortar containing mineral oxide pigment. This structure
produced higher strength than did the structure of the plain
portland cement mortar. There is less splotchiness, showing
greater homogeniety, especially in the upper center of the
photograph. The density appears to be greater also as
evidenced by the closely packed particles indicating a ten-
dency to get away from clumping of the mix as shown in the

photo-micrograph of the plain mortar briquet.

POZZOLITH

 

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This is a high power (450 diameter) photo-micrograph
of a suspension of cement in water with pozzolith added.
This photograph is remarkable in comparison with the plain
cement. The larger, dark particles are cement whereas the
light grey fine ones are the minute pozzolith particles. Due
to the near-transparency of the pozzolith, it does not show
up too well in the photograph, but the principle is there.

The pozzolith particles are practically spheroids in
shape contrasted with the highly irregular shape of the
cement particles. Pozzolith in the dry state practically
seeks its own level much as a pile of fine buckshot would,
being almost frictionless, but much lighter in weight than
the buckshot. The pozzolith, being so free-flowing, upon

mixing, penetrates the cement clusters and breaks them up

allowing for more thorough hydration of the particles and
at the same time, the fine pozzolith tends to fill the voids
producing a more dense mix.

While being viewed under a microscope, the slide was
moved slightly and the very small pozzolith particles were
observed to flow very freely, due to the lack of friction as
before stated. It was this motion which caused the agitation
and wearing on the clusters until they were thoroughly broken

up, producing complete dispersion.

 

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This 100 diameter photograph of the pozzolith speaks for
itself. Over the broader field, there are ng_clusters;-the
pozzolith mixes thoroughly with the cement particles and

complete dispersion is observed.

 

 

 

 

 

This photo-micrograph of 100 diameters is the cement
mortar containing 15% pozzolith. As shown, there is practically
no grouping of the mix in large clusters, but the opposite,
namely, the extremely even distribution of the particles and
on careful examination, the great density of the mix. It is
apparent that this admixture, administered in the proper

quantities, is the most desirable of the group of admixtures

under observation.

SUGAR

 

 

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This is a 450 diameter photo-micrograph of a suspension
of cement in water with 15% sugar added. Sugar does nothing
toward dispersion but instead, a certain amount of sugar is
absorbed on the outer surface of the chain-like clusters
which not onlyprevents correct hydration, but when the
concrete sets, the sugar dries out also and becomes

crystalline, and with a crystalline structure at the inter-

 

granular boundary of concrete particles it can readily be
seen the reason for the decrease in the strength of a concrete

by the use of sugar as an admixture, accidentally or other-

wise.

 

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This 100 diameter photo-micrograph of sugar in the
mix does not show a great deal except cover a broader
field and showing that while sugar will produce a very
plastic mix, it does not do it by good dispersion, as the

tendency is still toward clusters of the cement particles.

 

 

 

 

 

This 100 diameter magnification of the hardened cement
mortar briquet contains sugar as an admixture. This is the
poorest admixture that could be possible added, even though
it produces a very plastic mix. The deep shadows indicate
the place where a large clump has pulled out under the ten—
sion instead of failing on the face of a single plane. The
poor dispersion and the large number of cracks (not too clear
on this picture) are further reason for care being taken to

exclude sugar being used as,and for,an admixture.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Conclusions

The ambiguity of the bar graph is due to the fact that
although the freezing and thawing cycle was going on, the
briquets were also aging and gaining strength at the same
time. The poor showing of the pozzolith was due to the
fact that such a large percentage of this admixture was
used to show up conclusively in the photo-micrographs.
Ordinarily only about 2% or 5% is added, but in this case
it was about 15%.

After the freezing and thawing cycle it was the sugar
sample that definitely failed. This was due to the breaking
up of the sugar crystals under the expanding and contracting,-
leaving almost no binder between the sand particles. The
other briquets when examined after being subjected to the
freezing and thawing cycle showed no appreciable difference
under the microscope so they went un-photographed.

In general, it has been shown that this method of analysis
of concrete has proved satisfactory. It is common knowledge
that calcium chloride will give concrete high early strength
or that sugar in the mix will decrease the strength of a con-
crete, etc., etc., but this thesis has shown why admixtures

effect concrete in this manner.

 

 

 

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