THE EFFECT OF CARBONATION STRENGTH,
THE SHRINKAGE, AND PERMEABILITY
OF PORTLAND CEKENT CONCRETE
by
. ADOLFO LAU-CHANG
Submitted in Partial Fulfillment of
the Requirements for the Degree of BACHELOR OF SCIENCE
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOTf
May 1962
re of the Author. t I" - -*' D e p a r t,;""/nn o f . . . . * . * * Is Civil Engineering 18 ay 1962 e d by. ... P * * * . - 11 ___-___:V--_._j--;- a 0 0X. --. -"-I-- * 0 0 " -t Thesi Swo or ... I -11- -_ * 0 # *
Departmental Committee on Theses
d byo***'*O'.*#***
.
ABSTRACT
THE EFFECT OF CARBONATION ON THE SHRI!qKAGE,
STRENGTH3 AND ERMEABILITY OF PORTLAND CEkENT CONCRETE,
by
Adolfo Lau-Chang
Submitted to the De artment, of Civil Engineering on 18 May 1962 in partial fulfillment of the requirements for the Degree of Bachelor of Science.
The ex-oerimental work of this investigation as ade
using Portland 6e-i2eht mortar specimens' By using mortar
it i -o6ssib'le't(omdke
sall specimens'. If concrete ere
used' s-oecimens ofmi-ch larger dimensions would be
neces-sary. tut it is evident that the results obta"ed with'
mortar are ualitatively alicable to concrete.
This study is concerned with hree items: the
inves-tigation of the tme r'equired for secimens of Portland
cement mortar to reach moisture euilibrium when stored
in mediums of different relative I aidities; the
investi-gation of the effect that the following factors have on the weight stability and dimensional stability of Portland
cement mortar: time of exposure to carbon dioxide,
concen-tration of carbon dioxides and the curing time; and finally., the nvestigation of the effect of carbonation on the
strength and 'ermeabilit7 Of ortland cement mortar.
It was found that the amount of weight loss and shrink-ige due to drying increases s the relative hidity of the surrounding medium decreases. The relationship between the weight loss and shrinkage due to drying is not necessarily
direct. Portland cement mortar specimens stored at diffei-"dities reach dimensional equilibrium after
ent relative hum1 U_
100 days. in most of the cases sudied he eight continues to decrease after 100 days.
The data coll ted indicated e' 'hat -ore-carbonation in
mediums of 50% co concentrations for tmes of carbonation 2
no longer than 72 hours, in an atmosphere of 52% relative
humiditys has no significant effect on the dimensional sta-bility of Portland-cement mortar. Pre-carbonation in
mediu--ras of 100% Co concentration reduces shrinkage signi-ficantly. In the Ltter cases shrinkage decreases with
i
.
Curing tine does not modify the reduction of shrinIc-age caused by pre-carbonation. Curing tine afects the amount weight los's reduction-caused by pre-carbonation. For a constant carbonation time, the amount of eight loss
reduction increases wth increasing uring tine.
Weight loss decreases with ncreasing tine of
carbon-ation.
Experiments-'jo'roved that the compressive strength of Po'r"tland 'lerdeh-t-, 'mort6-r's decreases with decreasing caring tire T compressive strength of Portland cement mortars
increases with increasing title of carbonation. Hir7her
strengths are obtained for secimens carbonated in ediums
of higher CO 2 concatration.
Finally, it as fund that the permeability of Portland
cement mortars decreases ith increasing'time of
carbona-tion, until carbonation reaches 23 hours. Permeability
starts to irease with increasing time o carbonation after
2.3 hours*
The collected data is ortrayed br menas of graphs.
Photographs and drawings of pertinent aparatus and machines
are included to illustrate eerimental techniques. In-formation concerning the general testing method is
Presented
Thesis Su-pervisor: Lloyd Cutcliffe
Instru'ctor
Ttle:
ACKNO-VVLEDGE11fl-ENT
The author wishes to express his ratitude to the Supervisor of this Thesis J. Lloyd Cutcliffe, for introducing him and encouraging
his interest in the subject of carbonation of concret e
The author is very graUteful to Mr. Albert J. O'Neill, Technical Instructor and to r. Joseph E. White of the Building Dfiateria18 Laboratory
for their technical assistance in the experimental
work of --his investigation.
Finally,, to iss Mautilda Za-,,qanian for
TABLEOF CONTENTS N 0 E k fI i II i I
iv.
Chapter AB STRACT ... ACl\'lTOWLEDGEMENT ... I. INTRODUCTION ... II. PREVIOUS AND PRESENT WORK ONCARBONATION ... The echanism of Carbonation
Previous Work on Carbonation Present Work on Carbonation
.1) Stabilization ...
2) Vleigjat and Dimensional
i
iii
1 4 4 6 10 11 11 13 15 32 32 33 35 36 38 40Stabllity ...
3) Iler,-,,-eab- lity!II. RESULTS
...
IV. DISCUSSION OF RESULTS . . . .
Graphs 1 and 2 ... ura-oh 3 to 1 ...
Gra-phs 15 and 16 ...
Gra-oh 7
V. CONCLUSI0,',-iS
VI. R:E,,COl.,U,,lENDATIONS FOR DRTHEEIR ORK
APPEYDIX A'. APPARATUS AND EXIDERIDYTENTAL
TECHNIQb--,'--S ... 0
APPENDIX B: ClfiPUI-'=,, PROGRA1,11S * . . . . ,
BIBLIOGRAFTZ
41 2 50
V*
LIST OF GRAPHS
Gra-,oh
1 Percent Change in Weight vs.
COo Concentration %
Curing Time - 7 Days
Time
15
O * * *
2 Percent Change in Lngth vs. Time
n I'll n __ - _ I I .
ju oncentration - uxo
Cui`g Time - 7 Daysl -* . * * * 16
3 Percent Change in eight vs. Time
co2 Concentration - %
Curing Time - Dd7-- ''
R.H. - 52%, 710F .,
4 Percent Change in Length vs. Time
CO Concentration - %
Cuing Time - Day' .. ,
R.H. - 52%, 7OF O.. 0. *Is . 18
Time
5 . Percent Chan,re in Weight vs,
GO Concentratio 2 - 0%
Curing Time - 3 Days
R.H. - 52% 7F .... ..
. . . * * i S-1120
21
22
23
6 Percent Change in Length vs. Time
C02 Concentration - 50%
Curing Time - 3 Da'ys'
R.H. - 52/O", 7F
7 Percent Change in Weight vs. "Llime
CO, Concentration - %
Curing Time - D s-aY
R.H, - 5,,%., 7F
Percent Change in L.ength vs. Time
COo Concentration - 0%
Curing Tirie - 7 Days
R.H4 52%., 7F
Percent Chan'e in Weight vs. TimeI.
co 2 oncen'ration ;l - 00%
Curing Time - Day
k* -4 V.
f
-I... I .- :1 .i., II I I. vi. Gra-oh -' e-P10 Percent Change in LenF'h vs' Time.
co 2 Cncentration - 10%
Curin--, Time - Day - I
--- I I/
R.H. - 52/05 '70:Hl # . . . 24
11 Percent Change in Weight vs. Tine
C02 Concentration - 00%
Curing Tme - 3 Days .
R.H. - 52%, 7OF ... 0.90 25 26 12 Percent Cha,, C02 Co] 'Curing R.H
-age i Length vs. Time
acentration - 00%
Time - 3- D a'y s
5 2`/0.9 7F
- Change in Weigh' vs, Time
13 Percent U 0
co Concentratio - O 01,I,
2
Curing Time - 7 Da7,s
R.H - 52% 7F ...
14 Percent Change i Length vs. Tirne
CO Concentration - 100% -2. Time - 7 Ddys .... Curing ROH'a 5:2-/-,o 7F :,I , t :, z 28
15 Compressive S45rength vs. Time of
Carbonation
co2 Concentratio - 0C4
Age - 270 Days O*..* 29
30
O . * O
16 Compressive rength vs. Time f
Carbonation
CO Concentration 00,% 2
. Age - 278 Days *00 a- -0 .. 0 .. O I* * *001*
17 Permeabili-7 vs. Time of Carbonation,
and Com-pressive Strength vs. Tme of
Carbonaion .
LIST OF PHOTOGRAPHS
vii.
!'ho t o g, r a-ph Page
42
4 3
44
46
Snecimen
and Mold
... 4 ... 00Measurin' Sample in the Upriglat' -9
Gauge ...
1 2 Stabilization of Specimens 3 4 a 1P 0 0 * * U ),-,.ent Carbonation EquiSpecimens Cut -,'or Strength'
5
Measurements
60,000 Found
,draull 'Testin' '
-PI a c h -IL n e
Permeability Measuring Apparatus
-46
47 48
6
1
I. INTRODUCTION
Concrete has certain properties that have been
respons-ible for its widespread use as a building material.
Struct-ural members made of reinforced concrete can develop very
high strength. In addition it is a material of relatively
low cost, it can be molded into complex shapes, it is
dur-able and fire proof.
But concrete also has some shortcomings. One of the
most iportant ones is its relatively low dimensional
sta-bilit7- Concrete tends to cree-P and shrink. Creep is defined as the increased deformation with time due to sus-tained constant loads. Shrinkage is due to dimensional changes caused by water loss. The effect of shrinkage in
a structural member made of concrete can be serious. After
a structural member
is set in place, srinkage will
intro-duce tensile stresses. Concrete is weak in tnsion.
The
presence of tensile stresses in a concrete member usually causes cracks. These cracks can deteriorate the structural value of these-members severely.
There are several theories that describe the exact mechanism o f srinkage. Among the most aeptable ones are:
the capillary tension theor7* the'surface sorption theor72
the swelling pressure of the gel theoryy and the interla7er
3.
cost. In additions improved strength, and hardnesss and reduced ermeability may result from carbonation of
con-crete. The contribution that carbonation can give to con
crete technology seems excellent.
As it is explained in the following section of this
relports carbonation of concrete depends on vaA ous factors such as relative humidity of the surrounding medium,
con-centration of carbon dioxides length of exposure to carbon dioxide, curing conditions etc. The purpose of this
in-vestigation is to gather data concerning the agnitude of
the effect of each one of these factors in the carbonation, of concrete and the resulting improvement of the dimensional
stability of the material It is also the purpose of this
investigation to correlate the empirical information obtained
with the theory of the process of carbonation developed so
far, This'study is also concerned with the effect that
carbonation has on the strength and the permeability of the concrete.
4. II. PREVIOUS AND PRESENT WORK ON ARBONATION
The Mechanism of Carbonation
There has been substantial amount of research done
in the field f carbonation of concrete. Existing
evi-dence indicates that sstantially
all the constituents
of cement are sbject to ultimate carbonation under deal
conditions ().
At the present time2 tere is significant technological and theoretical interest in the effect of carbonation on
Portland cement based materials. The reaction between
car-bon dioxide and Portland cement might be the partial explana
tion for te slow shrinkage of these materials in service
over a long period of time. There is also enough evidence
Uo indicate that the low dimensional stability of concrete
may be improved by intentional pre-carbonat-2 on. In addition,,
pre-carbonation can increase the strength-and lower the permeability of concrete.
The slow shrinkage of concrete is due mainly to two causes: water loss due to drying and &ange in length due
J-to carbonation. To obtain coiTplete understanding of
car-bonation it is necessary to investigate both the process
5 The four principal compounds of Portland cement are as follows:
Tricalcium Silicate 3CaO*SiO 2 (C S)3
Dicalcium Silicate 2CaO*SiO 2 (C )2
Tricalcium Aluxainate 3CaO*Al 0 2 3 (C3A)
Tetracalcium Aluminoferrite 4CaO*A1 0 Fe (C AF) 2 3 2 3 4
All four compounds set by reacting with water and forming
hydrates, The rate of hydration of all four is quite
different. When water comes in contact with the cement
particles, it is expected that C A crystals on he surface3
of the particles react immediately, while the other surface
crystals will react at a slower rate. With time the hydra-tion will proceed towards the center of the particle some-times taking years for the particle to become completely hyd-ated. This is the reason hy fresh concrete should be cured for several days after it has been placed.
The product o he reaction of tricalcium silicate a3fd
water is a saturated solution with lime due to hydrolysis. 'In a short period of ime calcium hydroxide crystals begin
to appear in the hydrating mass, mixed with the usual
structure of less hydrated silicates, The reaction between
the calcium hydroxide and carbon dioxide is usually referred to as carbonation, This reaction occurs in the presence of water$ and the' products are calcium carbonate and more water:
Ca(OH) 2 00 2 CaC 03 + H 20 (2.1)
In the presence of excess carbon dioxide and wa- CaCO
3
reacts to yield calcium bicarbonate:
CaCO- C- + H o T,-T r 0
2 --2 -3'2
Usually the supply of C02 for these reactions to occur comes from the surrounding ar. The concentration of CO
2
in air is approximately .03%.
There are several factors which influence the extent
and the rate o carbonation. The mst important factors are: the relative hidity during exposure to carbonic gas, the concentration of carbonic gas in the surrounding medium, the length of the exposure the specimen size, and the
ur-ing conditions.
Previous Work on Carbonation
Mortars have been used in -most of the experimental
work done in carbonation. By using mortars it is ossible
to make small specimens to study the process of carbona-tion. If concrete were used much larger specimens would
be required, but it is obvious that the results obtained
with mortars are qualitatively applicable to oncretes. As it was mentioned before, the relative hmidity during exposure to carbonic gas is a very critical factor in carbonation. The relationship between carbonation and the relative hmidity is dependent on the specimen sire.
M
... I . .1-7 1
If the rate of carbonation is very rapidy secimen size
becomes very critical. Water is released during arbona-tion, If the specimen is not small, moisture ill not be able to diffuse out rapidly and the internal relative
humidity will differ from the relative humidity of the
surrounding medium,
Verbeck 4 studied the effect of relative humidity on carbonation using mortar secimens, I by by 111 in.
All s-Pecimens were cured for seven days. They were divIded.
in four groups, and each group as stored in atmospheres
of 100 7 50 and 25 per cent relative humidity. The
specimens attained moisture equilibrium after 52 days. After,
the specimens were exposed to carbonic gas at atmosphere
until the new equilibrium was reached. It took 60 additional days. The shrinkage due to drying increased with
decreas--ing relative humidity. The amount of shrinkage produced, by
carbonation was largest for the secimens kept at 50%
rela-'Cive humidity. Very little shrinkage was caused by CO in2
the specimens stored at 100 and 25 per cent relative humidity. At 100 per cent relative humidity the pores of' the
paste are filled with water$ soy t is difficult for the
CO to diffuse iard. In addition,, Calcium ions counter 2
diffuse toward the surface precipitating calcium carbonate and clogging the pores. As the relative humidity decreases
more pores become emp-tied and the diffusion of the carbonic
9as into the aste is increased. ""his also reduces the
pore clogging at the surface caused by the calcium
car-bonate precipitate.
Carbonation occurs. to a very sall extent a 25 per cent relative humidity due to low moisture in he paste. The presence of water is necessary for the reaction of
carbonation to occur, Consequently the amount shrink-age caused by carbonation is very small.
The effect of specimen size on the rate and extent of carbonation was mentioned before, The problem with large
specimens is that it takes a very long time to attain
moisture equilibrium between the inside a the outer media. The moisture gradient in he cross-section of a large
specimen will cause certain sections of the sioecimen to
carbonate at a faster rate than others.
Water is released in the carbonation reactions. If
he specimen has attained moisture equilibrium, the ater
released by the reaction will disturb the equilibrium In
the case of a small specimen, it is easy for the water to
diffuse out and the specimen will regain its equilibrium.
In alarge specimen this ould take a long period of time. The rate and extent of carbonation is dependent on the concentration of CO in the urrounding edium. Re-2
search done 4 has demonstrated that the shrinkage and
weight gain due to carbonation increases ith increasing
The concentration of CO also has an effect in the2 humidity of he interior of the pas-'U-e* Lower
concentra-tions of carbonic gas produce a slower reaction and hence
should evolve ater at a lower rate.
In the test described previously., secimens were stored at different relative humidities and allowed to reach
moist-ure equilibrium, nd ten subjected to carbonation while maintaining the same hidities until the new equilibrium was reached. est have also been.made,, using similar
spec-imens, where after U--he same initial curing time were
ex-posed to simultaneous drying and carbonation surrounded by
mediums at various relative humidities.
It is quite obvious that the hmidity inside the spe c
-imens will be higher than that of the surrounding medium during the period zPaere simultaneous drying and
carbona-tion is taking place. The result as that specimens
car-bonated and dried simultaneously in relative humidities
above 50% shrank less than the specimens that were subiJected
P-n-r"onnatinn procedure. The
U J L, I.L' Y J_.L.Lt5
reason was because carbonation actually occurred at higher humidity than that of the surroundino medium* This
reason-ing is supported by the fact that the samples kept at 25% relative humidity shrank more in the simultaneous drying
and carbonation procedure than in the drying and subsequeit carbonation procedure.
10
In the absence of carbon di-oxide, hydrated mortars and concretes suffer changes in length when the moisture conditions ae changed. Vrnen the s'-6ec-lmen is subjected to wetting and drying in normal air., -the changes in length are due to both carbonation and changes in the moisture
conditions. The rate of carbonation in normal air is ve'rT
low because of the low oncentration of carbon dioxide in the atmosphere.
Investigations made by Shideler (5) show that ore
-carbonated secimens shrink half as uch as un-carbonated
specimens hen subsequently exposed to alternate c7rcles of'
drying and wetting. Tests made by Verbeck () also show
that samples subjected to the drying and subsequent car-bonation procedure have proved volume sability. Results
also showed hat the increase in volume stability is not
directly related to the aount of carbonation of the
var-ious secimens
Present Work on Carbonation
The presont, study is concerned with three items: the investigation of the ime required for secimens Of
Portland cement mortar to reach moisture equilibrium when
stored in ediums of different relative huiwidities- the
Investimation of the effect that the _-Collowing facu-ors
stability and dimens
.
-Portland cement mortar specimens: tme of exposure to carbon dioxide$ concentration of carbon dioxide, and the curIng time; and the investigation of the effect of carbonation on the permeability of Portland cement mortar secimens.
1) Stabilization
In he -bests to determine the time required for the stabilization of Portland cei,,ient mortar, secimens 1 by I by 6-4- in. were used. All secimens 1,,iere moist cured
for 7 davs at 73F. The specimens were divided into 4 groups
and stored in-relative humidities of 7 76,, 52 and 33 per
cent. An initial measurement of length and we'igght as made
at the end ol"' the curing period. Subsequent easurements were ade at different intervals of tme to determine changes
in length and weight. This as stopped when apparent
equil-ibrium was reached. The results appear in Graphs and 2.
Each one of the curves represent the average easurements of six secimens. To calculate the values or these two
graphs, two programs were vriritten for the IBM 1620 A
listing of these programs, and Tables and 2 vvhich
corres-pond t-,-Graphs and 2 respectively, aear in Aptendix B.
These to Programs are listed as Program and Program 2.
2) Weight and Dimensional Stability
The eect of he tme 0- ex-posure -to carbon dioxide,
the concentration of carbon dioxide, and the curing time have on the eight stability and dimensional stability of
.. ...- ... 1 __
120
Portland cement ortar was investigated in the following
manner. Portland cement mortar secimens, by 1 b 6z in.
were cast. Three groups o 28 secimens were cast. These
groups were oist cured at 73F for 1, 3 and 7 days. Initial
Weight and length -measurements were taken. Tnen each one
of these groups as divided into two grouses of 14. One
group to be carbonated n a medivzn of-100% carbon dioxide
concentration the other group to be arbonated in a medium of 50% carbon dioxide concentration. In these-grou-ps of 4j, pairs of specimens ere carbonated for the following times:
0 min,, 10 min,, hr.$ 2 hrs., 4 hrs*, 24 1ros., and 2 hrs.
The relative humidity during sultaneous carbonation and drying was kept at 52% I-or all the secimens. Weight and
length measurements ere taken at different intervals of
time until apparent euilibrium was reached. The results appear in Graphs 3 to 14. To calculate he values for these
graphs. two more programs were ritten for the IBM 1620.
A listing of these progra-ms and Tables 3 to 14 w1iich
--orrE,-,s-pond to Graphs 3 to 14 respectively appear in Appendix B.
These to programs are listed as Program 3 and Progra 4.
To correlate the factors entioned in the paragraph above and the srength of the specimens, the ollowing
tests ere made. The length of all the secimens was reduced to 2 inches. Tese I y I by 2 in, risms were tested for
M
13.
3) Permeability
To investigate he effect of arbonation
on
the permeability of Portland cement mortar the following testswere made. wo 6 by 12 in. ylinders and twelve 2 in.
cubes ere cast. These specimens ere moist cured for seven days at 73F. At the end of the curing period, the
cylinders ere sliced into - in. thick slabs by eans of
a diamond saw. EQual aount of slabs and cubes ere
car-bonated in 100%
CO
2 atmospheres-for the following times:0, I., 14,, 40., and 87 hours.
U U
.. There was no adequate apparatus to measure 'he per
meability of the slabs available. One had to be designed. A diagram and a photograph of the appartus designed appears
in Appendix A. The procedure to measure permeability was
as follows. The slab was placed in the apparatus. The sab was subjected to a head of about 3 feet of water. The slab was allowed to absorb water ad stabilize for 24 hours
be--lore taking any readings. After the 24 hours passed he
head as raised acrain to about 3 feet and a reading 1,1ras
taken. Five more readings were taken at 10 hour intervals. The ermeability was computed after every one of the five readings. The average value was used as the permeability
of the slab. The results appear in Graph 17.
The 2 i. cubes ere stored n the constant tem-perature
i
14.
temperature room was F and the relative humidity 50 per
cent. When -IChe cubes reached 28 days of age (standard ASTM
specification) the7 were tested n conTpression to obtain
a correlation with their corresponding permeability. The
results appear in Graph 17.
I, r . I,N
m
IT i. 31 I wi 14) VI et -1 'AlMl
IM '
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IV. DISCUSSION OF RESULTS
Graph I and 2
The weight and length euilibrium curves represented., in these two-graphs correspond to specimens stored in
co -free mediums at 4 different relative 2 umidities. The
changes in length and changes in weight represented in
these graphs are due to drying only. As it is expected,
the secimens stored in mediums of lower
relative-humidi-ties loose more weight and shrink more. The only aparent discrepancy is in the case of the specimens stored in a surrounding medium of 97 per cent. In that case the speei-mens gained weight and shrank at the same time. In a
medium of hat ty-pe it is expected that secimens will gain
weight as more and more pores of the Paste bedome filled
with water, and the aste becomes hydrated. As it can be .seen in Graph 2 for samples stored in 97% relative humidity
there is an initial shrinkage in the first 27 days, but
after--that there is little change in lenr,-th.
It is interesting to note that after 151 days the
specimens have reached dimensional stability, hile their weight eilibrium has not been attained et. The only exception are the samDID s stored at 76% relative humidity,
where after 151 days, both weight" and dimensional stability
have been reached. This fact shows that the relationship
between shrinkage and weight change is not necessarily direct.
___
33.
Graphs 3 to 14
It is evident from Graphs 4 6 and that
pre-carbonation in ediums of 50% Co concentration 2 for tes
no longer -than 2 hours, and stored in an atmosphere of
52% relative hu-idit7l has no significant effect on the dimensional stability of the specimens. This is regardless of the curing time. Apparently under these conditions
the extent carbonation is insignificant,
The efect of curing time on the weight s11--ability of
specimens carbonated in mediums of 50%
CO
concentration2
is significant. This fact is evident from Graphs t5, 5 and 7.
The spacing betyieen the seven curves in these graphs in-creases with decreasing curing time. This can be explained in the ollowing manner. As it was discussed reviously,
Verbeck 4 found that specimens that were carbonated and
dried simultaneously had iternal humidities higher than
that of the surrounding medium 52%, in this case). It
is logical to say that the longer the curing time he larger
the internal umidity. Verbeck also found that the'rate
and extent of carbonation decreases with increasing relative
humidity (above 500. Therefore it can be concluded that
the extent of arbonation is higher for samples of low curing time because the internal h= id-ity s closer to
34,
From (raphs 3 4 6 7 and the effect of the
time of carbonation on weight and dimensional stability is not very clear. The general trend of these curves
indicates that shrinkage and weight loss decreases ith
increasing time of carbonation. This is eqected because
carbonation at high hmidities precipitataes calcium carbonate
in the' surface of the secimens, clogging the -cores of the paste. Tis reduces the -oermeabilit7. -in consequence the
amount of water lost due 'o drying is reduced nd the amount
of shrird-age that occurs is decreased. Therefore the longer the time of carbonation, the lower the permeability and
the srinkage
It can be seen from Graphs 10, 12 and 4 tat when
the-concentration of CO is 10001, the tLirie o carbonation2 /0 becomes significant to the dimensional sability of te
specimens. The extent of carbonation increases ,Keith in-crease in the time of carbonation. Therefore ermeability decreases with increasing time of arbonation. The result .is that shrinkage also decreases with increasing time of
carbonation. It should be noted from these three graphs that the uring time has no effect onluhe srinkage The curves on the three graphs are ver7 smilar.
From Graphs 9 11, and 3 it is evIdent that the
se-parat! on between the dif-Perent curves in each graph
.35.
behaviour has been discussed previously. It should be noted that for the same curing times,., the separation between the curves is larger for specimens that were
rbonated in mediums of 100%
CO
concentration than for specimens carbonated in mediums here the cncentrationof CO was only 50%.2
The effect of time of carbonaflon on shrinkage
and weight loss is clearer in Graphs 9 0 1 12 13
and 14 than it is in Graphs 3 43 53 63 7 and S. In the
former it is evident that shrinkage and weight loss de-crease with increasing time of arbonation. The
ex-plana-tion for this henomenon has already been discussed.
Gra-,ohs 15 and 16
Gra-phs 15 and 16 show that the compressive srength
of Portland cement -niortars decreases with decreasing ur-ing time. This penomenon is logical because the samples cured for a longer time have a larger supply of wter to
a'tain a more complete ydration. The strength depends
on the degree of hydration of the aste.
Tnie general trend of these curves is the increase in
compressive srength with the increase of the time of car-bonation. This is expected because the extent of
carbona-tion increases with increasing carbonation time. The
extent of carbonation is inverse17 related to te per-meability of-the paste. The smaller he erper-meability the
36. reases the it s an gths are ms of higher increases
it is
rea-s -o e c iraen rea-s ion. ofcarbona-ity obtained
tion. The average no eMeri-of arbon-of carbona-act can be ,,,later forces Ls calcium :)onate is When this s that were an increase I. . .71 : -, ". .. .7 -", I-I - I -,i
37.
The second explanation is concerned with the forma-tion of gel. Further carbonation causes the CO to react.2 with the hydrated product C S fo'rming a silicate gel*
3 22
This gel increases the pern. eability of the paste*
;i
."a
38,
V. CONCLUSIONS
1. The amount of weight loss and shrinkage due to
drying increases as e relative humidity of the
surround-ing, medium decreases (Graphs and 2.
2. The relationship between the weight loss and the
shrinkage due to drying is not -necessarily drect.
(Graphs and 2.
3. Portland cement mortar secimens stored at differ-ent relative humidities reach dimensional 'equilibrium after
100 da7s- In most of the casesstudied the ueight
contin-ued to decrease after 100 davs. (Graphs and 2.
4. Pre-carbonation in mediums of 50% co
concentra-tion, for times of carbonation no 1onger than 2 hours, in an atmosphere of 52% relative humidity, has
no'signi-ficant effect on the dimensional stability of Portland
cement mortar secimens. (Graphs 4 6 and 8).
5. Pre-carbonation in mediums of 100% concentration reduces shrin]kage significantly. Shrinkage decreases with
increasing time of carbonation (Graphs 6 10 12
and 14).
6. Curing time does not modify the reduction f
shrinkage caused by -ore-carbonation. Graphs 4 6 So
10, 12 and 14)*
-39.
7. Curing time affects he amount of weight loss
reduction caused by pre-carbonation. For a constant
carbonation time, the aount o weight loss reduction
increases with decreasing curing time. Graphs 3.t 5. '7.q
11 and 13),
B. Weight loss decreases with increasing time of'
carbonation (Graphs 3 7 11 and 13).
9, The compressive strength of Portland cement mortars decreases with decreasing curing time Graphs 15 and'16).
100- The compressive strength of Portland cement ortars increases with increasing time of arbonation. Higher
strengths are obtained for specimens carbonated in mediums
0f higher CO concentrations. (Graphs 15 and 16).
2
11, The permeability of Portland cement mortars
de-el-eases with increasing time of carbonation$ until car-bonation time reaches 23 hours. Permeability starts to increase with increasing time of carIz nation, after 2)
40.
VI. RECOMMENDATIONS FOR FURTHER STUDY
1. The carbonation equipment should be redesigned.
The new system should include the following features:
no leakage2 the internal pressures suld be controllable,,
a fluid here CO2 is not soluble should be used instead of water.
2, uture studies of pre-carbonation should include
as variables, the specimen sLze and water-cem-ent ratio.
Investigations should be-m'de by exposing secimens to longer times of carbonation (longer than 72 hours).
3. Specimens should be stored in ediums of differ-ent relative humidity or times longer than 151 day to determine the time required to reach eight equilibrium.
4. An analysis sould be made to determine the rela-tionship between the eight loss due to drying and he
shrinkage due to dryingo
5. After specimens have been cured, different
dry-ing periods should be allowed to occur before
"pre-carbonating". The effect of these drying Periods on the ultimate weight and dimensional stability, is o
con-siderable, interest.
6, The experimental investigations o the effect of
pre-carbonation on the permeability of Portland cement mortar should be done again to check the results obtained
41,
7. The effect on Derme4bility of other factors such
as wauer cement ratio., concentration of CO 2Y curing time.,
etc. in combination ith pre-carbonation slaould be sdied.
S. The permeability equipment should be ira-proved in
the following areas. A funnel should be used instead of
a cylindrical tin can to prevent air from getting trapped
inside the apparatus, The area of the s-tandpipe used was
2 any
0.254 cm To prevent possibility of capillarit7,,, a standpipe of larger area should be used. But it should
be kept in mind that the increase in te area of t-he stand-pipe ill increase the time interval between readings,
since the water level will not descend as fast.
