International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: 05
10
Degree of Hydration and Strength
Development of Low Water-to-Cement Ratios
in Silica Fume Cement System
Dillshad K.H. Amen

Abstract— Hydration of cement is very important to be
estimated, since there are a great relation of hydration with the
properties of hardened cement paste. Limited information
available about the rate of hydration of cement containing silica
fume with low water to cement ratio. In this investigation
strength development of cement paste with and without silica
fume described using gel-to-space ratio concept. Twenty seven
mixes of cement paste were prepared with low water-to-cement
ratios ranging from 0.23 to 0.35 and different silica fume contents
(0, 5, 10, 15, 20) % by weight of the binder. The mixtures were
maintained at different curing temperatures (10, 20, and 40 ) oC .
Non-evaporable water considered as combined water and
compressive strength were determined at different stages of
hydration. A best fit power equation used to describe the
relationship.
Index Term-- Degree of hydration, compressive strength,
silica fume cement system.
I. INTRODUCTION
Generally the overall rate of hydration of cement is a
summation of the rate of hydration of it’s individual
components. This phenomenon of cement has been studied in
the past leads to the formation of the hardened cement paste
with porosity, temperature rise and chemical shrinkage during
hardening [1], [2], [3], [4]. Hydration of cement is very
important to be estimated, since there are a great relation of
hydration with porosity, heat of hydration, strength
development, chemical shrinkage and autogenously
deformations which the later is a serious cause of cracks of
hardened cement based matrix of low water-to-cement ratios.
The most common way to estimate the degree of hydration of
cement in practice has been to measure the non-evaporable
water content (Wn) , and the degree of hydration can be
estimated by the following equation:
h
(1)
h
h: degree of hydration
Wn: Non-evaporable water content
Wcomb: Total combined water for full hydration of cement.
.
It has been estimated [5] that average combined water 23% by
weight of cement is required for full hydration of Portland
cement compounds. However if silica fume is incorporated in
a mixture. The non-evaporable water and the degree of
hydration of cement are not the same[6]. Whereas the degree
of hydration of the cement is slightly increased when the
moderate amounts of silica fume are added with the sufficient
water present in pore system. In mixture with low w/c ratio the
faster self desiccation and the reduced permeability caused by
silica addition affect the moisture state in a large specimen
even if it is water cured this consequently affect the degree of
hydration of the cement. It has been concluded [6], that some
of the non evaporable water is released as evaporable water
lowering the total amount of nonevaporable water per
hydrated cement content in silica fume blends, and In low w/c
ratios the hydration has been obstructed by the lack of free
water caused by an increased self desiccation.
It has been reported [7, 8], that there are many factors
influence on the rate of hydration such as: fineness,
admixtures, water to binder ratio and temperature of the
materials at the time of mixing and grinding methods .For
analysis several approaches are used to describe mechanical
properties of cement based matrix , such as the compressive
strength development process. These approaches are: water to
cement ratio concept, the gel-space ratio concept, the total
porosity concept, degree of hydration concept, maturity laws
and others.
For a fully compacted matrix, strength is found to be inversely
proportional to the water-to-cement ratio and expressed by
well known Abram’s law [9]. Other concepts have been
furthered taking into account the total heat liberated by the
major mineral components of the individual powder material
as the parameters affecting the strength, Kato[10]; has
proposed a model expressing the differential increase in the
strength as a weighted linear summation of the differential
amount of heat liberated by the major chemical compounds of
blended cement. It has been expressed [11] also the relative
strength versus the porosity of several materials with same
relationship, where In this case water-to cement ratio is the
main factor besides the degree of hydration which governs the
porosity of the cementitious matrix thereby influencing it’s
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International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: 05
strength.
Very limited information in the literature available dealt with
the rate of hydration in silica fume cement system, whereas
the rate of chemical reaction between silica fume and calcium
hydroxide produced from the hydration of cement is not well
understood. It has been reported [12] that pozzolanic materials
have a retarding effect on the hydration of Portland cement
especially at early stages of hydration, this reduction in the
rate of reaction caused by the shortage of calcium hydroxide
which is necessary for the reaction at any stage of hydration.
While at later ages when there are sufficient amount of
calcium hydroxide produced, rate of reaction will be preceded
with further amount of gel produced leading to increase the
strength of the matrix. Therefore the degree of hydration, of
silica fume cement system at early age is not well known. In
this investigation nonevaporable water which is considered as
combined water were determined experimentally for cement
paste with and without silica fume, then gel-to-space ratio
determined based on the total combined water considered as
equal to 0.23 that required for full hydration of 1 gm pure
cement, while for full hydration of silica fume cement system
total combined water calculated based on the amount of water
required for chemical reaction of silica fume to be transformed
to Silicic acid, and the percentage ratios of the materials
(cement + Silica fume) making the composite. The results of
gel-space ratio correlated with the strength of cement paste at
different stages of hydration using a best fit power or
exponential equation. In addition the effect of w/c ratio and
curing temperature on the nonevaporable water were taken
into account.
II.
EXPERIMENTAL WORK MATERIALS:
Cement: normal type cement from Japan used in this
investigation, some of chemical and physical properties of the
cement are shown in table (1),
Silica Fume: Fine powder silica fume with average size of
0.15 um, and specific gravity 2.2 used as a supplementary
cementitious material. Some properties of silica fume are
shown in table (2).
III.
PREPARATION OF THE MIXES
Table (3) shows the proportion and preparation of 27
mixtures of the binding material. Mixing time were done
about 4+ 1 minutes for all , then the mixes were placed into
the molds of prisms (40X40x160) mm according to the ASTM
C349-82 for measuring compressive strength from the failed
flexural prisms. The prisms were covered by a polyethylene
sheets and stored in a controlled room under the temperature
of 20 +1 and relative humidity about 80 % for one day , then
taken out from the molds , cured in a water bath until the
testing date.
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IV.
COMPRESSIVE STRENGTH
Modified cube method is used to determine compressive
strength after testing modulus of rupture and taking the failed
prisms of about 40 mm Cubes. Dimensions of the prepared
cubic specimens measured using digital vernier caliper with
accuracy of 0.01mm. Compressive strength determined at 12
hours, 1, 3, 7, and 28 days.
V.
NON-EVAPORABLE WATER
After placing the specimens in a controlled room, and after
the initial setting of cement, small pieces of the paste fractured
from the prisms, sealed and stored in a Lab. At different
conditions for different intervals of times [ 2.5 , 6, 12, 24, 48,
72, hours and 7 days] for measuring the combined water (nonevaporable water). The weight of the wet pieces was measured
at the end of the specified intervals of time by a digital
electronic balance with an accuracy of + 0.01. Specimens
were immersed in ethanol for about 7 days. Then, taken out
and dried in an oven at 105 oC for about 48 hrs. Dried
specimens were weighed and crushed; parts of the crushed
material in a ceramic cup were weighed, before placing in a
furnace for firing with a constant rate of increase 250o C /hr.
up to 1000 oC. Specimens were maintained at this temperature
for 2 hours. After heating specimens in a ceramic cup were
cooled naturally inside the furnace. Weight of specimens was
measured after firing, the difference in the weight is the non
evaporable water
.
Loss on ignition of dry cement and silica fume were
determined, the average of 5 specimens was 2.47 % for
cement and 3.01 % for silica fume. This subtracted from
the determined non-evaporable water,
VI. RESULTS AND DISCUSSION COMPRESSIVE STRENGTH
Results of compressive strength of hardened cementitious
composite at different ages are shown in Table (4-a &4-b), It
can be observed that compressive strength of the paste
increased with a high rate initially and slows down with
increase in curing time for all kinds of the specimens with and
without silica fume, this indicated that the hydration of cement
is continuous with a high rate initially and slows down with
age for different w/c ratios and silica fume contents.
increasing w/c from 0.23 to 0.26 showed an increase in
compressive strength for some specimens at ages of 3 and 7
days , while beyond 0.26 compressive strength has decreased,
this was attributed to stiff mix of 0.23, which decreased the
permeability and availability of mixing water for the hydration
of cement or might insufficient compaction during the
preparation of the mixes.
Very stiff mixtures were resulted when cement partially
replaced by silica fume. To enhance the flowability of the
mixtures containing silica fume; superplasticizer 2% was
added, The inclusion of silica fume as a partial replacement of
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International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: 05
cement decreased the compressive strength at early ages and
have been increased later, with increase level of silica fume in
the mixes, this was reported by Atlassi(6), who concluded that
for very dense systems with low w/c ratios the hydration may
be obstructed by the lack of free water, caused by an increased
self desiccation, also because silica fume react with Ca(OH)2
generated from the hydration of cement therefore they have a
diluting effect on PC during the period of
Ca(OH)2 accumulation causes the effect of silica fume in
favor of the system strength to arise. Greatest increase in
strength can be seen in the mixture with silica fume at 15%,
beyond this limit compressive strength slightly decreased.
This indicated that 15 % is the best content of silica fume for
the tested cement. It can be observed that (5, 10, and 20 ) %
silica fume mixtures have a lower compressive strength than
mixtures without.
VII. NON-EVAPORABLE WATER
Non-evaporable water of the cementitious composite
determined for all the mixes at different stages of hydration as
shown in Fig. (4). It is shown that the rate of hydration is high
at the initial stage and has lowered with time. Degree of
hydration can be determined by measuring the ratio of
combined water at any stage divided by the total combined
water for full hydration of cement which is considered as 0.23
for OPC.
Higher rate of hydration were observed relatively for
mixtures of lower w/c ratios at early age up to 48 hours
curing, that exposed to temperature 20o C , and no significant
difference in the hydration rate observed for all the mixtures
exposed to curing temperature 10o C and 40o C; beyond this
time of curing non evaporable (Combined) water increased
with increase of w/c ratio. This indicated the availability of
sufficient amount of mixing water and the presence of more
space available for the precipitation and growth of hydration
products , this increased the hydration rate and decreased the
compressive strength, due to the increase in total porosity.
Slower rate and higher rate of hydration were observed for
specimens cured in low temperature 10o C and high
temperature 40o C respectively as shown in Fig.(1) . For
sealed specimens if there is no loss of water by evaporation,
then adding the ratio of combined water to the moisture
content (evaporable water) at any time before curing of
cement, the result approximately was the total amount of
water or w/c ratio , except some mixtures showed a slight
difference ranged from 0.01 to 0.03.
At early age up to 7 days, the results showed an increase in
the nonevaporable water content per 1 gm binder of silica
fume cement systems when silica fume has increased from (015%); beyond this limit for mixture of 20 % silica fume which
is cured at 20 oC a slight decrease in the nonevaporable water
is resulted. This case followed the same trend of compressive
strength development.
12
VIII.
STRENGTH DEVELOPMENT
In this investigation, gel-to-space ratio concept was used to
describe strength development of hardened cement paste with
and without silica fume. It is assumed that 1 ml of cement on
hydration will produce 2.132 ml of gel , then gel to space ratio
determined by the equation:
Gel / Space =
h
h
(2)
Where:
G is the specific gravity of cement ; Gs is the specific
gravity of silica fume (2.2); h is the degree of hydration, it is
considered equal to the ratio of non-evaporable water-to-total
combined water required for full hydration. It is represents
about (5 23 percent of the mass of dry cement, while for silica
fume cement system total combined water is slightly different
which depends on the chemical reaction between silica fume
and Ca(OH)2 resulted from the hydration of cement, at
different stages, it is determined based on the molecular
weights of the elements in a pozzolanic chemical reaction
between calcium hydroxide (Ca(OH)2) and silicic acid , this
reaction can be schematically represented as follows
 H4SiO4 (Silicic acid)
100 gm + 30 gm  130 gm
Ca (OH) 2 + H4SiO4  CaH2SiO4 · 2 H2O (Calcium
silicate hydrate).
These reactions indicates that 1 gm of silica fume requires
0.3 ml of combined water for full transformation to Silicic
acid , which is consequently transforms to calcium silicate
hydrate (gel) , if sufficient amount of Ca(OH)2 is provided by
the hydration of cement compounds.
Using equation (2), specific gravity of cement is 3.16 , silica
fume 2.2 and the percentage of air entrapped in the paste due
to insufficient compaction approximately assumed equal to
2%. Gel to space ratio determined at different stages of
hydration.
Power equation of the type
used as a best fit
equation to estimate actual compressive strength of the paste
as a function of the gel to space ratio. The results is plotted as
shown in figure (2 ) &(3) . Based on this equation it is
possible to calculate the effect of increasing the water –cement
ratio at a given degree of hydration on both the porosity and
strength of low water –cement ratio with and without silica
fume.
IX.
CONCLUSIONS
Based on the results obtained from the investigation the
following conclusions can be drawn:
1) After the age of 48 hrs. higher water-to-cement
ratio , resulted to higher rate of non-evaporable
(combined) water while for too early age before 48
hrs. Converse effect of water-to-cement ratio were
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International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: 05
TABLE I
PROPERTIES OF TYPE ( I )CEMENT
resulted especially for curing temperature 20o C ,
and for other temperatures , the differences was
very slight.
2) Different contents of silica fume resulted a slight
increase in nonevaporable water with the reduction
of compressive strength in silica fume cement
mixes especially at early age
3) The best silica fume content was 15 % by weight
of the composite, whence resulted highest
compressive strength at ages of 7 and 28 days.
4) Silica fume cement mixes were influenced by the
change in curing temperature more than mixes of
pure cement paste.
5) The development of compressive strength of
cement with and without silica fume were
expressed as a function of the gel-to-space ratio
using a power equation, higher correlation
coefficient were resulted for cement paste mixes.
For further work on this area it is recommended to
determine and relate the effect of the degree of
hydration especially at early age , on both chemical
and autogeneous deformations, which the latter is a
serious cause for crack at early age of low water to
binder ratios in cement based matrix.
REFERENCES
Justnes, H., Sellevold, E.J., et, al “ The influence of cement
characteristics on chemical shrinkage”, Proceddings of the international
workshop on autogeneous shrinkage of concrete , Hiroshima, Japan,
June 13 -14, 1998, pp. 67-76.
[2] Justnes, H., Van Gemert, A., et al “ Total and external chemical
shrinkage of low w/c ratio cement paste” , Advances in cement research
, Vol. 8, No. 31, 1996 , pp. 121-126.
[3] Justnes, H., Hammer, T.A., et al “ Chemical shrinkage of cement paste
mortar and concrete”, Proceedings of the international workshop on
autogeneous shrinkage of concrete, Hiroshima , Japan, June 13-14 ,
1998, pp. 201-211.
[4] Justnes, H. Sellevold, E. J., et al “ Chemical shrinkage of cementitious
paste with mineral additives”, Proceedings of the second international
research seminar on self-desiccation and its important in concrete
technology, Lund, Sweden, June 18, 1999, pp.73-84.
[5] Neville A. M., “ Properties of concrete”4th and final edition , Wiley, 4
sub edition 25 July 1996.
[6] Atlasi,. E. H , “ Nonevaporable water and degree of cement hydration in
silica fume-cement system”, ACI SP 153-37, volume 153, June, 1995.
[7] Binici, H. , Cagatay, I. H. , Tokyay, M., Kose, M.M. “ The early heat
of hydration of blended cements incorporating GGBFS and ground
basaltic pumice (GBP). International Journal of physical sciences , Vol.1
(3) , November, 2006 , pp. 112 – 120.
[8] SH Kosmatka, WC Panarese, “ Design and control of concrete mixtures,
Portland Cement Association (PCA), Illinois , 1994.
[9] Shetty, M.S. “ Concrete Technology Theory and Practice” S Chand and
Company Ltd. Ram Nagar, New Delhi, Revised Edition, 2005
[10] Kato, Y. and Kishi, T., “ Strength development model for concrete in
early ages based on hydration of constituent minerals” Proceedings of
the JCI , 1994 , Vol. 16 , No. 1, pp. 503 -508.
[11] Bentz ,D. P. and Stutzman , P. E. “ Curing, Hydration and
microstructure of cement paste” , ACI materials Journal , Vol. 103,
No.5, September-October 2006.
[12] Maekawa, K. , Chaube, R. and Kishi, T. “ Modelling of Concrete
Performance – Hydration, Microstructure Formation and Mass
Transport, Published by E && FN an imprint of Routledge, London,
1999.
13
Properties
Specific Gravity g / cm3
Setting Time
Initial (h-min)
Final (h-min)
Compressive strength
3d
N/mm2
7d
28 d
Heat of hydration J/gm
7d
28d
Secondary oxides %
MgO
SO3
Loss on Ignition
Alkalies
Cl
3.16
2-21
3-28
30
45.7
62.3
328
376
1.23
2.12
2.08
0.51
0.015
TABLE II
SOME PHYSICAL PROPERTIES AND OXIDES CONTENT OF SILICA FUME
Oxides
L.O.I
H2O
C
PH
Na2O
MgO
Al2O3
K2O
CaO
Fe2O3
%
3.0
1.0
--2.5
3.0
1.5
3.0
2.0
3.0
Physical properties
Bulk density
Specific gravity
Surface Area
Average Size
values
200-350 kg/m3
2.2
200,000 cm2/gm
0.15 um
Table III
proportions of cement paste mixes
[1]
Mixes
w/c
A
0.23
0.23
0.23
0.26
0.26
0.26
0.29
0.29
0.29
0.32
0.32
0.32
0.35
0.35
0.35
B
C
D
E
% silica
fume
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Temp.
0C
20
10
40
20
10
40
20
10
40
20
10
40
20
10
40
mixes
w/c
D-5
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
0.32
D-10
D-15
D-20
% silica
fume
5
5
5
10
10
10
15
15
15
20
20
20
TABLE IV-A
COMPRESSIVE STRENGTH OF HARDENED CEMENT PASTE AT DIFFERENT AGES
Age (days)
0.5
1
3
7
28
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Compressive Strength (N/mm2) for the Mixes
A
B
C
D
E
15.87
14.25
8.1
5.8
4.33
27.72
24.59
18.29
16.99
17.44
40.46
42.14
42.17
34.47
22.35
50.59
61.1
45.77
48.84
45.65
102.11
89.19
99.03
88.11
82.29
IJENS
Tem
p. 0C
20
10
40
20
10
40
20
10
40
20
10
40
International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: 05
14
TABLE IV-B
COMPRESSIVE STRENGTH OF SILICA FUMES CEMENT SYSTEM AT DIFFERENT
AGES.
Age (days)
Compressive Strength (N/mm2) for the Mixes
D-5
D-10
D-15
D-20
0.5
4.45
5.31
4.81
7.94
1
8.45
12.21
16.23
15.37
3
35.32
24.35
36.18
34.2
7
40
42.79
47.87
41.22
28
70.88
69.5
93.99
66.56
0.8
0.7
T=20 0C
0.7
0.6
0.6
0.5
0.5
0.4
0.4
A
B
C
D
0.3
0.2
0.1
0.0
D-5
D-10
D-15
D-20
0.3
0.2
0.1
0.0
0
50
100
150
200
0
0.7
0.7
0
T=10 C
0.6
50
100
150
200
T=100
0.6
0.5
0.5
Degree of hydration
T=20 0C
0.4
A
0.3
B
0.2
0.1
0.4
D-5
D-10
D-15
D-20
0.3
0.2
0.1
0.0
0.0
0
50
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
100
150
200
T=400 C
A
B
C
D
0
50
100
150
200
0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
50
100
150
200
T=400 C
D-5
D-10
D-15
D-20
0
50
100
150
200
Curing time (hours)
Fig. 1-a. Degree of hydration of cement paste versus curing time at
different temperatures
Fig. 1-b. degree of hydration in silica fume cement system at different
temperatures
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120
15
y = 80.551x4.2644
R² = 0.8411
100
80
60
40
20
0
0
0.2
0.4
0.6
0.8
1
Fig. 2. Relationship between compressive strength and gel-to-space ratio for cement paste
100
y = 129.89x3.5017
R² = 0.7391
80
60
40
20
0
0.00
0.20
0.40
0.60
0.80
1.00
Fig. 3. Relationship between compressive strength and gel-to-space ratio for silica fume cement system
D. K.H. Amen, is with the College of Engineering, University of
Salahaddin - Erbil-Iraq, dillshadbzeni@yahoo.com
.
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