110205-7676-ijcee-ijens

International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: 05 10

110205-7676 IJCEE-IJENS © October 2011 IJENS I J E N S

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

Degree of Hydration and Strength

Development of Low Water-to-Cement Ratios

in Silica Fume Cement System

Dillshad K.H. Amen



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.

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 (non-

evaporable 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



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 (0-

15%); 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.

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



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

[1] 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.

TABLE I

PROPERTIES OF TYPE ( I )CEMENT

Properties

Specific Gravity g / cm3 3.16

Setting Time Initial (h-min) 2-21

Final (h-min) 3-28

Compressive strength N/mm2

3d 30

7d 45.7

28 d 62.3

Heat of hydration J/gm 7d 328

28d 376

Secondary oxides % MgO 1.23

SO3 2.12

Loss on Ignition 2.08

Alkalies 0.51

Cl 0.015

TABLE II

SOME PHYSICAL PROPERTIES AND OXIDES CONTENT OF SILICA FUME

Oxides % Physical properties values

L.O.I 3.0 Bulk density 200-350 kg/m3

H2O 1.0 Specific gravity 2.2

C -- Surface Area 200,000 cm2/gm

PH -- Average Size 0.15 um

Na2O 2.5

MgO 3.0

Al2O3 1.5

K2O 3.0

CaO 2.0

Fe2O3 3.0

Table III

proportions of cement paste mixes

TABLE IV-A

COMPRESSIVE STRENGTH OF HARDENED CEMENT PASTE AT DIFFERENT AGES

Age (days) Compressive Strength (N/mm2) for the Mixes

A B C D E

0.5 15.87 14.25 8.1 5.8 4.33

1 27.72 24.59 18.29 16.99 17.44

3 40.46 42.14 42.17 34.47 22.35

7 50.59 61.1 45.77 48.84 45.65

28 102.11 89.19 99.03 88.11 82.29

Mixes w/c % silica fume

Temp. 0C

mixes w/c % silica fume

Temp. 0C

A 0.23 0 20 D-5 0.32 5 20

0.23 0 10 0.32 5 10

0.23 0 40 0.32 5 40

B 0.26 0 20 D-10 0.32 10 20

0.26 0 10 0.32 10 10

0.26 0 40 0.32 10 40

C 0.29 0 20 D-15 0.32 15 20

0.29 0 10 0.32 15 10

0.29 0 40 0.32 15 40

D 0.32 0 20 D-20 0.32 20 20

0.32 0 10 0.32 20 10

0.32 0 40 0.32 20 40

E 0.35 0 20

0.35 0 10

0.35 0 40



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

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

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200

A

B

C

D

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 50 100 150 200

D-5D-10D-15D-20

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 50 100 150 200

A

B

T=10 0 C

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 50 100 150 200

D-5D-10D-15D-20

T=100

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200

ABCD

T=400 C

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 50 100 150 200

D-5

D-10

D-15

D-20

T=400 C

T=20 0C

T=20 0C

Deg

ree

of

hy

dra

tio

n



Fig. 2. Relationship between compressive strength and gel-to-space ratio for cement paste

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, [email protected] .

y = 80.551x4.2644 R² = 0.8411

0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8 1

y = 129.89x3.5017 R² = 0.7391

0

20

40

60

80

100

0.00 0.20 0.40 0.60 0.80 1.00

mailto:[email protected]

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