"EFFECTS OF WATER TO CEMENTITIOUS RATIO ON COMPRESSIVE STRENGTH OF CEMENT MORTAR CONTAINING FLY ASH"* By Tarun R. Naik, Ph.D., P.E. Director, Center for By-Products Utilization

Shiw S. Singh, Ph.D., P.E. Post-Doctoral Fellow Center for By-Products Utilization and Amr S. Hassaballah Research Associate Center for By-Products Utilization

Department of Civil Engineering and Mechanics College of Engineering and Applied Science The University of Wisconsin-Milwaukee P.O. Box 784 Milwaukee, WI 53201 Telephone: (414) 229-6904 Fax: (414) 229-6958

__________________________________________________________________ *Presented and Published at the ACI/CANMET-EPRI Sponsored Fourth

International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Istanbul, Turkey, May 1992.

"Effects of Water to Cementitious Ratio on Compressive Strength of Cement Mortar Containing Fly Ash" by Tarun R. Naik, Shiw S. Singh and Amr S. Hassaballah

ABSTRACT

This study was directed toward studying performance of ASTM Class

C and F fly ashes in mortars under varying water to cementitious

materials ratio. Four different basic mixtures were proportioned.

These mixes were proportioned to have cement replacements in the

range of 20 - 40 percent by the weight of fly ash. For each basic

mix, water to cementitious materials ratio varied between 0.25 - 5.0.

An ASTM Type I Portland cement obtained from one source was used

in all the tests. Mortar mixes containing 20% Class C fly ash

exhibited better results than that shown by both the control mix as

well as other mixes containing Class F fly ash. The optimum water

to cementitious materials ratio (weight of water divided by total

weight of cement plus Class C or Class F fly ash) was found to range

between 0.35 and 0.6 for mixes tested in this investigation.

INTRODUCTION

Researchers have shown that the addition of fly ash to concrete

mixes reduces water requirement for a given workability [1, 2, 3,

7]. The decrease in water demand has been attributed to increase

3

in workability due to decreased fiction between paste and large

aggregate particles resulting from ball bearing effects of spherical

particles of fly ash present in these mixes.

Minnick et al. [4] indicated that inclusion of fly ash can

increase or decrease the water requirement of mortar or concrete mixes

depending upon the carbon content (LOI), and the amount of material

retained by the 45 μm sieve. The increase in water demand results

due to water absorption by carbon particles and other porous

materials [4], and coarse particles cause increased frictional

resistance of the mix systems, especially between the paste and coarse

aggregate particles. Therefore, mixes containing large and/or coarse

fly ash particles, higher amounts of water will be required relative

to the control mix in order to produce mortars/concrete at a fixed

workability level. In general, mixes containing fly ash with finer

particles show decrease in water demand with increasing amount of

fly ash in the mixture.

Helmuth [4] reviewed critically the water-reducing properties

of fly ash in cement pastes, mortars, and concretes. Based on his

critical analysis of test data derived from several studies, he

concluded that the reduction in water requirement in these mixes may

not be because of ball bearing effects of spherical fly ash particles,

as generally described in the literature, but it may be primarily

due to absorption of very fine fly ash particles on cement particles

4

surfaces which in turn causes dispersion of the cement particles

similar to that obtained through addition of organic water-reducing

admixtures.

Variations in fly ash properties, physical, chemical, and

mineralogical, can have substantial effects on its performance in

mortar as well as in concrete. Also, considerable variations can

also occur due to variations in properties of cements and other mineral

admixtures depending upon their production processes, types and

brands. Therefore, it is of special importance to develop optimum

mix proportion for each fly ash type and source in order to make

effective utilization of fly ashes in mortars/concrete. This

research work was carried out to evaluate performance of fly ash in

mortar as a function of water requirements and water to cementitious

materials ratio. The result of this investigation would be useful

in determining optimum mix proportion for fly ash mortars, and later

concrete, containing either ASTM Class C or Class F fly ashes.

PREVIOUS STUDIES

Berry [6] studied development of compressive strength of mortars

made from blends of slag, fly ash and Portland cement. No significant

interaction was found to occur between them when granulated slag and

fly ash was used together. Mortars were made with binary or ternary

blends of cement, slag, and fly ash. Mortar cube specimens were tested

5

for compressive strength. The results showed that optimum amount

of fly ash containing finer particles (10.2% retained on Sieve # 325)

can be blended with cement and slag in the range of 10-30% of the

blend.

Lin and Hwang [8] evaluated effects of design parameters for

cement mortars containing Class F fly ash such as water to cementitious

ratio and curing temperature on the replacements of cement and fine

aggregates by fly ash. Based on the results obtained, they

recommended that in order to increase the early strength grain of

mortars containing fly ash: (a) replace part of cement by fly ash

and reduce water-cement ratio simultaneously; (b) replace part of

sand by fly ash and slightly increase the water-cement ratio to improve

its workability; (c) replace part of cement and sand simultaneously;

and, (d) cure with high temperature.

Compressive strength of mortars containing slag and fly ash have

been measured by Douglas et al. [9, 10]. Their study [9] revealed

that high-lime content fly ash develops higher compressive strength

than low-lime fly ash above 25% Portland cement replacement. They

further indicated that 50 percent cement replacement by the high-lime

content fly ash might provide an acceptable compromise with respect

to compressive strength and heat of hydration. Douglas et al. [10]

also reported that sulphate resistance of mortar containing 58 percent

6

of fly ashes with lime content up to 12.3 percent was significantly

higher compared to the reference mortars containing no fly ash.

TEST MATERIALS

For the research project reported herein, test materials

consisted of cement, sand, water, and fly ashes. ASTM Type I cement

was used throughout these tests. ASTM Class C and F ashes were

obtained from Pleasant Prairie Power Plant, Kenosha County, WI and

Valley Power Plant, Milwaukee, WI, respectively. Natural sand was

obtained from a local ready mix concrete producer.

MIX PROPORTIONING

In mixing the reference mortar mixes, cement to sand ratio was

taken as 1:2.75. This mix proportion for the control mix was

designated as Mix A.

Mortars were also proportioned to have cement replacement by

fly ash in the range of 20-40 percent. Three different basic mortar

mixtures containing fly ashes, designated as Mix B, Mix C, and Mix

D, were also proportioned, Table 1. For all mixes, except Mix B,

water to cementitious materials ratio was varied in the range of 0.25

- 5.0 (Tables 2 through 5). For Mix B water to cement cementitious

materials ratio varied in the range of 0.25 - 0.8.

7

PREPARATION AND TESTING OF CUBE SPECIMENS

For each mix, 2-in. mortar cubes were cast and moist-cured for

24 hours. Then demolded and stored in lime-saturated water until

time of testing. All cubes were tested for compressive strength at

7 and 28 days in accordance with the ASTM Test C-109.

RESULTS AND DISCUSSION

Test data are presented in Tables 2 through 5, and in Figures

1 through 7.

As expected, compressive strength increased with water to

cementitious ratio up to a certain level and then decreased. This

happens due to the well known fact that up to certain water to

cementitious ratio, below optimum, strength decreases because

insufficient amount of water is available for completion of hydration

reaction, and/or because of low fluidity of the mix, "dryness" of

the mix, compaction might be poor to achieve the potential strength.

Whereas above the optimum water requirement, strength decreases due

to increased porosity of the mortar mixture system resulting from

increased water content.

8

Generally, for optimum strength/performance, higher water to

cementitious materials is observed in the case of mortars compared

to concrete. Several factors can influence the water to cementitious

ratio of mortar and concrete mixes. The transition zone, the

interfacial region between aggregate and paste, is larger due to size

of coarse aggregate particles present in case of concrete with compared

to mortar. Also, for concrete, porosity of the transition zone

increases with increase in water content. The transition zone is

the weakest link and thus dictates the extent of concrete strength

that can be achieved due to the cementitious compounds hydration

reaction. Probably due to this factor, less than the required amount

of water for hydration could be added to concrete to avoid critical

porosity beyond which strength diminishes rapidly. This effect is

relatively small in case of mortars due to their finer aggregate

particles relative to coarse aggregate particles used in concrete.

Therefore, higher amount of water can be added to mortar mixes to

derive potential strength gain due to hydration reactions. The peak

strength, therefore, for mortars occurs at a higher value of water

to cementitious ratio.

Compressive strength data for Mix A as a function of water to

cementitious ratio by weight is presented in Table 2. The compressive

strength increased with increasing water to cementitious ratio up

to 0.57 and then diminished, for both 7-day and 28-day test ages.

9

Compressive strength of mortar Mix B containing Class C fly ash

at 20% cement replacement is shown in Table 3. This mixture showed

higher compressive strength compared to reference mixture, Mix A,

at both 7-day and 28-day ages, but the peak occurred at a lower water

to cementitious ratio of 0.37 at both test ages. The maximum

compressive strength exhibited by Mix B was 125 and 112 percent of

the compressive strength attained by the reference mortar without

fly ash at 7 days, and 28 days, respectively.

Compressive strength as a function of water to cementitious ratio

for Mix C having 20% cement replacement by Class F fly ash is given

in Table 4. The optimum water to cementitious materials ratio for

this mix was found to be 0.57 at both 7-day and 28-day ages. This

mix attained maximum compressive strength of 84% and 85% of maximum

strength of the reference mortar without fly ash at 7-day and 28-day

ages, respectively. Concrete Mix C (20% Class F fly ash) gave lower

strength compared to the reference Mix A (no fly ash) as well as Mix

B containing 20% Class C Fly Ash.

Test data for Mix D is presented in Table 5. The maximum

compressive strength of this mix was obtained at water to cementitious

ratio of 0.47. This mix attained maximum compressive strength of

56% at 7-day and 72% at 28 days of the corresponding maximum values

shown by the reference mortar without fly ash at both test ages.

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The data presented above revealed that increase in Class F fly

caused reduction in mortar strength. Furthermore, peak strength

occurred at a lower W/C+F ratio for the 40% Class F fly ash mix with

respect to the no fly ash mix and the 20% Class F fly ash mix (for

which peak occurred at the W/C+F ratio of 0.57. This may be because

of the fact that slower pozzolanic reaction occurs at early age due

to poor reactivity of the Class F ash used. Improvement in compressive

strength were not achieved even at 20% Class F fly ash replacement.

Mix B with 20% cement replacement by Class C fly ash showed encouraging

results due to its better cementitious and pozzolanic properties

relative to Class F fly ash. The higher pozzolanic activity in the

case of Class C fly ash is associated with its improved fineness,

and higher lime content. The test results showed that optimum water

to cementitious ratio by weight was in the range of 0.35 - 0.60 for

all the mortar mixes with and without Class C or F fly ash.

RELATIONSHIP BETWEEN WATER TO CEMENTITIOUS RATIO BY WEIGHT AND WATER

TO CEMENTITIOUS RATIO BY VOLUME

The water to cementitious ratio by weight can be expressed as:

RW =

WW

WCM

1

RW =

WW

(WC + W

FA) 2

11

Where

RW = Water to cementitious ratio by weight

WW = Weight of water

WCM = Weight of total cementitious materials (cement and fly

ash)

WFA = Weight of fly ash

P = Percent cement replacement by fly ash

RR = Replacement ratio, ratio of fly ash to cement

replacement used

The water to cementitious materials ratio by volume can be written

as

RW =

WW

WCM[(1 -

P

100) + P*RR

100 ]

3

RV =

VW

VC + V

FA

4

RV =

WW

ρW

[(1-P)

ρC

WCM +

P WCM RR

ρFA

]

5

12

where

Rv = Water to cementitious materials ratio by volume

ρW = Density of water

ρC = Density of cement

ρFA = Density of fly ash

SC = Specific gravity of cement

SFA = Specific gravity of fly ash

CF = Conversion factor, and it is given by the following

relation:

The values of CF can be multiplied by water to cementitious ratio

by weight to obtain the water to cement ratio by volume (Eq. 7).

Eq. 8 provides general conversion factor (CF) for converting

water to cementitious ratio by weight (RW) to water is cementitious

ratio by volume (RV). When the replacement ratio (RR) is set to zero,

RV =

WW

WCM

[1

(1-P)

SC

+ P*RR

SFA

] 6

RV = R

W * CF 7

CF = 1

[(1-P

SC

) + P*RR

SFA

]

8

13

then RW and RV become W/C by weight and W/C by volume, respectively

for mixes containing fly ash. For mixes containing no fly ash, both

P and RR will be set to zero to obtain the desired water to cement

ratios.

The general trend of the results remains the same as described

above for the weight ratio (Rw) when water to cementitious ratio is

expressed by volume (RV). However, the strength values will be

represented at a higher water to cementitious ratio by volume relative

to water to cementitious ratio by weight as determined by the factor

CF. The CF factor is a function of amount of cement replacement by

fly ash, replacement ratio (RR), specific gravity of cement and fly

ash. The computed values of CF is shown in Table 6. Compressive

strength data for mixes tested are plotted as a function of water

to cementitious ratio on both weight and volume basis (Figure 1 through

7).

CONCLUSIONS

(1) Addition of ASTM Class F fly ashes caused reduction in compressive

strength of cement mortars within the tested range of variables.

(2)Mortar made with mixes with 20% cement replacement by Class C fly

ash cement showed excellent results amongst all the mixes tested

in this investigation.

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(3)The optimum water to cementitious material ratio by weight varied

in the range of 0.35 - 0.6 for all mortar mixes tested.

REFERENCES

1.Berry, E.E., and Malhotra, V.M., "Fly Ash for Use in Concrete -

A Critical Review", ACI Journal, Vol. 77, No. 2, March/April,

1980, pp. 59-73.

2.Naik, T.R., and Ramme, B.W., "High-Strength Concrete Containing

Large Quantities of Fly Ash", ACI Materials Journal, Vol. 86,

No. 2, March-April 1989, pp 111-116.

3.Berry, E.E., Hemmings, R.T., Langley, W.S., and G.G. Carette,

"Beneficiated Fly Ash: Hydration, Microstructures, and Strength

Development in Portland Cement Systems", in "Fly Ash, Silica

Fume, Slag, and Natural Pozzolans in Concrete", V.M. Malhotra,

Ed., proceedings of the Third International Conference,

Trandheim, Norway, June 1989, pp 241-273.

15

4.Minnick, L.J., Webster, W.C., and Purdy, E.J., "Predictions of the

Effects of Fly Ash in Portland Cement Mortar and Concrete", ASTM

J. of Materials, Vol. 6, No.1, 1971, pp. 163-187.

5.Helmuth, R.A., "Water-Reducing Properties of Fly Ash in Cement

Pastes, Mortars, and Concretes: Causes and Test Methods", in

"Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete",

V.M. Malhotra, Ed., proceeding of the Second International

Conference, Madrid, Spain, Sp-91, Vol. I, April 1986, pp 723-740.

6.Berry, E.E., "Strength Development of Some Blended-Cement

Mortars", International Journal of Cement and Concrete

Research, Vol. 10, No. 1, 1990, pp. 1-11.

7.Naik, T.R. and Ramme, B.W., "Effects of High-Lime fly Ash Content

on Water Demand, Workability, Time of Set and Compressive

Strength of Concrete", Presented at the Third International

Conference on the Use of Fly Ash, Silica Fume, Slag and Natural

Pozzolan's, Trondheim, Norway, 1989.

8.Lin, C.Y., and Hwang, C.L., "The Effect of Fly Ash on Properties

of Cement Mortar" in "Fly Ash, Silica Fume, Slag and Natural

Pozzolans in Concrete", V.M. Malhotra, Ed., proceedings of the

Second International Conference, Madrid, Spain, April 1986,

pp 21-1 - 21-32.

16

9.Douglas, E., Elola, A., and Malhotra, V.M., "Characterization of

Ground Granulated Blast-Furnace Slag and Ashes and Their

Hydration in Portland Cement Blends", Cement, ASTM Journal of

Concrete and Aggregates, Summer, 1990, pp. 38-46.

10. Douglas, E., Huyssteen, E.V., and Malhotra, V.M., "Sulphate

Resistance of Mortars Made with High Volumes of Class F Fly Ash

or Granulated Blast-Furnace Slag - Progress Report", to be

Published in the proceedings of the Second International

Conference on Durability of Concrete, Montreal, Quebec, Canada,

Aug. 1991.

17

Table 1 - Description of Mix Proportions

════════════════════════════════

Mix A - no fly ash

Mix B - 20% Type C fly ash

Mix C - 20% Type F fly ash

Mix D - 40% Type F fly ash

════════════════════════════════

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Table 2 -Compressive Strength of Concrete as a Function of Water to

Cementitious Ratio for Mix A (no fly ash)

Water/ Cementitious

Ratio

C O M P R E S S I V E S T R E N G T H, p s i +

7 days 28 days

Mean S.D. C.V.* Mean S.D. C.V.*

0.27 490 25 5.1 750 13 1.8

0.32 1347 88.1 6.50 1743 38.0 2.2

0.37 2200 25 1.1 3295 243 7.4

0.47 3278 84 2.6 4317 63 1.5

0.57 3482 78 2.2 4742 227 4.8

0.67 2627 153 5.8 4008 76 1.9

0.77 2403 102 4.2 3650 161 4.4

0.87 1550 41 2.6 2613 135 5.2

0.97 1548 54 3.5 2478 28 1.1

1.22 905 79 8.7 1823 18 1.0

1.47 696 48 6.9 1868 338 18.1

1.72 540 30 5.6 1443 306 21.2

1.97 362 63 17.5 847 86 10.2

2.47 213 3 1.4 535 113 21.1

2.97 235 68 28.8 463 63 13.6

3.97 336 12 3.7 393 106 27.1

4.97 ** ** ** 451 85 18.8

+ 1000 psi = 6.895 MPa

* C V. = Coefficient of Variation S.D. = Standard Deviation

*C.V.=S.D.

Meanx100

19

** Cubes could not be made to an appropriate dimension.

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Table 3 -Compressive Strength of Concrete as a Function of Water to

Cementitious Ratio for Mix B (20% Class C Fly Ash)

Water/ Cementitious

Ratio

C O M P R E S S I V E S T R E N G T H, p s i +

7 days 28 days

Mean S.D.* C.V.* Mean S.D.* C.V.*

0.27 972 70 7.2 1150 61 5.3

0.32 3538 278 7.9 4408 118 2.7

0.37 4358 177 4.1 5283 101 1.9

0.47 3707 44 1.2 4867 76 1.6

0.57 3142 110 3.5 4675 43 0.9

0.67 2605 35 1.3 4162 88 2.1

0.78 1788 79 4.4 2872 109 3.8

+ 1,000 psi = 6.895 MPa

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Table 4 -Compressive Strength of Concrete as a Function of Water to Cementitious Ratio for Mix C (20% Class F Fly Ash)

Water/ Cementitious

Ratio

C O M P R E S S I V E S T R E N G T H, p s i +

7 days 28 days

Mean S.D.* C.V.* Mean S.D.* C.V.*

0.27 392 18 4.5 475 18 3.7

0.32 973 113 11.6 1063 55 5.2

0.37 1953 80 4.1 2462 46 1.9

0.47 2664 29 1.1 3348 46 1.4

0.57 2922 29 1.0 4010 49 1.2

0.67 1973 85 4.3 2880 123 4.3

0.77 1676 30 0.17 2388 124 5.2

0.87 1125 90 8.0 2194 70 3.2

0.97 1250 78 6.3 1647 340 20.7

1.22 684 35 5.2 1326 75 5.7

1.47 531 60 11.3 930 92 9.9

1.72 417.5 9 2.1 767 25 3.3

1.97 248 32 12.8 556 9 1.6

2.47 199 27 13.8 357 48 13.4

2.97 149 17 11.4 252 36 14.3

3.97 105 16 14.8 251 49 19.5

4.97 53 21 40.1 78 8 9.8

+ 1,000 psi = 6.895 MPa

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Table 5 -Compressive Strength of Concrete as a Function of Water to Cementitious Ratio for Mix D (40% Class F Fly Ash)

Water/ Cementitious

Ratio

C O M P R E S S I V E S T R E N G T H, p s i +

7 days 28 days

Mean S.D.* C.V.* Mean S.D.* C.V.*

0.27 414 19 4.5 573 20 3.6

0.32 703 47 6.7 1045 42 4.1

0.37 1767 263 14.9 2699 119 4.4

0.47 1955 61 3.1 3420 90 2.6

0.57 1819 8 0.42 3047 63 2.1

0.67 1417 33 2.3 1972 97 4.9

0.77 1271 31 2.5 2017 43 2.1

0.87 1004 66 6.5 1413 74 5.3

0.97 800 48 6.0 1443 150 10.4

1.22 598 31 5.1 973 13 1.3

1.47 293 4 1.2 588 79 13.5

1.73 313 25 7.9 560 13 2.4

1.97 280 31 11.2 508 93 18.2

2.47 103 3 2.8 278 8 2.7

2.97 95 15 15.8 312 76 24.5

3.97 83 4 4.3 205 91 44.5

4.97 48 4 7.4 123 15 11.9

+ 1,000 psi = 6.895 MPa

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Table 6 - Conversion Factor (CF) for Converting the Water to Cementitious Materials Ratio by Weight to Water to Cementitious Materials Ratio by Volume.

Conversion Factor (CF)*

Mix No. W/C by Volume W/C + F by Volume

A 3.15 -

B 1.25 3.00

C 1.25 2.94

D 1.67 2.76

*The specific gravity of cement, Class C fly ash, and Class F fly ash was taken as 3.15, 2.52, and 2.32, respectively.

REP-101