Indian Journal of Engineering & Materials Sciences Vol. 6, October 1999, pp. 274-278

Effect of silica fume on the slag cement containing wet cement dust

H EI-Didarnony"*, S A Abo-EI-Enein,bA H Alic & T M EL-Sokkar/

"Faculty of Science, Zagazig University, Zagazig, Egypt bFaculty of Science, Ain Shams University, Cairo, Egypt

cHousing and Building Research Centre, Dokki , Cairo, Egypt

Received 5 November 1997; accepted / JUli e 1999

The present work aims to study the effect of various amounts of silica fume on the physico-mechanical propcrties of slag cement with 5% raw cement dust. Slag cement is substituted by 4, 8 and 12% silica fume. The results reveal that the increase of silica fume tends to increase the water of consistency. On the other side, 4% silica fume decreases the sctting time (initial and final) . 8 and 12% silica fume tend to lengthen the setting time due to the increase of mi xing water. Thc free lime content increases up to 7 days then decreases for all mixes. As the amount of silica fume increases the free limc contcnl decreases. Also, the combined water content increases with the amount of silica fume. The compressive strength of ccmcnt paste decreases with silica fume due to the increase of mixing water.

Blended cements contain additive rather than that used in portland cement which have considerable scientific and technological interest, because such addition increases the chemical resistance to sulphate attack, impermeability, lowering heat of hydration and thermal expansion of the hydrated cement pastes. There are strong environmental and energy reasons for developing a wide range of hydraulic binders consisting of portland cement clinker and pozzolanic admixtures.

Condensed silica fume is a by-product of the manufacture of silicon or silicon alloys which are produced in the submerged-arc electric furnaces. A series of complex chemical reactions occur in the furnace. Condensed silica fume (Si02) particles appear to be formed by the oxidation and condensation of the gaseous silicon suboxide, SiO, which is formed in the reaction zone.

The kiln dust represents a mixture of cement raw mix as well as calcined mix, and clinker. The chemical composition of the dust also influenced by the size of particles carried away by the kiln gases. The exit dust shows a considerable concentration of alkalies volatilized in the burning zone and subsequently condensed on the kiln dust particles. In fact, approximately 12% of the kiln feed exists from the kiln with the gas (up steam of any air pollution control equipment). Throughout the industry, roughly 73% of the kiln dust is recycled to the cement-making

*For correspondance

processes I . Granulated blast-furnace slag by itself I ~

hydraulically very weak, due to its glassy structure, a highly alkaline medium is required in order to disintegrate the silicate aluminate network of the slag glass. Portland cement clinker is normally used to provide this alkalinit/.

It was reported' that the addition o f cement kiln dust to either ordinary portland or s lag cement certainly adversely affects the phys ico-mechanical properties of cement paste. The magnitude of thi s effect is generally proportional to the amount of cement dust which must not exceed rhan 5-8 % of portland cement.

The pozzolanic activity of silica fume was studied 4.

The hydration characteristics were fo llowed using DTA and the quantitative determination of free lime, free silica and combined water contents. The results showed that the activity of silica fume increases with lime content. At higher lime content the hydrati on products are characterized by a high CaO/Si02 rati o and the rate of reaction decreases.

EI-Didamony et al.5 found that the add ition of 5% cement dust calcined at 1000°C for 2 h can be used with 10% silica fume, 80% portland ce ment clinker and 5% gypsum. Al so, the substitution of granul ated slag in portland slag cement (35 % granulated slag) by calcined cement dust activates the hydration of cement paste. It can be said that the calcined cement dust is more hydraulic than granulated slag.

The by-products such as silica fume and granulated slag were used for the preparation of blended

EL-DIDAMONY et ai.: EFFECT OF SILICA FUME ON THE SLAG CEMENT CONTAINING WET CEMENT DUST 275

cements6. The results revealed that the addition of 5 to

10% silica fume improves the physico-mechanical properties of ordinary portland as well as granulated slag cement pastes. This is mainly due to the high pozzolanic activity of silica fume.

Amin et aL. 7 studied the effect of granulated slag with calcined dust In which the calcination temperature as well as the amount of cement dust on the activation of slag were studied. It was found that the activation increases with the amount of the cement dust and the calcination temperature up to l000°C.

The effect of silica fume on the hydration characteristics of portland slag cement has been studied8

• The results indicated that the free lime content decreases with the silica fume and curing time up to 90 days. The DT A results are in a good agreement with those of the chemical analysis. Moreover, the combined water content increases with the silica fume while decreases with the slag content.

The incorporation of silica fume in portland cement paste contributes the hydration process by pozzolanic reaction with liberated lime to form stable CSH and also by reaction with alkali and Ca ions. The pozzolanic activity of silica fume in cement pastes has been demonstrated by measuring the amount of Ca(OHh at different times with varying dosages of silica fume. The results showed high pozzolanic activity9.11. The aim of the present investigation is to study the effect of varying amounts of silica fume on the properties of slag cement in the presence of 5% cement kiln dust.

Experimental Procedure The materials used in this work were granulated

slag cement, cement kiln dust (wet process) as well as silica fume. The chemical oxide composition of each starting material is given in Table 1. The surface area of the slag cement was 3528 cm2g- l

. Raw cement dust from wet process and silica fume were mixed with slag cement. The ingredients of the different mixes in mass proportions were homogenized in a ball mill for an hour to assure complete homogeneity and are given in Table 2.

A weighed amount of cement was placed on a smooth, non- absorbent surface, and a crater was formed in the center. The water of consistency'2 was poured into the crater by the aid of a trowel, and troweled to absorb the water for about one minute. The mixing was then completed by vigorous mixing for three minutes by means of trowel and the setting time was measured. The paste was moulded in 2x 2x2 cm3 moulds, the surface of the paste was made smooth by the aid of thin edged trowel. Immediately after moulding, specimens were cured in (100% R.H .) at room temperature 23± I DC for 24 h then demoulded and cured in tap-water up to 3, 7, 28 and 90 days for further investigations.

Density (dp) was carried out before the specimens were subjected to the compressive strength determination . Samples were weighed suspended in water and in air (saturated surface dry). Each measurement was conducted on at least three similar cubes of the same blend and age. The density was calculated using the following formula:

B Ik d· Saturated weight (/ .1 )

U enslty = g cm Volume of sample

Bulk density = Saturated weight / .1 )

----------------~~--------(gcm Saturated weight - Suspended weight

In each case, this test was carried out on three samples following the procedure described by (ASTM, 1993). Three cubes from each cement paste was prepared with a certain W/C ratio and cured for a prescribed period. Compressive strength measurements were carried out using WYKEHAM FARRANCE ENG. The hydration of the cement paste was stopped by grinding in acetone-methanol mixture

Table 2-Mix composition of the investigated mixes

No. of mixes Slag cement Wet cement Silica fume dust

MO 95 5 ()

M4 91 5 4 M8 87 5 R MI2 83 5 12

Table I--Chemical oxide composition of starting materials, wt %

Oxides % Si02 AI 2O, Fe20, CaO MgO SO, L.O.I Na20 K10 CI- Free lime

Materials

Slag cement 21.81 6.18 3.25 60.24 2.14 2.43 2.39 0.53 0.11 0.02 2.70 Cement kiln 15.18 , 4.20 2.48 58.69 1.03 1.53 15.97 0. 11 0.83 0.86 3.90 Dust (wet. p) Silica fume 96.10 0.52 0.70 0.21 0.48 0.10 1.14 0.31 0.49 0.00 0.43

276 INDIAN J. ENG. MATER. SCI., OCTOBER 1999

as described elsewhere 13. The free lime was determined by using ammonium acetate method l4

.

The combined water content was estimated by igniting the hydrated cement paste at I DOO°C for I h.

Results and Discussion To ~how the effect of silica fume, on the properties

of blended cement in the presence of wet cement dust, different mixes were prepared with slag cement pastes and various proportions of silica fume at 5% wet cement dust. The water of consistency and setting times of the blended cement pastes made of 0, 4, 8 and 12% silica fume and slag cement at 5% wet cement dust are shown in Fig. I. The water of consistency of the cement pastes containing silica fume is higher than those without silica fume. As the silica fume replacement increases, the water of consistency increases due to the relatively higher surface area of silica fume II. The initial and final setting times of the blended cement pastes made with silica fume are shorter than those made without silica fume. The addition of 4% silica fume decreases the setting time (initial and final) due to the activation and consumption of liberated Ca(OHh of cement paste. The silica fume reacts with the liberated Ca(OHh, therefore, the setting times are decreased at 4% silica fume . Higher values of silica fume, such as 8% and 12% tend to lengthen the setting time. This is mainly

35

lit

[; 33 c ! en en c ~ . 31

'0

27

~ W.cons.

-+- Inll. •• I.

-*- Fln.aet .

- 245

195

145

25 L-------~---------~------~ 95

o 4 B 12

Silica fume content , %

c E

ai E -'" c -C1> ., 0; c: -

o(l

<11

c:

Fig. I-Water of consistency, initi al and fin al sett ing times of slag cement pastes made with various proportions of silica fume at 5% wet cement dust.

attributed to the effect of higher values of mlxmg water which affect the total porosity and then the setting time. As the water/cement ratio increases the setting time (either initial or final ) is lengthened due to the presence of larger pores in the cement paste.

The influence of silica fume addition all the free lime contents of the blended cement pastes made of 0, 4, 8 and 12% silica fume and slag cement at 5% wet cement dust is plotted in Fig. 2.The free lime content of the blended. cement pastes increases up to 7 days followed by gradual decrease up to 90 days. The rate of Ca(OHh liberation during the hydration is hi gher than its consumption by silica fume forming additional amounts of CSH. At later ages of hydration the rate of liberation of Ca(OH)2 is slower than its consumption, therefore, the free Ca(OH)l decreases. Also as the amount of silica fume increases, the free lime content decreases. Generally, the blended cement pastes containing silica fume possess lower values of free lime than those of free silica fu me blended cement pastes.

The combined water contents of the blended cement pastes made of 0, 4, 8 and 12% silica fume and slag cement at 5% wet cement du st are graphically plotted in Fig. 3. Obviously, the combined water content increases gradually with curing time for all cement pastes. This is due to the continuous hydration and accumulation of hydrat ion products

10 r------------

lit ; c ! c 0 o · Q) 6 E

C1>

~ U.

4

2 L-____ ~ __ _'____1....__L .. I I ~ _ ___L _ __'

3 3 0 90

Curing time, days

Fig. 2--Free lime contents of slag cement pastes made wi th various proportions of silica fume at 5% wet ce ment · dust with curing time.

EL-DIDAMONY et al.: EFFECT OF SILICA FUME ON THE SLAG CEMENT CONTAINING WET CEMENT DUST 277

24r.=====~----------------~

~ 22 -e ! e . o ~ 20 CD a; ~

'0 ~ 18 .0 E o (,)

. >- 16 / -

Iii .2 E Q)

t3 14

-+--+--e-

12L------L---L--L-~~~~ ______ _L __ ~

3 30 90

Curing time, days

Fig. 3--Combined water contents of slag cement pastes made with various proportions of silica fume at 5% wet cement dust with curing time.

t')

E (,) ..... co

>-... " e CD '0

~ ::l

,III

~

2.2

2. 1

2

1.9 3

~

~

~

1.40 -+- 1.4 4 -+ 1.48 -e- 1.412

30 90

Curing tirAe, days

Fig. 4--Bulk density of slag cement pastes made with various proportions of silica fume at 5% wei cement kiln dust as a function of curing time.

1200,----------------------------,

C\I

E 1000 0 ..... Cl

"" .1:. ' -CI e ~ 800 ... ., Q) >

OJ ., ~

' 0. E 0 (J ~

1.40 -+- 1.4 4 -+- 1.48 -e- 1.412

3 30 90

Curing timli! , days

Fig. 5-<::ompressive strength of slag cement pastes made with various 'proportions of silica fume at 5% wet cement du st with curing time.

mainly as calcium silicate and sulphoaluminate hydrates. It was also found that as the amount of silica fume increases, the combined water also increases. This is due to the fact that the addition of si lica fume activates the hydration of blended cement. The presence of a larger amount of silica fume leads to activate the hydration of cement pastes which is due to the consumption of free Ca(OHh ,depositi on of C­S-H and an increase of combined water content.

The effect of silica fume add iti on on the bulk density of the hardened blended cement pastes made of 0,4,8 and 12% silica fume and slag cement at 5% wet cement kiln dust is plotted in Fig. 4. Evidently, the values of bulk density of the hardened cement pastes made with silica fume increase with curing time up to 90 days. On the other side, with the increase of silica fume content, the va lues of bulk density decrease below that of the blended cement paste made without silica fume. Thi s is due to the fact that the water of consistency of blended cement pastes containing si lica fume was higher than that of blended cement paste made without si lica fume. Therefore, the total porosity increases, meanwhile, the bulk density decreases . The hardened blended cement paste made without silica fume has the lowest amount of water of

278 INDIAN 1. ENG. MATER. SCI. , OCTOBER 1999

consistency, so that it is expected to possess higher values of bulk density.

The influence of silica fume on the compressive strength of the blended cement pastes made of 0, 4, 8 and 12% silica fume and slag cement at 5% wet cement dust is plotted in Fig. 5. The values of compressive strength decrease with the increase of the amount of silica fume in the blended cement pastes. The water of consistency increases, with the amount of silica fume therefore, the free water as well as the total porosity are increased and the compressive strength decreased. It can be concluded that wet cement dust can be used as an activator for slag cement hydration. It can be concluded that in blended cements using silica fume in addition to cement kiln dust, a superplasticizer must be used to reduce the mixing water which affects the physico-mechanical properties such as water of consistency, setting time, porosity and finally compressive strength.

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