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Effect of fly ash and silica fume on hardened properties of foam concrete

Article in Construction and Building Materials · January 2019

DOI: 10.1016/j.conbuildmat.2018.11.036

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 JCBM 14341 No. of Pages 11, Model 5G
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Construction and Building Materials xxx (2018) xxx–xxx

1

Contents lists available at ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier.com/locate/conbuildmat

4
5

3 Effect of fly ash and silica fume on hardened properties of foam concrete
6 H. Süleyman Gökçe a,⇑, Daniel Hatungimana b, Kambiz Ramyar b
7 a
Bayburt University, Engineering Faculty, Civil Engineering Department, Bayburt, Turkey
8 b _
Ege University, Engineering Faculty, Civil Engineering Department, Izmir, Turkey

9
10

1 2
h i g h l i g h t s
13
14
Foam concrete has been an essential construction material for developing world.
15
Mineral admixtures can have remarkable effects on the properties of such products.
16
More satisfactory products is achieved by the introduction of silica fume rather than fly ash.
17
Ternary binder systems are recommended for products having high strength to density ratio.

18
a r t i c l e i n f o a b s t r a c t
2 4
3 0
21 Article history: In this study, the effects of foam content as well as fly ash and silica fume inclusion on some physical and 35
22 Received 12 June 2018 mechanical properties of foam concrete, subjected to various curing regimes, were researched. Totally, 36
23 Received in revised form 24 October 2018 forty-five series of foam concrete mixtures were prepared at three replacement levels of fly ash and silica 37
24 Accepted 2 November 2018
fume (0, 10 and 20%, by weight of cement), at three foam contents (0, 31 and 47%, by volume) and at three 38
25 Available online xxxx
curing regimes (7- and 28-day standard water curing and autoclave curing). Density, water absorption, 39
compressive strength and thermal conductivity values of the mixtures were determined on prismatic 40
26 Keywords:
specimens with dimensions of 40  40  160 mm. The density (oven dry), water absorption, 41
27 Foam concrete
28 Density
compressive strength and thermal conductivity results of the foam concrete were found to be between 42
29 Water absorption 873–1998 kg/m3, 3.5–35.9%, 1.5–88.1 MPa and, 0.239–0.942 W/m.K, respectively. A balanced benefit with 43
30 Compressive strength regard to both compressive strength and water absorption of foam concrete was found in the mix with 44
31 Thermal conductivity density of 1320 kg/m3. Particularly in the mixtures having high foam content, silica fume introduction 45
32 Fly ash and silica fume resulted in superior compressive strength values and greater compressive strength/thermal conductivity 46
33
ratios than fly ash introduction. 47
Ó 2018 Elsevier Ltd. All rights reserved. 48
49

50
51
52 1. Introduction the same air-void sizes but different porosity [5]. However, ensur- 65
ing an optimal air-void system in foamed concrete is essential to 66
53 Foamed concrete, conventionally having a plastic density of produce a material with a high strength/weight ratio. For the same 67
54 500–1600 kg/m3, is widely used for sustainable construction tech- water/binder (w/b) ratio, the density of foamed concrete can be 68
55 nologies [1]. Density of the cellular (foamed) concrete can be con- varied through the incorporation of different amounts of foam, 69
56 trolled by adding a calculated amount of proper foam into the which results in different air-void systems (i.e. air content, air- 70
57 slurry of water and cement, with and without addition of aggregate void size, air-void frequency and spacing factor) [6]. Any change 71
58 [2]. in the micro/macro structure of foamed concrete due to variations 72
59 Reduction in the density by adding foam is accompanied by the in the air-void system may significantly affect the mechanical 73
60 reduction of strength and thermal conductivity [2]. Air voids, properties as well as density [7]. Moreover, variability of the com- 74
61 which govern the porosity of foamed concrete, are generally con- pressive strength can be observed by the nature of foaming agent 75
62 sidered to have a significant effect on the compressive strength such as protein-based and synthetic-based due to their effect on 76
63 of the concrete [3,4]. It is possible to produce concrete mixtures stability of foam [8]. 77
64 of different air-void sizes having the same porosity and, conversely, Kearsley and Wainwright [9] established comprehensive rela- 78
tionships between porosity, density and compressive strength of 79
foam concrete containing different amounts of fly ashes having 80
⇑ Corresponding author. two Blaine fineness values (280 and 350 cm2/g). It was found that 81
E-mail address: suleymangokce@bayburt.edu.tr (H.S. Gökçe).

https://doi.org/10.1016/j.conbuildmat.2018.11.036
0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

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Table 1
Properties of cement, FA and SF.

Property Cement FA SF
Chemical SiO2 19.72 47.07 78.82
Al2O3 5.31 11.56 0.00
Fe2O3 3.37 7.22 0.98
CaO 62.33 15.94 2.35
MgO 2.33 7.77 6.41
SO3 3.33 2.78 1.21
Na2O 0.53 1.59 1.68
K2O 0.77 3.04 3.28
Loss on ignition 2.09 0.42 6.33
Physical Fineness cm2/g 3960 3050 200,000
Specific gravity 3.08 2.44 2.37

Table 2
Mix proportions of foam concrete.

Mixture Cement kg FA kg SF kg Water kg w/b ratio SP kg Flow mm Foam % (by vol.)

Ref-0 1600 0 0 480 0.3 0 195 0.0
Ref-I 1098 0 0 329 0.3 0 195 31.4
Ref-II 846 0 0 254 0.3 0 195 47.1
10FA-0 1440 160 0 480 0.3 0 200 0.0
10FA-I 988 110 0 329 0.3 0 200 31.4
10FA-II 762 85 0 254 0.3 0 200 47.1
20FA-0 1280 320 0 480 0.3 0 190 0.0
20FA-I 878 220 0 329 0.3 0 190 31.4
20FA-II 677 169 0 254 0.3 0 190 47.1
10SF-0 1440 0 160 480 0.3 1 180 0.0
10SF-I 988 0 110 329 0.3 1 180 31.4
10SF-II 762 0 85 254 0.3 1 180 47.1
20SF-0 1280 0 320 480 0.3 5 180 0.0
20SF-I 878 0 220 329 0.3 5 180 31.4
20SF-II 677 0 169 254 0.3 5 180 47.1

2400
7-day 28-day Autoclave
2000

1600
Density, kg/m3

1200

800

400

Mix ID

Fig. 1. Density of foamed concrete mixtures.

82 in the mixtures having low foam content finer fly ash resulted in a Understanding the effect of mineral admixtures having differ- 91
83 higher porosity and consequently a lower density. However, the ent fineness and characteristics on the major properties of the 92
84 effect of fly ash fineness diminished in the mixtures having high foam concrete is being essential in its design at various foam con- 93
85 foam content. Kunhanandan Nambiar and Ramamurthy [10] tents. A number of studies have been performed on the relation 94
86 showed that there is a significant effect of filler type and its fine- between hardened properties of foam concrete and on the effect 95
87 ness on the density, water absorption and compressive strength of the foam content, type of foaming agent, curing regime or filler 96
88 of foam concrete. Bing et al. [11] produced high-strength foam con- type and amount on some of the properties of foam concrete such 97
89 crete with foam content ranging from 20 to 50%, by means of fly as density, water absorption, compressive strength, air-void char- 98
90 ash and silica fume inclusion. acteristics or strength to density ratio of foam concrete up to 99
now [9,10,12–19]. However, multi-superior characteristics that 100

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101 are opposite to each other are still needed to be researched in foam thermal conductivity ratio) were evaluated for the optimization of 114
102 concrete produced with various mix design parameters. For exam- foam concrete mixtures. 115
103 ple, high compressive strength with less water absorption and less
104 density value, or high compressive strength to thermal conductiv-
105 ity ratio. 2. Materials and method 116

106 In this study, the effect of foam content (0.0, 31.4 and 47.1%, by
107 volume), and partial (0, 10 wt% and 20 wt%) replacement of cement 2.1. Materials 117

108 with fly ash (FA) and silica fume (SF) on the density, water absorp-
109 tion, compressive strength and thermal conductivity of foam con- In this study, a CEM I 42.5 R type Portland cement, a high-lime 118

110 crete was researched after 7-day and 28-day standard curing as FA and a SF were used as binder materials. The foam was prepared 119

111 well as autoclave curing. Moreover, abovementioned multi- in laboratory by using a synthetic-based foaming agent (amber 120

112 superior characteristics (high compressive strength with less water color) in accordance with ASTM C 869 Standard [20]. Besides, a 121

113 absorption and less density value, or high compressive strength to polycarboxylate ether-based superplasticizer (SP) was used to pro- 122
vide the required consistency in paste mixtures before foam intro- 123

200 Ref 200 Ref 200 Ref

10% FA 10% FA 10% FA
20% FA 20% FA 20% FA
Relaitive density, %

Relaitive density, %
10% SF 10% SF 10% SF
Relaitive density, %

150 150 150

20% SF 20% SF 20% SF
7-day 28-day Autoclave
100 100 100

50 50 50

0 0 0
0.0 31.4 47.1 0.0 31.4 47.1 0.0 31.4 47.1
a Foam content, %
b Foam content, %
c Foam content, %

200 200 200

0.0% 0.0% 0.0%
31.4% 31.4% 31.4%
47.1% 47.1% 47.1%
Relaitive density, %

Relaitive density, %
Relaitive density, %

150 7-day 150 28-day 150 Autoclave

100 100 100

50 50 50

0 0 0
0 10 20 0 10 20 0 10 20
d FA replacement, %
e FA replacement, %
f FA replacement, %

200 200 200

Relaitive density, %
Relaitive density, %
Relaitive density, %

150 150 150

100 100 100

50 7-day 50 28-day 50 Autoclave

0.0% 0.0% 0.0%
31.4% 31.4% 31.4%
47.1% 47.1% 47.1%
0 0 0
0 10 20 0 10 20 0 10 20
g SF replacement, %
h SF replacement, %
i SF replacement, %

Fig. 2. Effect of foam, FA and SF contents on the density of foam concrete cured in standard condition or in autoclave.

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124 duction. Properties of the cement and mineral admixtures are pre- 3. Results and discussion 168
125 sented in Table 1.
3.1. Density 169
126 2.2. Methods and mix proportions
Density values of the foam concrete specimens are given in 170
127 The substitution levels of FA and SF were 0, 10 and 20% by Fig. 1. The density values were found to be in the range of 873– 171
128 weight of cement. There is no a particular target density in the 1998 kg/m3. The reduction in mix density upon foam incorporation 172
129 design of the mixtures. Thus, the density values obtained by the in SF-bearing mixtures was lower than that of the control mixtures 173
130 designed mix proportions were only assessed in this study. The and FA-containing mixtures. Kunhanandan Nambiar and Rama- 174
131 flow values of the mixtures were determined according to EN murthy [10,12] showed that the type of filler material such as fly 175
132 1015-3 Standard [21]. In order to prevent the adverse effect of SP ash or fine sand may affect the density of the mixture. Owing to 176
133 on foam formation, the flow value of blended cement pastes prior its finer particles, fly ash resulted in a more uniform distribution 177
134 to foam inclusion were adjusted in the range of 190 ± 10 mm by of air voids by providing a well and uniform coating on each air 178
135 the addition of appropriate amounts of SP. Then, 0, 31.4 and bubble and preventing merging of the bubbles [13]. 179
136 47.1% by volume of the mix foam was added. After determination The effect of foam, FA and SF contents of the mixtures, cured in 180
137 of unit weight of the foam, calculated weights of the foam were different regimes, on their relative density are given in Fig. 2. The 181
138 added into constant volume of ordinary/blended cement paste in relative density of FA-bearing mixtures was found to be closer to 182
139 a container in order to theoretically ensure above mentioned foam that of the control mixtures, compared to the relative density of 183
140 contents for the fresh state of the mixtures. The foamed pastes SF-bearing mixtures. As it can be seen from Fig. 2g and h, the addi- 184
141 were mixed for about 3 min to obtain a visual homogeneity. The tion of SF in the mixtures containing no foam had a negligible 185
142 mix proportions and designation of mixtures are given in Table 2. effect on the relative density. However, in the mixtures containing 186
143 The mixtures were poured into the 40  40  160 mm pris- foam, an increase (up to 55%) in the relative density was occurred. 187
144 matic moulds by slight jolting, and then kept at 20 ± 2 °C at Besides, the curing at 100% relative humidity was found to be 188
145 90% relative humidity for 24 h. Different curing regimes were somewhat effective on the density of mixtures depending on the 189
146 applied to the specimens after demoulding. All of the specimens introduction of siliceous materials from FA and SF and foam con- 190
147 were found to be visually stable according to the assessments tent. Falliano et al. [8] reported that different relative humidity- 191
148 given by Jones et al. [1]. The specimens were exposed to 7- and curing regimes caused further variations with higher standard 192
149 28-day standard water curing, and autoclave curing. In standard deviations in compressive strength of limestone blended cement 193
150 water curing, specimens were kept at 20 ± 2 °C in water up to test- (CEM II A-L)-foam concrete produced with synthetic-based foam- 194
151 ing time. In autoclave curing regime, the specimens were subjected ing agent rather than its density results, especially when the den- 195
152 to high pressure steam curing at 200 °C and 1.4 MPa pressure for sity range increased. Kearsley and Wainwright [9] studied the foam 196
153 3 h. The autoclave reached to 200 °C in 2 h. concrete mixtures having densities in the range of 773–1751 kg/ 197
154 After 7- and 28-day standard water curing and autoclave curing, m3. It was concluded that the fineness of filler material affected 198
155 density (oven dry) and water absorption of the mixtures were the density of the foam concrete. However, the effect of SF that is 199
156 determined according to EN 12390-7 Standard [22] and Eq.1, extremely finer than FA on the density is contradictory due to com- 200
157 respectively. Compressive strength test was performed on the plexity of the air-void system arisen from either foam and SF con- 201
158 40 mm modified cubes obtained from the broken portions of the tent of the mix. It was stated that large macropores in the air-void 202
159 prismatic specimens according to EN 196–1 Standard [23]. The system may in turn affect the density and other properties such as 203
160 thermal conductivity of autoclaved specimens was determined water absorption, strength and thermal conductivity [7,13,25]. 204
161 by using hot-wire test method in accordance with EN 993-15 Stan-
162 dard [24].
163
3.2. Water absorption 205
165 wð%Þ ¼ ½ðSSD  ODÞ=OD  100 ð1Þ
166 w: water absorption, SSD: saturated surface dry weight, OD: Water absorption values of the foam concrete specimens are 206
167 oven dry weight. given in Fig. 3. The water absorption values were found to be 207

40
7 day 28 day Autoclave
35

30
Water absorption, %

25

20

15

10

Mix ID

Fig. 3. Water absorption values, %.

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208 between 3.5% and 35.9%. The significant effect of duration of water absorption values of foam concrete, SF significantly reduced 224
209 water-curing as well as curing regime on the water absorption of these values, irrespective of the curing regime. Ramamurthy et al. 225
210 foam concrete mixtures is clear from Fig. 3. The water absorption [26] stated that reporting the water absorption of the foam con- 226
211 of foam concrete is reported to be influenced by the paste phase crete as a percentage by mass may result in misleading due to large 227
212 but not by all of the artificial pores, since they are not intercon- variation of density of foam concrete mixtures. 228
213 nected [10,14]. Thus, reductions in water absorption values of
214 the mixtures containing mineral admixture may be resulted from 3.3. Compressive strength 229
215 the enhancement effect of the mineral admixture on the micro-
216 structure of the paste phase, especially in SF bearing series. Compressive strength values of the foam concrete specimens 230
217 The effects of foam content (Fig. 4a–c), FA content (Fig. 4d–f) are given in Fig. 5 The compressive strength values were found 231
218 and SF content (Fig. 4g–i) incorporation on the relative water to be between 1.5 and 88.1 MPa, depending on the mix proportions 232
219 absorption values of foam concrete subjected to different curing and the applied curing regime. Except for mixtures without foam, 233
220 regimes are presented in Fig. 4. The relative water absorption of the compressive strength results reached up to 23.3 MPa for refer- 234
221 the mixtures containing no foam was found to be lower (up to ence foam concrete mixtures without FA and SF, 28.4 MPa for FA- 235
222 4.2 times) than those of mixtures containing 47.1% foam. Although bearing foam concrete mixtures, and 63.8 MPa for SF-bearing foam 236
223 FA caused fluctuating (increment and reduction) effect on the concrete mixtures. Authors of this study think that foam can work 237

500 Ref 500 Ref 500 Ref

10% FA 10% FA 10% FA
Relaitive water absorption, %

Relaitive water absorption, %

Relaitive water absorption, %

20% FA 20% FA 20% FA

400 10% SF 400 10% SF 400 10% SF
20% SF 20% SF 20% SF
7-day 28-day Autoclave
300 300 300

200 200 200

100 100 100

0 0 0
0.0 31.4 47.1 0.0 31.4 47.1 0.0 31.4 47.1
a Foam content, %
b Foam content, %
c Foam content, %

500 500 500

0.0% 0.0% 0.0%
Relaitive water absorption, %

Relaitive water absorption, %

31.4% 31.4% 31.4%

Relaitive water absorption, %

400 47.1% 400 47.1% 400 47.1%

7-day 28-day Autoclave

300 300 300

200 200 200

100 100 100

0 0 0
0 10 20 0 10 20 0 10 20
d FA replacement, %
e FA replacement, %
f FA replacement, %
500 500 500
0.0% 0.0% 0.0%
31.4% 31.4%
Relaitive water absorption, %

Relaitive water absorption, %

31.4%
Relaitive water absorption, %

400 47.1% 400 47.1% 400 47.1%

7-day 28-day Autoclave

300 300 300

200 200 200

100 100 100

0 0 0
0 10 20 0 10 20 0 10 20
g SF replacement, %
h SF replacement, %
i SF replacement, %

Fig. 4. Effect of foam content (a–c), FA (d–f) and SF (g–i) introduction on water absorption values for 7-day, 28-day and autoclave curing regimes.

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238 more compatible with binder materials having low specific gravity ratio C-S-H which has higher strength than that of conventional 282
239 and small particle size in terms of segregation, pore structure and C-S-H [28]. 283
240 uniformity of the mixtures. Thus, the characteristics of the binder
241 materials as given in Table 1 cause significant variations in com- 3.4. Relationships between density and compressive strength or water 284
242 pressive strength results of the mixtures. Falliano et al. [8] simi- absorption 285
243 larly reported that the synthetic-based foaming agent showed
244 remarkably better performance in limestone blended cement Many physical properties and compressive strength of foam 286
245 (CEM II A-L) compared to that of ordinary Portland cement (CEM concrete depend upon its density in hardened state [16,26]. 287
246 I). It is seen that SF incorporation decreased the reduction of the Researchers reported an exponential relationship between density 288
247 compressive strength values causing by the foam introduction and compressive strength in the medium-to-high density range 289
248 when compared to those of the mixtures without SF. In high foam [16], a linear relationship in the low-to-medium density range 290
249 mixtures, merging of bubbles seems to produce larger voids, [8], a power relation in low density values [29], and a linear rela- 291
250 resulting in wide distribution of void sizes and lowering the tion in ultra-low density values [30]. Water absorption of the 292
251 strength [13]. Bing et al. [11] reported that the compressive mix can also show respectable relation with its physical character- 293
252 strength of the foam concrete containing silica fume is higher than istics [16,31]. On the other hand, high moisture content can 294
253 those of non-silica fume mixtures at any foam content ranging increase thermal conductivity of concrete [32]. Fig. 7 presents the 295
254 from 20 to 50%, by volume. Kearsley and Wainwright [15] stated relationship between density and compressive strength as well 296
255 the importance of the curing temperature on the time required as density and water absorption values of foam concrete. In order 297
256 for the contribution of the fly ash to the strength. The results of to determine a rational optimum intersection point, the curves 298
257 the present study show that the compressive strength of foam con- were plotted for the maximum density of 2000 kg/m3, a maximum 299
258 crete cured in autoclave is comparable to the strength of 7-day water absorption of 100% and a maximum strength value of 300
259 water cured specimens. 90 MPa. Kearsley [33] reported that compressive strength of the 301
260 The effects of foam, FA and SF contents on the relative compres- foam concrete reduced exponentially with the reduction of den- 302
261 sive strength of foam concrete are given in Fig. 6. Up to around 97% sity. Moreover, thermal conductivity that is one of the most essen- 303
262 reduction in the compressive strength was resulted upon 47.1% tial characteristics of lightweight concrete mixtures is strictly 304
263 foam addition. However, the adverse effect of foam incorporation related to density [31,34]. In foam concrete it is desirable to mini- 305
264 was less pronounced on the strength of SF-bearing mixtures. While mize the density meanwhile, to maximize the strength. For this 306
265 FA incorporation caused strength loss at high foam content purpose, the intersection point of the curves given in Fig. 7 corre- 307
266 (Fig. 6d–f), SF introduction increased the compressive strength val- sponds to an optimum density value of 1320 kg/m3. The mixture 308
267 ues up to 4.4 times, compared to that of the mixtures containing no containing 10%SF and 47.1% foam and water-cured for 7 days (10 309
268 silica fume. Ramamurthy et al. [26] similarly stated that partial SF-II) was found to show the closest density to 1320 kg/m3. This 310
269 replacement of cement with SF resulted in high long-term com- combination of parameters is specific to this study, thus the fact 311
270 pressive strength due to the pozzolanic and pore-filling effect of should be considered also for other studies provided such compa- 312
271 SF. The effect was more marked in foam concrete mixtures having rable mechanical and physical parameters of foam concrete 313
272 high densities. However, contradictorily in the present study the mixtures. 314
273 beneficial effect of SF was more pronounced in high foam-
274 bearing mixtures (low density foam concrete mixtures). Besides, 3.5. Thermal conductivity and compressive strength/thermal 315
275 the strength enhancement effect of SF was less pronounced in conductivity ratio 316
276 standard water-cured (either 7-day or 28-day) specimens com-
277 pared to that of the autoclave-cured samples in the mixtures hav- As it can be seen from Fig. 8, the thermal conductivity values 317
278 ing high volume foam content. The increment in strength upon and compressive strength/thermal conductivity ratios of the foam 318
279 autoclaving is attributed to the additional hydration of unhydrated concrete specimens cured in autoclave were found to be between 319
280 cement particles [27]. Moreover, pozzolanic reaction in mineral 0.2385 and 0.9418 W/m.K and between 10.9 and 94.3 MPa/(W/m. 320
281 admixture-bearing systems results in the formation of low Ca/Si K), respectively. In general, higher strength/weight ratio and lower 321

90
7-day 28-day Autoclave
75
Compressive strength, MPa

60

45

30

15

Mix ID

Fig. 5. Compressive strength values, MPa.

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322 coefficient of thermal conductivity are considered as advantages of the thermal conductivity. However, at high foam content, FA sig- 339
323 lightweight concrete when compared with normal weight concrete nificantly reduced the thermal conductivity values. Such irregular 340
324 [35]. The increase in compressive strength/thermal conductivity variations in the thermal conductivity was attributed to the com- 341
325 ratio indicates an increase in strength and/or a reduction in ther- plex nature of heat conduction through the solid and void compo- 342
326 mal conductivity. Thus, from this view point SF-bearing mixtures nents of foam concrete [17]. Regarding the results of this study, it 343
327 were found to be superior to those containing FA. seems that higher substitution levels for FA or SF can be recom- 344
328 The effects of foam content, FA or SF incorporation on the rela- mended for reducing the thermal conductivity of foam concrete. 345
329 tive thermal conductivity of autoclaved foam concrete mixtures The effects of foam, FA and SF contents on the relative compres- 346
330 are given in Fig. 9. Thermal conductivity of the mixture without sive strength/thermal conductivity (r/k) ratios of autoclaved foam 347
331 mineral admixture remarkably reduced by introduction of 31.4% concrete are given in Fig. 10. The ratio gives an idea about the effi- 348
332 foam compared to that of mixture without foam. Partial replace- ciency of foam concrete with regards to its hardened characteris- 349
333 ment of cement with mineral admixtures reduced the thermal con- tics. Foam content significantly reduced the efficiency of foam 350
334 ductivity of the mixtures without foam. Similarly, reduction in concrete. The loss of r/k ratio was more pronounced in the high 351
335 thermal conductivity values was reported by the use of FA and foam content mixtures containing FA. The efficiency increased by 352
336 SF in normal and lightweight cementitious systems [32,36,37]. increasing the SF incorporation level at high foam content. These 353
337 However, it was revealed in this study that in 31.4% results promise the higher incorporation levels of SF in foam con- 354
338 foam-bearing mixtures the addition of either FA or SF increased crete mixtures having high foam content. 355

500 Ref 500 Ref 500 Ref

10% FA 10% FA 10% FA
Relaitive comp. strength, %

Relaitive comp. strength, %

Relaitive comp. strength, %

20% FA 20% FA 20% FA
400 10% SF 400 10% SF 400 10% SF
20% SF 20% SF 20% SF
300 7-day 300 28-day 300 Autoclave

200 200 200

100 100 100

0 0 0
0.0 31.4 47.1 0.0 31.4 47.1 0.0 31.4 47.1
a Foam content, %
b Foam content, %
c Foam content, %

500 500 500

0.0% 0.0% 0.0%
31.4% 31.4% 31.4%
Relaitive comp. strength, %

Relaitive comp. strength, %

Relaitive comp. strength, %

400 47.1% 400 47.1% 400 47.1%

7-day 28-day Autoclave

300 300 300

200 200 200

100 100 100

0 0 0
0 10 20 0 10 20 0 10 20
d FA replacement, %
e FA replacement, %
f FA replacement, %
500 500 500
Relaitive comp. strength, %
Relaitive comp. strength, %

Relaitive comp. strength, %

400 400 400

300 300 300

200 200 200

100 0.0% 100 0.0% 100 0.0%

31.4% 31.4% 31.4%
7-day 47.1% 28-day 47.1% Autoclave 47.1%
0 0 0
0 10 20 0 10 20 0 10 20
g SF replacement, %
h SF replacement, %
i SF replacement, %

Fig. 6. Effect of foam content (a–c), FA (d–f) and SF (g–i) introduction on compressive strength values for 7-day, 28-day and autoclave curing regimes.

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100 90
Water absorption Optimum
Compressive strength
poorer 75
80 overweight

Compressive strength, MPa

strength
Water absorption, % 60
60

Mix ID: 10SF-II 45

(7-day)
40 y = 102.33e-0.001x y = 1E-12x4.1878
R² = 0.7264 R² = 0.9399 30

20
15

0 0
0 400 800 1200 1320 1600 2000
Density, kg/m3

Fig. 7. Relationships between density-compressive strength and density-water absorption.

1.2 100

Compressive strength/thermal conductivity

Comp. strength/ 90
Thermal conductivity ( ), W/m.K

1.0
80
70
0.8
60
0.6 50
40
0.4
30
20
0.2
10
0.0 0

Mix ID

Fig. 8. Thermal conductivity and compressive strength/thermal conductivity ratio.

200 200 200

Ref 0.0% 0.0%
Relaitive thermal conductivity, %

Relaitive thermal conductivity, %

Relaitive thermal conductivity, %

10% FA 31.4% 31.4%

20% FA 47.1% 47.1%
150 10% SF 150 150
20% SF

100 100 100

50 50 50

0 0 0
0.0 31.4 47.1 0 10 20 0 10 20
a Foam content, %
b FA replacement, %
c SF replacement, %

Fig. 9. Effect of foam content (a), FA (b) and SF contents (c) on thermal conductivity.

356 3.6. Relationships between thermal conductivity and compressive sity ratio of foam concrete cured in autoclave. The power 360
357 strength or density regression curves denote the strong relationship between thermal 361
conductivity and density or compressive strength of foam concrete. 362
358 Fig. 11 presents the relationships between thermal conductiv- Other researchers similarly reported that there is a strong relation- 363
359 ity, density, compressive strength and compressive strength/den- ship between thermal conductivity and density of lightweight con- 364

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500 500 500

Ref 0.0% 0.0%

Relaitive comp. strength/ , %

Relaitive comp. strength/ , %

10% FA 31.4% 31.4%

Relaitive comp. strength/ , %

400 20% FA 400 47.1% 400 47.1%
10% SF
20% SF
300 300 300

200 200 200

100 100 100

0 0 0
0.0 31.4 47.1 0 10 20 0 10 20
a Foam content, %
b FA replacement, %
c SF replacement, %

Fig. 10. Effect of foam content (a), FA (b) and SF (c) introduction on compressive strength/thermal conductivity ratio.

2500 100
Comp. strength×1000/density
Compressive strength
Density

Comp. strength×1000/density ratio

2000 80

Compressive strength (MPa) or

Density, kg/m3

1500 60
y = 2058x0.4889
R² = 0.8712
y = 49.349x1.7606
1000 y = 101.56x2.2494 R² = 0.8082 40
R² = 0.8402

500 20

0 0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Thermal conductivity ( ), W/m.K

Fig. 11. Relationships between thermal conductivity-density, compressive strength or strength/density ratio.

Fig. 12. Pore structure of some foam concrete mixtures cured in autoclave.

365 crete [31,36,38]. Thus, in the real design process of such light- ucts in addition to mechanical performance. The concave shape 368
366 weight cementitious systems, target densities are selected to of the curve expressing the relationship between density and ther- 369
367 ensure the desired theoretical isolation performance of the prod- mal conductivity denotes that in foam mixtures having a high den- 370

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371 sity, a small increase in density results in a great increase in ther-
Silica fume incorporation increased the thermal conductivity of 431
372 mal conductivity. On the other hand, the convex curve between foam concrete by 37%. However, the increase in thermal con- 432
373 thermal conductivity and strength/density ratio states that in mix- ductivity was significantly lower than the increase (up to 4.4 433
374 tures having high strength/density ratio, small changes in this ratio times) in compressive strength. Therefore, compressive 434
375 cause negligible effect on the thermal conductivity. strength/thermal conductivity ratios of the silica fume-bearing 435
products increased up to 4 times by the increase of foam con- 436
376 3.7. Effect of FA and SF on pore structure tent. While fly ash incorporation reduced the compressive 437
strength/thermal conductivity ratio of the mixtures containing 438
377 Fig. 12 presents the pore structures of some selected foam con- high foam content (47.1%) up to 38%, it resulted in around 439
378 crete mixtures cured in autoclave. The images mainly emphasise 33% increase in this ratio in low foam content (31.4%) mixtures. 440
379 the effect of foam content in FA-bearing mixtures (difference From this perspective, silica fume is superior to fly ash. 441
380 between 10FA-I and 10FA-II) and effect of mineral admixture type
The pore structure of the foam concrete was significantly 442
381 in mixtures having high foam volume (difference between 10FA-II affected by the introduction of foam, fly ash and silica fume. 443
382 and 10SF-II) in terms of air-void distribution. The air-void distribu- Due to the strong relations between the properties of foam con- 444
383 tion is one of the most important micro and macro characteristics crete and its mix proportions, optimization of the mixtures 445
384 influencing chemical, physical and mechanical properties of foam seems to be of greater importance in mineral admixture- 446
385 concrete. Wee et al. [6,7] reported that the air-void system, i.e. bearing foam concrete. 447
386 spacing factor, average air-void size and air content, controls the
The utilization of ternary cementitious systems (silica fume 448
387 compressive strength and strength/weight ratio of the foam con- + fly ash + cement) to balance the desired properties of foam 449
388 crete. As the filler becomes finer, a more uniform distribution of concrete having high foam contents is recommended. 450
389 air voids can be achieved probably by providing a uniform coating 451
390 of bubbles by the paste and consequently preventing the merging Conflict of interest 452
391 and overlapping of bubbles. Mixes with a narrower air void size
392 distribution showed higher strength [13,26]. As it can be seen from The authors declare that they have no conflict of interest. 453
393 Fig. 12 large bubbles were observed in 10FA-II mixture. Jones et al.
394 [1] schematically presented the movement of small bubbles References 454
395 towards large bubbles and instability of large bubbles arisen from
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