Properties of a Foamed Concrete with Soil As Filler

Construction and Building Materials 76 (2015) 61–69

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Construction and Building Materials

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Properties of a foamed concrete with soil as ﬁller ⇑ Ma Cong, Chen Bing

Department of Civil Engineering, Shanghai Jiaotong University, Shanghai 200240, PR China highlights

Soil-based foam concrete exhibits unusual physical properties, such as strength, thermal insulation and water resistance. Silica fume added into soil-based foamed concrete can greatly increase the physical properties. The ﬁt of mathematical expressions based on hygroscopic test results was investigated. article info abstract

Article history: Ordinary Portland cement, soil and foaming agent are the raw materials used to make soil-based foamed Received 12 May 2014 concrete. The effects of foam content and silica fume on the physical properties of soil-based foamed con- Received in revised form 27 September 2014 crete, such as the dry density, 28-day compressive strength, thermal conductivity, water resistance and Accepted 28 November 2014 pore structure, were studied. The experimental results indicate that the foam and silica fume contents Available online 12 December 2014 have a large impact on the physical properties of soil-based foamed concrete. The thermal conductivity, density, water resistance and compressive strength decrease with increasing volume fractions of foam. Keywords: The compressive strength, the thermal insulation and water resistance are all improved by increasing Soil the content of silica fume. Soil-based foamed concrete consisting of 20% silica fume with a density of Thermal conductivity 3 Compressive strength 800 kg/m , compressive strength of 7.5 MPa and thermal conductivity of 0.16 W/m K can be used as Foam water-resistant lightweight concrete. The hygroscopic tests were performed and the results indicate that Density the addition of silica fume has some effect on the hygroscopic property of soil-based foamed concrete. Water resistance Several fitting curves have been obtained, the fitted functions developed by the Kumaran model and Hygroscopic property Cubic function have better fitting parameters. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction lightweight concrete [6,7]. Goual et al. studied clayey cellular concrete and found that the compressive strength and thermal Soil-based construction buildings have been used in China for conductivity ranged from 0.95–3.85 MPa and 0.201–0.281 W/m K centuries, especially in rural regions of the Loess Plateau and Fujian for densities of 843–1038 kg/m3, respectively [7]. Porosity identifi- Province [1,2]. In recent years, the soil which can be used as an eco- cation revealed the existence of two distinct pores classes: inher- friendly building material has attracted lots of attention. Some ent micropores in the clay and macropores or gaseous cells, researchers have studied the mechanical properties of soil-based resulting from the addition of aluminium powder at the time of concrete used in modern buildings [3–5]. Adesanya’s tests showed mixing. On account of the only moderate strength requirement that the trend of compressive strength development was similar to for foamed concrete, soil can be used as eco-friendly and cheap that of conventional concrete. The 28-day compressive strength of filler for creating novel foamed concrete some excellent properties. laterite and clay based concrete ranged from 8 to 22 MPa for den- Soil-based foamed concrete is a novel lightweight construction sities of 1500–2350 kg/m3 [4]. material consisting of Portland cement, soil, water and foaming Furthermore, the soil-based ancient buildings have characteris- agent. Although the foamed concrete is not used as load-bearing tics of coolness in the summer and warmth in the winter for the wall material, it is necessary to improve the strength of soil-based existing of natural micro-pore structures in mud walls. Inspired foamed concrete without thermal insulation performance degrada- by this discovery, several researchers have introduced soil into tion. Generally, fly ash, silica fume and fibres are often added into the mixture to customise the mechanical properties of foamed concrete [8–10]. Fly ash has been used in concentrations of 30–70 wt% ⇑ Corresponding author. Tel.: +86 13917109022. [8] and silica fume (up to 10 wt% by weight of cement) has been E-mail address: [email protected] (C. Bing). added to intensify the cement strength [10]. http://dx.doi.org/10.1016/j.conbuildmat.2014.11.066 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved. 62 M. Cong, C. Bing / Construction and Building Materials 76 (2015) 61–69

The durability of building materials is very sensitive to humid- 3000 ity. And the use of water absorption as a measure of the potential Q durability of porous building materials has been proposed, which 2500 characterises the tendency of a porous material to absorb and transmit water by capillarity [11–15]. It was reported that the 2000 water movement into concrete is not a simple function of porosity but depends on the pore diameter, distribution, continuity and tor- tuosity. Furthermore, there is a great interest in the fundamental 1500 study on the moisture transfer between indoor air and porous building materials for the changes in the indoor humidity, and it 1000

Intensity (Counts) Q is clear that this phenomenon may improve air quality and comfort K I G K Q in buildings [16,17]. Since soil is a porous, hygroscopic material 500 Q Q that contains abundant micropores, it is expected that the soil- based foamed concrete absorbs moisture vapour form the air when 0 relative humidity increases and release this moisture when the humidity falls. Therefore, it is necessary to achieve experimental 5 101520253035404550556065707580 results to precisely analyse the moisture transfer between soil- 2 Theta (o ) based foamed concrete and air. The main objective of this paper is to provide some basic infor- Fig. 1. XRD patterns of the soil (Q: SiO2 quartz, K: kaolinite, I: illite, G: gismondine). mation on the properties of soli-based foamed concrete. In the experimental program, the physical properties of soil-based der; and Series III, which used the combination of cement and quick lime as the bin- foamed concrete were tested and analysed. In addition, the effects der. To obtain similar workability, different superplasticiser contents were added to of foam volume fraction and silica fume on the dry density, com- the Series II and Series III groups. Further details of the mix proportions are shown pressive strength, thermal conductivity, and water resistance of in Table 2. The mixing sequence consisted of combing the powder (including binder soil-based foamed concrete were investigated. According to the and filler) with water and mixing until a homogeneous base mix was achieved. The required weight of the foam, calculated by multiplying the foam density by the results of the hygroscopic tests, the best-fitting curves based on foam volume to be added, was then generated and added immediately to the base analytical functions have been obtained. mix and mixed until no foam appeared on the surface and the foam was uniformly distributed throughout the mix. The surface of test specimens was smoothed by hand only. The specimens were 2. Experimental details covered with wet gunny bags 10 h after casting, demoulded after 24 h and stored in a fog room (95 ± 3% relative humidity (RH), 22 ± 2 °C) to cure until testing. 2.1. Materials and mix proportions 2.3. Methodology The cementitious materials used in this study consist of ASTM Type I ordinary Portland cement (OPC), silica fume and quick lime. The 28-day compressive The thermal conductivity of each material was measured on dry strength of OPC is 56.5 MPa and used as a basic cementitious material for each mix- 20 20 5cm3 half-cubic samples using a non-stationary method [18]. The non- ture. Silica fume was obtained from Elken Materials. Quick lime was an oven-dry stationary method provided several measurement advantages in terms of the con- white powder with a CaO content of 94.3%. The soil sample is soft clay collected tact resistance, power and duration of the emitted signal. The experimental device from the Shanghai Jiao Tong University campus in Shanghai, China. Particle size was composed of a transient plane source (TPS) element, a power supply stabilised analysis indicates that about 95% of the soil is finer than 1 mm. The liquid and plas- in tension with a Wheatstone bridge, an acquisition power station and a microcom- tic limits are approximately 43% and 15%, respectively. According to the Unified Soil puter for data control and processing. To protect the probe against damage and Classification System, this soil is a CL. The detailed chemical composition of the ensure that the flow was distributed over a representative surface, two copper cementitious materials and soil are shown in Table 1. The mineralogical character- plates with thicknesses of 1 mm and areas of 5 5cm2 were introduced between ization of the soil is shown in Fig. 1. Throughout this experimental study, tap water the TPS sensor and two 20 20 5cm3 blocks composed of the same material. was used to produce all foamed concrete specimens. A protein-based foaming agent The surfaces of the two half samples were polished beforehand to minimise the was used, which was diluted with water in a ratio of 1:40 (by volume) and then aer- influence of contact resistance. A chucking device was used to ensure good contact ated to a density of 70 kg/m3. A naphthalene-based superplasticiser was used to between the various elements. The thermal conductivity was calculated directly by produce mixes of a flowable or highly flexible nature. The complete description a computer. Before testing, all specimens were kept in a hot air oven at 60 °C until of the foamed concrete mixes is presented in Table 2. they attained constant weight. Compressive strength tests to characterise the mechanical properties of foamed 2.2. Specimen preparation concrete were conducted on an MTS servo hydraulic testing machine with a 100 kN capacity and a constant displacement rate of 5 mm/min. At least three specimens Foamed concrete was produced in a laboratory using a paddle mixer by adding were measured for each concrete mix. In most cases, the results under the same the preformed foam to a base mix. Three different variations of the foamed concrete testing condition were reproducible with low mean standard deviations, SD were made: Series I, the control mixtures cast using soil, cement, foaming agent and (SD=x < 10%, where x ¼ the mean strength value). The compression tests were water; Series II, which used the combination of cement and silica fume as the bin- performed under soaked and unsoaked conditions. The soaked condition simulates the attack of complex overly wet weather or rainy weather, where large amounts of water filters into the specimens. After a desired curing (28 days ± 10 h), the specimens were immersed in deionized water Table 1 for 48 h in a room at the constant temperature of 22 ± 2 C. The water absorption Chemical composition of cementitious materials and soil. ° was tested after 1, 2, 3, 8, 24 and 48 h of soaked time [19]. To evaluate the water

OPC SF Soil resistance of soil-based foamed concrete, the ratio (RCS) of compressive strength under soaked condition (CS ) and unsoaked condition (CS ) is obtained taking into Chemical composition (%) S U account the following expression: Silicon dioxide (SiO2) 21.6 92.40 72.3

Aluminium oxide (Al2O3) 4.13 0.80 5.8 Rd RCS ¼ ð1Þ Ferric oxide (Fe2O3) 4.57 0.50 0.4 Rw Calcium oxide (CaO) 64.44 0.91 3.5 Magnesium oxide (MgO) 1.06 0.27 6.9 where RCS is the strength retention coefﬁcient, Rd is the strength of the specimens cured for 28 days in a fog room with no water absorption and Rw is the strength Sodium oxide (Na2O) 0.11 – 8.9 of the specimens with water saturation. Potassium oxide (K2O) 0.56 – – A variant of the air-conditioned room method indicated in the standard for a cli- Sulphur trioxide (SO3) 1.74 – – Manganese (Mn) – – 0.3 matic test chamber was used for hygroscopic test. This layout allows that the tem- Loss on ignition 0.76 2.0 1.9 perature inside the test chamber remains constant (22 ± 0.5 °C) as well as the relative humidity ranging from 8.5% to 100%. The moisture content of specimens M. Cong, C. Bing / Construction and Building Materials 76 (2015) 61–69 63

Table 2 Mixture compositions.

Series Mix no. Target density (kg/m3) Composition of mixture (per m3) Dry density (kg/m3) Binder (kg) Soil (kg) Water (kg) Foam (m3) Superplasticizer (kg) Cement Silica fume Quick lime Series I 1 1800 920 0 0 460 414 0 9.2 1835 2 1500 750 0 0 375 338 0.2 7.5 1473 3 1200 600 0 0 300 270 0.4 6.0 1130 4 800 380 0 0 190 171 0.6 3.8 755 5 500 220 0 0 110 99 0.75 2.3 460 Series II 6 800 361 19 0 190 171 0.6 3.8 758 7 800 342 38 0 190 171 0.6 3.8 764 8 800 323 57 0 190 171 0.6 3.8 778 9 800 304 76 0 190 171 0.6 3.8 790 Series III 10 800 361 0 19 190 171 0.6 4.75 1047 11 800 342 0 38 190 171 0.6 4.75 1433 12 800 323 0 57 190 171 0.6 4.75 1650 was determined by weighing until to obtain a constant mass so that the equilibrium with conventional lightweight aggregate foam concrete. Fig. 2 state is reached in each test. The constant mass was achieved when the mass var- shows the compressive strength and thermal conductivity of soil- iation between three consecutive weightings, carried out with a time separation of at least 15 h, becomes less than 0.1% of the total mass of the specimen. The hygro- based concrete for different dry densities. The strength and ther- scopic moisture storage is described using the sorption isotherm curves of the mal conductivity appeared to increase signiﬁcantly with increasing material, termed hygroscopic curves. These curves establish the relationship concrete density or decreasing foam volume. The dry density of between the moisture content for porous media in volume, V, or per unit mass, soil-based concrete without foam is 1835 kg/m3 and the corre- and the relative humidity of air in equilibrium at constant temperature, u, as fol- sponding thermal conductivity was 0.755 W/m K. When the dry lows [20]: density of soil-based foamed concrete with 60% foam was mðuÞm mðuÞm WðuÞ 3 WðuÞ¼ 0 or uðuÞ¼ 0 ¼ ð2Þ 755 kg/m , the strength and thermal conductivity were 4.0 MPa V m0 q and 0.19 W/m K, respectively. For the conventional foamed concrete, the thermal conductivity is essentially proportional to the density of concrete and the ther- 3. Test results and discussion mal conductivity may increase by 0.04 W/m K as the density increasing by 100 kg/m3 [21,22]. For soil-based foamed concrete 3.1. Dry density, thermal conductivity and compressive strength studied in this study, when the density decreases from about 1500 kg/m3 to 800 kg/m3, the thermal conductivity reduces from According to the data presented in Table 2, the soil-based 0.63 W/m K to 0.19 W/m K accordingly. According to Ref. [21], foamed concrete mixture consisting of quick lime requires more the thermal conductivity of conventional foamed concrete with a water during the mixing process than that consisting of silica fume density of 800 kg/m3 is greater than 0.3 W/m K and that of soil- to obtain the same workability. Therefore, more superplaticiser based foamed concrete is only 0.19 W/m K. This implies that soil- was added to the specimens of Series III during the mixing process, based foamed concrete has better thermal insulation performance keeping the ratio of water and binder the same as that of Series I than that of conventional foamed concrete. The particle shape and and II. This behaviour may be due to additional water being size play an important role in the thermal insulation performance. required to chemically react with quick lime as the following equation: 3.1.1. Effect of silica fume

CaO þ H2O ! CaðOHÞ2 ð3Þ Silica fume, an ultraﬁne activity admixture, is often added to concrete mixture to improve the strength of concrete [23]. In this The effect of the foam volume fraction on the dry density is also study, silica fume was used as a mineral admixture to improve shown in Table 2. It can be seen that the dry density decrease with the performance of soil-based foamed concrete. As shown in the increasing of foam volume, which shows the similar tendency Table 2, using a foam volume fraction of 60%, silica fume was used 1.0 to replace 5%, 10%, 15% and 20% of the cement. The effects of the silica fume on the compressive strength and thermal conductivity 30 0.9 Compressive strength of soil-based foamed concrete are presented in Fig. 3, which shows Thermal conductivity 0.8 25 that the compressive strength of soil-based foamed concrete 0.7 increased with silica fume addition. The 28-day compressive strength of soil-based foamed concrete without silica fume was 20 0.6 only 4.0 MPa, whereas that of the concrete with 20% silica fume 0.5 added was almost doubled to 7.8 MPa. However, silica fume 15 0.4 appears to reduce the thermal conductivity, which improves thermal insulation. The thermal conductivity of soil-based foamed con- 10 0.3 crete without silica fume is 0.19 W/m K; in contrast, it is 0.16 W/

0.2 Thermal conductivity (W/mK) m K after the addition of 20% silica fume, which is a reduction of Compressive strength (MPa) 5 0.1 18%. The addition of silica fume achieves a balance of compression strength and thermal insulation performance of soil-based foamed 0 0.0 400 600 800 1000 1200 1400 1600 1800 2000 concrete. This effect may be due to the ﬁner pores caused by silica Dry density (kg/m3) fume in the mixture. Previous research has shown that the number and distribution of pores are critical for thermal insulation such Fig. 2. Compressive strength and thermal conductivity with dry density for Series I. that ﬁner pores lead to better insulation [24]. Therefore, adding 64 M. Cong, C. Bing / Construction and Building Materials 76 (2015) 61–69

0.20 90 0 20% 8 Compressive strength Thermal conducticity 80 40% 60% 75% 0.19 7 70

0.18 60 6 50 0.17 5 40

Absorption (%) 30 4 0.16 Compressive strenght (MPa) Thermal conductivity (W/mK) 20

3 0.15 048121620 10 Silica fume content (%) 0 01020304050 Fig. 3. Effect of silica fume on the thermal conductivity and compressive strength of Soaking time (hours) soil-based foamed concrete for Series II. Fig. 5. Water absorption of soil-based foamed concrete with different foam volumes for Series I. 0.8 30 Compressive strength 0.7 Thermal conducticity 0.95 80 25 0.6 RCS 0.90 Absorption 20 0.5 60 0.85 0.4 15 0.3 0.80

CS 40

10 R 0.2 0.75 Compressive strength (MPa) Thermal conductivity (W/mK)

5 48 h Absorption (%) 0.1 0.70 20 0 0.0 0 2 4 6 8 10121416 0.65 Quick lime content (%) 0 0 1020304050607080 Fig. 4. Effect of quick lime on the thermal conductivity and compressive strength of Foam volume (%) soil-based foamed concrete for Series III.

Fig. 6. Water resistance of soil-based foamed concrete with different foam volumes silica fume is a very effective technical means to improve the com- for Series I. pressive strength and reduce the thermal conductivity of soil- based foamed concrete. 3.2. Water resistance

3.1.2. Effect of quick lime The soaked engineering properties of soil-based foamed con- Quick lime, an effective cementitious material which can per- crete is dependent upon the state of water content and the decrease form the pozzolanic reactions, is often used for stabilizing soil of compressive strength [27]. Water absorption of soil-based [25]. In this research, quick lime was used as the second binder foamed concrete with different foam volume fractions is shown in and replaced 5%, 10% and 15% of the cement (Table 2). Fig. 4 shows Fig. 5. For each mixture, water absorption increased remarkably the effects of the quick lime on the compressive strength and ther- within 3 h of soaking time, then the absorption kept approximate mal conductivity of soil-based foamed concrete. The compressive constant. This is in agreement with the results reported by Kampala strength and thermal conductivity both increased with the increas- [28]. In addition, as the foam volume increased, water absorption of ing content of quick lime. When the foamed concrete consists of soil-based foamed concrete increased sharply. This implies that 15% quick lime, the 28-day compressive strength could reach to more pores are inter-connected in the foamed concrete with the 28.2 MPa with the thermal conductivity increasing to 0.73 W/ increase of stable foam. Fig. 6 shows the water resistance of soil- m K. As shown in Table 2, although 60% foam was introduced in based foamed concrete with different foam volumes. As shown in the specimens for Series III, the density of soil-based foamed con- Fig. 6, the RCS of soil-based foamed concrete decreased approxi- crete increased from 1047 kg/m3 to 1650 kg/m3 as the content of mately linearly with the increasing foam volume. As the water quick lime increases from 5% to 15%. This implies that more foam absorption at 48 h increased from 13.1% to 69.1%, the RCS decreased fractured with the increasing content of quick lime. As reported in from 0.92 to 0.67, which indicates that the water resistance of soil- Ref. [26], the foam stability occurs on the premise of a small based foamed concrete decreased. amount of water absorbed on the surface of the foam (i.e. the sta- As mentioned before, silica fume can improve the compressive ble liquid membrane). During the mixing process, the reaction in strength and thermal insulation of soil-based foamed concrete. Eq. (3) occurs immediately and the water existing in the surface The effects of silica fume content on the water resistance of soil- of the foam can also be consumed by quick lime, which results in based foamed concrete are shown in Figs. 7 and 8. As shown in the broken of foam and the increasing of the dry density. Fig. 7, the water absorption of soil-based foamed concrete M. Cong, C. Bing / Construction and Building Materials 76 (2015) 61–69 65

0 5% 0 20% 50 10% 15% 20% 12 40% 60%

10 (a) =18.8% 40 ) 3 8

30 6 (kg/m ) 4 W( 20

Absorption (%) 2

200 10 (b) =58.3%

) 15 3 0 01020304050 10 (kg/m

Soaking time (hours) )

Fig. 7. Effect of silica fume content on the water absorption of soil-based foamed W( 5 concrete for Series II. decreased with silica fume addition. The water absorption of soil- 0 based foamed concrete without silica fume at 3 h reached up to 40 (c) =100% 38.3%, correspondingly, that of the concrete with 20% silica fume ) added was only 16.8%. As reported in Ref. [15], the water absorption 3 30 of the conventional foamed concrete with density of 800 kg/m3

changes from about 17.5% to 35%. Nevertheless, water absorption (kg/m

) 20 of the concrete with silica fume increased slightly after 3 h of soaking time, which is different from the concrete without silica fume. W( 10 This illustrates that it needs longer soaking time for water to permeate into the concrete, which indicates that there may be more 0 disconnected and ﬁne pores in the soil-based foamed concrete with 0 40 80 120 160 200 silica fume. As shown in Fig. 8, the RCS of soil-based foamed concrete Absorption time (hours) increased with silica fume addition. The RCS of the foamed concrete without silica fume was only 0.73, whereas that of the concrete Fig. 9. Hygroscopic moisture content of soil-based foamed concrete with different with 20% silica fume added was almost 0.86. The addition of silica foam volumes for Series I under different relative humidities. fume achieves better water resistance of soil-based foamed concrete and the soil-based foamed concrete with 20% silica fume humidity. When the relative humidities were 18.8% and 100%, added can be used as water-resistant lightweight concrete. the moisture contents of soil-based foamed concrete with 60% foam at 40 h were 7.48 kg/m3 and 22.86 kg/m3, and those at 3 3 3.3. Hygroscopic property 170 h were 9.65 kg/m and 38.32 kg/m , respectively. In addition, the moisture contents of each mixture under a relative humidity 3.3.1. Hygroscopic moisture storage versus time of 100% increased continuously with the increasing of absorption Figs. 9 and 10 show the hygroscopic moisture content time, but those under relative humidities of 58.3% and 18.8% expressed in kg/m3 of soil-based foamed concrete Series I and II. increased within 145 h and then decreased slightly. For soil-based As shown in Fig. 9, the hygroscopic moisture content of the foamed foamed concrete with different contents of silica fume, the mois- concrete increased with the increasing foam volume and relative ture contents under a range of relative humidity were lower than those of the foamed concrete without silica fume (Fig. 10). And 0.90 the reduction of the moisture content under a lower relative R CS 42 humidity was much greater than that under a higher relative Absorption humidity. The reduction of the moisture content of the foamed 0.85 concrete with the addition of 20% silica fume under a relative 36 humidity of 18.8% was 50% which was 28% higher than that under a relative humidity of 58.3%. It is notable that the reduction of the moisture content under a relative humidity of 100% was insigniﬁ- CS 0.80 30

R cant even when the content of silica fume was 20%. The moisture contents of the foamed concrete with 20% silica fume at 40 h and 3 3 24 170 h were 21 kg/m and 33.39 kg/m , respectively. This implies 0.75 48 h Absorption (%) that silica fume has little inﬂuence on the hygroscopic property of soil-based foamed concrete under high relative humidity. 18 0.70 3.3.2. Hygroscopic ﬁtted curves 0 5 10 15 20 The sorption isotherms of the specimens checked, the relative Silica fume content (%) humidity as a function of the moisture content are shown in Fig. 8. Effect of silica fume content on the water resistance of soil-based foamed Figs. 11 and 12. Table 3 shows the moisture content per unit mass concrete for Series II. (kg/kg) of soil-based foamed concrete for Series I and II under a 66 M. Cong, C. Bing / Construction and Building Materials 76 (2015) 61–69

0 5% 40 10% 15% 20% 12 35 0 5% 10% (a) =18.8% 15% 20% average 10 )

3 30 8

(kg/m 25 6 3 )

4 kg/m 20 W( 2 W( 15 0 20 (b) =58.3% 10 )

3 15 5

(kg/m 0 10 ) 0 102030405060708090100 relative humidity/ (%) W( 5 Fig. 12. Absorption isotherms of soil-based foamed concrete with different silica 0 fume contents for Series II. 40 (c) =100%

) Table 3 3 30 Experimental results of the moisture content per unit mass (kg/kg) at different relative humidities. (kg/m

) 20 Mix no. Relative humidity/u (%) 8.5 18.8 37.1 58.3 80.5 93.3 100 W( 10 1 0.0011 0.0017 0.0048 0.0078 0.0107 0.0116 0.0124 2 0.0035 0.0045 0.0090 0.0112 0.0135 0.0179 0.0193 0 3 0.0054 0.0079 0.0014 0.0015 0.0018 0.0026 0.0030 0 40 80 120 160 200 4 0.0089 0.0128 0.0215 0.0237 0.0336 0.0453 0.0508 Absorption time (hours) 6 0.0089 0.0128 0.0215 0.0237 0.0336 0.0453 0.0508 7 0.0072 0.0088 0.0174 0.0181 0.0284 0.0395 0.0493 Fig. 10. Effect of silica fume content on the hygroscopic moisture content of soil- 8 0.0062 0.0075 0.0165 0.0167 0.0257 0.0361 0.0466 based foamed concrete for Series II under different relative humidities. 9 0.0052 0.0061 0.0157 0.0176 0.0214 0.0361 0.0423

40 35 0 20% 40% 35 average Kunzel 60% average 30 Kumaran Cubic 30 25 25 3 3 20 kg/m kg/m 20 15 W( W( 15 10 10

5 5

0 0 0 102030405060708090100 0 102030405060708090100 relative humidity/ (%) relative humidity/ (%)

Fig. 11. Absorption isotherms of soil-based foamed concrete with different foam Fig. 13. Hygroscopic ﬁtted curves for Series I. volumes for Series I.

In order to obtain the hygroscopic ﬁtted curves, the three most range of relative humidity from 8.5% to 100%. The sorption iso- well-known theoretical models to represent the sorption isotherms of soil-based foamed concrete and the moisture content therms curves are used. per unit mass show clearly that the moisture absorption increased The Künzel model [30] is used by the WUFI moisture transport as their densities decreasing. This observation is consistent with simulation software and its mathematical expression is: earlier results presented in Ref. [29]. And the results obtained in this research are useful to determine the hygroscopic sorption of ðb 1Þu WðuÞ¼Wlim ð4Þ different specimens. b u M. Cong, C. Bing / Construction and Building Materials 76 (2015) 61–69 67

40 where Wlim and b are the coefﬁcients of the ﬁtted curve. The Kumaran model [31] was reported in the Annex 14 of the 35 average Kunzel International Energy Agency (IEA), proposes the following Kumaran Cubic function: 30 u Wðu; A; B; CÞ¼ ð5Þ Au2 þ Bu2 þ C

3 25

where A, B and C are the coefﬁcients of the ﬁtted curve. kg/m 20 The Cubic function [32] is another model and its mathematical

W( 15 expression is:

3 2 10 WðuÞ¼au þ bu þ cu ð6Þ where a, b and c are the coefficients of the fitted curve. 5 In order to fit the experimental data of hygroscopic sorption 0 tests, the functions of the above mentioned theoretical models 0 102030405060708090100 are used. The hygroscopic fitted curves for Series I and II are shown relative humidity/ (%) in Figs. 13 and 14. The values of the coefficients and R-square obtained in each of the fitted curves are shown in Table 4. From Fig. 14. Hygroscopic fitted curves for Series II. the results shown in Figs. 13 and 14, and Table 4, the coefficient of determination indicates that the fitted curves to the theoretical

Table 4 Coefﬁcients and R-square of the ﬁtted curves for the hygroscopic tests.

Series Künzel model Kumaran model Cubic function Coefficients R-square Coefficients R-square Coefficients R-square

5 Series I Wlim 28.9953 0.9701 A 0.04904 0.9936 a 5.8419 10 0.9985 b 6.8098 B 0.06968 b 0.0085 C 0.01195 c 0.5760

5 Series II Wlim 32.2827 0.9292 A 0.07868 0.9886 a 9.8557 10 0.9971 b 1.4101 B 0.09853 b 0.0130 C 0.00748 c 0.6765

Fig. 15. Optical micrographs of the pores distributed in soil-based foamed concrete containing (a) 20% foam, (b) 40% foam, (c) 60% foam and (d) 75% foam. 68 M. Cong, C. Bing / Construction and Building Materials 76 (2015) 61–69

Fig. 16. Effect of silica fume on the pores distributed in soil-based foamed concrete containing (a) 5% silica fume, (b) 10% silica fume, (c) 15% silica fume and (d) 20% silica fume. models developed for the Kumaran model and the Cubic function concrete, thus improving the compressive strength of soil-based are appropriate to reproduce the hygroscopic behaviour of soil- foamed concrete. Silica fume leads to more uniform soil pores based foamed concrete. Specifically, the fitted function developed and closed circular pores, which significantly improves and regu- by Cubic function is a bit better than the Kumaran model. In addi- lates the insulation performance of foamed concrete. As the diam- tion, the function developed by Künzel model is the worst-fitting eter of the aperture decreases, it is not so unrestricted for the liquid curve and it does not fulfil the requirements of the minimum water to permeate into the foamed concrete, which improves the required goodness. water resistance of soil-based foamed concrete. Moreover, it is more difficult for the foamed concrete to absorb water vapour from 3.4. Optical micrograph of the pore structure of soil-based foam the air when relative humidity is relatively low. concrete 4. Conclusions The pore structure of the foamed concrete is crucial to its mechanical properties, thermal insulation and hygroscopic prop- In this study, the effects of foam and silica fume contents on the erty. Optical micrographs of the pores distributed in the soil-based properties of soil-based foamed concrete were investigated. Based foamed concrete are presented in Fig. 15, wherein (a)–(d) are the on the experimental results and numerical fitting study of this optical micrographs of the pores for soil-based foamed concrete investigation, the following findings can be drawn: containing 20%, 40%, 60% and 75% foam, respectively. The foam formation in foamed concrete is clearly better with increasing foam (1) For the same water cement ratio, soil-based foamed concrete contents. As the aperture grows, it become more uniform as the with different densities can be produced by introducing dif- groove becomes increasingly circular. Compared with the other ferent foam volume fractions. The thermal conductivity three mixtures, there are more interconnected pores in the mixture decreases and the compressive strength increases as the with 75% foam added. A high foam and micropore content greatly foam volume fraction increases. Accordingly, the water improves the thermal insulation and hygroscopic property of soil- resistance decreases and the water absorption increases based foamed concrete. We know that the strength of soil-based with the decrease of the dry density. foamed concrete depends mainly on the matrix pore structure (2) Silica fume added into soil-based foamed concrete can and bonding interface. Although the foam formation improves, greatly increase the strength and improve the thermal insu- the changes in the foam volume enlarged the inner pores, signifi- lation performance. Besides, the soil-based foamed with 20% cantly reducing the compressive strength of concrete. silica fume added can be regarded as water-resistant light- The effects of silica fume content on the pores distributed in weight concrete. Based on an analysis of the internal micro- soil-based foamed concrete with 5%, 10%, 15% and 20% silica fume scopic pores, silica fume improves the aperture distribution, are presented in Fig. 16(a)–(d), respectively. The aperture clearly making pores more uniform and closed circular, which grows smaller and the foam becomes more compact with the addi- improves the insulation performance and the strength. tion of silica fume. Adding silica fume can improve the pore struc- (3) Quick lime added into soil-based foamed concrete can ture of concrete gel by reducing the aperture while also reducing greatly increase the strength and density because the foam the volume of harmful pores and improving the density of the stability is degraded. Although the formation of calcium M. Cong, C. Bing / Construction and Building Materials 76 (2015) 61–69 69

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