Synthesis and Performance Evaluation of a Synthetic Based Foaming Agent

Barugahare Javilla, SHUI Zhong-he,CHEN Wei

(1.State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology，Wuhan 430070，China;2.School of Material Science and Engineering, Wuhan University of Technology, Wuhan 430070,China)

Synthesis and Performance Evaluation of a Synthetic Based Foaming Agent

Barugahare Javilla1, 2, SHUI Zhong-he1, 2,CHEN Wei1, 2

(1.State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology，Wuhan 430070，China;2.School of Material Science and Engineering, Wuhan University of Technology, Wuhan 430070,China)

The aim of this study was to create a foaming agent and then evaluate its performance.This foaming agent is made up of Sodium lauryl sulfate, Ethanol, Lauryl alcohol and Water.On aeration, the diluted foaming solution produces a stable foam whose properties， density, capacity and drainage were studied.The compressive strength, water absorption and density of foamed concrete were also studied.Test results show that the initial foam density is 104 kg/m3at an optimum dilution ratio of 1:40.Foamed concrete’s density can vary from 450 kg/m3to 1 950 bkg/m3and its compressive strength is in 0.5～23 MPa.Compressive strength test results of this foaming agent are higher than those produced with EABASSOC foaming agent.According to ASTM 869-91, this chemical composition qualifies to be used as a foaming agent.

surfactant;chemical abstracts service (CAS);foaming agent;foamed concrete

From literature, little to no information is reported about chemical compositions of foaming agents.Manufactures of foaming agents only provide instructions best of use.Since foam affects the properties of foamed concrete, to improve the properties of foamed concrete, it's best to know and study the chemical composition of foaming agents.

Foaming agents are surfactants that aid the creation of foams[1].Synthetic based foaming agents are manufactured from hydrocarbon based on surfactants.These surfactants are classified according to the nature of the polar head group as either Ionic, Non-Ionic, Cationic or Amphoteric as noted by Brady and Greene (1997)[2].The head of an anionic surfactant carries a negative charge while that of cationic surfactant carries a positive charge.An amphoteric surfactant contains a head with two oppositely charged ions.Synthetic based surfactants are known to have a longer working life, produce foams that are more suited in the production of heavier foamed concrete and allow for a greater control over the density of foamed concrete[3].

G.Indu Siva Ranjani and K.Ramamurthy (2010) reveal that foam produced using Sodium Lauryl Sulfate (a synthetic surfactant), has a foam density of 25 kg/m3and a drainage rate of 11.6% in 5 minutes at optimum concentration of 1.8.When 18% of this foam volume was mixed with cement sand mortar, it produced foamed concrete with the highest compressive strength of 12.5 MPa and density 1 500 kg/m3[3].According to the 28 day compressive strength target of this paper (above 15 MPa) and compared to test results of EABASSOC foaming agent[4], a compressive strength of 12.5 MPa is low.

It has been proposed that, for a foaming agent to be used in the production of foamed concrete, foam properties like optimum dilution ratio, foam density, foam capacity and foam drainage need be studied and foamed concrete properties like compressive strength, density, and water absorption also need be studied[3, 5,6].Foam capacity is the measure of the expansion ratio of foaming solution and dilution ratio is the concentration of surfactant in the foaming premix solution[3].

The purpose of this research is to design a suitable compositional formula of a synthetic based foaming agent, assess its performance in the production of foamed concrete and its performancerelationship with foaming agents already in use in the foamed concrete construction industry.

1　Material and methods

1.1Foaming agent composition

From the study of surfactants, emulsifiers and solvents, three chemicals were chosen in the design of this foaming agent; Sodium lauryl sulfate, Lauryl alcohol and Ethanol.Sodium lauryl sulfate is an anionic surfactant.Its Chemical Abstracts Service (CAS) number is 151-21-3.Its toxological studies reveal it’s not carcinogenic but it can irritate the skin and eyes on constant exposure[7].Lauryl Alcohol a nonionic surfactant and an emulsifier.Its chemical formula is CH3(CH2)10CH2OH and its Chemical abstract Service number 112-53-8.It is not carcinogenic though it’s been found to be a mild skin irritant[8].Ethanol is an organic solvent with a chemical formula CH3CH2OH.Its Chemical Abstract Service number (CAS NO) is 64-17-5.

The above table shows a chemical compositional range within which, this proposed foaming agent can form a stable foam on dilution in water and aeration using a foaming generator.(Section 3.1 explain the results of table1).Based on the cost of individual chemicals and foam trial tests, values in column 3 (25% Sodium lauryl sulfate, 7% Lauryl alcohol, 15% Ethanol and 53% water) were chosen to make a foaming agent that was used for this paper.Sodium lauryl sulfate and Lauryl alcohol were weighed, put in a container and stirred until the mixture was uniformly mixed.Ethanol was added and stirring sustained till when the mixture was uniform mixed again and lastly, water was added and the solution stirred until it was uniformly mixed.

1.2Foamed concrete composition

Foamed concrete composition included Ordinary Portland Cement (OPC) of grade 42.5 manufactured according to Chinese standards GB 175—2007.Its chemical composition based on X-ray fluorescence analysis is shown in Table 2 and well graded fine aggregates as according to ASTM C 33 and clean water as specified by BS 3148-1980.

Table 2　Chemical composition of cement (OPC 42.5)

1.3Design formulation of foamed concrete

At present, there is no standard method of design specified for the design of foamed concrete.In this design, the following assumptions were made in the design; a density lossLkg/m3as foamed concrete dries from its fresh state and a 30% volume reduction during mixing of the raw materials.

The mass of foam is density of foam multiplied to the volume of the mechanically entrained foam.The design losses (L) in density assumed are 150, 200 and 100 kg/m3for design target densities of 1 800,1 600, and 1 400 kg/m3respectively.

1.4Foam generator

The foam generatoris is used for the production of foam.It was run at a compressed air pressure of 0.4～0.7 MPa and suction pump rate of 260 to 630 r/min (27.2 to 66.0 rad/s) as according to ASTM C 869-91, Philadelphia, 1991[5].

1.5Tests

The foam drainage was assessed according to Def Standard 42～40 (2002), the compressive strength according to BS-1881 (BS 1881-11) and the water absorption test according to BS 1881: Part 122: 1983.Specimens for water absorption and compressive strength tests were 100 mm×100 mm× 100 mm cubes.

2　Results and discussion

2.1Foam agent composition

Table 1 shows a range of values within which a foaming agent may be made.Within this range, a stable foam with an appropriate foam drainage as required by Def Standard 42-40 (2002) can be made.Preliminary foam stability tests were carried out to determine a weight composition range of Sodium lauryl sulfate, Ethanol, Lauryl alcohol and water from which, a foaming agent can be made whose foam is stable and has a drainage rate of less that 15% within the first five minutes.Table 1 shows a weight composition range of individual chemicals that were successful as according to the above stated condition.

Sodium lauryl sulfate (SLS)used is an anionic surfactant with Chemical Abstracts Service (CAS) number of 151-21-3.It could produce foam though, its foam had a high drainage rate of about 25%～30% within the first five minutes.Lauryl alcohol is both a nonionic surfactant, an emulsifier and a foam booster[13].When used with SLS, the foam drainage rate was reduced to 5.9% within the first five minutes (results shown in table 3).Lauryl alcohol is insoluble in water and thus a relatively inexpensive solvent ethanol, was selected and used to dissolve it.

2.2Formation of foam

Sodium Lauryl Sulfate(SLS) is an anionic surfactant with both hydrophilic (water loving) and hydrophobic (water hating) parts.When dissolved in water, Sodium lauryl sulfate’s hydrophilic head (sulfate head) is attracted to the water surface (positive side of the polar water molecule) and its hydrophobic tail points out of the water surface.The attraction of the hydrophilic end of SLS to the surface water molecules, reduces the interaction force between surface water molecules and neighboring water molecules.The created new bonding between SLS and surface water molecules disrupts and affects the intermolecular hydrogen bonding energy of surface water molecules.When this energy is reduced, the surface tension of water is automatically reduced[14, 15].That is the purpose of SLS, to reduce the surface tension of water.

Micelles are formed within the internal water molecules when SLS is mixed or diluted with water but, they are exposed to the water surface when the foaming solution is agitated with compressed air in high density and porous restriction material.When they are exposed to the water surface, SLS molecules realign themselves as before (the hydrophilic part is attracted to the water surface and the hydrophobic part points out of the water surface).When the surface tension of water is reduced, it becomes easier for compressed air mixed with the foaming solution in high density restriction areas begin to create thin water skins that trap air[14, 15].A foam bubble is air trapped in a thin water skin.

2.3Relationship between dilution ratio, foam density, foam capacity and foam drainage

Table3　Dilution ratio effect on foam density, foam capacity and foam drainage

Note:FA is foaming agent.

From observation, an increase in concentration of the foaming solution resulted in production of a more viscous foam.The higher the concentration (less water involved in dilution), the drier the foam and as a result of the lower the foam density.Foam is relatively dry at higher concentration (less water is available to be drained) than at lower concentration and as a result, foam drainage decreases with increase in concentration.From observation, more foaming solution is drained from the foam bubbles with increase of foam bubble exposure in air and, foam bubbles are also seen to become bigger in size with increase in air exposure time within the first 15 minutes.This explains why the foam density decreases with increase with air exposure in the first 15 minutes.

The pressure difference between the inside and outside of an air bubble is given by the following relationship; ΔP= 4γ/D[14], where ΔPis pressure difference between inside and outside of air-void,γis surface tension andDis bubble diameter.It can be noted that smaller bubbles contain air at a higher pressure compared to bigger bubbles and, there’s a tendency for smaller bubbles to merge to form larger bubbles so as to lower the internal air pressure.When foam is exposed to air, the foaming solution begins to drain out of the foam and bubbles are seen to become bigger and lighter which results in a reduction in foam density.

2.4Relationship between dilution ratio, foamed concrete density and compressive strength

Table 4　 Relationship between dilution ratio, foamed concrete density and compressive strength

Cement to Sand ratio used was 1:2 and foam volume 30%.

1∶10 dilution ratio has the lowest 28 day dry density and 1∶40 has the highest 28 day dry density.From table 3, since the initial foam densities increased with increase in concentration, it is expected that, its related foamed concrete densities will follow the same increase in trend but it was not the case.It is not clear as to why the same foamed concrete mix design, mixed with different foam produced from different foaming agent dilution ratios, resulted in non-related dry and wet densities.

The same design mix was used for all dilution rations.No standard relationship was established between compressive strength and dilution ratio.It may be expected that since 1:60 has the highest foam density then the corresponding foamed concrete should have the lowest compressive strength but, it was not the case.It not yet clear still as to why foam produced from different dilution ratios of the foaming agent in water result in foamed concrete with different compressive strength.

2.5Relationship between dilution ratio and 10 day water absorption capacity

From the above table, it can be noted that the lowest 10 day water absorption was 11.8% at dilution ratio 1∶30.The loss in weight after oven drying is due to forceful water evaporation from capillary pores and the pores created by foam within foamed concrete.There’s no linear relationship between dilution ratio and 10 day water absorption.It is not yet understood as to why the same mix design of foamed concrete resulted in foamed concrete with different water absorption at different dilution ratios of the foaming agent in water.

Table 5Relationship between dilution ratio and 10 day water absorption capacity

DilutionratioTargetdensity/(kg·m-3)DryDensityat28days/(kg·m-3)Dryweightafter4daysofOvendryingat108℃/kgWeight(after10daysofcompletewaterimmersionat25℃/kgWaterabsorptionafter10days1∶10140013371207135812.51∶2014001440.51303147413.11∶30140014471307146111.81∶40140014951347151512.51∶50140014681320150313.91∶60140014551305148413.7

2.6The optimum conditions

G.Indu Siva Ranjani, K.Ramamurthy(2010) define optimum conditions of foam production as conditions under which foam produced has better characteristics and can be used to produce stable cellular structure when used in foamed concrete[3].Optimum conditions may also be extended to include conditions under which, foamed concrete produced has better mechanical properties at the lowest economical cost.

Based on the results of compressive strength tests in table 4, and the results in table 3 that show the variation effect of dilution ratio on foam density, foam capacity and foam drainage, it can be concluded that the foaming agent dilution ratio of 1∶40, produces foam with the highest 7 day compressive strength, the second highest 28 day compressive strength, the second highest 10 day water absorption rate, the fourth highest foam capacity, a stable foam density and a stable foamed concrete density.Compared to results produced with other dilution ratios, 1∶40 performed better and was selected as the optimum dilution ratio for this foaming agent.

2.7Evaluation of the foaming agent’s foam performance in cement paste as according to ASTM C 869

At an optimum dilution ratio of 1∶40, the foaming agent’s foam was evaluated according to its performance in cement paste as required by ASTM C869[5].Results were noted and recorded in the Table 6 below.

All test results passed the requirements of appropriate foaming agents as according to ASTM 869-91.It can thus be concluded that this foaming agent is suitable in the production of foamed concrete.

Table 6　Comparison of foam cement paste test results with ASTM specifications

2.8Performance of foamin cement sand mortar

The target design densities were 1 800, 1 600, 1 400 and 450 kg/m3.The volume of foam added to the raw mix of foamed concrete was varied from 20%～30%.Table 7 below shows the results after 28 days of curing of foamed concrete.

Table7　Test results of foam in cement sand mortar

Note: C: W is Cement to Water ratio and C: S is Cement to Sand ratio.

From table 7 above, it can be seen that at the same foam volume, the 28 day compressive strength increases with increase in density (1 400～1 800 kg/m3).It can also be seen that an increase in foam volume results in a decrease in the 28 day compressive strength for specimens having the same density, C∶W ratio and C∶S ratio.The more the pores in cement sand mortar the less the compressive strength.The lowest 28 day compressive strength is 0.5 MPa at design density 450 kg/m3and foam volume 55% and the highest 28 day compressive strength is 23.5 MPa at design density 1 800 kg/m3and foam volume 20%.

These compressive test results are higher compared to test results gotten by G.Indu Siva Ranjani, K.Ramamurthy (2010)[3]and construction site foam test results shown by EABASSOC Lightweight Foamed Concrete company in U.K[4].EABASSOC’s highest compressive strength lies between 16～18 MPa for foamed concrete of density 1 800 kg/m3made from sand to cement ratio of 2∶1.It can thus be noted that this foaming agent can make an appropriate foaming agent that can be used in the production of foamed concrete.

3　Conclusion

1)The optimum dilution ratio for this foaming agent composition in water is 1∶40.

2)This foaming agent’s foam is acceptable in the production of foamed concrete as according to requirements of foaming agents specified in ASTM 869-91.

3)This foaming agent’s foam can produce foamed concrete within a density range of 450 to 1 900 kg/m3.

4)This foaming agent’s foam is desirable in producing foamed concrete with a 28 day compressive strength of 0.5 MPa to 23.0 MPa as shown by the test results.

5)As compared to compressive strength test results of a foaming agent already in use in the construction industry (EABASSOC foaming agent), this foaming agent performed better.

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10.3963/j.issn.1674-6066.2015.02.002

Received：2015-03-04.

Author：Barugahare (1986-)，Master.E-mail:makorogo200@yahoo.com