Generation and stability of bubbles in a cement based slurry

Cement and Concrete Research 60 (2014) 37–44

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Generation and stability of bubbles in a cement based slurry Pauline Petit a, Isabelle Javierre b, Pierre-Henri Jézéquel b, Anne-Laure Biance a,⁎,1 a b

Institut Lumière Matière, Université de Lyon, UMR5306 Université Lyon 1-CNRS, 69622 Villeurbanne, France Lafarge Centre de Recherche, St. Quentin Fallavier, Isère, France

a r t i c l e

i n f o

Article history: Received 21 August 2013 Received in revised form 20 February 2014 Accepted 28 February 2014 Available online 3 April 2014 Keywords: Foamed concrete Particles at interfaces Surfactant

a b s t r a c t In this article, the fabrication of a single stable cement bubble is investigated. To achieve this goal, the stability of model particle covered bubbles is firstly experimentally studied, by bubbling in a pool filled with micrometric silica particles. Bubble stability is shown to be governed mainly by particle covering rate, which is maximized when particle wetting angle prior to liquid approaches π/2. This angle can be adjusted in situ by electrostatic adsorption of cationic surfactant on silica if proper amount of surfactant is added in the silica suspension. The covering rate is also shown to be governed by the time spent by the bubble in the pool, allowing us to define a timescale for particle adsorption at the liquid/gas interface. In the end, this method is shown to be successful with other types of foamed granular materials such as cement, and the fabrication of a stable and fully covered solid cement bubble is for the first time demonstrated. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Buildings currently account for 40% of the global primary energy consumption, mainly through its daily use during its life time [1], and in particular through thermal regulation. In this context, designing a material with large insulation properties keeping a good mechanical resistance remains a crucial challenge. Solid foams, and particularly foamed concrete, gather these characteristics [2], which explains the renewed interest for this material. These particular foamed materials also possess a light weight with densities between 1600 and 400 kg/m3 [2] thus reducing primary material consumption and consequently decreasing the greenhouse-gas emissions due to the manufacture of Portland cement, which today represents 5% of all human generated emissions [3]. Solid foams are constituted of air voids captured in a solid matrix, with a gas fraction which can reach up to 97% [4,5]. Foam is a multiscale material, whose macroscopic properties, like mechanical strength [6], are strongly related to the microstructure of the foam. For example, thermal and acoustical insulations are governed by the structure of closed or interconnected bubbles [6,7], an open porosity being in some situation a drawback for a good insulation. Moreover, material durability is strongly affected by this porosity because of fluid and ion transport [8] within the foam matrix. Consequently, to achieve a performant material, foam porosity must be controlled to obtain disconnected air bubbles within the matrix [5,9,10]. Many factors such as

⁎ Corresponding author. E-mail address: [email protected] (A.-L. Biance). 1 Tel.: +33 4724 48228.

http://dx.doi.org/10.1016/j.cemconres.2014.02.008 0008-8846/© 2014 Elsevier Ltd. All rights reserved.

drainage, coarsening or rupture affect the interconnection between bubbles, mainly during foam generation and solidification. The first step to achieve such a material is the possibility to create one single, closed and solidified, bubble. Then, a prerequisite is the formation of a longstanding stable concrete bubble that will in a second step solidify via concrete hydration reaction. The conditions required for these two processes to be achieved, the stable bubble formation and its subsequent solidification, are discussed in the following. Indeed, a bubble, which consists of a thin liquid film separating two gas regions, is intrinsically an out-of-equilibrium system that will eventually collapse. To stabilize a bubble, amphiphile molecules such as surfactants, which adsorb on the interface and allow spontaneous cicatrization of some holes created by external perturbations, are added in the liquid solution. These molecules allow to stabilize foams or bubbles for times ranging from a few minutes to a few hours. Another type of surface active stabilizers are nano or micro particles, which are more protective as they remain attached on the interface. Indeed, the desorption energy of a particle attached to a liquid/gas interface depends on the wetting property of the solid with respect to the liquid and the gas and reads W = πR2pγlg(1 − cosθ)2, with Rp the radius of the particle, γlg the surface tension of the liquid/gas interface, and θ b 90° the contact angle [11–13]. This desorption energy is maximal around 90° and of thousands of kT for microparticles. Thus, foam stabilized only by solid particles can last for months as the particle layer at the interface inhibits film collapse and then bubble coalescence and/or coarsening [14]. The main challenge of this method is to adsorb the particles on the interface, which can be achieved by tuning in situ the relative solid/liquid wetting properties. Martinez et al. [14] changed the liquid properties by adding a small amount of volatile ethanol in the initial mixture that will

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P. Petit et al. / Cement and Concrete Research 60 (2014) 37–44

eventually evaporate, and then reduce the particle affinity with the liquid. Another common method is to modify the wetting properties of particles by electrostatic adsorption of oppositely-charged short-chain surfactants on the solid surface [15–21]. We investigate in the following the possibility of this processes on particle and cement stabilized bubbles. The second prerequisite is to keep a stabilized bubble during solidification. Indeed, a solid thin film and then a closed bubble are obtained only if the hydration reaction occurs within the thin film covering the bubble: the film needs to be filled with particles but also requires a sufficient quantity of water for the hydration reaction to occur. Moreover, the reaction adds new destabilization factors to the foam, because of water movement, modification of particle properties, cement reactivity and shrinkage. To investigate these two steps, we first study the fabrication of stable long standing bubble in a model suspension that is constituted of silica beads whose wettability is changed in situ by surfactant adsorption. The effect of suspension wettability, concentration and bubble rising time are investigated. The methodology employed is then applied to bubbles created in a cement paste where the solidification process is tested. The article is organized as follows: after a first part dedicated to the material and method employed, the fabrication of bubbles fully covered with solid spherical silica beads is considered as a model system. Then, the conditions for obtaining a bubble stabilized with cementitious particles and its subsequent solidification are successfully experimentally investigated. 2. Materials and methods 2.1. Materials In order to study bubble formation in a model paste, silica beads (Tecosphere) with a mean diameter of 36 μm and a standard deviation of 37 μm are suspended in an aqueous solution of surfactants at different concentrations between 5 × 10−4 and 2 times the critical micellar concentration (CMC) of the system. Sodium dodecyl sulfate (SDS) is used as a model anionic surfactant (Sigma, 98.5%, No. L4509, CMC = 8 × 10 − 3 mol L − 1 [22]), and tetradecyltriammonium bromide (TTAB) is used as a model cationic surfactant (Sigma, 99%, No. T4762, CMC = 3.5 × 10− 3 mol L− 1). Glycerol is added to vary the bulk viscosity η of the suspending phase between 1 and 10.7 mPa s [23]. The solid volume fraction ϕ in the suspension is varied between 26% and 52%. The reference compositions are given in Table 1. TTAB and SDS are not used simultaneously and the different parameters of the system are varied independently. The viscosity of the suspending fluid is measured with a capillary viscosimeter (Ubbelohde) with a precision of 15% because of temperature variations and the surface tension is measured by the Wilhelmy plate method (Nima) with a precision of 5%. In our real cement paste, Portland cement CEM I 52.5 R is mixed with a limestone filler (Betocarp HP Orgon de Omya) and the water/powder mass ratio is 0.25. Solutions of two different admixtures are made in deionized water: polycarboxylate polymers (PCP) containing grafted PEO chains as superplasticizer, and surfactants (either SDS or TTAB). Small

Table 2 Physical properties of the particles, with d the density, Σ the specific surface and Rp the particle radius. Material

Silica

Cement

Limestone

d (kg m−3) Σ (m2 g−1) Rp (μm)

2500 – 5–40

3110 0.42 1–50

2750 1 1–50

quantities of paste are used, so the powder is gently stirred by hand during 1 min in the aqueous solution. Tables 1 and 2 summarize respectively the different reference compositions and solid phase physical properties used in our experiments. 2.2. Methods Air bubbles are injected through an aperture at the bottom of a cylindrical tank of diameter 26 mm initially filled with the paste. After detaching, bubbles rise by gravity in tubes of height h = 3.1 cm. A syringe pump allows to inject a controlled air volume (30 μL) at a given injection rate (0.1 mL s− 1), in order to obtain single bubbles. When the bubble reaches the interface, a liquid film is formed, which is fully covered with particles. The movement of the particles on the bubble thin film is then observed through a stereomicroscope, as depicted in Fig. 1. When the bubble reaches the interface, a liquid film is formed, which is fully covered with particles. The movement of the particles on the bubble thin film is then observed through a stereomicroscope, as depicted in Fig. 1. The particles rearrange in two zones, a covered one and a bare one as reported in Fig. 2. Obtained with reflected light, the stereomicroscope images allow to distinguish the area covered by particles from the bare area, as soon as the bubble reaches the surface. Then, the evolution of the covering rate of particles on the bubble is determined as a function of time. The covering rate is at equilibrium generally 15 s after the bubble reaches the surface, and a mean value of the covering rate is measured at equilibrium for 5 to 10 different bubbles. As wettability of our suspending solution versus particles is a crucial point on our experiments, two types of wettability measurements are performed, depending on the case considered. The contact angles of the different solutions on silica surface are determined by drop deposition on a clean plane silica slide, and side view image analysis as shown in Fig. 3-top. Each value is a mean of six measurements. Wettability of cement particles is measured by imbibition [24,25] of the liquid through the powder. A cylindrical column of 6 cm of height and 1.5 cm of diameter is filled with 13 g of packed cement powder. The bottom of this column is put in contact with a solution of PCP and SDS, as shown in Fig. 3-bottom. An estimation of powder wettability is deduced from the impregnation dynamics through Washburn law, with hw the height of column filled by liquid, dp the pore diameter, and t the time:

hw ðt Þ≃

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dp tγ lg cosθ 2η

ð1Þ

Table 1 Reference composition of the suspensions in the different experiments: concentration of TTAB, SDS (mol L−1) and PCP (g L−1), glycerol content (wt.%), solid volume fraction ϕ (%), mass of silica beads, cement and filler in the solid phase (%). System

a

b

c

d

e

f

TTAB SDS PCP Glycerol ϕ Silica Cement Filler

[10−5;10−2] – – [10;60] 52 100 – –

– 8 × 10−4 – 10 52 100 – –

5 × 10−4 – 4.8 – 58 – 33.7 66.3

– [10−6;10−2] 4.8 – 58 – 33.7 66.3

5 × 10−4 – – – 35 – 33.7 66.3

– [10−6;10−2] – – 35 – 33.7 66.3

P. Petit et al. / Cement and Concrete Research 60 (2014) 37–44

39

a

Camera

Stereomicroscope

θ Reflecting light

Bubble Paste

h ~ 3,1 cm

1 mm

b

Syringe pump Gas injection Fig. 1. Experimental set-up: a bubble is blown by the syringe-pusher in the tank containing our suspension. Bubble stability is investigated by top view images through a stereomicroscope.

This law can be quantitative if a geometrical factor is introduced depending mainly on the permeability of the powder. In practice, for comparison between the different solutions tested, and as the geometry of the powder is preserved in the experiments, a 6 cm liquid rise duration is measured, which will vary only due to particle wettability modifications. In the end, the solidification of the bubbles is achieved in Petri dishes. Post-mortem observations are also performed by SEM visualization. Then, the solidified bubbles are casted in a resin and polished sections of diameter 30 mm are prepared. Then, the samples are metalized under vacuum with a carbon evaporator. The microstructure of the obtained solid bubbles is analyzed by scanning electron microscopy images of sample slices (FEG-quanta400, HI-vacuum mode, 15 kV, with backscatter electron sensor). In the following, the covering rate of the rising bubble is measured for different compositions and surfactant concentrations of our model suspension of silica beads. It is shown to be correlated to particle wettability. A similar method is then employed to maximize the covering rate of bubble rising in a cement paste, where the cohesion of this particle layer is then tested through solidification.

hcement =6cm

hw(t)

1 cm

liquid

Fig. 3. Top: picture of a droplet on silica surface used for contact angle measurement. Bottom: picture of the imbibition experiment used to determine powder hydrophobicity.

3. Results and discussion 3.1. Model system The effect of the nature of surfactant on the evolution of the covering rate with time is first studied with aqueous solutions and either anionic

surfactant (SDS, system (b) of Table 1) or cationic surfactant (TTAB, system (a) of Table 1 with 5 × 10 − 4 mol L − 1 and containing 10 wt.% of glycerol). The measurement values are reported in Fig. 4, which show that particles are expelled out of the film in less than 1 s

Fig. 2. Left: Picture observed in reflected light from the stereomicroscope. Right: Image treatment used to determine the film covering rate.

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P. Petit et al. / Cement and Concrete Research 60 (2014) 37–44

100

Covering rate [%]

80

60

40

20

0 0

2

4

6

8

10

12

can be estimated by considering that each available site of silica bead is covered by a TTAB molecule. Indeed, when silica beads are mixed with milli-Q water, the water pH increases from 6.3 to 10, indicating that the silanol (SiOH) groups on the silica surface are in the state of the conjugate base SiO−. When TTAB is added, as related in Fig. 6, the ammonium ions (TTA+) adsorb on the negative SiO− sites of the silica creating a hydrophobic monolayer and an increased contact angle. When each silanol site interacts with one surfactant molecule, a second layer of surfactant can adsorb on the previous one in order to minimize the number of aliphatic chains in contact with water: the silica surface becomes covered by a bilayer as depicted in Fig. 6, and the contact angle decreases again. The minimum of wettability can be estimated considering that it occurs when the number of silanol groups is equal to the number of surfactant molecules in solution:

t [s] Fig. 4. Evolution of a bubble particle covering rate with time for suspending solutions at 52% of solid volume fraction with SDS at 8 × 10−4 mol L−1 (system b of Table 1, green triangles) and TTAB at 5 × 10−4 mol L−1 (system a of Table 1, red diamonds and blue squares).

in the case of SDS, while bubbles stay fully covered with TTAB during more than 10 s. The contact angle of the suspending fluid containing 60 wt.% of glycerol on the silica slide is 18 ± 6° in the case of SDS (8 × 10−4 mol L−1), while it is 32 ± 6° for TTAB (5 × 10−4 mol L−1). Indeed, TTAB has a positively charged head, which can adsorb on the negative silanol groups of the silica surface [20], and therefore increase its contact angle. When the particles have a larger contact angle, due to energy balance, they prefer to adsorb on the interfaces and then create a protective layer which stabilizes bubbles provided that the contact angle remains below 90° [12]. Then, to test the optimum of this process, the bubble covering rate is measured for suspending liquids containing 60 wt.% of glycerol and various TTAB concentrations in water (system a of Table 1). Silica beads are shown to adsorb on the liquid film when TTAB concentration remains between 2 × 10−4 mol L−1 and 2 × 10−3 mol L−1, as shown in Fig. 5. Particles are expelled out of the film and covering rate collapses if TTAB concentration is set out of this range i.e. for larger and lower concentrations. The evolution of covering rate with concentration is compared to the one of the contact angle on the glass plate, as reported in the inset of Fig. 5. The variations of the wettability versus concentration are smoother than the ones of the covering rate, but both are correlated and show the same qualitative tendency. If adsorption of TTAB surfactant molecules on silica beads is the main mechanism of wettability variations, the optimum TTAB concentration 100

40

Covering rate [%]

θ [°]

30

80

20 10 0

60

−4

−2

10

10

C [mol.L−1]

1 1 3ϕ 1 NA Σsilanols 1−ϕ Rp

20

ð2Þ

with C max the concentration corresponding to the maximum contact angle, N A the Avogadro number, 1/Σ silanol the surface charge density, and ϕ the solid volume fraction. Therefore, with 1/Σsilanols = 0.5SiO−/nm2 [22], ϕ = 52%, and Rp ∼20m , one obtains Cmax ∼ 1 × 10−4mol ⋅ L−1. This estimation is slightly lower than the maximum of the experimental covering rate and contact angle, around 5 × 10−4 mol L−1 (Fig. 5). However, this rough estimation does not take into account the loss of entropy associated to surfactant adsorption, which limits the balance between the adsorbed and bulk species and consequently might explain the discrepancy observed. Requirements for foam fabrication generally involve dynamical parameters among which the duration of particle adsorption is usually a crucial limiting factor for stable foam fabrication. We investigate the effect of this duration on the covering rate by varying the height h of the tank and consequently the time spent by the bubble in the paste. Fig. 7 shows the results obtained for a fixed TTAB concentration of 5 × 10−4 mol L−1 and particle volume fraction ϕ = 44%. The height of rise appears to be a crucial parameter: the covering rate increases with h, a plateau being observed for height below 3 cm. To disentangle the effect of time spent in the solution with the one of the number of particle encountered during bubble rise, the solid content of the paste is varied for a fixed value of h. According to Fig. 7, particles are expelled out of the film if ϕ b 35%, but are stuck on the film if ϕ N 50%. Additional experiments (not shown here) confirm these observations for different TTAB concentrations, so this effect is not a consequence of the shift on Cmax (Eq. (2)), but it is linked to the mechanism of bead adsorption at the liquid/gas interface during the bubble rise. Indeed, silica beads have to overcome an interfacial energy barrier to absorb on the interface. Then, the probability to have a full coverage is expected to increase with the number of particles met during the rise. When h or ϕ are modified, both the number N of particles met by the rising bubble and the rising duration t are respectively varied. Indeed, when the solid content ϕ of the paste is increased, its viscosity and consequently the bubble rise velocity are modified. The number of particles Nbeads met by the bubble during the rise then reads: Nbeads ϕh ∼ πR2b 43πR3p

40

0

C max ¼

ð3Þ

and the duration of rise tr writes: −4

−3

10

10

C

−2

10

[mol.L−1]

Fig. 5. Evolution of the equilibrium covering rate with TTAB concentration with system a of Table 1 (inset: contact angle of the suspending fluid on a silica slide for different TTAB concentrations).

tr ¼

hη ðϕÞ 9  paste 2 gR2 ρ −ρ b

air



ð4Þ

paste

where ηpaste is the viscosity of the suspension estimated by the law of Krieger–Dougherty [26], Rb is the bubble radius, ρair is the density of the gas inside the bubble, and ρpaste is the density of the suspension.

P. Petit et al. / Cement and Concrete Research 60 (2014) 37–44

TTAB

41

TTAB +

θ=

+

+

silica

+

silica

θ

+

+

silica

θ

Fig. 6. Mechanism of surfactant adsorption at the surface of silica beads. Blue and red circles represent respectively negative and positive charges.

The covering rates are then represented as a function of the duration of rise tr or the number of particles met Nbeads in Fig. 7. For our two sets of experiments (with variable tank height or variable solid content in the tank), the collapse of the data when plotted in function of the duration of rise assumes that it is the parameter which controls the bubble covering rate at the end of the rise. Moreover, these experiments allow us to define a timescale for particle adsorption of about 0.4 s, to be compared to the timescale of foam generation processes.

3.2. Concrete bubbles The same experiments of the covering rate measurements are performed for bubbles rising in a cement paste. Our cement based slurry is a complex material as described before, as it contains cement, filler particles and PCP. To disentangle the effect of cement and filler particles on the covering rate, benchmark experiments in a paste (ϕ = 50% and ϕ = 45%) of cement particles alone or filler particles alone respectively have been performed, always in the presence of PCP in the liquid. These experiments show that filler and cement particles have similar behaviors, very comparable to the one of a mix between the two species.

In both cases, the particles stay on the bubbles, which break as soon as a liquid thin film appears, in about 17 ± 3 s. Then, as performed with silica beads, TTAB and SDS are tested at the concentrations of system (c) and (d) of Table 1 with a mix of cement and filler, in order to deduce which kind of surfactant can interact with our cementitious mix of particles. The covering rate is measured in the presence of PCP, in order to obtain a paste comparable with the model system concerning the solid volume fraction, but with a controlled rheology necessary for the bubble rise. With TTAB, a liquid soap film is observed after 2 ± 0.5 s of particle migration, while a stable covered bubble is obtained with SDS at 8 × 10−4 mol L−1. In the case of cement particles, it seems that wettability modification and particle adsorption is more efficient when a negative surfactant is added in the liquid phase, in good agreement with a positive zeta potential of cement particle [27]. By analogy with the study on silica beads, the effect of SDS concentration on the bubble covering by particles is investigated with the system (d) of Table 1. The covering rate measured for different concentrations 15 s after a bubble appears at the liquid surface is reported in Fig. 8. The particles are stuck in the thin film for SDS concentrations ranging between 4 × 10−5 and 1 × 10−3 mol L−1, where fully covered bubbles are observed. Moreover, the bubble remains closed after

100

Covering rate [%]

Covering rate [%]

100 80 60 40 20 0

0

2

4

80 60 40 20 0 0.2

6

0.3

0.5

0.6

0.6

0.8

100

Covering rate [%]

Covering rate [%]

100 80 60 40 20 0

0.4

φ

h [cm]

0

2

4

N [mm−2]

6

8

x 10

4

80 60 40 20 0

0

0.2

0.4

t [s]

Fig. 7. Top: covering rate at equilibrium, as a function of the height h of rise (left) and of the solid volume content ϕ (right). Bottom: covering rate versus the number of particles met during the rise N ¼ NπR (left) and versus the duration of the bubble rise tr (right). Blue squares and red diamonds correspond to experiments at different heights of rise and solid volume fraction respectively, data from above. beads 2 b

42

P. Petit et al. / Cement and Concrete Research 60 (2014) 37–44 Table 3 Rise duration in 6 cm of cement powder with solutions containing 4.8 g L−1 of PCP and various concentrations of TTAB.

95

C [mol L−1]

t [min]

η [mPa s]

γlg [mN m−1]

90

5 × 10−4 1 × 10−2 2.5 × 10−2

16 ± 10 23 ± 10 20 ± 10

0.99 1.14 1.07

43.7 36.6 37

Covering rate [%]

Covering rate [%]

100

100

85

80

50

0

−5

10

75 −6 10

10

−5

10

−4

C

10

−3

−3

10

C [mol.L−1] 10

−2

−1

10

10

−1

[mol.L−1]

Fig. 8. Covering rate on cement bubble measured 15 s after the bubble rise for different SDS concentrations (systems (d) of Table 1). Inset: Covering rate on cement bubble measured 15 s after the bubble rise for different SDS concentrations (systems (f) of Table 1).

solidification if maintained during 1 day in a closed Petri dish, as reported in Fig. 9. Outside of this range of SDS concentration, particle migration is limited but bubble breakage is observed as soon as a bare area without particles grows up, usually after 22 ± 7 s, depending on the surfactant concentration. However, the mix contains PCP and SDS molecules, which are both negatively charged, and should then be in competition concerning the adsorption on cement particles. Therefore, TTAB and SDS are tested alone at the concentrations of system (e) and (f) of Table 1. The same qualitative behavior is observed with TTAB and SDS. With TTAB, a liquid soap film is observed after 2 ± 0.5 s of particle migration, while a stable covered bubble is obtained with SDS. Moreover, the variation of the covering rate with SDS concentration shows an increase at low concentrations (insert of Fig. 8). The curve maximum seems also to be slightly shifted towards a higher surfactant concentration. We can deduce that with or without PCP, the effects of negatively charged surfactant concentration are qualitatively similar. To validate the mechanism proposed for model silica beads on the concrete material, based on particle wettability modification by surfactant adsorption, cement wettability with respect to water must be measured. Contact angle measurement by droplet deposition method is difficult to achieve as water drops deposed on a bed of cement particles spread on the surface in less than 1 min because of the material porosity

[28], even on a solidified material, i.e. a hardened and sanded cement surface. A more suitable method to quantify the wettability is the study of imbibition dynamics [29], as defined previously (see Section 2.2). The results of imbibition duration over 6 cm of powder are reported in Tables 3 and 4 for water + PCP mixtures with different surfactant types (SDS or TTAB) and concentrations. One can first notice that the rise duration is always shorter than 40 min, except for two concentrations of SDS. The variations of solution viscosity and surface tension cannot explain these variations of imbibition dynamics over more than one order of magnitude. Therefore, the surface wettability is altered by the introduction of SDS in the solution, as TTAB remains inert. It correlates with the covering rate measurements: positively charged particles are expelled from the film by repulsive interactions with positive surfactants (TTAB) and by drainage, while particle surface is made partially hydrophobic by adsorption of negative surfactants (SDS). The effect is observed even with PCP in solution. It shows that when negative surfactants and superplasticizers are simultaneously mixed in the solution, an adsorption competition can occur. We can compare this effect with the competition between sulfate ions and PCP adsorption which reduces rheological modification of superplasticizer on the slurry [30]. As in the model silica particle solution, particle wettability depends again on surfactant concentration, here SDS. The rise duration is less than 20 min for solutions of low concentration in SDS until 8 × 10−4 mol L−1. At 1.6 × 10−2 mol L−1 and 4 × 10−2 mol L−1, the solutions have imbibed only 3 cm after 2 h. For higher concentrations, the rise duration decreases again to 35 min, where adsorption of multiple layer is invoked, as depicted in Fig. 6. The SDS concentrations at which we obtain fully covered bubbles and at which imbibition is lower are not in a satisfying agreement. However, these discrepancies can be attributed to differences between the two experiments such as the timescale involved, the number of particles in contact with the liquid or powder composition. Indeed, while a mix of cement and filler is used in the covering rate measurements, cement alone has been characterized for imbibition, because of dry powder homogenization issues. Surfactant adsorption is directly related to the zeta potential of the particles, which is related to the different mineralogic phases of the cement powder composition [27]. Furthermore, cement is reactive and its properties, in particular the ionic concentrations within the paste, evolve with time. The ionic composition of the solution has an effect on the adsorption and zeta potential because of the effects of charge screening [31,32]. Therefore, as the timescales of both experiments differ from one order of magnitude, discrepancies between the two methods are expected. Moreover, the critical concentration for which wettability modification occurs is a function, as shown in our Table 4 Rise duration in 6 cm of cement powder with solutions containing 4.8 g L−1 of PCP and various concentrations of SDS.

Fig. 9. Solid bubble obtained with a cement paste containing PCP and SDS at 8 × 10 − 4 mol L− 1 , after setting in a Petri dish (systems d of Table 1).

C [mol L−1]

t [min]

η [mPa s]

γlg [mN m−1]

0 4 × 10−5 8 × 10−4 8 × 10−3 1.6 × 10−2 4 × 10−2 8 × 10−2

17 ± 18 ± 20 ± 37 ± N120 N120 35 ±

1.15 1.10 1.12 1.13 1.16 1.30 1.51

58.5 57 52.4 40.9 40.1 39.2 38

5 5 10 10

5

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a consequence of which is that Portlandite phase has disappeared because of its reactivity with ambient carbon dioxide [8]. 4. Conclusion The attachment of particles on a single bubble has been studied, firstly with a model system containing silica particles, and then with cementitious particles. In the two cases, it was experimentally demonstrated that the particles stay on the liquid film if their surface is partially hydrophobic. In order to obtain such surface properties, the surfactants need to adsorb on the particles in a monolayer. Therefore, they can be chosen with a charge opposite to the one of the particle surface, and with an intermediate concentration, slightly lower than the CMC, about 0.1 CMC. The experiment highlights also that an equilibrium state of the particles on the bubble is reached in less than 1 min after production: a cement bubble fully covered after this first phase results most of the time in a closed solid bubble after setting. The parameters governing particle adsorption dynamics at the interfaces have also been identified, as they play a crucial role in the foaming process, i.e. to produce foam with particles at the industrial scale. Our study shows that the interface must be in contact with the particles at least 0.5 s to obtain, if conditions are optimized, a fully covered and stable bubble. This first demonstration of solidification of a thin cementitious liquid film is a first step in the achievement of a concrete foam at small solid fraction and with closed bubbles. However, in the macroscopic material, interactions of the thin films with foam structure (i.e. contacting channels between the bubbles, the so-called Plateau borders) must be taken into account. Acknowledgments Anne-Laure Biance thanks CNES for a partial funding of this project through convention CNRS/CNES number 127233. References

Fig. 10. SEM images of solid cement bubble after 6 months of hydration.

model experiments with silica particles, of solid fraction within the paste ϕ (Eq. (2)). In the imbibition experiments, this concentration evolves with time (as more powder is in contact with the liquid when the liquid front is rising). A complete characterization of surfactant diffusion and adsorption kinetics would allow to define the total dynamics of wettability variations, which is not in the scope of our paper. The solid bubble obtained with the composition of system (d) in Table 1 with 8 × 10−4 mol L−1 of SDS is observed by scanning electronic microscopy (SEM – Fig. 10). It shows that the solid film has a thickness of 55 ± 5 μm. Therefore, the liquid film after drainage is filled with particle multilayers homogeneously distributed. At larger magnification, filler (bright) and cement (gray) grains can be observed mixed with hydrated phase (CSH). These CSH phases are the signature that during the process, the liquid film retains a sufficient amount of liquid to allow hydration reaction to occur within the protective layer. These phases are mandatory to maintain good mechanical properties of the layer. One must notice also that the observation of the solid bubble has been performed 6 months after the beginning of the experiment,

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