Stability criterion for fresh cement foams

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Abstract

We prepare and study cement foam samples with well-controlled structure, i.e. containing monodisperse bubbles. We observe that the foam structure often changes before cement setting and identify ripening as the major destabilization mechanism at stake. Drainage plays only a minor role in cement foam destabilization except when bubble size is large. Then we show that a single stability criterion can be defined, for a large range of cement foams with different formulations. This criterion involves the bubble radius and the yield stress of the cement paste such as confined by and between the bubbles, at a given characteristic time after sample preparation.

Introduction

When it is unconstrained, a bubble has a spherical shape because of the air-liquid surface tension. Though, in a foam, bubbles are deformed by their neighbors. The structure of a foam was studied in 1873 by Joseph Plateau, who stated three laws known as Plateau's laws [1]: (1) two bubbles are separated by a soap film of constant average curvature, (2) three films join in a channel, called Plateau border, forming 120° angles, (3) four Plateau borders join into a node at angle 109.5°. The resulting morphology of foam tends to evolve with time due to downward flow of the interstitial fluid due to gravity (drainage), air exchange between bubbles (ripening) and film breakage (coalescence).

In cement foams, these destabilization mechanisms are expected to occur until cement paste hardening. Thus, to control the final bubble size and air distribution in the foam, one has to stop or slow down the three mechanisms. These mechanisms are affected by initial bubble size. For instance, the increase of bubble size for a given gas volume fraction results in the increase of the size of the film areas between the bubbles, which enhances coalescence. It also increases the size of the Plateau borders and nodes, which favors drainage [1]. Ripening, on the contrary, is reduced when bubble size increases. Indeed, it is caused by the capillary pressure inside the bubbles Pc ≈ 2γ/R0, where γ is the air-liquid surface tension and R0 the bubble radius.

To avoid coalescence, liquid film must be stabilized by molecules or partially hydrophobic particles, which adsorb at air-water interfaces. The molecules, called surfactants, must be compatible with the highly alkaline cement solution and be present in sufficiently high amount [1].

Consistency of cement paste is also expected to play a major role in foam stability. High yield stress can stop drainage and ripening [2, 3]. However, we have shown in a previous paper [4] that, in a cement foam, the effective yield stress of the cement paste confined between the bubbles, noted τy,int, can differ significantly from the yield stress of the reference cement paste τy,0, measured in the bubble-free paste. On the one hand, when τy,0 is low, i.e. a few Pascals, cement grains remain stuck in the channels and nodes between the bubbles, whereas gravity makes the liquid flow to the bottom of the foam. This drainage of the liquid leads to a decrease of the water-to-cement ratio of the interstitial cement paste, and therefore, to an increase of the interstitial yield stress τy,int up to about 100 Pa. On the other hand, when the yield stress of the reference cement paste τy,0 is high, i.e. a few tens of Pascals, no densification of the cement paste through drainage occurs, so that τy,intτy,0 during the first 10 min after sample preparation. For the cement foam formulations studied in Ref. [4], cement paste densification through liquid drainage was found to be essential to ensure the foam stability.

Therefore, the stability of fresh cement foams is expected to be observed for pastes with sufficiently high yield stress values, but stability can be observed also for low yield stress values. The aim of this paper is to reconcile those contradictory results and to propose a single stability criterion for cement foams. In the materials and methods, we describe how we prepare cement foams with controlled morphology and formulation, which allows for the factors controlling the stability of these cement foams to be investigated. First, the leading destabilization mechanism is identified. Then, the effects of bubble size and of cement paste yield stress are investigated. Finally, a global criterion for cement foam stability is defined.

Section snippets

Cement

We use two cements. The first will be referred to as C1, it is manufactured by Lafarge, in Saint-Vigor factory and C2 is a CEM I cement from Lafarge, Lagerdorf. Their compositions and physical properties are specified in Table 1.

Surfactants

Two surfactants are used to produce the precursor foam. Tetradecyltrymethyl ammonium bromide (TTAB) is a cationic surfactant at purity above 99% provided by Sigma-Aldrich. Its molar mass is 336 g/mol. Steol® 270 CIT is an anionic surfactant provided by Stepan. Its molar

Stability of aqueous foams

During the six experiments presented in Fig. 4, we observe that the foam becomes more and more dry and that the size of the air bubbles increases. However, the height of the foam did not decrease for 10 h in all cases. Foams made with synthetic cement pore solution are less stable than those made from distilled water, and the presence of C6F14 decreases the collapse velocity; however, both these effects can be seen only after 10 h.

Smaller bubbles (R0500μm)

Let us first consider the unstable samples, whose properties are

Destabilization mechanisms

Let us first consider coalescence. Experiments performed on aqueous foams made with both surfactants have shown that the foam volume was constant for more than 10 h with C6F14 and without. Stability of a thin liquid film depends on the ability of the surfactant layers on both interfaces to repel each other. Film breakage occurs when the disjoining pressure Πd, i.e. the pressure in the liquid film due the repulsion on the air-liquid interfaces, reaches a critical value Πd,crit. In an aqueous

Conclusion

We have investigated the mechanisms at stake in the destabilization of cement foam samples. We first note that a proper choice of surfactant can prevent coalescence of the bubbles up to cement hardening.

For most of the cement foam samples, prepared either with anionic or cationic surfactant, ripening is the leading destabilization mechanism. Drainage is also sometimes observed when the yield stress of cement paste is very low or bubble size is large. In these cases, we observed that drainage

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgments

The authors wish to thank David Hautemayou and Cédric Mézières for technical support for the foam generation and mixing devices, Sabine Carré for support on synchrotron measurements, Nicolas Ducoulombier for help for the 3D reconstructions of tomography measurements, and Michel Bornert for giving us time to study our samples on the synchrotron line and for useful comments on this paper. This work has benefited from two French government Grants managed by the French National Research Agency

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Current address: Department of Mathematics, University of Oslo, Oslo, Norway.

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