Contribution of non-adsorbing polymers to cement dispersion
Introduction
Colloids represent a class of nano- and micrometer sized particles which exhibit some unique properties [1]. Among them is their ability to scatter light when dispersed in a medium, which is commonly referred to as the Tyndall effect [2]. Typical colloidal materials are the different kinds of clays including bentonite, kaolinite, and attapulgite, as well as cement powder. When dispersed in water, colloids form suspensions exhibiting different viscosities depending on their particle size, density, pH-dependent surface charge, and the ionic strength of the aqueous phase. Cement dispersed in water produces an exceptionally high viscosity when compared with similar suspensions prepared from silica (SiO2) or limestone (CaCO3) powder of the same particle size. The reason behind is the heterogeneous surface charge of a cement particle stemming from the presence of different clinker phases (the aluminates C3A and C4AF exhibit a positive and the two silicate phases C3S and C2S a negative surface charge) [3]. Contrary to cement, SiO2 or CaCO3 possess homogeneous surface charges and thus do not much agglomerate in water via attraction of oppositely charged surface sites, as is the case for cement [4]. Therefore, to achieve a concrete or mortar of high fluidity and low viscosity, it is necessary to add a superplasticizing chemical admixture, which can disparage the large agglomerates into individual cement particles.
Generally, for the dispersion of particles in a medium, three principle mechanisms are being discussed in the literature: electrostatic, steric, and depletion stabilization [5], [6].
- (1)
Electrostatic stabilization can be achieved when two particles exhibit the same electrical charge. This concept was introduced by Derjaguin, Landau, Verwey, and Overbeek, who independently worked on the stability of lyophobic colloids (oil/water emulsions) [7], [8]. According to their model, which is now known as DLVO theory, the electrostatic repulsion potential is expressed by the equation as follows:where d is the interparticle distance, a is the particle radius, ε is the permittivity, ε0 is the permittivity of vacuum, σ is the charge of ions, F is Faraday's constant, R is the universal gas constant, T is the temperature, and γ = (ez/2 − 1) × (ez/2 − 1).
The electrostatic repulsion between two particles strongly depends on the Debye length 1/κ and thus on the thickness of the diffuse ion cloud around a particle. Therefore, high ionic strength caused by high electrolyte concentrations as are present in cement pore solution reduce the repulsive force between particles [9].
It is commonly believed that polycondensate-based superplasticizers such as, e.g., β-naphthalensulfonate formaldehyde, melamine formaldehyde sulfite, or acetone formaldehyde sulfite predominantly achieve dispersion of cement via this mechanism, although a partial steric effects is involved as well, as was evidenced from the thermodynamic parameters ΔG, ΔH, and ΔS for the adsorption of these polymers [10], [11].
- (2)
Steric stabilization arises from polymers adsorbed on the surface of a colloidal particle and is considered to be more effective than electrostatic repulsion [12], [13]. According to Ottewill and Walker, the steric repulsion potential between two particles is represented as follows [14], [15]:where Cν is the concentration of the adsorbed polymer, δ is the molecular volume of the solvent molecules, δ is the adsorbed layer thickness, f2 is the density of the adsorbate (polymer), ψ1 is the entropy, κ is the enthalpy, R is the radius of the adsorbate, and d is the distance between two adsorbate particles.
This model clearly suggests that the thickness of the adsorbed polymer layer, δ, plays a prominent role for stabilization as it impacts Vsteric in the third power. The superior dispersing effect of polycarboxylate superplasticizers is commonly attributed to a strong steric effect arising from their extended side chains [16]. This is supported by the observation that PCEs exhibiting longer side chains represent more powerful dispersants than those with shorter side chains [17]. Unfortunately, at present, only data on the adsorbed layer thickness of PCE polymers obtained from atomic force microscopic (AFM) measurements are available, which are surprisingly low (1–4 nm) and might be specific for this test method [18]. More data from additional methods will be needed to gain a complete understanding of the dimensions of various adsorbed polymer layers.
Attempts have been made to quantify the total repulsive force resulting from the sum of electrostatic and steric (the “electrosteric”) contributions by using AFM measurements [19], [20], [21]. It was found that PCEs can provide a much higher repulsive force than linear polycondensate-based superplasticizers, such as, e.g., naphthalene- or melamine-based ones. Furthermore, it was confirmed that the total repulsive force is dependent on the surface charge of the substrate and the ionic strength of the aqueous phase, as was predicted by theory.
- (3)
Depletion stabilization is assigned to non-adsorbing polymers when present in high concentrations in the dispersion phase. When particles approach each other at distances less than the radius of gyration, Rg of the polymers present, then the polymer molecules are depleted from the interparticle region. As a consequence, a concentration gradient develops which leads to an osmotic pressure and reduction in entropy. This way, depletion of the macromolecules is prevented [1]. This mechanism is considered for systems only which contain relatively high concentrations (2–3%) of water-soluble macromolecules [5], [6]. Because of this condition, it does not seem to apply for cementitious systems admixed with superplasticizers because their dosages typically lie at 0.05–0.5% by weight of cement and thus are far below what the depletion mechanism requires to become effective.
To conclude, the current models on stabilization of colloidal systems suggest that for a concrete superplasticizer to be effective, it needs to include charged groups, which can facilitate its adsorption on cement and install an electrostatic repulsive force between particles, and possibly pendant groups, which can provide a steric dispersion effect.
The aim of the present research project was to investigate on earlier observations by the group of Daimon and Sakai [22], [23] as well as our group [24] that non-adsorbed PCE polymers can also contribute to cement dispersion. This phenomenon cannot be explained by the existing theories as presented before. For this purpose, a cement paste fluidized with conventional MPEG PCE polymer was further admixed with three types of nonionic, non-adsorbing polymers: a polyester prepared by homopolymerization of the corresponding MPEG methacrylate ester macromonomer, the macromonomer as is, and a polyethylene glycol of a chain length as is present in the polyester and the macromonomer. From these tests, it was hoped to obtain a better understanding of potential interactions between the non-adsorbed polymers, the PCE, and the cement. Furthermore, it was sought to develop a model describing the fluidizing mechanism of such polymers present at subcritical concentrations in comparison to the depletion mechanism. Lastly, it was hoped to stimulate ideas on how to design more powerful admixtures for highly solids loaded systems such as self-compacting or ultra-high strength concrete.
Section snippets
Cement sample
A CEM I 52.5 N sample (HeidelbergCement, Milke® brand from Geseke plant, Germany) was utilized to assess the dispersing power of the polycarboxylate samples. Its phase composition as determined via Q-XRD using Rietveld refinement is exhibited in Table 1. Average particle size (d50 value) was 12 ± 0.2 μm (Laser granulometer Cilas 1064, Silas, Marseille, France), and density was 3.14 g/cm3 (Helium pycnomentry). Its specific surface area according to Blaine's method was 3300 cm2/g.
Chemicals
As mixing water,
Characterization of the polymers
At first, the molecular properties and purity of the synthesized copolymers were checked via gel permeation chromatography. The SEC spectra are displayed in Fig. 2, and the molecular properties retrieved from the spectra are listed in Table 2.
In the chromatograms, the blue line represents the refractive index signal which is proportional to the polymer concentration, whereas the red line represents the static light scattering signal which is mass dependent. Using the information from the RI and
Conclusion
When cement is dispersed in water at low w/c ratio, then non-adsorbing polymers can augment the dispersing effect of conventional PCE superplasticizers. It is surprising that simple PEG can provide this effect. This opens up new concept for blends of PCEs and polyglycols to be used as admixtures in such highly solids-loaded systems.
Still, the mechanism behind this synergistic action of both components is not fully understood. In the future, the range of nonionic, non-adsorbing polymers to be
Acknowledgment
This work was performed at and financed by the “TUM Center for Advanced PCE Studies”. A. Lange gratefully acknowledges the support from this institution.
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