Elsevier

Cement and Concrete Research

Volume 82, April 2016, Pages 100-106
Cement and Concrete Research

Impact of welan gum stabilizer on the dispersing performance of polycarboxylate superplasticizers

https://doi.org/10.1016/j.cemconres.2015.12.009Get rights and content

Abstract

Welan gum, a microbial biopolymer produced via fermentation, represents a common stabilizer (VMA) for highly dispersed concretes such as e.g. self-compacting concrete (SCC). Here, interaction between welan gum and two methacrylate ester-based polycarboxylate (PCE) superplasticizers was studied. It was found that the stabilizing effect of welan gum solely derives from its strong viscosifying effect on the aqueous phase of concrete (i.e. cement pore solution). The dispersing effectiveness of PCE is negatively influenced by this increased viscosity. A mechanistic study revealed that the stabilizer does not adsorb on cement and that it does not reduce adsorption of PCE. Thus, no competitive adsorption between the two admixtures occurs. This behavior differentiates welan gum from copolymerized VMAs based on AMPS® which adsorb on cement and can perturb PCE adsorption. When formulating SCC with welan gum as stabilizer, its dosage should be kept as low as possible to preserve the fluidity provided by PCE.

Introduction

Self-compacting concrete (SCC), also referred to as self-consolidating concrete, is an extremely flowable, self-leveling and self-deairing concrete which generally contains a relatively high amount of powder constituents and a superplasticizer to provide such extremely high flowability [1]. SCC can be used for densely reinforced concrete structures (e.g. those with complex formworks) where vibration and compaction are difficult. However, in order to successfully formulate such powder-type SCC it has been found that accurate dosing of the admixtures and precise analysis of the moisture content present in the aggregates is mandatory. Therefore, some producers encountered difficulties to deliver an SCC of consistent high quality. As a consequence, an SCC incorporating a stabilizing agent as an additional admixture was developed [2], [3]. The stabilizer (in literatures also referred to as viscosity modifying agent, VMA) provides coherence and anti-settling properties to the concrete. Such stabilized SCC exhibits a significantly higher tolerance for variations in the content of mixing water and therefore was found to be more practical in daily operations.

Generally, stabilizers for SCC consist of high molecular weight compounds such as welan gum, diutan gum, curdlan, hydroxypropyl cellulose, polyethylene glycol, and 2-acrylamido methane propane sulfonic acid (AMPS®)-based copolymers [4]. In Europe, AMPS®-based copolymers and welan gum are more prominent than any other kind of stabilizer. Stabilizing admixtures are commonly used at much lower dosages (typically ~ 0.01–0.05% by weight of cement, bwoc) than the PCE superplasticizers present in the SCC formulation.

Welan gum, a microbial polysaccharide which is produced by aerobic fermentation using bacteria from the Alcaligenes species (ATCC-31555), presents a common VMA that can increase the yield stress and the shear thinning property of SCC while mitigating the effect of changes in the water content [5]. In contrast to cellulose-based VMAs, welan gum is less affected by temperature changes and does not negatively impact e.g. the air void system [6]. The main chain of welan gum consists of glucose units holding L-rhamnose or L-mannose as pendant groups. They shield the carboxylate group present in the backbone which forms a double helix [7]. Therefore, welan gum can impart high viscosity to alkaline solutions, even at significant concentrations of calcium ions [8], [9], [10]. The chemical structure of the welan gum is shown in Fig. 1.

Most chemical admixtures applied in concrete present anionic polyelectrolytes which physically adsorb onto positively charged surface sites of the cement particle, thereby developing their specific function (e.g. fluidifying, anti-bleeding, viscosifying, water retention, retardation, etc.) [11], [12].

It has been reported before that the adsorption of superplasticizers (e.g. of PCEs) can be impeded by free sulfate anions present in the cement pore solution through a mechanism called “competitive adsorption” [13]. There, sulfate ions occupy the positively charged anchoring sites on cement, thus preventing the PCE from adsorbing in significant amount. In such case, the polymer can adsorb only on surface sites which are not occupied by sulfate, resulting in weak steric repulsion between the cement particles and low effectiveness. The phenomenon of competitive adsorption has been verified as well for the interaction between another type of VMA, a copolymer consisting of AMPS® and N,N-dimethyl acrylamide (NNDMA), and PCE [14]. Experiments revealed that the AMPS®-NNDMA stabilizer adsorbs on cement and therefore hinders the PCE to adsorb quantitatively, thus leading to lower fluidity. Fortunately, at increased dosages (> 0.06% bwoc) this kind of VMA starts to slightly disperse cement. This effect makes it possible to develop combinations of PCE and the AMPS®-NNDMA stabilizer which at the same time exhibit high fluidity and good coherence of the SCC at reasonable dosages of both admixtures.

Accordingly, it was speculated whether in SCC competitive adsorption between PCE and welan gum might occur as well. Two commonly used PCEs, i.e. one which is frequently applied in the precast concrete industry and possesses a high anionic charge, and another one which is used in ready-mix concrete and exhibits a low anionic charge, were synthesized and characterized with respect to their molecular properties and their adsorption behavior on cement. The impact of welan gum on the dispersing performance of the two PCE superplasticizers was determined via ‘mini slump’ testing and its influence on PCE adsorption on cement was quantified as well. From this data, compatibility between the PCE polymers and welan gum VMA was assessed and their mutual interactions were identified.

Section snippets

Cement

An ordinary Portland cement CEM I 52.5 N from HeidelbergCement, Geseke plant/Germany was utilized in this study. Its phase composition was determined via Q-XRD utilizing Rietveld refinement (see Table 1). The physical properties of this cement sample such as density, specific surface area (Blaine) and d50 value (average particle size) are also presented in Table 1. Particle size distribution was measured utilizing a laser granulometer (Cilas 1064; Cilas, Marseille, France), while the Blaine

Properties of admixtures

When several admixtures are combined in one system, competitive adsorption between them may occur. To a great extent, this effect depends on the relative anionic charges of the admixtures. In Table 2, the anionic charge amounts of the admixtures used in this study are displayed. According to this data, in cement pore solution (CPS) the specific anionic charge amount of the PCE sample 45PC6 (the kind of PCE used in precast concrete) is higher than that of 45PC2 (the ready-mix concrete product),

Conclusion

Our experiments revealed that welan gum VMA can significantly reduce the dispersing effectiveness of PCE superplasticizers. Generally, welan gum viscosifies the liquid phase (pore solution) of SCC, even at low dosages of 0.01–0.08% bwoc. While this effect helps to avoid settling and bleeding of SCC, it can destroy the fluidity achieved by PCE when substantial dosages of this stabilizer are used. Thus, when applying welan gum VMA in concrete, its concentration should be kept to a minimum to

Acknowledgement

The authors gratefully acknowledge these companies for their support: Lubrizol Company for supplying the AMPS® monomer, Clariant Deutschland GmbH for providing the PEG-MA 2000 ester macromonomer, and Hebei Henbo Biotechnology Co., Ltd. China for donating the welan gum powder sample. Dr. Ahmad Habbaba is thanked for his help in preparing the paper and for the 1H NMR measurements.

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