Effect of the cross-linker structure of cross-linked polycarboxylate superplasticizers on the behavior of cementitious mixtures

Abstract

At present, several researchers mainly focus on the synthesis of new micro-cross-linking structure, or explore the effect of cross-linkers with different molecular weight and similar structure on the performance of cross-linked polycarboxylate superplasticizers. In order to explore the influence of the different molecular structure of the cross-linker on the performance of polycarboxylate superplasticizers, two kinds of cross-linked polycarboxylate superplasticizers were synthesized in this study. These two superplasticizers incorporated either a linear cross-linker triethylene glycol diethyl ether or a cyclic cross-linker 3,9-divinyl-2,4,8,10-tetroxyspiro[5,5] undecane as one of the monomers. Then the polymers were respectively coded as SPT and SPD. Furthermore, the dispersion performance of the cross-linked polymers was evaluated by cement slurry fluidity tests and scanning electron microscopy (SEM) characterization. Consequently, cementitious mixtures incorporating SPT had better dispersibility than those incorporating conventional comb-type polycarboxylate superplasticizers (SPC) and SPD. In addition, the XRD, thermogravimetric analysis (TGA), hydration heat, and setting time tests confirmed that SPT exhibited a stronger retardation effect on cement hydration than that was provided by SPD. It was also found that SPT enhanced the compressive strength of concrete, compared to SPC and SPD. This study provides a reference for the development of cross-linked polycarboxylate superplasticizer.

Introduction

The development of polycarboxylate superplasticizers has advanced rapidly in recent years due to their superior performance and pollution-free production [[1], [2], [3], [4]]. Previous reports have demonstrated that the common characteristics of polycarboxylate superplasticizers are their effectiveness at low dosages, high water reducing rates, good plasticity and better concrete shrinkage properties [[5], [6], [7], [8], [9]]. In addition, the molecular structures of polycarboxylate superplasticizers can be readily adjusted, so that they can improve the performance of concrete, by providing various desirable features such as adaptability to various temperatures or compatibility with different types of cements, clay resistance, and so forth [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]].

The dispersion effect of traditional water reducing agents, including lignin-based and naphthalene-based water reducers, is primarily due to the repelling effect of double electric layers [21,22]. Commonly, polycarboxylate superplasticizers are composed of a backbone chain along with side-chains, which contain both hydrophobic and hydrophilic groups. The hydrophilic groups can be extended into the water phase to form a polymer-based adsorption layer, while the hydrophobic groups can be adsorbed onto the surface of cement particles extend into the air In this context, the interfacial energy between water and a solid or a liquid phase can be substantially reduced, thus promoting dispersion [3,14,[23], [24], [25], [26], [27], [28], [29]]. It should be noted that steric hindrance is considered to have an important influence on the performance of polycarboxylate superplasticizers [5,[30], [31], [32]]. The polymeric structures of polycarboxylate superplasticizers are usually designed so that they bear long polyoxyethylene ether side-chains to achieve strong steric repulsion, which plays a crucial role in dispersing cement particles. Therefore, the side-chain length of a polycarboxylate superplasticizer has a great influence on its dispersing performance.

Several studies have shown that polycarboxylate superplasticizers with longer side-chains can enhance the initial dispersion of cement pastes [[33], [34], [35]]. However, Ran et al. [17] reported that polycarboxylate superplasticizers with longer side-chains could accelerate the cement hydration process and change the crystal state of the resultant hydration products, but their initial water-reducing capabilities were rather weak. Zou et al. [36] demonstrated that a better dispersibility and workability retention could be achieved by grafting long side-chains onto polycarboxylate superplasticizers. In addition, cross-linked polycarboxylate superplasticizers derived from hyperbranched cross-linking agents have also been synthesized to explore the effect of superplasticizers with greater steric resistance on the comprehensive properties of cement [11,13,37]. However, the steric resistance and molecular weight of the hyperbranched cross-linking agent may also affect how the superplasticizers interact with cement. As Zhao et al. [13] showed that with the increase of branching degree of cross-linking agent, the steric hindrance of superplasticizers increased, which could reduce the aggregation of cement particles in cement slurries and reduce the viscosity of the system. In addition, Lin et al. [37] found that with the increase of molecular weight of cross-linkers in a certain range, cross-linked polycarboxylate superplasticizers could better promote the dispersion of cement particles and improve the fluidity of cement pastes. Therefore, in order to avoid the interference arising from molecular weight differences among various hyper-branched cross-linking agents, the effect of the cross-linking agents structure on the dispersing and mechanical performance of superplasticizers merits investigation. This can be achieved by using two cross-linking agents with the same molecular weight but different structures.

In this paper, cross-linked polycarboxylate superplasticizers were synthesized using acrylic acid, polyethylene glycol methacrylate and sodium methylallyl sulfonate as comonomers, while triethylene glycol divinyl ether (TEGDE) and cyclic 3,9-divinyl-2,4,8,10-tetrahydrospiro[5,5]undecane (DTU) were used as cross-linking agents. The effects of different cross-linking agent structures (with cyclic and linear topologies) on the dispersing performance and mechanical properties of the resultant superplasticizers, as well as their influence on cement paste microstructure were investigated.