Effects of core-shell polycarboxylate superplasticizer on the fluidity and hydration behavior of cement paste

Highlights

• The polycarboxylate superplasticizer nanomicelles with hydrolysable cores were prepared via emulsion copolymerization.

• The slow release of carboxyl groups is modulated with polystyrene segment for fluidity retention.

• The fluidity of cement paste is maintained in 3 h with a nano-PCE dosage of 0.20 % bwoc and a water/cement ratio of 0.35.

Abstract

Polycarboxylate superplasticizer nanomicelles (nano-PCEs) with a core-shell structure were prepared via aqueous emulsion copolymerization in one pot. The shells are constructed with hydrophilic segments of poly(acrylic acid)-co-poly(isobutenyl polyethenoxy ether) (PAA-co-PHPEG), offering the water-reducing performance and stability for nano-PCEs. The cores are self-assembled with hydrophobic segments of polystyrene-co-poly(hydroxyethyl acrylate) (PS-co-PHEA), endowing nano-PCEs with good loss resistant of fluidity for cement pastes. The chemical structure of nano-PCEs was verified by the nuclear magnetic resonance spectrum (¹H NMR) and fourier transform infrared spectroscopy (FTIR), and the 16−48 nm diameter of nano-PCE nanomicelles was determined by dynamic laser scattering (DLS) and transmission electron microscopy (TEM). Compared with comb PCEs, the cement paste containing nano-PCEs exhibited better fluidity retention of three hours by mini-slump measurements, lower hydration heat and more delayed hydration heat evolution by isothermal calorimetry tests. Furthermore, the hydrolysis and adsorption behavior of nano-PCEs in alkaline cement pastes were deduced, and a working mechanism of nano-PCEs was theoretically explained. This new type of superplasticizer nanomicelles can be used as a long time rheology modifying agent in fresh cementitious systems.

Introduction

Polycarboxylate-ether superplasticizers (PCEs) are nowadays widely used in cementitious systems to improve the workability of fresh mixes, the compactness and the strength of hardened concrete [[1], [2], [3], [4], [5], [6]]. The positive effects are based on the unique comb structure of PCEs, which are constructed with polycarboxylate main chains and pendant polyether grafted groups [[7], [8], [9], [10], [11], [12], [13]]. After the polycarboxylate adsorbing onto the surface of cement particles, these particles are well dispersed as to the steric hindrance effect of grafted polyether in PCEs, thus the initial high-fluidity of cementitious mixes is guaranteed. Nevertheless, it is not easy to maintain the initial fluidity of fresh mixes for a long time, because of the free PCEs are exhausted and cannot disperse cement particles and hydrate phases [[14], [15], [16], [17]]. In real-life projects, managing the workability loss of fresh concrete is highly critical for long distance deliveries, hot weather placement, finishing operations, and strength or durability improvement [18,19]. As such, the chemical and physical structure modifications of PCEs have been attempted for sufficient workability retention [[20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]].

For chemical modification, the polycarboxylates of PCEs are esterified firstly via functional-group-modification or grafting copolymerization [[20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]], and then hydrolyzed in strong alkaline pastes to produce linear PCEs for controlling the fluidity. It has been verified in both academic and business field that ester-functional acrylic/maleic monomers [[20], [21], [22]], such as hydroxyethyl acrylate (HEA) and monomethyl maleate (MMM), have excellently dispersing effect with the hydrolysate of carboxyl groups in highly alkaline cementitious pastes. Nevertheless, a majority of ester groups in these monomers are hydrolyzed in two hours, which is detrimental for fluidity retention [22]. As a consequences, complex morphological structures with a lower ester hydrolysis rate have been developed, such as star-shape [[23], [24], [25]], crosslinking [[26], [27], [28], [29]], branch/hyperbranch [[30], [31], [32]], claw-shape [33], and jellyfish-like-shape [34]. Typically, Zhao et al. employed the multi-armed-graft structure to delay the dissociation of comb polycarboxylates from the hydrolysable central cores in star-shaped PCEs [25]. Moreover, they have confirmed that the star-shaped PCEs with 3–5 arms retained a good paste fluidity for 2 h (>150 mm). Besides, the slightly crosslinked structure is another candidate for slump retention of cement pastes [26]. Accompanied with the decrosslinking of crosslinked networks, a slow release of PCEs is usually implemented, because of the slow hydrolyzing of ester crosslinkers in alkaline conditions.

For physical modification, the PCEs are firstly reserved in the laminated, hollowed or spherical structures, and then slowly released to reduce the fluidity loss [[35], [36], [37], [38]]. For example, Cao et al. designed a calcium/aluminum type-layered double hydroxide (Ca/Al-LDH) to regulate the sustained-release of interbedded anionic PCEs via anion-exchange intercalation reaction [35]. Ballweg reported a new coextrusion technique (the vibrational nozzle and UV-curing technique) to encapsulate PCEs, which can be controlled release for more than 72 h [36]. Kong and co-workers employed the spherical nanoparticles to avoid the one-time release of PCEs by the high content (>50 wt%) hydrophobic polystyrene chains [37,38]. Unfortunately, whether chemical modification or physical modification, it is still a challenge to maintain the initial fluidity for 3 h with superplasticizer.

In this paper, a new method combined chemical and physical modifications is presented to maintain workability in 3 h. Firstly, the ester-functional acrylate polymers, i.e. poly(hydroxyethyl acrylate) (PHEA), was adopted to release absorbable carboxyl groups after hydrolyzing in alkaline paste. Secondly, the spherical nanomicelles, i.e. polycarboxylate superplasticizer nanomicelles (nano-PCEs), were prepared by the emulsion copolymerization of acrylic acid, isobutenyl polyethenoxy ether (HPEG), HEA and styrene, to retard the hydrolyzation of PHEA. Finally, the nano-PCEs transformed into comb polymer chains to disperse hydrate phases, maintaining the workability over 2 h. The morphological effects of these nano-PCEs on their dispersibility and workability retention were evaluated by mini-slump tests of cement pastes. Besides, the hydration process of cement pastes were checked, and a possible action mechanism of the nano-PCEs was proposed.