Fundamental mechanisms for polycarboxylate intercalation into C3A hydrate phases and the role of sulfate present in cement
Introduction
Polycarboxylate-based superplasticizers (PCs) are recognized as important admixtures for use in modern concrete technology [1]. They allow the production of a highly flowable concrete or to reduce the water to cement ratio of concrete, resulting in higher compressive strength. For optimum use, it is essential to profoundly understand all potential ways of interaction between PCs and the mineral compounds formed during cement hydration. It has been generally accepted that the high range water reduction and the high fluidity of concrete containing PC is attributed to the PCs adsorbing onto the surface of cement hydrate phases [2]. Thus, a layer exercising a steric effect is formed which leads to the dispersion of the cement particles [3], [4], [5], [6]. The hydration of tricalcium aluminate (C3A) and tetracalcium alumoferrite (C4AF) present in cement can produce hydrocalumite-type layered double hydroxides (LDHs) which have the potential to intercalate various anions between the cationic main layers.
C3A accounts for approx. 5–10% of the clinker mass of ordinary Portland cement (OPC). During its hydration, the layered phases C2AH8 and C4AH13 which belong to the family of calcium aluminum layered double hydroxides (Ca–Al–LDHs), are initially formed as metastable compounds [7], [8]. They contain hydroxide as interlayer anion. Depending on temperature, they convert within minutes or hours to the cubic katoite phase C3AH6 which is the most stable calcium aluminate hydrate at room temperature. Sulfate, e.g. in the form of gypsum which is commonly present in any OPC to control its setting behavior, can intercalate into the layered calcium aluminate hydrates as well, resulting in [Ca4Al2(OH)12](SO4) · 6 H2O, or monosulfoaluminate which is also called AFm phase [9]. Its interlayer is occupied by sulfate anions and water molecules. β-Naphthalene sulfonate (BNS) formaldehyde condensate, a linear unbranched polymer commonly used as superplasticizer, was the first concrete admixture for which intercalation into calcium aluminate hydrate phases has been experimentally confirmed [10]. Also, novel hybrid LDH materials incorporating arene sulfonates such as nitrobenzoic acid, naphthalene-2,6-disulfonic acid and naphthalene-2 sulfonic acid have been reported [11]. Recently, we described the intercalation of comb-type PC superplasticizers into calcium aluminum layered double hydroxides formed during the hydration of C3A in the absence of sulfate [12], [13].
Generally, LDHs are host–guest materials consisting of positively charged metal oxide/hydroxide sheets with intercalated anions and water molecules. Their general composition can be expressed by the formula [M1-xIIMxIII (OH)2]x+ [Ax/nn−] · z H2O, where MII and MIII represent metal cations and An− the interlayer anion. Allmann and Brown et al. were the first to elucidate the structure of LDHs [14], [15]. A schematic illustration of the lamellar structure of Ca–Al–A–LDH is shown in Fig. 1. The steric size and orientation of the anions intercalated between the main layers determine the interlayer distance. The 001 reflection shown in the X-ray powder diffractograms of these compounds allows one to calculate the basal spacing d between the main layers.
Owing to the highly tunable LDH main layer and interlayer composition coupled with a wide possible choice of organic anions, a large variety of LDH hybrid materials has been reported. Various kinds of polymers such as linear polymers, poly(ethylene oxide) derivatives [16], poly(α,β-aspartate) [17], poly(acrylic acid), poly(vinyl sulfonate), poly(styrene sulfonate) [18] and bimolecular DNA [19] have been intercalated between double hydroxide layers. The formation of these hybrid materials may proceed via different pathways such as coprecipitation, anion exchange, surfactant-mediated incorporation, rehydration, or restacking.
In this study, we investigated potential mechanisms of PC incorporation into calcium aluminate hydrates formed during early cement hydration. It is important to understand these potential processes because intercalation reduces the amount of PC available for adsorption and thus decreases its dispersing power. Out of all potential formation processes for LDH compounds mentioned above, rehydration of C3A and anion exchange between AFm phases and PC may occur during early cement hydration. Therefore, these two processes were chosen for the study. Coprecipitation was not investigated because of the low concentration of Al3+ commonly present in cement pore solution (0.2–0.3 µm/L). Thus, it was concluded that in industrial cements, this potential route of PC intercalation will not be significant. First, the ability of PC to intercalate as a function of its steric size (side chain length) was investigated. Then, C3A (re)hydration experiments were carried out in presence and absence of PC and with varying amounts of sulfate, representing cements with different ratios between C3A and sulfate. Finally, anion exchange reactions between PC intercalates and OH− and SO42−, resp., as well as between monosulfoaluminate and C4AH13, resp. and PC were performed. Our goal was to gain an understanding of conditions favorable for PC intercalation and to develop a scheme of potential reaction patterns involved in the intercalation of PCs into cement hydrate phases.
Section snippets
Materials and methods
Pure tricalcium aluminate (C3A) was synthesized via a sol–gel process followed by calcination of the gel for 14 h at 1260 °C with intermediate grindings [20]. Analysis by X-ray powder diffraction confirmed the resulting product to be pure tricalcium aluminate.
The Ca–Al–A–LDH-phases (A = SO42− or OH−) described in the following were synthesized from freshly prepared calcium oxide obtained by calcination of calcium carbonate for 12 h at 950 °C (p.a., Merck). Additional raw materials were aluminum
Hydration of C3A at different sulfate concentrations
To begin with, we examined the hydration products of C3A in the absence and presence of various amounts of sulfate. The XRD powder diffractograms of the reaction products obtained at SO42−/C3A molar ratios between 0 and 2 are depicted in Fig. 4. In the absence of sulfate, C3A is quantitatively transformed to katoite (C3AH6) within less than an hour. The small reflection denoted MC (monocarboaluminate or [Ca4Al2(OH)12](CO3) · 8 H2O) is an impurity which is formed through the uptake of carbon
Mechanisms for PC intercalation into C3A hydrates
Based on the results discussed above, we can now draw general schemes for the interaction of PCs with C3A. This interaction strongly depends on the concentration of sulfate present. The different possibilities are illustrated in Fig. 13, Fig. 14.
First, there are two direct routes for the formation of PC intercalates (Fig. 13). Both are based on rehydration of C3A. When C3A hydrates in the presence of PC and at zero or low SO42− concentration, intercalation of PC or interstratification of both
Conclusions
The formation of PC intercalation compounds can occur via two completely different mechanisms: First, in a direct synthesis between hydrating C3A and PC, and second, through an indirect process involving calcium aluminate hydrates (Ca–Al–OH–LDH phases) as precursors which undergo anion exchange with PC in releasing initially intercalated OH− anions. However, no matter which mechanism prevails, intercalation is generally possible only in undersulfated systems where the amount of quickly soluble
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