Article pubs.acs.org/IECR
Effect of Polycarboxylate−Ether Admixtures on Calcium Aluminate Cement Pastes. Part 2: Hydration Studies María del Mar Alonso,† Marta Palacios,†,‡ and Francisca Puertas*,† †
Eduardo Torroja Institute for Construction Science (IETcc-CSIC), Madrid 28033, Spain Institute of Building Materials, ETH Zurich, CH-8093 Zurich, Switzerland
‡
S Supporting Information *
ABSTRACT: In part 1, the effect of different polycarboxylate ether (PCE) admixtures with Portland-based binder (OPC) and calcium aluminate cement (CAC) pastes has been studied; indicating a low CAC−PCEs compatibility. In this part, 2, the effect of PCE admixtures with different molecular structure on the hydration and microstructural development of CAC pastes has been studied. The findings showed that the PCE admixtures used retarded CAC hydration and this effect is independent of the admixture carboxylate/ester ratio. The results obtained have proven that the presence of PCE admixtures in OPC and CAC pastes does not modify the nature of hydrates formed. However, in CAC pastes, the presence of PCE admixture has an important effect on the amount of hydrates generated, it being 45% lower at the same time than in CAC pastes without admixture.
1. INTRODUCTION A prior study1 revealed substantial differences in the interaction of polycarboxylate ethers (PCEs) with calcium aluminate cement (CAC) with respect to Portland cement (OPC). The admixture C/E ratio plays a less important role in adsorption in the former. Moreover, while fludity is attained in CAC pastes at low admixture doses, its decline over time denotes low CAC− PCE compatibility. This second paper discusses the effect of PCE admixtures on CAC paste hydration and microstructural development. While the use of any type of superplasticiser in cement pastes induces changes in their fluidity, it may also alter the hydration process itself. When the admixture adsorbs onto cement particles, it affects the liquid−solid interface, possibly varying cement hydration, which may alter hydrate nucleation and hydration kinetics,2 as well as reaction product morphology and paste microstructure. The effect of superplasticisers on reaction processes depends on the characteristics of the admixtures and cements used. Admixture dosage and structure, among others, are determinants in this respect.2−8 The cement factors involved include fineness, chemical composition (aluminate and sulfate content in particular), the presence of alkalis, and the existence of possible mineral additions.8−12 In OPC pastes, previous studies8,9 have confirmed that the higher the C3A content, the lower is the admixture-induced delay in hydration. CAC hydration has been widely studied.13−20 This exothermal process is governed essentially by the hydration of its main phase, monocalcium aluminate (CA), to give rise to different hydration products, depending on humidity and temperature. At temperatures under 15−18 °C, the only hydrate formed is CAH10, while at somewhat higher temperatures, C2AH8 and AH3 are also formed in cements containing C12A7.21 The hexagonal crystalline hydrates CAH10 and C2AH8 are metastable at ambient temperature and tend to convert to stable CAH10-type cubic hydrates. This conversion entails the © 2013 American Chemical Society
formation of aluminum hydroxide (AH3) and water, as shown in the following reactions:22 6CAH10 → 3C2AH8 + 3AH3 + 27H 2O 3C2AH8 + 3AH3 + 27H 2O → 2C3AH6 + 4AH3 + 36H 2O
Since C3AH6 cubic aluminate is denser and 53% less voluminous than hexagonal aluminates such as CAH10,18 the dimensional changes attendant upon conversion raise porosity, consequently lowering mechanical strength and durability. Such conversion is expedited at higher temperatures and relative humidity.13 This change can be prevented by using water/ cement under 0.4 . However, this means an important loss of workability of the concrete, so it is recommended the use of plasticizers or superplasticizers admixtures. Kinetic changes have been observed in CAC hydration in the presence of accelerating and retarding compounds such as citric acid, lithium carbonates or sulfates, or sodium carbonate, among others.23−26 Retarded hydration in these pastes is associated with the complexes formed by the −COO− and Ca2+ in the solution. Some authors have also proven that lignosulfonate-based and naphthalene- and melamine-based superplasticisers retard CAC paste hydration substantially.27−29 Very little information is available on the effect of PCE-type superplasticisers on CAC hydration, however. Furthermore, information is wanted not only on the effect of PCE admixtures on CAC system hydration (to determine their effect on conversion processes), but also on how these admixtures impact the nature and microstructure of the reaction products formed. The present study consequently aimed to explore the effect of three PCE admixtures with Received: Revised: Accepted: Published: 17330
May 22, 2013 October 31, 2013 November 12, 2013 November 12, 2013 dx.doi.org/10.1021/ie401616f | Ind. Eng. Chem. Res. 2013, 52, 17330−17340
Industrial & Engineering Chemistry Research
Article
Figure 1. Isothermal conduction calorimetry curves. Heat flow for OPC and CAC pastes.
selected: 4, 8, and 16 h. Four hours marked the beginning of the acceleration period, heat flow peaked at approximately 8 h, and by 16 h deceleration was underway. The w/c ratios were the same as in the calorimetric trials. The admixture was batched to the optimal dose determined in the rheological trials: 1.2 mg polymer/g OPC and 0.4 mg/g CAC. At the selected ages, paste hydration was detained by submerging the samples first in acetone (for 45 s) and then in ethanol (for 15 s). The samples were then vacuum-dried, ground, and sieved to a particle size of under 45 μm for the mineralogical and microstructural studies. The X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses to determine the effect of admixtures on cement paste mineralogy were performed with admixture PC1 only. The instrumental conditions were: BRUKER AXS D8 Advance diffractometer with an RX Lynxeye super speed detector, 2.2-kW copper anode and without monochromator. The samples were scanned at 2θ angles of 5 to 60° in 24 min. The voltage generator (X-ray tube) operated at 40 kV and 30 mA. KBr pellets (1 mg sample/300 mg KBr) were analyzed on a Thermo Scientific Nicolet 6700 FTIR spectrometer. The spectra were recorded after 64 scans in the 4000−400 cm−1 range. 2.2.4. Study of Si and Al Environments in Anhydrous Cements and Hydration Products. Anhydrous OPC and its 16-h hydrated paste were characterized with 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) in the presence and absence of PC1. Anhydrous and hydrated CAC pastes were studied with 27Al MAS NMR. The 29Si and 27 Al MAS NMR spectra were recorded on a Bruker MSL 400 spectrometer at resonance frequencies of 79.49 and 104.2 MHz, respectively. Chemical shift values were found using tetramethylsilane (TMS) and a 1-M solution of AlCl3·Cl3· 6H2O as standards for 29Si and 27Al, respectively.
different molecular structures on CAC hydration and determine the changes induced by these superplasticisers on paste mineralogy and microstructures.
2. EXPERIMENTAL SECTION 2.1. Materials. The materials used in the present study were the same as in the previous paper1 • A calcium aluminate cement (CAC) and, as the reference or control material, European standard EN 197-1:2011classified CEM I 42.5R (OPC) Portland cement. • Three poly(acrylic acid) derivatives, here labeled PC1, PC2, and PC3, as the PCE admixtures. 2.2. Effect of PCEs on the Hydration of OPC and CAC Pastes. 2.2.1. Conduction Calorimetry. Calorimetric measurements were recorded on a TAM Air conduction calorimeter at 25 °C with cement pastes with w/c ratio of 0.4 in OPC and 0.35 in CAC, having both pastes similar consistency. Dosages of 0.4 and 1.2 mg polymer/g OPC were added to the mixing water. In all cases, the dosages refer to milligrams of actual active polymer (dry admixture extract) per gram of cement. In CAC pastes, a dosage of 1.2 mg polymer/g CAC led to sedimentation. The test duration was 65 h. 2.2.2. Differential Thermal Analysis with Simultaneous Thermogravimetric Analysis (DTA/TG) Study of OPC and CAC Pastes in the Presence and Absence of PCE Admixtures. OPC and CAC pastes in presence and absence of PC1 were analized after 4, 8, and 16 h of hydration on a TA Instruments TGA-DSC-DTA Q600 simultaneous thermogravimetric analyzer and differential scanning calorimeter to determine the effect of the admixture on hydration and quantify the products formed. After ramping at 10 °C/min to 80 °C, the pastes were heated for 1 h to eliminate all the moisture from the sample. The temperature was subsequently ramped up to 1000 °C at 4 °C/min. Tests were carried out using platinum crucibles and under nitrogen atmosphere. 2.2.3. Effect of Admixtures on Hydration Product Mineralogy. The manner in which PCE admixtures may affect hydration product mineralogy was also studied. After analyzing the cement paste calorimetric curves, three trial ages were
3. RESULTS 3.1. Conduction Calorimetry. The heat flow curves for OPC and CAC hydration in the presence and absence of PC1, PC2, and PC3 are plotted in Figure 1. The calorimetric data 17331
dx.doi.org/10.1021/ie401616f | Ind. Eng. Chem. Res. 2013, 52, 17330−17340
Industrial & Engineering Chemistry Research
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The sole signal on the CAC hydration heat flow curve (Figure 1, right) was attributed to the mass precipitation of reaction products. At a dosage of 0.4 mg polymer/g cement, the peak on the main signal was retarded by approximately 3 h. In this case, admixture PC3 induced the longest delay. Adding the superplasticisers to the CAC pastes, however, had practically no effect on the heat released (see Table 1). As discussed in the Experimental Section, from the isothermal conduction calorimetry findings, admixture PC1 at dosages of 1.2 mg polymer/g OPC and 0.4 mg polymer/g CAC were chosen for the mineralogy studies on the cement pastes. In addition, the three trial ages of 4, 8, and 16 h were selected. 3.2. DTA/TG Study of OPC and CAC Pastes in the Presence and Absence of PCE Admixtures. By way of example, Figures 2 and 3 show the DTA/TG signals observed in the 16-h OPC and CAC pastes, respectively, in the absence of PC1. The same signals were observed in presence of PC1. The DTA curve for OPC presents five endothermal peaks. First and second ones, at 133 and 330 °C associated with water loss of ettringite and C−S−H gel. The third signal at 426 °C corresponds to portlandite (Ca(OH)2) dehydroxylation, while the endothermic signals at 649 and 830 °C are attributed to the decomposition of calcium carbonate32 The DTA curve for CAC presents two endothermal signals at 131 and 193 °C, attributed to the decomposition of CAH10 and C2AH8, respectively, and the signal at around 226 °C to AH3 decomposition.33−35 This third signal had a small shoulder (244 °C) that might have been the result of C 3 AH 6 decomposition. 3.3. Effect of Admixtures on Hydration Product Mineralogy. Figures 4 and 5, respectively, show the diffractograms obtained for the anhydrous and hydrated OPC and CAC. Each figure reproduces the diffractogram for the anhydrous cement and for the 4-, 8-, and 16-h pastes in the presence and absence of admixture PC1. The semiquantitative analysis of these XRD patterns is given in Table 2. Figures 6 and 7 contain the FTIR spectra for the OPC and CAC anhydrous cements and pastes at the same ages and likewise in the presence and absence of the admixture. The diffractograms for the OPC pastes (Figure 4) and their analysis (Table 2) show that in the 4-h samples, the relative intensities of the diffraction lines characteristic of the main
resulting from an analysis of these curves (based in all cases on the acceleration signal) are given in Table 1. Table 1. Calorimetric Findings for Cement Pastes OPC admixture none 0.4 mg polymer PC1/g cement 1.2 mg polymer PC1/g cement 0.4 mg polymer PC2/g cement 1.2 mg polymer PC2/g cement 0.4 mg polymer PC3/g cement 1.2 mg polymer PC3/g cement
heat flow rate peak time (h)
signal intensity (J/g·h)
total heat at 65 h (J/g)
6.7 7.0
13.9 13.8
251 262
7.4
15.7
282
7.5
13.7
262
8.4
14.2
268
7.0
13.4
251
7.5
13.7
262
heat flow rate peak time (h)
signal intensity (J/g·h)
total heat at 65 h (J/g)
9.0 11.8
75.5 82.8
252 251
12.0
78.2
256
12.4
74.1
259
CAC admixture none 0.4 mg polymer PC1/g cement 0.4 mg polymer PC2/g cement 0.4 mg polymer PC3/g cement
The most prominent signal on the heat flow curve for OPC (Figure 1, left) was associated with the acceleration induced by the mass precipitation of the main reaction products (C−S−H gel and Ca(OH)2). The shoulder on this signal was attributed to ettringite conversion to monosulfoaluminate.30 The presence of PCE-type admixtures slightly retards OPC hydration, although the scale of the effect depends on admixture dosage7,31 and characteristics. Here, the longest delay in the reactions (1.7 h) was prompted by admixture PC2, while PC1 had the greatest impact on the intensity of the main acceleration signal.
Figure 2. DTA/TG for 16-h OPC pastes. 17332
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Figure 3. DTA/TG for 16-h CAC pastes.
Figure 5. Diffractograms showing the hydration-induced variations in CAC in the presence and absence of PC1.
Figure 4. Diffractograms showing the hydration-induced variations in OPC in the presence and absence of PC1.
The FTIR findings confirmed the XRD results. An analysis of these spectra (Figure 6) showed no significant differences between the OPC pastes bearing PC1 and the pastes hydrated in the absence of the polymer and that the spectral variations over time (4−16 h) were due to the hydration reactions. The wide band at around 923−930 cm−1, present in the spectrum for anhydrous OPC and attributable to the ν3 asymmetric stretching vibrations generated by the (Si−O) bond in silicates (essentially C3S or alite), was shifted to higher wavenumbers (around 978 cm−1) as cement hydration progressed. This shift was associated with the formation of C−S−H gel, further confirmed by the gradual weakening of the ν4 (O−Si−O) absorption at 520 cm−1, attributed to alite and belite. Moreover, as hydration progressed, the bands at 1144 and 1107 cm−1,
anhydrous compounds (C3S, C2S, C3A, C4AF, and gypsum) remained essentially unchanged. The 8-h patterns contained reflections that concurred with the earliest forming crystalline reaction products: portlandite and ettringite. In the 16-h traces, the portlandite reflection (2θ = 18.05°) was the most intense; the gypsum signal disappeared; the intensities of the most significant C3S or alite reflections declined (2θ = 32.22° and 51.88°); and ettringite signals were identified (2θ = 9.06° and 22.87°). In general terms, the presence of PC1 did not alter the mineralogy of the hydrated OPC pastes, for the reaction products formed at the ages studied were not significantly different from the products formed in the pastes without the admixture. 17333
dx.doi.org/10.1021/ie401616f | Ind. Eng. Chem. Res. 2013, 52, 17330−17340
Industrial & Engineering Chemistry Research
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Table 2. Semiquantitative Analysis of OPC and CAC Diffractogramsa OPC pastes compound anhydrous 4h 8h 16 h
no admixture with PC1 no admixture with PC1 no admixture with PC1
compound anhydrous 4h 8h 16 h a
no admixture with PC1 no admixture with PC1 no admixture with PC1
C3 S
C2S
gypsum
C3 A
C4AF
portlandite
ettringite
++ ++ ++ ++ ++ ++ ++
0 0 0 0 0 0 0
+ 0 0 0 − − − CAC pastes
+ + + + + + +
0 0 0 0 0 0 0
− 0 0 ++ ++ +++ +++
− + + + + + +
CA
Al(OH)3
FeO
CaTiO3
CAH10
C2AH8
C3AH6
+++ +++ +++ +++ +++ + +
− − − − − − −
+ + + + + ++ ++
++ ++ ++ ++ ++ +++ +++
− − − 0 0 + ++
− − − 0 0 0 +
− −− − − 0 −
(0)
