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Early C3A Hydration in the Presence of Different Kinds of Calcium Sulfate. Sylvie Pourchet, Laure Regnaud, André Nonat, Jean-Philippe Perez

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Early C3A Hydration in the Presence of Different Kinds of Calcium Sulfate. Sylvie Pourchet, Laure Regnaud, André Nonat, Jean-Philippe Perez Early C3A hydration in the presence of different kinds of calcium sulfate. Sylvie Pourchet, Laure Regnaud, André Nonat, Jean-Philippe Perez To cite this version: Sylvie Pourchet, Laure Regnaud, André Nonat, Jean-Philippe Perez. Early C3A hydration in the presence of different kinds of calcium sulfate.. Cement and Concrete Research, Elsevier, 2009,39, pp.989-996. 10.1016/j.cemconres.2009.07.019. hal-00452779 HAL Id: hal-00452779 https://hal.archives-ouvertes.fr/hal-00452779 Submitted on 3 Feb 2010 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 1 Early C 3A hydration in the presence of different kinds of calcium 2 sulfate. 3 4 5 S. Pourchet a*, L. Regnaud b, J.P. Perez c, A. Nonat a 6 7 a Institut Carnot de Bourgogne, UMR 5209 CNRS-Université de Bourgogne, 8 9 Av. A. Savary, BP 47 870, F-21078 DIJON Cedex, France 9 b Chryso , 7 rue de l’Europe - Zone Industrielle - 45300 Sermaises , France 10 c LCR Lafarge, 95 rue Montmurier, 38070 St Quentin Fallavier, France 11 12 Abstract: 13 14 Hydration reactions of C 3A with various amounts of calcium sulfate hemihydrate, 15 gypsum or a mixture of the two, were investigated by isothermal microcalorimetry, 16 and a monitoring of the ionic concentrations of diluted suspensions. This study 17 shows that sulfate type used modifies the early C 3A-CaSO 4 hydration products and 18 the rate of this hydration. The fast initial AFm formation observed before ettringite 19 precipitation in the C 3A-gypsum system is avoided as soon as hemihydrate is 20 present in the suspension. This was attributed to.higher super saturation degrees and 21 then higher nucleation frequency with regard to the ettringite obtained in the 22 presence of hemihydrate. Moreover, replacement of gypsum by hemihydrate also 23 leads to an increase of the ettringite formation rate during at least the five first hours 24 under experimental conditions. 25 26 Keywords: Hydration, Ca 3Al 2O6, Ettringite, Kinetics. 27 28 29 30 31 32 33 34 35 36 37 38 39 1 40 1 Introduction 41 42 43 Tricalcium aluminate, C 3A, is known to be the most reactive mineral present in 44 Portland clinker. Its early hydration leads to the formation of calcium 45 hydroaluminate (3CaO-Al 2O3-Ca(OH) 2-nH 2O or hydroxy-AFm) which induces a 46 stiffening of the hydrating paste. In order to prevent this phenomenon, calcium 47 sulfate is added to the clinker to slow down the early C 3A hydration, which then 48 leads to the formation of ettringite (Ca 6Al 3(SO 4)3(OH) 12 , 26H 2O) [1]. Because the 49 liquid phase composition in this system is at early ages usually saturated or 50 supersaturated with respect to gypsum, thermodynamic calculations show that 51 ettringite should be the initial hydrate formed during hydration of C3A-CaSO 4 52 mixtures at room temperature. However, prior work has shown that the hydroxy- 53 AFm can also be formed during this step. Brown et al [2] showed that C 3A 54 hydration carried out in different solutions containing various amounts of sodium, 55 calcium, hydroxide and sulfate ions gives rise to at least two hydrated phases 56 (ettringite and hydroxy-AFm) precipitating in the same time, the proportions and the 57 rate of precipitation of each phase depending on the composition of the solution. 58 These findings are also in good accordance with the work of Eitel [3] showing 59 hexagonal hydrate phases precipitating in direct contact with C 3A. More recently, 60 Minard et al [4] confirmed the formation of both ettringite and hydroxy-AFm at 61 early ages in C 3A-gypsum hydration and determined the quantity of hydroaluminate 62 precipitating by using a method based on microcalorimetry in stirred diluted 63 suspension. 2 64 Even if the factors which control the kinetics of the hydration of C 3A-CaSO 4 65 mixtures are still under discussion [2, 4-8] nevertheless most authors conclude that 66 the initial rate of C3A hydration is significantly influenced by the type of sulfate 67 source used [5, 9, 10]. This is interpreted as a consequence of the solubility and the 68 rate of dissolution of the particular form of calcium sulfates used. For example, 69 Bensted [9] showed that increasing the grinding temperature of a Portland cement 70 made with gypsum results in increased quantities of ettringite determined by DTA at 71 all the hydration times examined up to 2 hours. He attributed this effect to the 72 increased solubility rate of the calcium sulfate, because when gypsum, is 73 interground with clinker to produce cement, some of the gypsum can dehydrate to 74 hemihydrate, depending on the temperature and humidity conditions. Hemihydrate 75 has a higher solubility and solubility rate than gypsum, so a higher initial calcium 76 sulfate concentration is expected in the pore fluid when gypsum is partially 77 dehydrated to hemihydrate, and this could induce an increase of the ettringite 78 precipitation rate. 79 Because early C 3A hydration is known to have a significant influence on the 80 rheological properties of Portland cement pastes, and thus of concrete, it is of great 81 interest to have a better understanding of this phenomenon. The purpose of this 82 study is to examine how the nature of the calcium sulfate used (gypsum, 83 hemihydrate, or a mixture of the two) can influence early C 3A hydration reactions, 84 and more specifically, if it affects AFm formation. This work is based on 85 calorimetry of stirred dilute C 3A suspensions, using Minard’s procedure [4]. Using 86 stirred suspensions instead of paste avoids possible heterogeneities in the mixture 87 and decreases the concentration gradients at the interface between solids and 88 aqueous phase. Moreover this method allows for quantification of the 3 89 hydroaluminate formed at the very beginning. Parallel experiments were carried out 90 in a thermoregulated reactors allowing frequent sampling of solids and liquid for 91 analysis. Results are discussed by considering the effect of the type of calcium 92 sulfate used on the pore solution composition. 93 94 95 2. Materials and Method 96 97 98 2.1. Materials 99 100 101 Most of the experiments were made with the same batch of cubic C 3A. Its specific 102 surface area determined by Blaine’s method was 370 m 2/kg and its particule size 103 distribution is depicted below (Figure 1). Another batch of cubic C 3A was used for 104 supplementary experiments led in the presence of ettringite. In this case its specific 105 area determined by Blaine’s method was 330 m 2/kg. Both were supplied by Lafarge 106 LCR. 6 5 4 3 volume(%) 2 1 0 0.1 1 10 100 1000 Diameter (µm) 107 4 108 Figure 1: Particle size distribution of C 3A determined in ethanol by laser light 109 scattering. 110 111 112 As calcium sulfate sources, gypsum (RP Normapur, Merck) and pure hemihydrate 113 (Fluka) were used. Ettringite was synthesized by Lafarge according to [11] and 114 analysed by XRD. 115 116 2.2. Methods 117 118 119 2.2.1. Heat evolution rate in diluted suspension 120 121 122 The hydration of C 3A was followed at 25°C with a high sensitivity (0.1µW) 123 isothermal Tian-Calvet microcalorimeter in diluted suspension (Setaram, M60, and 124 Setaram MS80). A tube with a calorimetric cell as presented in figure 2 is 125 introduced in the calorimeter which transmits the heat evolution. C3A hydration was 126 studied in a portlandite saturated solution in order to mimic the pore solution during 127 early cement hydration. Then for the experiments, 50 mL of a saturated calcium 128 hydroxide solution were introduced into the cell, and 2g of C 3A placed in the tank. 129 When C 3A hydration was carried out with gypsum, the appropriate amount of solid 130 gypsum was either introduced in the cell with the solution or dry-mixed with C 3A in 131 a turbula mixer and this mix was then placed in the tank. In the case of hemihydrate- 132 C3A hydration, hemihydrate was first dry-mixed with C3A, in order to avoid the 5 133 hydration of hemihydrate into gypsum before the introduction of C 3A in the cell. 134 This mix was then placed in the tank. In the case of the C 3A-ettringite-gypsum 135 hydration experiment, 0.2g of ettringite was added to the initial gypsum suspension. 136 To begin each experiment, the calorimetric tube was introduced in the calorimeter 137 and the agitation of the solution begun. Once the thermal equilibrium was reached 138 (after about 4 hours), C 3A (respectively C 3A + hemihydrate, or C 3A + gypsum) was 139 introduced into the cell from the tank, and the heat evolution rate was then 140 registered as a function of time. Agitation Tank for the solid System allowing the introduction of the solid (C 3A or C 3A+ gypsum or C3A+hemihydrate) Calorimetric cell 141 142 Figure 2: schematic view of the calorimetric tube. 143 144 A typical heat evolution rate curve recorded during the hydration of C 3A-gypsum 145 mix (20% by weight related to the C 3A) is shown in Figure 3.
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