HYDRATION OF CALCIUM SULFOALUMINATE CEMENT IN PRESENCE OF ZINC CHLORIDE

Transcription

1 HDRATION OF CALCIUM SULFOALUMINAT CMNT IN PRSNC OF ZINC CHLORID S. Berger (1), C. Cau Dit Coumes (1), P. Le Bescop (2), D. Damidot (3) (1) CA/DN/MAR/DTCD/SPD, BP 17171, Bagnols/Cèze cedex, France (2) CA/DN/SAC/DPC/SCCM, Gif/vette, France (3) Civil & nvironmental ngineering Department - cole des Mines de Douai, 764 Bd Lahure, Douai, France Abstract The structural flexibility of the two mains hydrates, ettringite and calcium monosulfoaluminate hydrate, produced by calcium sulfoaluminate cements (CSA cements) hydration may give rise to chemical entrapment of some heavy metals salts, such as ZnCl 2. The properties of mortars made with CSA cement (mechanical strength and length changes) and their hydration process were thus investigated (by calorimetric and XRD measurements) as a function of various parameters: the content of the CSA cement (from 0 to 35%), the composition of the mixing solution (either pure water, or a ZnCl 2 solution with a variable concentration ranging from 0 to 0.5 mol/l) and the thermal evolution at early age, with a view to develop a cementitious material to stabilize Zn-rich waste. Increasing the content accelerates the hydration process but decreases the total heat of hydration. The precipitation of ettringite is promoted and the porosity increased. The influence of zinc chloride on hydration depends both on the zinc concentration in the mixing solution and on the content. The thermal history of the cement-based material is also a key parameter since it modifies the rate of hydration, the mineralogy of the cement paste and the macroscopic properties of mortar samples as mechanical strength. 1. INTRODUCTION Calcium sulfoaluminate cements are obtained by mixing CSA clinker, which is mainly composed of yeelimite (C 4 A 3 S), C 2 S and an Al-rich ferrite, with [1]. ttringite (AFt, C 3 A.3CS.H 32 ) and calcium monosulfoaluminate hydrate (, C 3 A. CS.H 12 ) are the main hydrates formed by the hydration process and the amount of produced ettringite increases with the content while the quantity of decreases [2]. Sharp and al [1] categorize CSA cements from rapid-hardening (below 15% of ) to shrinkage-compensating (beyond 25% of ). 67

2 At early age (less than 24h), hydrating of CSA cement produces rapid heat release, (more than an ordinary Portland cement) which can induce significant temperature increase in massive structures [3]. The fast temperature rise is likely to affect the mineralogy, microstructure and mechanical properties of the hardened products. ttringite stability is indeed very dependent on temperature and relative humidity [4]. ttringite and are known to accommodate many substitutions in their structure: Ca 2+, Al 3+, SO 2-4 and OH - are suitable to be replaced by many cations or oxyanions of heavy metals, such as Cd 2+, Cr or CrO 4 [5-7]. In this article, particular attention is paid to the stabilization of zinc chloride, which is present in large amounts in ashes resulting from the incineration of technological wastes including neoprene and polyvinylchloride. According to previous studies, Zn 2+ /Ca 2+ and 2Cl - /SO 2-4 substitutions in the structure of AFt and may be involved [8]. During leaching of Zn-containing cement pastes by acid solution, it was observed that the zinc release and the ettringite destabilization occurred simultaneously [9-11]. In other studies [12, 13], CSA cement pastes were prepared with ZnCl 2, ZnSO 4.7H 2 O and Zn(NO 3 ) 2 solutions. Hydrated samples were ground and mixed with de-ionized water during 24 to 72h. Analysis of the solutions did not show the presence of zinc. The authors thus concluded that CSA hydrated cements had a good capacity to retain zinc. Moreover, according to DS analyses on fractures from cement pastes prepared with a 1 mol ZnCl 2 solution, Cau-dit-Coumes [14] suggested that some ettringite crystals could be partially substituted by Zn and Cl. The 2Cl - /SO 2-4 substitution is well known: a chloro-ettringite was precipitated and studied by Schwiete and al [15]. Furthermore, a chloro- termed Friedel s salt (3CaO-Al 2 O 3 -CaCl 2-10H 2 O) and a mixed compound containing chloride and sulfate referred as Kuzel s salt (3CaO-Al 2 O 3-0.5CaCl 2-0.5CaSO4-10H 2 O) were also synthesized and described [16, 17]. The aim of this work was to investigate the hydration process of CSA cement in presence of zinc as a function of content of the binder, ZnCl 2 concentration of the mixing solution and thermal history of the cementitious material at the early age. 2. XPRIMNTAL 2.1 Materials and preparation of specimens CSA cements were prepared by mixing a ground CSA clinker (which composition is summarized in Tab. 1) with the appropriate amount of analytical grade (from 0 to 35% by mass of cement) during 15 min. Both cement pastes and mortars were made with the same water to cement (w/c) weight ratio of A blend of two siliceous sands (0.1 to 1.2 mm) was used with a send to cement (s/c) ratio of 3 to optimize the workability and limit the heat release of fresh mortars. Table 1: Mineralogical composition of the CSA cement (KTS 100 provided by Bellitex). Minerals (% weight) C 4 A 3 S C 2 S C 3 FT C 12 A 7 Periclase CS Quartz The mixing solution was prepared by dissolving the appropriate amount of analytical grade ZnCl 2 salt (0, 0.01 or 0.5 M) into distilled water. This solution (with a temperature comprised 68

3 between 20 and 25 C) was introduced in a standardized laboratory mixer (uropean standard N 196-1) with the cement, possibly pre-mixed with sands for mortars preparation. Mixing was performed at low speed for 3 min and at high speed for 2 min. The fresh mortars were then cast in 4*4*16 cm 3 metal moulds, vibrated during a few seconds and cured in sealed bag to prevent drying. The fresh pastes were cast into polystyrene airtight boxes (10 ml of paste per box). Samples were cured during 7 days at 20 C, or were submitted to a thermal cycle in an oven (Memmert UFP 500), before being stored in sealed plastic bag or under water at ambient temperature and pressure until testing. 2.2 Thermal cycles Thermal cycles were temperature profiles applied on pastes and mortars in a programmable oven. The objective was to reproduce the temperature rise and decrease which may occur in a massive structure (such as a 400 l drum containing a stabilized waste) during cement hydration. This temperature evolution was estimated by recording the temperature of 800 ml mortar samples placed in semi-adibatic Langavant calorimeters as a function of time (Fig. 1.1). The temperature profiles were then defined by interpolating in 40 segments at the most the curves achieved. In the case of pastes, which produced more heat than mortars, some corrections were brought to the temperature profiles, so as to keep the same effective temperature evolution in the heart of the materials (Fig. 2). As shown in Fig. 1.2, the differences between the thermal histories of two 4*4*16 cm 3 mortar samples respectively cured at 20 C and under semi-adiabatic conditions were very significant. Temperature ( C) curing under semiadiabatic conditions Time (h) Figures 1.1 and 1.2: Temperature evolution recorded on CSA mortars. 80 semi-adiabatic curing Temperature ( C) C curing Time (h) 80 inner temperature of the paste Temperature ( C) calorimetry recorded on mortar temperature profile applied to the paste Time (h) Figure 2: Application of a thermal cycle on a CSA paste ( content of the binder: 20%, 0.5 M ZnCl 2 mixing solution). 69

4 2.3 Analytical methods Hydration of mortars was followed by calorimetry according to the semi-adiabatic method (uropean standard N 196-9). This latter consisted in introducing 1575g ± 1 g of fresh mortar into a cylindrical container which was then placed into a calibrated calorimeter in order to determine the quantity of heat emitted in accordance with the development of temperature. Inner temperature of pastes and mortar were measured with waterproof penetration probes TC type K and recorded with a Testo thermometer. Ultimate compressive strengths of 4*4*16 cm 3 specimens were measured according to uropean standard N at 1, 7, 28, 90, 180 and 360 days using a 3R testing machine. Lengths changes were measured after the same periods of time with displacement gauges consisting of LVDT (linear variable differential transducer). Porosity accessible to water was determined following the AFPC-AFRM [18] protocol with a drying temperature of 105 C. Hydration was stopped after fixed periods of time (5 min, 1h for pastes with, 2h, 5h, and 1, 7, 28, 90, 180 and 360 days) by successively immersing crushed pastes into isopropanol and drying them into a controlled humidity chamber (with relative humidity of 20% at ambient temperature). Crystallised phases were identified using X-ray diffraction (Siemens D8 Copper anode λ Kα1 = Å generated at 40 ma and 40 kv) on pastes ground to a particle size less than 100 µm. The acquisition range was from 5 to 60 2θ at θ steps with integration at the rate of 2s per step. 4. RSULTS AND DISCUSSION 4.1 Influence of on CSA hydration The cumulated heat evolution versus hydration time is shown in Fig. 3 for mortars prepared with an increasing amount of (from 0 to 20%). The heat output which mainly resulted from cement dissolution (yeelimite giving the major contribution), decreased when the content increased whereas the induction period was reduced. From 0 to 3% of, the total cumulated heat did not change significantly but the induction period and the times for maximum heat fluxes (inflexion point of the curves in Fig. 3) were strongly reduced. From 3 to 7%, the delays before the acceleration period were approximately the same, but the times for maximum heat fluxes went on decreasing when the content increased, but more slightly. The total cumulated heat distinctly decreased. From 7 to 10%, the induction period, the time for maximum heat flux and the total cumulated heat reached a minimum. Increasing the content still further (20%) resulted in a slight increase of the induction period, whereas the total cumulated heat did not vary anymore. 70

5 % 5% Cumulated heat (J/g of cement) % 20% 3% 2% 1% 0% Time (h) Figure 3: Heat produced by CSA mortars with various contents during hydration. The higher cumulated heat without or with a low content, that is mainly due to the heat released by dissolution, may result from a higher hydration degree before the diffusioncontrolled regime. The influence on CSA cement pastes mineralogy was investigated by XRD (Fig. 4). The samples followed the same evolution than the corresponding mortars. The XRD diagrams showed that the presence of promoted the precipitation of ettringite instead of calcium monosulfoaluminate hydrate. Moreover, in the absence of, the residual amount of yeelimite was higher 5h after the beginning of hydration, but significantly lower after 24h. 5h, 0% C A A 24h, 0% 5h, 10% 24h, 10% θ-scale Figure 4: XRD diagrams of CSA pastes subjected to thermal cycles, ettringite (), calcium monosulfoaluminate hydrate (), yeelimite (), AH 3 (A), C 3 FT (C). 71

6 4.2 Thermal cycle at early age The mineralogies of paste samples cured at 20 C or undergoing a thermal cycle were compared (Fig. 5). The diagrams show that and yeelimite reacted faster when the temperature increased: after two hours, was totally consumed to form mainly ettringite. At 5h (corresponding to the maximal temperature of the cycle) and 24h, diagrams were similar but, with a thermal cycle, calcium monosulfoaluminate hydrate was favoured against ettringite which has a smaller stability domain with an increase of the temperature. 2h 2h, cycle G G G G C 5h 5h, cycle 24h A Q 24h, cycle θ-scale Figure 5: XRD diagrams of CSA pastes prepared with 10% of with and without thermal cycle; ettringite (), calcium monosulfoaluminate hydrate (), yeelimite (), (G), AH 3 (A), C 3 FT (C), quartz (Q). The thermal cycles had also an influence on the microstructure of the mortars. Table 2 shows that the compressive strength after 28 and 90 days of curing decreased, especially without. The compressive strength was not directly correlated with the total porosity accessible to water. This latter, which increased with the content, did not vary when the thermal cycle was applied. This means that changes in the pore volume distribution should also be taken into account. Investigations with mercury intrusion porosimetry are currently under way. Table 2: Influence of thermal cycles on porosity and compressive strength of mortars Compressive strength (MPa) Porosity accessible to water (%) 0% 10% 20% 0% 10% 20% Cure cycle 20 C cycle 20 C cycle 20 C cycle 20 C cycle 20 C cycle 20 C 28d d

7 4.3 Influence of zinc chloride Influence of zinc chloride on CSA cement hydration was investigated by semi-adiabatic calorimetry on mortars which were prepared with a solution containing 0.01 mol/l or 0.5 mol/l of ZnCl 2 (Fig.6). The results depended on amount of present in the mix. When the cement contained 20%, adding ZnCl 2 in the mixing solution did not change the rate of hydration, even if the concentration reached 0.5 mol/l. In this case, the main effects were a rapid heat output a few minutes after mixing and an increase in the total cumulated heat. The initial heat production, occurring simultaneously with a stiffening of the mortar, might indicate the precipitation of an amorphous Zn-containing compound (such as Zn(OH) 2 ). The increase in the total heat could not be explain only by the initial extra-heat release, and thus probably resulted also from the precipitation of a higher amount of hydrates. Cumulated heat (J/g of cement) ,5 M 0 M 0,01 M Time (h) 0,5 M Figure 6: Heat evolution recorded from CSA mortars with various contents (20% for the three first curves and 0% for the others) mixed with a solution of ZnCl 2. In the absence of, retardation was observed when the mixing solution contained ZnCl 2. The induction period increased with the zinc concentration. However, the hydration rate in the acceleration period seemed to increase when the ZnCl 2 concentration reached 0.5mol/L. The total cumulated heats were very close to those produced by reference without. XRD (Fig.7) revealed some important changes in the mineralogy, occurring between 5h and 24h after mixing. At 5h, the main phases were yeelimite and a small amount of ettringite. At 24h, yeelimite was almost totally hydrated and two other phases were observed: calcium monosulfoaluminate hydrate and Friedel s salt. However, a third phase, characterized by intense peaks, is not clearly identified (d=8.47 strong, d=4.22 weak, d=2.88 strong). It could be a chloro-substituted close to the Kuzel s salt. Crystallized Zncontaining phases were not identified. 0 M 0,01 M 73

8 A A C 0 M ZnCl 2 P 2θ-scale ?? + F F A? A? +A C C 2 S 2θ-scale Figure 7: XRD diagram of CSA pastes (0%, 0 and 0.5 M of ZnCl 2 ) submitted to a thermal cycle, ettringite (), calcium monosulfoaluminate hydrate (), yeelimite (), AH 3 (A), C 2 S (C 2 S), C 3 FT (C), Periclase (P), Friedel s salt (F), Unknown (?). Another consequence of the ZnCl 2 addition (0.5 mol/l) in the mixing solution was a strong expansion of the mortars cured under water when the cement did not contain any (Fig.8). This dimensional instability was strongly reduced by adding to the clinker. Characterizations of the mineralogy and microstructure are required to explain this phenomenon. C 2 S A P 0.5 M ZnCl 2 Length change (µm.m-1) M, 0% 0.5 M & 0.01 M, 20% Time (jours) 0.01 M, 0% Figure 8: Influence of on the length change of ZnCl 2 containing mortars cured under water. 74

9 The total porosity accessible to water and the compressive strength of mortars after 28 days of curing in sealed bag with or without ZnCl 2 are reported in table 3. The porosity slightly decreased in the presence of ZnCl 2. and the compressive strength was significantly enhanced especially for a content of 20%. Table 3: Influence of ZnCl 2 concentration on porosity and compressive strength of mortars. Compressive strength (MPa) Porosity accessible to water (%) 0% 20% 0% 20% ZnCl mol/l mol/l mol/l mol/l 0 28 days CONCLUSIONS Hydration of CSA cement was investigated as a function of content of the binder, thermal history of the sample at early age, and ZnCl 2 concentration in the mixing solution. Hydration was strongly accelerated by the presence of, but slightly slowed down by ZnCl 2 when the binder did not contain any. In that case, the mineralogy observations revealed the precipitation of chloro- such as Friedel s salt and possibly Kuzel s salt. Complementary investigations are under way to determine the zinc location. The thermal cycles accelerate the rate of hydration, promote the precipitation of instead of ettringite and affect the macroscopic properties of the mortars such as compressive strength and probably the porosity distribution. From a more practical point of view, CSA cements showed a much better compatibility with ZnCl 2 than Ordinary Portland Cement. Hydration inhibition was never observed, even at high Zn concentration. Adding a 20% content to cement was recommended since it improved the compressive strength of mortars prepared with a 0.5 mol/l ZnCl 2 solution and reduced their expansion under wet-curing. RFRNCS [1] Sharp, J.H., Lawrence, C.D., and ang, R., Calcium sulfoaluminate cements - low-energy cements, special cements or what? Advances in Cement Research, (1): [2] Glasser, F.P. and Zhang, L., High-performance cement matrices based on calcium sulfoaluminate-belite compositions. Cement and Concrete Research, (12): [3] Zhang, L. and Glasser, F.P., Hydration of calcium sulfoaluminate cement at less than 24 h. Advances in Cement Research, (4): [4] Zhou, Q. and Glasser, F.P., Thermal stability and decomposition mechanisms of ettringite at < 120 degrees C. Cement and Concrete Research, (9): [5] Kumarathasan, P., Mccarthy, G.J., Hassett, D.J., and Pflughoefthassett, D. "Oxyanion substituted ettringites: synthesis and characterization; and their potential role in immobilization of As, B, Cr, Se and V". Proceedings of Materials Research Society Symposium [6] Chrysochoou, M. and Dermatas, D., valuation of ettringite and hydrocalumite for heavy metal immobilization: Literature review and experimental study. Journal of Hazardous Materials, :

10 [7] Gougar, M.L.D., Scheetz, B.., and Roy, D.M., ttringite and C-S-H Portland cement phases for waste ion immobilization: A review. Waste Management, (4): [8] Bonen, D. and Sarkar, S.L. "The present State of the Art of Immobilization of Hazardous Heavy Metals in Cement Based Materials". Proceedings of ngineering Foundation Conference. Durham. 1994: Advances in Cement and Concrete [9] Poon, C.S., Clark, A.I., Peters, C.J., and Perry, R., Mechanisms of Metal Fixation and Leaching by Cement Based Fixation Processes. Waste Management & Research, (2): [10] Albino, V., Cioffi, R., Marroccoli, M., and Santoro, L., Potential application of ettringite generating systems for hazardous waste stabilization. Journal of Hazardous Materials, (1-3): [11] Berardi, R., Cioffi, R., and Santoro, L., Chemical effects of heavy metals on the hydration of calcium sulphoaluminate 4CaO.3Al 2 O 3.SO 3. Journal of Thermal Analysis, (3): [12] Pera, J., Ambroise, J., and Chabannet, M., Valorization of automotive shredder residue in building materials. Cement and Concrete Research, (4): [13] Peysson, S., Pera, J., and Chabannet, M., Immobilization of heavy metals by calcium sulfoaluminate cement. Cement and Concrete Research, (12): [14] Cau-Dit-Coumes, C., Courtois, S., Malassagne, N., and Benameur, N. "Influence of Zn(II) ions on the hydration of calcium sulfoaluminate cement, application to nuclear waste conditionning". in 12th International Congress on the Chemistry of Cement. Montreal, Canada: [15] Schwiete, H.. and Ludwig, U. "Crystal structures and properties of cement hydration product (Hydrated calcium aluminates and ferrites)". Proceedings of 5th International Symposium on the Chemistry of Cement [16] Glasser, F.P., Kindness, A., and Stronach, S.A., Stability and solubility relationships in phases - Part 1. Chloride, sulfate and hydroxide. Cement and Concrete Research, (6): [17] Brown, P. and Bothe, J., The system CaO-Al2O3-CaCl2-H2O at 23 +/- 2 C and the mechanisms of chloride binding in concrete. Cement and Concrete Research, (9): [18] Ollivier, J.P. "Les resultats des essais croisés AFRM pour la determination de la masse volumique apparente et de la porosité accessible à l'eau des bétons". Proceedings of AFPC- AFRM Durabilité des bétons. Toulouse, France