Study of the process of carbonation of cement with mineral additives The 9th International Conference on the Occurrence, Properties and Utilization of Natural Zeolites June 8-13 2014, Belgrade, Serbia Study of the process of carbonation of cement with mineral additives during early hydration period (minutes) up to 24 hours V. Lilkov1, O. Petrov2, D. Kovacheva4, I. Rostovsky3, Y. Tzvetanova2, P. Savov1, M. Vatzkitcheva1 1University of Mining and Geology ‘‘St. Ivan Rilski’’, Sofia, Bulgaria 2Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Sofia, Bulgaria

Introduction When cement composites are subjected to the action of CO2 the latter is dissolved in the pore-liquid of the cement pastes ensuring CO32– ions, which react with Ca2+ ions in the gel to form calcite (and/or other carbonates). In case of prolonged contact of the cement pastes with air and water it is possible to reach complete carbonation . The depth of the formed carbonate layer depends on the time of contact with СО2 and its concentration in the surrounding environment and on the coefficient of diffusion of the hardened cement paste (Wang and Lee, 2009).

Early times of hydration of cement composites Lilkov, V., E. Dimitrova and O.Petrov (1997) have studied the hydration process of cement with SF and FA in the first 24 hours and stated that the Pz mineral additive (SF+FA) actively affects the early hydration of cement expressed in increased total amount of hydration products and decreased amount of calcium hydroxide. R. Snellings, G. Mertens, R. Adriaens, J. Elsen (2013) have studied the early age hydration of cement blended with clinoptilolite tuff (30%) monitored in situ by time-resolved synchrotron X-ray powder diffraction starting from 10 min to 8 h (continuously) and then at intervals up to 48 h of hydration. Main conclusion was that the addition of the zeolite tuff accelerated the hydration rate of the main C3S cement component.

Aims of this investigation The present study is on processes of carbonation of cement with natural zeolite (clinoptilolite) during the first minutes, tens of minutes up to the 24th hour of hydration of cement. The carbonation is studied on the surface of the cement paste, where the reactions with CO2 from air are most intensive.

Materials and Methods - PC 100% (PC) - PC 90% and 10% B (PC+B) The used materials are cement (PC45), natural zeolite – clinoptilolite (B) – Beli plast deposit, Bulgaria and silica fume (SF – waste product from the steel production) (Table 1) Table 1. Chemical composition of the starting materials (wt.%) Sample Al2O3 CaO Fe2O3 K2O MgO MnO Na2O P2O5 SO3 SiO2 TiO2 H2O PC 5,37 54,9 2,86 1,12 1,08 0,20 0,24 0,14 3,46 23,22 0,22 8.4 B 9,68 6,7 0,74 2,79 2,90 0,03 0,29 ˂ 0,03 62,74 0,12 18.5 SF 1,13 1,4 2,27 0,86 1,75 0,02 0,49 0,40 89,70 <0,05 2.4 The following compositions with water-to-solid ratio (by mass) of w/s = 0.5 were prepared for study: - PC 100% (PC) - PC 90% and 10% B (PC+B) - PC 90% and 10% SF (PC+B) - PC 90% and 5% B and 5% SF (PC+B+SF)

Sample preparation and XRD approach The dry mixtures were mechanically stirred. The samples were wetted with water and intensively homogenized for 2 min to obtain a paste . The paste was placed in the sample holder for powder XRD measurement. The sample was placed in an desiccator and periodically scanned starting after 3 min of relaxation and at time intervals of 10 min up to the 50th min and then at 30 min up to the 24th hour. The XRD measurements were performed at room temperature on a Bruker D8 Advance diffractometer (Cu Kα radiation and LynxEye detector). The detector scans the pattern (8÷50° 2 theta) for less than 1 min, which ensures registration at exact time intervals revealing the status of the hydrating cement samples.

Super Speed detector, the LYNXEYE™

Initial materials Powder XRD pattern of SF Powder XRD pattern of cement Powder XRD pattern of clinoptilolite

Results and Discussion Figs. 1 and 2 demonstrate the phase changes during hydration of the sample PCBp (cement plus clinoptilolite) and PC During the entire range of time intervals there are no portlandite (Po) and/or ettringite (Et) formed. A constant and steady increase of calcite (Cc) is observed and at 24 h calcite is dominating as a formed product. Fig. 1 Fig. 2

Additionally, the crystallization of calcite was checked by determining its the crystallite size during the times of the experiment by profile-fitting of the strongest diffarction line of calcite at d=3.03 Å. It was found that (for samples plain cement and cement+clinoptilolite) the crystallite sizes are similar – up to the 120 min they are in the range 50÷60 nm and after that up to 480 min. they lower twice - 20÷30 nm. The latter indicates that with time the depth of the formed carbonate layer depends on the time of contact with CO2 and its concentration in the surrounding environment and the diffusion in the hardening composite pastes is slowed down. There is no specific role of clinoptilolite to be elucidated.

Hydration in depth After 24 h the hydrating cement composites were isolated from the water, dried at 105 °C for 1 h and placed in alcohol to stop further hydration. Then the samples were analyzed by XRD. It was registered that the processes in the bulk volume for all 4 cases were normally proceeding with the formation of portlandite and ettringite.

Hydration of the cement composites for 24 h XRD pattern of cement (24 h) XRD pattern of cement +SF (24 h) XRD pattern of cement +zeolite (24 h) XRD pattern of cement +SF+zeolite (24 h)

Conclusions The missing of the diffraction lines of portlandite and ettringite in both pastes indicates that the process of carbonation on the surface of the cement paste takes place directly between the calcium ions from the solution and the carbon dioxide from air without formation of portlandite and ettringite. The cement composite with clinoptilolite is characterized by slowed process of gypsum dissolution. The diffraction lines of gypsum are preserved up to the 60th min, while for the plain cement gypsum disappears after 30th min. Probably the zeolite particles are barrier for direct contact of water with gypsum thus hindering the gypsum dissolution. The plain cement sample displays more intense calcite peaks due to the higher concentration of Ca ions, while with zeolite the carbonation is slowed down, possibly because clinoptilolite may attract Ca ion for some ion-exchange.

Acknowledgements O. Petrov thanks Gravelita OOD for the financial support to participate in ZEOLITE 2014 The work of Yana Tzvetanova was supported by ESF (Grant BG051PO001-3.3.06-0027).

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