Y. Tzvetanova

Study of the pozzolanic activity and hydration products of cement pastes with addition of natural zeolites

Abstract This paper presents results from comparative thermogravimetric, calorimetric and pozzolanic activity analyses of five natural zeolite samples from Bulgaria, Slovakia, Philippines, USA and North Korea. The zeolites actively participate in the hydration processes of cement. Their activity in the early stage of hydration is based mainly on the large surface area of the particles while, in the later stages of activation, chemical reactions occur between the products of the hydration of cement and the soluble SiO2 that is present in the bulk of the zeolites. It has been shown that in all cement pastes which contain zeolite additives, the quantity of portlandite is lower than that in pure cement paste or is even totally absent. The amounts of hydration products are greater when 30% zeolite is used than when 10% zeolite is added (excluding the sample with chabazite). The lowest pozzolanic activity is shown by chabazite, which possessed the lowest SiO2/Al2O3 ratio.

Mössbauer, XRD, and Complex Thermal Analysis of the Hydration of Cement with Fly Ash

Hydration of cement with and without fly ash is studied with Mossbauer spectroscopy, XRD, and thermal analysis. Iron in cement is present as Fe3

Evaluation of Bulgarian clinoptilolite as ion-exchanger for Cs+ removal from water solutions

Cesium uptake by natural zeolite clinoptilolite from Bulgaria was studied using batch technique and model solutions. The optimal conditions of interaction were determined. The pseudo-second-order rate model better describes the kinetic data obtained at different concentrations. The intraparticle diffusion and the surface diffusion models were tested to identify the rate-controlling step. The sites in the structure of clinoptilolite that are preferable for exchange were studied by application of Rietveld structural approach and the sequence of site occupation by cesium was followed. The Langmuir isotherm model provides a good fit of the equilibrium experimental data. The thermodynamic parameters for the system were calculated.

Rheological, porosimetric, and SEM studies of cements with additions of natural zeolites

Abstract Rheological, porosimetric, and SEM studies of cements with additions of natural zeolites (chabazite, mordenite and clinoptilolite) were investigated. The addition of 5% zeolite to cement pastes had a plastifying effect. The increase in the rheological parameters of aqueous cement-zeolite suspensions began after additions of more than 10% zeolite. At a water/solid ratio (w/s) = 0.5 all compositions exhibited similar rheological behaviour for yield stress and maximum shear stress values. At lower w/s values the rheology depended on the zeolite used, decreasing in the order Sl (clinoptilolite, Nižny Hrabovec deposit, Slovakia), M (mordenite, Philipines), Bp (clinoptilolite, Beli Plast deposit, Bulgaria), NM clinoptilolite, Saint Cloud deposit, New Mexico) and Ch (chabazite, Nasm Yan deposit, Korea). After 28 days of hydration the mixtures containing 10% Bp clinoptilolite, mordenite, and Sl clinoptilolite (up to 30%) had unchanged specific pore volumes but additions of 10% and 30% of chabazite and NM clinoptilolite and of 30% mordenite and Bp clinoptilolite lead to increases in the total specific pore volume compared to the neat cement paste. Between the 28th and 180th days of hydration the specific volume of the pores in all the cement-zeolite pastes decreased due to the filling of the pores with products from the pozzolanic reaction between the zeolites and the hydration products of the cement. The pozzolanic reaction between zeolite and the hydration products of cement is enhanced by the zeolite content in the samples and depends on the Si/Al ratio (clinoptilolite (Si/Al >4.5), mordenite (Si/Al >5), chabazite (Si/Al <2.5)) and the surface area (clinoptilolite from Slovakia has the smallest crystallites and, hence, the greatest reactive surface area).

Evaluation of the possible use of a Bulgarian clinoptilolite for removing strontium from water media

Abstract The sorption of Sr2+ ions by natural clinoptilolite was investigated using the batch method. The effects of pH, contact time and strontium concentrations were tested and the optimal conditions for sorption were determined. The process was very fast initially and equilibrium was reached within 24 h. Kinetic sorption data were fitted to pseudo-first-order, pseudo-second-order and intra-particle diffusion models. The simple pseudo-second-order rate model provides good agreement with the experimental data for Sr2+ uptake. The respective rate constants, k2, calculated at different initial Sr2+ concentrations, were obtained. The equilibrium data were analysed by applying the Langmuir, Freundlich and Dubinin-Raduchkevich isotherm models. The Langmuir model describes the observed Sr2+ uptake most accurately and the value of the monolayer sorption capacity is 32.81 mg Sr2+/g. The effect of Na+ ions and the effectiveness of uptake from low saline groundwater simulated solutions were also studied and discussed.

Sorption of Silver Cations by Natural and Na-Exchanged Mordenite

Sorption of Ag+ by natural mordenite and its Na-exchanged form was investigated by the batch method. Maximum Ag+ uptake was observed at initial pH > 3.0 and contact time 90 min. The kinetic data fitted very well to the pseudo-second-order rate model with values of k 2 from 37.31 to 0.487 and 48.32 to 0.491 g/meq min for the natural and Na-exchanged mordenite, respectively. The Langmuir model is in good correlation with the isotherm data up to initial concentration of 500 mg Ag+/L (q m = 57.41 (natural sample) and 87.72 mg/g (Na-exchanged sample). The obtained data are promising for clean-up of polluted water.