Thaumasite formation due to atmospheric SO 2 –hydraulic mortar interaction M.T. Blanco-Varela a, * , J. Aguilera a , S. Mart  ınez-Ram  ırez a , F. Puertas a , A. Palomo a , C. Sabbioni b , G. Zappia b , C. Riontino b , K. Van Balen c , E.E. Toumbakari c a Instituto Eduardo Torroja-CSIC, C/Serrano Galvache s/n, 28033 Madrid, Spain. b Institute ISAO, National Research Council, Via Gobetti 101, 40129 Bologna, Italy. c Department of Civil Engineering, KU Leuven, De Croylaan 2, 3001 Heverlee, Belgium. Abstract The objective of this paper was to reproduce the formation of thaumasite due to the reaction of atmospheric SO 2 with hydraulic mortars. The research was carried out on mortars made with ordinary Portland cement (OPC), mineralized white Portland cement, hydraulic lime and a mixture of lime and pozzolana. Mortars underwent sulfation by exposing the samples to 300 ppm SO 2 at 25 °C and 95% RH for 2 days. Subsequently, half of the sulfated samples were kept for 6 and 12 months in a chamber with 0.3 ppm, SO 2 as pollutant (0.50 l min 1 flow gas velocity), 5 °C and 95% RH. The other halves of the sulfated samples were kept partially immersed in water at 5 °C for 4, 9 and 14 months. The process of thaumasite formation in hydraulic mortars due to the interaction of the material with atmospheric SO 2 was reproduced in all the hydraulic mortars kept partially immersed in water at low temperature, except in the lime–pozzolana mixture. Gypsum was the first reaction product formed as a result of that interaction. Subsequently, gypsum reacted with calcium carbonate and C–S–H gel resulting in the formation of thaumasite. The formation of thaumasite was easier and quicker in sulfated samples kept at low temperature partially immersed in water. Only in OPC mortars was thaumasite formation observed in samples exposed to 0.3 ppm of SO 2 for 12 months. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Thaumasite; Durability; Sulfate attack; Hydraulic mortar 1. Introduction The sulfation of mortars and concretes is a function of diverse environmental conditions; among those it is necessary to highlight SO 2 and aerosol concentration in the air, temperature, relative humidity, speed of the wind and presence of liquid water. It also depends on the characteristics of the material, including their chemical and mineralogical composition, porosity and permeability [1]. Phases from hydraulic mortar pastes can react in a different way with atmospheric SO 2 and consequently diverse compounds can be formed including calcium sulfite hydrate, gypsum, and syngenite [2,3]. One of the phases that could be formed as a consequence of the sulfation of the hydraulic binder is thaumasite. Obvi- ously this phase cannot be formed by direct interaction between the SO 2 and the material; a series of interme- diate stages (SO 2 solution in water filling the pores of material, SO 2 oxidation, H 2 SO 3 and H 2 SO 4 acids for- mation, etc.) should occur. As a consequence of the reaction among the previously mentioned acids and the binders, the conditions of precipitation of the thauma- site could be achieved. In the literature no data exist on the formation of thaumasite being atmospheric SO 2 the source of sulfation. The main objectives of this work is to study the evolution of carbonated hydraulic mortars when they are exposed to an atmosphere containing SO 2 and to reproduce the formation of thaumasite through atmo- spheric SO 2 –hydraulic mortars interaction, by mean of simulation tests carried out on mortars into a laboratory exposure chamber. * Corresponding author. Tel.: +34-91-3020440; fax: +34-91- 3026047/49. E-mail address: blancomt@ietcc.csic.es (M.T. Blanco-Varela). 0958-9465/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0958-9465(03)00122-7 Cement & Concrete Composites 25 (2003) 983–990 www.elsevier.com/locate/cemconcomp