Journal of Hazardous Materials 192 (2011) 234–245 Contents lists available at ScienceDirect Journal of Hazardous Materials j our na l ho me p age: www.elsevier.com/locate/jhazmat Microencapsulation of phosphogypsum into a sulfur polymer matrix: Physico-chemical and radiological characterization Félix A. López a,∗ , Manuel Gázquez b , Francisco José Alguacil a , Juan Pedro Bolívar b , Irene García-Díaz a , Israel López-Coto b a Centro Nacional de Investigaciones Metalúrgicas (CENIM), CSIC, Avda. Gregorio del Amo, 8, 28040 Madrid, Spain b Departamento de Física Aplicada, Universidad de Huelva, Campus de El Carmen, 21071 Huelva, Spain a r t i c l e i n f o Article history: Received 22 December 2010 Received in revised form 3 May 2011 Accepted 5 May 2011 Available online 11 May 2011 Keywords: Phosphogypsum Natural radioactivity Radon Sulfur polymer cement Microencapsulation a b s t r a c t The aim of this work is to prepare a new type of phosphogypsum-sulfur polymer cements (PG-SPC) to be utilised in the manufacture of building materials. Physico-chemical and radiological characterization was performed in phosphogypsum and phosphogypsum-sulfur polymer concretes and modeling of exhalation rates has been also carried out. An optimized mixture of the materials was obtained, the solidified mate- rial with optimal mixture (sulfur/phosphogypsum = 1:0.9, phosphogypsum dosage = 10–40 wt.%) results in highest strength (54–62 MPa) and low total porosity (2.8–6.8%). The activity concentration index (I) in the PG-SPC is lower than the reference value in the most international regulations and; therefore, these cements can be used without radiological restrictions in the manufacture of building materials. Under normal conditions of ventilation, the contribution to the expected radon indoor concentration in a stan- dard room is below the international recommendations, so the building materials studied in this work can be applied to houses built up under normal ventilation conditions. Additionally, and taking into account that the PG is enriched in several natural radionuclides as 226 Ra, the leaching experiments have demonstrated that environmental impact of the using of SPCs cements with PG is negligible. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Phosphogypsum (PG), CaSO 4 ·2H 2 O, is a by-product coming from the processing of fluoroapatite resulting in H 3 PO 4 production. In the process, fluoroapatite is dissolved using sulfuric acid. Phospho- ric acid (PA), phosphogypsum, and hydrofluoric acid are obtained: Ca 5 (PO 4 ) 3 F + 5H 2 SO 4 + 10H 2 O → 3H 3 PO 4 + 5CaSO 4 •2H 2 O + HF (1) Phosphate rocks contain high concentrations of some metals as As, Cd or Sr, and natural radionuclides from 238 U decay-series, in secular equilibrium, which are about 50 times higher than the Abbreviations: PR, phosphate rock; PG, phosphogypsum; PCC, portland cement clinker; SPC, sulfur polymer concrete; PG-SPC, phosphogypsum-sulfur polymer con- crete; STX TM , sulfur modified; I, external risk index; Ra(eq), equivalent radium concentration; Ac (%), coefficient of absorption with respect to the pH; Lc (%), leaching coefficient for 238 U and 210 Po at the different pHs. ∗ Corresponding author at: Centro Nacional de Investigaciones Metalúrgicas (CENIM), CSIC, Department of Materials Recycling, Avda. Gregorio del Amo, 8, E- 28040 Madrid, Spain. E-mail address: flopez@cenim.csic.es (F.A. López). ones in typical soils [1,2]. During the industrial process is produced a fractionation of radioelements contained in PR. In the factory of Huelva (Southwestern Spain), 226 Ra remains in PG (practically 100%), 210 Pb– 210 Po (about 90%), and 230 Th (70%) [3]. World PG production is around 200–280 × 10 6 t per year [4,5]. Spanish fertilizer industry produces annually about 3 × 10 6 t of PG, which is the result of processing 2 × 10 6 t of phosphate rocks [6]. PG is deposited in regulated stacks, which may have a negative impact on the environment, being necessary a valorization and recycle of phosphogypsum. Nowadays a number of researches are focused on the search of new uses of PG: (1) agricultural fertilizer or for soil stabilization amendments [7–10]; (2) cement industry as a setting regulator in place of natural gypsum [11–13], in the gypsum industry to make gypsum plaster [14,15], as mineralizer in the burning Port- land cement clinker (PCC) [16], as raw material in the raw mix of cement [17–19] and in other binder materials [20–23]. Only the 15% of the world-wide production is recycled [1,4,6]. In this sense, we have studied a new application of PG in civil engineering, consisting in to use this by-product as additive of sulfur polymer cement (SPC). The SPC and SC (sul- fur cement) has studied during last three decades [24–26], because of its high strength and excellent corrosion resis- tance. 0304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2011.05.010