Strontium Retention in Ordinary Portland Cement, Low pH Cement and Model Compounds

Introduction: Within the framework of geological disposal for radioactive waste, the interaction between the host rock and the structural materials (cement) leads to enhanced chemical perturbation at the interface. The use of low-pH cement reduces the chemical perturbation. However, the impact on the retention properties of such materials is not known. The objective of this work is to examine the microstructure and the Sr (strontium) retention properties of CEM I cement pastes and low-pH cement pastes, together with model tri-calcium silicate (C3S) pastes. This approach is useful to discriminate the impact of individual phases, such as portlandite, ettringite and C-S-H, on the whole set of the Portland cement retention properties.

Methods: Four paste formulations were chosen: CEM I paste (w/c = 0.47), low-pH CEM I paste (50 % wt silica fume, 2.2 % wt superplasticizer, w/b = 0.6), C3S paste (w/c = 0.47) and low-pH C3S paste (40 % wt colloidal silica, 6.6 % wt superplasticizer, w/b = 0.65). During the casting, all the samples were spiked with non-radioactive Sr ions to simulate the immobilized waste ions. A solution of SrCl2 was introduced to reach a concentration of 3.6 mg Sr/ g binder. The samples were demolded after two days and cured for four months under N2 atmosphere to prevent carbonation. Calorimetry of the fresh pastes was measured using a TAM-AIR isothermal calorimeter. The cured samples were analyzed by X-ray diffraction (XRD), thermogravimetric analyses (TGA) and X-ray micro-tomography. 1D monolith leaching experiment was performed according to EPA method 1315 using deionized water as the leachant. pH-dependent leaching was performed according to EPA method 1313, providing liquid-solid partitioning curves as a function of pH for the different elements as well as pH behavior and buffer capacity of the cementitious matrices. Major ions concentrations (Na, K, Mg, Ca, Ba, Mn, Al, Fe, Si, P, and S) as well as Sr content of the leachates were measured using ICP-OES, for both leaching experiments.

Preliminary Results: In CEM I paste and C3S paste, portlandite was observed by TGA and XRD. Portlandite buffers the pH of the interstitial solution, as observed by the pH-dependent leaching experiments. For the low-pH CEM I paste and the low pH C3S paste, the absence of portlandite is confirmed by TGA and XRD due to the pozzolanic reaction. Preliminary results from the 1D monolith leaching tests show that low-pH systems have the best Sr retention property. Compared to CEM I paste, the Sr leachability increases by a factor of 2.5 for the C3S paste, while it decreases up to 30% for the low-pH systems (CEM I and C3S). The effect of the microstructure variation on the leaching behavior is yet to be determined. Geochemical modelling will be used to bridge the gap between the leaching experiment results and the chemical and mineralogical characterizations.