Effect of Chain Lengths at Constant Molecular Weights of Water Reducing Admixture on Properties of Cementitious Systems Containing Fly Ash

In this study, the effect of main chain and side chain length of polycarboxylate-ether based high range water reducing admixture (WRA) on the fresh properties, compressive strength and water absorption of cementitious systems containing 0, 15, 30 and 45 wt.% fly ash was investigated. For this purpose, 3 WRAs with same molecular weight but different chain lengths were produced. According to test results, flowability of paste and mortars was negatively affected when the length of the main chain and side chains of the admixture was longer or shorter than a certain value. This adverse effect is thought to be arisen from the weakening of the adsorption of admixture with increase of its chain lengths. However, when the main chain and side chain lengths of the admixture were shorter or longer than a certain value, the time-dependent flow properties of the mortar mixtures improved. The main chain and side chain lengths of the WRAs had not a significant effect on the compressive strength and water absorption capacity of the mortar mixtures. However, irrespective of the admixture characteristics, with the increase of fly ash substitution the flow and time-dependent flow properties of the mixtures were negatively affected but their water absorption decreased.

Uptake of Be(II) by Cement in Degradation Stage I: Wet-Chemistry and Molecular Dynamics Studies

The uptake of beryllium by hardened cement paste (HCP, with CEM I 42,5 N BV/SR/LA type) in degradation stage I was investigated with a series of batch sorption experiments with 10−6 M ≤ [Be(II)]0 ≤ 10−2.5 M and 2 g·L−1 ≤ [S/L] ≤ 50 g·L−1. All experiments were performed under Ar atmosphere at T = (22 ± 2) °C. Solubility limits calculated for α-Be(OH)2(cr) in the conditions of the cement pore water were used to define the experimental window in the sorption experiments. Beryllium sorbs strongly on HCP under all of the investigated conditions, with log Rd ≈ 5.5 (Rd in Lkg−1). Sorption isotherms show a linear behavior with a slope of ≈ +1 (log [Be(II)]solid vs. log [Be(II)]aq) over four orders of magnitude (10−8 M ≤ [Be(II)]aq ≤ 10−4 M), which confirm that the uptake is controlled by sorption processes and that solubility phenomena do not play any role within the considered boundary conditions. The similar uptake observed for beryllium in calcium silicate hydrate (C-S-H) phases supports that the C-S-H phases are the main sink of Be(II) in cement. The strong uptake observed for Be(II) agrees with the findings reported for heavier metal ions, e.g., Zn(II), Eu(III), Am(III), or Th(IV). The exceptional sorption properties of beryllium can be partially explained by its small size, which result in a charge-to-size ratio (z/d) of the same order as Eu(III) or Am(III). Kinetic experiments confirm the slow uptake of Be(II), which is characterized by a two-step process. In analogy to other strongly sorbing metal ions such as Zn(II) or Th(IV), a fast surface complexation (t < 4 days) followed by a slower incorporation of Be(II) in the C-S-H structure (t ≥ 60 days) are proposed. The surface complexation was studied in detail with molecular dynamic simulations, and the most common surface species are identified and described. This work provides the first experimental evidence supporting the strong uptake of Be(II) by HCP in degradation stage I, further extending previous findings on C-S-H phases and HCP in degradation stage II. These results overcome previous conservative estimates assuming no or only a weak uptake in cementitious systems and represent a relevant contribution for the quantitative assessment on the retention/mobilization of beryllium in the context of nuclear waste disposal.