| Immobilization of radioactive nuclide like Sr and Cs is one of the important keys in the application of nuclear technology. Alkali-activated slag-clay minerals composite cement (AASCM) is a new matrix for solidifying Sr and Cs which has characteristics of low C/S, Na and Al enrichment and integrates high strength, low porosity, better resistance to corrosion of alkali-activated slag and good cation exchange adsorption of modified attapulgite(M-ATT) and zeolite(ZEO). The research on mechanism and properties of immobilizing Sr and Cs based on AASCM aims at obtaining theoretical basis and accumulating experimental data for its composition design and engineering application.The adsorption capacity of Sr and Cs in several kind of minerals such as ATT , ZEO etc. were studied by static adsorption method. The results reveal that the adsorption capacity of Sr and Cs are dependent on the structure and sorts of cation of the minerals. The substiruted-structural adsorption centers (SSAC) formed from substitution of high-valency cations by low-valency cations (such as Si4+ by Al3+ or Al3+ by Mg2+ ) in the structure of the are a prerequisite to the high adsorption capacity of the minerals. The cations with low-valency and small radius (such as Na+) in the structure are more easily exchangeable and are more helpful to the adsorption capacity of Sr and Cs in the minerals. The channels in the structure of the minerals such as ZEO and ATT are favorable for adsorbing cation whose diameter is smaller than that of the channels. When the interaction of SSAC and the cations is long-range neutralization in the charge compensation way, the layer-structural clay minerals have higher cations exchange capacity. The adsorption capacity of Sr in ZEO group minerals is higher than that of Cs while the adsorption capacity of Sr in M-ATT is lower than that of Cs.The adsorption & step-by-step leaching experimental method was applied to the formula optimization of AASCM. The adsorption capacity and resistance to leaching in deionized water of AASCM which is integrated with ZEO, M-ATT, heat-activated kaolinite (HAK) and alkali-activated slag cement(AAS) are higher than AAS. Under the experimental conditions, the formula of AASCM has significant effects on adsorption & step-by-step leaching properties. When HAK/GBS ratio is in range of 0.176-0.25 in AASCM, the adsorption capacity of Sr is higher than at any other ratio while that of Cs at a HAK/GBS ratio of 0.25 isthe highest in all investigated HAK/GBS ratios. In consideration of adsorption & step-by-step leaching and strength properties, the suggested formula HAK:ZEO: M-ATT:GBS ratio of AASCM =15:5:10:70 (HAK/GBS ratio is 0.214). The average C/S is 0.80 in the AASCM with suggested formulation while (C+N)/(S+A) is 0.75 and A/S is 0.24. The adsorption capacity of Sr and Cs in hydrated paste power of AASCM is higher than that of PC, AC and PSC. The 28-day compressive strength of the hardened AASCM paste is more than 80MPa and its long-term compressive strength of AASCM does not decrease.The hydration products of AASCM, the hydrothermally synthesized products of SrO-CaO-SiO2-H2O systems were studied by means of XRD, TG-DSC, FTIR and SEM. The results indicate that AASCM have characteristics of low C/S, Al and Na enrichment by introducing HAK. The major hydration products of AASCM cured at room temperatures are considered to be Na and Al substituted calcium silicate hydrate gels [(Na+Al)-C-S-H] and poorly-crystallized and well-dispersive products featuring zeolite may exist. Besides (Na+Al)-C-S-H, there are phillipsite-Ca and chabazite in the hardened AASCM cured at 80 °C. Under the hydrothermal conditions at 80 °C, when SrO reacts with CaO and SiC>2 which are elemental compositions of cements, calcium-strontium silicate hydrate solid solution can be produced and the structure of C-S-H gel will be more disorderly.The static adsorption behaviors of Sr and Cs in hydrated paste powder of AASCM were firstly investigated. The relationship between the adsorption capacity of Sr and Cs in the hydrothermally synthesized powder and the composition of Na2O-CaO-Al2O3-SiO2-H2O systems was studied. The immobilization mechanism of Sr and Cs was discussed. The results reveal that, under experimental conditions, the adsorption equilibrium time of Sr in hydrated paste powder of AASCM is longer than that of Cs. The adsorption capacity of Sr and Cs increases and adsorption ratios decrease with initial concentration of Sr and Cs. The adsorption capacity of Sr is more than twice that of Cs at the same conditions. So far as Cs is concerned, the adsorption equilibrium time shortens and the adsorption capacity and ratio decrease with temperature. The adsorption capacity reduces with the decrease of pH. In a certain range of Cs concentration, the adsorption isothermal curve of Cs in hydration products powder of AASCM is in conformity to Freudlich formula and the adsorption process is exothermal. The compositions of C-S-H have an obvious effect on adsorption properties. In the C-S-H with low C/S, Al and Na enrichment, a part of Na+ can replace Ca2+ in the Ca-0 sheets and Al3+ can replace Si4+ in the Si-0 tetrahedral. So, two kinds of SSAC are formed. Part of Ca2+ and the rest part of Na+ both exist between layers. The former have poorer exchangeability while the latter is exchangeable. The C-S-H with low C/S, rich-Na and Altogether can possess high adsorption capacity of Sr and Cs. The high alkaline in hydrated AASCM paste benefits precipitation of Sr and adsorption of Cs.AASCM exhibits high resistance to gamma irradiation and better resistance to sulfate and acid corrosion than PC. Immobilization of simulated radioactive slurry (SRS) of Fe2O3#nH2O using AASCM is feasible. The application of AASCM in the solidification engineering is suitable below 20°C. When cement/sand ratio is 1:1 and W/C is 0.45, the flowability of the mixture meets the case of solidification engineering and the compressive strength of the waste forms containing 20% SRS meets the needs of GB14569.1-93 and its drying shrinkage rate is approximately equal to waste forms based on PC. When the compressive strength of the AASCM waste forms is approximately equal to that of PC, the former loads 5% SYS more than the latter does. The leaching rate of AASCM based waste forms is lower than that of PC. Sr2"1" in waste forms is bound more strongly than Cs+ in them. The accumulative leaching fractions of 137Cs in the waste forms based on AASCM and PC in 192 d are 1.21 * 10'2 cm and l^OMO'1 cm respectively, the former is 7.56% of the latter. The accumulative leaching fraction of 90Sr in the waste forms based on AASCM and PC are 9.65*10"4 cm and 3.42*10"3 cm respectively, the former is 28.22% of the latter. Compared with waste forms based on PC, the improvement degree of Cs retention in AASCM matrix is higher than that of Sr in it.
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