The Immobilization of Radioactively Contaminated Soil in Cementitious Materials

The Immobilization of Radioactively Contaminated Soil in Cementitious Materials Shuxin Bai, Shengliang Yang National University of Defense Technology, P.R.China 2nd CRM

Introduction/Background • In China, a mass of radioactively contaminated soil is required to be disposed. • From the point of effectiveness and economy, cementation is a promising candidate approach to immobilize such kind of waste. 2nd CRM

Introduction/Background • Soil in the cement paste may deteriorate its allover properties. • Especially when soil is present in the form of clump. • But if well designed and processed, the contaminated soil can be mixed with cement and additional agents to form clay cement concrete of good durability. 2nd CRM

Research Methodology • The physical and chemical processes occurring during the harden process of soil cement system • including: the cementation of cement, soil and other agents, and role of them, interactions of cement and soil, the state of radionuclides in the concrete, etc. • Specific of cementation process for soil: • pretreatment of the soil, mixing device and method, post-treatment of the cementation product. 2nd CRM

Research Methodology • The influence of cementitious materials and processes on properties of concrete • such as strength, microstructure, porosity, cracking etc. • The degradation of the soil cement concrete in water at different conditions • including: water penetration and transport in pores of the concrete, dissolution of different components (C-S-H, clay .etc) in the concrete block, the leaching and retention behaviors of different radionuclides (actinides specially). 2nd CRM

Pretreating of Soil Wasterform Preparing Cementing Materials XRD SEM+EDS Aging for 28d Testing Properties Compressive Strength Shocking Properties Leaching Behavior Thermal Cycling Other aspects Research Methodology 2nd CRM

Experimental Techniques and Equipment • Techniques and Equipment • XRD: D-Advance, Germany • SEM-EDS: S-4800 SEM, HORIBA EDS • ICP-AES: IRIS Advantage1000, USA • Universal Materials Testing Machine • Mixing Device • Materials • various cements, cement additives, contaminated soil samples. 2nd CRM

Research Approach and Results • Different types of cement was adopted to immobilize the contaminated soil. • Ordinary Portland Cement (OPC) is used as the main composition. Additives such as super-plasticizers, pulverized fly ash (PFA), zeolite, silica fume etc. was introduced to enhance the cement properties. • Substituent elements for radionuclides were added to the soil samples • Cs+, Sr2+, Ce3+ were added to substitute the 137Cs, 90Sr and 239Pu • concentration of each element: ~0.01%(wt) 2nd CRM

A — CaCO3 B — SiO2 C — CaSO4·2H2O D —CaAl2Si2O8 soil

Cemented samples Good compatibility High soil volume (2.0~2.5:1, or 67~70%wt) Density, but pores visible 55cm no vibration 4416cm aged wasteform samples As fabricated wasteform samples Cross-section

Research Approach and Results • The compressive strength of each group show good results. Aged 28d 2nd CRM

Leaching rate results GB7023-86 Standard: 2nd CRM

Research Approach and Results • Free water check: no free water observed after aged 7d • Thermal cycle durability: • Loss of compressive strength (-25~15C, 5cycles): <25% • No obvious change after 60 cycles (-40C ~ 40C, quenching in LN2) 60 cycles from -40C to 40C 2nd CRM

Influence of additives on the cement hydraulic process • Influence of soil • Influence of PFA • Influence of silica fume • Influence of zeolite 2nd CRM

Influence of soil Pure OPC cement AFm decreased for the exist of CaSO42H2O in the soil prevents the formation of AFm from AFt. Cement+soil (1:1) 2nd CRM XRD of hydrated cement and cement-soil grout aged 21d

Influence of PFA No obvious difference for 21d samples, however for 60d samples phase C remains little amount, indicates reaction between cement and PFA at later period of setting cement cement Cement +PFA Cement +PFA XRD of samples aged for 21d XRD of samples aged for 60d 2nd CRM

Influence of PFA The adding of PFA improves leaching properties: 2nd CRM

Influence of silica fume cement Cement +SF Active SiO2 reacts with Ca(OH)2 to form C-S-H SiO2 + Ca(OH)2 = CaSiO3 + H2O CaSiO3 + nH2O = CaSiO3·nH2O XRD of samples aged for 21d 2nd CRM

Influence of silica fume The adding of suitable amount of SF decreases the leaching rate of waste forms The adding of SF increases the compressive strength of waste forms 2nd CRM

Influence of zeolite A — SiO2 B — montolite The leaching rate decreases with the adding of zeolite 2nd CRM

Influence of other aspects • water/cement ratio: plasticizer is necessary • Vibration of cemented waste form before setting increase the strength and decrease the leaching rate 2nd CRM

Research results and conclusion • cemented waste forms with high durability and low cost: • Soil/cement ratio: 2.0~2.5 • Compressive strength: >10MPa • Leaching rate of Cs+, Sr2+ <410-3, 110-3 cm/d • Thermal cycle durability: the same after 60 freeze thaw cycles (-40,+40℃) • Unit cost of immobilization: <50€/t of soil 2nd CRM

Research results and conclusion • the soil cemented in this work which is mainly composed of sand and stone has good compatibility with cement and a high encapsulating ratio can be achieved. • At a ratio of cement/soil =1:2.5 the cementing process is still viable and the properties of the waste form including the strength and leaching rate of Sr2+ and Cs+ are accordant with the requirement of the standards . • The investigation of the hydraulic process of the cemented specimen indicates that the exist of soil has apparent effect on the hydraulic process. The CaSO44H2O in the soil retards the formation of hydraulic product AFm . 2nd CRM

Research results and conclusion • The effects of the addition of silica fume, zeolite and PFA on the properties of the cemented waste forms were studied and the results show that the addition of these additives can improve the properties of the waste forms including enhancing the strength and reducing the leaching rate. • Some other cementation processing aspects such as water/cement ratio and whether the specimen being vibrated or not also have important effects on the properties of the waste form. The water/cement ratio can be reduced by adding super-plasticizer to improve the anti-leaching behavior. The vibration during the cementation process can raise the strength and reduce the leaching rate. 2nd CRM

Further works • Massive waste form process of soil immobilization • Suitable method to test the leaching of 239Pu • Leaching behavior and immobilizing mechanism 2nd CRM

Thanks for your attention! 2nd CRM

The Immobilization of Radioactively Contaminated Soil in Cementitious Materials