Effect of calcium sulfates on the early hydration of calcium sulfoaluminate cement and the stability of embedded aluminium

Conventional Portland cement-based systems have been considered un-suitable for immobilising nuclear wastes containing reactive metals, such as alu-minium, due to the high pH of the pore solution (usually around 12.5) and free moisture. On the contrary, calcium sulfoaluminate cement (CSA) produces a low-er pH (10.5-12) environment and has an excellent water binding capability as a re-sult of the formation of its main hydration product, ettringite. Therefore, it offers a good potential to immobilise aluminium. However, the pore solution pH and ettringite formation depend largely on the raw materials used to formulate the CSA, which is usually a blend of 75%-85% of CSA clinker and 15-25% of calci-um sulfate (in the form of gypsum or anhydrite). In this paper, it was found that, compared to anhydrite, gypsum (15%wt of the blend) demonstrated the highest reduction in the corrosion of embedded Al, possibly due to its lower initial pH (around 10.5) and self-desiccating nature at the early stage of hydration. Whilst the CSA/anhydrite had a higher Al corrosion rate, the initial set was more ac-ceptable than CSA/gypsum. Nonetheless, overall, it was concluded that CSA with gypsum (15%wt) should be considered as a base formulation for the encapsulation of Al waste. The unfavorable rapid set and high heat generation, however, demon-strated that modifications are required, potentially by using mineral additions

Development of calcium sulfoaluminate cement composites for nuclear waste encapsulation

In the UK, nuclear wastes are usually ‘cemented’ before disposal so that harmful radionuclides can be physically and chemically contained. In this process, conventional Portland cement is blended with high levels of relatively inert mineral additions, mostly to reduce the high heat evolution in large pours. Calcium sulfoaluminate cement (CSAC) has recently attracted interest in various applications due to its lower pH and ability to bind significant quantities of water compared with conventional Portland cement. Such qualities are particularly suited to the encapsulation of legacy wastes such as aluminium and uranium, which would otherwise corrode if embedded within a Portland cement environment. While some early trials have demonstrated 111 good potential of CSAC, the rapid reaction rate (and associated heat generation) is still restricting its use. In this paper, common mineral additions such as ground granulated blastfurnace slag (GGBS), pulverised fuel ash (PFA) and limestone powder (LSP) were incorporated at very high replacement levels (up to 75%) in an attempt to dilute the cement matrix and hence reduce the heat of hydration. Studies of compressive strength, heat of hydration and aluminium corrosion revealed that these CSAC composites demonstrate excellent potential for aluminium waste encapsulation. Keywords: Calcium sulfoaluminate cement, composites, nuclear waste encapsulation, corrosion, aluminiu

Application of fibre Bragg gratings for the optimization of microwave-cured concrete

In this paper, the suitability of using ‘intelligent’ fibre Bragg gratings (FBGs) as sensors for the temperature feedback control of concrete cured in a microwave environment has been presented and experimentally demonstrated. In this novel approach, the temperature data provided by the embedded FBGs are processed on the fly (using a feedback control algorithm) in order to regulate the microwave power so that an internal curing temperature of 70°C is maintained. The immunity of the FBGs to microwave radiation ensures that the embedded sensors remain stable, unlike conventional metallic/electrical probes

Potential of Microwave Curing for Precast Concrete Manufacture

In this paper, an ongoing research sponsored by Innovate UK on developing an intelligent microwave system for manufacturing concrete products is introduced. The key features of this microwave system include using optical fibre Bragg grating (FBG) sensors to control the temperature and the humidity inside concrete and microwave oven cavity, respectively. Using this tailor-made microwave system, research is ongoing in the Advanced and Innovative Materials (AIM) Group at University College London to explore the potential of curing two types of low-carbon cementitious systems, namely high-volume fly ash (HVFA) and alkali-activated fly ash (AAFA). In both systems, thermal curing is essential for the strength development, in particular, early strength development. Their results indicate that, by using the microwave curing technique, the low-carbon performance can be achieved from both systems whilst the early strength was not affected. The current results also demonstrate a good potential of microwave curing for manufacturing low-carbon precast concrete elements in the future

Fibre Bragg Grating-based Cascaded Acoustic Sensors for Potential Marine Structural Condition Monitoring

This paper explores the potential of using multiple Fibre Bragg grating (FBG)-based sensors for acoustic emission (AE) detection, thus offering an effective alternative to conventional piezoelectric (PZT) sensors, especially where they have shown limitations in use, such as in the marine sector. A cascaded fibre optic acoustic sensor system, using optical filter signal demodulation has been developed and its performance extensively evaluated. To undertake this under standardized conditions, the optical sensor system was evaluated using a glass plate to detect the acoustic signal, followed by an evaluation using a metal plate to identify the location of acoustic sources, when subjected to sonotrode excitation, mimicking acoustic detection in cavitation detection. Under these circumstances, a very good agreement has been reached between the outputs of the optical acoustic sensors and of the co-located PZT acoustic sensors. This work confirms the utility of these sensors – they can detect not only weak AE signals, but also enable multipoint simultaneous measurement, showing their potential for condition monitoring applications, especially in the marine sector

Optimization of the accelerated curing process of concrete using a fibre Bragg grating-based control system and microwave technology

In this paper, an investigation into the suitability of using fibre Bragg gratings (FBGs) for monitoring the accelerated curing process of concrete in a microwave heating environment is presented. In this approach, the temperature data provided by the FBGs are used to regulate automatically the microwave power so that a pre-defined temperature profile is maintained to optimize the curing process, achieving early strength values comparable to those of conventional heat-curing techniques but with significantly reduced energy consumption. The immunity of the FBGs to interference from the microwave radiation used ensures stable readings in the targeted environment, unlike conventional electronic sensor probes

Immobilisation of caesium in magnesium phosphate-based blends

Disposal of Caesium (Cs) by incorporating it into host matrices has been proved as a promising concept. Even though Portland cement (PC) based systems can be used to encapsulate low (LLW) or intermediate level waste (ILW), they are not efficient for immobilising Cs due to the very high water-solubility of Cs. Magnesium phosphate cement (MPC), a chemically bonded ceramic consists of struvite families produced by the acid based reactions between dead-burnt magnesium oxide (MgO) and phosphates (e.g., KH2PO4), can be a potential candidate for hosting Cs, since it has been reported that Cs can be incorporated in the Kstruvite structures by substituting potassium (K) to form (K,Cs)-struvite. However, the acid-based reaction to form MPC under ambient temperature between magnesium oxide (MgO) and phosphates (e.g., KH2PO4) is violent and exothermic, which raises concerns about industrial application of MPC in real world. In this work, ground-granulated blast-furnace slag (GGBS) and pulverised fuel ash were used to replace MPC in order to reduce the heat released. The feasibility of MPC, as well as MPC-based blends, i.e. GGBS-MPC or PFAMPC matrices, for immobilising Cs was assessed by their leaching behaviour at the ages of 3d, 7d and 28d. In addition, two typical PC based systems currently used in nuclear waste immobilisation, i.e., pure PC and PC-GGBS (1:9) mixes, were also produced and tested as controls. The results indicated that all the MPC mixes, including pure MPC, GGBS-MPC and PFA-MPC, demonstrated superb capability for immobilising Cs, with the immobilisation rates achieved more than 99.5% at all curing age investigated. Compared to the traditional PC and PC/GGBS mixes, the MPC mixes nearly doubled the Cs immobilisation rate, which is rather encouraging

A statistical investigation of the rheological properties of magnesium phosphate cement

Magnesium phosphate cement (MPC) is a promising material applied for rapid patch repairing in civil engineering and waste immobilisation in nuclear industry. However, the rheological properties of this new binder material which highly affects its engineering application, is to be explored. The current work aims at investigating the rheological properties of MPC along 98 with determining the optimum conditions to obtain MPC materials with desirable rheological performances. The Response Surface Methodology (RSM) accompanied by Central Composite Design (CCD) were adopted to establish mathematical model describing the rheological characteristics of MPC in terms of yield stress (Pa) and plastic viscosity (Pa.s), as a function of three independent variables namely W/S ratio, M/P ratio and Borax dosage. The analysis of variance (ANOVA) was also conducted to assess the significance and adequacy of the regression models attained. The results showed that the M/P ratio and Borax dosage could affect significantly the yield stress of MPC, while W/S ratio was the significant coefficient influencing the plastic viscosity. The numerical optimised values of the W/S ratio, M/P ratio and Borax dosage were 0.25, 8.97 and 0.17 respectively, and a MPC paste with desirable rheological characteristics (yield stress of 0.40 Pa and plastic viscosity of 0.93 Pa.s) can be obtained. Further experiments will be carried out to verify the predicted optimum conditions and study the interactions between the factors in relation to the responses

Potential of Microwave Curing for Precast Concrete Manufacture

This paper introduces research to develop an intelligent microwave system for manufacturing concrete products. The key features of this system include the use of optical fibre Bragg grating sensors to control the temperature and humidity inside the concrete and microwave oven cavity respectively. Using this tailor-made microwave system, research is ongoing in the Advanced and Innovative Materials Group at University College London to explore the potential for curing two types of low-carbon cementitious systems: high-volume fly ash and alkali-activated fly ash. In both systems, thermal curing is essential for strength development, particularly early strength development. Results indicate that, by using the microwave curing technique, low-carbon performance can be achieved from both systems, whilst early strength was not affected. The results demonstrate the potential of microwave curing for manufacturing low-carbon precast concrete elements in future

Environmental surveillance results for 1995 for the Hazelwood Interim Storage Site

This memorandum presents and interprets analytical results and measurements obtained as part of the 1995 environmental surveillance program for the Hazelwood Interim Storage Site (HISS) under the Formerly Utilized Sites Remedial Action Program (FUSRAP). The discussion provides a comparative analysis of average historical background conditions and applicable regulatory criteria to the 1995 results reported for external gamma radiation and for samples from the media investigated (air, surface water, sediment, groundwater, and stormwater). Results from the 1995 environmental surveillance program at HISS indicate that, with the exception of thorium-230 in streambed sediment, applicable US Department of Energy (DOE) guidelines were not exceeded for any measured parameter or for any dose calculated for potentially exposed members of the general public. In the absence of sediment guidelines, DOE soil guidelines serve as a standard of comparison for data obtained from stream bed sediment; two samples from downstream locations contained concentrations of thorium-230 that exceeded DOE soil guidelines. All stormwater sample results were in compliance with permit-specified limits. Other radioactive materials include radium 226 and natural uranium