N.B. Milestone

Engineering Properties of Alkali-Activated Natural Pozzolan Concrete

The development of alkali-activated binders with superior engineering properties and longer durability has emerged as an alternative to ordinary portland cement (OPC). It is possible to use alkali-activated natural pozzolans to prepare environmentally friendly geopolymer cement leading to the concept of sustainable development. This paper presents a summary of an experimental work that was conducted to determine mechanical strength, modulus of elasticity, ultrasonic pulse velocity, and shrinkage of different concrete mixtures prepared with alkali-activated Iranian natural pozzolans—namely Taftan andesite and Shahindej dacite, both with and without calcining. Test data were used for Taftan pozzolan to identify the effects of water-binder ratios (w/b) and curing conditions on the properties of the geopolymer concrete, whereas the influence of material composition was studied by activating Shahindej pozzolan both in the natural and calcined states. The results show that alkali-activated natural pozzolan (AANP) concretes develop moderate-to-high mechanical strength with a high modulus of elasticity and a shrinkage much lower than with OPC.

Cementitious Systems for Encapsualation of Intermediate Level Waste

Encapsulation in cement is the favoured method in the UK for disposal of intermediate and low level radioactive wastes. It is usual to use composite cement systems incorporating blast furnace slag (BFS) or pulverised fuel ash (PFA) as these offer several advantages over Portland cement, notably a lower heat of hydration. The use of these mineral additions utilises a waste product which would itself need a disposal route and, because of the decreased amount of Portland cement used, provides a reduction in cost and energy consumption. Cementitious systems have many attributes which make them suitable for encapsulation and immobilisation, including:  Inexpensive and readily available  Assist immobilisation of radionuclides by: a) acting as a diffusion barrier b) providing sorption and reaction sites c) maintaining a high pH which in turn decreases radionuclide solubility  Provide radiation shielding which is not degraded by the radiation  Controllable permeation and diffusion characteristics over a wide range via selection of constituents and components. Where physical adsorption is a significant factor for immobilisation, the calcium silicate hydrate gel (C-S-H) formed on hydration of a Portland cement is advantageous as it has a high surface area and large micropore volume. Composite cements based on blast furnace slag will produce a higher proportion of C-S-H than ordinary Portland cement increasing the sorption capacity, and reducing the capillary porosity so that the diffusion resistance is increased. Intermediate level waste covers a wide range of materials, for example, metals and ion exchangers, each with differing chemical properties. It is, therefore, necessary to access the ability of the cementitious system to immobilise different wastes and to characterise the products formed. It is also necessary that alternative encapsulant materials be considered for immobilising wastes not suited to the composite cements already being used. The techniques employed to do this include x-ray diffraction (XRD), to identify standard and non-standard hydration products, isothermal conduction calorimetry (ICC) and scanning electron microscopy (SEM).

Corrosion of aluminium and magnesium in BFS composite cements

Abstract Legacy radioactive wastes arising from reprocessing of nuclear fuels in the UK are classified as intermediate level waste (ILW), which contain things such as aluminium and magnesium. Blast furnace slag (BFS) composite cements are used to encapsulate ILW. These cements have a high pH which is advantageous to limit the mobility of some of the radioactive species but can cause corrosion of metals. The present paper describes some fundamental aspects of corrosion of aluminium and magnesium in BFS composite cements. The corrosion of aluminium produced an interface between aluminium and cement which was porous with a series of zones containing bayerite (Al(OH)3) and strätlingite (2CaO.Al2O3.SiO2.8H2O). With magnesium, the main corrosion product was found to be brucite (Mg(OH)2) and the porous zone was less pronounced. The hydration of the bulk cement did not appear to be affected by the corrosion of these metals.

Oxygen and Chloride Permeability of Alkali-Activated Natural Pozzolan Concrete

This paper describes how one of the most important factors in the use of portland cement concrete is its durability. Most of the situations where durability is lacking have been identified and strategies to manage durability have been implemented. Geopolymer concrete, made from an alkali-activated natural pozzolan (AANP), provides an important opportunity for the reduction of carbon dioxide (CO2) emissions associated with the manufacture of concrete. However, there is a limited history of durability studies. Until its different properties are well understood there is no desire to adopt this new technology of unknown provenance by the concrete industry. This paper presents an experimental study of oxygen and chloride permeability of AANP concrete prepared by activating Taftan andesite and Shahindej dacite (Iranian natural pozzolans), with and without calcining, and the correlations between these properties and compressive strength. The results presented in the paper show that compared to ordinary portland cement (OPC) concrete, AANP concrete has lower oxygen permeability at later ages; but it shows moderate to high chloride ion penetrability.

The Immobilisation of Clinoptilolite Within Cementitious Systems

The zeolitic ion exchanger clinoptilolite was encapsulated within various cementitious systems in order to assess their suitability for the retention of the radioelements, Cs and Sr. The pozzolanic reaction of clinoptilolite is reduced in composites containing BFS and PFA and appears not to continue after 7 days of hydration. Ca(OH) 2 persists up to 360 days of hydration in a 9:1BFS:OPC system with 10% clinoptilolite added, despite the presence of unreacted pozzolan. This may be due to low pH of the pore solution, if Na and K act as counter cations in the aluminous C-S-H, a product of pozzolanic hydration or are exchanged onto the clinoptilolite. Saturation of the pore solution with Ca may prevent further dissolution of Ca(OH) 2 . Cs leaching occurs in all samples during accelerated tests due to breakdown of the clinoptilolite structure. The alternative cement system calcium sulfoaluminate cement (CSA) has a different hydration chemistry and properties to OPC and OPC composites with a lower pore solution pH. Clinoptilolite appears to react in a hydrating CSA system with significant reaction continuing between 28 and 90 days of hydration. Leaching of Cs from CSA is higher than from an OPC system, in which almost all of the clinoptilolite crystallinity is lost. The major product of CSA hydration is ettringite. Cs may be adsorbed within cation sites of the C-S-H in an OPC system but not by ettringite which does not retain Cs so Cs has high mobility and leachability through the CSA matrix.

Simplified Model for Prediction of Compressive Strength of Alkali-Activated Natural Pozzolans

AbstractThe assessment of pozzolanic activity is essential for estimating the reaction of a material as pozzolan. Natural pozzolans can be activated and condensed with sodium silicate in an alkaline environment to synthesize high performance cementitious construction materials with low environmental impact. In this paper, the pozzolanic activities of five natural pozzolans are studied. The correlation between type and chemical composition of natural pozzolan, which affects the formation of the geopolymer gel phase, both for the calcined and untreated natural pozzolans, have been reviewed. The improvement in pozzolanic properties was studied following heat treatment including calcinations and/or elevated curing temperature by using alkali solubility, and compressive strength tests. A model was developed to allow prediction of the alkali-activated pozzolan strength versus their chemical compositions, alkali solubility, and crystallinity.

Sodium Sulphate Activated GGBS/PFA and Its Potential as a Nuclear Waste Immobilisation Matrix

The UK currently uses Portland cement composite blends to immobilise/encapsulate intermediate and low level radioactive wastes (ILW and LLW). However, among other things, the high pH of these systems causes the corrosion of some metals, which can lead to expansion and excess generation of hydrogen. Therefore, in order to immobilise nuclear waste where corrosion is an issue, a near neutral cementing system is desirable. Among the activators which can be used in the alkali-activated slag (AAS) systems, a solution of Na 2 SO 4 is near neutral but the ground granulated blast-furnace slag (GGBS) itself has a pH about 11, increasing the pH within the Na 2 SO 4 activated AAS. As low calcium Pulverized Fuel Ash (PFA) only has a pH about 9, using a GGBS/PFA blend activated by Na 2 SO 4 offers the potential to develop a near neutral cementing system for nuclear waste immobilisation purposes. In this paper, the replacement of GGBS in the Na 2 SO 4 activated AAS system with PFA at 0%, 10%, 20% and 30% by mass was examined. The pH and corrosion of Al were determined and used as primary criteria for judging the feasibility for further development of a Na 2 SO 4 activated GGBS/PFA matrix for immobilising nuclear wastes. The microstructure of the matrices was studied by SEM. Leaching studies were carried out to examine the possibility of immobilising Cs + within these Na 2 SO 4 activated GGBS/PFA matrices. The potential of using Na 2 SO 4 activated GGBS/PFA for immobilising nuclear wastes is discussed.

Immobilisation matrices for intermediate level nuclear wastes using sulphate activated BFS/ OPC and PFA/OPC composite cements

Abstract Abstract The highly alkaline pore solution of ordinary Portland cement composites currently used by the nuclear waste immobilisation industry causes corrosion of encapsulated reactive metals which generates hydrogen, causes loss of wasteform integrity and may release radioactive material to the environment. This work has investigated using near neutral salts as additives to composite cement powders in order to develop less alkaline cement pastes and reduce the corrosion of reactive metals. The effect of adding anhydrite and gypsum to ordinary Portland cement partially replaced by blast furnace slag or pulverised fuel ash was investigated. Adding the sulphate salts significantly changed the composition and microstructure of the bulk cement and the aluminium corrosion layer with ettringite the main phase detected. The amount of aluminium hydroxide formed during corrosion was significantly reduced by adding the sulphate salts as was the rate of hydrogen generation.

Lessons Learned from the Development of Cementitious Grouts for Deep Borehole Disposal Applications

AbstractThe performance of grouts made using oilwell cement is markedly different above 90°C than at lower temperatures, and the rapidity with which grouts thicken can cause failures in well cement...

The Knoo Research Consortium: Work package 3-An integrated approach to waste immobilisation and management

The Keeping the Nuclear Option Open (KNOO) research consortium is a four-year research council funded initiative addressing the challenges related to increasing the safety, reliability and sustainability of nuclear power in the UK. Through collaboration between key industrial and governmental stakeholders, and with international partners, KNOO was established to maintain and develop skills relevant to nuclear power generation. Funded by a research grant of £6.1M from the “Towards a Sustainable Energy Economy Programme” of the UK Research Councils, it represents the single largest university-based nuclear research programme in the UK for more than 30 years. The programme is led by Imperial College London, in collaboration with the universities of Manchester, Sheffield, Leeds, Bristol, Cardiff and the Open University. These universities are working with the UK nuclear industry, who contributed a further £0.4M in funding. The industry/government stakeholders include AWE, British Energy, the Department for Environment, Food and Rural Affairs, the Environment Agency, the Health and Safety Executive, Doosan Babcock, the Ministry of Defence, Nirex, AMEC NNC, Rolls-Royce PLC and the UK Atomic Energy Authority. Work Package 3 of this consortium, led by the University of Leeds, concerns “An Integrated Approach to Waste Immobilisation and Management”, and involves Imperial College London, and the Universities of Manchester and Sheffield. The aims of this work package are: to study the re-mobilisation, transport, solid-liquid separation and immobilisation of particulate wastes; to develop predictive models for particle behaviour based on atomic scale, thermodynamic and process scale simulations; to develop a fundamental understanding of selective adsorption of nuclides onto filter systems and their immobilisation; and to consider mechanisms of nuclide leaving and transport. The paper describes highlights from this work in the key areas of multi-scale modeling (using atomic scale, thermodynamic and process scale models), the engineering properties of waste (linking microscopic and macroscopic behaviour, and transport and rheology), and waste reactivity (considering waste hosts and wasteforms, generation IV wastes, and waste interactions).Copyright