A database of sources of information on mineral reaction kinetics

The rate and magnitude of geochemical reactions can be described by two main processes; thermodynamics which determines the end point of reaction (i.e. approach to equilibrium conditions), and kinetics which determines how rapidly the reaction proceeds. There have been many studies that have investigated equilibrium conditions and have generated a wealth of data. However, for many systems the rate at which the end point of the reaction is reached is of equal, and possibly greater importance (e.g. the behaviour of waste products stored within the geosphere or during weathering). Predictive geochemical computer modelling is becoming increasingly important for investigating many different scenarios relatively rapidly. Although such models are capable of modelling rate-controlled dissolution and precipitation, there is no standard database of kinetic functions. As a consequence, it is sometimes difficult to locate sources of information to aid modelling. The overall aim of this report, is to provide information on literature sources of mainly mineral reaction rate data. It is hoped that this will facilitate predictive modelling exercises, or laboratory experimental studies addressing gaps in data coverage. Information on the sources of literature information has been produced in the form of an EndNote electronic database (see Appendix I). This study does not however, go as far as extracting and tabulating all the individual data points within these references

Results of laboratory carbonation experiments on Nirex Reference Vault Backfill cement

Some repository concepts envisage the use of large quantities of cementitious materials – both for repository construction and as a buffer/backfill. However, some wastes placed within a subsurface repository will contain a significant amount of organic material that may degrade to produce carbon dioxide. This will react with cement buffer/backfill to produce carbonate minerals such as calcite, which will reduce the ability of the buffer/backfill to maintain highly alkaline conditions and as a consequence its ability to limit radionuclide migration. The reaction may also alter the physical properties of the buffer/backfill. The work involved in this study investigates these processes through elevated pressure laboratory experiments conducted at a range of likely future in situ repository conditions. These will provide information on the reactions that occur, with results serving as examples with which to test predictive modelling codes. This report details a series of batch experiments to study carbonation of Nirex Reference Vault Backfil (NRVB) cement. Thirty-two static batch experiments were pressurised with either CO2, or with N2 for ‘nonreacting’ comparison tests at 20°C or 40°C, and 40 or 80 bar. Twenty-six of these were left to react for durations of between 10-40 days, with six more left to react for a year. The aim of them was to help investigate mineralogical and fluid chemical changes due to the diffusional ingress of CO2 into unconfined NRVB samples measuring 2.5 cm in diameter and 5 cm long. All the cement samples showed rapid reaction with CO2, manifested by a colour change from grey to light brown. Petrographic analysis of the reacted cement revealed that this colour change reflected the breakdown and dissolution of primary calcium ferrite and calcium alumina-ferrite (CAF) cement clinker phases (e.g. brownmillerite, Ca2(Al,Fe)2O5 to form calcium carbonates and finely-disseminated free ferric oxide (probably hematite, Fe2O3), as a result of reaction with CO2 to give a ‘rusty’ colour. It should be noted that his is not an oxidation reaction as the iron is present as Fe3+ in the original cement phases. The cement blocks remained intact, even after prolonged exposure to CO2-rich fluids. Carbonation was associated with an increase in weight of up to 8.5% during CO2 uptake, though the samples did not change in overall size. There is potential therefore, for carbonation to immobilise 14CO2 if that were present. Free-phase CO2 gave slightly more reaction than dissolved CO2, possibly because of its higher concentration and greater ability to penetrate the samples. In terms of major reactions during carbonation, these were the breakdown of portlandite, calcium silicate hydrate (CSH) phases, calcium aluminate (or calcium aluminate hydrate) phases, and ettringite-like phases, and the formation of carbonate phases and silica gel. Carbonation also revealed that heterogeneity within the cement samples had a major impact on migration pathways and extent of carbonation. This heterogeneity may have been a result of casting, and was only observed in some of the samples studied. It led to faster carbonation in some areas, and may account for some of the differences observed in the reacted cement samples. Such heterogeneity may be present within a repository, and should be taken into account when assessing repository performance

Meeting report : harnessing volcanic and geothermal resources for sustainable development in the East African Rift

This report is the published product of the British Geological Survey (BGS) and summarises key observations from a workshop held at Niavasha, Kenya, 17-20 September 2018. Its primary purpose was as a ‘getting to know you’ workshop focused on sharing knowledge about volcanoes and geothermal in the East African Rift. This was made possible because of a £25k Global Challenges Research Fund grant awarded Prof David Pyle of Oxford University, who linked with Prof Nicholas Marita of Dedan Kimathi University of Technology, Nairobi. This allowed 6 UK participants to travel to Kenya for the meeting, and approximately three times that number to attend from a range of countries along the East African Rift

A long-term experimental study of the reactivity of basement rock with highly alkaline cement waters: reactions over the first 15 months

A series of long-term laboratory experiments was started in 1995 to investigate longer-term dissolution/precipitation reactions that may occur in the alkaline disturbed zone surrounding a cementitious repository for radioactive waste. They consist of samples of UK basement rock reacting with either Na-K-Ca-OH water (‘young’ cement porewater) or Ca-OH water (‘evolved’ cement porewater) at 70°C. This paper summarizes results of reactions occurring over the first 15 months. Experiments of both fluid types showed many similar features, though primary mineral dissolution and secondary mineral precipitation were more extensive in the experiments involving Na-K-Ca (younger) cement porefluids compared to more evolved (Ca-rich) cement porefluids. Dissolution of dolomite, and to a lesser extent silicates (probably K-feldspar, but also possibly mica) occurred relatively rapidly at 70°C. Dolomite dissolution may have been a key factor in reducing pH values, and may be a key mineral in controlling the extent of alkaline disturbed zones. Dissolution was followed by precipitation of brucite close to dolomite grains, at least two generations of C-S-H phases (which may have contained variable amounts of K, Al and Mg); overgrowths of calcite; small crystals of hydroxyapophyllite; and elongate crystals of celestite. Though hydroxyapophyllite was observed (a phase commonly associated with zeolites), there was no evidence for the formation of zeolites in the experiments. Fluid chemical changes track the mineralogical changes, with C-S-H phases being a major control on fluid chemistry. In the ‘young’ porewater experiments there were decreases in pH, and K, Ca and Mg concentrations, together with transitory increases in SiO2 concentrations. In the ‘evolved’ porewater experiments there were decreases in pH, Mg, Ca and Sr concentrations, together with small increases in K and SiO2 concentrations. A number of experiments are still running, and will be sampled in coming years

An experimental and analogue study of iron release from red sandstones

The Jurassic Entrada sandstone at Salt Wash Graben, Utah, USA, a red sandstone contains significant rock bleaching. The cause of the bleaching has been thought to be associated with the modern day CO2-rich fluids in the area which present on the surface by utalising the local fractures, some of which are filled with calcite and iron rich minerals (e.g. Jarosite). An experimental study was conducted to determine the cause of the bleaching. CO2 was found not to cause sandstone bleaching. However, the CO2 was found to mobilize significant amounts of iron from the fracture minerals suggesting that this is a possible source of the iron in the modern pore fluids