Evolution of microstructure and transport properties of cement pastes due to carbonation under a CO2 pressure gradient: a modeling approach

Most carbonation models only account for diffusion as the main transport mechanism rather than advection. Nevertheless, in the case of concrete used for underground waste disposal facilities, concrete may be subjected to a high hydrostatic pressure and the surrounding environment may contain a high dissolved CO2 concentration. Therefore, a combination of diffusion and advection should be taken into account. This is also the case in accelerated carbonation where a high CO2 pressure gradient is applied in which advection in the gas phase has a significant contribution to the carbonation process. This study aims at developing a model to predict the evolution of the microstructure and transport properties of cement pastes due to carbonation under accelerated conditions in which a pressure gradient of pure CO2 is applied. The proposed model is based on a macroscopic mass balance for carbon dioxide in gaseous and aqueous phases. Besides the prediction of the changes in transport properties (diffusivity, permeability), the model also enables to predict the changes in microstructure. Data from accelerated tests were used to validate the model. Preliminary verification with experimental results shows a good agreement

Decalcification of cement paste in NH4NO3 solution: microstructural alterations and its influence on the transport properties

Leaching of cement-based materials changes its properties such as a reduction in pH, an increase in porosity and transport properties and a detrimental effect on properties related to long-term durability. Therefore, a better understanding of leaching process is important including the relevant long-term effects for concretes used in waste disposal systems. However, the decalcification process is not easy to capture because it is extremely slow. In this study, an ammonium nitrate (NH4NO3) solution of 6 mol/l was used to accelerate the leaching kinetics. The experiments were performed on cement paste samples with different water/powder and limestone filler replacement ratios. The change of sample mass over time was monitored, and the amount of calcium ion leached out during the test was determined. Different post-analysis techniques like SEM, MIP and N2-adsorption were used to characterize the microstructural changes, while the degraded front was determined by phenolphthalein spraying. The effect of accelerated leaching on transport properties was studied by measuring the change in water permeability. Results show that (i) NH4NO3 solution is an aggressive but suitable agent to be used to accelerate the Ca leaching in cementitious materials while still keeping the “nature” of leaching; (ii) the square-root-time law of degradation is applicable under accelerated conditions; (iii) the porosity of the leached samples increases significantly and the critical pore size is shifted to larger radius; and (iv) the BET specific surface area of the leached sample is also significantly increased. These changes result in a significant increase in water permeability

Effects of W/P ratio and limestone filler on permeability of cement pastes

Because of environmental and economic benefits, a fraction of cement is increasingly replaced by limestone fillers raising a question on to what extent limestone fillers affect the durability of cementitious materials. This work aims at understanding the effects of water/powder (w/p) ratio and limestone filler replacement on water permeability of cement pastes. A newly proposed technique using a controlled constant flow concept was applied to measure permeability of hardened cement paste samples following a factorial experimental design. It was observed that both limestone filler and w/p ratio significantly influence the water permeability. At a given w/p ratio, adding limestone filler made the microstructure coarser, especially for high w/p ratio. Nevertheless, if the comparison is based on a given water/cement (w/c) ratio instead of w/p ratio, the limestone filler replacement refined the microstructure in terms of capillary porosity and pore size distribution, resulting in permeability decreases of cement pastes. Furthermore, a modified Carmen-Kozeny relation was established which enables prediction of the permeability from capillary porosity and the critical pore diameter

Microstructural and permeability changes due to accelerated Ca leaching in ammonium nitrate solution

Although Ca leaching in cement-based materials is an extremely slow process, this process is relevant for long-term assessments of the evolution of concrete used in radioactive waste disposal systems. In the present work, an ammonium nitrate solution of 6 mol/l was used to accelerate the leaching kinetics of cement paste. The change of sample mass over time was monitored by weighing, whereas the amount of calci-um ion leached out during the test was followed by ion chromatography. A variety of post-analysis tech-niques like XRD, MIP and BET were used to characterize the microstructural changes and portlandite content, while the degraded front was determined by phenolphthalein spraying. The effect of accelerated leaching on transport properties was studied by measuring changes in water permeability. Results showed that (i) the porosity of the leached samples increased significantly, (ii) the critical pore size was shifted to larger radius and (iii) the BET specific surface area of the leached sample was also significantly increased. These changes resulted in a one to two order increase in water permeability depending on the immersed time

Influence of Micro-Pore Connectivity and Micro-Fractures on Calcium Leaching of Cement Pastes — A Coupled Simulation Approach

A coupled numerical approach is used to evaluate the influence of pore connectivity and microcracks on leaching kinetics in fully saturated cement paste. The unique advantage of the numerical model is the ability to construct and evaluate a material with controlled properties, which is very difficult under experimental conditions. Our analysis is based on two virtual microstructures, which are different in terms of pore connectivity but the same in terms of porosity and the amount of solid phases. Numerical fracturing was performed on these microstructures. The non-fractured and fractured microstructures were both subjected to chemical leaching. Results show that despite very different material physical properties, for example, pore connectivity and effective diffusivity, the leaching kinetics remain the same as long as the amount of soluble phases, i.e., buffering capacity, is the same. The leaching kinetics also remains the same in the presence of microcracks

Report of RILEM TC 281-CCC: outcomes of a round robin on the resistance to accelerated carbonation of Portland, Portland-fly ash and blast-furnace blended cements

Many (inter)national standards exist to evaluate the resistance of mortar and concrete to carbonation. When a carbonation coefficient is used for performance comparison of mixtures or service life prediction, the applied boundary conditions during curing, preconditioning and carbonation play a crucial role, specifically when using latent hydraulic or pozzolanic supplementary cementitious materials (SCMs). An extensive interlaboratory test (ILT) with twenty two participating laboratories was set up in the framework of RILEM TC 281-CCC ‘Carbonation of Concrete with SCMs’. The carbonation depths and coefficients determined by following several (inter)national standards for three cement types (CEM I, CEM II/B-V, CEM III/B) both on mortar and concrete scale were statistically compared. The outcomes of this study showed that the carbonation rate based on the carbonation depths after 91 days exposure, compared to 56 days or less exposure duration, best approximates the slope of the linear regression and those 91 days carbonation depths can therefore be considered as a good estimate of the potential resistance to carbonation. All standards evaluated in this study ranked the three cement types in the same order of carbonation resistance. Unfortunately, large variations within and between laboratories complicate to draw clear conclusions regarding the effect of sample pre-conditioning and carbonation exposure conditions on the carbonation performance of the specimens tested. Nevertheless, it was identified that fresh and hardened state properties alone cannot be used to infer carbonation resistance of the mortars or concretes tested. It was also found that sealed curing results in larger carbonation depths compared to water curing. However, when water curing was reduced from 28 to 3 or 7 days, higher carbonation depths compared to sealed curing were observed. This increase is more pronounced for CEM I compared to CEM III mixes. The variation between laboratories is larger than the potential effect of raising the CO concentration from 1 to 4%. Finally, concrete, for which the aggregate-to-cement factor was increased by 1.79 in comparison with mortar, had a carbonation coefficient 1.18 times the one of mortar

Effects of carbonation and calcium leaching on microstructure and transport properties of cement pastes

During the past decades, a significant progress has been made in the development of concrete materials. With the development of high performance and self-compacting concrete, the design of concrete structures for hundred-year service life becomes possible. In recent years, concrete has been considered as a useful material for facilities with extremely long-term service life such as radioactive waste repositories. Therefore, the assessment of the long-term performance of such concrete structures is of utmost importance. Within its service environment, these structures undergo chemical degradation processes which are very slow but they significantly change the physical integrity (e.g. transport and mechanical properties) and the chemical condition (e.g. pH) of the structures in the long-term. Chemical degradation is typically the result of alteration of the cement matrix mineralogy caused by the interaction with environmental conditions. The interaction disturbs the equilibrium between the pore solution of the cementitious materials and the solid phases of the cement matrix which results in dissolution and/or precipitation of minerals. The chemical degradation of cementitious materials is mostly followed by alteration of the microstructure and, thereby, transport properties. The transport properties such as permeability and diffusivity are the key parameters to evaluate whether the concrete still retains its function as a barrier against the transport of radionuclides and other hazardous products out of the disposal system. Although a lot of effort has been spent on studies concerning the use of cement-based materials in such structures, the evolution of the microstructure and transport properties under chemical degradation over long time periods (up to thousands of years) is still unclear due to the limited experimental timeframe available to capture these processes. This thesis presents a comprehensive study of the consequences of exposure of cementitious materials to carbonation and calcium leaching. Due to the extremely slow nature of these degradation processes, accelerated methods are needed to reach a certain degradation stage in order to study the behaviour of concrete representative for the long-term. In the present work, an ammonium nitrate solution was used to accelerate the Ca-leaching degradation kinetics, while pure CO2 at high pressure was applied to speed up the carbonation. The changes in permeability and diffusivity of the degraded materials were measured by novel methods. Microstructural and mineralogical alterations were qualitatively and quantitatively examined by a variety of complementary techniques including SEM/SEM-EDX, MIP, TGA, N2-adsorption, XRD/QXRD and ion chromatography. The experiments were performed on cement pastes with different water/powder ratios and limestone filler replacement. In parallel, phenomenological models were also developed to better understand the transient state of degradation and to predict the evolution of the cementitious materials during the degrading processes, which is difficult to capture with experiments. By using the accelerated techniques, it was able to obtain degraded mateials which are representative for long-term degradation states and as such it allowed for studying the microstructure and hence its relation to the transport properties. The novel methods to measure transport properties proposed in this study are promising in terms of the required experimental time, the control of parameters (pressure, flow, concentration) and reliability. Moe importantly, the techniques allowed for a convenient capturing of the changes in transport properties of the degraded materials, thanks to their high compatibility with the accelerated degradation techiques. Results showed that leaching and carbonation significantly changed the microstructure and transport properties of cementitious materials but in different manners. The leaching significantly altered the microstructure of the cement paste to a material with a higher specific surface area, increased total porosity and a shift to larger pore sizes resulting in a significant increase in transport properties depending on the degradation state. In contrast, carbonation led to a porosity reduction, shift of pore size distribution to smaller ranges and lower permeability and diffusivity. However, both leaching and carbonation processes induced a lower pH with possible loss of beneficial condition for waste package integrity (e.g. rebar corrosion). The phenomenological models developed enabled us to simulate and predict the evolution of the microstructure and related transport properties (permeability, diffusivity). In combination with accelerated experiments, it provides us the possibility to assess the long-term performance of cement-based materials used in disposal systems

Insights and issues on the correlation between diffusion and microstructure of saturated cement pastes

The objective of this study is to quantify the contributions of microstructure and molecular size of diffusing species to tortuosity, constrictivity and effective diffusivity. The microstructural effect is simulated with different sound, leached or carbonated cement pastes with varying water to cement ratios and limestone replacement filler replacements. Leached and carbonated samples were obtained by accelerated experiments: leaching by immersing samples in ammonium nitrate solution and carbonation by subjecting the samples to pure CO2 at elevated pressure. To characterise the microstructural properties, Mercury Intrusion Porosimetry (MIP), and N-2-adsorption were used. The effect of molecular size is quantified with a recent developed diffusion setup allowing for simultaneous measurement of the diffusion of species with different molecular sizes. Previously developed models were also used to verify and give insights into the evolution of diffusion of degraded materials. In addition, a larger dataset from literature is used to evaluate a model which accounts for molecular size as well to predict diffusivity. Resutls show that because of the significant contribution of the molecular size of the diffusing species to the diffusion process, the constrictivity and thereby geometric factor may not be considered as intrinsic properties of the cement pastes. The geometric factor and/or constrictivity of cement pastes depends on the interrelationship of the molecular size of the diffusing species with the microstructure of the cementitious materials. A smaller diffusing species or/and a higher porosity of the sample results in a lower value of geometric factor. Interestingly, constrictivity is significantly influenced by the molecular size, but not the porosity

Diffusive Transport of Dissolved Gases in Potential Concretes for Nuclear Waste Disposal

In many countries, the preferred option for the long-term management of high- and intermediate level radioactive waste and spent fuel is final disposal in a geological repository. In this geological repository, the generation of gas will be unavoidable. In order to make a correct balance between gas generation and dissipation by diffusion, knowledge of the diffusion coefficients of gases in the host rock and the engineered barriers is essential. Currently, diffusion coefficients for the Boom Clay, a potential Belgian host rock, are available, but the diffusion coefficients for gases in the engineered concrete barriers are still lacking. Therefore, diffusion experiments with dissolved gases were performed on two concrete-based barrier materials considered in the current Belgian disposal concept, by using the double through-diffusion technique for dissolved gases, which was developed in 2008 by SCK CEN. Diffusion measurements were performed with four gases including helium, neon, methane and ethane. Information on the microstructure of the materials (e.g., pore size distribution) was obtained by combining N2-adsorption, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and water sorptivity measurements. A comparison was made with data obtained from cement-based samples (intact and degraded), and the validity of existing predictive models was investigated