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There is a growing demand for sustainable solutions in civil engineering concerning the carbon footprint of cementitious composites. Alkali-Activated Binders (AAB) are materials with great potential to replace ordinary Portland cement (OPC), with similar strength levels and lower environmental impact. Despite their improved environmental performance, their durability remains a gap in the literature, influenced by aspects of mechanical behavior, physical properties, and microstructure. This paper aims to assess the impact of steel slag aggregates and curing temperature of a proposed AAB based concrete formulation by characterizing fresh state, mechanical behavior, and microstructure. The proposed AAB is composed of fly ash (FA) and basic oxygen furnace (BOF) steel slag (SS) as precursors, sodium silicate and sodium hydroxide solution as activators, in total replacement of OPC, using baosteel slag short flow (BSSF) SS as aggregate in comparison with natural aggregate. The concrete formulation was designed to achieve a high-performance concrete (HPC) and a self-compacting concrete (SCC) behavior. Mechanical characterization encompassed hardened (compressive strength and Young’s modulus), fresh state (J-ring, slump flow, and T50), and durability tests (scanning electronic microscopy, water penetration under pressure, and chloride ion penetration). The compressive strength (64.1 ± 3.6 MPa) achieves the requirements of HPC, while the fresh state results fulfill the SCC requirements as well, with a spread diameter from 550 mm to 650 mm (SF-1 class). However, the flow time ranges from 3.5 s to 13.8 s. There was evidence of high chloride penetrability, affected by the lower electrical resistance inherent to the material. Otherwise, there was a low water penetration under pressure (3.5 cm), which indicates a well-consolidated microstructure with low connected porosity. Therefore, the durability assessment demonstrated a divergence in the results. These results indicate that the current durability tests of cementitious materials are not feasible for AAB, requiring adapted procedures for AAB composite characterization.

The need for a vast quantity of new buildings to address the increase in population and living standards is opposed to the need for tackling global warming and the decline in biodiversity. To overcome this twofold challenge, there is a need to move towards a more circular economy by widely using a combination of alternative low-carbon construction materials, alternative technologies and practices. Soils or earth were widely used by builders before World War II, as a primary resource to manufacture materials and structures of vernacular architecture. Centuries of empirical practices have led to a variety of techniques to implement earth, known as rammed earth, cob and adobe masonry among others. Earth refers to local soil with a variable composition but at least containing a small percentage of clay that would simply solidify by drying without any baking. This paper discusses why and how earth naturally embeds high-tech properties for sustainable construction. Then the potential of earth to contribute to addressing the global challenge of modern architecture and the need to re-think building practices is also explored. The current obstacles against the development of earthen architecture are examined through a survey of current earth building practitioners in Western Europe. A literature review revealed that, surprisingly, only technical barriers are being addressed by the scientific community; two-thirds of the actual barriers identified by the interviewees are not within the technical field and are almost entirely neglected in the scientific literature, which may explain why earthen architecture is still a niche market despite embodying all the attributes of the best construction material to tackle the current climate and economic crisis. This article is part of the theme issue 'The role of soils in delivering Nature's Contributions to People'.

Due to the environmental impact of building materials, researches on sustainable materials, such as bio-based and earth materials, are now widespread. These materials offer numerous qualities such as their availability, recyclability and their ability to dampen the indoor relative humidity variations due to their hygroscopicity. As these materials can absorb large amount of humidity, numerical and experimental studies of their hygrothermal behaviour are crucial to assess their durability. To validate a hygrothermal model, numerical and experimental data have to be confronted. Such confrontation must take into consideration the uncertainties related to the experimental protocol, but also to the model. Statistical tools such as uncertainty and global sensitivity analysis are essential for this task. The uncertainty analysis estimates the robustness of the model, while the global sensitivity analysis identifies the most influential input(s) responsible for this robustness. However, these methods are not commonly used because of the complexity of hygrothermal models, and therefore the prohibitive simulation cost. This study presents a methodology for comparing the numerical and experimental data of a rammed earth wall subjected to varying temperature and relative humidity conditions. The main objectives are the investigation of the uncertainties impact, the estimation of the model robustness, and finally the identification of the input(s) responsible for the discrepancies between numerical and experimental data. To do so, a recent and low-cost global variance-based sensitivity method, named RBD-FAST, is applied. First, the uncertainty propagation through the model is calculated, then the sensitivity indices are estimated. They represent the part of the output variability related to each input variability. The output of interest is the vapour pressure in the middle of the wall to confront it to the experimental measurement. Good agreement is obtained between the experimental and numerical results. It is also highlighted that the sorption isotherm is the main factor influencing the vapour pressure in the material.

Unfired earth is a sustainable construction material with low embodied energy, but its development requires a better evaluation of its moisture–thermal buffering abilities and its mechanical behavior. Both of them are known to strongly depend on the amount of water contained in its porous network and its evolution with external conditions (temperature, humidity), which can be assessed through several sorption–desorption curves at different temperature. However, the direct measurement of these curves is particularly time consuming (up to 2 month per curve) and thus, indirect means of their determination appear of main importance for evident time saving and economical reasons. In this context, this paper focuses on the prediction of the evolution of sorption curves with temperature on earth plasters and compacted earth samples. For that purpose, two methods are proposed. The first one is an adaptation of the isosteric method, which gives the variation of relative humidity with temperature at constant water content. The second one, based on the liquid–gas interface equilibrium, gives the variation of water content with temperature at constant relative humidity. These two methods lead to quite consistent and complementary results. It underlines their capability to predict the sorption curves of the tested materials at several temperatures from the sole knowledge of one sorption curve at a given temperature. Finally, these predictions are used to scan the range of temperature variation within which the evolution of water content with temperature at constant humidity could be neglected or should be taken into account.

Due to their various applications in geo-environmental engineering, such as in landfill and nuclear waste disposals, the coupled chemo-hydro-mechanical analysis of expansive soils has gained more and more attention recently. These expansive soils are usually unsaturated under field conditions; therefore the capillary effects need to be taken into account appropriately. For this purpose, based on a rigorous thermodynamic framework (Lei et al., 2014), the authors have extended the chemo-mechanical model of Loret el al. (2002) for saturated homoionic expansive soils to the unsaturated case (Lei, 2015). In this paper, this chemo-mechanical unsaturated model is adopted to simulate the chemo-elastic-plastic consolidation process of an unsaturated expansive soil layer. Logical tendencies of changes in the chemical, mechanical and hydraulic field quantities are obtained.

Earthen building materials, with their hygroscopic properties conferred by clay minerals, interact with the indoor environment, regulating factors such as relative humidity. While their moisture buffering capacity is well documented, there is limited data on their potential for CO 2 buffering. Whilst previous research has investigated the CO 2 retention mechanism in dry air conditions, this study focuses on the effect of relative humidity (RH) and temperature on the CO 2 retention process. Experimental comparisons were carried out at 0%, 51% and 71% RH with a CO 2 concentration of 20,000 ppm at a temperature of 25  ∘ C and 35  ∘ C using a TG-DSC instrument. The results show that the presence of water reduces CO 2 retention and slows down the kinetics of mass uptake. However, CO 2 is still retained in the presence of water, indicating the availability of adsorption sites and potential CO 2 –water interactions. Furthermore, the study shows that the effect of temperature is less pronounced at 51% than at 0% RH, and higher CO 2 retention is observed at 71% than at 51% RH, indicating the possibility of CO 2 dissolution in water. This paper presents the first analysis of the complex interactions between CO 2 earth and water, elucidating their dependence on relative humidity and ambient temperature conditions. Graphic abstract

In unsaturated soils, the gaseous phase is commonly assumed to be continuous. This assumption is no more valid at high saturation ratio. In that case, air bubbles and pockets can be trapped in the porous network by the liquid phase and the gas phase becomes discontinuous. This trapped air reduces the apparent compressibility of the pore fluid and affect the mechanical behavior of the soil. Although it is trapped in the pores, its dissolution can take place. Dissolved air can migrate through the pore space, either by following the flow of the fluid or by diffusion. In this context, this paper present a hydro mechanical model that separately considers the kinematics and the mechanical behavior of each fluid species (eg liquid water, dissolved air, gaseous air) and the solid matrix. This new model was implemented in a C++ code. Some numerical simulations are performed to demonstrate the ability of this model to reproduce a continuous transition of unsaturated to saturated states.

In developing countries, most of the population cannot afford conventional building blocks made with the sand-cement mixture. In addition, these blocks do not provide thermal comfort and have a high embodied energy compared to vernacular materials. The main objective of this work was to produce low cost, resistant and durable (good resistance to water) blocks with a thermal behaviour enabling quality comfort indoor. For that purpose, the effects of cow-dung on microstructural changes in earth blocks (adobes) are investigated by means of X-ray diffraction, thermal gravimetric analyses, scanning electronic microscopy coupled with energy dispersive spectrometry, and video microscopy. The effects of these changes on the physical properties (water absorption and linear shrinkage) and mechanical properties (flexural and compressive strengths) of adobe blocks are evaluated. It is shown that cow-dung reacts with kaolinite and fine quartz to produce insoluble silicate amine, which glues the isolated soil particles together. Moreover, the significant presence of fibres in cow-dung prevents the propagation of cracks in the adobes and thus reinforces the material. The above phenomena make the adobe microstructure homogeneous with an apparent reduction of the porosity. The major effect of cow-dung additions is a significant improvement in the water resistance of adobe, which leads to the conclusion that adobes stabilized by cow-dung are suitable as building materials in wet climates.

A set of tests is proposed to contribute to the experimental identification of the failure surfaces of an elasto-plastic model for a rammed earth along different stress paths such as compression, extension and tensile stress paths. The constitutive model involves two failure surfaces reflecting two different modes of failure within the material, a shear mode of failure and a tensile mode of failure typical of quasi-brittle materials. Secondly, the influence of water content on these failure surfaces is addressed. Such an influence is important to understand when stability of walls against unexpected storage of humidity is modelled since such storage is the main cause of failure of rammed earth construction. Three different water contents were considered in this study. The results show that for the studied material, the dissymmetry of behaviour between compression and extension is far greater than another quasi-brittle material such as concrete, which is new. As a first attempt, the influence of the water content can be modelled by a mere shift of the shear and of the tensile failure surface along the hydrostatic axis. Particularly, in the range of the investigated water contents, the shape of the failure surface can be stated as independent of the water content where just the apex will shift towards smaller values.