Bio-Stabilised Earthen Blocks: A Critical Study on Compression Tests of Immersed Samples

Currently, much consideration is given to earthen building materials regarding their highly sustainable properties. Numerous studies have highlighted their structural ability but their water sensitivity is still limiting a potentially more spread use. To limit this sensitivity several studies have recently brought out the positive effects of bio-stabilisers such as linseed oil or xanthan gum. These recent developments allow bio-stabilized earthen materials to be resistant to immersion in water. Also, a French experimental standard (XP P 13-901) for compressed earth blocks already asks for a minimal compressive strength after a two-hour immersion that is overly severe and is difficult to satisfy without the addition of high contents of hydraulic binders. In this paper, a critical study of this compressive test after immersion is conducted on bio-stabilized (linseed oil and xanthan gum) samples of different Breton earths. Some testing adjustments are suggested and the water-diffusion in the samples is followed and linked to previously obtained capillary absorption coefficients. It is shown that the effect of immersion on the mechanical strength depends on the sample size and that an equivalence between size and immersion time can be made based on an equivalent penetration depth. Linseed oil and xanthan gum help to significantly increase the compressive strength of the earthen materials after immersion and allow to avoid the addition of hydraulic binders in earthen blocks to obtained a strong water resistance. The water diffusion in the sample during the immersion can be linked to capillary absorption behaviour, thus a water content and a compressive strength after a given time of immersion could be easily predicted.

Erosion Behaviour of Bio-Stabilised Earthen Materials

Development of earthen building materials is one of the answers that the construction sector can provide to tackle the accelerated climate change issue. However, these materials present a wide variability, even at the local scale, and their water durability can be difficult to ensure. In order to improve their durability regarding water and avoid its prejudicial effect on earthen material’s properties, the stabilisation with bio-polymers is an increasingly studied solution. In this paper a ten-minute erosion drip test is developed and performed for various combinations of Breton earths and bio-based additions or surface treatments (linseed oil, xanthan gum, casein, alginate, vegetal varnish and tannins). The final pitting depths and eroded volumes are compared and the evolution of erosion during the test is monitored. These results are also linked to previously obtained water capillary absorption coefficients. The obtained results enable to highlight the impact of bio-based additions on erodibility of earthen materials: linseed oil and xanthan gum help to protect the earth-based samples from erosion. Other original parameters characterizing the erosion of the samples during the drip test are suggested. Limitations of this type of erosion tests are also brought out.

Water Sensitivity of Hemp-Foam Concrete

The necessity to build energy-efficient and low environmental impact buildings favors the development of biobased light-weight materials as hemp-foam concretes. In this context, experimental protocols were developed to study the effects of hemp shiv and the production methods on the water sensitivity of bio-based foamed concrete (BBFC). Foam concrete incorporates several materials and compounds: cement, protein-based foaming agent, ground granulated blast–furnace slag, metakaolin as a binder, and hemp shiv as bio-based aggregates. The study investigated first the effect of the incorporation of hemp shiv (from 0 to 15 vol%) and then the elaboration method, comparing direct method versus preformed method on the resulting physical properties, the isotherms sorption-desorption and the capillary water absorption of hemp-foam concretes. We observe an increasing porosity of the concrete with hemp shives content. Additionally, hemp shives increase the adsorption and the capillary absorption of water. Moreover, the preformed method produces concretes more sensitive to water than the direct methods since it increases its porosities.

Thermal Insulation Blocks Made of Sunflower Pith Particles and Polysaccharide-Based Binders: Influence of Binder Type and Content on Their Characteristics

Co-product of sunflower cultivation, pith of stem has a little exploited insulating potential. Blocks in which pith particles are glued together using a starch-based binder have already been obtained. However, they are highly water-sensitive. Replacing this binder with others has been considered here. Polysaccharide-based binders were tested, chosen for their more hydrophobic character: sodium alginate, chitosan, Citrus pectin, and a modified starch. Like starch, these binders are physically binding. They are first solubilised in water (except chitosan, dissolved in 2% acetic acid). The solution is then mixed with pith particles before cold compression molding for 90 s. A 10% binder content was initially considered. The blocks were all cohesive with a dry density from 36 to 42 kg/m3). Their performances were assessed through water absorption capacity and resistance via capillary absorption tests on wet sponges, mechanical test and thermal conductivity. Chitosan and pectin-based blocks show the best properties, particularly concerning water resistance and mechanical properties. The pectin-based block has improved its elastic modulus by 40% compared to a starch-based block. The pectin-based block in its case absorbs 2.7 times less water than starch. Finally, thermal conductivities of pectin and chitosan-based pith blocks are in the same order of magnitude as for starch (39.8-40.1 mW/m.K), and close to values from commercial materials (e.g., polystyrene). Pectin and chitosan were also tested at three rates (5%, 10% and 15%). A significant improvement in the blocks' compressive strength was observed with the increase in binder rate, while thermal conductivities varied little.

Effect of Viscosity Modifying Agent on the Performance of Hybrid Bio-Based Concrete

An experimental investigation was conducted in order to study the water absorption, mechanical performance, thermal properties and durability of hybrid hemp-rapeseed composite materials. The hybrid composite material is made with 50% hemp shives and 50% rapeseed fibres. The purpose of this study is to investigate the influence of the incorporation of viscosity modifying agent (VMA) on hybrid concrete. Four mixes were made for: shuttered walls and roof insulation with and without VMA. The water absorption of the hybrid composite fibres was, first, measured. The compressive strengths of these mixes at 7 and 28 d were then determined in order to compare the mechanical behaviour of the hybrid composite materials made with VMA. In addition, the capillary absorption and coefficient of thermal conductivity were also measured. The results revealed that VMA improved the performance of the hybrid concrete.

Effect of varying silica-limestone sand fines on the physical-mechanical performance of concrete

The use of crushed limestone sand in the concrete industry will be quite possible and imperative for environmental reasons. Many researchers around the world have found that concrete based on 50% substitution of river sand by limestone sand gives better physico-mechanical characteristics. The main objective of this investigation is to search for an optimal percentage of silica-limestone fines resulting from the substitution of half in quantity of alluvial sand by crushed limestone sand in ordinary concrete. The proportions of fines that were tested in this work are 6%, 8%, 10%, 12% and 14%. The obtained results revealed that concrete based on silica-limestone sand and containing 14% of the same type of fines strongly improves the different mechanical strengths and participates in the reduction of 10% and 13%, of the coefficient of capillary absorption and of the porosity accessible to water, respectively, compared to the control concrete. In addition, good statistical relationships between the studied parameters were also found

A Critique on “Measurement of the Hygric Resistance of Concrete Blocks with Perfect Contact Interface: Influence of the Contact Area”

Background: In March 2021, this journal published the article “Measurement of the hygric resistance of concrete blocks with perfect contact interface: influence of the contact area”. That article reports on a study on the impact of ‘perfect contact’ between concrete blocks on moisture absorption, with a focus on the impact of the sample cross-section. Objective: This critique aims at formulating several essential concerns on the hygric aspects of that article, thus expressing the discusser’s reservations on the reliability of the presented outcomes in particular and the published article in general. Methods: The data, as provided in the graphs of the critiqued article, are digitally extracted and further analysed by the discusser. Results: That analysis results in serious concerns with regard to 1) the magnitude of the quantified post-interface flows, 2) the distinguishability of the moisture absorption in the monolithic and perfect contact samples, 3) the robustness of the knee-point identification algorithm, 4) the dependability of the capillary absorption measurements, and 5) the consistency of the capillary absorption processing. These are finally translated into 8 concrete questions to be addressed by the authors of the critiqued article in order to placate these doubts and establish the reliability of their work. Conclusion: This critique formulates appreciable apprehension with respect to an earlier publication in the journal and invites its authors to respond to that via answering the 8 concrete questions. If not satisfactory, then the critiqued article’s findings cannot be considered reliable, and the journal should reconsider its prior publication.

Influence of Wood Fly Ash on Concrete Properties through Filling Effect Mechanism

This paper presents the results of an experimental study aimed at determining the influence of wood fly ash (WFA) from three Croatian power plants on the properties of concrete. First, the chemical and physical properties of WFA’s were determined. It was found that these properties are highly influenced by combustion technology, the type and parts of wood used as fuel, and the local operating conditions. Subsequently, workability, heat of hydration, stiffness development, 28-day compressive strength, apparent porosity, and capillary absorption were determined on concrete mixes prepared with WFA as cement replacement from 5–45% by weight. Cement replacement up to 15% with the finest WFA accelerated hydration, stiffness development, and increased compressive strength of concrete up to 18%, while replacement with coarser WFA’s led to a decrease in compressive strength of up to 5% and had more gradual heat liberation. The dominant effect that could explain these findings is attributed to the filler and filling effect mechanisms. At the same time replacement content of up to 45% had very little effect on capillary absorption and could give concrete with sufficiently high compressive strength to be suitable for construction purposes.

Mechanisms in Carbon Nanotube Growth: Modelling and Molecular Dynamics Simulations

<p><b>A selection of nanoscale processes is studied theoretically, with the aim of identifying themechanisms that could lead to selective carbon nanotube (CNT) growth. Only mechanisms relevant to catalytic chemical vapour deposition (CVD) are considered. The selected processes are analysed with classical molecular dynamics (MD) simulations and continuum modelling.</b></p> <p>The melting and pre-melting behaviour of supported nickel catalyst particles is investigated. Favourable epitaxy between a nanoparticle and the substrate is shown to significantly raise themelting point of the particle. It is also demonstrated that substrate binding can induce solid-solid transformations, whilst the epitaxy may even determine the orientation of individual crystal planes in supported catalysts. These findings suggest that the substrate crystal structure alone can potentially be used to manipulate the properties of catalyst particles and, hence, influence the structure of CNTs.</p> <p>The first attempt at modelling catalyst dewetting, a process where the catalyst unbinds from the inner walls of a nucleating nanotube, is presented. It is argued that understanding this process and gaining control over itmay lead to better selectivity in CNT growth. Two mutually exclusive dewetting mechanisms, namely cap lift-off and capillary withdrawal, are identified and then modelled as elastocapillary phenomena. The modelling yields an upper bound on the diameter of CNTs that can stem from a catalyst particle of a given size. It is also demonstrated that cap lift-off is sensitive to cap topology, suggesting that it may be possible to link catalyst characteristics to the structural properties of nucleating CNTs. However, a clear link to the chiral vector remains elusive.</p> <p>It is shown that particle size, as well as binding affinity, plays a critical role in capillary absorption and withdrawal of catalyst nanoparticles. This size dependence is explored in detail, revealing interesting ramifications to the statics and dynamics of capillary-driven flows at the nanoscale. The findings bear significant implications for our understanding of CNT growth from catalyst particles, whilst also suggesting new nanofluidic applications and methods for fabricating composite metal-CNT materials.</p>