Estabilização de solos residuais graníticos através da adição de cal

A correcta estabilização dos solos de fundação constitui um aspecto cada vez mais importante no panorama actual da construção. De facto, a preocupação com o ambiente assume proporções cada vez maiores, existindo hoje em dia a clara consciência de que todos os esforços devem ser feitos no sentido de minorar os estragos ambientais causados pelo desenvolvimento das mais variadas infra-estruturas. Nessa óptica, é urgente não só encontrar métodos construtivos que permitam atingir este objectivo, como ac elerar a sua implementação. O aproveitar dos materiais existentes no local da obra, independentemente da sua qualidade e adequabilidade ao projecto em questão, permite não só evitar a sua substituição por materiais de melhor qualidade, recolhidos noutro local, com graves consequências ao nível dos impactos ambientais; como também eliminar a necessidade de um local de depósito dos materiais não compatíveis. O recurso à estabilização de solos através da inclusão de materiais como o cimento ou a cal permite atingir os objectivos propostos e, para além de constituir um método relativamente pouco dispendioso e de fácil aplicação, pode ser aplicado em praticamente todos os tipos de solo. Os solos residuais graníticos podem ser encontrados em extensas áreas da zona norte do país, possuindo uma fracção argilosa que, embora significativa em termos quantitativos, é constituída maioritariamente por caulinite. Ora, este mineral é pouco reactivo com a cal, quando comparado com outros tipos de minerais argilosos. Por outro lado, o elevado volume da fracção argilosa diminui a adequabilidade deste tipo de solo à estabilização com cimento. Neste trabalho pretende assim quantificar-se os benefícios alcançados com a estabilização com cal de um solo residual granítico, quer no que respeita à sensibilidade à água (melhoramento), quer no que respeita aos ganhos de resistência mecânica (estabilização). Foram estudadas as características de dois solos, distintos na relação material arenoso / material argiloso, tendo sido monitorizada a evolução dessas características após a mistura com cal. A cal utilizada neste trabalho era do tipo calcítica hidratada, e foi adicionada aos solos nas percentagens de 2, 6 e 10%, em função dos efeitos que se pretendiam estudar. São apresentados e analisados os resultados obtidos em provetes com períodos de cura até 10 semanas. Em termos de resistência mecânica, registaram -se aumentos até 10 vezes o valor inicial da resistência do solo. Foram também testadas as respostas dos dois solos sob condições de saturação, sendo que também neste caso se registaram melhorias, relativamente às condições originais. Foi ainda estudado o efeito da mistura da cal com cloreto de sódio, verificando-se que esta constitui uma opção viável, na medida em que permite uma redução dos custos, sem perda de qualidade em termos de resistência mecânica e susceptibilidade à água. Foi também feita uma revisão do estado da arte respeitante aos métodos de estabilização de solos, nas vertentes de estabilização mecânica, estabilização física e estabilização química.The correct stabilization of foundation soils constitutes an increasingly important issue in the present civil engineering world. Concerns over the environment have taken significant proportions, and there is the awareness today that efforts must be made to diminish the environmental damage caused by the development of infrastructures. Therefore, it has become urgent not only to find building procedures, which will allow this objective to be achieved, but also to accelerate their implementation. Recycling material from the original site, independently of the simplicity avoids its substitution by better materials, recovered elsewhere; which has severe consequences for the environment. This also eliminates the need for a deposit site of unused material. Soil stabilization with cement and/or lime is a way of achieving the proposed goals, which is at the same time inexpensive and easy to apply in practically any type of soil. Residual granite soils can be found in large areas of northern Portugal. Their clay fraction, being significant in quantity, is formed almost exclusively by kaolin, a mineral which in comparision with other types of clays does not react very much with lime. On the other hand, the high percentage of clay reduces the fitness of the this type of soil for stabilization with cement. The main goal in this research work is to quantify the benefits achieved with the lime stabilisation of a residual granite soil, with respect to water sensibility and mechanical resistance. The characteristics of two soils, with different clay fractions are discussed and the evolution of their properties after mixing with 2, 6 and 10% lime is studied. A calcitic hydrated lime was used in this research work. The results were monitored up to ten weeks after the admixture, at which point resistance had improved more than ten times the initial values. The compressive strengths of both soils in saturated conditions were also tested, once again significant improvements relatively to the initial values were observed. The effects of the addition of sodium chloride and lime are also reported. It was concluded that this is an efficient way of reducing costs, without losing the strength gains and water sensibility already achieved.. Furthermore, a review of the state-of-the art was also undertaken with regard to mechanical, physical and chemical stabilization of soils

Study on guardrail post behavior located on organic soil using simplified experimental and numerical methods

For the purpose of road safety, it is vital to reduce the severity of road accidents and increase safety around the roadway area by deploying guardrails. In case of a car crash, a guardrail post must be deformable so that such restraint is not too abrupt due to the occupant’s sensitivity. Soil type influences on the guardrail post behavior have been a somewhat unfounded variable due to the high soil heterogeneity and challenging interpretation of its real implications on the safety of guardrail systems. Since little attention is concentrated on evaluating the guardrail post behavior through simplified procedures, this article aims to provide a simplified experimental and numerical approach to study the behavior of guardrail posts located on organic soil. Results of laboratory and in-situ tests indicated that guardrail posts behavior located on organic soil depends on section orientation, driving depth, and loading speed. To confirm and compare the in-situ tests, simplified numerical simulations through Plaxis 3D software were carried out, and data from numerical modeling approved the accuracy of in-situ results.The authors acknowledge the financial support by FCT / MCTES through national funds (PIDDAC) under the R&D Unit Institute for Sustainability and Innovation in Structural Engineering (ISISE), under reference UIDB/04029/2020; and ANI through national funds (Portugal 2020), under project “BARROD - Barreiras de Segurança Rodoviária”, reference 33497

Stabilisation of a plastic soil with alkali activated cements developed from industrial wastes

The development of alternative materials for the construction industry, based on different types of waste, is gaining significant importance in recent years. This is mostly due to the need to increase sustainability of this heavily polluting activity, thus mitigating the dependence on, for instance, Portland cement. The present paper is related to the development of an alkaline activated cement (AAC) exclusively fabricated from industrial by-products (both precursor and activator). Coal combustion fly ash, a common residue from thermoelectric powerplants, and glass waste, from the manufacture of ophthalmic lenses, were used as precursors. These precursors were activated with a recycled alkaline solution, resulting from the cleaning of aluminium extrusion dies, instead of the more common commercial reagents usually applied for this type of binder. Several pastes were studied, combining the precursor and alkaline solution in different proportions. When the most-performing cements were defined, they were used to stabilise a cohesive soil. The experimental procedure and subsequent analysis were designed based on a Response Surface Methodology model, considering the Activator/Solids and Soil/Precursor ratios as the most relevant variables of the stabilisation process. It was observed that, depending on the type of alkaline cement used, there was an optimum precursor and activator contents to optimise the mechanical properties of the stabilised soil. The reliability of this prediction was especially dependent on the type of precursors and, also, on their respective dissolution process right before the homogenization with the soil, under the working conditions available.This work was funded by the R&D Project JUSTREST- Development of Alkali Binders for Geotechnical Applications Made Exclusively from Industrial Waste, with reference PTDC/ECM-GEO/0637/2014, financed by the Foundation for Science and Technology—FCT/MCTES (PIDDAC)

Constitutive behaviour of a clay stabilised with alkali-activated cement based on blast furnace slag

Alkaline cements have been extensively tested for soil stabilisation in the last decade. However, only a few studies have focused on the assessment of such performance by establishing the constitutive behaviour of the cement. In this paper, we focus on the mechanical behaviour, using triaxial testing of a clay with high water content stabilised with an alkali-activated binder and the subsequent prediction of the experimental stress–strain response using a kinematic hardening constitutive model initially developed for natural clays. Monotonic consolidated undrained triaxial tests were conducted on reconstituted and stabilised clay specimens cured for 28 days to evaluate the effects of cementation on the overall shear behaviour. Alkali-activated binder was synthetised from blast furnace slag and sodium hydroxide. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were performed to study the microstructure, whereas leachate analyses were performed after 28 and 90 days of curing to investigate the contamination potential. The main product formed was calcium aluminosilicate hydrate (C-A-S-H) with a low CaO/SiO2 ratio, and no risk of soil contamination was found. The compressibility and undrained shear strength in the pre-yield state was found to be independent of the initial mean effective stress (p’0), unlike what was observed in the post-yield state, where the shear strength seemed to be affected by p’0. The model provided reliable predictions of the experimental results and captured the main features of the artificially cemented clay for the tested p’0 range. Such studies are fundamental to establish adequate confidence in such alternative binders—an essential aspect if their use is to become widespread in the near future.This work was funded by the Project “MINECO- New Eco-innovative Materials for Mining Infra” with reference ERA-MIN/0002/2018 and by a Ph.D. scholarship with reference SFRH/BD/132692/2017 financed by the Portuguese Foundation for Science and Technology (FCT) and the European Social Fund (FSE)

Unsaturated response of clayey soils stabilised with alkaline cements

The influence of suction on the mechanical behaviour of unsaturated chemically stabilised soils is still mostly unknown and unquantified. This is also motivated by the difficulties associated with the experimental procedure required to fully characterise the unsaturated response of the soil, including its direct influence on traditional strength tests. The present paper presents the soil water retention curves obtained for a Portuguese soil before and after being stabilised with Portland cement (OPC) and an alkali-activated cement (AAC). Saturated undrained triaxial tests were also performed for the same curing conditions (0, 28, and 90 days). Previous attempts to characterise the retention curve of soils stabilised with AAC are unknown, and the results showed that the pore volume structure is already formed after 28 days, prior to the full development of the gel matrix responsible for the strength increase between 28 and 90 days. The curve changed after stabilisation, and with each binder, as the OPC presented a higher air-entry value and a narrower suction range compared to the AAC solution. The significant differences between the curves obtained from each binder suggest the future development of a methodology to assess the quality of the AAC stabilisation.ERDF -European Regional Development Fund(undefined

Improvement of a clayey soil with alkali activated low-calcium fly ash for transport infrastructures applications

The improvement of geotechnical properties is often achieved by the addition of traditional binders, such as cement or lime. However, the use of such binders implies a considerable financial and environmental cost that needs to be mitigated. An unconventional solution, similar to cement in terms of performance but more environmentally friendly, consists in the use of binders made from alkaline activated industrial residues. The technique consists on the activation of raw materials (such as fly ash or blast furnace slag) rich in Si, Al, or even Ca, with high pH alkaline solutions. The present work was developed aiming the possible stabilisation, using different fly ash contents, of a clayey soil with sand. The activator solution was composed of sodium hydroxide and sodium silicate. The extended experimental campaign included unconfined compressive strength (UCS), California Bearing Ratio (CBR), pulse velocity tests and triaxial tests to assess the geomechanical improvement induced by the new binder. As a mean of comparison, the experimental campaign included also the stabilisation of the same soil with either cement or lime. The obtained data indicates that the use of alkaline activation as a soil stabilisation technique provides competitive geomechanical results, when compared with those obtained with traditional binders.(undefined

Sustainable soil stabilisation with ground granulated blast-furnace slag activated by olivine and sodium hydroxide

Ground granulated blast-furnace slag (GGBS), activated with olivine (Mg2SiO4) and sodium hydroxide (NaOH), was used to stabilise a clayey soil. Mechanical and microstructural properties of the stabilised soil were assessed through uniaxial compression strength (UCS) tests, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDS), after curing periods of 7, 18 and 90 days. The UCS of the GGBS-treated soil (without activation with NaOH), even at the highest slag dosage (G20S), after 90 days, showed only a slight increase (142 kPa) relatively to the original soil. When olivine was added to the GGBS-treated mixture (O20G20S), the UCS increased to 444 kPa, after 90 days. However, when NaOH was used as an activator, the UCS of the olivine–GGBS-treated soil (NO20G20S) increased to more than 6000 kPa, after 90 days. This significant strength increase was attributed to the higher reaction degree provided by the NaOH, which enabled a more effective exploitation (dissolution) of the Ca and Mg present in the slag and olivine, respectively, forming a mixture of C–S–H and M–S–H gels

Characterization of stainless steel spent pickling sludge and prospects for its valorization

Fluorspar is considered a critical raw material for the European Union, due to its industrial uses and lack of sufficient extraction in European countries. It is a source for hydrofluoric acid manufacture, this latter chemical being employed, among other uses, in the pickling of stainless steels. From this latter activity, sludge is generated due to the need for used water treatment. In this article, we report a full characterization of this residue, obtained in an industrial plant in Vieira de Leiria, Portugal. Its chemical and mineralogical characteristics were determined, showing that it is mostly a mixture of calcium fluoride and calcium sulfate with some heavy metals content. Thermal behavior allowed us to determine that the material melts at around 950 ◦C. The influence of calcining operation on the residue was determined, especially concerned with the leachability of some elements. Taking into account the results of the characterization of this residue, some considerations are presented about the potential for the valorization of this industrial residue.Bollinghaus Steel, S

Indirect tensile behaviour of fibre reinforced alkali-activated composites

There are currently still some sustainability-related issues that need to be tackled within the construction sector. Namely, cement production is accountable for nearby 5% of the worldwide total CO2-eq release. Therefore, environmentally viable and economically sustainable solutions need to be pursued in order to mitigate the use of Portland cement. The incorporation of industrial waste in concrete compositions, such as fly ash (from coal combustion in power stations) is a feasible alternative. The properties of these residues may be enhanced through alkaline activation, which is able to yield aluminosilicate-based materials with excellent physico-chemical properties. Nonetheless, these materials exhibit a brittle behaviour. Therefore, the present work addresses the study of alkali-activated composites reinforced with sisal fibres. For that purpose, alkali-activated Class F fly ash was mixed with natural fibres and the composite mechanical behaviour was assessed through both indirect tensile and compressive tests. Four different fibre contents, in wt % of fly ash (0, 0.2, 0.6 and 1%), two fibre lengths (13 and 50 mm) and four curing periods (14, 28, 56 and 112 days) were considered. Results confirm that the post-cracking response of these composites was improved with the inclusion of sisal fibres. In general, higher residual tensile strengths and dissipated energy were observed for the lengthier fibres, i.e., 50 mm.info:eu-repo/semantics/publishedVersio

Application of the response surface method to optimize alkali activated cements based on low-reactivity ladle furnace slag

Steel-making slags, resulting from basic oxygen furnaces or electric arc furnaces are heavily applied in the construction industry, as an aggregate for pavements or concrete. Although possessing a significant crystalline content, it is expected that, if properly milled, the reactivity of these slags can increase up to a point when they are viable to produce alkaline cements. The aim of this study was the application of a response surface method to design the experimental work required to optimise the composition of an alkaline cement based on ladle furnace slag, a specific type of steel slag (SG). Fly ash (FA) was also added, in a precursor role, and the activation was achieved with an alkaline solution prepared with sodium silicate (SS) and sodium hydroxide (SH). The factors/variables considered were the activator index X = SS/(SS + SH), the precursor index Y = SG/(SG + FA) and the SH concentration (Z). The output variables were the unconfined compression strength and the flexural strength, after 7 and 28 days curing. Results indicate that the activator index (X) was the most influential variable, followed by the precursor index (Y). Microstructural analysis of selected pastes was also performed, using scanning electron microscopy and energy dispersive spectroscopy. The ideal composition obtained for the alkaline cement was the mixture constituted by X = 0.75, Y = 0.5 and Z = 10 (activator: 75% SS and 25% SH; precursor: 50% SG and 50% FA; SH concentration = 10 molal). This mixture achieved 8.70 MPa of flexural strength and 44.25 MPa of compressive strength which is reasonable for the required application (soil stabilisation)