When appropriately designed and maintained, passive systems can provide long-term, efficient, and effective treatment for many acid mine drainage (AMD) sources. Passive AMD treatment relies on ...natural processes to neutralize acidity and to oxidize or reduce and precipitate metal contaminants. Passive treatment is most suitable for small to moderate AMD discharges of appropriate chemistry, but periodic inspection and maintenance plus eventual renovation are generally required. Passive treatment technologies can be separated into biological and geochemical types. Biological passive treatment technologies generally rely on bacterial activity, and may use organic matter to stimulate microbial sulfate reduction and to adsorb contaminants; constructed wetlands, vertical flow wetlands, and bioreactors are all examples. Geochemical systems place alkalinity-generating materials such as limestone in contact with AMD (direct treatment) or with fresh water up-gradient of the AMD. Most passive treatment systems employ multiple methods, often in series, to promote acid neutralization and oxidation and precipitation of the resulting metal flocs. Before selecting an appropriate treatment technology, the AMD conditions and chemistry must be characterized. Flow, acidity and alkalinity, metal, and dissolved oxygen concentrations are critical parameters. This paper reviews the current state of passive system technology development, provides results for various system types, and provides guidance for sizing and effective operation.
Structure from Motion (SfM) photogrammetry, on-site hardness and water absorption tests, and petrographical and porosimetric analyses on targeted samples were carried out on representative areas ...within six different locations of Maltese fortifications affected by salt weathering. The objective was to quantify the limestone loss over the last half-millennium and to attempt to identify the controlling factors of durability. The results highlight the existence of a two-rate weathering regime within the Miocene Globigerina Limestone which is the main building stone of the Maltese Islands. With a median stone recession of 1.4 mm/century, the subtype locally called Franka has resisted atmospheric aggression on average 36 times better than the Soll subtype (median stone recession of 50 mm/century). The Oligo-Miocene Coralline Limestone has also resisted well, with a stone recession of on average 1.3 mm/century. Two main controls of the varying durability of these limestones have been identified. First, the water uptake capacity of the least durable, already weathered limestone (Soll) is two to three times higher than that of the weathered but more resistant Franka and Coralline Limestone (6.3 g.m−2.s−1 against 2.5 g.m−2.s−1). Second, there is a marked difference in cohesiveness between these limestones. Whereas the weathered, susceptible Soll facies is poorly-cemented or microcracked, the exposed durable Franka displays an abundant micritic and microsparitic matrix. The statistical relationship between durability and surface hardness is not straightforward, and other controls such as the clay content of limestones should be explored. Investigating a larger number of sites would allow the refining of the limestone durability scale inferred from the present study.
•SfM photogrammetry and on-site tests were carried out on Maltese fortifications.•A two-rate weathering regime was highlighted within the building limestones.•Cohesiveness and water uptake capacity were identified as controlling factors.•There is no straightforward correlation between durability and surface hardness.
High-strength strain-hardening cement-based composites (HS-SHCC) demonstrate excellent mechanical and durability properties. However, high cement content typical to HS-SHCC results not only in high ...carbon footprint, but also in excessive hydration heat and severe autogenous shrinkage. In this investigation, Limestone Calcined Clay Cement (LC3) was used to produce sustainable HS-SHCC. The LC3 substitution resulted in higher energy consumption during mixing and in shorter setting times of the fresh, plain matrices. Although the LC3 substitution slightly reduced the compressive strength, the formation of highly polymerized C-A-S-H gel and abundant ettringite benefited the flexural strength of the plain matrices. Additionally, single-fiber pullout experiments showed that the use of LC3 led to increased fiber-matrix bond strength and pullout energy. Finally, the replacement of Portland cement by LC3 resulted in HS-SHCC with similar mechanical performance to the reference composite, indicating a high potential for using LC3 in high-performance cement-based composites.
•Use of LC3 slightly reduces flowability and shortens the setting time of fresh matrices.•Highly polymerized C-A-S-H gel and abundant ettringite benefit flexural strength of LC3-matrices.•Use of LC3 enhances the bond strength between fiber and high-strength matrix.•LC3-based HS-SHCC yields similar flexural performance as the composite with Portland cement.
This article reviews the rapidly developing state-of-the-art literature available on the subject of the recently developed limestone calcined clay cement (LC3). An introduction to the background ...leading to the development of LC3 is first discussed. The chemistry of LC3 hydration and its production are detailed. The influence of the properties of the raw materials and production conditions are discussed. The mixture design of concrete using LC3 and the mechanical and durability properties of LC3 cement and concrete are then compared with other cements. At the end the economic and environmental aspects of the production and use of LC3 are discussed. The paper ends with suggestions on subjects on which further research is required.
Abstract
The karst cavern is common undesirable geological body in limestone areas, which can bring great safety hazards to the construction and operation of tunnels, bridges and highways. Transient ...electromagnetic method (TEM) can effectively detect hidden karst structures based on the difference in electrical property between the karst cavern and the surroundings. Traditionally, 1D inversion is commonly used for the interpretation of TEM data due to its high efficiency. However, this may lead to artifacts due to 2D or 3D effects in practical surveys. In this study, we present a Gaussian-Newton approach to 3D inversion of TEM data for karst cavern detection. The numerical test on synthetic TEM data demonstrates that 3D inversion can effectively delineate the location and geometry of concealed karst cavern compared to the 1D inversion results.
•The action mechanism of limestone powder includes four effects.•The physical effect of limestone powder depends on its particle size and amount.•The chemical effect of limestone powder relies on the ...alumina content.
Limestone powder (LS) has been widely used in cement-based materials; and reportedly, can influence their properties by filler, nucleation, dilution and chemical effects. The action mechanism of LS mainly depends on its particle size and amount. The filler effect of LS refines the microstructure and reduces the porosity of cement-based materials. Its nucleation effect accelerates the hydration of C3S, increases the amount of hydration products and reduces the porosity of cement-based materials at early ages. Its dilution effect reduces the hydration peak of C3S, decreases the amount of hydration products and increases the porosity of cement-based materials. Its chemical effect promotes the appearance of third hydration peak, forms carboaluminate and reduces the porosity of cement-based materials.
This study compares the influence of two limestone fineness on strength development and cement hydration. The replacement of clinker by 20% of fine limestone (dv,50 = 2 μm and SSA = 4,21 m2/g) gives ...similar strength development to PC up to 7 days. Fine limestone enhances cement hydration and offsets the dilution effect caused by the decrease of clinker amount. The accelerating effect of the limestone was explained by a higher degree of undersaturation with respect to C3S. The influence of coarse limestone (dv,50 = 130 μm and SSA = 0,46 m2/g) on cement hydration is limited to dilution effect. The combination of fine and coarse limestone was also studied. A linear correlation was obtained between strength and gel space ratio for all investigated systems. Furthermore, the results showed that despite the similarity of specific surface area, limestone is more effective than quartz for the enhancement of cement hydration.
Recent research have recognized that coupled use of calcined clay, limestone and cement clinker in concrete is viable to reduce environmental footprints at manufacture and to enhance material ...durability. In this study, a novel application of the limestone calcined clay cement (LC3) is demonstrated by substituting the Ordinary Portland Cement (OPC) in Engineered Cementitious Composites (ECC). The composite mechanical properties including σ-δ and σ-ε relationships and residual crack widths were evaluated to 28 days under uniaxial tension. Matrix chemistry was characterized using thermogravimetric analysis and X-ray diffraction, while the pore structure of matrices and composites was analyzed using mercury intrusion porosimetry. The LC3-based ECC showed more rapid early strength development but lower 28-day strength (~32 MPa) due to a 20% higher water-to-solid ratio for attaining adequate workability and fiber dispersion. Nevertheless, the tensile strain capacity of LC3-based ECC achieved over 6% with an average residual crack width less than 50 μm. Additionally, the composite pore structure exhibited a decreasing volume fraction of large pores and voids (>100 nm) after substituting LC3 for OPC. The use of LC3 marginally decreased the embodied material energy and cost, but led to about 32% and 28% reductions in CO2 emissions compared to traditional OPC-based ECC and concrete, respectively. As a preliminary study, LC3-based ECC shows promise as a greener ductile concrete compared with OPC-based ECC.
Halloysite is a kind of 1:1 clay mineral having a special nanosized tubular morphology and pore structure. In this work, nanosized tubular halloysite calcined at 750 °C (Hal750) and limestone (LS) ...were used to partially replace the ordinary Portland cement (OPC) for the preparation of limestone calcined clay cement (LC3). The mechanical properties and microstructure of LC3 were studied. The results revealed that the obtained LC3 had higher early compressive strengths on 3 and 7 days than did plain OPC. LC3 with a replacement ratio of 22.5% (containing 15.0% Hal750 and 7.5% LS) resulted in maximum compressive strength of 46.38 MPa, being 9.4% higher than OPC's after 28 days of curing. Further, LC3 featured a compact microstructure and smaller critical pore size than OPC, which is mainly because more additional C-(A)-S-H, hemicarboaluminate (Hc), and monocarboaluminate (Mc) were formed in LC3 system, thus contributing to the matrix densification and strength development. However, when the replacement ratio exceeded 37.5%, the insufficient portlandite (CH) limited the pozzolanic reaction with Hal750, leaving excess Hal750 in the LC3 that led to their inhomogeneous microstructure, and thus weakened the mechanical properties. These results show that halloysite is a promising material for the preparation of LC3 whose properties are sensitive to the replacement ratio used.
•Calcined halloysite is feasible to prepare limestone calcined clay cement (LC3).•The compressive strength of LC3 is sensitive to the replacement ratio by Hal750 and LS.•LC3 prepared from calcined halloysite features a higher compressive strength than OPC.
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