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Red beds (RBs) are widely distributed in all continents of the world. With the rapid development of road and railway construction, the construction of roads and railways in RB areas is increasing. However, when RB is the filler, it will expose three aspects of serious deficiencies in compaction, mechanical bearing capacity, and water stability, which will cause fatal harm to the subgrade. Therefore, RB has to be abandoned and cannot be used as a subgrade filler. In this paper, different microwave irradiation temperatures (DMITs) from room temperature of 25°C to 700°C were used to reveal the enhancing mechanism of RB in three aspects based on the analysis results of microscopic composition and structure. The microscopic composition and structure show that RB can be divided into a low-temperature section (25°C–400°C) and a high-temperature section (500°C–700°C) under the microwave condition. The low-temperature section has some improvement in the engineering performance of RB, but the influence is limited. The engineering performance of RB is improved completely in the high-temperature section. Specifically, the coarse size of RB increases with the rise in microwave temperature, which is beneficial for improving fine size. The compaction property of RB is not largely affected by the water content and can still meet the most stringent subgrade filler requirements, even under saturated conditions or 0. The California bearing ratio (CBR) value can maintain more than 20% under the ultimate working condition, far higher than the code requirements. The increase of internal friction angle and cohesion can enhance the stability of the cut slope, which is conducive to construction in remote mountainous areas. The water resistance of RB is positively correlated with the microwave irradiation temperature, especially at 700°C; RB experiences secondary hardening after encountering water, the mechanical strength increases, and the softening coefficient reaches 107.44%. The research shows that after microwave high-temperature stabilization, the engineering characteristics of the RB are thoroughly strengthened, and it becomes a high-quality subgrade filler to eliminate the three diseases of RB subgrade completely. Compared with the traditional method of adding stabilizers, microwave in situ RB stabilization reduces the site construction's complexity, saves a lot of stabilizer use and transportation costs, and can effectively reduce carbon emissions. This study provides a positive perspective on the in situ application of the RB soft rock subgrade. It could be a starting point for understanding the microwave-based stabilization of soft rock soils subgrade filler in situ. [ABSTRACT FROM AUTHOR]

Most geopolymer research claims a substantial reduction in carbon dioxide emissions compared with ordinary portland cement concrete. However, geopolymer concretes are not all equally eco-friendly. The environmental benefits of geopolymer concrete cannot be taken for granted. Modeling emissions is necessary to assess the environmental impact of each geopolymer mix. Unfortunately, only studies concerned with carbon dioxide emissions care to provide sophisticated emissions modeling. Such modeling is practically nonexistent for other geopolymer studies; because function-oriented geopolymer research tends to assume the environmental eminence of their designed mixes. This research reviews the last 7 years of geopolymer concrete research to assess the sustainability of the mixes designed by researchers worldwide and to highlight any bad environmental practices in the mix design. The nominal CO2-e equivalent emissions for geopolymer concrete mixes varied from 56 to 661 kg-CO2/m3 for the surveyed mixes. SOx and NOx emissions reached 1,865 g-SOx/m3 and 1,161 g-NOx/m3 , respectively. Finally, this paper recommends a simple method for estimating a nominal carbon dioxide emissions value, which can be easily implemented in future geopolymer research. [ABSTRACT FROM AUTHOR]

This paper represents a series of laboratory tests to illustrate the effects of natural zeolite and sulfate environment on the mechanical behavior, hydraulic properties, and microstructure of plastic concrete. Plastic concrete (Pl-C) is comprised of cement, water, aggregate, and bentonite. Because cement production is responsible for a significant amount of carbon dioxide emissions into the atmosphere, cement was replaced in five different percentages of 0%, 10%, 15%, 20%, and 25% by natural zeolite (Z). Moreover, all the specimens were cured in chambers simulating sulfate attack. The mechanical behavior of plastic concrete containing natural zeolite (Pl-CZ) was studied using a series of confined and unconfined compression tests. Hydraulic properties were evaluated using three different confining pressures of 200, 350, and 500 kPa. Scanning electron microscopy (SEM) images were utilized to explore the microstructure of Pl-CZ specimens. The test results show that, although increasing the zeolite content decreased the unconfined strength, peak strength, and the elastic modulus in the early ages, at the later ages, it increased the unconfined strength as well as the peak strength and the elastic modulus, while it decreased the permeability. As illustrated in SEM images, specimens cured in the sulfate environment indicated lower porosity which led to having higher unconfined strength, elastic modulus, and peak strength, along with lower permeability. [ABSTRACT FROM AUTHOR]

MgO and Na2CO3 are used instead of strong alkalis (e.g., NaOH, NaOH+Na2SiO3) to prepare alkali-activated slag cement, achieving the promotion and use of green-economic alkali activators. In this study, the hydration reaction kinetics, hydration products, mechanical evolution, and microstructure characteristics of MgO-Na2CO3 activated slag (MNAS) system are studied. The results show that adding Na2CO3(>1%) prolongs the time of the second exothermic diffraction peak, and inhibits the hydration of the MNAS system in the early stage. The addition of Na2CO3 increases the 28 day compressive strength of the MNAS system. However, at 90 days, with the higher content of Na2CO3 , the SiO4, and AlO4 at the Q2 site are dechained, the peak value at the Q0 site increases, the mean chain length (MCL) of the gel phase decreases, the total pore volume increases, cracks appeared in the internal structure, and the compressive strength decreases. Na2CO3 mixing of 3%, 5%–8%, and 10% is appropriate when using S-type, M-type, and R-type MgO. The MNAS system is excellent from the aspects of CO2 emission, energy consumption, and cost of the alkali-activated materials. [ABSTRACT FROM AUTHOR]

Massive infrastructure constructions have resulted in scarcity of river sand resources. Previous studies and applications proved dune sand can partially replace river sand used in engineering to alleviate the supply and demand contradiction of river sand. However, due to the dune sand is finer, rounder and smoother than river sand, determining the appropriate proportion of dune sand for designing engineered dune sand concrete (DSC) remains challenging. To tackle this challenge, a mix design method for DSC was proposed via close packing model and mortar film thickness theory. Specifically, the close packing model was used to confirm dune sand content and sand ratio, then the dune sand and river sand were mixed to obtain engineering-standard mixed sand. The mortar film thickness theory was utilized to determine the mortar volume and to tailor the concrete slump. By proper design, the DSC achieved the 28-d compressive strengths of 50 MPa along with good workability. Increasing the thickness of the mortar film from 0.3 to 0.9 mm leads to significant changes in DSC performance. For per 0.1 mm increase in thickness, there is an average change of 14.25 mm in slump, 11.35 kg/m3 in dry density, 0.90 MPa in compressive strength at 28 d, 15.06 kg/m3 in CO2 emission, 85.3 MJ/m3 in energy consumption, and 1.2 USD/m3 in cost. The designed DSC achieved a relatively low cement intensity (CI) value of 6.23 kg/m3/MPa , implying that DSC exhibits good eco-efficiency. Therefore, the proposed hybrid design approach can guide the design of engineered DSCs with excellent mechanical properties as well as environmental and economic performance. [ABSTRACT FROM AUTHOR]

Considering CO2 emissions during production and re-mineralization during hardening, natural hydraulic lime (NHL) has been deemed an ideal cementitious material based on CO2 cycling. However, the preparation of NHL has rarely been reported. In this study, NHL clinker was prepared by calcining calcite tailings, and NHL was obtained by digesting the clinker. The results showed that the performance of the prepared NHL when calcining the calcite tailings at 1,200°C for 1 h and digesting the clinker at 20°C±2°C and 70%±5% RH (relative humidity) for 8 days met the requirements of NHL5 standard lime mortar. Furthermore, carbonation of the prepared NHL5 was studied, showing that the pore size of the hardened NHL5 mortar was refined during carbonation, and its compressive strength also improved. The prepared NHL5 could be an environmentally friendly cementitious material owing to its utilization of waste, relatively lower energy consumption, and higher CO2 recycling rate. [ABSTRACT FROM AUTHOR]

Emission of carbon dioxide (CO2) either from the firing of clay bricks or from cement production, contributes considerably toward global warming. Conversely, the production of bricks is inevitable since a large number of bricks are needed to fulfill the housing sector demand. In this study, silty clay-based geopolymer bricks were produced incorporating fly ash and sugarcane bagasse ash. This was accomplished in two stages: the laboratory phase that comprised of production of cylindrical specimens, and the industrial phase whereby full-size bricks were produced based on the results obtained in the laboratory phase. The silty clay-based geopolymer bricks were developed with lesser energy input, i.e., forming pressure of 7 MPa with curing at ambient temperature. The whole set of mechanical and durability properties of the newly developed geopolymer brick yielded satisfactory results conforming to the standard codes. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results revealed the coexistence of sodium aluminosilicate gel (N─ A─ S─ H) and calcium aluminosilicate hydrate (C─ A─ S─ H), which led to a dense microstructure resulting in increased mechanical strength and ensuring enhanced durability of the brick structure. The environmental impact assessment confirmed the ecofriendly utilization of sugarcane bagasse ash in combination with fly ash in clay-based geopolymer bricks. The newly developed geopolymer can have a broad range of applications, including wall panel making, jet grouting, deep mixing, mortar for masonry constructions, canal lining, and grouting material used in backfill grouting during shield tunneling. [ABSTRACT FROM AUTHOR]

Concrete with coal gangue as aggregate has a self-sensing ability with addition of carbon fiber (CF), which has an outstanding effect in terms of dealing with the large amount of coal gangue discharge and monitoring the concrete structure. However, studies on the development of CF-based gangue concrete (CF-GC) with self-sensing capabilities are limited. Therefore, the uniaxial compression tests of CF-GC were carried out and changes in the electrical resistivity (ER) and acoustic emission (AE) characteristics were monitored throughout testing. The test results suggested that the addition of CF can significantly improve the electrical conductivity properties of gangue concrete. Value of gage factor for CF-GC and gangue concrete is 2.3 and 16.6, which the value of gage factor for the CF-GC increased by 14.3 compared with the GC. The research showed a good correlation between the ER and AE of CF-GC, and an increase in curing time led to a decrease in in the electrical conductivity of CF-GC. This study shows that it is of great significance to be able to predict deformations in gangue concrete structures by monitoring changes in ER and AE. [ABSTRACT FROM AUTHOR]

Peat is geotechnically problematic due to high organic content leading to low strength, high compressibility, high rate, and magnitude of creep. The nontraditional materials like a geopolymer can be used to stabilize the peat due to its low CO2 emission and high strength gain than traditional materials like cement. Therefore, this study presents the viability of rice husk ash (RHA)–based geopolymer to stabilize the Indian peat with a wide range variation of organic content (OC) (21%–79%) and fiber content (FC) (6%–73%). The geopolymer was prepared by adding extra aluminum oxide (Al2O3) with RHA to maintain the atomic weight of silica to alumina (Si/Al) equal to 3. The specimens with binder (RHA+Al2O3) percentages of 10%, 20%, and 30% by weight of dry peat and alkali/binder ratios of 0.5, 0.7, and 0.9 were activated by sodium hydroxide (NaOH) with molarities (M) of 3, 6, and 9. The results show that the RHA-based geopolymer stabilized peat depends on various factors like the amount of organic content in peat, amount of binder content, concentration of activator, A/B ratio, curing period, and pH. The optimum combination of binder content, concentration, and A/B ratio was found as 20%, 6 M, and 0.7 respectively. At this combination, the unconfined compressive strength (UCS) of treated peat was found 133, 125, and 136 times more than untreated sapric, fibric, and hemic peat, respectively. Further, it was observed that UCS decreases with OC and increases with the curing period The field emission scanning electron microscopy (FESEM) micrograph and X-ray diffraction (XRD) patterns confirm that the pore spaces are filled with cementitious products like zeolite which is further represented by a reaction mechanism. [ABSTRACT FROM AUTHOR]