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This paper characterizes the efficacy of borax on different properties of alkali-activated materials (AAMs). Fly ash (FA) and ground granulated blast slag (GGBS) were used as base substances at different ratios. The effects of borax on workability, compressive strength, efflorescence formation and crack resistance are the main ideas of the study. The results indicated the positive effects of borax on the properties of one-part fly ash/slag-based alkali-activated materials such as increasing workability and decreasing efflorescence. The microscopic images and scanning electron microscope (SEM) showed that the addition of borax reduced the cracks and a higher amount of slag made the samples denser with less unreacted raw materials, respectively. The compressive strength test results showed that borax had a detrimental impact at early ages with a positive effect at later ages. Binders with borax and 20-25% replacement of FA with GGBS were found to have the best performance when workability, compressive strength, efflorescence formation, and cracks on the samples were taken into account. [ABSTRACT FROM AUTHOR]

Aiming at the problems of low mechanical properties and poor water resistance of hardened hemihydrate phosphogypsum, magnesium oxysulfate cement was used to modify hemihydrate phosphogypsum. The effects of magnesium oxysulfate cement content on setting time, mechanical properties and water resistance of hemihydrate phosphogypsum were investigated. The mechanism of magnesium oxysulfate cement modified hemihydrate phosphogypsum was analyzed from a microscopic point of view. The results show that adding magnesium oxysulfate cement significantly delays the final setting time of hemihydrate phosphogypsum and ensures construction operation time of hemihydrate phosphogypsum. Sulfate ions that dissolved and ionized in hemihydrate phosphogypsum participate in the hydration process of magnesium oxysulfate cement, forming a stable 517 phase filled in dihydrate gypsum, resulting in a great improvement of mechanical properties and water resistance of hemihydrate phosphogypsum. [ABSTRACT FROM AUTHOR]

Recycling of phosphate tailings (PT) in cement production can reduce the amount of this solid waste and its consequent pollution. This paper used high content of calcined PT as an alternative to light-burned magnesia to prepare magnesium oxysulfate (MOS) cement. Thermal decomposition properties of PT at calcination temperature of 600°C–900°C were studied with X-ray diffraction (XRD) and scanning electron microscopy (SEM). Compressive strength, XRD, Fourier-transform infrared spectroscopy (FT-IR), and SEM were conducted to study the effects of the calcination temperature of PT (600°C–800°C) and the dosage of calcined PT (50%–100%) on the strength development and hydration mechanism of MOS cement. Results showed that the main component of PT, dolomite, partially decomposed into magnesium calcite, periclase, and calcite at 700°C; decomposed further into calcite, periclase, and lime at 800°C; and finally decomposed into lime and periclase at 900°C. The periclase sourced from calcined PT reacted with the MOS system to form brucite and 5Mg(OH)2·MgSO4·7 H2O (517 phase). Gypsum also was formed in the reaction. Brucite, 517 phase, and gypsum worked with the unreacted PT to build the structure and provide the source of strength. MOS paste prepared with 60% PT calcined at 700°C had the highest compressive strength, 63 MPa, at 28 days of all the mixtures. The paste prepared with 50% PT calcined at 700°C had the highest strength retention value, about 68%, after 56 days of water immersion. This study provides a method for the efficient use of PT by incorporating it into MOS cement after calcination. [ABSTRACT FROM AUTHOR]

This study investigates the mechanical behavior of cement-stabilized dredged sediment (CDS) improved by nano-MgO (NM) and polypropylene fiber (PF). The effects of NM content, PF content and PF length on the mechanical behavior of NM-modified and PF-reinforced CDS (CNPDS) were evaluated by conducting unconfined compressive strength (UCS, qu) and splitting tensile strength (STS, qt) tests. Furthermore, optical microscope scans were conducted to detect the interface friction evolution. The results indicated that adding NM can significantly improve UCS and STS of CDS, and the optimum NM content was 1.2%. The optimum PF content tended to increase with increasing curing age, and the 3 mm-length PF was the most effective in improving UCS and STS. Increasing NM content mainly increased the strength, but had little influence on ductility. The inclusion of PF can effectively improve the ductility of CNPDS, and contributed to its tensile strain-hardening characteristic. The relation of q t = 0.23 q u was obtained regardless of other influence factors. The combination of NM-modification and PF induced "bridging" effect provided a coupling effect inside CNPDS, and a concept model representing this process was established. The key findings in this study can be used as an effective method for dredged sediment stabilization in practical engineering. [ABSTRACT FROM AUTHOR]

The strength, durability and swelling behavior of cement-solidified dredged sludge (CDS) has been attracted more and more attention in recent years. This study performed a series of laboratory tests including unconfined compressive strength (UCS), water absorption, swelling tests and pH to investigate the effectiveness of using fly ash (FA) and nano-SiO2 (NS) blended with Portland cement (PC) for DS solidification. Microstructural properties were also analyzed by conducting scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The results showed that the addition of FA and NS can significantly improve the UCS of CDS. Using FA and NS together as PC admixture can effectively improve the water resistance. Compared with FA, NS was more effective to reduce the water absorption and swelling of CDS. Adding 0.9% NS to CDS notably reduced the 7-day water absorption and swelling value by up to 40% and 67.5%, respectively. Furthermore, the addition of NS is beneficial to control the pH level of CDS. The SEM and XRD results showed that CSH and CAH gels were the major cementitious products inside solidified DS. The addition of NS contributed to the formation of additional CSH gels in CDS. [ABSTRACT FROM AUTHOR]

This study investigated the feasibility of using lime (L) and carbide slag (CS) to activate ground granulated blast-furnace slag (GGBS), as a potential substitute to Portland cement (PC) for stabilizing dredged sludge (DS). A series of unconfined compressive strength (UCS), scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests were performed to evaluate the strength development and micro-mechanisms of stabilized DS. The results showed that the optimum lime content was 20-25% for L-GGBS stabilized DS, which can achieve the highest UCS. The 28-, 60- and 90-day optimum UCS of stabilized DS with 20% L-GGBS were 1.38, 1.14 and 1.06 times that of 15% PC stabilized DS. Similarly, the optimum CS content of 25-30% was recommended for achieving the highest UCS of CS-GGBS stabilized DS. The 28-, 60- and 90-day optimum UCS of 20% CS-GGBS stabilized DS were 1.45, 1.12 and 1.02 times that of 15% PC stabilized DS. It was suggested using 20% L-GGBS or CS-GGBS instead of 15% PC for stabilizing DS. The SEM and XRD results revealed that the CSH and CAH gels were the main hydration products for stabilized DS, and the ettringite was also obviously detected in CS-GGBS stabilized DS. [ABSTRACT FROM AUTHOR]

Agricultural by-products are increasingly used in the production of bio-based concrete. Compaction forming technology was used in this study to develop a sustainable biocomposite made from corn stalk (CS) and magnesium phosphate cement (MPC). The goal was to experimentally evaluate the effect of forming pressure (FP), CS content, and fly ash (FA) content on the apparent density, compressive strength, and thermal conductivity of CS-MPC biocomposite (CMB). Furthermore, the microstructure changes of typical CMB samples were analyzed by conducting scanning electron microscopy (SEM) tests. The results indicated that the apparent density and thermal conductivity of CMB were less influenced by the FP, while the compressive strength can be effectively improved by increasing FP. The increase of CS content from 25% to 45% led to a 47% reduction in thermal conductivity and a significant decrease of compressive strength. The inclusion of FA had little influence on the apparent density, but resulted in 9.48% reduction in thermal conductivity by adding 20% FA. The optimum FA content of 10% was recommended as MPC admixture, which can increase compressive strength of CMB by 4.17%. The linear empirical model for predicting thermal conductivity by apparent density was established. The microstructure changes inside CMB showed the bonding properties between MPC and CS, and the corresponding micromechanisms were analyzed. The findings provide a basic theory for using CMB as thermal insulation material in buildings. [ABSTRACT FROM AUTHOR]

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• Physical and compressive strength properties of FRMC mortars were analyzed. • Addition of 0.6% and 0.8% fibres improved the physical and CS characteristics. • Air cured specimens exhibited less mass loss in abrasion test than static cured in water. • XRD and SEM corroborated with the observed findings of strength loss in water regime. Fibre reinforced magnesium phosphate cement (FRMPC) composites are drawing attention day by day in the practical applications due to their excellent strength performance. Keeping this issue in mind, this study aimed to add a little contribution on this area by examining the physical and strength properties of FRMPC mortars containing micro-steel fibre (MSF), polyvinyl alcohol fibre (PVA) and basalt fibre (BF). Each fibre content with consecutive four dosages such as 0.6%, 0.8%, 1% and 1.2% of the total quantity of binders and aggregate, were added in the designated combinations. The analyzed results exhibited that porosity, pore degree of saturation, reduction of permeable voids, density and water absorption properties were well improved for adding 0.6% and 0.8% MSF and PVA, and 0.6% BF fibres in the matrices, whereas the rest two selected higher fibre contents made the microstructure of MPC specimens sponginess by forming the substantial quantity of internal pores. Mass loss was recorded about 0.6%−1% for air cured FRMPC samples at 28 d by adopting abrasion test, where the static immersion liquid condition revealed around 1.5% − 3.0%. In addition, air cured samples containing 0.8% MSF showed the highest compressive strength around 54.8 MPa and 82.6 MPa at 1hr and 28 d, respectively than other considered combinations. Moreover, FRMPC syntheses exposed around 10% − 15% strength loss in water environment as compared to air. SEM observations presented the well interfacial closeness of MSF by coating the hydration products that probably enhanced the noteworthy strength quality of MSF-MPC mortars. XRD investigations also corroborated the possible explanation for reducing the strength loss in water regime by presenting the low peaks of struvite minerals, which was happened due to the dissolution of mass that accorded with the experimental results. These findings might show a path for potential use of FRMPC specimens to enhance the durability properties. [ABSTRACT FROM AUTHOR]

This work addresses the preparation of self-flowing lightweight aggregate foamed concrete (LAFC) resulting from the mixing of stable foam, expanded clay aggregate (ECA) and silica fume (SF) as main raw materials. Formulated LAFC mixtures were tested for the mechanical, thermal, sorption and high-temperature resistance properties. Results showed that the increase in the volume of foam from 0% to 20% considerably improved the flowability of LAFC and LAFC mixtures were categorized into high stable visual stability index. Compressive strength of LAFC mixtures was reduced from 24.75 MPa to 21.10, 15.95 and 12.05 MPa respectively when foam volume was increased from 0% to 10, 15 and 20%. Thermal conductivity of LAFC mixtures was 0.84, 0.77, 0.65 and 0.53 Wm−1K−1 for the 0, 10, 15 and 20% foam volume respectively. The addition of SF enhanced the strength and thermal conductivity of LAFC mixtures. Sorption coefficient and porosity of LAFC were gradually increased from 0.22 to 0.85 kgm−2min−0.5 and 25.68–39.74% by increasing the volume of foam from 0% to 20%. However, incorporation of SF reduced the sorptivity and porosity of LAFC. Effect of high temperature was more prominent on the compressive strength of LAFC as compared to the density of LAFC. [ABSTRACT FROM AUTHOR]

• Different percentages of red mud were added in UHPC. • 20% replacement of red mud was preferable with respect to pumpability and buildability. • Upcycling the red mud in UHPC is an energy-saving and low-cost strategy to address disposal of solid waste. This paper introduces a new application of industrial solid waste red mud to use as a cementitious material in the development of ultra-high performance concrete (UHPC) performances. In addition, utilization of red mud can reduce the environmental impact of red mud landfill. For gaining this aims, UHPC specimens were prepared with different contents of red mud and analyzed the effects of different RM contents on the fluidity, setting time, mechanical properties, durability, drying shrinkage and microstructure of the red mud-based UHPC. The results indicated that fluidity was reduced to 310 mm at 20% red mud level, but it still belongs to enormous fluidity, which is convenient for construction. In addition, it is worth noting that using red mud instead of cement can significantly shorten the setting time, providing a new method for applying the UHPC rapid-repairing process. Red mud-based UHPC mortar prepared with appropriate red mud instead of cement exhibits higher mechanical properties, better durability and reduced drying shrinkage of UHPC during curing. After curing for 28d, the highest compressive strength of UHPC samples containing 20% red mud was 129.5 MPa, the compressive strength of the UHPC samples' sulfate resistance was 136.6 MPa, and the drying shrinkage value was around 414 µm. Moreover, observed information from scanning electron microscopy (SEM) noted that red mud can fill the micro-cracks between particles of cementitious materials, make the structure more compact and form new hydrates. Finally, the two characteristics of low cost and energy consumption further promote the sustainable application of red mud-based UHPC. Therefore, replacing OPC with 20% RM is meaningful to produce a sustainable UHPC mixture and field applications. [ABSTRACT FROM AUTHOR]

It is generally known that the physical and mechanical properties would be greatly degraded after water or moisture transferring into the foamed concrete. In this study, a foamed concrete with a low density of about 550 kg/m 3 is prepared using ordinary Portland cement, and three types of water repellents including potassium trimethylsilanolate (PT), calcium stearate (CS) and siloxane-based polymer (SP) are employed to decrease the water absorption of the foamed concrete. The effects of the water repellent on the mechanical and physical properties of the foamed concrete, such as 7-day and 28-day compressive strength, thermal conductivity, sorptivity and hygroscopicity, are studied. The laboratory results indicate that the water repellents improve the compressive strength to some extent without sacrificing the thermal insulation property of the foamed concrete. The sorptivity evaluated by 48-h water absorption and strength retention coefficient ( R S ) is significantly improved as increasing the content of water repellent. When 1.0% SP is used, the water absorption and R S value of the foamed concrete with 28-day strength of 1.77 MPa and thermal conductivity of 0.150 W/m K are 2.5% (by volume) and 0.989, respectively. In addition, the contents of hygroscopic moisture [ W ( φ )] also decrease with the increasing content of water repellent. The hygroscopic fitted curves with high coefficients of determination obtained the KUM and CUB models have been proved applicable in exploring the relationship of the W ( φ ) to the relative humidity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Compressive strength prediction of one-part fly ash slag based AAMs. • Evaluation of contribution of ingredients in compressive strength development of one-part AAMs. • SHAP dependency analysis for interpretation of machine learning model results. In recent years, alkali activated material (AAM) or geopolymer has emerged as a sustainable and eco-friendly alternative to cement. It is owing to its low power consumption and greenhouse gas emissions, as well as good mechanical and durability features. However, due to the nature and diversity of available source materials, developing an AAM mix to attain desirable fresh properties, sufficient strength characteristics, and touted environmental benefits is quite challenging. It demands a precise selection of input material and mix proportions based on several trials, which requires a large quantity of material, time, and effort. Therefore, employing machine learning techniques could facilitate and accelerate the development of one-part AAM binder with the desired properties. This study evaluates the performance of various machine learning models (Ridge regression, RF, LightGBM, and XGBoost) for accurate compressive strength prediction of one-part AAM binder. Extreme Gradient Boost (XGBoost) outperformed all other algorithms in terms of prediction efficacy and accuracy. In addition, SHapley Additive exPlanations (SHAP) is also used to interpret the predicted compressive strength through XGBoost and the effect of various parameters, independently and in relation with other parameters, is evaluated and discussed in detail. The interpretable ML strategy used in this study will aid in the production and performance tuning of durable and sustainable one-part AAMs for widespread applications. [ABSTRACT FROM AUTHOR]

Phosphate tailings (PT) and acid water, by-products of phosphorus chemical industrial chain, are usually identified as waste because they can hardly be reused in cement production. This paper proposes an approach of recycling solid waste PT and acid wastewater in preparing a high value-added magnesium oxysulfate (MOS) cement. Results show that the phosphorus in acid water promotes the formation of 5 Mg(OH) 2 ·MgSO 4 ·7H 2 O (517) phase, thereby improving compressive strength of MOS cement. MOS cement prepared with undiluted acid water present excellent resistance to water and acid. The incorporation of calcined PT lowers the strength of MOS cement but improves the volume stability. The phosphorus and fluorine in acid water are effectively immobilized by the cement matrix. Moreover, recycling of PT and acid water to produce MOS cement not only largely consumes the waste and benefits the environment, but also reduces the cost of waste disposal and cement production. [Display omitted] • Phosphate tailings and acid wastewater are recycled to prepare a high value-added MOS cement. • The phosphorus in acid water promotes the formation of 517 phase in MOS cement. • MOS cement with undiluted acid water shows high compressive strength and excellent resistance to water and acid. • The phosphorus and fluorine in acid water are effectively immobilized by MOS cement. [ABSTRACT FROM AUTHOR]

This paper presents an investigation on a new material composed of magnesium phosphate cement (MPC), fly ash, and rape stalk. The main goal is to evaluate the viability of using this new material as an insulation material for buildings. Several composites with different weight concentrations and three different rape stalk sizes were prepared. The density, compressive strength, and thermal conductivity of the material were measured. Experimental results show that the proposed MPC rape stalk concrete possesses required material properties for production of lightweight concrete in structural and non-structural applications. The physical properties of the rape stalk indicate that it is a promising alternative to hemp hurd for producing plant concrete. Results also indicate that MPC might be a better cementitious material for producing plant aggregate concrete compared with portland cement or lime. [ABSTRACT FROM AUTHOR]

The fire-resistance of magnesium oxysulfate (MOS) cement is important when it is serviced for the structures exposed to high temperatures. This paper investigated the effect of material molar ratios on properties of MOS cement during high temperature exposure. Compressive strength, X-ray diffraction, thermogravimetric analysis and scanning electron microscopy were conducted to study strength development and phase changes of MOS cement. Results showed that thermal stability of MOS cement was mainly attributed to thermal decomposition behavior of 5 Mg(OH) 2 ·MgSO 4 ·7H 2 O (517) phase and brucite. For MOS cement with a constant H 2 O: MgSO 4 molar ratio, the increase of a-MgO: MgSO 4 molar ratio lowered the proportion of 517 phase but improved compressive strength. Before 200 °C, needle-like 517 phases interlaced with each other and worked with healed cracks and pores by thermal expansion to densify the microstructure. MOS specimens all presented a higher compressive strength at 200 °C, and the highest strength value was over 150 MPa. After 300 °C, the dehydrated 517 phase and brucite resulted in a loose structure as well as a strength reduction. • Strength development and phase changes of MOS cement subjected to high temperature were studied. • Thermal stability of MOS cement was attributed to decomposition behavior of 517 phase and brucite. • The 517 phases became short and slender before 200 °C and tended to be flaky at over 300 °C. • Compressive strength of MOS cement with molar ratios of 12:1:20 and 15:1:20 was 150 MPa at 200 °C. [ABSTRACT FROM AUTHOR]

Abstract: In China, accidental fires are known to occur during construction, causing concrete to be exposed to high temperatures when it is at an early age (i.e. “young”). In this paper, compressive and splitting tensile strengths of concretes cured for different periods and exposed to high temperatures were obtained. The effects of the duration of curing, maximum temperature and the type of cooling on the strengths of concrete were investigated. Experimental results indicate that after exposure to high temperatures up to 800°C, early-age concrete that has been cured for a certain period can regain 80% of the compressive strength of the control sample of concrete. The 3-day-cured early-age concrete was observed to recover the most strength. The type of cooling also affects the level of recovery of compressive and splitting tensile strength. For early-age concrete, the relative recovered strengths of specimens cooled by sprayed water are higher than those of specimens cooled in air when exposed to temperatures below 800°C, while the changes for 28-day concrete are the converse. When the maximum temperature exceeds 800°C, the relative strength values of all specimens cooled by water spray are lower than those of specimens cooled in air. [Copyright &y& Elsevier]

Expanded polystyrene (EPS) concrete is a lightweight, low strength material with good energy-absorbing characteristics. However, due to the light weight of EPS beads and their hydrophobic surface, EPS concrete is prone to segregation during casting, which results in poor workability and lower strength. In this study, a premix method similar to the ‘sand-wrapping’ technique was utilized to make EPS concrete. Its mechanical properties were investigated as well. The research showed that EPS concrete with a density of 800–1800 kg/m3 and a compressive strength of 10–25 MPa can be made by partially replacing coarse and fine aggregate by EPS beads. Fine silica fume greatly improved the bond between the EPS beads and cement paste and increased the compressive strength of EPS concrete. In addition, adding steel fiber significantly improved the drying shrinkage. [Copyright &y& Elsevier]

• An artificial intelligence-based gene expression programming approach was used for modelling the performance of bio-composites. • Mathematical models were proposed for density, compressive strength and thermal conductivity of bio-composites. • Performance of the proposed mathematical models was validated using statistical parameters and performance indices. • The proposed models depicted high degree of generalization capability and predictability. Giving the high amount of carbon and energy emission from the use of traditional building materials, the use of bio-composites made from industrial crops especially hemp has caught attention from researchers in recent years. These bio-composites not only enhance the thermal performance of buildings but also promote sustainable development due to their eco-friendly nature. Due to their highly heterogeneous nature, however, most of the existing studies on the bio-composites have only focused on experimental investigations, while mathematical modeling of physical, thermal and mechanical properties of bio-composite remains a challenge for the researchers. In this paper, an artificial intelligence (AI) based gene expression programming (GEP) technique is used to develop the mathematical models for predicting the dry density, compressive strength and thermal conductivity of hemp-based bio-composites. A large amount of database was established based on past studies and the most influential parameters were identified by several trial analyses. The proposed mathematical models showed a high correlation with the experimental results. All the models passed the statistical and performance index checks showing strong predictability, generalization capability and high accuracy of GEP-AI models. Comparison of results with the regression analysis techniques further proved the superiority of GEP-AI models over the traditional methods. [ABSTRACT FROM AUTHOR]

• UHPC specimens were prepared incorporation with BF, PPF and GF. • Compressive strength was increased for 0.5% inclusion of micro-fibres. • Microstructures presented the well interfacial relation between the fibres and hydration products. • The results of the current test can be applied for ultra-high-performance (UHPC) panels and vibration-free UHPC material. The research aims to enhance the ductility characteristics of ultra-high performance concrete fibre material (UHPC-FM) comprising with different micro-synthetic fibres such as basalt fibre (BF) polypropylene fibre (PPF) and glass fibre (GF). As a sequence, consecutive dosages of these fibres such as 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% of volumetric fractions were introduced into the mixes for preparing the UHPC-FM to achieve the goals. The observed results showed that both the type and amount of added fibres influenced on the properties of fresh and hardened UHPC-FM concretes. In addition, higher contents of fibre filament (FFs) resulted a decreasing trend in flow of fresh UHPC-FM mixtures, where pastes with PPF demonstrated the highest fluidity property. Moreover, compressive strength (CS) increased upon addition of 0.5% FFs and linearly decreased for introducing the higher amounts. Compared with the control specimen (UHPC-A0), the flexural strength of samples containing 2.5% BF, PPF and GF increased by 20.8%, 26.9%, and 27.9%, respectively. Modulus of rupture increased by 20.04% in case of BF, 24.92% for PPF and 26.05% for GF when compared with reference sample (8429.5-kN). Toughness indexes of BF, PPF and GF based UHPC-FM samples were around 4.64, 4.75 and 4.86 times higher respectively, in comparison to the control specimens. The microstructure observation and drying shrinkage test illustrate fiber can modify UHPC well, which corroborated the results of mechanical properties exactly. In addition, the results of the current test can be applied for ultra-high-performance (UHPC) panels and vibration-free UHPC material. [ABSTRACT FROM AUTHOR]

• High content of untreated PG is used as cementitious component for grouting materials. • The setting time and flowability of PG-based grouting slurry are investigated. • Strength, permeability and water-resistance along with microstructure are analyzed. The industrial by-product phosphogypsum (PG) is one of the bulk solid wastes with the risk of environmental pollution. Recycling PG as filling and binding material in preparing grouting materials is not only an effective solution to the PG disposal, but also a new approach to replacing the conventional cement-based grouts that conforms to the sustainable development. The flowability, setting time and early strength are vital characteristics for the grouts. Water-resistance should also be considered in underground construction. This paper works on the workability of the PG-based grouting slurry and the mechanical strength of the hardened paste. The water-resistance and permeability of the grouts are investigated. XRD analysis and SEM observation are used to study the hydration products and the microstructure. Experimental results show that the increase of water/binder ratio and PG dosage leads to a longer setting time and a higher flowability. The grouts with superplasticizer dosage of <0.8% and accelerated agent dosage of 0.3 ~ 0.5% obtain an ideal final setting time and flowability for grouting construction. The addition of OPC and silica fume improves both the compressive strength and the water-resistance of grouting materials. Microstructure observation shows that the formation of the ettringite and C-S-H gel is responsible for the early strength. The filling effect and pozzolanic reaction of fly ash and silica fume contribute to the compactness of the structure. [ABSTRACT FROM AUTHOR]

The strength, durability and swelling behavior of cement-solidified dredged sludge (CDS) has been attracted more and more attention in recent years. This study performed a series of laboratory tests including unconfined compressive strength (UCS), water absorption, swelling tests and pH to investigate the effectiveness of using fly ash (FA) and nano-SiO2 (NS) blended with Portland cement (PC) for DS solidification. Microstructural properties were also analyzed by conducting scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The results showed that the addition of FA and NS can significantly improve the UCS of CDS. Using FA and NS together as PC admixture can effectively improve the water resistance. Compared with FA, NS was more effective to reduce the water absorption and swelling of CDS. Adding 0.9% NS to CDS notably reduced the 7-day water absorption and swelling value by up to 40% and 67.5%, respectively. Furthermore, the addition of NS is beneficial to control the pH level of CDS. The SEM and XRD results showed that CSH and CAH gels were the major cementitious products inside solidified DS. The addition of NS contributed to the formation of additional CSH gels in CDS. [ABSTRACT FROM AUTHOR]

• Increasing slag content increase the early compressive strength but reduced flexure strength and workability. • Surficial cracking is induced if slag content is high or retarder content is reduced. • Increase in curing temperature led to significant improvement in early strength. • Later age strength was somewhat independent of curing temperature. • Microstructural testing affirms that heat curing does not significantly alter the structure of mix but only accelerates the rate of reaction. One-part Alkali activated material (AAM) is eco-friendly replacement of OPC with adequate strength and durability characteristics. In this study, the impact of slag content, activator dosage, retarder content and curing temperature was evaluated on fresh and early-age mechanical properties of one-part fly ash/slag-based alkali activated mortar. For assessing the effect of curing temperatures, three temperatures were selected: 20 °C low ambient temperature, 35 °C high ambient temperature and 65 °C replicating heat curing. Consistency, setting time, mix temperature and pH measurements were recorded for fresh mix using standard procedures. In addition, hardened mortar specimens were tested for compressive and flexure strength in conjunction with microstructural characteristics (SEM, XRD, and FTIR analysis). The test results depicted that among all parameters, curing temperature governed the development of early age strength of one-part AAM mortar. Microstructural testing confirmed the enhancement of the geopolymerization reaction through heat curing thus leading to a substantial increase in the early strength. [ABSTRACT FROM AUTHOR]

• Lower w/b ratio increased compressive strength, but lowered workability and caused surficial cracks. • A w/b ratio of 0.3 resulted in the highest strength without surficial cracking. • Both the mechanical strength and workability were significantly improved with the addition of the borax. • A borax dosage between 4 and 6% was found to be most appropriate in terms of both strength and workability. One-part alkali-activated fly ash/slag binders have been promoted as an environmentally friendlier alternative to Ordinary Portland Cement (OPC). However, its tendency towards flash setting and high sensitivity of the mixture's strength to water content have been sources of major concern in the industry. This study investigates the effects of different w/b ratios, admixtures (such as superplasticizers and/or retarders) and the dosage of the selected admixture on the fresh and hardened properties of one-part alkali activated fly ash/slag based binders. To evaluate the fresh properties of binders, setting time and flowability tests were carried out. The compressive and flexural strength tests were conducted to determine the mechanical strength of the samples. The mineralogy and microstructure of the samples were studied using X-ray diffraction (XRD) and scanning electron microscope (SEM) in conjunction with energy dispersive spectroscopy (EDS) which was used to observe the elemental composition. Additionally, an optical microscope was also used to study the surficial physical properties of the hardened binders. The experimental results showed that an increase in w/b ratio led to an improvement in fresh properties of binder but significantly decreased the mechanical strength. A w/b ratio of 0.3 was deemed to be optimal in this study because at this ratio the hardened samples showed no surficial cracking and had enough compressive strength to be viable as a construction material. Test results also showed borax to be superior to all the other admixtures tested when both mechanical strength and workability are considered. [ABSTRACT FROM AUTHOR]

Highlights • Use of steel slag powder in magnesium phosphate cement (MPC) increased its water resistance. • Significant increase in the compressive strength of MPC mortar specimens with addition of steel slag powder. • Formation of new hydration products which supported the improvement mechanism of steel slag powder. Abstract Magnesium phosphate cement (MPC) belongs to the family of chemically bonded phosphate ceramics and exhibits certain outstanding qualities. The current study investigated the effects of steel slag powder (SSP) on the properties of MPC mortar. MPC mortar specimens with different content of SSP were prepared and their workability, mechanical properties and water resistance were tested. Moreover, microstructure and phase composition of MPC paste were examined by Scanning Electronic Microscopic (SEM) and X-ray diffraction (XRD) techniques. It was found that the incorporation of SSP can promote early hydration rate. A significant improvement in the later stage mechanical strength was observed but early strength was reduced. Water resistance of MPC mortar specimens was improved by increasing the percentage of SSP from 0% to 30%. Finally, XRD and SEM analysis of MPC pastes corroborated with the mechanical strength results and confirmed the formation of secondary reaction products along with the main hydration products struvite and residual magnesia. [ABSTRACT FROM AUTHOR]

The study of dynamic impact properties of Mg–Gd alloy is important for its application in aerospace, automotive manufacturing and weaponry fields. In this work, the dynamic behaviors of the casting solid-solution (T4) and peak-aged (T6) Mg-6.5Gd-3Y-0.5Zr alloys were investigated at different strain rates by using optical microscopy(OM), scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), differential scanning calorimetry(DSC), X-ray diffraction(XRD), transmission electron microscopy(TEM), Vickers hardness testing, and split Hopkinson press bar(SHPB). Due to the dynamic precipitation phase during the SHPB and the residual rare earth hydride(REH) particles produce in the T4 treatment, the compressive strength of the casting T4-sample can reach 485 MPa at a strain rate of 3500 s−1. The casting T6-alloy precipitated a reinforced phase β′ and therefore exhibited an ultra-high compressive strength of 519 MPa at a high strain rate of 4500 s−1. The dynamic compressive strength of a T4 sample is unaffected by strain rates, whereas the dynamic compressive strength of a T6-sample is sensitive. Based on fracture morphology, the cracks are made up of a 45° loading direction crack on the cylindrical surface and a compressed surface crack. The fracture surface is characterized by cleavage planes and dimples. As the deformation strain-rate increases, the number of cleavage planes increases, while the proportion of dimples decreases. Under varied strain rates, it can likewise be deduced that cleavage fracture is the main fracture mechanism of the T6-alloy. • The dynamic compressive strength of T4 sample is not sensitive to strain rates, while that of T6 sample is sensitive. • The dynamic precipitation phases and rare earth hydrides synergistically improve impact resistance of the T4 alloy. • The precipitated reinforcing phase β′ accounts for the ultra-high compressive strength of T6 alloy at high strain rate. [ABSTRACT FROM AUTHOR]

Highlights: [•] A new type of lignosulfonate-based graft copolymer OLSAI was studied by single factor analysis and orthogonal experiment. [•] The OLSAI has good retarding ability, thermal-resistant properties and little impact on the strength of MPC. [•] The comprehensive evaluation indicates that the OLSAI will enable MPC to be applied to high-CO2 oil-well cementing. [ABSTRACT FROM AUTHOR]

[Display omitted] • Vegetal concrete was exposed to extreme weather conditions. • Red mud and fly ash were added to enhance the sustainability of concrete. • Loss in mass and compressive strength of vegetal concrete was measured. • Aging of vegetal concrete does not affect thermal properties of vegetal concrete. This paper studies the durability properties of vegetal concrete exposed to extreme environmental conditions including immersion weathering, outdoor weathering, salt attack, and freeze-thaw attack. Magnesium phosphate cement (MPC) binder containing high percentage of red mud and fly ash was mixed with the corn stalk based vegetal aggregates. It was found that all samples exposed to immersion weathering, outdoor weathering, and freeze-thaw attack showed a loss in mass, reduction in sample area, and decrease in compressive strength, which was associated with the dissolution and leaching out of the binder. Increase in the mass and swelling was observed for the vegetal concrete mixtures immersed into salt solution due to formation of salt layer on the surface of binder and vegetal aggregate. It was concluded that incorporation of fly ash, high compaction of samples, and pretreated aggregates improved the durability performance of vegetal concrete against extreme weather conditions. [ABSTRACT FROM AUTHOR]

The current study is taken an attempt to reuse the waste river sludge (WRS) with high moisture content in the production of cost-effective unfired green bricks. For this purpose, hemihydrate phosphogypsum (HPG) was innovatively applied to dehydrate WRS, and different consecutive dosages of Ordinary Portland cement (OPC), sodium metasilicate (SMS) and pozzolanic waste materials like ground granulated blast-furnace slag were mixed with WRS. The observed results indicated the excellent performance of unconfined compressive strength (UCS) of the brick specimens that was obtained around 31 MPa employing the combinations 6: 4 of WRS and HPG along with the 16% slag and 4% OPC. Water absorption resisting capacity and UCS of this hard mixture satisfied the requirements of the Chinese standard. Moreover, the scanning electron microscopic (SEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) corroborated the strength development properties of bricks through progressing the microstructures along with calcium silicate hydrate (CSH) and ettringite minerals formations. The result of this study indicated that the unfired green bricks comprising the mixtures mentioned above are economical viable. Furthermore, the utilization rate of waste materials in the optimal ratio can reach 96%. Therefore, the outcomes of this research might be a potential application in the field of sludge treatment for manufacturing the cos-effective green bricks. [ABSTRACT FROM AUTHOR]

• Application of nano-SiO 2 to cement-solidified dredged sludge (CDS) can significantly improve its strength development. • Roles of cement, nano-SiO 2 , humic acid, lime and curing age on the solidification effect were evaluated. • Using lime together with cement for solidifying dredged sludge containing humic acid had advantages over using cement alone. • The nano-SiO 2 content of 1.0% was considered the most cost-effective for improving the strength development of CDS. The disposal of dredged sludge (DS) poses an increasingly difficult problem for sediment dredging engineering. This study investigated the strength development of solidifying DS containing humic acid (HA) with cement, lime and nano-SiO 2 (NS). A range of unconfined compressive strength (UCS) and pH tests were conducted to explore the roles of cement, NS, HA, lime and curing age on the solidification effect of DS. Furthermore, microstructures and crystalline phases of typical mixes were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The results showed that the addition of NS can significantly improve the strength development of cement-solidified dredged sludge (CDS). The 60-day UCS of CDS with 1.0% NS was more than twice that without NS, concurrently, the addition of NS also reduced the pH of CDS. The HA seriously affected the strength development of CDS, and the influence threshold value of HA content was in the range of 4.5–5.0%. Using lime together with cement for solidifying DS containing HA had advantages over using cement alone, and the optimum mass ratio of lime to cement is 6:9. The optimum NS content of 1.0% was determined to be the most cost-effective for improving the strength development of CDS. The microstructure and mineralogy analysis confirmed that adding NS to CDS can effectively accelerate the hydration reaction and produce more calcium silicate hydrate (CSH), leading to a significant improvement in the strength of CDS. [ABSTRACT FROM AUTHOR]

Large amounts of coastal silt produced annually is urgent to be treated with a feasible strategy. This study converted it into subgrade soil by cement solidification for resource utilization. Biochar was used as exogenous additive for enhancing compressive strength of the product, simultaneously achieving carbon sequestration. Three biochars derived from peanut shells (PSBC), cow dung (CDBC) and sewage sludge (SSBC) at 300 °C, 500 °C and 700 °C pyrolysis, were added into raw materials with 1%, 2% and 5%, respectively. All biochars significantly improved the compressive strength of the subgrade soil by 20–110%. Biochar catalyzed cement hydration reactions to produce more Ca(OH) 2 , CaCO 3 and calcium silicate hydrates (C–S–H gel). The catalytic capacity of different biochars followed the order of SSBC > PSBC > CDBC. Addition of 2% SSBC500 induced the greatest increase in 28 d-strength from only 1.0 MPa–2.1 MPa, which was due to that 500 °C biochar had a suitable specific surface area and porosity. Biochar facilitated CO 2 capture (absorption) during the hydration reactions at the initial 48 h with 55–70 mg g−1. The high alkalinity and water holding capacity of biochar contributed to the absorption of CO 2 ; the high content of minerals in SSBC compared to CDBC and PSBC promoted chemical conversion of CO 2 to carbonate. Besides, the biochar itself as carbon rich material was encapsulated in the subgrade soil, which can be regarded as a long-term carbon sequestration strategy. Carbon budget analysis demonstrated that converting one ton dry silt into subgrade soil with addition of 2% biochar could increase CO 2 sequestration from 11 kg to 36–94 kg. This study proposes a novel strategy of using biochar to strengthen the subgrade soil simultaneously achieve long-term carbon sequestration. • Coastal silt was converted into subgrade soil by cement for resource utilization. • Biochar addition improved compressive strength of subgrade soil by 20–110%. • Biochar induced more CO 2 absorption during the initial 24-h hydration reactions. • Biochar catalyzed hydration reactions due to alkalinity, pores and water retention. • More Ca(OH) 2 , CaCO 3 and calcium silicate hydrates (C–S–H gel) were produced. [ABSTRACT FROM AUTHOR]

The earth-based construction with unique advantages of wide source of raw material and low carbon emission has attracted growing attention recently In South China, raw soils which have been used for hundreds of years are mainly organic clays with high contents of water and organic matter. This research uses reconstituted organic soil technique to prepare earth-based construction and investigate the key variables controlling compressive strength by measuring compressive strength ( q u ) and physical indexes including unit weight ( γ t ), water content ( w t ) and specific gravity ( G st ). The cementitious material used is a high-efficiency stabilizer named as CSCN, and organic clay is obtained by mixing humic acid powder (HAP) with raw clay. The presence of HAP is regarded as the consumption of CSCN, and total CSCN content ( C T ) can also be calculated. Firstly, a parameter taking into account the key variables is expressed as the ratio of void ratio to total CSCN content ( e t / C T ) and the relationship of q u to e t / C T is analyzed by a power function. Physical indexes of stabilized earth are dependent upon the initial physical properties and influenced by C T value, actual CSCN content, HAP content and curing time. And then, the calculation formulae for physical indexes are obtained by the multiple linear regression and power regression analysis by defining dimensionless ratios of the after-curing physical indexes to the initial ones. Ultimately, a strength-calculation model with a deviation between calculated strength and experimental results less than 10% is established based on e t / C T . The model is proved to be very useful for the design and preparation of earth-based construction with organic matter. [ABSTRACT FROM AUTHOR]

The earth-based construction with unique advantages of wide raw material origin, energy conservation and environment protection has attracted growing attention recently. Although the mechanical properties can be improved by adding a high-efficiency stabilizer named as CSCN, it is difficult to generalize the application of CSCN due to the relatively high cost of several chemical admixtures. In this study, fly ash is introduced, and its influences on mechanical and physical properties of earth-based construction are investigated. Judged from laboratory results, mechanical properties including compressive strength and secant modulus increase as stabilizer contents and curing time increase. Water content of stabilized earth decreases due to more significant cement hydration and pozzolanic reactions by adding more stabilizer. Additionally, unit weight increases slightly with the increase of stabilizer content and curing time. The relationships of mechanical and physical properties to curing time have been estimated by hyperbolic models, and the calculation formulae for strength, modulus and physical indexes are developed. The controlling parameters of the hyperbolic models have been expressed through CSCN and FA contents by percent based on exponential regression analysis. The precision of calculation formulae is confirmed through comparing calculated values with experimental results, and the deviation is mostly lower than 10%. These results can guide the design of earth-based construction and help engineers keep track of changes in physical properties with service time. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sewage sludge incineration ash was used to prepare lightweight concrete with foam. • Incorporating biochar into lightweight concrete for long-term carbon sequestration. • Biochar significantly enhanced the mechanical performance of these concretes. • Biochar reduced Cr (VI) to Cr (III) and reinforced the immobilization of all metals. • Biochar of high pyrolysis temperature displayed higher effect in the above benefits. Sewage sludge incineration ash (SSIA) is a solid waste containing heavy metal pollutants, which could be recovered as construction material via cement solidification. Biochar, as a carbon sequestration material, has properties of water absorption, metals immobilization, electron donor, etc. This study produced lightweight concrete (LC) with the ratio of SSIA to Portland cement being 1:1 (w/w), and incorporated peanut shell biochar produced at 500℃ and 700℃ pyrolysis (BC500, BC700) with addition rates of 1–5 %. Both BC500 and BC700 increased the mechanical properties of the LC at their optimal addition rates, i.e., 5 % BC500 and 3 % BC700 respectively, achieved 10.8 MPa and 11.1 MPa compressive strength, 2.1 MPa and 2.8 MPa flexural strength, being much higher than the control (6.2 MPa compressive strength and 1.35 MPa flexural strength). Biochar also significantly enhanced the water resistance of LC by 56.3 % (BC500) and 60.4 % (BC700). Instrumental analysis (TGA, XRD, SEM-EDS and heat flow) revealed that the alkaline and porous biochar promoted and prolonged the hydration reactions with forming more cementitious materials such as calcium silicate hydrate and ettringite. Biochar simultaneously strengthened the metals immobilization in LC including Cr, Cu, Zn, Mn, As and Cd, especially it acted as electron donor to reduce Cr(VI) into Cr(III) with less toxicity and mobility. Based on leaching test, it was confirmed that converting 1-ton biomass into biochar and incorporating into SSIA-based FC could achieve 541 and 980 kg CO 2eq carbon sequestration for BC500 and BC700 respectively. [ABSTRACT FROM AUTHOR]

The introduction of metakaolin into magnesium phosphate cement (MPC) can improve the compressive strength, volume stability, water resistance and freeze-thaw resistance significantly. Two possible reasons were proposed in this study. Firstly, metakaolin decreases the total porosity of MPC and improves the pore structure which is directly related to the mechanical properties. Secondly, part of metakaolin can dissolve in MPC and take part in hydration, even though no new hydration products can be observed in XRD patterns. This study not only gives a compressive evaluation on the application of metakaolin in magnesium phosphate cement, but also explains the possible reason for the improvement of the properties of magnesium phosphate cement. • Strength and durability of MPC incorporating metakaolin were systematically studied. • Possible mechanism of metakaolin influencing MPC was proposed by multi-analysis. • The more homogeneous microstructure in metakaolin modified MPC can explain the property improvement. Metakaolin has been generally used as a filler in Portland cement-based materials, and it was proved to be able to enhance the performance of magnesium phosphate cement composites. In this research, metakaolin was introduced into MPC, and compressive strength, tensile bonding strength, porosity and durability of MPC were measured. The microstructure of MPC before and after water immersion was evaluated. The experimental results indicate that the presence of metakaolin can lead to higher plastic viscosity of fresh MPC mortar, 28-day compressive strength, higher tensile bonding strength, better freeze-thaw resistance, better water resistance, lower drying shrinkage and better optimized microstructure. Physical effects of metakaolin including filling effect and inducing effect play the leading role in MPC. Metakaolin particles can fill in the pores or cracks, resulting in the decrease in porosity. In addition, metakaolin particles can provide lots of sites for crystallization of hydration products, inducing in the generation of more homogeneous microstructure. Finally, the dissolution and chemical reaction of metakaolin particles were confirmed by BSE analysis. [ABSTRACT FROM AUTHOR]