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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]

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 use of portland cement (PC)–ground-granulated blast-furnace slag (GGBS) binary cement (BC), which was improved by chemical activation and nanomodification, as a potential alternative to PC for stabilization of dredged sediment (DS). The activators of Na2SiO3 (SM), Ca(OH)2 (CH), Na2SO4 (SS), and nanomodifiers of nano-SiO2 (NS) and nano-MgO (NM) were used to improve the strength gaining of BC-stabilized DS (BCDS). A series of unconfined compressive strength (UCS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) tests were conducted to investigate the strength development and associated micromechanisms of nanomodified and activated BCDS. The results indicated that chemical activation can effectively improve the UCS of BCDS, and optimum single activator contents of SM, CH, and SS were respectively 6%, 4%, and 10%. Composite activators SM/SS with mass ratio of 1/9 exhibited more advantages than a single activator. Nanomodification can further improve the UCS of SM/SS-BCDS. The optimum single nanoparticle contents of NS and NM for SM/SS-BCDS were 6% and 8%, respectively. Compared with a single nanoparticle, composite nanoparticles NS/NM with mass ratio of 5/5 were more effective in improving UCS of SM/SS-BCDS. The 15% optimum NS/NM-SM/SS-BC can be used to replace 30% PC for DS stabilization. XRD and SEM results confirmed that the main hydration products of NS/NM-SM/SS-BCDS were calcium silicate hydrate (CSH), calcium aluminate hydrate (CAH), and ettringite, which contributed to the strength enhancing. [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]

The combination of chemical stabilization and fiber reinforcement can simultaneously improve the strength and ductility of dredged sediment. The polypropylene fiber (PF) and straw fiber (SF) were respectively used as artificial and natural reinforcements of cement-stabilized dredged sediment (CDS). A series of unconfined compressive strength (UCS) tests were conducted to investigate the effects of cement content, fiber content, fiber length and water content on the mechanical properties of PF-reinforced CDS (CPFDS) and SF-reinforced CDS (CSFDS). Furthermore, the cementation-reinforcement mechanism was explored and analyzed via macro failure characteristics and micro interfacial morphologies inside typical CPFDS and CSFDS samples. The results showed that increasing cement content or decreasing water content significantly improved the UCS and aggravated the brittleness of CPFDS and CSFDS. The suitable addition of PF can effectively improve the UCS of CDS, while incorporating SF exhibited the opposite role. The maximum 7d-, 28d-, 60d- and 90d-UCS of CPFDS were respectively 17.7%, 43.6%, 10.7% and 9.7% higher than that of CDS. The optimum length of PF inside CPFDS and SF inside CSFDS was 3 mm and 5–10 mm, respectively. Both incorporating PF or SF can effectively improve the ductility of CDS. Based on the proposed parameters of total-water/cement ratio and fiber cementation factor, the effective strength development models of CPFDS and CSFDS considering cement content, fiber content, water content and curing time were empirically established. The fiber "bridge" effect and interfacial friction between fiber and cemented soil particles were mainly responsible for the strength evolution and ductility improvement of CPFDS and CSFDS. [ABSTRACT FROM AUTHOR]

• The alkali-activated GGBS was developed to replace cement for stabilizing dredged sludge (DS) with different water contents. • Effects of activator type, water content and curing age on the strength evolutions of stabilized DS were investigated. • Composite activators were found to be more effective than single activator in activating GGBS for DS stabilization. • The micro-mechanisms explaining strength development of stabilized DS were analyzed and discussed. The variation of water content significantly affects the strength development of stabilized dredged sludge (DS), as well as the rational use of binder. This paper experimentally studies the strength and microstructure evolutions of DS stabilized with alkali-activated ground granulated blast-furnace slag (GGBS) under different water contents, via a range of unconfined compressive strength (UCS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests. The sodium hydroxide (SH) and sodium silicate (SS) were used as single and composite activators for GGBS, and their roles on the stabilization efficacy were mainly focused. The results illustrated that the optimum single activator content tended to increase with the increase of water content. Composite activators were more effective than single activator in activating GGBS for achieving higher UCS of stabilized DS. The optimum mass ratio of SH to SS (H/S) in composite activators was 2/3, 2/3 and 1/4 at water content of 60, 70 and 80%, respectively. The linear empirical equations for predicting 60- and 90-day UCS based on 28-day UCS were proposed. XRD and SEM results confirmed that calcium silicate hydrate and calcium aluminate hydrate were the major hydrations, which undertook the cementation and filling roles. Furthermore, the micro-mechanisms revealing the strength evolutions of stabilized DS were analyzed and discussed. The results recommended that using 20% SH/SS-GGBS binder with optimum H/S of 2/3, 2/3 and 1/4 as the substitute of equivalent PC for stabilizing DS with water content of 60, 70 and 80%, respectively. [ABSTRACT FROM AUTHOR]

• The unfired sludge bricks (USB) were prepared by chemical solidification and compaction forming technology. • A series of physical, mechanical and thermal conductivity tests were conducted to evaluate the engineering properties of USB. • The optimum compressive strength of USB was achieved by adding 1.5% nano-SiO 2. • The microstructure and crystalline phases of typical USB were explored by conducting SEM and XRD tests. This paper investigated the engineering properties of unfired sludge bricks (USB) made from dredged sludge and cement-fly ash-lime admixed nano-SiO 2 binders under compaction forming technology. A series of laboratory tests were conducted to evaluate the effects of cement content, mass ratio of lime to fly ash (L/FA) and nano-SiO 2 (NS) content on the physical, mechanical and thermal insulation performance of USB. Furthermore, scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests were also performed to explore the microstructural properties of USB. The results indicated that the binder composed of 15% cement, 1% lime, 9% fly ash and 1.5% NS was recommended to replace 25% cement for preparing USB, which can increase 7-, 28- and 60-day compressive strength by 1.37, 12.94 and 23.30%, respectively. The bulk density of USB increased with the increase of cement and NS content, but decreased with the L/FA. The water absorption of USB decreased with increasing cement and NS content, but increased with the L/FA. The lime content used in USB should be less than 3%. The pH of USB decreased with increasing NS content, and tended to be stable as the NS content increased to more than 1.5%. The increase of cement and NS content had adverse effect on the thermal insulation performance, but this can be effectively improved by the inclusion of lime and fly ash. A positive linear relationship was achieved between thermal conductivity and bulk density. The microstructure analysis confirmed that the CSH and CAH gels were the main cementitious products inside the USB, and the calcite (CaCO 3) may be responsible for the micro-cracks appeared in USB. [ABSTRACT FROM AUTHOR]

• The addition of steel slag powder can effectively improve the strength development of CSDS. • The inclusion of 5–10%, and 15–20% SSP were recommended as cement admixture for stabilizing DS and humic acid-containing DS, respectively. • Using 15% cement admixed 20% SSP was conducive to the long-term strength development of CSDS. • Microstructure and crystalline phases analyses were conducted by SEM and XRD tests. Waste steel slag is an industrial by-product during the process of steelmaking, and it is usually stacked in steelmaking plants. Employing steel slag in treating dredged sludge (DS) would be an economic and environmental protection solution. This study investigated the effectiveness of steel slag powder (SSP) on the strength development of cement-SSP-stabilized DS (CSDS). The effects of cement content and type, SSP content, humic acid (HA) content and curing age on the strength development of CSDS were analyzed via a series of UCS and pH tests. Additionally, microstructure analyses were conducted by scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. The results indicated that the cement content and type significantly affected the strength gain of CSDS. The UCS of CSDS increased first and then decreased with an increasing in SSP content. The SSP content of 5–10% was the optimal to improve the strength of CSDS, but for CSDS containing HA, the 15–20% SSP was recommended as cement admixture. The HA seriously reduced the strength of CSDS, but this strength loss can be effectively relieved by adding SSP. Based on the strength difference (ΔUCS) and strength growth rate (UCS gr), it was suggested that using 15% cement admixed 20% SSP was the most effective to the long-term strength development of CSDS. The variation of UCS with pH exhibited oscillation characteristics, and the pH of 9.00–10.00 was conducive to the strength development of CSDS. The microstructural analyses confirmed that the addition of SSP can contribute to the formation of cementitious products in CSDS, especially needle-like gel ettringite, and then leading to a dense inter-locked structure in CSDS. [ABSTRACT FROM AUTHOR]

• The small-strain dynamic shear modulus (G) and damping ratio (D) of CSC and CFSC were investigated by resonant column tests. • The G and D increased and decreased respectively, with an increase in the confining pressure and curing age. • The D was significantly affected by the cleanliness and cementation of soil particles. • It is desirable to use 5% cement together with 30% fly ash instead of 15% cement alone for the stabilization of silty clay. This study examines the small-strain dynamic properties of cement-stabilized silty clay (CSC) and cement-fly ash-stabilized silty clay (CFSC). A series of resonant column tests were performed to investigate the effect of confining pressure, binder content and curing age on the small-strain dynamic shear modulus (G) and damping ratio (D) of CSC and CFSC. Furthermore, the maximum dynamic shear modulus (G max) and maximum damping ratio (D max) of CSC and CFSC were also evaluated. The results showed that the G of CSC and CFSC increased with the increase of confining pressure, binder content and curing age. The G decreased slowly and then rapidly with shearing strain (γ), and the γ of 10−4 was the turning point of evaluating the small-strain dynamic behavior of CSC and CFSC. Using 5% cement together with 30% fly ash as stabilizer has advantageous over using 15% cement alone in improving the stiffness properties of silty clay. The D of CSC and CFSC decreased with the increase of confining pressure and curing age, but increased with an increase in cement and fly ash content. This was attributed to the weakly cemented and rough of soil particles led to more energy dissipation of vibration wave propagating through the sample. The microstructural analysis revealed that the micro-aggregate effect and hollow structure of fly ash particles not only contributed to the improvement in G, but also effectively improve the energy dissipation performance of CFSC. [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]

The environmental pollution resulted from the stacking of solid wastes, and the 250 kg CO 2 emission caused by calcined a ton of kaolinite have become an urgent problem to be solved. In this study, a novel phosphoric acid-activated geopolymer was developed, which was composed of fly ash (FA), mechanically activated-kaolinite (MAK). The influences of phosphoric acid concentration, mechanical grinding duration, and curing temperature were investigated. Microstructural properties of FAMAK geopolymers were examined through a serious of tests. Furthermore, the Gibbs free energy and nano-scale structure characteristics of acid-activated hydration products were calculated for the first time by using density functional theory. The results show that the optimal mass ratio of FA to MAK is 7:3 with 8 mol/L phosphoric acid, which led to the maximum compressive strength of 36.6 MPa at 56-day. Besides, the optimal grinding time is 3 h and temperature is 40 °C. Moreover, the content of silicon promotes the polymerization reaction. The valence and bonding of aluminum are primarily affected by its coordination number. The key outcomes in this study provides a low-carbon cementitious materials, which can achieve the double benefits of carbon reduction and the utilization of solid wastes. • Mechanically activated kaolinite (MAK) can be used to replace metakaolin. • A novel high-strength FA-MAK acid-activated geopolymer was developed. • The optimal synthesis conditions are 3 h grinding and 40 °C curing. • The strength development and polymerization mechanism were revealed. • The nano-scale structure and properties of hydration products were calculated by DFT. [ABSTRACT FROM AUTHOR]

• Recycled glass powder (GP) can be used as Portland cement (PC) admixture in dredged sediment (DS) stabilization. • The effects of PC content, GP content, GP fineness and curing age on the strength development of PC-GP stabilized DS (CGDS) were investigated. • The optimum GP content inside CGDS tended to increase with the curing age evolution. • Incorporating suitable GP contributed to the formation of dense cemented soil matrix inside CGDS. Using glass powder (GP) as Portland cement (PC) helper in dredged sediment (DS) stabilization can achieve the double benefit of reducing PC dosage and recycling waste GP. This study evaluated the feasibility of using GP together with PC in improving the strength properties of DS. The effects of PC content, GP content, GP fineness and curing age on the strength development of PC-GP stabilized DS (CGDS) were investigated via a series of unconfined compressive strength (UCS) tests. Furthermore, scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests were conducted to analyze the strength evolution micro-mechanisms inside CGDS. The results showed that the UCS of CGDS increased monotonously with increasing PC content, and incorporating GP led to the strength gain of CGDS regardless of PC content. The optimum GP content tended to increase with increasing curing age and water content. The highest 28-day UCS of CGDS with water content of 50, 60 and 75% were respectively achieved at GP content of 15, 15 and 20%. Improving GP fineness can obviously improve the strength development of CGDS. The maximum strength growth rate (UCS gr) of CGDS with water content of 50, 60 and 75% were respectively achieved at PC content of 8, 8 and 10%, and increasing GP content and fineness resulted in UCS gr improvement. The 28-day UCS of CGDS can be predicted via the known 7-day UCS using proposed linear relation. The microstructural test results confirmed that incorporating suitable GP contributed to the formation of dense cemented soil matrix inside CGDS. The 10% PC admixing 15–20% GP with 2000-mesh number fineness is recommended to replace 15% PC for stabilizing DS containing 50–75% water content. [ABSTRACT FROM AUTHOR]