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This study investigated the potential for efficient and resourceful utilization of phosphogypsum (PG) through the preparation of a High-volume Phosphogypsum Cement Stabilized Road Base (HPG-CSSB). The investigation analyzed the unconfined compressive strength (UCS), water stability, strength formation mechanism, microstructure, and pollutant curing mechanism of HPG-CSSB by laser diffraction methods (LD), X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and inductively coupled plasma-mass spectrometry (ICP-MS). The optimal mix ratio of HPG-CSSB was 4% cement, 1% CA2, 35% PG, and 60% graded crushed stone. The UCS reached 6.6 MPa, 9.3 MPa, and 11.3 MPa at 7, 28, and 60 d, respectively. The alkaline curing agent stimulated cement activity and accelerated the release of Ca2+ and SO42− from the PG. This formed many C-S-H gels and ettringite (AFt). The curing agent converted Ca2+ to C-(A)-S-H gels due to high volcanic ash activity. The diverse hydration products strengthened HPG-CSSB. The HPG-CSSB exhibits favorable water stability, demonstrating a mere 7.6% reduction in strength following 28 d of immersion. The C-S-H gel and AFt generated in the system can carry out ion exchange and adsorption precipitation with F− and PO43− in PG, achieving the curing effect of toxic and hazardous substances. HPG-CSSB meets the Class A standard for integrated wastewater discharge. [ABSTRACT FROM AUTHOR]

Steel slag powder, S95 mineral powder, fly ash and desulfurization gypsum were used as raw materials, and multifaceted solid waste cementitious materials were prepared through the optimized design of the simple shape center of gravity method. The effect of composition on the mechanical properties of solid-waste mortar was analyzed by compressive and flexural strength tests of the mortar. X-ray diffraction (XRD), Fourier infrared spectroscopy (FT-IR) and thermogravimetric (TG) methods were used to investigate the strength formation mechanism of the multifunctional solid waste-based cementitious materials. The results show that the optimal mixing ratio of multifunctional solid waste-based cementitious materials is steel slag powder∶ S95 mineral powder∶ fly ash∶ desulfurization gypsum = 31∶ 38∶ 27∶ 4. The mechanical properties of multifunctional solid waste-based glued sand specimens show a tendency to increase and then decrease with the increase of steel slag powder admixture. S95 mineral powder contributed more to the early strength of the glued sand specimens, while excessive fly ash admixture is detrimental to the formation of the strength of the glued sand. The infrared absorption peak of SFMD showed the telescopic vibration peak of Si—O in C-S-H at 969 cm - 1, and the weight loss of C-S-H in SFMD is the most, which indicated that the synergistic effect among the components of the system is better. In addition, desulfurization gypsum can induce a more complete synergistic hydration reaction of the steel slag powder-S95 mineral powder-fly ash system to produce caliche that provides strength. [ABSTRACT FROM AUTHOR]

To solve the problems of large storage requirements for iron and steel solid waste piles, environmental pollution, and low utilization, an alkali-activated ground granulated blast furnace slag (GGBFS)-steel slag (SS) grouting material (AAGM) was prepared, quicklime was used as an additive, and the activity of SS and GGBFS was activated by NaOH and water glass. Through a full factor analysis test, the effects of alkali content (AC), GGBFS/SS, and quicklime content (QC) on the fluidity, initial setting time (IST), and final setting time (FST) of AAGM were investigated. The strength formation mechanism of AAGM was analyzed using mechanical property testing, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). These studies showed that the fluidity of the slurry decreased with increasing QC, AC, and GGBFS/SS, and the IST and FST of the slurry continuously shorten with increasing QC, increasing AC, and decreasing GGBFS/SS. The 28-day unconfined compressive strength (UCS) of AAGM decreased with decreasing QC and increasing AC, and first increased and then decreased with increasing GGBFS/SS. As the age increased, the hydration reaction inside the specimen was sufficient and the compactness was stronger, reducing the porosity inside the specimen; the hydration products inside AAGM were mainly plate-like crystals (anhydrite, calcite), a small amount of columnar crystals (ettringite), and a large amount of gel-like substances (C-S-H/C-A-S-H) attached to its surface. When the GGBFS/SS ratio was 1:1.3, the QC was 3 %, the AC was 13 %, the water glass modulus was 1.6, and the water/cement ratio was 0.6; additionally, the fluidity, IST, FST, and 28-day UCS of AAGM were 137 mm, 255 min, 280 min, and 27.07 MPa, respectively, which can meet various performance indicators required for engineering grouting.

The poor grading and significant differences in the mechanical properties of laterite granules in West Africa limit the application of this typical material in road structures. In this paper, the poorly graded laterite granule was blended with fine sand and stabilized with cement to improve the strength of laterite granules. An innovative combination of laboratory tests and numerical simulation analysis using the numerical simulation software PFC2D 5.0 was used to investigate the cement-stabilized laterite granule's strength formation and failure mechanisms. Laboratory test results show that when the sand content is 40%, the grading curve is close to the maximum theoretical density lines (MTDLs), and the unconfined compressive strength of cement-stabilized laterite granules increased by 35.2% as the cement dosage increased from 2.5% to 5.0%. Numerical simulation tests show that the strength failure of cement-stabilized laterite granules is shear. The failure surface is tilted and through with the axial direction, and the compressive and tensile stresses between the particles cause the failure of the specimen. The simulation test results are close to the laboratory test results, and the deviation rate is less than 10%. It can guide the optimal grading design and strength prediction of cement-stabilized laterite granular materials and provides a basis for reducing laboratory tests and accurate design for such research in the future. [ABSTRACT FROM AUTHOR]

Optimizing the performance of alkali-activated slag backfill grout (ASBG) is crucial for engineering applications. Fluidity and 28 d unconfined compressive strength (28 d UCS) are the two most important performances and multi-objective optimization is existed. In this research, the response surface method (RSM) for experiment design and model regression, non-dominated sorting genetic algorithm II (NSGA-II) for optimization, and entropy-based Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) for decision-making were implemented. The strength formation mechanism of the optimal ASBG was investigated via microstructure evolution. The results show that the quadratic RSM models for fluidity and 28 d UCS exhibit excellent predictive fitting capabilities, with the determination coefficients of 0.9713 and 0.9588 respectively. NSGA-II coupled with entropy-based TOPSIS determined the optimal mix proportion with water-binder ratio of 0.877, binder-sand ratio of 1.000 and alkali dosage of 4.138%. The porosities at 1–28 d decrease from 10.06% to 2.62%. And the mode of microstructure evolution can be divided into two stages according to the probability distribution index and fractal dimension. In early stage, probability distribution index and fractal dimension decreased from 0.9974 to 0.4833 and 1.2498 to 1.2022. This indicated that the number of the small pores decreased and the pore boundary transformed into the softer one as the irregular plate-like GGBS dissolved. In the later stage, probability distribution index and fractal dimension increased from 0.4833 to 1.4732 and 1.2022 to 1.3273, indicating a relatively denser microstructure with fewer large pores formed.

The application and promotion of the traditional micro-surfacing is limited by the defects of high noise and poor wear resistance. In order to improve the adhesive strength and noise reduction properties of the micro-surfacing, an attempt was made to add waste rubber powder (WRP) in dry form to the micro-surfacing. Firstly, the mixtures were prepared by orthogonal tests taking into account the number of rubber powder grades, rubber powder dosages, asphalt-aggregate ratio and gradation. The microscopic strength formation mechanism at the micro-surfacing of the waste rubber powder was investigated by means of Scanning Electron Microscope (SEM). Secondly, the effect of waste rubber powder (WRP) on the modification of asphalt mastic was investigated with the Dynamic Shear Rheometer (DSR), both as unaged and after short-term ageing. Finally, the effect of waste rubber powder on the performance improvement and noise reduction characteristics of the micro-surfacing was analyzed through road performance tests, and to establish the correlation between microstructure and road performance. The results showed that 2% of 40-mesh rubber powder mixed with 8% asphalt-aggregate ratio of medium grade MS-3 micro-surfacing mixture, the densest structure of the slurry. Significantly improved high-temperature and ageing performance at the micro-surfacing reduced noise generation and suppressed noise transmission, but no significant improvement in water stability performance. The waste rubber powder enhances the strength of the micro-surfacing by interweaving with cement hydration products and asphalt mortar. It also optimizes the inter-aggregate embedding and void characteristics, and has an excellent noise reduction effect.

By adjusting the content of geopolymer in geopolymer stabilized phosphogypsum (GSP) as roadbed filler, along with the mixing ratio, this paper mainly explores tendencies in the mechanical properties and water stability of GSP. This research is based on macro-mechanical properties such as unconfined compressive strength, resilience modulus, California bearing ratio and shear strength. It is also based on water stability tests, such as the water soaking test, dry and wet cycle test and expansion test, to explore changes in water stability. As for the durability of GSP, this paper is mainly based on the realization of a long time observation of mechanical properties and water stability. In the existing research, most of the stabilized phosphogypsum (PG) base material or roadbed filler consists of cement, lime, etc. In this paper, a new exploration is carried out on the composition of stabilized PG material, realized without the participation of cement. The 28 d compressive strength of GSP reaches 2.5 MPa, and over time this strength grows, which prevents the phenomenon of strength inversion that may occur in conventional cement-stabilized PG. In addition, a long-term soaking experiment was designed in this study based on the material after the strength was stabilized for up to 90 d. After the strength was steady, the GSP with the best water stability still had a softening coefficient of 80% after experiencing water immersion for 7 d. After determining the feasibility of the mechanical properties and water stability of GSP as roadbed filler, we further explored the strength formation mechanism of GSP by microscopic tests (XRD and SEM). This shows that geopolymer can stabilize PG in two main ways: one is the hydration reaction with PG to generate C-S-H gel and ettringite, and the other is to connect PG not involved in the chemical reaction to form a dense whole through generated hydration products. Geopolymer, stabilizing a high amount of PG, not only provides a new method for the consumption of PG, but also has more stable performance than cement, and has certain advantages in economy. In addition, the advantage of this study is that good performance can be achieved by simply sieving PG and adjusting the geopolymer ratio in practical engineering projects. [ABSTRACT FROM AUTHOR]

In order to realize the recovery and stabilization of detoxified municipal solid waste incineration fly ash (DIFA) in roadbases, the synergetic hydration behavior of cement DIFA composite system and the mechanism of DIFA on the development of compressive strength of cement-stabilized macadam were investigated by means of macro-mechanical properties and microscopic characterizations of SEM, and the water stability, frost resistance and heavy metals' leaching toxicity were also studied. Results showed that the compressive strength of cement-stabilized macadam increased first and then decreased with the increase of DIFA substitution percentage. The soluble chlorides introduced by DIFA were the key factors that affect the amount of hydration products and the development of macro-mechanical properties. The reasonable content of soluble chlorides not only accelerated the hydration of cement, but also produced a new product Friedel's salt, which made the structure more compact and optimized, and the macro-mechanical properties increased. However, when the substitution percentage of DIFA was high, the free chlorine appeared, forming a coating layer on the surface of cement particles, which hindered the further cement hydration and the development of macro-mechanical properties. The water stability of cement-stabilized macadam was improved by adding suitable DIFA, and the frost resistance was decreased, but it still met the technical requirements. The heavy metals in DIFA were firmly solidified in cement-stabilized macadam, and the leaching concentration was greatly reduced, thus realizing the stabilization and clean utilization of DIFA. [ABSTRACT FROM AUTHOR]

The poor grading and significant differences in the mechanical properties of laterite granules in West Africa limit the application of this typical material in road structures. In this paper, the poorly graded laterite granule was blended with fine sand and stabilized with cement to improve the strength of laterite granules. An innovative combination of laboratory tests and numerical simulation analysis using the numerical simulation software PFC2D 5.0 was used to investigate the cement-stabilized laterite granule’s strength formation and failure mechanisms. Laboratory test results show that when the sand content is 40%, the grading curve is close to the maximum theoretical density lines (MTDLs), and the unconfined compressive strength of cement-stabilized laterite granules increased by 35.2% as the cement dosage increased from 2.5% to 5.0%. Numerical simulation tests show that the strength failure of cement-stabilized laterite granules is shear. The failure surface is tilted and through with the axial direction, and the compressive and tensile stresses between the particles cause the failure of the specimen. The simulation test results are close to the laboratory test results, and the deviation rate is less than 10%. It can guide the optimal grading design and strength prediction of cement-stabilized laterite granular materials and provides a basis for reducing laboratory tests and accurate design for such research in the future.

Surface paste disposal (SPD) can utilize substantial coal gangue while effectively disposing of active coal mine collapse pits. The response surface method was used for mixture ratio design, and the desirability function method was used for the multiobjective optimization of SPD materials. For the optimal mixture ratio of SPD materials, microstructure analysis was performed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). A pore (particle) and crack analysis system was utilized to detect microstructural pore characteristics with increasing curing age. The optimal characteristics are a water-to-binder ratio of 4.0, an aggregate-to-binder ratio of 16.91, and a fine gangue rate of 60%. The porosities at 1–28 d decrease from 20.50% to 2.62%. Hydration products fill the pores randomly, leading to a probability entropy of pores >0.95. Ettringite (Aft) formed by cement hydration at 3–14 d exhibits a skeleton structure with complicated pore boundaries, increasing the probability distribution index from 2.0725 to 3.1523 and fractal dimension from 2.2625 to 4.4169. In late hydration, coal gangue powder acts as a supplementary cementitious material (SCM) that works with cement to generate calcium silicate hydrate (C-S-H) gel to cover the gaps in the AFt skeleton. Reducing the number of pores and softening the pore boundaries decrease the pore probability distribution index from 3.1523 to 2.0203 and the fractal dimension from 4.4169 to 2.1983. A denser “skeleton-gel” network structure eventually forms.

Under the compaction effort, asphalt-coated aggregate particles migrate as the aggregate-asphalt particle system undergoes structural rearrangement, thereby forming a stable spatial structure. To analyze the strength composition of asphalt mixtures on mesoscopic level, the self-developed experimental devices and discrete element compaction model were employed. A skeleton contact model for asphalt mixtures was established adopting three-dimensional dynamic virtual compaction test. The mechanism of asphalt mixture strength was investigated from the perspectives of skeleton structure and asphalt mortar. The research indicates that the skeleton contact index exhibits a good linear relationship with the composite angularity index, maximum migration force, skeleton bonding index, and skeleton lubrication index. Aggregate of variously-sized mutually interlocking and friction, driving the skeleton structure to adjust with the synergistic effect of particle migration and contact behaviors to achieve the stable state. Except for AC-13 L aggregate-asphalt system, the skeleton contact index evinced a robust correlation with the strength indices. A skeleton-dense model for asphalt mixtures was developed, incorporating the stability of the aggregate skeleton stability and the bonding-filling effects of the asphalt mortar. The skeleton-dense index has a good linear correlation with the maximum migration shear stress, and can good discriminates the strength of different mixtures. • A skeleton contact model for asphalt mixtures was established. • The effects of mesoscopic interface characteristics on skeleton contact properties was analyzed. • The mechanism of asphalt mixture strength was investigated from the perspectives of skeleton structure and asphalt mortar. [ABSTRACT FROM AUTHOR]

To meet the needs of the road industry for maintenance operations, a new cement emulsified bitumen mixture (CEBM) with early-strength, self-compacting, and room-temperature construction characteristics was designed. The strength formation mechanism of CEBM was revealed with a scanning electron microscope (SEM) and the surface free energy (SFE) theory. The mechanical properties and road performance of the CEBM were investigated extensively. The results show that before the demulsification of emulsified bitumen, the SFE of the bitumen–aggregate–water three-phase system was reduced due to the replacement of the bitumen–aggregate interface with water. The adhesion work between the emulsified bitumen and the aggregate is negative, which means the adhesion between the emulsified bitumen and the aggregate will not occur spontaneously due to the existence of water. The liquid emulsified bitumen improves the workability of the mixture and ensures that the mixture can be evenly mixed and self-compacted. After demulsification, the work of adhesion between the residual bitumen and the aggregate is positive, which means residual bitumen and aggregate can bond spontaneously. In addition, the hydration products of cement and aggregate form a skeleton, and the emulsified bitumen film wraps and bonds the cement and aggregate together, creating strength. The emulsified bitumen, cement content, and curing conditions have significant effects on the stability of CEBM. The recommended dosage of emulsified bitumen and cement is 8% and 8–10%, respectively. This material integrates the hardening effect of cement and the viscoelastic performance of bitumen and has good workability, mechanical properties, and road performance. Therefore, the CEBM is technically feasible for application to bitumen pavement. [ABSTRACT FROM AUTHOR]

At present, the research on rice hull ash and cement-based materials as cementitious materials continues to deepen. Low-cost rice hull ash replaces part of Portland cement, which plays a dual role in saving material costs and improving environmental benefits. In this study, alkali-activated rice husk ash (RHA) and ground granulated blast furnace slag (GGBS) were used to prepare cementitious material. The influence of RHA dosage on the strength, slump degree, and coagulation time of cementitious material was studied. On this basis, tailing was used as an aggregate based on the orthogonal design method and the bone–gel ratio, modulus, and alkali content were taken as variable factors, with strength and slump degree taken as the targets. A new cemented paste backfill (CPB) was prepared and mix ratio optimization was carried out. The strength formation mechanism of cementitious material and CPB was explored by combining scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The results of this study show that with the increase in RHA mixing, the unconfined compressive strength (UCS) of the gelling material purification slurry showed a tendency first to increase and then decrease. When the amount of RHA was about 10%, the internal structure became denser, more C-S-H gel was generated, and greater strength could be obtained. The specific surface area of RHA is high, and a small amount of RHA can fill the internal pores, making the internal structure of concrete more dense. The active silica content in RHA is relatively high. The addition of RHA can appropriately improve the strength of the material, which is of certain significance to our material research. Finally, the micro-analysis of RHA-GGBS clean slurry, the analysis of influencing factors of fluidity and strength, and the optimal mix proportion of alkali-activated RHA-GGBS-based backfill are put forward. [ABSTRACT FROM AUTHOR]

Cement-stabilized cold recycled mixtures (CCRMs) are widely used to rehabilitate and repair asphalt pavements. During laboratory production, the mechanical behaviors of these CCRMs are affected by the compaction method of the two types of compaction methods used in CCRM production, namely static pressing compaction method (SPCM) and vertical vibration compaction method (VVCM), and the latter has caught the attention of researchers owing to its advantages. However, current studies have only investigated the mechanical behaviors of CCRMs produced by SPCM but not through VVCM. The current research aimed to evaluate the mechanical behaviors of CCRM produced by VVCM. A total of 18 CCRMs were designed with different ratios of recycled cement base to recycled asphalt pavement, virgin aggregate contents and cement contents. The mechanical behaviors of CCRM produced by VVCM were evaluated, and the influencing factors were investigated. Scanning electron microscope was also used to analyze the strength formation mechanism of CCRM. The study results are useful for engineering practice, particularly for identifying the properties of CCRM produced by VVCM. [ABSTRACT FROM AUTHOR]

China produces a large amount of red mud (RM) and phosphogypsum (PG) each year. At this stage, there is an urgent need for a process that can use a large number of these wastes to reduce the pressure of stacking cost and environmental pressure. In this study, an improved high-strength and non-calcined RM-PG cementitious binder was developed. Under a certain mixture ratio, the cementitious binder is prepared from non-calcined Bayer RM and PG, with granulated blast furnace slag (GBFS) and alkaline activator. The RM-PG cementitious binder has a compressive strength of more than 40 MPa. A large amount of RM and PG were added, and their activities could be better stimulated under strong alkaline condition. The performance of the cementitious binder can be improved by adding different substitutes instead of GBFS. The strength formation and hydration mechanism of the non-calcined cementitious binder were discussed by analyzing the X-ray diffraction (XRD), scanning electron microscope (SEM) and pH of the leaching solution. Results indicate that various raw materials undergo different degrees of dissolution in an alkaline environment. They continuously generate C–S–H, C-A-H gels and Ettringite (AFt) to develop mechanical strength of the RM-PG cementitious binder. The result provides a new way for the coordinated utilization of two kinds of industrial wastes. The new cementitious binder is more environmentally friendly and has a great application prospect. • A novel non-calcined RM-PG cementitious binder was developed. • The RM-PG cementitious binder has a compressive strength of more than 40 MPa. • The strength formation and hydration mechanism of the RM-PG cementitious binder were discussed. • C–S–H, C-A-H gels and AFt are continuously generated to develop mechanical strength. [ABSTRACT FROM AUTHOR]

A new type of cemented paste backfill (CPB) was prepared using sodium hydroxide (NaOH) as the activator, slag and silica fume (SF) as the binder, and tailings as the aggregate. The effects of proportion of replacement of 0%, 5%, 10%, 15%, and 20% silica fume on the properties of CPB were studied. The strength formation mechanism of CPB was explored through a combination of scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and Fourier transform infrared (FTIR) spectroscopy. The SEM images were analyzed by IMAGE J software, and the porosity of CPB with different silica fume contents was obtained. The results show that as the amount of silica fume increases, the unconfined compressive strength (UCS) increases first and then decreases. When the amount of silica fume was approximately 5%, CPB with a larger UCS can be obtained. When the silica fume content increased from 0% to 5%, because silica fume has good activity and small particles, more calcium silicate hydrate (C–S–H) gels and Mg-Al type layered double hydrotalcites (LDHs) were generated in CPB, which made it denser and improved its strength compared with the non-silica fume group. C–S–H gels were the main source of CPB strength. With a further increase in the amount of silica fume, thaumasite produced inside of CPB, reducing the content of C–S–H gels. Moreover, due to the expansion of thaumasite, it is easy to generate a large number of micro cracks in CPB, which weakens the strength of CPB. [ABSTRACT FROM AUTHOR]