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The development of a modified cement stabilized calcareous sand (MCSCS) material with high strength, low carbon and heavy metal ions adsorption capacities is crucial for the eco-friendly construction of islands pavement bases. This study utilized natural zeolite powder (ZP) and three supplementary cementitious materials (SCMs) to develop MCSCS. The macro properties and micro mechanisms of MCSCS were analyzed using various experiments. The results indicated that while ZP alone reduced the mechanical properties of MCSCS, making it unsuitable for airport pavement bases, combining ZP with SCMs significantly improved these properties due to the synergistic effects of the pozzolanic reaction, achieving strengths up to 5.31 MPa. Microscopic analysis revealed that the competitive adsorption of Ca2+ by ZP resulted in a looser microstructure, whereas SCMs promoted the formation of calcium silicate hydrate, calcium aluminate silicate hydrate and tobermorite, leading to a denser microstructure. The cation-exchange and hydroxyl group binding properties of ZP, combined with the physisorption and chemical binding of the hydration products from SCMs, significantly enhanced the heavy metal ion adsorption of MCSCS. The maximum removal rates of Pb2+, Cu2+, Cd2+ and Zn2+ by MCSCS were 81.64 %, 76.26 %, 71.39 % and 65.11 %, respectively. [Display omitted] • A modified cement stabilized calcareous sand material was developed with high strength and eco-friendly properties. • The macro properties and micro mechanisms of the modified cement stabilized calcareous sand were analyzed. • The modified cement stabilized calcareous sand has excellent strength and heavy metal ions adsorption properties. [ABSTRACT FROM AUTHOR]

Loose and uncemented calcareous sand slopes are prone to collapse under rainstorm erosion. In order to improve the erosion resistance stability of slopes, it is crucial to enhance the erosion resistance of calcareous sand. In this study, a new method of cementing calcareous sand with zinc sulfate solution (ZSS) is proposed. The ZSS reinforcement technique can effectively cement calcareous sand, enhance the mechanical properties and help reduce erosion on calcareous sand slopes. A series of laboratory experiments were conducted, including uniaxial compression tests, Brazilian splitting tests, surface penetration tests, microscopic tests, and rainfall scouring tests. The test results show that the uniaxial compressive strength of calcareous sand achieved 8.3 MPa reinforced with ZSS. Microscopic analyses revealed the mechanism of reinforcement, discovering the formation of environmentally friendly compounds such as ZnCO 3 and CaSO 4 ·2 H 2 O between calcareous sand particles, which enhanced the soil mechanical properties. The calcareous sand slope reinforced with ZSS forms a hard shell on the slope surface, which effectively improves the erosion resistance of the slope. After being reinforced with a ZSS concentration of 1.0 mol/L, the slope remained stable after 20 min of scouring at a rainfall intensity of 80 mm/h. This method provides a quick solution for reinforcing calcareous sand slopes and holds promising potential for practical engineering applications. • A innovative method was proposed to improve the erosion resistance of calcareous sand slope. • The microscopic mechanism of strengthening calcareous sand with zinc sulfate is revealed. • The anti-erosion performance of calcareous sand is effectively improved. • This method has good reinforcement effect, high efficiency. [ABSTRACT FROM AUTHOR]

Calcareous sand is a favored unbound granular fill-in island project where complicated stresses are often applied. Traffic and ocean loadings are frequent in praxis, but poorly understood about the normalization of the pore pressure for both. This paper deals with an experimental simulation study on the pore pressure of calcareous sand subjected to ocean waves, traffic, and cyclic loading. Particular attention is devoted to the effect of the initial shear stress and the dynamic stress ratio (CSR). Results showed that, owing to the features of traffic loading and initial shear stress, the soil skeleton can still withstand partial external loading when reaching the failure criterion, hence the pore pressure at failure(r uf) is much smaller than liquefaction. In terms of the influence mechanism, unlike the initial shear stress, the increase in CSR accelerates the destruction of the soil skeleton, reducing the r uf , but having less effect on the critical pore pressure. Expressions for the number of cycles at failure(N f) and r uf were obtained and the exponential model was simplified by changing N / N f to ε 1/0.05 to reflect the characteristics of traffic loading. To normalize the pore pressure under different loadings, each stress component on N f and r uf has been analyzed and the noteworthy finding is that torque has a very minor impact on the soil skeleton. Based on this finding, a new normalization method was proposed in which N f needs to combine all loads including torque, while r uf only needs to consider axial forces. Hence, a pore pressure equation under three different loadings was established, taking into account the role of CSR and the initial shear stress. • Failure under traffic loading is governed by strain, allowing the soil skeleton to partially sustain external loads without complete destruction. • The influence mechanisms of the initial shear stress and CSR on pore pressure have been found to be distinct. • A model of pore pressure considering the initial shear stress to dynamic stress ratio under different loading types is derived. [ABSTRACT FROM AUTHOR]

Calcareous sand, a foundation material of marine origin, is an indispensable component in marine construction and has found extensive applications in building foundations and airport runways. The complex nature of the marine environment and the unique characteristics of calcareous sand present significant challenges for ensuring the stability and durability of engineering structures. To address these challenges, multistage loading triaxial creep tests were conducted on calcareous sands with different particle gradations to analyze their creep characteristics and underlying mechanisms. The result shows that the amount of ultimate axial creep strain is positively correlated with deviatoric stress but negatively correlated with the coefficient of uniformity (Cu). The creep process demonstrates a significant creep structure effect, characterized by creep rate lag and creep failure lag. The volumetric creep strain-time relationship at different creep stress ratios can be classified into three types. When the creep stress ratio is less than the stress ratio at the dilative point, the sample is first compacted followed by dilatancy; when the creep stress ratio is equal to the stress ratio at the dilative point, only compaction is obtained; as the creep stress ratio becomes larger than the stress ratio at the dilative point, pure dilatancy is found. The particle crushing modes and creep mechanisms of calcareous sand are also examined through meso-observations. It is found that the particle crushing modes can be categorized into three distinct types: grinding, angular breakage, and whole particle breakage. The particle gradation not only impacts the primary particle crushing mode but also governs the movement of particles and fragments, thereby influencing the occurrence of creep deformation. Specifically, a smaller Cu value corresponds to a higher fragmentation rate of the specimen, an increased number of overall crushing and angular fractures, a larger space for fragment migration after crushing, and eventually a greater creep deformation. Finally, a modified Burgers model is proposed based on the test results, which can effectively capture the creep deformation in the presence of creep structure effects and creep failure stages. • The ultimate creep strain is negatively correlated with coefficient of uniformity. • A significant creep structure effect is demonstrated by creep rate and failure lag. • The particle gradation impacts both crushing mode and movements of particles. • A modified Burgers model captured creep structure effect and failure is proposed. [ABSTRACT FROM AUTHOR]

Low–concentration of colloidal silica (usually from 5 to 20 wt%) for liquefaction mitigation has been widely studied. But fewer literatures have focused on higher concentration of colloidal silica for cementing sand. For calcareous sand seeped by 40 wt% colloidal silica, cyclic loadings were performed to explore its anti–liquefaction, long–term settlement and self–sensing characteristics. Cyclic triaxial apparatus were to perform anti–liquefaction and long–term settlement tests, and use self-built setup to perform self-sensing tests. The new findings are as follows: (1) for anti–liquefaction, specimens seeped by 40 wt% colloidal silica can't achieve liquefaction, i.e., the excess pore pressure ratios cyclically fluctuates between 1 and negative, which is not discovered by lower concentration of colloidal silica in the previous literatures; (2) for long–term settlement, cumulative strain is small after 20,000 cycles, and the cumulative strain is less than 1 % even the cyclic stress ratio (CSR) is greater than 1.1, indicating that the colloidal–silica–cemented sand has good resistance to deformation; (3) self–sensing characteristics have been discovered, i.e., a stress change of 90 kPa can lead to 59.37 % change in electrical resistivity, compared to Portland–cement-treated sand without change of resistivity under the same stress change. When Portland–cemented materials are added with conductive fillers in the previous literatures, they can exhibit self-sensing characteristics, but the stress sensitivity is still two orders lower than colloidal–silica–cemented sand. For colloidal–silica–cemented sand, based on the fact that the dry silica gel is non–conductive and only the salt solution in the micro porous media is conductive, the self–sensing feature can be attributed to the stress–induced deformation of micro conductive–salt–solution–filled channels (i.e., the deformation of conductive network). The above research indicates that colloidal–silica–seeped sand has a potential for self–sensing subgrades in the marine environments. • The self–sensing sensitivity is two orders higher than cement-based materials. • A new phenomenon that negative excess pore pressure ratio is observed. [ABSTRACT FROM AUTHOR]

Cement-bonded calcareous sand (C-BCS) demonstrates promising potential for marine island construction due to its exceptional mechanical properties. However, challenges such as freshwater scarcity and the erosive effects of seawater hinder the extensive utilization of C-BCS. To address this, this study comprehensively examines the feasibility of incorporating supplementary cementitious materials (SCMs) and seawater in C-BCS, focusing on macroscopic properties, microscopic mechanisms, and strength prediction. Macroscopic tests reveal that blast furnace slag and silica fume effectively enhance the hydration process and mechanical properties of C-BCS. Moreover, the cemented specimens developed in this study partially meet the strength standards for road pavement sub-base and airport runway upper-base applications. Microscopic analyses indicate that calcareous sand serves as a plentiful source of calcium, facilitating the formation of calcium aluminate silicate hydrate (C-A-S-H) and tobermorite when combined with Al 2 O 3 and reactive SiO 2 from slag and silica fume. This enhances the mechanical properties of the samples and their resistance against seawater erosion. Additionally, a back propagation (BP) neural network model accurately predicts the unconfined compressive strength of the cemented specimens. The findings of this study support the broader implementation of SCMs and seawater in practical island engineering projects involving C-BCS. [Display omitted] • The influence of seawater and SCMs on the physicochemical properties of cement -bonded calcareous sand was investigated. • Calcareous sand provided an adequate calcium source for the formation of calcium aluminate silicate hydrate and tobermorite. • A BP neural network model to predict mechanical properties of cemented calcareous sands was developed. [ABSTRACT FROM AUTHOR]

Considering economic and environmental factors, incorporating rubber into calcareous sand as a backfill material in marine engineering construction is a favorable approach. The research on rubber-modified sand has been extensive in the past, but it has not been fully understood. There is a need for a comparative study between impact loads and static loads. This study employs cyclic impact breakage tests and triaxial compression breakage tests to investigate the inhibitory influence of rubber on particle breakage and the Impact on the Volumetric Deformation of calcareous sand under distinct loading conditions. The experimental results indicate that under impact loading, as the impact energy increases, the relative breakage rate of calcareous sand increases, but the rate of increase gradually decreases. The addition of rubber up to a content of 10% notably suppresses calcareous sand breakage, while the impact of a rubber content between 10% and 20% on breakage is weaker. A higher rubber content results in a reduced sensitivity of sample breakage, which may be related to the coordination number of the calcareous sand and rubber. Under the final impact energy, a lower rubber content leads to a greater volumetric deformation of samples, while this trend reverses during the early loading stages due to the superior deformability of rubber. Furthermore, the dependency of calcareous sand particle breakage on volumetric deformation gradually decreases with increasing rubber content. Unlike under impact loading, under triaxial compression loading, along with the increase in input energy, the relative breakage rate of calcareous sand shows continuous growth, and its rate of increase progressively intensifies, which is related to the decreasing degree of participation of rubber. During the consolidation deformation phase, a linear correlation emerges between the input energy and deformation for rubber-modified samples, referred to as the consolidation deformation curve. In the compression phase, a positive linear correlation between volumetric deformation and input energy is observed. The findings of this study offer certain theoretical guidance for understanding the breakage and deformation patterns of rubber-modified calcareous sand in engineering applications. • A study on the inhibition of calcareous sand breakage by rubber. • Comparative experiments between impact and triaxial tests. • The mechanism of rubber varies under different loading conditions. • Under impact loading, the 10% rubber content effectively preserves the original PSD of calcareous sand. • Introducing the concept of "dependence of particle breakage on volumetric deformation". [ABSTRACT FROM AUTHOR]

Due to the unique properties of calcareous sand in reclamation projects, soil foundations consisting of calcareous sand-filled land are often exposed to reversible tidal waves. This may induce continuous long-term internal erosion of the soil that differs from monotonic seepage conditions, which has not yet been studied from a microscopic perspective. In this study, a selected particle size distribution (PSD) based on the breakage experiments of calcareous sand is adopted. Then, a series of reversed seepage flows is generated using CFD-DEM, taking into account both cyclic frequency and hydraulic gradient. Numerical results examine the overall suffused fine mass, the migration and suffusion degree across the divided layers along seepage, and the microscopic origins of continuous suffusion under reversed hydraulic conditions. Furthermore, the micro-mechanical and energetic evolution of soil skeletons is analyzed. The findings reveal that hydraulic gradient is the primary factor influencing suffusion outcomes at the initial stage (PEM increases from roughly 21% to 24%), while the cyclic frequency determines the long-term result (PEM increases from roughly 30% to 37%). From a microscopic point, the elimination of clogging, combined with energetic instability and force chain network rearrangement, is identified as the origin of continuous suffusion under reversed hydraulic loading. • Amplitude and frequency of cyclic hydraulic loading play dominant roles in the suffusion results at the initial and terminal stages separately. • The contribution to total percentage of eroded mass (PEM) fits a symmetric power function curve from the outer to the inner side of the erosion boundary. • Clogging elimination, combined with energetic instability and force chain network rearrangement, is detected as the origin of continuous suffusion under the reversal of the hydraulic gradient from a microscopic perspective. [ABSTRACT FROM AUTHOR]

[Display omitted] • EIS, XRD, and SEM are used to characterize Cement-Bonded Calcareous Sand. • Microstructure and macroscopic properties are evaluated in freshwater and seawater. • Optimal cement content and curing conditions for construction are determined. Cement-bonded calcareous sand (C-BCS) can effectively enhance the strength of calcareous sand, which is widely in infrastructure construction on islands in the South China Sea, and can fulfil the engineering requirements of island pavement subbase. However, the enhancement mechanisms and meso -generalized structural characteristics of C-BCS under different water environments and cement dosages are unclear. In this study, we consider the unconfined compressive strength as an evaluation index to quantify the degree of hydration reaction, change of hydration products, and meso -generalized structure of C-BCS in freshwater and seawater. We performed analyses using electrochemical impedance spectroscopy, X-ray diffraction, and scanning electron microscopy. The results demonstrate that C-BCS can satisfy the strength requirements of highway and airport runway subbases with 10% and 15% cement admixture, respectively. Furthermore, the corrosion deterioration phenomenon occurs and spreads in C-BCS in seawater. The internal hydration process of C-BCS can be continuously monitored using the electrochemical impedance spectroscopy test, and a correlation between the electrochemical and mechanical parameters at different ages is proposed. The meso -generalized structure inside the C-BCS can be densified by the bridging and pore filling effects of hydration products on calcareous sand particles, thereby improving the strength of calcareous sand. Additionally, we provide a concrete mechanistic explanation underlying the forming strength of C-BCS, and an optimized design approach of C-BCS considering several complex engineering properties. [ABSTRACT FROM AUTHOR]

• Crude urease was extracted from soybean powder with artificial seawater for the EICP. • A parallel injection method was proposed for the calcareous sand cementation. • Evolution of the hydrodynamic diameter of particles in solutions was analyzed. • The cementation uniformity of cemented sands was evaluated by the shear wave velocity. • The parallel injection method produced a relatively uniform cementation and high strength. Calcareous sands are widely distributed on continental shelves, coastal zones, islands and reefs in the tropical oceans. For being used as construction materials, calcareous sands should be artificially treated to improve their engineering properties. Enzymatically induced carbonate precipitation (EICP) is an innovative soil treatment technique. In this paper, urease was extracted from soybean powder using artificial seawater for calcareous sands cementing by the EICP technique. Cementation tests were conducted with three injection methods (the pre-mixed, staged and proposed parallel injections) and two injection rates (2 and 5 mL/min). The hydraulic conductivity, shear wave velocity (SWV) and unconfined compressive strength tests were conducted and the cementation effects were evaluated. The hydrodynamic diameter of the particles generated in the mixed solutions was measured with reaction time by a laser particle size analyzer and the influence of injection rate on the CaCO 3 distribution in the specimen was discussed. The coefficient of variation of the SWV values along the cemented specimen height was calculated for evaluation of the cementation uniformity. Results show that a high injection rate was conducive to reducing clogging and improving the cementation uniformity by completing injection within the urea hydrolysis phase. The promising parallel injection method proposed in this study provided a lag time for reaction by simultaneous injection of crude urease and cementation solutions using two separate tubes. Specimens treated at a high parallel injection rate were uniformly cemented with high mechanical strengths and high overall hydraulic conductivities because of a high proportion of CaCO 3 in contact-cementing distribution. [ABSTRACT FROM AUTHOR]

[Display omitted] • Effects of particle size on the particle breakage behavior under impact loadings were studied by experimental tests. • The critical energy was proposed to explain the impact resistance of sand. • The "breakage sensitivity index" B d was proposed to evaluate the breakage sensitivity of samples. Particle breakage of calcareous sands under impact loadings, closely related to marine and geotechnical engineering, has attracted a great deal of attention. This paper presents the results of a study on the effect of particle size on particle breakage characteristic of calcareous sands that underwent different degrees of particle breakage. Calcareous and quartz sands of four different particle size (i.e., 5.00–2.00 mm, 2.00–1.00 mm, 1.00–0.50 mm and 0.50–0.25 mm) were subjected to impact loadings with different input energies. By defining particle size distribution (PSD) using fractal methods, it is found that the PSD evolution of quartz sands is similar to that of calcareous sands. Compared with quartz sands, the particle size related fractal behavior of calcareous sands is more easily affected by the initial particle size. The compressibility of calcareous sands is greater than that of quartz sands. The volumetric deformation of the above two kinds of sand can be divided into three typical stages: initial compression stage, failure stage and stability stage. Unlike calcareous sands, quartz sands can reach the stability stage at a low input energy. For calcareous sands or quartz sands, the breakage sensitivity of fine particles is more significant than that of coarse particles. This could be attributed to their different coordination numbers. Particles with smaller coordination numbers undergo larger impact loadings, resulting in the higher breakage sensitivity than particles with larger coordination numbers. Finally, an empirical model for estimating the degree of particle breakage by volumetric deformation is proposed. [ABSTRACT FROM AUTHOR]

Abstract: Three cements were examined: (i) Portland cement, (ii) limestone cement containing 15% limestone and (iii) limestone cement containing 30% limestone. Concrete specimens, prepared with siliceous or calcareous sand, were immersed in a 1.8% MgSO4 solution and cured at 5°C and 25°C for 3years. Limestone cement concrete is susceptible to thaumasite form of sulfate attack at 5°C. In concrete with siliceous sand the damage is greater, the higher the limestone content of the cement. In concrete with calcareous sand the sulfate resistance seems to be independent on the limestone content. Concrete with calcareous sand showed better performance than concrete with siliceous sand. No damage was observed in the specimens exposed to sulfate solution at 25°C. [Copyright &y& Elsevier]

• Small-strain dynamic properties of polyester and hemp MICP-fiber combined reinforced calcareous sand were studied. • The maximum dynamic shear modulus is positively correlated with the calcium carbonate with the increase of fiber content. • Fibers and its content accelerate the stiffness degradation of MICP cemented calcareous sand based on the bonding between fibers and calcareous sand. • Hemp fiber with rougher surface results in greater damping ratio of MICP cemented calcareous sand than polyester fiber by AFM test. Microbially induced calcite precipitation (MICP)–fiber combined reinforcement through the addition of fibers during the MICP process is a novel, natural, and green technique for foundation improvement, which can effectively enhance the mechanical properties of cemented soil. However, the small-strain dynamic properties of this method remain unexplored. The effects of different polyester fiber and hemp fiber contents (0%, 0.1%, 0.2%, and 0.4%) on the dynamic properties of the MICP–fiber combined reinforced calcareous sand under small-strain conditions were studied using resonant column tests, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The maximum dynamic shear modulus of the MICP–fiber combined reinforced calcareous sand under small-strain increases with the fiber content; the stiffness degradation rate of the dynamic shear modulus ratio of the test sample increases with the addition of two types of fibers, and the addition of hemp fibers greatly boosts the increase rate of the damping ratio. The SEM results indicate that excessive fibers distributed between the sand particles occupy the nucleation sites of bacteria, hinder the bonding between sand particles, and ultimately affect the dynamic shear modulus parameters of the sample. The AFM results show that the hemp fiber has much greater surface roughness than the polyester fiber. Therefore, the change rate of the damping ratio of the hemp fiber-doped MICP–fiber combined reinforced calcareous sand is much greater than that of the polyester fiber-doped sample. This is the first report on the basic mechanism of the small-strain dynamic properties of the MICP–fiber combined reinforced calcareous sand with different fiber contents and types. Furthermore, it provides a new theoretical basis for related research on improving the dynamic properties of sand thus reinforced. [ABSTRACT FROM AUTHOR]

• Strength and stiffness of calcareous sand is enhanced by using PFA for chemical stabilization. • The influence of PFA on calcareous sand characteristics are different for varying particle gradation. • The shear strength as well as CBR of calcareous sand is greatly improved when stabilized by PFA. • The test results are useful in determining the optimum content of PFA to be used in a particular calcareous sand with specific particle gradation. The calcareous sand existing at the South China sea is characterized by low shear strength with high compressibility. Associated with particle breakage, foundations built on such problematic soil initiates undrained failure with uneven foundation settlement, unless adequate ground improvement technique is adopted. In this paper, chemical soil stabilization is carried out by applying Polyurethane Foam Adhesive (PFA) to the virgin soil. Improvement of strength and stiffness of the treated soil have been investigated in detail by a series of triaxial consolidation and drainage tests followed by California bearing ratio tests. The effect of particle gradation on the stress–strain curve and CBR values of the stabilized calcareous sand as well as the influence on peak shear stress and shear strength index has been studied. The test results show that the PFA is effective in improving the mechanical properties of calcareous sand. The particle gradation has significant effects on the stress–strain response of the stabilized calcareous sand under different PFA contents. Optimal PFA content for each grading of calcareous sand was proposed. The test results of this work would be beneficial for engineering practice in dealing with the bearing capacity and settlement of foundations involving calcareous sand. [ABSTRACT FROM AUTHOR]

• The crucial feasibility of using calcareous sand as a filler material was assessed. • The cyclic behavior divergence of calcareous sand and siliceous sand was discussed. • The replacement of siliceous sand with calcareous sand as fillers is cost-effective. • The allowable cyclic strength ratio was suggested for determining the suitable load level. • A relationship between the relative particle breakage and plastic work was developed. Calcareous sand has been used as an emerging unbound granular filler to replace siliceous sand in the construction of marine transportation infrastructure. However, the critical feasibility of and differences between using calcareous sand as a filler instead of siliceous sand have yet to be suitably assessed. In light of this, this paper presented a comparative study on the cyclic behavior of calcareous sand and siliceous sand through a series of high-cycle drained triaxial tests. To facilitate comparison, the same grain-size distribution and relative density were used for calcareous sand and siliceous sand to investigate any underlying cyclic behavior divergence caused by their respective distinctive particle shapes and breakage. Owing to the irregular and rough properties of calcareous sand particles, results showed that it exhibited stronger shear strength and resistance to permanent deformation than siliceous sand. However, during the early and middle stages of cyclic loading, the accumulation rate of permanent strain in calcareous sand was found to exceed that of siliceous sand due to its violent particle breakage habit, whereas a more severe dilatancy of calcareous sand induced by irregular particle shape was observed during the subsequent loading stage. Moreover, particle shape and particle breakage were found to have different competitive contributions to the increase of resilient modulus during different loading stages. In this work, the allowable cyclic strength ratio capable of better determining the suitable load level for a practical design was suggested. Furthermore, to capture the particle breakage extent under long-term cyclic loads, a refined hyperbolic equation was projected to model the relationship between relative particle breakage index and plastic work. [ABSTRACT FROM AUTHOR]