Comparison between reactive MgO- and Na2SO4-activated low-calcium fly ash-solidified soils dredged from East Lake,China

Abstract

A novel approach to mitigate the environmental concerns associated with cement industry is to replace Portland cement with low carbon alternative materials such as fly ash-based geopolymer cement. Hence, reactive MgO-activated low-calcium Class F fly ash was employed in comparison to Na₂SO₄-activated fly ash to stabilize a lacustrine soil reused potentially in soft coastal reclamation projects and as reinforced aggregates for anti-corrosion in marine engineering. The microstructural and strength properties were investigated with series of tests including X-ray diffraction (XRD), thermogravimetry/differential thermogravimetry (TG/DTG), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and unconfined compressive strength (UCS). The results demonstrate that the main hydration products in reactive MgO- and Na₂SO₄-fly ash-solidified soils are, respectively, magnesium silicate hydrate (M-S-H) gel and sodium aluminosilicate hydrate (N-A-S-H) gel. This finding is reconfirmed by the weight loss of solidified samples at 40–200?°C, which is correspondingly attributed to the dehydration of magnesium silicate hydrate (M-S-H) gel and sodium aluminosilicate hydrate (N-A-S-H) gel. The morphology and bonding ability of hydration products affects the microstructure and long-term strength of solidified soils. The microstructural change identified from SEM images coincides well with the quantitative evolution of pore structure. The pores with radius of 0.01–1?µm, i.e., micropore and mesopore, are supposed to be the dominant pores in reactive MgO- and Na₂SO₄-activated fly ash-solidified soils. The comparison of UCS indicates reactive MgO-activated low-Ca fly ash behaves much superior to Na₂SO₄-activated fly ash in enhancing the long-term compressive strength of soils. This study provides insight into the promising potential of low-Ca fly ash activated by immerging material – reactive MgO to replace cement in soil improvement.     

Enhancing the mechanical properties of marine clay using cement solidification

Abstract

The use of soft clay and dredged marine clays as the construction material is challenging. This is because the high water content, high compressibility and low permeability of the clay causing the instability of ground and structure. This detrimental effect of soft clay can be improved by the cement solidification process, which is relatively cheap and efficient. This paper mainly focuses on the study of improvement on the mechanical behavior of cement mixed marine clay. The soil is reconstituted by using ordinary Portland cement of 5%, 10%, 15% and 20% by its mass. The study reveals that cementation of clay significantly improves the peak and residual strength of soil. Similarly, the primary yield stress of the soil is also improved from 16 to 275?kPa as cement content increases from 5% to 20%, respectively. By using statistical tools, the relationships between various parameters are established, which are very important to define the mechanical behavior of the clay. This study reveals that the yield surface of the solidified marine clay is not a smooth elliptical surface. Rather it is composed of two linear surfaces followed by a log-linear surface which can be modeled by using simple parameters obtained from triaxial tests.     

Effect of salinity on rheological and strength properties of cement-stabilized clay minerals

Abstract

This paper presents an experimental investigation into the effect of salinity on Atterberg limits, flowability, viscosity and strength properties of cement-stabilized clay minerals. Three groups of clay minerals (illite, kaolinite and montmorillonite) were obtained. Specimens with different porewater salinities were prepared by mixing the air-dried clays with sodium chloride (NaCl) at various salt concentrations (i.e., 0%, 2% 4%, 6% and 8%). Atterberg limits test results indicated that liquid limit and plasticity index decreased insignificantly with increasing salinity for Kaolinite and illite but significantly for montmorillonite. Flow test results indicated that of all specimens of three groups of clay minerals with or without adding cement consistently increase with increasing salinity. The flow value of montmorillonite increased more significantly than kaolinite and illite. Viscosity test results indicated that all the specimens tested behave as Bingham plastic. Flow value consistently decreased with increasing dynamic viscosity or yield stress, regardless of clay mineralogy, porewater salinity and cement amount. Strength test results indicated that all cement-stabilized specimens exhibit strain softening behavior. Unconfined compressive strength for three groups of clay minerals stabilized with cement consistently decreased with increasing salinity indicating that the presence of salt had an adverse effect on the development of strength.     

Laboratory study on cement admixed marine silt bricks

For the purpose of efficient utilization of sediments dredged from harbor, a new method was proposed in this study. Marine silt bricks were made by mixing sediments with cement and gypsum, placing it in a cubic mold with 240 mm in length, 115 mm in width, and 53 mm in height, and curing for certain days. To investigate the effects of cement and initial water content of soil on the mechanical behavior of marine silt bricks, unconfined compressive and flexural strength tests were carried out. Given the same curing time and cement content, the higher the initial water content, the lower the compressive and flexural strength. After 60 days of curing, the compressive strength of marine silt bricks with cement content = 20% and water content = LL (liquid limit) reached approximately 5 MPa. The flexural strength was relatively low. The flexural strength of marine silt bricks with 20% cement and water content = LL was around 1.5 MPa. The compressive and flexural strength decreased with the increase of water/cement ratio. As for the curing time, longer curing time had a positive impact on the compressive strength. The ratio of flexural to compressive strength varied slightly in the range of 0.4–0.5.     

Using recycled calcium sources to solidify sandy soil through microbial induced carbonate precipitation

Abstract

The use of calcium solutions is a cost-limiting factor for bio-cement production from microbially induced carbonate precipitation (MICP). The aim of this article is to analyse the feasibility of using recycled calcium sources to solidify sand, including oyster shells, scallop shells and eggshells, by comparing the physical and mechanical properties and microstructural characteristics of solidified sand with different recycled calcium sources and chemical calcium nitrate. The results show that oyster shells have the optimal effect on MICP, with values of permeability, dry density, unconfined compressive strength and calcium carbonate precipitation of 1.12?×?10^?4 m s^?1, 2.09?g cm^?3, 1454.6?kPa and 15.28%, respectively. Strength values of bio-cemented sands made from different recycled calcium sources in this article range from 845.1 to 1454.6?kPa. According to the SEM and XRD analysis, calcium carbonates originating from the above recycled calcium sources precipitate as globular vaterite, whereas the precipitation from calcium nitrate is a cluster mixture of vaterite and calcite. Oyster shells, scallop shells and eggshells derived from kitchen waste, which is more economical and environmentally friendly than calcium nitrate, can be applied as recycled calcium solutions in MICP.     

Elevated curing temperature-associated strength and mechanisms of reactive MgO-activated industrial by-products solidified soils

Abstract

Alkali-activated industrial by-products (granulated blast furnace slag, Class F fly ash) by traditional alkali activator (such as NaOH and Na₂SiO₃) serves as a partial replacement for Portland cement in soil stabilization projects and suffers from environmental and technical problems. Reactive MgO – a greener and more practical alternative has recently emerged as a potential activator for slag and fly ash, but its micromechanisms of alkaline activation still need to be deeply investigated for strength improvement of soils. Hence, this study focuses on the strength and hydration properties of reactive MgO-slag and MgO-fly ash solidified soils, especially incorporating the impact of elevated curing temperature. Reactive MgO is proved to be excellent as a novel activator for activation of slag and fly ash, and their activating efficiency increases with elevated curing temperature that helps to remarkably enhance the compressive strength of soils. The major hydration products for reactive MgO-slag solidified soils, detected jointly by X-ray diffraction, scanning electron microscopy and thermogravimetric/differential thermogravimetric tests, are calcium silicate hydrate gels and hydrotalcite-like phases. The primary hydration products for MgO-fly ash solidified soils are magnesium silicate hydrate gels and Mg(OH)₂. That is just the intrinsic reason why the microstructure of solidified soils becomes much denser and the mechanical behavior is significantly improved. The minor carbonate phases such as magnesium carbonate and/or calcite are also observed in reactive MgO-slag and MgO-fly ash solidified soils, depending on the period of exposure to air. The curing temperature and binder amount are proved to be the two major factors governing the hydration process of reactive MgO-slag and MgO-fly ash blends. A higher curing temperature and binder amount can generate more hydration products, but their chemical compositions such as accurate element ratios need to be investigated in the future study.     

Effects of fly ash and slag content on the solidification of river-dredged sludge

Abstract

River-dredged sludge has a high water content and minimal bearing capacity and strength. Adding cement, fly ash, and slag to dredged sludge as a combined curing agent can quickly reduce its water content and improve its strength. This study experimentally investigates the solidification effectiveness of different proportions of curing agents using methods including electron microscopy, particle size analysis, water ratio limit, and water content and direct shear tests. The water content and shear strength of different combined curing agents are obtained at different ages. We find that an optimum curing agent combination exists. With increases in fly ash and slag content, test results indicate that the water content of solidified sludge first decreases and then increases, whereas the shear strength first increases and then decreases, allowing an optimal combination curing agent to be obtained. When using industrial waste residue as curing agent, it is necessary to consider the negative effects of the curing agent to better control the dosage so as to achieve better curing effect.     

A dredged material solidification treatment for fill soils in East China: A case history

Large amounts of sediments are dredged annually from Chinese oceans. Dredged materials (DMs) possess poor geotechnical properties and are normally treated as waste. This paper presents the first large-scale engineering application of DM solidification treatment in China. The technique has been used to treat approximately 1.8?×?10⁶?m³ of DM from Taihu Lake to produce fill soils. Portland cement was chosen as the solidification material, the amount of which is confirmed through indoor unconfined compressive strength (UCS) tests. Special solidification machines process DM at 120?m³/hours. Field-based DM solidification engineering began in September 2006. Curing specimens were examined over 28 days. Results show that both UCS and failure strain of solidified DM could meet fill soil requirements. Bearing capacity was also assessed with a cone penetrometer test. Samples were examined after 2 years (after project completion), and the mean UCS of the specimens was 237.2?kPa, which completely satisfied the engineering request. Wuxi Taihu City Science and Technology Industrial Park has now been established on top of the solidified DM storage yard. The successful engineering of such facilities results in economic and environmental benefits; thus, engineering applications of DM solidification treatment are widely promoted in China.     

Stress-Strain Behavior of Light-Weighted Soils

Unconfined and triaxial compression tests were carried out to examine the behavior of light-weighted soils (LWS) consisting of expanded polystyrene (EPS), dredged soils, and cement with respect to initial water content. The stress-strain behavior of LWS are analyzed with varying initial water content and silt contents of dredged soils, cement ratio, and confined stress. As initial water contents increase, the compressibility index increases and the preconsolidation pressure was vice versa. As initial water contents increase, the slope of stress-strain curve in elastic zone increases and strain rate at failure decreases and the strain rate at failure was not changed by the being of foams. As initial water contents increase, a compressive strength of LWS decreases. The decrement ratio of compressive strength of LWS with foams increases as cement content increases and initial water contents decreases. The compressive strength increases as silt contents increases.     

Long-Term Consolidation and Strength Behavior of Marine Clay Improved with Fly Ash

This paper presents an investigation of the long-term consolidation and strength behavior with fly ash as an additive in improving soft marine clay in Wando, Korea. 0%, 5%, 10%, 20% and 25% of the soil was replaced with fly ash. Consolidation tests were performed as incremental loaded tests. In addition, unconfined compressive strength were determined after 1, 14, 28 and 90 days. A series of forty-two long-term consolidation tests that lasted for 60 days under the constant loading were also conducted. Creep settlements of the blends decreased significantly with an increase in fly ash content. The shear strength properties increased with an increase in fly ash content. Statistical evaluation reveals an excellent correlation between the measured and predicted undrained shear strengths.     

Mechanical Characteristics of Light-Weighted Soils Using Dredged Materials

This study investigates the mechanical characteristics of light-weighted soils (LWS) consisting of expanded polystyrene (EPS), dredged clays, and cement through both unconfined and triaxial compression tests. The mechanical characteristics of the compressive strength of LWS are analyzed with varying initial water contents of dredged clays, EPS ratio, cement ratio, and curing pressure. In the triaxial compression test, it is found that the compressive strength of LWS associated with EPS is independent on the effective confining pressure. When both EPS ratio is less than 2% and cement ratio is more than 2%, the compressive strength rapidly decreases after the ultimate value. This signifies that the compressive strength-strain behavior is quite similar to that of the cemented soil. The ground improved by LWS has the compressive strength of 200 kPa associated with the optimized EPS ratio of 3-4% and initial water content of 165-175%. The ultimate compressive strength under both triaxial and unconfined compression tests is almost constant for a cement ratio of up to 2%.     

Mechanical Characteristics of Light-Weighted Soils Using Dredged Materials

This study investigates the mechanical characteristics of light-weighted soils (LWS) consisting of expanded polystyrene (EPS), dredged clays, and cement through both unconfined and triaxial compression tests. The mechanical characteristics of the compressive strength of LWS are analyzed with varying initial water contents of dredged clays, EPS ratio, cement ratio, and curing pressure. In the triaxial compression test, it is found that the compressive strength of LWS associated with EPS is independent on the effective confining pressure. When both EPS ratio is less than 2% and cement ratio is more than 2%, the compressive strength rapidly decreases after the ultimate value. This signifies that the compressive strength-strain behavior is quite similar to that of the cemented soil. The ground improved by LWS has the compressive strength of 200 kPa associated with the optimized EPS ratio of 3–4% and initial water content of 165–175%. The ultimate compressive strength under both triaxial and unconfined compression tests is almost constant for a cement ratio of up to 2%.     

Equivalent void ratio controlling the mechanical properties of cementitious material-clay mixtures with high water content

Abstract

This research develops a parameter defined as the equivalent void ratio, e?_st, which is able to accurately describe the dependence of the mechanical properties of cementitious material-clay mixtures on the influencing parameters, i.e., the mixing proportion, curing time, and initial state of the mixture, for different types of cementitious materials based on the results of unconfined compression, oedometer, and triaxial tests. Besides Portland cement, cementitious materials, such as fly ash and rice husk ash, are considered supplementary cement with different levels of efficiency. This equivalent cementitious material concept is then adapted for parameter development in conjunction with the effective void ratio proposed from our previous study. The developed single parameter, e?_st, can assess the mechanical properties of cementitious material-clay mixtures with different types of cementitious materials and under different test conditions.     

Strength evaluation of marine clay stabilized by cementitious binder

Abstract

Evaluation of the strength of cement-treated clay with a broad range of mix ratios and curing periods was conducted using unconfined compression tests (UCTs). The influence of cement content, total water content, and curing period on the unconfined compressive strength of cemented clay are investigated. It is found that, at constant total water content, higher cement content results in higher unconfined compressive strength, while the total water content has an opposite effect. A power function can be used to correlate the unconfined compressive strength with the cement content or the total water content. For a fixed mix ratio, the unconfined compressive strength of cement-stabilized clay increases with the curing period, the effect of which can be characterized by a semi-log formula. Also, a strength-prediction model that considers both mix ratios and curing periods for cement-admixed marine clay is developed and validated; the model can capture the effect of clay type by considering the plastic index of untreated soils. It is also proved that the proposed framework for strength development is also applicable for other cement types.     

Biogrouting of hydraulic fill fine sands for reclamation projects

ABSTRACT

Biogrouting, which is a new method for soil improvement, was used in an attempt to cement a type of hydraulic fill fine sands (called black sands) in reclamation projects in Tianjin, China, to form a working layer for mechanical equipment. Several factors influencing biogrouting with regard to cementing solution, including injection frequency, reaction time, concentration, and flow rate, were controlled to prepare black sand columns. This paper reports on an investigation of bacterial fixation, calcium ion utilization, and calcium carbonate distributions of biogrouted sand specimens. At the end of the tests, the geotechnical performances of the sand specimens were determined. The results showed that the biogrouting method effectively solidified black sands, by increasing the unconfined compressive strength of a sand column to 1.91?MPa and reducing the permeability coefficient by three orders of magnitude. A relationship between the unconfined compressive strengths and calcium carbonate contents was put forward, in addition to a relationship between the permeability coefficients and the calcium carbonate contents. According to the experimental results, some reasonable suggestions regarding the application of biogrouting to the consolidation of hydraulic fill fine sands in reclamation projects were proposed.     

Properties of sediments deposited in a fluid with different pH

Abstract

The effect of pH on the physical and mechanical properties of a sediment was investigated through a set of experimental tests. The sediment was formed from deposition of suspended particles in a fluid. Two different types of clay soil were suspended in fluids with different pH (2, 4, 7, 9 and 11) in cylindrical tubes with volume of 1?liter and also in special cylindrical reservoirs. The height of the sediment was measured in the cylindrical tube until equilibrium was achieved. The sediment deposited in the reservoirs was dried in air and then Atterberg limit, compaction and unconfined compressive strength (UCS) tests were conducted on samples prepared from each sediment. The results showed that the final height of the settled sediment is a function of pH; the height of sediment is increased with increasing the pH. Also, the Atterberg limits increased with increasing the pH. The maximum dry unit weight and optimum water content decreased and increased with increasing the pH. The final strength of the sediment decreased with increasing pH. Based on the SEM analysis, it was found that the values of pH influence the properties of the formed sediments.     

Effect of nano-MgO on mechanical performance of cement stabilized silty clay

Abstract

A series of direct shear tests were performed on cement-admixed silty clay to investigate the effect of cement content and nano-magnesia (MgO) on its shear strength properties. For each normal stress, shear strength increased with cement content. However, an obvious increment in shear strength was achieved when the cement content was adjusted from 13% to 17%. Both cohesion and friction angle of cemented soil increased with cement content, and exponential function was adopted to correlate both the factors with cement content. For cement content of 10% investigated in this study, the optimum nano-MgO content was 10‰, wherein the cohesion could reach the peak value. The microstructure of the mixture revealed that the structure of the mixture was compacted for the optimum nano-MgO content. However, micro-cracks were formed when the amount of nano-MgO exceeded its optimum content.     

Strength Characterization of Sand-Bentonite Mixtures and the Effect of Cement Additives

This article studies the strength properties of compacted sand-bentonite landfill barrier material with and without cement addition at different periods of aging. Test results indicated that strength values, both in compression and tension, increased up to threefold in cement added samples, as well as enhancing the ductile behavior. Cubic modulus, described as the slope of the elastic portion of the cubic compressive stress versus strain curves, is determined along with initial and flexural moduli, and all the strength and moduli values were correlated with each other. Finally, it is concluded that there is a marked improvement in strength properties and moduli with cement inclusion and that the effect of aging has been very effective.     

Mechanical Behavior of Light-Weight Soil Under Consolidated Drained Shear Condition

To reveal the influence of material composition on mechanical properties of light-weight soil, stress-strain -volumetric strain characteristics and Poisson's ratio of mixed soil were researched by consolidated drained shear tests. The results show that light-weight soil is a kind of structural soil, so its mechanical properties are affected by mixed ratio and confining pressure, and mixed soil possesses structural yield stress. When confining pressure is less than the structural yield stress, strain softening occurs; when confining pressure is more than the structural yield stress, strain hardening is observed. There are two kinds of volume change behavior: shear contraction and shear dilatancy. Shear dilatancy usually leads to strain softening, but there isn't an assured causal relationship between them. Poisson's ratio of mixed soil is a variational state parameter with the change of stress state, it decreases with increased confining pressure, and it increases with increased stress level. When axial strain is near 5%, Poisson’ ratio is gradually close to a steady value. The main range of Poisson's ratio is 0.25～0.50 when confining pressure changes from 50 to 300 kPa. When unconfined compressive strength of mixed soil is less than 328 kPa, its stress-strain-volumetric strain characteristics can be predicted very well by Duncan-Chang model (E-B model). However, when the range of unconfined compressive strength is [328 kPa, 566 kPa], the model can't predict stress-strain characteristics accurately when confining pressure is under 200 kPa, and it also can't predict the strong shear dilatancy phenomenon of mixed soil under low confining pressure.     

Use of Posidonia Oceanica Ash in Stabilization of Expansive Soils

Posidonia oceanica (PO) is the most plentiful seaweed of the Mediterranean Sea, which grows all along the coastal areas, forming widespread meadows. The leaf rejuvenation process of Posidonia oceanica typically occurs in fall when an increase in wave action causes the dead seaweeds to be transported and usually piled up along the coastal areas. This paper investigates the effect of PO ash stabilization on behaviour of an expansive clay. The ash was obtained by combustion of crushed PO pieces in a muffle furnace at 550°C. Atterberg limits, linear shrinkage, particle size distribution, one-dimensional swell, and unconfined compression tests have been carried out on natural soil as well as soil mixtures with 5% and 10% PO ash. There has been no significant improvement in the soil properties with 5% ash inclusion, whereas 10% ash has noticeably reduced the swell amount and increased the compressive strength. It is therefore concluded that there is a potential for the use of PO ash in geotechnical engineering applications.