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.     

Effect of sulphates and curing period on stress–strain curves and failure modes of soil–lime–natural pozzolana mixtures

Abstract

An experimental investigation was undertaken in order to assess the effect of sodium (Na₂SO₄) and calcium (CaSO₄·2H₂O) sulphates and curing period on stress–strain curves and failure modes of grey (GS) and red (RS) clayey soils stabilised by lime (L), natural pozzolana (NP) and their combinations (L–NP). Several soil–L–NP mixtures were studied to be used as subgrade soils for road pavements. Stress–strain curves were obtained from unconfined compressive strength (UCS) test made on several soil–L–NP specimens after curing for 7 and 120 days. Tests results showed that the use of L or L–NP without sulphates produced a significant increase in peaks stress of both clayey soils and then modified their stress–strain curves from nonlinear to linear behaviour almost up to 70% of peak stress after a longer curing period. However, the presence of 2% Na₂SO₄ or any CaSO₄·2H₂O content provided beneficial effects on peaks stress and stress–strain curves of both stabilised clayey soils and then improved their linearity almost up to 95% of peak stress after curing for 120 days. In contrast, the presence of 6% Na₂SO₄ caused undesirable effects. In addition, both sulphates greatly affected the failure modes of soil–L–NP specimens, particularly at a later stage.     

Effect of aluminate content in cement on the long-term sulfate resistance of cement stabilized sand

Abstract

Durability of cement-based materials for marine/coastal structures is an increasingly challenging problem. Sulfate ions in seawater can react with aluminate in cement to form erosion products, causing cracks and spalling. When cement is used to stabilize loose erodible sand in coastal areas, the resistance to sulfate attack is questionable. In this study, four cements with different aluminate contents were used to stabilize sand. Cement stabilized sands were immersed in 5% Na₂SO₄ solution for 300-days to simulate long-term sulfate attack process. The deterioration of engineering performance was evaluated based on expansion ratio, mass change, uniaxial compressive strength, and ultrasonic velocity. The deterioration mechanisms were analyzed through mineralogical and microstructural observations including X-ray diffraction, EDS, scanning electronic microscopy, and nuclear magnetic resonance. The results showed that the development of macro-scale mechanical performances could be divided into two stages (initial stage and erosion stage) when subjected to 300-days immersion in 5% Na₂SO₄ solution. Sand stabilized by low-aluminate-content cement displayed better engineering performance especially at the erosion stage. Mechanistically, more ettringite was formed in high-aluminate-content cement stabilized sand, leading to swelling and cracking. The formation of ettringite and gypsum were accompanied with the consumption of portlandite, leading to further strength loss.     

Properties and performance of a high volume fly ash grout

Abstract

This study investigated the penetrability of high volume fly ash cement suspensions prepared with and without superplasticizer into sandy soil having different relative densities with 30%, 60%, 73%, and 83% through permeation grouting. Class C fly ash was used due to its pozzolanic activity and fineness. Due to engineering characteristics and cost, cementitious grouts are the most commonly used grout in both waterproofing and ground strengthening. Fly ash-cement grouts have relatively constant and low viscosity values for a reasonable period after preparation, exhibit limited or negligible bleed capacity and set and develop satisfactory strength within a relatively short period. Modeling of grouting of soil was done in laboratory and improvements in physical and mechanical properties of grouted soil were analyzed. Unconfined compressive strength, shear strength and permeability characteristics of grouted soil were studied as a result. Unconfined compressive strength values of grouted sand with high volume fly ash ranged between 410 and 1107?kPa. Morover, cohesion values were comparable to microfine cement grouting ranging from 373 to 511?kPa. Furthermore, permeability values were also approximately equal to the permeability of impervious liners, which is around 1?×?10^?7?cm/s. The findings support the applicability of grouting in different applications.     

Effect of salt on strength development of marine soft clay stabilized with cement-based composites

Abstract

Marine soft clay with a high salt concentration is widely distributed in coastal areas. In this study, cement-based composites consisting of cement, silica fume, plant ash and NaOH were used as a substitute for ordinary Portland cement, and the effect of salt (sodium chloride) on the strength development of clay was investigated by unconfined compressive strength (UCS) testing and scanning electron microscopy (SEM). With the addition of sodium chloride (NaCl), the amount of cementitious materials decreased, and the salt (sodium chloride) was considered to consume the cement-based composites. The consumption effect could be quantitatively evaluated by the consumption index of salt (CIS) and the clay-water/cement ratio hypothesis. The relationship between the CIS and curing period and an UCS prediction model of clay stabilized with cement-based composites with different salt contents and curing times were established. The CIS gradually decreased with increasing curing time and cement-based composites content. The accuracy of the prediction model was evaluated by a comparative analysis between the measured strengths and predicted strengths; the deviation was mostly within 10%. SEM analyses were employed to describe the changes in the microstructure of the specimens and the influencing mechanism of salt on clay stabilized with cement-based composites.     

Sulphate complexation in seawater: A relation between the stability constants of sodium sulphate and magnesium sulphate in seawater from a determination of the stability constant of hydrogen sulphate and a calculation of the single ion activity coefficient of sulphate

The conditional stability constant of HSO₄^? has been determined at 25°C, 1 atm and a formal ionic strength of 0.7 M in solutions containing sodium, magnesium, chloride and sulphate. This was done spectrophotometrically (UV), using diphenylamine as indicator. The value obtained was 17.0 ± 0.1 (molar scale). Single ion activity coefficients for Na₂SO₄, K₂SO₄ and MgSO₄ have been calculated according to the Bates et al. (1970) model, assuming that the sulphate ion is not hydrated. It was found that the single ion activity coefficient of sulphate changes very little between Na₂SO₄, K₂SO₄ and MgSO₄ when the formal ionic strength is kept constant.These results have been used to obtain relations between the stability constants of NaSO₄^? and MgSO₄ valid for seawater.     

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.     

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.     

Experimental Investigation of Unconfined Compression Strength and Stiffness of Cement Treated Salt-Rich Clay

Soft clay with high sodium chloride salt concentration is a problem encountered by geotechnical and highway engineers. Chemical stabilization using cement is an attractive method to improve the engineering properties of soft soil. However, very limited studies have been conducted to reveal the effect of salt concentration on the engineering properties of cement-stabilized soil and the reported results in literature are not consistent. The impact of sodium chloride salt on the strength and stiffness properties of cement-stabilized Lianyungang marine clay is studied in this study. The clay with various sodium chloride salt concentrations was prepared artificially and stabilized by various contents of Ordinary Portland cement. A series of unconfined compressive strength (UCS) tests of cement stabilized clay specimen after 7, 14, and 28 days curing periods were carried out. The results indicate that a high sodium chloride salt concentration has a detrimental effect on the UCS and stiffness of cement-stabilized clay. The detrimental effect of salt concentration on the strength and stiffness of cement-stabilized clay directly relates to cement content. Soils mixed with high cement content are more resistant to the negative effect of salts than soils mixed with low cement content. The ratio of modulus of elasticity to UCS of cement treated soil does not have an obvious relationship with salt concentration. The findings of this study present a rational basis for the understanding of the impact of salt on the engineering properties of cement-treated soil.     

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.     

Inhibitory effects of chloride ions on concrete sulfate attack in the marine adsorption environment

To study the inhibitory effects of chloride ions on concrete sulfate attack in the marine adsorption environment, a series of cubic concrete specimens was fabricated. These specimens were totally submerged or partially submerged in 10% Na₂SO₄ solutions with 0, 0.5, 2, and 5% NaCl concentration. Three specific aspects, namely, superficial phenomenon, compressive strength degradation, and X-ray fluorescence spectrometry analysis, were studied during 450 days of long-term exposure. Results showed that Na₂SO₄ solution had a strong corrosive effect on concrete in both the adsorption and submersion zones, especially in the adsorption zone. Chloride ions exerted inhibitory effects on concrete sulfate attack in both the submersion and adsorption zones, and higher concentration of chloride ions will produce stronger inhibitory effects. However, given the different inhibitory effects of chloride ions on concrete sulfate attack with different corrosion mechanisms, the inhibitory effect of the same chloride concentration on concrete sulfate attack was a little lower in the adsorption zone than in the submersion zone.     

Thermodynamics of the system H2ONaClMgCl2Na2SO4MgSO4 at 25°C

Osmotic coefficients and Gibbs energies of mixing for the system H₂ONaClMgCl₂MgSO₄ have been calculated from isopiestic vapor pressure measurements. These results have been combined with earlier results from the system H₂ONaClNa₂SO₄MgSO₄ to construct Jänecke diagrams giving osmotic coefficients at constant ionic strength. Osmotic coefficients calculated from the theory of Reilly and Wood are in good agreement with measured values for the four-salt system, for compositions and ionic strengths comparable to those of natural brines.     

Seawater: an explanation of differential isothermal compressibility measurements in terms of hydration and ion-water interactions

Hydration, ion-water interactions, and water structure effects in seawater were studied by determining differences (Δβ) between the compressibilities of test salt solutions and the compressibilities of reference solutions. The reference solutions were distilled water and seawater (35%0), and the test salt solutions were either 0.13 m or 0.26m with respect to one of the following test salts: LiCl, NaCl, KCl, CsCl, NaF, NaI, MgCl₂, CaCl₂, BaCl₂, Na₂SO₄, K₂SO₄, and MgSO₄. The compressibility measurements (to 900 bars) were carried out at 2°C and also at 15°C using a differential method in which a pressure increase or a temperature increase causes Δβ to become less negative. At 1 bar and 15°C, the Δβ (0.26 m, distilled water reference) values ranged from ?1.14 × 10^?6 bar^? for NaI to ?3.84 × 10^?6 bar^?1 for Na₂SO₄, and the Δβ (0.26 m, seawater reference) values ranged from ?1.30 × 10^?6 bar^?1 for NaCl to ?3.04 × 10^?6 bar^?1 for Na₂SO₄. The Δβ values were used to calculate hydration numbers. Entropy of transfer, excess hydrogen bond breaking (determined by NMR), and effective radii of ions are properties which can be used to describe the influence of ions on water structure. The extent to which these properties correlate with Δβ values depends upon whether the ion is an anion or a cation, and this correlation forms the thesis that anions alter water structure in a different way than do cations.     

Experimental studies on the possible influences of composition changes of pore water on the stability conditions of methane hydrate in marine sediments

Experiments with a set of electrolyte solutions have been carried out to investigate the effects of pore water composition changes on the stability conditions of methane hydrate in marine sediments. The results reveal that (1) SO₄²⁻ and Cl⁻ concentration changes can affect hydrate stability slightly, (2) the changes in both the type and the concentration of cations, which occur in normal diagenetic processes, do not exert a significant influence on the methane hydrate stability conditions, and (3) the shift of hydrate stability in pore water can be expressed as a function of the Cl⁻ concentration only. Based on the results above, an empirical equation ΔT (K)=0.00206 Cl⁻ (mmol/dm³) has been obtained for estimating the shift in the equilibrium temperature of methane hydrate in pore water at a given pressure.     

黄河三角洲浅层地下水化学特征与演化

2009年4月~2010年3月连续2年对黄河三角洲19口地下水监测井进行观测并对地下水离子化学成分分析,探讨了该区域浅层地下水化学成分、水化学类型及演化规律,为揭示区域地下水环境特征和演化具有现实意义。结果表明,黄河三角洲地区地下水p H值变化较小,地下水埋深较浅且年内变化幅度为1.0~3.0 m,但矿化度和各离子差异明显,地下水以Na+、Cl–占绝对优势;矿化度较低的测井的离子浓度变幅较小,反之,矿化度高的测井离子浓度变幅较大;地下水化学类型分为氯化物型、重碳酸盐氯化物型、重碳酸盐氯化物硫酸盐型和硫酸盐重碳酸盐型四大类型,主要包括Na+-Cl–、Na+-Cl–.3HCO?、Na+-Cl–.3HCO?、Na+-Cl–.3HCO?.24SO?、Na+-3HCO?.Cl–.24SO?和Na+-24SO?.3HCO?六种子类型,氯化物型主要分布于广饶县咸水入侵区和滨海区域,重碳酸盐氯化物型主要分布于黄河三角洲保护区内,重碳酸盐氯化物硫酸盐型分布在广饶县咸水入侵区,硫酸盐重碳酸盐型主要分布在靠近黄河流路附近;根据黄河三角洲流路变迁和Gibbs模型,黄河三角洲地区水样化学组成均落在Gibbs提出的Boomerang Envelope模型右上翼,表明研究区水样化学组成主要受蒸发和沉淀作用,海水控制起次要作用,土地利用变化、灌溉、施肥等人为活动的影响亦不能忽视。     

Experimental research on the mechanical properties of methane hydrate-bearing sediments during hydrate dissociation

This paper describes studies of the effect of hydrate dissociation on the safety and stability of methane hydrate-bearing sediments. Methane hydrates within the sediments were dissociating under the conditions of a confining pressure of 0.5 MPa, 1 MPa, 2 MPa and a temperature of −5 °C. After 6 h, 24 h, or 48 h, a series of triaxial compression tests on methane hydrate-bearing sediments were performed. The tests of ice-clay and sediments without hydrate dissociation were performed for comparison. Focusing on the mechanical properties of the sediments, the experimental results indicated that the shear strength of the ice-clay mixtures was lower than that of the methane hydrate-bearing sediments. The strength of the sediments was reduced by hydrate dissociation, and the strength tended to decrease further at the lower confining pressures. The secant modulus E_S of the sediments dropped by 42.6% in the case of the dissociation time of the hydrate of 48 h at the confining pressure of 1 MPa; however, the decline of the initial yield modulus E₀ was only 9.34%. The slower hydrate dissociation rate contributed to reducing the failure strength at a declining pace. Based on the Mohr–Coulomb strength theory, it was concluded that the decrease in strength was mainly affected by the cohesive reduction. Moreover, the mathematical expression of the M–C criterion related to the hydrate dissociation time was proposed. This research could be valuable for the safety and stability of hydrate deposits in a permafrost region.     

GEOCHEMICAL CHARACTERISTICS OF INTERSTITIAL WATER OF THE BOHAI GULF

This paper discusses the geochemical characteristics of the interstitial water of the Bohai Gulf, where the main salt contents (K+, Na+, Ca++ Mg+ + , C1-, HCO3-, SO4-) in 55 sedimentary layers of 13 cores (3-4 m in length) and those of bottom seawater have been determined and the exchange capacity and exchange cations (K+, Na+, Ca++, Mg++) of the layers analysed (see Fig. 1)Fig. J. The stations of core samplings in the Bohai Gult.     

The effects of sodium sulfate on the emissions characteristics of an emulsified marine diesel oil-fired furnace

Degraded diesel oils are commonly used in marine power plants to conform to the demands of shipowners for fuel economy. The burning of these marine fuel oils, which frequently contain various extents of oxides of iron, silicon, calcium, vanadium and potassium, such as Na₂SO₄, Fe₂O₃, SiO₂, CaO, V₂O₅, etc., are susceptible to form much more complex compounds of either gaseous or solid phases. The release of these emissions to the environment may cause atmospheric pollution and a health hazard to human beings. Emulsification of a fuel oil with water to produce a micro-water-particles-dispersed-in-oil (W/O) emulsion has been considered as one of the promising techniques to improve combustion characteristics of low-grade marine oils and in turn effectively help to reduce the release of air pollutants. Marine fuel oil A, which approximates ASTM No. 2D oil was used as the test oil and the surfactant Span 80 was used to promote the affinity and integrating force between the components of the emulsion. An emulsifying/homogenizing machine was employed to stir the emulsion mixture of the marine oil, distilled water, surfactant Span 80 and sodium sulfate (Na₂SO₄) powder of 300 ppm. The mechanically blended emulsion mixture was injected, atomized and burned in an oil-fired furnace using an automatic burner. Burning gas composition, burning efficiency and gas temperature were measured and analyzed. Compared to neat marine diesel oil, W/O emulsions had higher combustion efficiencies, higher concentrations of O₂ and SO₂, while gas temperatures were lowered and CO and NO_x production was reduced. The addition of sodium sulfate decreased combustion efficiency and NO_x concentration and increased O₂, CO, and SO₂ concentrations.