Sorption and diffusion studies with low molecular weight organic compounds in cementitious systems

The uptake of methanol, ethanol, formaldehyde, acetaldehyde, formic acid and acetic acid by cement paste was determined in hydrating cement after 1 h, 28 d and 390 d hydration. The sorption values determined for formate and acetate were critically assessed by investigating through- and out-diffusion of these compounds in fully hydrated cement paste and their uptake by individual cement phases. Diffusion studies included inverse modelling of four data sets for each compound and an uncertainty analysis based on a Latin hypercube sampling procedure. Solid–liquid distribution ratios determined from the hydration experiments are on the order of 10⁻⁴ m³ kg⁻¹ in the case of alcohols and aldehydes indicating non-specific (very weak) bonding onto the surface of the cement phases, e.g. through hydrogen bonding. Hydration and diffusion studies reveal slightly higher distribution ratios and reversible uptake by cement paste and cement phases in the case of acetate indicating specific adsorption (electrostatic interaction) onto partially positively charged surface sites of the cement phases. Selective binding of a small fraction of formate is evidenced from both sorption and out-diffusion experiments suggesting the presence of sorption sites capable of strongly bonding the anion, presumably by SO₄²⁻/HCOO⁻ replacement in the ettringite structure.     

小导管注浆处理顶管涌水技术

结合实际顶管工程,从地质条件出发分析涌水原因和注浆处理涌水的作用机理,详细讨论了小导管处理涌水的施工工艺。注浆效果检查表明小导管注水泥-水玻璃双液浆处理裂隙和地下水发育的围岩涌水地质条件效果明显,加固措施可行,为顶管工程在施工中和后期运营提供安全保证。     

A dual-porosity model for ionic solute transport in expansive clays

A microstructure model of dual-porosity type is proposed to describe contaminant transport in fully-saturated swelling clays. The swelling medium is characterized by three separate-length scales (nano, micro, and macro) and two levels of porosity (nano- and micropores). At the nanoscale, the medium is composed of charged clay particles saturated by a binary monovalent aqueous electrolyte solution. At the intermediate (micro) scale, the two-phase homogenized system is represented by swollen clay clusters (or aggregates) with the nanoscale electrohydrodynamics, local charge distribution, and disjoining pressure effects incorporated in the averaged constitutive laws of the electro-chemo-mechanical coefficients and the swelling pressure, which appear in Onsager’s reciprocity relations and in a modified form of Terzaghi’s effective principle, respectively. The microscopic coupling between aggregates and a bulk solution lying in the micropores is ruled by a slip boundary condition on the tangential velocity of the fluid, which captures the effects of the thin electrical double layers surrounding each clay cluster. At the macroscale, the system of clay clusters is homogenized with the bulk fluid. The resultant macroscopic picture is governed by a dual-porosity model wherein macroscopic flow and ion transport take place in the bulk solution and the clay clusters act as sources/sinks of mass of water and solutes to the bulk fluid. The homogenization procedure yields a three-scale model of the swelling medium by providing new nano and micro closure problems, which are solved numerically to construct constitutive laws for the effective electro-chemo-hydro-mechanical coefficients. Considering local instantaneous equilibrium between the clay aggregates and micropores, a quasisteady version of the dual-porosity model is proposed. When combined with the three-scale portrait of the swelling medium, the quasisteady model allows us to build-up numerically the constitutive law of the equilibrium adsorption isotherm, which governs the instantaneous immobilization of the solutes in the clay clusters. Moreover, the constitutive behavior of the retardation coefficient is also constructed by exploring its representation in terms of the local profile of the electrical double layer potential of the electrolyte solution, which satisfies the Poisson–Boltzmann problem at the nanoscale.     

Alteration processes of cement induced by CO2-saturated water and its effect on physical properties: Experimental and geochemical modeling study

The objective of this study is to understand cement alteration processes with the evolution of porosity and hardness under geologic CO₂ storage conditions. For this study, the cylindrical cement cores (class G) were reacted with CO₂–saturated water in a vessel (40 °C and 8 MPa) for 10 and 100 days. After the experiment, the CO₂ concentration and Vickers hardness were measured in the hydrated cement core to estimate the carbonation depth and to identify the change in hardness, respectively. Diffusive-reactive transport modeling was also performed to trace the alteration processes and subsequent porosity changes. The results show that cement alteration mainly results from carbonation. With alteration processes, four different reaction zones are developed: degradation zone, carbonation zone, portlandite depletion zone, and unreacted zone. In the degradation zone, the re-dissolution of calcite formed in the carbonation zone leads to the increase of porosity. In contrast, the carbonation zone is characterized by calcite formation resulting mainly from the dissolution of portlandite. The carbonation zone acts as a barrier to CO₂ intrusion by consuming dissolved CO₂. Especially in this zone, although the porosity decreases, the Vickers hardness increases. Our results show that cement alteration processes can affect the physical and hydrological properties of the hydrated cement under CO₂-saturated conditions. Further long-term observation is required to confirm our results under in-situ fluid chemistry of a CO₂ storage reservoir. Nonetheless, this study would be helpful to understand alteration processes of wellbore cements under CO₂ storage conditions.     

膨胀土膨胀特性的变化规律研究

研究了击实膨胀土的膨胀压力p与50 kPa下的膨胀率 随干密度、饱和度及含水量的变化规律。结果表明：p和 与干密度 、含水量 、饱和度 的关系及p- 间的关系皆可用幂指数函数描述;在不同条件下, , , 中的某一个或两个因素可更好地描述p和 的变化规律,一般在高含水量范围含水量因素与干密度因素等价;在低含水量范围含水量因素与饱和度因素等价;含水量一定时干密度因素与饱和度因素等价;存在一临界干密度,干密度大于临介干密度时,膨胀力随饱和度的增加而减小,反之则增加。     

CO2驱煤层气中煤层膨胀对套管稳定性的影响

随着中国经济的快速发展,能源供需矛盾日益突出,由于过度依赖化石能源,大量排放CO2,致使面临的环境压力越来越大。CO2驱煤层气(CO2 enhanced coal-bed methane)技术可同时增产煤层气和大规模减排CO2,在我国具有良好应用前景,但应用该技术在注采生产过程中因煤层吸附CO2产生的膨胀效应对套管稳定性的影响尚未引起重视。通过建立考虑CO2煤岩吸附膨胀效应的套管-充填材料-煤岩平面应变模型计算的套管应力和变形理论解与ABAQUS数值模拟结果具有良好的一致性,并分析了CO2吸附膨胀条件下套管应力的影响因素。分析结果表明,膨胀对套管应力和变形具有显著影响。在给定CO2吸附浓度和套管内径条件下,套管和充填材料的壁厚、充填材料和煤岩的力学参数是影响套管应力的主要因素,可通过增加套管或（和）充填材料壁厚以及改变充填材料力学参数来降低套管应力。开发超低弹模和较大泊松比的充填材料是降低套管应力的有效途径。     

泥质岩膨胀各向异性与循环胀缩特征

采用自主开发研制的软岩膨胀试验装置,对新生代煤系地层中泥质岩进行膨胀试验研究,分析了泥质岩膨胀各向异性以及循环胀缩特性,并结合SEM试验结果,探讨了膨胀各向异性和循环胀缩特性的形成机制。结果表明:泥质岩膨胀性随岩样端面与层理面夹角的增大而减小,具有明显的各向异性;泥质岩所含黏土矿物颗粒排列的择优取向导致了泥质岩吸水膨胀的各向异性,可通过膨胀各向异性系数进行定量描述;随着干湿循环次数的增加,泥质岩绝对膨胀率增加,趋于某一稳定值;相对膨胀率和相对收缩率降低,亦趋于某一稳定值;泥质岩循环胀缩特性是干湿循环过程中矿物颗粒排列方式改变和微裂隙萌生扩展的能量耗散共同作用的结果。     

软板岩膨胀特性试验及微观机制分析

针对软板岩内部特殊的构造及其遇水膨胀的特点,以湖南省怀化芷江地区的板岩为例,选取14组岩样分别进行矿物成分、微观结构测试和侧向约束膨胀试验,研究软质板岩的膨胀特性及其微观机制。结果表明:软质板岩侧向约束膨胀率与吸水时间之间呈指数函数关系,在试验早期,侧向约束膨胀率随吸水时间的延长而增大,且增长速率较快,侧向约束膨胀率-吸水时间关系曲线较陡;在试验后期,侧向约束膨胀率趋于稳定,侧向约束膨胀率-吸水时间关系曲线由陡变缓。侧向约束膨胀率随着吸水率的增长呈乘幂函数关系增长,在试验早期增长较快,而后增长速率变小,直至最后趋于稳定。轴向膨胀率随轴向荷载的增加呈负对数函数减小,初期减小幅度较大,后期逐渐变低。结构面方向和试样轴向的夹角α在区间[0°,90°]之间变化时,极限侧向约束膨胀率随着夹角α的增大而减小。     

Use of shredded tyre waste in improving the geotechnical properties of expansive black cotton soil

In recent years, geotechnical engineers are using tyre waste to alter the properties of soil for providing sustainable solutions to complex engineering problems. It is noted that black cotton soil, which is expansive in nature, covers a major portion of the Indian sub-continent, such as the Deccan plateau, Malwa plateau and a portion of Gujarat and poses challenging problems to infrastructural development in the region. In the present study, authors have characterised the geotechnical properties of black cotton soil, which is partially replaced with 4.75 mm passing – 2 mm retained (coarse fraction) and 2.0 mm passing - 75 micron retained (fine fraction) shredded tyre waste. The results of the experimental studies clearly show that partial replacement of black cotton soil with shredded tyre waste alters the geotechnical properties favourable to engineering applications and greatly reduces the swelling potential of the black cotton soil. It is noted that instead of fine fraction (2.0–0.075 mm), replacement with coarse fraction (4.75–2.00 mm) of shredded tyre waste can better be used for light weight fill behind the retaining walls as well as for embankment construction as it helps in reducing swelling pressure as well as improving the strength properties.     

浅议非饱和土基坑无支护开挖深度的确定

在D.G.Fredlund等的非饱和土抗剪强度理论及其延伸的Rankine土压力公式以及国内学者卢肇钧等近年来的研究成果(用膨胀力代替基质吸力来刻画非饱和土的抗剪强度)的基础上,推导了含膨胀力的非饱和土竖直边坡临界高度的计算公式,初步探讨了非饱和土基坑无支护开挖深度的确定方法。