Geopolymer as well cement and variation of its mechanical properties under different curing temperature and curing mediums

Geo sequestration of carbon dioxide (CO2) has been found to be one of the best solutions to reduce anthropogenic amount of greenhouse gases to the environment. Well integrity of sequestration wells should be maintained for the success of any sequestration projects. Well cement plays a vital role in well integrity for any sequestration projects, and ordinary Portland cement (OPC) based well cement has been used in underground wells. There are many problems, such as cement degradation, chemical attacks, durability issues, leakage, etc., associated with OPC based well cement. One of the best replacements for OPC based well cement would be the use of geopolymer cement, as it is economical in production, sustainable in reducing waste products, consumes less energy, doesn‟t undergo chemical attacks, durable, resistive in acidic environments and possess higher strength compared to OPC. This paper will review suitability of geopolymer as well cement under downhole conditions, and analyse the advantages of using geopolymer over OPC-based well cement. Moreover, well cement will be exposed to range of temperatures, pressures and fluid medium from the ground surface to sequestration depths of more than 1 km. Therefore, this paper aims to study the mechanical behaviour of geopolymer under different curing temperatures (from 23 ºC to 80 ºC) and curing mediums (brine, water and CO2 saturated brine). It has been found that optimum curing temperature for higher strength is 60 ºC and geopolymer exhibits high strength compared to class G cement above ambient temperature. In addition, water saturated samples showed higher strength reduction compared to brine saturated geopolymer samples

Numerical simulation of settlement behaviour of axially loaded piles used for high-rise building

The reliable prediction settlement of pile foundation at typical working load remains one of the major geotechnical engineering problems. In this research, settlement behaviour of a pile foundation located in sandy-silt, under the loads from high-rised building is simulated in 2D using a finite element program (PLAXIS). Three different types of analysis were investigated: a linear elastic (LE) analysis where the soil was assumed as linear-elastic material, a simple nonlinear analysis where the soil was completely assumed as Mohr- coulomb (MC) model and an advanced nonlinear analysis where the soil was completely assumed as Hardening-Soil (HS) model. A comparison was done between the predicted settlement from Finite element analysis and field settlement values. Based on the results of analysis, it is suggested that although complete MC model shows good agreement with the settlement behaviour obtained from field static load test at lower working loads, MC model is not adequate to capture the settlement prediction at higher working loads. In addition, modelling the soil completely using HS model is required to capture the safe settlement prediction at higher working loads. Finally, this scenario can be applied for the similar problems in settlement prediction using numerical methods

Geopolymer as well cement for geological sequestration of carbon dioxide

Carbon capture and storage (CCS) is one of the feasible solutions to reduce carbon dioxide (CO2) emission levels without affecting the usage of fossil fuels. Wellbore integrity needs to be maintained for leak-free storage and well cement plays a major role in wellbore integrity as it provides the necessary zonal isolation. To date, ordinary Portland cement (OPC)-based sealant has been used in injection wells and it has been found that it experiences cement degradation and is unstable under CO2-rich down-hole conditions. Therefore, current research has focused on geopolymer, an alkali-activated inorganic binder, as an alternative well sealant material to OPC. The main objective of the research is to study the flow and mechanical behaviour of geopolymers under CO2 sequestration conditions using experimental studies and numerical modelling techniques. Comparisons were made with the outcomes of traditional OPC-based well cement materials. Uniaxial compression and triaxial flow experiments were conducted as part of the experimental program and the COMSOL mulitiphysics numerical package was used to conduct numerical analysis. A series of uniaxial compressive strength tests were performed to study the mechanical behaviour of the wellbore materials geopolymer (G), formation rock sandstone (S) and G-S composite materials under different in-situ conditions. The mechanical behaviour of three different materials in water and brine with two different NaCl concentrations (5% and 15%) was experimentally studied. As expected, peak compressive strength of all three materials reduces in water and brine water, and lowest reduction rates were observed for geopolymer saturated in 15% NaCl brine. In addition, the strength reduction rates of geopolymer are less than that of traditional OPC-based well cement materials. The lowest reduction in compressive strength of geopolymer is due to the increased resistance to alkali leaching of geopolymer in brine water compared to water. The compressive strength of sandstone saturated in brine water does not vary significantly compared to that of water, and this is due to the higher quartz content and lower NaCl concentration in brine water. Previous researchers have also noticed that higher quartz content sandstone is not sensitive to microstructural changes in brine water. As a typical wellbore experiences a range of temperatures with injection depth, the mechanical behaviour of geopolymer and class G cement was studied at different curing temperatures (23-80 ºC). The optimum curing temperature of both geopolymer and class G cement leading to higher mechanical strength is between 55-60 ºC. Geopolymer shows higher compressive strength and Young’s modulus values at elevated temperatures (> 36 ºC), whereas G cement possesses higher values at lower curing temperatures (< 36 ºC). The failure of geopolymer is shear at lower temperatures, while splitting failure is observed at elevated temperatures. On the other hand, class G cement fails in shear manner regardless of the curing temperature. Therefore, geopolymers can be employed at deeper depths of the well, where the temperature is higher, while G cement is suitable at shallow depths. A numerical study was performed to predict the mechanical behaviour of geopolymer at different temperatures. First, the model was calibrated with the experimental results and then a parametric study was performed to predict the mechanical behaviour under higher confining pressures. The structural mechanics module of COMSOL multiphysics was used, and a confining pressure range of 5-25 MPa was applied to geopolymer at different curing temperatures (23-80 ºC). As the temperature is increased from 23-60 ºC, the resulting failure strength increases by 205-320 % at different confining pressures (5-25 MPa), and a reduction of 9-10 % was observed from 60 ºC to 80 ºC. Increase in confinement and temperature increases the ductility and mechanical integrity of geopolymers due to plastic flow. When geopolymer is used as well cement in CCS wells, it is exposed to CO2 rich environment. Therefore, an experimental program was conducted to study the mechanical behaviour of geopolymer saturated in CO2. Based on the experimental findings, geopolymers show excellent mechanical integrity in CO2 as there was no sign of either strength reduction or cement degradation in geopolymer up to 6 months in CO2. In addition, geopolymer could not be fractured in hydraulic fracturing experiments, which employed different water injection pressures, confining pressure and tube lengths. Under any stress exposure conditions, the primary objective of well cement is to provide zonal isolation in the well. Since geopolymer could not be fractured under extreme stress exposures, it can provide good mechanical integrity and zonal isolation under severe loading conditions. Permeability tests were conducted with the high pressure triaxial rig available in the laboratory to investigate the permeability of wellbore materials under different in-situ stress conditions. A set of drained experiments was performed on geopolymer samples, and it was noticed that the apparent CO2 permeability of geopolymers (2×10-21 m2 to 6×10-20 m2) is lower than that of traditional American Petroleum Industry (API) class well cements (10-20 to 10-11 m2). In addition, increases in injection and confining pressures reduce the permeability of geopolymers. Another set of undrained flow experiments was conducted to study the sub- and super-critical CO2 permeability of wellbore materials (geopolymer (G), sandstone (S) and G-S composite). The permeability of sandstone and G-S composite materials is approximately thousands of times higher than that of geopolymer. Both the permeability and percentage permeability reduction (per 1 MPa increase in downstream pressure) reduce significantly from sub-critical to super-critical CO2 pressure conditions, implying the importance of super-critical CO2 phase conditions for effective and leak-free underground storage. An attempt was made to model the CO2 flow through geopolymer under laboratory triaxial stress conditions. The CO2 flow through geopolymer could be modelled using COMSOL multiphysics. The permeability values obtained from the model were consistent with the experimental outcomes. In addition, CO2 pressure and concentration distributions in geopolymer were also studied under various injection and confining pressures. The permeability of geopolymer depends on many factors, one being the mix composition of the geopolymer. Therefore, a series of undrained flow experiments was performed to study the effects of three different types of geopolymer on CO2 permeability, and existing class G cement was also tested for comparison purposes. Different types of geopolymers were made by adding 0, 8 and 15% of slag to fly ash (by mass). It was noted that the CO2 permeability values of geopolymers were approximately 100-1000 times lower than class G cement. In addition, the permeability of 15% slag added geopolymer was approximately 10 times lower than fly ash-based geopolymer, and this is explained by the reduction in pore diameter and increase in pore area of geopolymer with the addition of slag. Finally, the effect of temperature on the permeability of fly ash-based geopolymer was experimentally studied for a range of temperatures. The apparent CO2 permeability of geopolymer increases with increases in temperature from 23-70 ºC, however the permeability values (0.0004-0.04 μD) are well below the API recommended limits of 200 μD. An empirical formulation was developed to predict the permeability of geopolymer at different temperatures under various injection and confining pressure conditions

Compressibility behaviour of peat stabilized with low calcium fly ash an experimental study

Peat is a kind of soft organic soil having partially disintegrated plant remains hence it is not good for constructions. Chemical stabilization is the commonly used ground improvement technique by adding chemical admixtures such as ordinary Portland cement, fly ash, natural fillers etc. In Sri Lanka, annually 150 metric ton of fly ash is produced in Nuraicholai coal fired power plant and only about 20 % is usable for cement production, leaving huge amount of fly ash ends up in landfills. Thus, our research focused on stabilizing peat using a combination of fly ash and well graded sand. An experimental study was conducted to analyse the stabilization of peat with 125 kg/m3 dosage of well graded sand and fly ash at three various proportions 10, 20 and 30 % by weight. A series of experiments including Unconfined Compressive Strength (UCS) and Rowe cell test were conducted to evaluate the compressibility behaviour of stabilized peat. UCS increases up to 10 % fly ash addition and increases with curing period for all sample types. There is an improvement in settlement behaviour of peat after the stabilization using fly ash and well graded sand

Effect of low calcium fly ash (ASTM class f) on the stabilization behaviour of expansive soil

Expansive soil experiences swelling with the addition of water and then shrink after the removal of water. These alternate wetting and drying impose lot of problems to the structures built on expansive soils. Ground improvement techniques for expansive soil include chemical and mechanical method of soil stabilization. In this paper, chemical stabilization has been used as a ground improvement technique. Testing such as compaction, unconfined compressive strength (UCS) and swell pressure were conducted for expansive soil stabilized with ASTM Class F fly ash as a chemical stabilizer at 8%, 16% and 24% of total weight. Based on the outcome of this study, it was noticed that maximum dry density (MDD) increases up to 16% and then decreases beyond that. Effect of fly ash on variation of UCS value was observed with three different curing periods (7, 28 and 45 days) as well as three different percentages of fly ash (8%, 16%, 24%). UCS values increase up to 16% and then they decrease with any further addition of fly ash. Further, increment of curing period helps to increase the UCS value for a given percentage of fly ash mixture. Reduction of swell pressure was observed with addition of fly ash. On the whole, fly ash can be successfully used as soil stabilized to improve the geotechnical engineering properties of expansive soil

Geopolymer as well cement and its mechanical behaviour with curing temperature

Carbon capture and storage (CCS) technique is found as a best solution to reduce the emission of CO2 to the atmosphere. In this technique, the CO2 emitted from large industries is captured, and pressurized, and finally injected into deep underground reservoirs. In a geological sequestration project, integrity of injection well play an important role. It means the well cement is a key factor that affects the well integrity. In typical injection wells, Ordinary Portland cement (OPC) based cement is used as well cement and it has been found that it undergoes degradation in CO2 rich environment. Geopolymer can be a good alternative to existing OPC based well cement as it has been found that geopolymer possess high strength and durability compared to OPC. Geopolymer is a binder produced through the process called geopolymerization of alumino- silicate materials and alkaline activators. In the sequestration wells, well cement is exposed to different curing temperatures with a geothermal gradient of 30°C/km. Therefore, it is important to study the mechanical behaviour of well cement with curing temperatures expected deep under the ground. Therefore, this research aims to study geopolymer as well cement and its mechanical behaviour at different curing temperatures (25, 40, 50, 60, 70, 80 °C). In addition, effect of ageing on the mechanical behaviour was also studied. The OPC samples were tested for the comparison of results with geopolymer. The results showed that the optimal curing temperature for higher strength of geopolymer and OPC are 60 °C and 50 °C respectively. Geopolymer possess highest strength at elevated temperatures whereas OPC possess higher strength at ambient temperatures. Moreover, at elevated temperature curing, geopolymer develops ultimate strength within short curing period and it does not gain significant strength with further ageing

Geopolymer as well cement for geological sequestration of carbon dioxide

Carbon capture and storage (CCS) is one of the feasible solutions to reduce carbon dioxide (CO2) emission levels without affecting the usage of fossil fuels. Wellbore integrity needs to be maintained for leak-free storage and well cement plays a major role in wellbore integrity as it provides the necessary zonal isolation. To date, ordinary Portland cement (OPC)-based sealant has been used in injection wells and it has been found that it experiences cement degradation and is unstable under CO2-rich down-hole conditions. Therefore, current research has focused on geopolymer, an alkali-activated inorganic binder, as an alternative well sealant material to OPC. The main objective of the research is to study the flow and mechanical behaviour of geopolymers under CO2 sequestration conditions using experimental studies and numerical modelling techniques. Comparisons were made with the outcomes of traditional OPC-based well cement materials. Uniaxial compression and triaxial flow experiments were conducted as part of the experimental program and the COMSOL mulitiphysics numerical package was used to conduct numerical analysis. A series of uniaxial compressive strength tests were performed to study the mechanical behaviour of the wellbore materials geopolymer (G), formation rock sandstone (S) and G-S composite materials under different in-situ conditions. The mechanical behaviour of three different materials in water and brine with two different NaCl concentrations (5% and 15%) was experimentally studied. As expected, peak compressive strength of all three materials reduces in water and brine water, and lowest reduction rates were observed for geopolymer saturated in 15% NaCl brine. In addition, the strength reduction rates of geopolymer are less than that of traditional OPC-based well cement materials. The lowest reduction in compressive strength of geopolymer is due to the increased resistance to alkali leaching of geopolymer in brine water compared to water. The compressive strength of sandstone saturated in brine water does not vary significantly compared to that of water, and this is due to the higher quartz content and lower NaCl concentration in brine water. Previous researchers have also noticed that higher quartz content sandstone is not sensitive to microstructural changes in brine water. As a typical wellbore experiences a range of temperatures with injection depth, the mechanical behaviour of geopolymer and class G cement was studied at different curing temperatures (23-80 ºC). The optimum curing temperature of both geopolymer and class G cement leading to higher mechanical strength is between 55-60 ºC. Geopolymer shows higher compressive strength and Young’s modulus values at elevated temperatures (> 36 ºC), whereas G cement possesses higher values at lower curing temperatures (< 36 ºC). The failure of geopolymer is shear at lower temperatures, while splitting failure is observed at elevated temperatures. On the other hand, class G cement fails in shear manner regardless of the curing temperature. Therefore, geopolymers can be employed at deeper depths of the well, where the temperature is higher, while G cement is suitable at shallow depths. A numerical study was performed to predict the mechanical behaviour of geopolymer at different temperatures. First, the model was calibrated with the experimental results and then a parametric study was performed to predict the mechanical behaviour under higher confining pressures. The structural mechanics module of COMSOL multiphysics was used, and a confining pressure range of 5-25 MPa was applied to geopolymer at different curing temperatures (23-80 ºC). As the temperature is increased from 23-60 ºC, the resulting failure strength increases by 205-320 % at different confining pressures (5-25 MPa), and a reduction of 9-10 % was observed from 60 ºC to 80 ºC. Increase in confinement and temperature increases the ductility and mechanical integrity of geopolymers due to plastic flow. When geopolymer is used as well cement in CCS wells, it is exposed to CO2 rich environment. Therefore, an experimental program was conducted to study the mechanical behaviour of geopolymer saturated in CO2. Based on the experimental findings, geopolymers show excellent mechanical integrity in CO2 as there was no sign of either strength reduction or cement degradation in geopolymer up to 6 months in CO2. In addition, geopolymer could not be fractured in hydraulic fracturing experiments, which employed different water injection pressures, confining pressure and tube lengths. Under any stress exposure conditions, the primary objective of well cement is to provide zonal isolation in the well. Since geopolymer could not be fractured under extreme stress exposures, it can provide good mechanical integrity and zonal isolation under severe loading conditions. Permeability tests were conducted with the high pressure triaxial rig available in the laboratory to investigate the permeability of wellbore materials under different in-situ stress conditions. A set of drained experiments was performed on geopolymer samples, and it was noticed that the apparent CO2 permeability of geopolymers (2×10-21 m2 to 6×10-20 m2) is lower than that of traditional American Petroleum Industry (API) class well cements (10-20 to 10-11 m2). In addition, increases in injection and confining pressures reduce the permeability of geopolymers. Another set of undrained flow experiments was conducted to study the sub- and super-critical CO2 permeability of wellbore materials (geopolymer (G), sandstone (S) and G-S composite). The permeability of sandstone and G-S composite materials is approximately thousands of times higher than that of geopolymer. Both the permeability and percentage permeability reduction (per 1 MPa increase in downstream pressure) reduce significantly from sub-critical to super-critical CO2 pressure conditions, implying the importance of super-critical CO2 phase conditions for effective and leak-free underground storage. An attempt was made to model the CO2 flow through geopolymer under laboratory triaxial stress conditions. The CO2 flow through geopolymer could be modelled using COMSOL multiphysics. The permeability values obtained from the model were consistent with the experimental outcomes. In addition, CO2 pressure and concentration distributions in geopolymer were also studied under various injection and confining pressures. The permeability of geopolymer depends on many factors, one being the mix composition of the geopolymer. Therefore, a series of undrained flow experiments was performed to study the effects of three different types of geopolymer on CO2 permeability, and existing class G cement was also tested for comparison purposes. Different types of geopolymers were made by adding 0, 8 and 15% of slag to fly ash (by mass). It was noted that the CO2 permeability values of geopolymers were approximately 100-1000 times lower than class G cement. In addition, the permeability of 15% slag added geopolymer was approximately 10 times lower than fly ash-based geopolymer, and this is explained by the reduction in pore diameter and increase in pore area of geopolymer with the addition of slag. Finally, the effect of temperature on the permeability of fly ash-based geopolymer was experimentally studied for a range of temperatures. The apparent CO2 permeability of geopolymer increases with increases in temperature from 23-70 ºC, however the permeability values (0.0004-0.04 μD) are well below the API recommended limits of 200 μD. An empirical formulation was developed to predict the permeability of geopolymer at different temperatures under various injection and confining pressure conditions

A numerical study of triaxial mechanical behaviour of geopolymer at different curing temperatures: an application for geological sequestration wells

To date, ordinary Portland cement (OPC)-based cement has been used as well cement in carbon dioixide sequestration wells, although its durability and survival have been questioned by many researchers. Therefore, this work aims to study the use of geopolymer as well cement and predict the tri-axial mechanical behaviour of geopolymer cured from 23 to 80 &#0176;C, as a typical wellbore experiences such a range of temperatures with sequestration depth. The COMSOL multi-physics numerical simulator was first used to simulate the uniaxial experimental results, and then to model the tri-axial behaviour of geopolymer cured at different curing temperatures. Based on the modelling results, for all confining pressures (from 5 to 25 MPa), the deviatoric stress of geopolymer increases up to 60 &#0176;C curing temperature, and beyond that it reduces to 80 &#0176;C. When the curing temperature is increased from 23 to 60 &#0176;C, the deviatoric strength also increases by 300%, and then reduces by 15%&#8211; towards 80&#0176;C for all confining stresses. Generally, for a given curing temperature of geopolymer, the volume of geopolymer experiencing plastic deformation increases with the increase in confining pressure

Geotechnical Characterization of Peats in Muthurajawela Region in the Western Coast of Sri Lanka

Peatland covers a total area of 2500 hectares in Sri Lanka, and a more significant portion of it spreads in the western coast of the country through Muthurajawela wetland, which is the largest wetland in Sri Lanka extending from Colombo to Negombo. Colombo Katunayake Expressway (CKE) is a recently completed project in this region, and a lot of peatlands were encountered in the construction. To date, there are no studies focusing on characterization of peats in Muthurajawela region. Therefore, this study focused on investigating the geotechnical properties of peats in Muthurajawela region. Peat samples were obtained from six different locations in this area, and the samples were subjected to investigate the index properties including moisture content, density, specific gravity, fiber content, ash content, organic content and acidity, and consolidation characteristics. Further, Scanning Electron Microscopy (SEM) image analysis and Energy Dispersive X-ray (EDX) analysis were conducted to analyze the structure and to identify the elemental composition of peat samples. According to the basic investigations; Muthurajawela peats were classified as “Fibric and Hemic” peats with high acidity. The results further analysed to develop correlations for the easy prediction of some properties and, the developed correlations were compared with those reported in the literature. The obtained consolidation properties were indicated that the large portion of settlement occurs during the primary consolidation stage within a short time. On the whole, findings of this research will enable engineers to better understand Muthurajawela peat for the design and construction in this region.</p