Coupling Effect of Fly Ash and Lime Sludge Geopolymer in Stabilizing Kaolin Clay: Mechanical and Microstructural Characteristics

Jeremiah J.J., Abbey S.J., Booth C.A., Eyo E.U.

Problematic ground conditions constituted by weak or expansive clays are commonly encountered in construction projects and require some form of chemical treatment such as lime and cement to re-engineer their performance. However, in the light of the adverse effects of these traditional additives on the climate, alternative eco-friendlier materials are now sourced. In the current study, the viability of calcinated wastepaper sludge ash geopolymer in enhancing the engineering behaviour of a problematic site condition is evaluated. A highly expansive clay (HEC) constituted with a blend of kaolinite and bentonite clays is treated with calcinated wastepaper sludge ash (CPSA) geopolymer. Activation of the precursor is actualised at room temperature using a combination of NaOH and Na2SiO3 at various activator to soil+binder ratios (AL/P), and molarity (M). The mechanical, microstructural, and mineralogical characteristics of the treated clay were investigated through unconfined compressive strength (UCS), swell, water absorption, SEM, and EDX analysis. The performance of the stabilised samples was then compared with the requirements for road subgrade and subbase materials and that of OPC and lime-GGBS treatment. The results showed that CPSA-geopolymer enhanced the engineering properties of the treated clay better than traditional binders (OPC and Iime-GGBS). UCS improvement of 220% was observed in the CPSA-stabilised soil over that of OPC-treated ones, while the swell potential and water absorption were drastically reduced by over 95 and 97% respectively after 28-day soaking. The SEM and EDX results showed improved crystallisation of earth-metal-based cementitious flakes (NASH) with increasing CPSA, molarity, and AL/P ratios, which enhanced the inter-particle bonds with simultaneous reduction in porosity. The modified characteristics of the stabilised materials meet the requirements for pavement subgrades. Further, the equivalent carbon emission (CO2-e) from the stabilised materials were also evaluated and compared with that of traditional binders. The results also showed that CPSA-geopolymer had lower CO2-e at higher subgrade strengths than OPC, making it more eco-friendly. Therefore, wastepaper sludge, a common landfill waste from paper recycling is a viable geopolymer precursor that could be utilised in enhancing the engineering properties of subgrade and sub-base materials for road and foundation construction.

Li H., Tang X., Zhang X., Li M.

To address the geological hazard posed by unstable loess slopes prone to collapse and landslides, a high-strength geopolymer cementing material was developed utilizing green steel slag–fly ash as its primary constituent and activated through the application of sodium silicate alkalinity. The mechanical properties and microstructure changes of loess under varying dosages of steel slag–fly ash geopolymers and curing age were investigated through a series of tests, including unconfined compressive strength, direct shear, disintegration, electron microscope scanning, and X-ray diffraction. The findings indicate that the incorporation of geopolymers can significantly enhance the internal friction angle, cohesion, and unconfined compressive strength of loess, while mitigating the disintegration quantity and rate of stabilized soil. When 20% geopolymer is mixed into the solidified soil and cured for 28 days, the resulting solidified soil exhibits an internal friction angle of 31.12°, a cohesion of 81.09 kPa, and an unconfined compressive strength of 570.86 kPa. These values are 1.62 times, 1.76 times, and 3.36 times higher than those of loess, respectively. Moreover, the solidified soil shows minimal disintegration within 1800 s, with only 1.97% disintegration. The curing age of solidified soil has a significant impact on its curing effect. Enhancing the curing time can considerably enhance the mechanical properties of solidified soil. When the geopolymer content is 20% and the curing time is extended to 28 days, the internal friction angle, cohesion, and unconfined compressive strength increase by approximately 0.23 times, 0.48 times, and 1.61 times, respectively, compared to a curing time of 7 days. By analyzing SEM and XRD, it was found that the hydration of steel slag–fly ash geopolymer produces C-S-H and C-A-S-H cementing materials, which effectively fill the gaps between soil particles and enhance the mechanical properties of solidified soil. The research findings can serve as a theoretical foundation for the consolidation of loess subgrade utilizing steel slag–fly ash geopolymer.

Nabizadeh Mashizi M., Bagheripour M.H., Jafari M.M., Yaghoubi E.

In this study, a combination of geopolymers including Rafsanjan Natural Pozzolan (RNP), Cement Kiln Dust (CKD), and an activator such as Calcium Carbide Residue (CCR) or NaOH was used to stabilize and improve the poorly graded sandy soil. Factors such as the activator type, activator concentration, CKD and RNP content were studied. Chemical compounds of the soil and abovementioned materials were investigated using X-Ray Diffraction (XRD) and X-Ray Fluorescence (XRF) detection tests. Unconfined Compressive Strength (UCS) tests were carried out to evaluate the mechanical behavior of the specimens. The findings revealed that CKD, which is a hazardous byproduct, could be turned into an eco-friendly construction material through geopolymerization. The presence of CKD along with NaOH significantly increased the UCS of the samples compared to unstabilized specimens (control 1). Microstructural analyses using Scanning Electron Microscopy (SEM) confirmed the desirable distribution of the geopolymer gel in the stabilized soil. According to the SEM images, it was observed that the samples stabilized with CKD had a higher strength than those stabilized with CKD combined with RNP due to the formation of a greater amount of gel and a stable microstructure. The findings of this research promote sustainable ground improvement techniques using waste by-products.

Disu A.A., Kolay P.K.

Soft and highly compressible soils beneath civil engineering structures are often responsible for severe problems such as excessive settlement, cracking, sinking, etc. Over the years, these soft soils have been stabilized with Ordinary Portland Cement (OPC). Unfortunately, a known demerit of OPC as a traditional stabilizer is its energy-intensive nature and emission of large quantities of CO2, which has immediate- and long-term detrimental effects to the environment. This shortcoming has led to the emergence of geopolymers as a viable alternative to OPC with desirable properties such as high strength, stiffness, reduced shrinkage, and low energy consumption. This paper presents a critical appraisal (including a statistical approach) of geopolymer-stabilized soils, from relevant published literature sources covering various soil types and industrial by-products as activators for geopolymers. A review of 50 selected references on geopolymer-stabilized soil revealed that a maximum Unconfined Compressive Strength (UCS) of 82.5 MPa at an elevated temperature of 150 °C and a minimum UCS of 0.38 MPa at room temperature (23 °C) were obtained. The molarity of the alkali precursors (NaOH, KOH, Na2SiO3, Ca(OH)2) used by different researchers ranged from 1.7 to 23.0 M, while the stabilized soil types ranged predominantly from silty to lateritic clays. From the statistical analysis performed on 50 selected references, a statistically significant difference between molarity, curing temperature, and UCS was observed; while, there was a statistical correlation of 0.539 obtained between UCS and curing temperature. Furthermore, a regression equation was developed to predict the UCS of geopolymer-stabilized soil. However, a major limitation affecting the wide utilization of geopolymers in soil stabilization is the absence of design templates compared to the well-established OPC or lime stabilization parameters. Future research should focus more on long-term durability of geopolymer-stabilized soils, especially wet–dry, freeze–thaw durability, and sustainability assessment.

Nehdi M.L., Yassine A.

While alkali-activated materials (AAMs) have been hailed as a very promising solution to mitigate colossal CO2 emissions from world portland cement production, there is lack of robust models that can demonstrate this claim. This paper pioneers a novel system dynamics model that captures the system complexity of this problem and addresses it in a holistic manner. This paper reports on this object-oriented modeling paradigm to develop a cogent prognostic model for predicting CO2 emissions from cement production. The model accounts for the type of AAM precursor and activator, the service life of concrete structures, carbonation of concrete, AAM market share, and policy implementation period. Using the new model developed in this study, strategies for reducing CO2 emissions from cement production have been identified, and future challenges facing wider AAM implementation have been outlined. The novelty of the model consists in its ability to consider the CO2 emission problem as a system of systems, treating it in a holistic manner, and allowing the user to test diverse policy scenarios, with inherent flexibility and modular architecture. The practical relevance of the model is that it facilitates the decision-making process and policy making regarding the use of AAMs to mitigate CO2 emissions from cement production at low computational cost.

Burra S.G., Kolay P.K., Kumar S.

The paper investigates the use of lime sludge (LS), fly ash (FA) on unconfined compressive strength, and shrinkage behavior of commercially available kaolinite clay (EPK). The stabilization process was done using lime sludge alone with varying percentages i.e., 2, 4, 6, and 8% by dry weight, as well as a combination with fly ash in varied percentages i.e., 5, 10, 15, and 20% by dry weight. In order to evaluate the strength properties, standard Proctor tests, and unconfined compressive strength (UCS) tests were performed. For estimating the shrinkage, linear shrinkage test was carried out. The UCS samples were prepared based on the optimum moisture content and dry unit weight from standard Proctor tests. The samples were tested after a curing period of 0, 7, and 14 days. In addition, Scanning Electron Microscopy (SEM) tests were performed on UCS samples after shearing. The test results indicated an increase in UCS value with an increase in lime sludge content. Also, the UCS value increased with curing period following a slight decrease. The combination of LS and FA in varied percentages, also had an increase in UCS strength. Also, the UCS value showed an increment in values with the curing period. The reduction in shrinkage behavior of EPK clay was observed with LS and FA. The SEM analysis was conducted to evaluate the reason for strength increase, which was due to the chemical reaction developed between EPK clay, lime sludge, and fly ash.

Abdullah H.H., Shahin M.A., Sarker P.

This paper focuses on stabilisation of kaolin clay at ambient temperature using fly-ash based geopolymer incorporating ground granulated blast-furnace slag (GGBFS). Comprehensive experimental programme was conducted including soil plasticity, compaction, unconfined compressive strength, durability and leaching. These tests were followed by a microstructural analysis using scanning electron microscopy (SEM) technique. An optimisation study using several combinations of geopolymer ingredients was performed, and the role of GGBFS in enhancing the geopolymer-stabilised clay was evaluated. The results indicated that introducing partial replacement of class (F) fly-ash by GGBFS assists, when synthesised in certain ratios, in achieving strength properties of geopolymer-stabilised clay comparable to those of cement stabilised clay. Although a small percentage of geopolymer can improve the soil strength, a larger amount was essential to enhance the wetting–drying durability performance. Under freezing–thawing conditions, low durability performance was detected indicating retardation in the geopolymer reaction at low temperature. For simulated water infiltration, leaching of the activator from geopolymer-stabilised clay was a minor concern in relation to the gel formation and long-term strength gain. Finally, SEM results clearly demonstrated a clay fabric modification attributed to the inter-particle contacts and the corresponding bonding due to the gel formation and hardening.

Salih H., Patterson C., Li J., Mock J., Dastgheib S.A.

The feasibility of lime-softening sludge utilization for flue gas desulfurization in coal-fired power plants was evaluated through a supply-demand analysis and a life cycle assessment (LCA). To evaluate the demand and supply of lime sludge to replace limestone on a national scale, the annual amount of lime sludge generated by water treatment utilities in the United States was estimated and compared with the annual amount of limestone used by coal-fired power utilities. To evaluate the environmental sustainability of reusing lime sludge in power plants, an LCA study was performed in which the environmental impact and water footprint of the proposed approach were quantified and these results were compared with the conventional approaches for limestone mining, grinding, and transportation to power plants and lime sludge disposal in landfills. Water utilities across the United States are currently generating approximately 3.2 million tons of lime sludge per year at an estimated disposal cost of approximately US$90 million, whereas power utilities are using approximately 6.3 million tons of limestone per year. The potential savings that would result from partial replacement of limestone with lime sludge was estimated to be approximately US$97 million per year. The LCA study showed that the environmental impact of lime sludge utilization in power plants under different scenarios was 2 orders of magnitude lower than that of the landfill disposal option. Furthermore, the water footprint for lime sludge reuse in power plants was almost negligible compared with that of the conventional approaches of disposing of lime sludge at water utilities or using limestone at power utilities.

Moghal A.A.

AbstractThermal power stations in most countries are burdened with the problem of fly ash disposal, and unless suitable uses are found for fly ash, it will pose a gigantic problem for the power sec...

He P., Wang M., Fu S., Jia D., Yan S., Yuan J., Xu J., Wang P., Zhou Y.

Geopolymer with Si/Al ratios from 2 to 4 were prepared by adding different contents of fused silica into geopolymer matrix. Effects of Si/Al ratios on the structure, mechanical properties and chemical stability in air of the obtained geopolymer were systematically investigated. The results showed that all the geopolymer samples were XRD amorphous. Geopolymer with Si/Al ratios of 2 and 2.5 showed similar structure and property and they were classed as KGP-I; and geopolymer with Si/Al ratios of 3, 3.5 and 4 were similar and they were class as KGP-II. In alkaline solution, reactivity of fused silica were higher than that of metakaolin, resulting in higher content of both residual metakaolin and free alkaline cation in KGP-II than in KGP-I. Fused silica partially reacted with the alkaline solution in KGP-II indicating chemical interfacial bonding between silica and binder phase. With the increase in Si/Al ratios, KGP-II especially for geopolymer with Si/Al of 4 showed much higher mechanical properties than KGP-I due to the increased Si-O-Si bonds and residual silica as reinforcement. However, KGP-II showed worse chemical stability in air than KGP-I, with the presence of efflorescence on the surface, which was attributed to their higher residual free K+.

Phummiphan I., Horpibulsuk S., Phoo-ngernkham T., Arulrajah A., Shen S.

AbstractTwo waste by-products, fly ash (FA) and calcium carbide residue (CCR), are used to develop geopolymer binders for stabilizing marginal lateritic soil as a sustainable pavement base. The liquid alkaline activator is a mixture of sodium silicate solution (Na2SiO3) and sodium hydroxide (NaOH) at a concentration of 10 molars. Unconfined compressive strength (UCS) and scanning electron microscopy (SEM) images of lateritic soil–FA geopolymers at different influential factors (curing times, Na2SiO3∶NaOH ratios, and CCR replacement ratios) are measured. The soaked 7-day UCS of lateritic soil–FA geopolymers meets the strength requirement for both light and heavy traffic pavement specified by the local national authorities. The early 7-day UCS and cementitious products increase with increasing CCR replacement ratio, and the cementitious products are clearly observed at CCR = 30% (the highest CCR replacement ratio tested). However, the CCR replacement ratio providing the maximum 90-day strength is found at 2...

Pavithra P., Srinivasula Reddy M., Dinakar P., Hanumantha Rao B., Satpathy B.K., Mohanty A.N.

Timakul P., Rattanaprasit W., Aungkavattana P.

In this study, ASTM Class C fly ash used as an alumino-silicate source was activated by metal alkali and cured at low temperature. Basalt fibers which have excellent physical and mechanical properties were added to fly ash-based geopolymers for 10–30% solid content to act as a reinforced material, and its influence on the compressive strength of geopolymer composites has been investigated. XRD study of synthesized geopolymers showed an amorphous phase of geopolymeric gel in the 2 θ region of 23°–38° including calcium-silicate-hydrate (C-S-H) phase, some crystalline phases of magnesioferrite, and un-reacted quartz. The microstructure investigation illustrated fly ash particles and basalt fibers were embedded in a dense alumino-silicate matrix, though there was some un-reacted phase occurred. The compressive strength of fly ash-based geopolymer matrix without basalt fibers added samples aged 28 days was 35 MPa which significantly increased 37% when the 10 wt%. basalt fibers were added. However, the addition of basalt fibers from 15 to 30 wt% has not shown a major improvement in compressive strength. In addition, it was found that the compressive strength was strong relevant to the Ca/Si ratio and the C-S-H phase in the geopolymer matrix as high compressive strength was found in the samples with high Ca/Si ratio. It is suggested that basalt fibers are one of the potential candidates as reinforcements for geopolymer composites development.

Liu Z., Cai C.S., Liu F., Fan F.

AbstractLoess, which primarily consists of wind-transported silt and clay particles, is widely distributed around the world, including the central Asia, central Europe, the northwestern and central United States, Alaska, and South America. The metastable structure of natural loess has resulted in construction delays and catastrophic failures. To explore the potential applications of soil improvement with green materials, this paper presents an experimental study of applying fly ash–based geopolymers for the loess stabilization. Two different precursors are employed to the investigation. It is found that potassium hydroxide renders a higher unconfined compressive strength than sodium hydroxide to the samples when the same fly ash/loess ratio is applied. This agrees with the results of the mechanical properties of the two different geopolymers. With an increasing fly ash/loess ratio, the compressive strength and Young’s modulus increase. The microstructural characterization unveils that a compact and stable...

Rios S., Cristelo N., Viana da Fonseca A., Ferreira C.

AbstractAlkaline activation of fly ash (FA) was used to improve the mechanical performance of a silty sand, considering this new material as a replacement for soil-cement applications, namely, bases and subbases, for transportation infrastructures. For that purpose, specimens were molded from mixtures of soil, FA, and an alkaline activator made from sodium hydroxide and sodium silicate. Uniaxial compression tests showed that strength is highly increased by the addition of this new binder. The results described a high stiffness material, with an initial volume reduction followed by significant dilation. All specimens have clearly reached the respective yield surface during shearing, and peak-strength Mohr–Coulomb parameters were defined for each mixture. The evolution of the microstructure during curing, responsible for the mechanical behavior detected in the previous tests, was observed by scanning electron microscopy. These results were compared with soil-cement data obtained previously with the same soi...

He L., Li J., Yun X., Wang S., Liu X., Yang J., He R.

Alkali-activated materials, serving as alternative cementitious materials, showed great mechanical properties and excellent durability. Nevertheless, their practical application was limited due to their rapid setting and loss of workability. To adjust the workability and setting time, Na2HPO4 and Ba(NO3)2 were used as retarders in the alkali-activated ternary binders incorporating fly ash (FA), ground blastfurnace slag (GGBFS), and extracted titanium tailing slag (TS). The influence of retarder content on the fresh and hardening properties, as well as the microstructure development of the binder, was investigated. The results showed that both Na2HPO4 and Ba(NO3)2 could prolong the setting time of the binder, but the latter was more effective. When these retarders’ content was 1.5 wt.%, the initial setting time was extended by 21% and 45% to 103 min and 123 min, respectively. Ba(NO3)2 was harmful to the strength development of the binder, and the values of specimens containing 1.5 wt.% Ba(NO3)2 decreased by 9.1%, 22.2%, and 22.2% at 1, 3, and 28 days, respectively. Whereas the addition of Na2HPO4 was slightly negative to the 1- and 3-day strength of the binder, it benefitted the 28-day strength. Adding 1.0 wt.% Na2HPO4 would promote the formation of reaction products, resulting in an increase in the 28-day compressive strength by 8.5% to 53.5 MPa. The primary phases of this binder were C-A-S-H and C-(N)-A-H gels.