Rapid Strength Development Research Articles

Engineering characteristics of one-part alkali-activated slag-based paste incorporating nano-silica and calcined oyster shells ash

ABSTRACT This study investigates the influence of nano-silica up to 5% by mass on the hydration, engineering properties, and microstructure of one-part alkali-activated slag-based paste with up to 20% calcined oyster shell ash (COSA). Adding nano-silica reduced workability and accelerated setting times. A mixture with 3% nano-silica and 20% COSA showed a 31.55% increase in compressive strength, improved thermal conductivity from 0.434 W/(m · K) to 0.715 W/(m · K), and reduced water absorption from 10.43% to 3.02% when compared to those mixtures with only COSA. These improvements are due to nano-silica interacting with calcium hydroxide, forming hydration products such as ettringite and C-S-H gel, which fill micropores and strengthen the microstructure. This rapid hydration and strength development make it ideal for precast manufacturing. Thus, incorporating nano-silica allows efficient use of up to 20% COSA, making it a sustainable and eco-friendly construction material.

Design, modelling and optimisation of ultra high-performance fibre reinforced concrete incorporating waste materials

The adoption of ultra-high-performance fibre-reinforced concrete (UHPFRC) in modern-day construction has gradually increased because of its high compressive and flexural strength at the early production ages. The UHPFRC's high initial cost discourages its production and widespread use. This cost can be reduced by incorporating waste materials in UHPFRC production. This study aims to design, model, and optimise a three-day heat-cured UHPFRC incorporating waste materials. To produce a sustainable UHPFRC mixture, this study combines rice husk ash (RHA) and recycled tire steel fibre (RTSF) as the primary waste components, with Portland limestone cement, river sand, superplasticizer, and water. The concrete is heat-cured in a hot water bath at 90 °C for 3 days, resulting in rapid strength development. A D-optimal mixture design technique was used to optimise the mix proportions, and mathematical models were developed to predict the compressive and flexural strengths. The non-significant lack-of-fit results and the high values of coefficient of determination (R2) revealed the accuracy of the models in predicting the compressive and flexural strengths of the UHPFRC. The low values of standard deviation (SD) and coefficient of variation (CV) confirmed the consistency and reliability of the prediction models. A numerical optimisation revealed that it is possible to design a UHPFRC mixture with a lower cement content of 38.29% of the UHPFRC mix, resulting in a compressive strength of 117.62 MPa and a flexural strength of 36.32 MPa. The study reveals that integrating RHA and RTSF into UHPFRC can yield high-performance properties, offering innovative solutions for resource scarcity and environmental sustainability in the construction sector.

Selection of exercises for physical development for first-year male students of Duy Tan University in Danang, Viet Nam

This study aimed to select and evaluate exercises for the physical development of first-year male students at Duy Tan University in Da Nang, Vietnam. Utilizing an experimental design, documentary analysis, interviews, and pedagogical testing, the study involved 32 experienced physical education lecturers to ensure exercise reliability. The experimental phase included 46 first-year male students, who were divided into two groups: a control group (n = 23) following the standard curriculum and an experimental group (n = 23) incorporating 19 additional exercises categorized into five groups: rapid strength development (4 exercises), strength development (6 exercises), endurance (4 exercises), flexibility (3 exercises), and dexterity (4 exercises). Over five months, both groups attended 36 classes. Statistical analysis revealed significant differences between the groups (p < 0.05), indicating that the experimental group showed notable improvements in physical fitness due to the newly selected exercises. As a result, the selected exercises were highly effective in developing the physical fitness of first-year male students at Duy Tan University in Da Nang, Vietnam.

Performance assessment of pervious concrete incorporated with calcium silicate hydrates (C-S-H) cultivated on rice husk ash substrates – A trend surface analysis interpretation

Pervious concrete is a special type of concrete designed to allow water to pass through by omitting fine aggregates from the mix, or by using special techniques to create interconnected voids within the concrete. The current study investigates a novel method of cultivating calcium silicate hydrate (CSH) precipitates on a supplementary cementitious material (SCM), Rice husk ash (RHA), substrate and integrating the RHA substrate into pervious concrete to improve mechanical and durability properties. Scanning electron microscopy (SEM) images indicates the formation of precipitates of calcium silicate hydrates on the surface of RHA particles. The silica to RHA (Si/RHA) ratio and curing age of pervious concrete specimens serve as focal points for assessing the mechanical characteristics. Trend surface analysis is utilized to analyse the results, using a polynomial equation to identify underlying trends or patterns within the dataset. The analysis reveals that the Si/RHA ratio and curing days have minimal impact on permeability, while an increase in RHA substrate volume reduces permeability, indicating microstructure densification rather than macro void filling. For compressive strength, the results show that higher Si/RHA ratios and longer curing times positively correlate with strength, with accelerated hydration suggested by the quadratic coefficients of curing age. In terms of split tensile strength, increasing substrate volume enhances strength, while linear terms of Si/RHA ratio and curing age also exhibit positive correlations, although quadratic terms indicate minimal change in strength gain rates. Flexural strength correlates positively with both Si/RHA ratio and curing age, while abrasion resistance negatively correlates with Si/RHA ratio and substrate volume, suggesting enhanced resistance with increased values. CSH seeds grown on RHA enhances the microstructure of concrete, resulting in a denser material facilitating rapid strength development and improved durability.

Innovative approaches to enhancing crack repair: Performance optimization of biopolymer-infused CXT

Abstract Biopolymer-modified cementitious repair material (CXT) is a composite material designed to enhance the properties of traditional cementitious materials through the incorporation of biopolymers. CXT is formed by blending cement (C) with biopolymers such as xanthan gum (XG) and additives like tartaric acid (TA), which improve the material’s performance characteristics. This study investigates the effects of varying dosages of XG, TA contents, and curing durations on the performance characteristics of CXT. We evaluated key performance indicators, including the flowability, workability, setting time, compressive strength, and shrinkage. The optimal formulation XG 1:2 TA0.6 achieved a maximum compressive strength of 25 N·mm−² and a flexural strength of 9.4 N·mm−², with negligible shrinkage. The results showed rapid early strength development, with the material reaching over 77% of its final compressive strength within the first 14 days. These findings highlight the potential of CXT for crack-repair applications, offering improved workability, accelerated setting times, and reduced environmental impact compared to traditional repair materials. This study provides valuable insights for optimizing the formulation of CXT for enhanced performance in real-world applications.

Formulation of high-recycled-content hydraulic cements based on alkali aluminosilicate chemistry and mechanochemical processing techniques

The current study investigates the potential of various waste materials and industrial by-products for formulating high-recycled content hydraulic cement, leveraging alkali aluminosilicate chemistry principles. Through rigorous characterization employing X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) Spectroscopy, thermogravimetric analysis (TGA), and Scanning Electron Microscopy (SEM), alongside alkalinity and alkali solubility assessments, the chemical and mineralogical compositions of waste brick, lime kiln dust, cement kiln dust, waste glass, gypsum, foundry sand, granulated blast furnace slag, and impounded coal ash were unveiled. Silicon and aluminum were found abundantly in these materials, with lime kiln dust and slag contributing calcium. A thoroughly crafted blend, enriched with alkalis, underwent mechanochemical activation to yield a hydraulic cement finely tuned to meet specific chemical ratios required for effective alkali aluminosilicate reactions. Upon hydration, this innovative cement exhibited rapid early strength development, achieving a commendable compressive strength of 32 MPa within 28 days a performance on par with standard Portland cement. Remarkably, the heat of hydration profiles revealed a prompt onset of hydration reactions, marked by a significant exothermic peak within the initial hours, in contrast to the gradual peak observed with Portland cement. TGA/DTA results further corroborated the thermal stability of the hydration products formed. Alkali-solubility assessments of the raw materials emerged as reliable indicators of their reactivity within the cement matrix, with solubility values exceeding 20 % for waste glass and impounded coal ash, signifying their high potential for alkali activation. The culmination of this research underscores the feasibility of re-composing waste materials to craft alkali aluminosilicate cement, offering performance akin to conventional options. These findings advocate for the adoption of sustainable material utilization in construction, paving the way for enhanced industrial symbiosis in cement production.

Structuration and yield strength characterization of hybrid alkali-activated cement composites (HACC) for ultra-rapid 3D construction printing

This study investigates the critical time-dependent rheological properties, including structuration rate and yield stress, of hybrid alkali-activated cement composites (HACC). HACC, a blend of geopolymer and cement paste, is examined to evaluate its suitability for ultra-rapid 3D construction printing (3DCP), which refers to a material’s ability to attain rapid structuration and buildability within 5–10 min after mixing, essential for additive manufacturing of 3D structures. The investigation extends to the impact of filler materials like silica sand and PVA fibers on HACC’s rapid structuration. Vicat needle, fast- and slow-penetration resistance measurements indicate that setting time increases with decreasing the Silicon/Aluminum (S/A) molar ratio and the addition of silica sand. Conversely, the addition of fibers reduces setting time while enhancing yield strength. Specifically, HACC mixtures with 0.5% and 1% PVA fiber additions, alongside combined sand (30%, 40%) and fiber (0.5%) mixtures, demonstrate ultra-rapid setting behavior within 3–5 min. Notably, mixtures with 40% silica sand and 0.5% PVA fiber exhibit highest yield strength development, i.e., 14 MPa at 12 min. Remarkably, incorporating 1% fibers into the HACC mixture yields the highest 28-day compressive strength (∼80 MPa) and flexural strength (∼11 MPa). The synergistic interplay between geopolymer and cement facilitates rapid setting time and high early-age strength development in HACC mixtures. The varying effects of silica sand and PVA fiber on setting time and yield strength provide opportunities for designing suitable structuration profiles. These findings underscore the exceptional rapid setting, structuration, and mechanical properties of HACC, positioning it as a promising material for ultra-rapid 3D construction printing.

The excellent adaptability to large temperature differences of alkali-activated slag using Na2SO4+ Ca(OH)2 as the activator

The thickening time of NaOH-activated slag system is difficult to control by retarders, which limits its application in oil and gas well cementing. To facilitate the application of alkali-activated slag, the high-temperature thickening and strength characteristics of Na2SO4+Ca(OH)2 activated slag, Na2CO3+Ca(OH)2 activated slag and NaOH activated slag were studied in this paper. It is found that the strength properties of the three are similar, with a strength of about 18 MPa for 72 h. The Na2SO4+Ca(OH)2 activated slag system has superior high-temperature thickening performance (up to 180℃) and extensive temperature difference adaptability (25–180℃). With similar thickening time at 120℃, the Na2SO4+Ca(OH)2 activated slag system has a strength of up to 15 MPa at 48 h at a low temperature of 25℃, and the Portland cement don’t set at 48 h. The Na2SO4+Ca(OH)2 activated slag system shows good thickening properities at 180℃ while still maintaining rapid strength development at 25℃. The large temperature difference adaptability of Na2SO4+Ca(OH)2 activated slag is apparently better than that of Portland cement. The mechanism analysis shows that the effective control of the retarder on the reaction of Na2SO4 and Ca(OH)2 to generate NaOH, and the low-temperature sensitivity of the reaction of Na2SO4 and Ca(OH)2 under the action of the retarder are the main reasons why the ease of adjusting the high-temperature thickening time of the Na2SO4 and Ca(OH)2 activated slag, and the good performance of large temperature difference adaptability. This provides a new way to solve the problem of over-retarding at the top of cement slurry column for long sealing section in deep wells.

Impact of Alkaline Concentration on the Mechanical Properties of Geopolymer Concrete Made up of Fly Ash and Sugarcane Bagasse Ash

Geopolymer concrete (GPC) is a novel and environmentally friendly type of concrete that eliminates the use of cement, resulting in a significant reduction in carbon emissions and a more sustainable construction material. Alkaline activators are used in GPC to achieve rapid strength development. The most popular alkaline activators are sodium/potassium silicate and sodium/potassium hydroxide, which are known contributors to carbon emissions, hence limiting the advantages of GPC; therefore, reducing the amount of these alkaline activators that contribute to carbon emissions is necessary for developing a more sustainable geopolymer concrete. In this study, the influence of the variation in sodium hydroxide molarities on the performance of fly ash/sugarcane bagasse ash-based-geopolymer concrete was investigated. The different molarities used were 10 M, 12 M, 14 M, and 16 M sodium hydroxide solutions. In addition, the effect of sugarcane bagasse ash content (0%, 5%, 10%, 15%, and 20%) on the fresh and hardened geopolymer concrete properties were examined. The slump test, compression test, split tensile test, and flexure test were conducted on the cast samples. The results of this study showed that raising the concentration of NaOH from 10 M to 16 M while maintaining a sodium silicate to sodium hydroxide ratio of 2.5 resulted in a 3.75–10.2% improvement in compressive strength after 28 days. It is worth noting that, even at a concentration of 10 M, the concrete still achieved high strength.

Effects of 12-week gait retraining on plantar flexion torque, architecture, and behavior of the medial gastrocnemius in vivo.

This study aims to explore the effects of 12-week gait retraining (GR) on plantar flexion torque, architecture, and behavior of the medial gastrocnemius (MG) during maximal voluntary isometric contraction (MVIC). Thirty healthy male rearfoot strikers were randomly assigned to the GR group (n = 15) and the control (CON) group (n = 15). The GR group was instructed to wear minimalist shoes and run with a forefoot strike pattern for the 12-week GR (3 times per week), whereas the CON group wore their own running shoes and ran with their original foot strike pattern. Participants were required to share screenshots of running tracks each time to ensure training supervision. The architecture and behavior of MG, as well as ankle torque data, were collected before and after the intervention. The architecture of MG, including fascicle length (FL), pennation angle, and muscle thickness, was obtained by measuring muscle morphology at rest using an ultrasound device. Ankle torque data during plantar flexion MVIC were obtained using a dynamometer, from which peak torque and early rate of torque development (RTD50) were calculated. The fascicle behavior of MG was simultaneously captured using an ultrasound device to calculate fascicle shortening, fascicle rotation, and maximal fascicle shortening velocity (Vmax). After 12-week GR, 1) the RTD50 increased significantly in the GR group (p = 0.038), 2) normalized FL increased significantly in the GR group (p = 0.003), and 3) Vmax increased significantly in the GR group (p = 0.018). Compared to running training, GR significantly enhanced the rapid strength development capacity and contraction velocity of the MG. This indicates the potential of GR as a strategy to improve muscle function and mechanical efficiency, particularly in enhancing the ability of MG to generate and transmit force as well as the rapid contraction capability. Further research is necessary to explore the effects of GR on MG behavior during running in vivo.

A New Approach for Durable and Sustainable Infrastructure Concrete

The durability of reinforced concrete infrastructures is a major concern related to several aspects of the sustainability. The actual concrete design is still largely based on prescriptive indications, such as the cement content and the water cement ratio, while the durability investigation started to be oriented towards a more performance-based concept. The actual laboratory durability tests depend on the environmental exposure classes. Nonetheless, the actual stringent durability limits force to produce concrete with a too rapid strength development. This fact adversely affects the rigidity of the infrastructure and reduce the durability. Furthermore, the actual concrete requirement concepts partially inhibit the use of more sustainable concrete. This work proposes a new approach and a new concrete framework to produce more eco-efficient and more durable concrete. This will be attained by introducing a better laboratory-field relationship with respect to the durability by means of combined laboratory environmental attacks, which better simulate the real infrastructure site condition.

CONTROL OF SETTING TIMES OF SLAG-CONTAINING CEMENTS ACTIVATED WITH SODIUM METASILICATE

In global construction practice, theneed for high-strength concrete with intensive earlystrength gain is increasing due to increased requirements for the physical, mechanical and operationalcharacteristics of concrete and the desire to reduceconstruction time. Alkali-activated cements madeusing soluble sodium silicates are capable of rapidstrength development and high standard strength,which is due to the modifying effect of amorphoussilica present in soluble glass. However, the problem with the effective use of such cements is theshort setting time.The purpose of this work is to study the influenceof the method of introducing low-basic sodium silicate into slag-containing compositions, the composition of the compositions and the amount of modifying additive on the setting time. As a result of thework performed, a mechanism for controlling thesetting lines in the “slag + clinker + sodium metasilicate” binder system was developed.The dependence of setting times on the methodof introducing sodium metasilicate into the “slag +clinker” system, on the composition of cement andon the type and quantity of modifier additives hasbeen established. It has been shown that the introduction of sodium metasilicate to cement in theform of an aqueous solution leads to a much greaterintensification of structure-forming processes in theprocess in comparison with the option of usingmetasilicate in the form of a powder with mixingwith water. It has been established that an increasein the clinker content in the slag-clinker mixtureleads to a significant reduction in setting time.A study of the influence of the LSTM additiveshowed the possibility of effectively controlling thesetting lines, subject to an increased amount of useof these additives, namely up to 4%. As a result,technologically acceptable setting times were obtained within the range of 0–38…2–24 hours-min when using LSTM additives in an amount of 2…4%.The results of the study of the structure formation of alkali-activated slag-containing cement confirmed the above conclusions and were in goodcorrelation with the obtained physical and technological results.

Experimental and theoretical studies on the hydration of ordinary portland cement/calcium sulfoaluminate cement blends containing biological admixtures

The present study investigated the hydration properties of ordinary portland cement (OPC) and calcium sulfoaluminate (CSA) cement blends with the biological admixtures to examine the feasibility of the blends as a carrier for microbes, Sporosarcina pasteurii. The hydration kinetics of the blends were explored by X-ray powder diffractometry, thermogravimetry, and 27Al nuclear magnetic resonance analysis. The test outcomes revealed that the blended CSA cement expedited the early hydration of OPC clinker, which was delayed by the biological admixtures in the sole OPC system. In particular, blending with the CSA cement improved rapid strength development, lowered pH, and increased the crack healing capacity. Furthermore, the hydration of the systems was thermodynamically modeled to provide the fundamental idea of the systems and to compare with experimental phase assemblages. The hydration kinetics and development of physical and chemical properties of the blends were simulated and discussed along with the experimental data.

Evaluation of neutron shielding performance of cementless binder manufactured with desalination reject brine

This study investigated the feasibility of desalination reject brine as a new water resource for the nuclear construction industry by evaluating the thermal neutron shielding properties of Ca(OH)2–Na2CO3-activated fly ash (FA-NC) binder manufactured with reject brine since the high chloride concentration is believed to promote neutron shielding performance. High Cl− concentration encouraged rapid and greater early-age strength development of FA-NC binder mixed with reject brine, which is the main mechanism of the accelerator. A series of chemical and microstructural analyzes were conducted, namely, XRD, TG, and MIP tests, to investigate the influence of the reject brine. The use of reject brine remarkably improved the thermal neutron attenuation performance of the FA-NC binder and achieved 98.76 % of thermal neutron shielding efficiency. Furthermore, the reject brine provided an increased efficacy in fast neutron attenuation. To further analyze the practical application potential, a 190 × 90 × 57 mm sized brick sample was made using rejected brine mixed FA-NC binder and tested with the followings: water absorption and TCLP.

Mechanical performance of concrete canvas reinforced with insert-bar structures under quasi-static and impact loading

Concrete canvas (CC) is a promising composite based on a combination of three-dimensional fabric and cement-based powder, with the comprehensive advantages of easy storage and transport, convenient construction, rapid strength development, and low dust pollution. In civil and military engineering, interest and exploration work within the field of CC have been ongoing for many years. While external measures exist to enhance the mechanical properties of CC, substantial internal measures are still lacking. In this paper, an internal stiffening strategy was proposed by coupling warp-knitted spacer structure with a series of insert-bar structures, attempting to improve the mechanical performance of CC. The reinforcing effect of flexible bar stiffeners were investigated through quasi-static bursting test and impact test. Damage morphology analysis was then performed to reveal the progressive failure mechanism. The obtained results demonstrated that flexible bar stiffeners with low modulus were highly beneficial reinforcements for CC due to their favorable synergy and great stress dissipation capacity. The load-bearing capacity and toughness of CC were increased with the insertion height and groups of bar combination. The vertical combination could further enhance the penetration resistance by altering stress transfer.

Effect of gypsum addition on acid resistance of ye'elimite rich calcium sulfoaluminate cement

Calcium sulfoaluminate (CSA) cements exhibit useful properties such as shrinkage compensation and rapid strength development. Acid resistance of CSA cement is majorly governed by its phase composition that varies significantly depending on the desired properties. In this study, acid resistance of CSA cement (i.e., non-expansive) and gypsum blended CSA cement (i.e., expansive) is explored. The current work emphasizes that a small change in phase assemblage can cause large difference in the performance under acidic environment. Conventional acid immersion tests with hydrochloric acid (HCl) of 1 % and 2 % conc., and sulfuric acid (H2SO4) of 2.95 % conc. were considered in the study. Furthermore, a recently developed acid consumption method was used to rank the binders with regard to their acid resistance in HCl and H2SO4 environments. It was found that the gypsum-blended CSA cement showed poorer performance than CSA cement in acidic environment despite having marginally higher compressive strength before the acid exposure.

The influence of microwave curing on the strength of silica fume-added fly ash-based geopolymer mortars

Geopolymer concretes a substitute for traditional Portland cement as a more environmentally sustainable alternative. In addition, one of the most significant advantages of geopolymer concrete is the high setting speed that can be achieved with thermal curing, that is, the short curing period. Research on geopolymer concrete has focused on curing, where curing is usually done with conventional ovens. Microwave curing initiates heat generation at the molecular level, which results in a minimal heat increase on the surface of the specimens compared to traditional thermal curing methods. In addition, it provides internal, rapid, and homogeneous heating and rapid strength development. This study is based on research that examined the effect of microwave cure on the compressive strength of silica füme-modified geopolymer mortar. In the study, sodium silicate and sodium hydroxide were used for activation, and first of all, fly ash substituted geopolymer mortar specimens were subjected to conventional thermal curing in an oven. On the other hand, the mortar specimens with the same composition were subjected to short-duration (10 minutes) microwave curing at power levels of between 200-600 W. When the results were compared regarding compressive strength, void and percent water absorption, UPV, and SEM-internal structure examination, it was revealed that microwave curing could be used faster than the conventional oven curing method in silica füme-added fly ash-based geopolymer mortars. This research can be considered an essential step for further optimizing and popularising geopolymer concrete and microwave curing and proposes a method that should be considered in the future development of more sustainable construction materials.

Multiscale insights into mechanical performance of a granite-based geopolymer: Unveiling the micro to macro behavior

Geopolymers (GPs), based on rocks and similar (waste) materials, have shown promising features as alternatives to Ordinary Portland Cement (OPC) in many applications. The properties of these materials can be tailored to a large degree, to meet the requirements of specific applications. This study presents a two-part granite-based GP system developed for wellbore sealant applications in Carbon Capture and Storage (CCS) operations. The system was modified by incorporating different concentrations of a Zn–K-based retarder. The performance of the GP system was then evaluated through consistency analyses, and uniaxial compressive strength (UCS) measurements. Further, the in-situ mechanical attributes of the GP system were assessed through tri-axial experiments at 90 °C and under varying confining pressures. Additionally, the morphology, mineralogy, and chemical composition of the hardened GP system were analyzed using X-ray diffraction (XRD), scanning electronic microscopy with energy dispersive spectroscopy (SEM-EDS), and Fourier transform infrared spectroscopy (FTIR). Consistency analyses revealed that slurries with higher retarder content exhibited improved fluidity but also longer setting times, which could result in fluid invasion and integrity issues. Accordingly, slurries with appropriate setting behavior were identified, striking a balance between rapid strength development and sufficient fluidity for downhole applications. The appropriate flow characteristics of the slurries and the stability of the solidified materials were also verified through rheological assessments, free-fluid tests, and sedimentation analyses. UCS measurements showed a continuous increase in strength for all GP samples, with optimized retarder content yielding the highest late-age mechanical strength. Tri-axial experiments showed that the in-situ Poisson's ratio, compressive strength, and compressive strength to Young's modulus ratio (C/E) of the GP system could be enhanced by 86.79%, 40.26% and 13.05%, respectively, through retarder content optimization. Furthermore, compressive strength and C/E ratios increased with higher confining pressure, while Poisson's ratio remained relatively constant. Moreover, XRD analyses revealed a transition from amorphous to crystalline phases within the GP system, depending on the retarder content, indicating enhanced network connectivity and the formation of new crystalline structures. FTIR analyses supported these observations, demonstrating the formation of new crystals and robust amorphous gels, indicating improved polycondensation and network connectivity in the GP system. Besides, SEM analysis of fractured samples exhibited distinct microstructural characteristics depending on the retarder content, with the optimized retarder content leading to higher crystallinity and more interconnected gel structures. These findings highlight the significance of microscale analysis alongside macroscale assessments when studying GP systems.

Rapid early age strength development of in-line activated geopolymer for concrete 3D printing

The in-line activation using print head mixing technology involves two parts pumping and mixing at the print head. This technology can be applied in geopolymer concrete, where the base mix (precursors, sand and water) and alkaline activators can be pumped as two separate parts and mixed at the print head for activation. While such methods provide prolonged open time, the early age strength development is low since water from the base mix dilutes the activator solution during print head mixing. Besides, choosing an activator solution with high alkalinity to combat the dilution effect reduces the immediate static yield strength development after print head mixing, which is essential for rapid building. Therefore, this study investigates the hydrated lime as an additive in the base mix to overcome the challenge to attain high 1-day compressive strength along with immediate static yield strength development. The effect of hydrated lime dosage on the pumpability (i.e. evolution of viscosity with time), static yield strength development after print head mixing (buildability) and hardened properties were assessed. The optimum hydrated lime dosage of 1 wt% of the precursors exhibited 1-day compressive strength of 20 MPa and static yield strength of 38.8 kPa after 5 min from print head mixing with minimal changes in pumpability of the base mix for up to 6 h. Moreover, the effect of rapid static yield strength development of the printed layers on the interlayer bond strength was assessed with varying cycle times between 5 s and 40 min. It was found that the interlayer bond strength reduced by 63% when the cycle time was increased from 5 s to 40 min, however, the reduction can be decreased to 15% by surface wetting of the previous layers.

On the use of one-part geopolymer activated by solid sodium silicate in soft clay stabilization

The application of geopolymers in ground improvement has garnered significant attention in recent years. However, most geopolymer preparations have focused on the two-part method, which not only has negative environmental impacts but also falls short of meeting practical requirements. This study aimed to address these limitations by employing solid sodium silicate (Na2O·nSiO2, n is the molar ratio, NS) to activate binary precursors (fly ash [FA] and ground granulated blast furnace slag [GGBFS]), along with water, to synthesize a one-part geopolymer (OPG) for soft clay stabilization. The primary factors on the properties of the OPG binder were firstly identified through macro- and micro-tests. Subsequently, the optimization of the OPG mixing proportion was achieved by reducing the molar concentration of NS, and was further used for soft clay stabilization. The effects of the FA/GGBFS ratio (0, 0.1, and 0.2), curing period (7, 14, and 28 days), and binder content (0.15, 0.20, and 0.25) on the mechanical properties of the OPG-stabilized soft clay were then evaluated using unconfined compressive strength (UCS) tests. Furthermore, mercury intrusion porosimetry (MIP), scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDS), and X-ray diffraction (XRD) techniques were employed to examine the evolution of microstructure, hydrate composition, and mineral/phase of the OPG-stabilized soft clay samples. The experimental results indicated that an appropriate OPG stabilizer proportion was achieved with an FA/GGBFS ratio of 0.1, water/precursor ratio of 0.8, molar ratio of NS of 1.0, and molar concentration of 3 mol/L. The high-early-strength feature of OPG binder contributed to the rapid strength development of the stabilized soft clay at an early age. A noticeable pozzolanic reaction was observed in the OPG-stabilized soft clay sample with an FA/GGBFS ratio of 0.1. Additionally, a binder content of 0.20 was recommended for the stabilization of soft clay due to its optimal balance between economic benefits and meeting the required UCS criteria in soil stabilization. Finally, a reliable nonlinear relationship between UCS and porosity of OPG-stabilized soft clay was established to assess the mechanical properties and stabilization effects of the OPG-stabilized soft clay for practical applications. This study greatly enhances the scientific understanding of geopolymerization in the OPG system by using a combination of the binary precursor of FA and GGBFS and the solid NS activator. It sheds light on the stabilization mechanism of OPG-stabilized soft clay. The outcomes of this study make a valuable contribution to the advancement of environmentally friendly soil stabilizers, promoting green and low-carbon practices in the field of ground improvement.