Strength Development Research Articles

Experimental investigation on properties of gold tailings recycled brick-concrete aggregate concrete under microwave curing

Tailings are the low-value solid waste by-products, which can be expected to serve as sustainable material through activation. In this study, activated gold tailings (GT) were utilized as supplementary cementitious materials to produce GT-cement slurry and gold tailings recycled brick-concrete aggregate (GT-RBCA) concrete. By varying curing methods including microwave curing and steam curing, the effects of resting time and curing temperature on compressive strength of GT-cement slurry specimens were experimentally analyzed. Also, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to analyze the hydration products and microstructure. Additionally, the compressive strength of GT concrete including GT-RBCA concrete under these curing methods were explored, followed by an assessment of environmental and economic benefits. The results demonstrated that a curing temperature around 40 °C with microwave method and 10%–20 % GT will benefit the strength development of GT-cement slurry specimens, reach the highest 28-day compressive strength of 42.6 MPa compared with steam curing and standard curing. This could be attributed to the “hot spot" effect of GT under microwave curing, leading to effective promotion of the pozzolanic reaction and subsequent enhancement of the hydration reaction. For GT-RBCA concrete, microwave curing exhibited the best effect on early compressive strength of GT-RBCA concrete compared with other curing methods. Particularly, whether the standard water-cement ratio condition (w/c = 0.42) or the low water-cement ratio condition (w/c = 0.35), GT-RBCA concrete under microwave curing at 40 °C and 10 % gold tailings substitution rate, achieved a maximum 12.3 % increase in 28-day compressive strength compared to steam curing and standard curing conditions. In addition, the GT-RBCA concrete under microwave curing obtained 11.7 % CO2 emissions reduction compared with steam curing.

High performance illitic clay-based geopolymer: Influence of thermal/mechanical activation on strength development

The scientific community's interest in geopolymers keeps on growing under environmental and climate change pressures. Although most studies concern metakaolin or industrial by-products as aluminosilicate precursors, few are those concerning clay cuttings characterized by little or no kaolinite content. This study proposes a method to activate illitic clay in order to obtain a geopolymer with high mechanical performances. The tested illitic material contained 53 wt% of illite/muscovite, 22 wt% quartz, 7 wt% feldspars and low content of calcite. The innovation consists in coupling thermal and mechanical activations favoring both the clay dehydroxylation and the destruction of the clay crystalline structure. Such increase of amorphous rate in the precursor is required to make the material reactive. The evolution of the precursor material after activation was followed by ThermoGravimetric Analysis (TGA/DTG), X-Ray Diffraction (XRD), Fourier-Transform InfraRed spectroscopy (FTIR) and Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectrometry (SEM-EDX). The impact of the variations of the mechanical activation duration and ball:powder mass ratio (b:p) were mainly explored, as well as the Liquid:Solid mass ratio (L:S) which is a key parameter of the geopolymerization. Finally, an optimized geopolymer with a mechanical compressive strength reaching 126 MPa after 28 days has been manufactured with a grinding duration of 240 min, a b:p = 15 and a L:S = 0.5. The demonstration was done that a geopolymer can be manufactured from 100% illitic clay precursor, in addition to a relatively low amount of alkaline solution (L:S mass ratio of 0.5) then lowering the cost and the environment impact of the final geopolymer material. The resulting illitic clay based geopolymers can display very high mechanical resistance (compressive strength up to 126 MPa after 28 days), fully competitive with conventional building materials.

Influencing factors of the temperature rise of direct electric curing concrete and its effect on concrete properties

Direct electric curing (DEC) is one of concrete's most promising rapid curing methods. However, the factors influencing temperature variations in concrete specimens during the curing process have not yet been fully elucidated. This study investigates the impact of electric voltage, specimen dimensions, energized cross-sectional area, and water-to-cement ratio on the temperature rise of concrete during the DEC process. It also examines the pore structure and strength development of specimens with different water-to-cement ratios under various curing voltages. The temperature rise mechanism is explored through the Joule heating effect, cement hydration heat, and specimen heat dissipation. Research findings reveal that, during the DEC curing process, the peak temperature of specimens gradually rises in conjunction with increasing curing voltage and energized cross-sectional area. The impact of the water-to-cement ratio on the temperature variation of DEC concrete depends on the voltage of DEC. Higher voltages tend to promote Joule heating. Adjusting the water-to-cement ratio contributes to achieving a higher temperature rise in the paste. At lower voltages, the opposite effect is observed. The electrical current of specimens under various curing conditions demonstrates an initial increase followed by a subsequent decrease over time. DEC facilitates early strength enhancement, with different water-to-cement ratios yielding varying curing efficiencies. Through DEC, the 1-day strength of the specimens shows a remarkable increase of up to 89.89 % compared to the standard curing methods. MATLAB simulations using Joule heating, heat dissipation formulas, and the equivalent age function effectively model temperature variations during curing. This study provides a reference for designing lower-carbon, more efficient, and intelligent DEC regimes.

Strength and muscle mass development after a resistance-training period at terrestrial and normobaric intermittent hypoxia

This study investigated the effect of a resistance training (RT) period at terrestrial (HH) and normobaric hypoxia (NH) on both muscle hypertrophy and maximal strength development with respect to the same training in normoxia (N). Thirty-three strength-trained males were assigned to N (FiO2 = 20.9%), HH (2,320 m asl) or NH (FiO2 = 15.9%). The participants completed an 8-week RT program (3 sessions/week) of a full body routine. Muscle thickness of the lower limb and 1RM in back squat were assessed before and after the training program. Blood markers of stress, inflammation (IL-6) and muscle growth (% active mTOR, myostatin and miRNA-206) were measured before and after the first and last session of the program. Findings revealed all groups improved 1RM, though this was most enhanced by RT in NH (p = 0.026). According to the moderate to large excess of the exercise-induced stress response (lactate and Ca2+) in HH and N, results only displayed increases in muscle thickness in these two conditions over NH (ES > 1.22). Compared with the rest of the environmental conditions, small to large increments in % active mTOR were only found in HH, and IL-6, myostatin and miR-206 in NH throughout the training period. In conclusion, the results do not support the expected additional benefit of RT under hypoxia compared to N on muscle growth, although it seems to favour gains in strength. The greater muscle growth achieved in HH over NH confirms the impact of the type of hypoxia on the outcomes.

Influence of interface agent and form on the bonding performance and impermeability of ordinary concrete repaired with alkali-activated slag cementitious material

Alkali-activated slag cementitious materials (AASCMs) exhibit fast hardening, early strength development, high strength, and low carbon content, making them suitable for repairing damaged concrete. However, the bonding performance of the repaired interface and the impermeability of the repaired specimens require further evaluation. Accordingly, this study performed splitting tensile, water penetration, chloride ion penetration, and microanalysis tests on ordinary concrete specimens repaired with AASCMs. The results demonstrated that epoxy resin and cement mortar improved the interfacial bonding strength, water permeability, and chloride ion permeability of the alkali-activated slag cementitious material-Portland cement (AASCM-PC) specimens. The rectangular mortise joint and S-shaped interface improved the interfacial bonding strength and water permeability of the AASCM-PC specimens but slightly reduced their chloride ion permeability. Compared with ordinary concrete, AASCM has fewer harmful voids and denser microstructure, improving the impermeability of the repaired specimens.

The effect of concurrent high-intensity interval training and resistance training on the lower body maximal strength and explosive power: A updated systematic review and meta-analysis

BACKGROUND: Endurance training can have a negative impact on strength training and may lead to reduced strength gains, known as the interference effect. However, high-intensity interval training (HIIT) as an endurance training mode may reduce this interference effect. OBJECTIVE: This systematic review and meta-analysis aim to investigate the effects of concurrent HIIT and resistance training (RT) on lower body explosive strength and maximum strength. METHODS: Five electronic databases were searched. Subgroup analyses were performed to assess the effects of HIIT modality, training status, and training duration on strength development following concurrent HIIT and RT. RESULTS: Meta-analysis showed that compared to RT alone, concurrent HIIT and RT will not affect the development of countermovement jump (CMJ) (WMD =-0.17, 95%CI =-1.45 to -1.11) and half squat (WMD =-0.05, 95%CI =-2.42 to 2.32). Further, subgroup analysis revealed that HIIT-running workout was conducive to the development of both CMJ and half squat. Longer training duration was found to be more effective in developing CMJ, while shorter training duration was more suitable for developing half squat. Additionally, athletes showed greater improvement than non-athletes. CONCLUSIONS: Combining HIIT and RT can enhance CMJ and half squat. The results of intervention are moderated by training variables and training status.

Highly reactive carbonated recycled concrete fines prepared via mechanochemical carbonation: Influence on the early performance of cement composites

Carbonated recycled concrete fines (RCF) possess the potential to serve as supplementary cementitious materials (SCMs). However, calcium carbonate (Cc) formed during traditional carbonation methods generally shows low reactivity. Moreover, the remaining calcium silicate hydrate (C-S-H) with a low Ca/Si ratio exhibits enhanced resistance to decalcification, leading to the formation of lower content of silica gels. These factors limit the reactivity and utilization of carbonated RCF in cementitious systems. In the present work, mechanochemistry was introduced into the RCF carbonation process, a technique named mechanochemical carbonation (MC), to enhance the reactivity of carbonated RCF. This study delved into the influence of MC-treated RCF (MCR) on the early performance of cement composites. The findings reveal that MCR exerted a more pronounced promotional effect on cement hydration, as demonstrated by the earlier appearance of hydration peaks for silicates and aluminates, coupled with increased peak intensity. This then resulted in a notable enhancement in early compressive strength. Specifically, with 10 wt% MCR, the compressive strength increased by approximately 19.9 % and 24.7 % at 1 d and 3 d, respectively. In comparison, samples incorporating wet carbonated RCF (WCR) demonstrated lower enhancements, with increases of only 4.5 % and 12.0 % at 1 d and 3 d, respectively. The enhanced performance of MCR can be attributed to the increased formation of nano-sized amorphous silica gels and metastable Cc. These components exhibit elevated reactivity and contribute to seeding effects, thereby promoting the overall reactivity and strength development of the cementitious system. These findings underscore the promising application potential of MC in transforming RCF into highly reactive SCMs.

Green Manufacturing of UHPFRC Made with Waste Derived from Scrap Tires and Oil Refineries

Ultrahigh-performance fiber-reinforced cement-based composite (UHPFRC) made with waste derived from scrap tires and oil refineries was tested in this study. The UHPFRC sample exhibited a maximum compressive strength of about 189 MPa at the end of 28 days. Steel fibers were recovered from scrap tires and were added up to 3% by volume in the UHPFRC samples. Such additions reduced cement flow by 11% but improved compressive strength by 21%. The equilibrium catalyst particles (ECAT) disposed of by oil refineries were used in amounts of up to 15% by weight as a replacement for cement in such UHPFRC samples. These aluminosilicate materials are spherical in shape and have a porous microstructure, which was found to reduce the cement flow by absorbing more free water onto their surfaces. They also reduced the heat and strength developments at early stages. However, the total cost of the final cement-based mixture and associated CO2 emissions were reduced by up to 7% and 15% due to the inclusion of the ECAT particles. These findings help to optimize the ECAT and recovered steel fibers in the UHPFRC mix design, and such waste valorization strategies can help achieve the goal of becoming carbon neutral.

Sustainable approach to quality assurance in cement-improved soil: applications of handheld XRF for geotechnical solutions

Traditional methods for evaluating the quality of the Deep Mixing Method (DMM) in Japan, such as the phenolphthalein tracer method, present limitations in ensuring strength improvement. This study explores handheld X-ray fluorescence (XRF) as a sustainable alternative for quality assessment. The elemental composition of cement-treated soil was investigated, focusing on Calcium (Ca), Silicate (Si), and Sulfate (S), which play crucial roles in the formation of hydrated products, aiming to accurately determine their influence on the strength development of the improved soil. A controlled protocol was employed using commercial Kaolin clay and the needle penetration test for strength assessment. Our laboratory experiments, conducted with a cement-based binder (C) dosage of 110 kg/m3 and a water-cement (W/C) ratio of 1, furthered our understanding of the hydration process. Results indicate an apparent increase in Ca amount over time, correlating with improved strength, while the apparent amount of Si decreases, suggesting its integration into hydration products such as C-S–H gel. Strength assessment rose significantly between day 1 and 28, aligning with variations observed in Si, Ca, and S, underlining their role in strength evolution. This research underscores the potential of handheld XRF as a sustainable substitute for conventional methods in field-quality assurance, offering real-time data on the formation of possible hydrated products during the hydration process for improved soil in geotechnical engineering.

Application of Compressible Carbon in Cement to Mitigate Cement Pore Pressure Reduction and Improve Cement Bonding

Summary A quality cement job is essential to ensure long-term zonal isolation in oil/gas and carbon storage wells. This may be affected by the cement hydration process, during which water is consumed, causing pore pressure reduction and shrinkage, which increases the risk of fluid/gas invasion, formation of microannuli, and even the risk of shear bond strength reduction. In this paper, we present the application of compressible carbon (CC) particles as a cement additive to mitigate cement pore pressure reduction and improve zonal isolation. We designed cement formulations with CC and evaluated them using API standard tests and industry-wide recognized tests to ensure they met functional requirements for well cementing, such as mixability, thickening time, rheology, compressive strength development, fluid loss, and free water. The concept is as follows: When cement is pumped into an annulus, the CC particles will compress under hydrostatic pressure. When the cement is hydrating, the CC will expand to mitigate the pore pressure reduction. To validate the proof of concept, we developed a benchtop high-pressure, high-temperature setup and a pilot-scale test setup. A cement formulation with 9% CC by volume was found to be stable and have controllable performance properties such as thickening time, fluid loss, and free water. We also designed and tested a control slurry without CC for comparison. Both the benchtop and pilot-scale tests demonstrated that adding CC into the cement formulation mitigated the pore pressure reduction during cement hydration. This may reduce the risk of fluid/gas invasion that could result in migration in the set cement. Comparing the cement containing carbon with the control system without carbon, the gas permeability was reduced. The bond strength obtained from shear bond tests improved significantly, which may reduce the risk of debonding and be an indication of the reduction of shrinkage. In addition, a better hydraulic seal was found in one test, but further investigation is needed. Similar mechanical properties such as compressive strength were measured for both cements with and without CC. The data showed that adding CC does not have a negative impact on the hardened cement properties. In fact, adding CC decreased Poisson’s ratio slightly. In this paper, we present the results of proof-of-concept testing for the novel application of CC in cement to improve zonal isolation by mitigating pore pressure reduction. We also present new benchtop and pilot-scale experimental setups to measure pore pressure changes during cement hydration.

Influence of Steam Curing on the Performance of High-strength Concrete Incorporating Metakaolin

This study aims to investigate the influence of steam curing on the performance of high-strength concrete incorporating metakaolin. An experimental study was conducted to investigate the compressive strength, chloride permeability, initial surface absorption, and water absorption, as well as thermal degradation under various steam curing scenarios. Four concrete mixtures were prepared in this study, each with different percentages of metakaolin, as partial substitution of cement weight of 0%, 5%, 10%, and 15%. The water–binder ratio was set to 0.32 in all mixtures. The specimens underwent steam curing for 0, 4, 8, and 16 hours at a temperature of 55 °C. Results indicate that the specimens that have been steam cured and contained metakaolin exhibit improved compressive strength during the initial stages of their strength development. Steam curing has been demonstrated to enhance the concrete’s resistance to chloride penetration and reduce initial surface absorption while increasing water absorption. The optimal replacement of cement with metakaolin to enhance strength and transport properties in high-strength concrete under steam curing regimes is between 10% and 15%. Future research could examine the effects of long-term steam curing on the durability and performance of concrete incorporating an optimal proportion of metakaolin.

USE OF SPORTS EQUIPMENT IN PHYSICAL EDUCATION CLASSES AT UNIVERSITY

The article describes a study to determine the methods used for using athletics equipment in physical education classes at a university. The study showed that this sports equipment is most often used in physical education classes for warming up, promoting the development of strength and coordination skills, speed, as well as for improving specific skills.

Performance and microstructural development of 3D printable MgO-SiO2 mixes containing magnesium silicate monohydrate

This study investigated the inclusion of magnesium silicate monohydrate (MSMH) crystal in MgO–SiO2 mixes designed for extrusion-based 3D printing. Considering the adjustment and optimization of rheological properties are fundamental for the printability of cement pastes, the dynamic rheology, thixotropy and strength development of the prepared mixes were analyzed to characterize their fresh properties, workability, buildability, and mechanical performance. Furthermore, isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric analysis (TG/DTG), and scanning electron microscopy (SEM) were employed to investigate the phase composition and microstructural evolution of the pastes, with and without the presence of MSMH. Addition of MSMH at 2 % of the binder component significantly enhanced the plastic viscosity, dynamic yield stress and thixotropy. Pastes incorporating 2 % MSMH exhibited excellent structural build-up without affecting extrudability, with a minimal strain deformation in the printed structure. The compressive strength of all pastes exceeded 20 MPa after 3 days, reaching ∼50 MPa at 28 days with 2 % MSMH inclusion. The introduction of MSMH improved the peak heat flow measured by isothermal calorimetry and enhanced the formation and growth of hydration products after 7 days of curing, resulting in a more compact microstructure.

3D printed sulfur-regolith concrete performance evaluation for waterless extraterrestrial robotic construction

By leveraging the capabilities of construction 3D printing, building structures in harsh extraterrestrial environments is conceivable. Sulfur concrete is a waterless construction material that offers great potential to replace Portland cement concrete (PCC) in extraterrestrial construction. The shape stability of 3D printed Martian sulfur-regolith concrete (SRC) was found to benefit from a lower substrate layer temperature. However, this comes at the cost of flexural strength, resulting in up to 53% strength loss. The printed SRC specimens demonstrated a significantly faster strength development rate (gaining about 85% of the ultimate strength after only 12 h) compared to the printed PCC. The printed SRC specimens also outperformed the PCC specimens in vacuum conditions at higher temperatures. Furthermore, modifying the SRC materials with Dicyclopentadiene resulted in up to 44% strength increase and minimized the sublimation rate of the printed specimens in vacuum, especially at an elevated temperature.

Effect of multiple environmental conditions on the formation process and adhesion strength of ice on asphalt binder surface

Road icing caused by freezing rain significantly impacts driving safety. Since asphalt binder plays a pivotal role in pavement infrastructure, understanding the formation process and adhesion strength of ice on asphalt binder surfaces is crucial for assessing road icing risk. This work investigated the effect of different environmental conditions on asphalt-ice interface adhesion strength via a self-designed low-temperature adhesion testing system. In addition, the icing process on asphalt binder surfaces was observed to analyze the asphalt-ice adhesion strength formation mechanism. Results show that the completion time of water-ice phase transition signified the initial development of asphalt-ice adhesion strength. Moreover, as the internal structure of the ice evolved from transparent to needle-like fine lines to dense, the asphalt-ice adhesion strength gradually increased until it reached stability. At asphalt-ice adhesion began forming after 40 min and stabilized after 80 min, whereas at −8°C, adhesion strength started forming after 20 min and stabilized by 60 min. The growth rates of asphalt-ice adhesion strength for temperatures ranging from −2°C to −10°C were approximately 7.92 kPa/°C, 6.56 kPa/°C, 5.14 kPa/°C, 3.29 kPa/°C, and 1.49 kPa/°C, respectively, showing a positive correlation with temperature and a negative correlation with time. According to heterogeneous nucleation theory, the influence mechanism of temperature and water depth on the nucleation icing process and asphalt-ice adhesion strength is analyzed. lower temperatures lead to asphalt-ice adhesion strength increase originating from the larger supercooling, which reduces the critical nucleation energy and thus accelerates the ice nucleation rate. In addition, the increase in water depth promotes the formation of nucleation sites and further enhances the asphalt-ice adhesion strength. Finally, an asphalt-ice adhesion strength prediction model was developed, with PSO-SVM-G achieving over 90 % accuracy. These results provide insights for evaluating ice adhesion behavior on asphalt surfaces and designing anti-icing asphalt materials.

Curing process monitoring of cementitious grout using time domain reflectometry (TDR) system

Curing process monitoring of the cementitious grout is necessary for its successful use and to ensure the serviceability of the constructed, repaired, and stabilized infrastructures using that material. In this study, time domain reflectometry (TDR) system is applied to monitor the curing process of the cementitious grout based on the changes in relative permittivity (εr). To utilize the TDR system to electrically conductive cementitious grout, a TDR probe with coated core electrode (coated probe) is manufactured, and a calibration is performed to evaluate the formal εr by using the coated probe. The coated probes are installed in the middle and at the bottom of the cementitious grouts with a length of 950 mm, and the εr have been evaluated with curing time using both middle probe (εr(middle)) and bottom probe (εr(bottom)). In addition, compressive strength tests are performed on separately prepared specimens for comparative analyses. Experimental results show that the εr(middle) and εr(bottom) decrease with curing time and converge from the fully cured time, and the curing process of cementitious grout monitored by the TDR system is consistent with the compressive strength developments of the specimens tested with curing time. Therefore, the TDR system with a coated probe may effectively be used for monitoring the curing process of the cementitious grouts based on the changes in electromagnetic property.

Improving thermal conductivity and drying shrinkage of foamed concrete with artificial ceramsite from excavation soil and sewage sludge

Foamed concrete (FC) is susceptible to crack induced by shrinkage, which results in a degradation of the thermal insulation. This study proposes a solution by incorporating artificial ceramsite (AC) from excavation soil and sewage sludge in FC for improving thermal conductivity and drying shrinkage. The characteristics of AC and its effects with varying particle size distributions (PSDs) as well as volume contents on the fluidity, strength development, drying shrinkage, thermal conductivity, pore structure and morphology of FC were explored. Both the water absorption and porosity of AC increased whereas its apparent density reduced as the PSD rose. The fluidity tended to decrease as the replacement elevated, but the rate of fluidity loss decreased. Compared to FC, fine AC with high replacement was detrimental to strength, while coarse AC with low replacement had no marked adverse impact on strength. More importantly, the presence of AC performed better in mitigating drying shrinkage and thermal conductivity, especially for the higher replacement of coarse AC. Furthermore, the existence of AC with tailored porosity increased the porosity and average pore diameter but improved the pore circularity and pore connectivity.

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.

Fresh and hardened properties for a wide range of geopolymer binders – An optimization process

The objective of this study was to identify the optimal geopolymer binder compositions produced from local industrial by-products and compare them with commercially available materials. The first part investigated the optimal binder composition by studying 27 mixtures. In this part of the research, three series of binders were created: the first series of 18 mixtures was Na-based fly ash-slag, the second series consisted of 8 mixtures of K-based fly ash-slag, and the third series were a mixture of metakaolin-slag based geopolymer. The compressive strengths of the mixtures at the age of seven days ranged from 2.18 to 96.23 MPa. The strength development is clearly defined by the components and proportions of the blends. Geopolymers reach about 80% of their 28-day strength in seven days. The 25% water content was optimal for slag-fly ash geopolymers. The strength of the material increased from 68.08 to 96.23 MPa when the Blaine surface area of the slag increased from 3500 to 4500 cm2/g. The optimal proportions of the alkali solution were the intermediate ratios: SiO2/Na2O = 2.0 and SiO2/K2O = 1.5. In the case where Visonta fly ash is prepared for blending, the fly ash content can be maximized by 30%–50% in addition to the blast-furnace slag. In the second part of the research, mortars were prepared from the selected binders: 4 mixtures were prepared with two different binders. In one of these mortars, the solid part of the binder consisted of local raw materials originating from Hungary. This mixture was prepared with 43 m% fine aggregates. The optimal composition of the tested 27 binders tested was selected as the geopolymer matrix for the production of three further mortar mixtures. In these geopolymer mortars, 55, 65, and 75 m% of aggregate applied. The flowtable values of fresh mortars decreased when the proportion of sand increased. The lower the additive content was, the higher the strength of the geopolymer mortar. The 28-day compressive strengths of filtered fly ash-slag mortar sample with 55%, 65%, and 75% of fine aggregate (F–S-a55, F–S-a65, and F–S-a75) made with the selected optimised filtered fly ash-slag geopolymer binder of group A-I.3 (F–S-A-I.3) varied between 11.39 and 40.15 MPa, whereas the Visont fly ash-slag mortar sample with 43% of fine aggregate (V–S-a43) has been 25.05 MPa. For 28 days, the density ranged between 1870 and 2204 kg/m3. The V–S-a43 is found to be the lowest-density blend. The chloride migration test revealed that geopolymer mortars with higher slag content have higher resistance to Cl− ions.

Carbonation strengthening mechanism of dry cement-based materials under supercritical carbon dioxide

Long-term stability of concrete as a wellbore sealing material is enhanced by the interaction between Portland cement and supercritical carbon dioxide. This study is to explore the mechanism of the strengthening effect of supercritical carbon dioxide on cement-based materials. After supercritical carbonation curing, the compressive strength of dry samples is increased by 93 %∼123 %. By using regional characterization analysis, the relationship between chemical reaction process and local mechanical property changes is established. The supercritical carbonation process and reaction kinetics are uncovered. On the one hand, the carbonation products form a compacted layer in the external area of cement paste, with a local microhardness increase of 30 % for 7d and 102 % for 28d, playing hoop effect on strengthening; On the other hand, the inward aggregation of carbonation produced water promotes the local “rehydration”, increasing the degree of hydration by 3.89 %∼15.98 %, which benefits strength development. The transport and accumulation of the produced water are the main limiting factors which obstruct the continuous penetration of supercritical carbonation.