Development of Waste-Based Alkali-Activated Cement Composites

Nowadays, global warming and the ensuing climate change are one of the biggest problems for humanity, but environmental pollution and the low ratio of waste management and recycling are not negligible issues, either. By producing alkali-activated cements (AACs), it is possible to find an alternative way to handle the above-mentioned environmental problems. First, with a view to optimizing experimental parameters, metakaolin-based AACs were prepared, and in it, waste tire rubber was used as sand replacement (5–45 wt %). Insufficient wetting between the rubber particles and the matrix was corrected through different surface treatments of the rubber. For improving the mechanical/strength properties of the specimens, fibrous waste kaolin wool (0.5–1.5 wt %) was added to the AAC matrix. Considering the results of model experiments with metakaolin, blast-furnace-slag-based AAC composites were developed. The effects of storage conditions, specimen size and cyclic loading on the compressive strength were investigated, and the resulting figures were compared with the relevant values of classic binders. The strength (44.0 MPa) of the waste-based AAC composite significantly exceeds the required value (32.5 MPa) of clinker saving slag cement. Furthermore, following cyclic compressive loading, the residual strength of the waste-based AAC composite shows a slight increase rather than a decrease.     

Mechanical Properties of Furnace Slag and Coal Gangue Mixtures Stabilized by Cement and Fly Ash

The mechanical properties and strength formation mechanism of cement–fly-ash-stabilized slag–coal gangue mixture were examined using an unconfined compressive strength test, splitting strength test, triaxial test, and scanning electron microscopy to solve the limitations of land occupation and environmental pollution that is caused by fly ash from the Xixia District thermal power plant in Yinchuan, slag from the Ningdong slag yard, and washed coal gangue. Its performance as a pavement base mixture on the road was investigated. The results demonstrated that as the slag replacement rate increased, the maximum water content increased while the maximum dry density decreased. The addition of slag reduced the unconfined compressive strength and splitting strength of the specimens; furthermore, the higher the slag substitution rate, the lower the unconfined compressive strength and splitting strength of the specimens. As the cement content increased, the specimen’s unconfined compressive strength increased. Based on the principle of considering the mechanical properties and economic concerns, the slag replacement rate in the actual construction should be ~50% and should not exceed 75%. Based on the relationship between the compressive strength and splitting strength of ordinary concrete, the relationship model between the unconfined compressive strength and splitting strength of cement–fly-ash-stabilized slag–coal gangue was established. The failure mode, stress–strain curve, peak stress, and failure criterion of these specimens were analyzed based on the triaxial test results, and the relationship formulas between the slag substitution rate, cement content, peak stress, and confining pressure were fitted. As per the SEM results, the mixture’s hydration products primarily included amorphous colloidal C-S-H, needle rod ettringite AFt, unhydrated cement clinker particles, and fly ash particles. The analysis of the mixture’s strength formation mechanism showed that the mixture’s strength was the comprehensive embodiment of all factors, such as the microaggregate effect, secondary hydration reaction, and material characteristics.     

Incorporation of Cement Bypass Dust in Hydraulic Road Binder

This article describes utilization of a cement kiln bypass dust utilization as an added component in a hydraulic road binder. Three experimental binder mixes (BM1–BM3) with variation in the composition of the main constituents (cement clinker, ground limestone and ground granulated blast furnace slag) and constant content of bypass dust (10%) were prepared under laboratory conditions. The properties of binder constituents, fresh experimental binder mixes and hardened specimens were tested according to STN EN 13282-2 for a normal hardening hydraulic road binder. The physical and chemical properties of all binder mixes (fineness: +90 µm ≤ 15 wt.%; SO₃ content: <4 wt.%) met the standard requirements. The bypass dust addition led to an increase in the water content for standard consistency of cement mixes (w/c = 0.23) and to a shortening of the initial setting time for two experimental blended cement pastes (BM1 and BM3) compared with the value required by the standard. Only BM2 with the lowest SO₃ content (0.363 wt.%) and the highest percentage of granulated blast furnace slag (9.5 wt.%) and alkalis (Na₂O and K₂O content of 5.9 wt.%) in the binder mix met the standard value for the initial setting time (≥150 min). The results of compressive strength testing of experimental specimens after 56 days of hardening (59.2–63.9 MPa) indicate higher values than the upper limit of the standard requirement for the N4 class (≥32.5; ≤52.5 MPa).     

Preparation and Swelling Behaviors of High-Strength Hemicellulose-g-Polydopamine Composite Hydrogels

Hemicellulose-based composite hydrogels were successfully prepared by adding polydopamine (PDA) microspheres as reinforcing agents. The effects of PDA microsphere size, dosage, and nitrogen content in hydrogel on the mechanical and rheological properties was studied. The compressive strength of hydrogel was increased from 0.11 to 0.30 MPa. The storage modulus G’ was increased from 7.9 to 22.0 KPa. The gaps in the hemicellulose network are filled with PDA microspheres. There is also chemical cross-linking between them. These gaps increased the density of the hydrogel network structure. It also has good water retention and pH sensitivity. The maximum cumulative release rate of methylene blue was 62.82%. The results showed that the release behavior of hydrogel was pH-responsive, which was beneficial to realizing targeted and controlling drug release.     

Residual Compressive Behavior of Self-Compacting Concrete after High Temperature Exposure—Influence of Binder Materials

This paper presents an experimental investigation of the compressive behavior of high-strength self-compacting concrete exposed to temperatures up to 600 °C. Ten different concrete compositions were tested, in which part of the cement (by weight) was replaced by three different mineral additives (5–15% metakaolin, 20–40% fly ash and 5–15% limestone). The stress–strain curves, compressive strength, modulus of elasticity and strain at peak stress were evaluated from uniaxial compression tests. Scanning electron microscope micrographs were also taken to evaluate the damage caused by the high temperatures. A sharp decrease in mechanical properties and an increase in peak strain were observed already after 200 °C for all mixes tested. The different mineral additives used in this study affected the variations of residual compressive strength by 24% and peak strain by 38%, while the variations of residual modulus elasticity were 14%. Comparing the obtained results with the recommendations for compressive strength given in regulatory code EN 1992-1-2 for high strength concrete, it can be concluded that the strength loss observed in EN 1992-1-2 at temperatures up to 400 °C is too conservative. The Popovics model for the relationship between stress and strain provided a good approximation for the experimentally determined stress–strain curves at different temperatures.     

Thermophysical and Mechanical Properties of Hardened Cement Paste with Microencapsulated Phase Change Materials for Energy Storage

In this research, structural-functional integrated cement-based materials were prepared by employing cement paste and a microencapsulated phase change material (MPCM) manufactured using urea-formaldehyde resin as the shell and paraffin as the core material. The encapsulation ratio of the MPCM could reach up to 91.21 wt%. Thermal energy storage cement pastes (TESCPs) incorporated with different MPCM contents (5%, 10%, 15%, 20% and 25% by weight of cement) were developed, and their thermal and mechanical properties were studied. The results showed that the total energy storage capacity of the hardened cement specimens with MPCM increased by up to 3.9-times compared with that of the control cement paste. The thermal conductivity at different temperature levels (35–36 °C, 55–56 °C and 72–74 °C) decreased with the increase of MPCM content, and the decrease was the highest when the temperature level was 55–56 °C. Moreover, the compressive strength, flexural strength and density of hardened cement paste decreased with the increase in MPCM content linearly. Among the evaluated properties, the compressive strength of TESCPs had a larger and faster degradation with the increase of MPCM content.     

High Temperature Performance of Concrete Confinement by MWCNT Modified Epoxy Based Fiber Reinforced Composites

The conventional method of fiber reinforced polymer (FRP) wrapping around concrete columns uses epoxy as the binder along with synthetic or natural fibers such as carbon, glass, basalt, jute, sisal etc. as the reinforcement. However, the thermal stability of epoxy is a major issue in application areas prone to fire exposure. The current work addressed this major drawback of epoxy by modifying it with a nanofiller, such as multiwalled carbon nanotubes (MWCNT), and reinforcing it using basalt and sisal fibers. The effect of exposure to elevated temperature on the behavior of concrete cylinders externally confined with these FRP systems was analyzed. Three types of specimens were considered: unconfined; confined with sisal fiber reinforced polymer (SFRP); and confined with hybrid sisal basalt fiber reinforced polymer (HSBFRP) specimens. The test samples were exposed to elevated temperature regimes of 100 °C, 200 °C, 300 °C and 400 °C for a period of 2 h. The compressive strengths of unconfined specimens were compared with various confined specimens, and from the test results, it was evident that the mechanical and thermal durability of the FRP systems was substantially enhanced by MWCNT incorporation. The reduction in the compressive strength of the FRP-confined specimens varied depending on the type of the confinement. After two hours of exposure at 400 °C, the compressive strength corresponding to the epoxy–HSBFRP-confined specimens were improved by 15%, whereas a 50% increase in strength corresponding to MWCNT-incorporated epoxy–HSBFRP-confined specimens was observed with respect to unconfined unexposed specimens. The MWCNT-modified epoxy-incorporated FRP-confined systems demonstrated superior performance even at elevated temperatures in comparison to unconfined specimens at ambient temperatures.     

Effect of Manufactured Sand with Different Quality on Chloride Penetration Resistance of High–Strength Recycled Concrete

High–strength manufactured sand recycled aggregate concrete (MSRAC) prepared with manufactured sand (MS) and recycled coarse aggregate (RCA) is an effective way to reduce the consumption of natural aggregate resources and environmental impact of concrete industry. In this study, high–, medium– and low–quality MS, which were commercial MS local to Changzhou and 100% by volume of recycled coarse aggregate, were used to prepare MSRAC. The quality of MS was determined based on stone powder content, methylene blue value (MBV), crushing value and soundness as quality characteristic parameters. The variation laws of compressive strength and chloride penetration resistance of high–strength MSRAC with different rates of replacement and different qualities of MS were explored. The results showed that for medium– and low–quality MS, the compressive strength of the MSRAC increased first and then decreased with increasing rate of replacement. Conversely, for high–quality MS, the compressive strength gradually increased with increasing rate of replacement. The chloride diffusion coefficient of MSRAC increased with decreasing MS quality and increasing rate of replacement. The chloride diffusion coefficient of MSRAC basically met the specifications for 50–year and 100–year design working life when the chloride environmental action was D and E. To prepare high–strength MSRAC, high–quality MS can 100% replace RS (river sand), while rates of replacement of 50–75% for medium–quality MS or 25–50% for low–quality MS are proposed. Scanning Electron Microscope (SEM) images indicated that an appropriate amount of stone powder is able to improve the compressive strength of RAC, but excessive stone powder content and MBV are unfavorable to the compressive strength and chloride penetration resistance of RAC.     

Modeling Uniaxial Bond Stress–Slip Behavior of Reinforcing Bars Embedded in Concrete with Different Strengths

This paper aims to study the uniaxial bond stress–slip characteristics of reinforcing bars embedded in concrete with different strengths. Tests were conducted on tension–pull specimens that had a cross-sectional dimension with a reinforcing bar embedded in the center section. The experimental variable was the concrete compressive strength (20, 40, and 60 MPa). The test results show that in the specimen subjected to any fixed load, the maximum value of the concrete strain occurred around the central position, and its value increased as the compressive strength of the concrete increased. Depending on the embedded position of the steel bars, the bond stress–slip relationship was also different. In addition, the analytical results indicate that the proposed bond stress–slip constitutive relationship is very accurate in describing the true bond stress–slip relationship.     

Synthesis and Characterization of Anatase TiO2 Microspheres Self-Assembled by Ultrathin Nanosheets

In this paper, we report a novel and simple method for synthesizing the microspheres self-assembled from ultrathin anatase TiO₂ nanosheets with a high percentage of (001) facets via the hydrolysis process of the single-reagent (potassium fluorotitanate). We then used optical microscopy, scanning electron microscopy, and high-resolution confocal laser Raman spectroscopy to characterize the microspheres generated under different conditions. The study found that the size of the anatase TiO₂ microspheres synthesized was 0.5–3 μm. As the synthesis time increased, the corroded surface of the microspheres gradually increased, resulting in the gradual disappearance of the edges and corners of the anatase nanosheets. The exposure percentage of the (001) facets of ultrathin anatase nanosheets synthesized for 2 h at 180–200 °C are close to 100%. The microsphere whose surface is completely covered by these anatase nanosheets also has nearly 100% exposed (001) facets. This new anatase nanosheet-based self-assembled microsphere will have great application potential in pollution prevention, environmental protection, and energy fields.     

Mechanical and Microstructural Characterization of Quarry Rock Dust Incorporated Steel Fiber Reinforced Geopolymer Concrete and Residual Properties after Exposure to Elevated Temperatures

The purpose of this research is to study the effects of quarry rock dust (QRD) and steel fibers (SF) inclusion on the fresh, mechanical, and microstructural properties of fly ash (FA) and ground granulated blast furnace slag (SG)-based geopolymer concrete (GPC) exposed to elevated temperatures. Such types of ternary mixes were prepared by blending waste materials from different industries, including QRD, SG, and FA, with alkaline activator solutions. The multiphysical models show that the inclusion of steel fibers and binders can enhance the mechanical properties of GPC. In this study, a total of 18 different mix proportions were designed with different proportions of QRD (0%, 5%, 10%, 15%, and 20%) and steel fibers (0.75% and 1.5%). The slag was replaced by different proportions of QRD in fly ash, and SG-based GPC mixes to study the effect of QRD incorporation. The mechanical properties of specimens, i.e., compressive strength, splitting tensile strength, and flexural strength, were determined by testing cubes, cylinders, and prisms, respectively, at different ages (7, 28, and 56 days). The specimens were also heated up to 800 °C to evaluate the resistance of specimens to elevated temperature in terms of residual compressive strength and weight loss. The test results showed that the mechanical strength of GPC mixes (without steel fibers) increased by 6–11%, with an increase in QRD content up to 15% at the age of 28 days. In contrast, more than 15% of QRD contents resulted in decreasing the mechanical strength properties. Incorporating steel fibers in a fraction of 0.75% by volume increased the compressive, tensile, and flexural strength of GPC mixes by 15%, 23%, and 34%, respectively. However, further addition of steel fibers at 1.5% by volume lowered the mechanical strength properties. The optimal mixture of QRD incorporated FA-SG-based GPC (QFS-GPC) was observed with 15% QRD and 0.75% steel fibers contents considering the performance in workability and mechanical properties. The results also showed that under elevated temperatures up to 800 °C, the weight loss of QFS-GPC specimens persistently increased with a consistent decrease in the residual compressive strength for increasing QRD content and temperature. Furthermore, the microstructure characterization of QRD blended GPC mixes were also carried out by performing scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS).     

The Effect of Crystalline Waterproofing Admixtures on the Self-Healing and Permeability of Concrete

This paper investigates the effectiveness of a specific crystalline waterproofing admixture (CWA) in concrete as a function of a water–binder ratio. Four concrete mixes with and without CWA were prepared; two of them with a water–binder ratio of 0.45 and two of them with a water–binder ratio of 0.55. Water permeability and compressive strength were tested on hardened concrete specimens and self-healing of cracks over time was observed. Cement paste and CWA paste were prepared to clarify the results obtained on the concrete specimens. SEM and EDS and XRD and FTIR were performed on the hardened pastes to explain the mechanism of CWA working. The results show that the addition of CWA had no significant effect on the compressive strength of the concrete, but reduced the water penetration depth in the concrete, and the reduction was more effective for mixes with lower water–binder ratio. Regarding the self-healing effect, it can be concluded that the addition of CWA improves the crack healing in concrete, but the efficiency of self-healing is highly dependent on the initial crack width. The mechanisms involved in the reduction of water penetration depth and crack healing in concrete can be explained by different mechanisms; one is creation of the CSH gel from unreacted clinker grains, then formation carbonate, and additional mechanism is gel formation (highly expansive Mg-rich hydro-carbonate) from magnesium based additives. The presence of sodium silicate, which would transform into carbonate/bicarbonate, also cannot be excluded.     

Experimental Research on Mechanical Properties of Carbon Fiber-Reinforced Reactive Powder Concrete after Exposure to Cryogenic Temperatures

This study aims to evaluate the mechanical properties of carbon fiber-reinforced reactive powder concrete (CFRPC) after exposure to cryogenic temperature. The mechanical properties of plain RPC and CFRPC with carbon fiber volume contents of 0, 0.5%, 1.0%, and 1.5% were examined after exposure to 20 °C, −5 °C, −15 °C, and −25 °C for 72 h. The effect of fiber contents and exposure temperatures on the cubic and axial compressive strength, splitting tensile strength, elastic modulus, and peak strain were systematically reported and analyzed. The results showed adding carbon fiber to RPC could significantly enhance the strength and slightly improve ductility performance. Additionally, CFRPC with 1.0% fiber content showed the best mechanical properties. The maximum increases in cubic and axial compressive strength and tensile strength were 26.0%, 25.7%, and 21.8%, the elastic modulus was 13.2%, and the peak strain was 13.0% over the plain RPC. Additionally, all mechanical properties continued to degrade with decreasing temperature. After exposure to −25 °C, the cubic, axial compressive strength, and tensile strength of CFRPC degraded to 82.2–84.9%, 80.7–87.5%, and 72.7–73.7% of the normal temperature strength, respectively. In addition, the linear relationship equation between the discount factor of each mechanical property and the temperature was established. Finally, the equation for the stress–strain ascending curve of CFRPC described by a quadratic polynomial was proposed, which fitted well with the experimental results.     

Experiment on the Properties of Soda Residue-Activated Ground Granulated Blast Furnace Slag Mortars with Different Activators

Soda residue (SR), a solid waste generated in the production of Na₂CO₃ during the ammonia soda process, with a high pH value of 12, can be used as an activator of alkali-activated ground granulated blast furnace slag (GGBFS) cementitious materials. Three groups of experiments on SR-activated GGBFS mortars were designed in this paper to assess the role of the dominant parameters on fluidity and compressive strength of mortars. The results indicate that for fluidity and mechanical properties, the optimal scheme of SR-activated GGBFS mortars is 16:84–24:76 S/G, 0.01 NaOH/b, 0.05 CaO/b, and 0.50 w/b, with fluidity and compressive strength (28 d) of the mortars being 181–195 mm and 32.3–35.4 MPa, respectively. Between 2.5–10% CaCl₂ addition to CaO (5%)-SR (24%)-activated GGBFS mortar is beneficial to the improvement of the compressive strength of C2, whereas the addition of CaSO₄ is harmful. The main hydration products of mortars are ettringite, Friedel’s slat, and CSH gels. The results provide a theoretical basis and data support for the utilization of SR.     

Mechanical Properties and Uniaxial Failure Behavior of Concrete with Different Solid Waste Coarse Aggregates

To reveal the differences between the mechanical properties of solid waste coarse aggregate concrete and natural coarse aggregate concrete (NCAC) under equal strength, the basic mechanical properties of coarse aggregate concrete with seven different solid wastes (i.e., self-combusted coal gangue, uncombusted coal gangue, marble sheet waste, granite sheet waste, iron waste rock, recycled concrete, and self-combusted coal gangue ceramicite) were tested, and the trends in failure morphology, elastic modulus, and the stress–strain full curves of the different solid waste coarse aggregate concretes were analyzed and compared with NCAC. Finally, the interfacial structure of the concrete was characterized by SEM. The results showed that C30 strength grade concrete was prepared with different solid waste coarse aggregates; however, the 28 d compressive strength, split tensile strength, axial compression strength, flexural strength, and elastic modulus of the concrete was 35.26–47.35, 2.13–3.35, 26.43–42.70, 2.83–3.94, and 17.3–31.2, respectively. The modulus of elasticity of the solid waste coarse aggregate concrete was smaller than the NCAC under equal strength, with a maximum difference of 45%. The peak compressive strain and ultimate compressive strain were larger than the NCAC, with a maximum difference of 43%. The crushing value of the solid waste coarse aggregate affected the splitting tensile strength, flexural strength, and modulus of elasticity of the concrete to a greater extent than the compressive strength. The transition zone at the concrete interface of the coarse aggregates with different solid wastes varied widely. The porous micro-pumping effect of the self-combusted gangue and self-combusted gangue vitrified reinforced the concrete interface transition zone, and the polished surface of sheet waste, uncombusted gangue, and recycled concrete aggregate surface adhesion weakened the interface transition zone; Finally, the uniaxial compressive stress–strain curve model for concrete with different solid waste coarse aggregates was established based on the Guo Zhenhai model.     

Evaluation and Multi-Objective Optimization of Lightweight Mortars Parameters at Elevated Temperature via Box–Behnken Optimization Approach

In this research, the mechanical properties of lightweight mortars containing different percentages of additional powder materials has been investigated using response surface methodology (RSM). Box–Behnken design, one of the RSM techniques, was used to study the effects of silica fume content (5, 10, and 15%), vermiculite/cement (V/C) ratio (4, 6, and 8), and temperature (300, 600, and 900 °C) on the ultrasonic pulse velocity (UPV), bending strength, and compressive strength of lightweight mortars. Design expert statistical software was accustomed to determining and evaluating the mix-design of materials in mortar mixtures and temperature effect on mortars. After preliminary experimental research of the relationships between independent and response variables, regression models were built. During the selection of the model parameters, F value, p-value, and R² values of the statistical models were taken into account by using the backward elimination technique. The results showed a high correlation between the variables and responses. Multi-objective optimization results showed that the critical temperatures for different levels of silica fume (5–10–15%) were obtained as 371.6 °C, 306.3 °C, and 436 °C, respectively, when the V/C ratio kept constant as 4. According to the results obtained at high desirability levels, it is found that the UPS values varied in the range of 2480–2737 m/s, flexural strength of 3.13–3.81 MPa, and compressive strength of 9.9–11.5 MPa at these critical temperatures. As a result of this research, RSM is highly recommended to evaluate mechanical properties where concrete includes some additional powder materials and was exposed to high temperature.     

Hydration–Strength–Workability–Durability of Binary,Ternary, and Quaternary Composite Pastes

At present, reducing carbon emissions is an urgent problem that needs to be solved in the cement industry. This study used three mineral admixtures materials: limestone powder (0–10%), metakaolin (0–15%), and fly ash (0–30%). Binary, ternary, and quaternary pastes were prepared, and the specimens’ workability, compressive strength, ultrasonic pulse speed, surface resistivity, and the heat of hydration were studied; X-ray diffraction and attenuated total reflection Fourier transform infrared tests were conducted. In addition, the influence of supplementary cementitious materials on the compressive strength and durability of the blended paste and the sustainable development of the quaternary-blended paste was analyzed. The experimental results are summarized as follows: (1) metakaolin can reduce the workability of cement paste; (2) the addition of alternative materials can promote cement hydration and help improve long-term compressive strength; (3) surface resistivity tests show that adding alternative materials can increase the value of surface resistivity; (4) the quaternary-blended paste can greatly reduce the accumulated heat of hydration; (5) increasing the amount of supplementary cementitious materials can effectively reduce carbon emissions compared with pure cement paste. In summary, the quaternary-blended paste has great advantages in terms of durability and sustainability and has good development prospects.     

Experimental Evaluation of the Concrete Damage and Pore Characteristics under Salt-Freezing Cycles

Herein, ordinary silicate concrete specimens are prepared to study the damage law of a cement-concrete material under the effects of salt erosion and a freeze–thaw environment. NaCl, NaHCO₃, and Na₂SO₄ solutions are separately produced, according to the characteristics of saline soil, to conduct an experimental study on the concrete characteristics during quick salt freezing cycles, and to analyse the changes in its compressive strength, mass loss, and dynamic elastic modulus (DEM) under freeze–thaw cycles. Low-field nuclear magnetic resonance (NMR) and scanning electronic microscopy are used to investigate the change in the microstructure of concrete specimens under salt freeze–thaw cycles (FTCs). The results show the loss in compressive strength, mass, DEM, and NMR spectrum signal increased by 1.5–3 times, 3–5 times, 1.5–2.5 times, and 2–4 times, respectively, for concrete specimens under 50–100 FTCs in 6.8% composite salt solution, in comparison to fresh water. Apparent spalling, decreases in the DEM, and reductions in the compressive strength occur in concrete when increasing the number of salt FTCs. The number of internal cracks in the concrete structure increase under the combined action of salt crystallization, moisture absorption, and freeze–thaw. The changes in the internal microscopic pore volume in concrete structures exhibit the same trend with changes in the macro mechanical properties of concrete. The correlation coefficients between the changes in each peak in the NUR spectrum and the changes in the compressive strength of concrete specimens under FTCs in freshwater or low-concentration salt solutions are both larger than 0.7, calculated using the grey correlation degree method. Therefore, these changes could be used as a potential evaluation index for salt frozen damage to concrete structures.     

Dynamic Behaviors of Mortar Reinforced with NiTi SMA Fibers under Impact Compressive Loading

In this study, a compressive impact test was conducted using the split Hopkinson pressure bar (SHPB) method to investigate SMA fiber-reinforced mortar’s impact behavior. A 1.5% fiber volume of crimped fibers and dog-bone-shaped fibers was used, and half of the specimens were heated to induce recovery stress. The results showed that the appearance of SMA fibers, recovery stress, and composite capacity can increase strain rate. For mechanical properties, the SMA fibers reduced dynamic compressive strength and increased the peak strain. The specific energy absorption of the reinforced specimens slightly increased due to the addition of SMA fibers and the recovery stress; however, the effect was not significant. The composite behavior between SMA fibers and the mortar matrix, however, significantly influenced the dynamic compressive properties. The higher composite capacity of the SMA fibers produced lower dynamic compressive strength, higher peak strain, and higher specific energy absorption. The composite behavior of the dog-bone-shaped fiber was less than that of the crimped fiber and was reduced due to heating, while that of the crimped fiber was not. The mechanical properties of the impacted specimen followed a linear function of strain rate ranging from 10 to 17 s⁻¹; at the higher strain rates of about 49–67 s⁻¹, the linear functions disappeared. The elastic modulus of the specimen was independent of the strain rate, but it was dependent on the correlation between the elastic moduli of the SMA fibers and the mortar matrix.     

Effect of Cementitious Materials on the Engineering Properties of Lightweight Aggregate Mortars Containing Recycled Water

With the trend toward taller and larger structures, the demand for high-strength and lightweight cement concrete has increased in the construction industry. Equipment for transporting ready-mixed concrete is frequently used to bring concrete to construction sites, and washing this equipment generates a large amount of recycled water, which is an industrial by-product. In this study, we recycled this water as the pre-wetting water for lightweight aggregate and as mixing water, and we substituted blast furnace slag powder (BS) and fly ash (FA) as cementitious materials (Cm). In addition, we evaluated the fluidity, compressive strength, tensile strength, drying shrinkage, and accelerated carbonation depth of lightweight ternary cementitious mortars (TCMs) containing artificial lightweight aggregate and recycled water. The 28-day compressive strengths of the lightweight TCM specimens with BS and FA were ~47.2–51.7 MPa, except for the specimen with 20% each of BS and FA (40.2 MPa), which was higher than that of the control specimen with 100% OPC (45.9 MPa). Meanwhile, the 28-day tensile strengths of the lightweight TCM specimens containing BS and FA were ~2.81–3.20 MPa, which are ~13.7–29.5% higher than those of the control specimen. In this study, the TCM specimen with 5% each of BS and FA performed the best in terms of the combination of compressive strength, tensile strength, and carbonation resistance.