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Concrete due to its special feature, is the most widely consumed material in the world, after water. But the production process of ordinary Portland cement as a main component of conventional concretes, has major disadvantages such as high amount of carbon dioxide emission and high energy consumption. Therefore, it seems necessary to find an alternative to ordinary Portland cement. In recent years, geopolymer has been introduced as a novel cementing agent and green alternative to the Portland cement which can eliminate the extensive negative of ordinary Portland cement production process. According to the needed engineering characteristics perspective in civil engineering, the geopolymer concretes have better chemical and mechanical properties than the ordinary ones such as high compressive, flexural and tensile strength, rapid hardening, resistance against high heat and firing, low penetration, resistance against salts and acids attacks and low creep. Compressive strength is considered as one of the important characteristics of concrete. In geopolymer concretes, according to the ingredients, several factors have been identified as important parameters affecting the compressive strength like: the type of aluminosilicate source, the molar composition of the oxides present in the aluminosilicate source, the curing regime, the water content, the weight ratio of alkaline activator solution to aluminosilicate source, alkaline activator solution parameters and etc. Hence, in this experimental research, several factors affecting the compressive strength of metakaolin-based geopolymer concrete including: the type of alkaline activator solution, the weight ratio of water to solid material participated in geopolymerization, sodium hydroxide concentration, the weight ratio of alkaline activator solution to aluminosilicate source and sodium silicate to sodium hydroxide weight ratio, were studied. In this regard, geopolymer concrete specimens were made and cured in 80 °C for 24 hours. After curing, specimens were placed in the ambient condition and compressive strength test, were performed. The obtained results indicated that using potassium hydroxide and potassium silicate as an alkaline activator solution, result in higher 28-day compressive strength of geopolymer concrete compare to sodium-based alkaline activator solution. On the other hand, using sodium hydroxide and sodium silicate as an alkaline activator solution, result in higher 3- and 7-day compressive strengths and also, faster hardening compare to potassium-based alkaline activator solution. Furthermore, increasing the weight ratio of water to solid material result in significant decreasing geopolymer concrete compressive strength. Also, 7-and 28-day compressive strength of geopolymer concrete is increases with increase in concentration of sodium hydroxide up to 14 M, but for 16 M, there is no remarkable changes in compressive strength. Besides, increasing sodium hydroxide concentration, causes faster hardening of geopolymer concrete. It is also absorbed that increasing the alkaline activator solution to metakaolin weight ratio result in decreasing geopolymer concrete compressive strength. Moreover, Increasing the weight ratio of sodium silicate to sodium hydroxide up to 1.5 (the optimum ratio), leads to achieve the highest 7-and 28-day compressive strengths of geopolymer concrete, but 7-and 28-day compressive strengths of geopolymer concrete is decreases noticeably, with further increase in weight ratio of sodium silicate to sodium hydroxide ratio up to 3. Compressive strength of geopolymer concrete is increases with increase in curing temperature up to 80 °C, but further increase up to 90 °C, result in decreasing geopolymer concrete compressive strength.

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Concrete due to its special feature, is the most widely consumed material in the world, after water. But the production process of ordinary Portland cement as a main component of conventional concrete, has major disadvantages such as high amount of CO₂ emission and high energy consumption. Therefore, it seems necessary to find an alternative to ordinary Portland cement. In recent years, geopolymer has been introduced as a novel green cementing agent and environment-friendly alternative to the Portland cement which can eliminate the extensive negative of ordinary Portland cement production process. According to the needed engineering characteristics perspective in civil engineering, the geopolymer concretes have better chemical and mechanical properties than the ordinary ones such as high compressive, flexural and tensile strength, rapid hardening, resistance against high heat and firing, low penetration, resistance against salts and acids attacks, and low creep. On the other hand, in terms of technical characteristics, concrete has some disadvantages, most notably low tensile strength and consequently low ductility. Therefore, the use of different fibers in the concrete mixture and the manufacture of fiber reinforced concrete is considered as an appropriate solution to eliminate these defects. Also, fiber reinforced geopolymer concrete is known as a novel type of concretes with higher ductility than ordinary concretes. In this experimental study, two types of polymer fibers, including simple polypropylene fibers and 4-element polyolefin hybrid fibers, were used to manufacture fiber reinforced geopolymer concrete specimens. In this regard, fiber reinforced and non-fiber geopolymer concrete specimens were made and cured in 80 °C for 24 hours. After curing, specimens were placed in the ambient condition and associated tests including: density, 3-days water absorption, 7-and 28-days compressive, Brazilian indirect tensile and three point flexural strengths, were performed to study effect of fibers on metakaolin-based geopolymer concrete mechanical properties. Also, to study effect of fibers on high-temperature resistance of metakaolin-based geopolymer concrete, specimens weight and compressive strength loss percentage after exposure to high temperatures up to 800 °C, were measured. The obtained results indicated that using fibers in geopolymer concrete mixture, result in increasing compressive, indirect tensile and flexural strengths and also decreasing in density and 3-days water absorption. Further, the use of hybrid fibers due to their ability to inhibit the cracking process from both micro and macro levels, significantly improved compressive, indirect tensile and flexural strengths compared to simple fibers. The optimum amount of 4-element polyolefin fibers for compressive, tensile and flexural strength improvement was measured 0.2%, 0.2% and 0.15% (by volume), respectively. Also, the optimum amount of polypropylene fibers for compressive, tensile and flexural strengths improvement was measured 0.2% (by volume). In term of high-temperature resistance, although the polymer fibers reduced the risk of the explosive sapling of geopolymer concrete specimens due to generation micro channels which were randomly distributed in concrete because of melting of fibers, resulting in less weight loss than non-fiber specimen, but on the other hand, the compressive strength loss of polymer fiber reinforced specimens were higher than non-fiber one. Overall, it can be concluded that these fibers did not have a significant effect on the high-temperature resistance of geopolymer concrete.
 

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Nowadays, protecting various structures including commercial, medical, industrial, and residential infrastructures against fire is a very complex issue. High heat causes microstructural changes and decreases compressive strength of the concrete containing conventional portland cement, but geopolymers as the third generation of cement due to amorphous structure and aluminosilicate 3D networks lead to more stable behavior under high heat conditions considering the conventional concrete. Calcium silicate hydrate (C-S-H) and calcium aluminosilicate hydrate (C-A-S-H) nanostructures are products of the hydration and geopolymerization processes that play an important role in increasing the strength of conventional and geopolymeric concrete, But heat, either in transient or steady state, changes the mechanical properties and microstructure of the concrete. Hence for a deeper understanding of the behavior of C-S-H and C-A-S-H nanostructures affected by high temperatures, geopolymer concrete has been compared with conventional concrete. In this regard, about 300 samples were cured in the humidity bath for 1, 3, 7, 14, and 28 days. All samples were then put in of 25, 50, 100, 200, 300, 500, 700, and 900°C temperatures for 2 hours. Length and weight change percentages, compressive strength, and ultrasonic and cracking behavior tests were performed on all samples. Images from the scanning electron microscope (SEM) and the energy-dispersive X-ray (EDX) analysis were also used to evaluate the microstructural behavior of samples in various temperatures. According to the results, the strength of both types of concrete decreases with increasing temperature. By increasing the temperature to more than 700 °C, the geopolymer concrete structure has transformed to a porous and semi-stable ceramic structure. This change in the ceramic structure has made a difference in the high heat compressive strength of geopolymer concrete vs. conventional concrete. The compressive strength of 28-day aged geopolymer concrete and conventional concrete samples at 900 °C was 7.35 and 4.31 MPa, respectively.

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Concrete is the most widely used building material in construction industry worldwide and its constituents are easily accessible everywhere. However, cement industry, as the producer of the primary binder of concrete, is one of the effective sources of environment degradation. Cement production needs extraction of mineral resources and burning fuel and causes extensive greenhouse emission due to disintegration of raw materials. Cement production alone is responsible for 7% of global CO₂ emission with estimated annual growth of 4%. Toward environmental sustainability, one way is partially or totally replacing cement by waste or byproducts of other industries such as fly ash, ground granulated blast-furnace slag (GGBS), waste water, metakaolin, and silica fume. Geopolymer is a cementitious material with comparable characteristics to those of ordinary cement produced by alumina- and silica-rich waste materials. Therefore, it does not require energy-intensive and pollutive calcination process. Geopolymerization is formed by reaction of silica-alumina under an alkaline solution which creates three dimensional Si-O-Al-O polymeric chains to attain compressive strength, compared to the ordinary cement which develops calcium silicate hydrates (C-S-H) as the main adhesive. Extensive research has conducted on geopolymer concrete. However, more investigations are needed to better understand characteristics of geopolymer concrete containing additives. Fibers are proved to have a positive effect on mechanical strength of concrete. As well, fillers such as microsilica can improve mechanical and durability of concrete. Moreover, most studies in this area are focused on fly ash-based geopolymers and the investigations on GGBS-based geopolymer are rare in the literature. In this study, mechanical and durability of GGBS-based geopolymer concrete containing CFRP fibers and microsilica is investigated. Different concrete samples with 0-3% CFRP fibers and 0-10% microsilica are prepared and experimentally tested. Sodium Hydroxide (NH) and Sodium Silicate (NS) solutions are used as alkali activators. 8 M NH as well as NS with 14.7 Na₂O and 29.4 SiO₂are used with the NS/NH ratio of 2.5. Since no standard exists for mix design of geopolymer concrete, proposed mix design by Venkatesan and Pazhani (2015) is used. Alkaline to binder ratio of 0.4 is selected with 430 kg/m³binder.The specimens were tested after 28 days of curing. Next, mechanical and durability tests including compressive strength, tensile strength, ultrasonic pulse velocity, water absorption, RCPT, and acid resistance are conducted on the samples. Also, microstructure of the geopolymer concrete is investigated. Results of experimental tests show that, compressive and tensile strength of geopolymer samples decrease by adding microsilica. However, 5% microsilica is the best value to enhance mechanical properties of geopolymer concrete. On the other hand, microsilica can enhance durability properties of geopolymer concrete so that adding 5% microsilica causes moderate improvement of water absorption and chloride penetration. The greatest impact of microsilica is on acid resistance by which adding 5% microsilica resulted in 67% improvement of compressive strength loss. However, unlike the microsilica, CFRP fibers have detrimental effect on mechanical properties and durability of the geopolymer concrete since adding fibers can yield disruption of concrete integrity .On the other point of view, microstructure study shows that all the specimens exhibit micro cracks that can inversely affect the performance of concrete. Also, SEM images show that there is not a strong bond between CFRP fibers and binder paste which yields lower performance of concrete specimens containing fiber.

 

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Today, in order to reduce the harmful effects of the environment and increase the mechanical properties and durability of concrete, particles with high pozzolanic properties are used as a suitable alternative to ordinary cement in concrete. And filler, as an alternative to cement, has attracted the attention of researchers. In this laboratory study to investigate the effects of slag and nanosilica slag consumption on the microstructure of geopolymer concrete and compare it with the characteristics of control concrete containing Portland cement, 1 mixing design of control concrete and 3 mixing designs of geopolymer concrete containing 92, 96 and 100% composite kiln slag was fabricated with 0, 4 and 8% nanosilica, respectively. X-ray fluorescence (XRF) was performed. In order to investigate the effect of microstructural changes on the macro structure of concrete, compressive strength and tensile strength tests were performed on concrete samples at 90 days of age. Examination of the images obtained from the SEM test shows the superiority of the microstructure of the geopolymer cement matrix in all designs, compared to the microstructure of the control concrete containing Portland cement. Celsius), the effects of improvement and cohesion in the microstructure of geopolymer concrete are evident due to the presence of silica nanoparticles, in this regard, the presence of 8% nanosilica in mixture 4 (geopolymer concrete), accelerates the reactivity process and increases the volume of hydrated gels Geopolymerization was compared to other geopolymer concrete mixtures (containing 0 and 4% nanosilica). Images of concrete samples heated to 500 ° C show signs of weakening of the concrete microstructure compared to images taken of concrete at room temperature. The results of XRF test indicate the presence of the highest amount of oxidilica and aluminum oxide (the main factors in improving the density in the microstructure of concrete), in the combination of designs 4 and 2 by 36 and 8%, respectively. The high peaks created in the XRD spectrum diagram often occur in areas with angles (θ2) of 28 °, and their height varies according to the presence of aluminosilicate particles in the concrete mix. The application of high heat to the concrete specimens caused a decrease in the results of the XRD test. Evaluations performed on the results of the test to determine the compressive strength and tensile strength in concrete, showed coordination and overlap with the results of microstructural tests in this study.

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Considering the high consumption of concrete, especially in structures, and the increasing need for cement production, it seems necessary to pay attention to the harmful environmental effects of this material. In the construction industry, to solve this problem, alternative adhesives are used in concrete, and geopolymers are one of these alternatives. Making geopolymer concretes based on slag can be one of the ways to produce environmentally friendly materials that reduce the harmful effects of cement production. Also, using lightweight concrete has valuable advantages, such as reducing the structure's dead load, and combining geopolymer with lightweight concrete can be beneficial. In this research, two series of lightweight geopolymer concrete have been used. In the first series, by keeping geopolymer ratios constant (Al/Bi=0.65, SS/SH=1 and sodium hydroxide concentration 2 M), Geopolymer concretes with different percentages (50, 60, 70, 80, 90, and 100) of scoria were made instead of coarse aggregate. Then, the designs with structural conditions (compressive strength above 17 MPa and specific weight below 2000 Kg/m3) were selected by comparing the samples' specific weight and compressive strength. In the second series of making lightweight geopolymer concretes by adding steel fibers (0.5, 1, and 1.5%), polypropylene fibers (0.1%), and hybrid, the mechanical characteristics of the samples were evaluated. By examining the compressive strength test, as expected, the compressive strength of the light geopolymer samples increases with the increase in the percentage of steel fibers. Also, samples containing 0.1% of polypropylene fibers face decreased compressive strength. When combined with steel fibers with percentages (0.5, 1, and 1.5), this decrease in compressive strength will be increased. By checking the compressive strength and specific weight, the samples containing 70, 80, and 90% scoria has structural conditions.  By examining the tensile strength test, it can be concluded that adding steel and polypropylene fibers both increase the tensile strength can be seen. In the flexural strength test, flexural strength increases with an increase in the percentage of steel fibers.  It can also be seen that the effect of steel fibers is more significant than polypropylene fibers in increasing the bending strength. An ultrasonic pulse speed test determines the quality of manufactured concrete. According to the observed results, it can be concluded that with the increase in the percentage of fibers (steel and polypropylene), the speed of the ultrasonic pulse also increases. The samples of 70% scoria-containing steel fibers are of excellent quality; this indicates that this design has a better quality geopolymer paste than other samples. Also, the geopolymer concrete sample containing 90% scoria has the lowest value of ultrasonic pulse speed. This reduction can be attributed to many voids in the scoria aggregate. The modulus of elasticity test results showed that the samples containing steel fibers were far more than the other samples, and the most significant increase was seen in steel fibers with a percentage of 1.5. Finally, by examining the microstructure of fiber geopolymer lightweight concrete using scanning electron microscope (SEM) images, it can be seen that the geopolymer sample without fibers contains voids, which are filled to a large extent when steel and polypropylene fibers are added.




 

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The cement-based stabilization/solidification (S/S) method is widely used in modifying soils polluted by heavy metals (HMs), although it may face technical, economic, and environmental limitations. Therefore, the present work was designed to investigate the effectiveness of a type of geopolymer based on the steel slag (SGP) and its combination with microparticles of zeolite (SGPZ), compared to cement (as a traditional additive), in enhancing the stability of S/S products. In so doing, different percentages (0 to 250 mg/g-soil) of SGP, SGPZ, and sole cement were separately added to the S/S samples containing different concentrations of lead (including 5000, 10000, 20000 and 40000 mg/kg-soil). After adequate curing (up to 28 days), a set of macro and micro scale experiments were performed to assess the long-term performance of the amended soil samples using a laboratory accelerated aging procedure that simulated 25, 50, 75 and 100 years of exposure to the acid rain and wet and dry (W-D) cycles in the field. It was found that, while low amounts of cement (PC) would greatly reduce the initial bioavailability of pollution in the pore fluid of soil, increasing the contact time of the PC-treated specimens upon harsh conditions, especially in the presence of high level of Pb, would dramatically diminish the efficiency of the precipitation mechanism as well as the degree of encapsulation process which play a significant role in increasing the ability of S/S sample to release the toxic ions stabilized/solidified previously. At simulated 100 years, the toxicity characteristic leaching procedure leached Pb from the PC-treated sample with 250 mg/g-soil binder would exceed the permitted threshold of pollution leaching (≥ 5 mg/L) by 508%, indicating that meeting the S/S regulation limits requires a large consumption of cement. The study showed that, unlike treatment conditions with the same level of PC, the use of novel cement-free S/S binders (especially SGPZ) would significantly limit the negative influences of the environmental changes on HM remobilization risks. In addition, the mechanical characteristics of those series of samples were sometimes up to 1.4 times higher than that of the soil modified with cement alone. Based on the X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) images, this enhancement can be mainly due to i) reduction in the adverse HM-binder interactions, ii) intensification in the level of hydration reactions, iii) formation of secondary complex hydrated phases (e.g., Hydrotalcite: Mg₆Al₂CO₃(OH)₁₆.4H₂O), and iv) creation of a three-dimensional network of solidification in the system containing geopolymer, wrapping the matrix of S/S products against the structure disintegration upon contact to the aggressive environments. Therefore, under the destroying impacts of acid attack and W-D scenario, adding 25% SGPZ composite could pass the S/S regulation limits. In general, based on the obtained results, the use of geopolymer (especially containing zeolite) is suggested as an effective and environmentally friendly alternative for sustainable soil improvement, even with the high contents of HM ions. Following the USEPA and UKAE standards to achieve the safe S/S performance, the optimal dosage of GP binder was determined to be approximately 6 mg/g-soil per 1 g/kg of lead in the sample.        

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Prior research has not explored the creation of concrete with superior thermal insulation properties using calcium oxide-activated materials. Furthermore, the impact of substituting large proportions of sand with worn rubber powder and PET on the mechanical and thermal characteristics of high-performance geopolymer concrete remains uninvestigated. This study addresses these gaps by examining the development of geopolymeric concrete with enhanced thermal insulation properties using calcium oxide-activated materials. A novel mixing method has been devised to improve the compaction of thermally insulating concrete, which includes calcium oxide-activated slag. For the purposes of this research, worn rubber powder and PET powder have replaced 10%, 20%, 30%, 40%, and 50% of the aggregates. The mechanical properties of the concrete were determined through compressive strength, four-point bending, and tensile strength tests. Lastly, the thermal conductivity coefficient was tested to ascertain the thermal properties of the developed concrete.
The findings revealed that in the developed concrete, substituting 10% of the aggregates with worn rubber powder or PET powder increased the energy absorption capacity of the concrete by 143% and 107%, respectively, while its mechanical properties decreased by 10% and 7%, respectively. Moreover, using 50% worn rubber powder and PET as aggregate substitutes reduced the samples’ thermal conductivity by 70% and 60%, respectively.