Metakaolin-based Geopolymer Mortars Research Articles

Behavior of Metakaolin-Based geopolymer concrete at ambient and elevated temperatures

Geopolymer or alkali-activated binders are emerging as a potential green sustainable alternative for ordinary Portland cement (OPC). In addition to the environmental benefits of geopolymers, it possesses excellent mechanical properties, including good resistance to elevated temperatures. Although many studies were conducted on the thermal performance of metakaolin-based geopolymer mortar and pastes, the performance of metakaolin-based geopolymer concrete under elevated temperature has been rarely studied. Therefore, the present study investigates the behavior of MK-based geopolymer concrete under ambient and elevated temperatures of 200 °C, 400 °C, and 600 °C. The experimental program of this study comprised five mixes with different formulations to primarily investigate the influence of varying Na2O/Al2O3 and SiO2/Al2O3 molar ratios. The results are discussed in terms of the visual observations for the fresh and hardened specimens, compressive strength development, stress–strain behavior, splitting tensile strength, water absorption, and weight loss percentage due to exposure to elevated temperatures. The residual compressive strength varied from 56% to 63%, 38% to 51% and 28% to 34% after exposure to 200 °C, 400 °C, and 600 °C, respectively. The rate of degradation in the modulus of elasticity with the increase in the exposure temperature is much higher than the compressive strength. Models are proposed for predicting the residual compressive strength and modulus of elasticity of geopolymer concrete after exposure to elevated temperature, which show good correlations with experiments. The SEM-EDS analysis confirms the formation of dense homogeneous geopolymerization gel and corroborate the trends of strength degradation after exposure to elevated temperature.

The effect of polyvinyl fibers on metakaolin-based geopolymer mortars with different aggregate filling

Environmental pollution and high energy loss during cement production are important factors threatening concrete production. In order to prevent these problems, geopolymer binders have been accepted as an alternative to cement in recent years. In this study, recycled concrete aggregate, basalt sand, Rilem sand, blast furnace slag, and polyvinyl alcohol additive metakaolin based geopolymer mortars with 12 different mixtures were produced. Sodium hydroxide and sodium silicate were used for the activation of metakaolin. Mechanical properties of the produced specimens at 7, 28, and 56-days were examined. Also, ultrasonic pulse velocity, high temperature, density, and freeze–thaw tests of geopolymers were carried out. In addition, SEM, XRD and FT-IR analyzes were performed to examine the internal structures of geopolymer composites. According to the results obtained, polyvinyl alcohol fibers improved all properties of the specimens. In general, the best mechanical results were obtained for geopolymers containing 1.2% PVA. It has been observed that samples exposed to 800 °C maintained their stability. The 180 cycle freeze–thaw effect did not create any negative effects on the geopolymers. From the SEM analysis results, it has been confirmed that BS filled geopolymers have a more stable and dense structure than others. Satisfactory results were obtained for recycled concrete aggregate filled geopolymers. Intense quartz peaks were observed in the crystal structure XRD results of the samples exposed to a high temperature of 800 °C. In the FT-IR results, Si–O–H bonds and other expected peaks were observed and the precipitated product was determined as CaCO3 and calcium hydroxide (Ca(OH)2).

Effect of basalt fiber on metakaolin-based geopolymer mortars containing rilem, basalt and recycled waste concrete aggregates

With the development of technology, it is important to recommend new environmentally friendly materials instead of cement, which is the main binder of concrete. Recently, geopolymer mortars have been the focus of many researchers to achieve this goal. In this study, the effects of three different basalt fiber (BF) ratios on the metakaolin (MK)-based geopolymer mortars produced with three different aggregate types were investigated. By adding 0.0, 0.4, 0.8, and 1.2% BF for each group aggregate, a total of 12 different geopolymer mixtures were obtained. In the mixtures, metakaolin activated with sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) was used as the main binder. Rilem sand (RS), basalt Sand (BS), and recycled waste concrete (RWC) were used separately as filling materials. Mechanical strengths of 7, 28, and 56-day specimens obtained from all mixtures were tested. Ultrasonic pulse velocity (UPV), high-temperature, abrasion, freeze–thaw, and porosity tests were performed for the mortars produced. According to the results, positive effects were observed on all properties due to the more systematic performance of geopolymer samples with the addition of basalt fiber. According to the results, basalt fiber had positive effects, especially when 0.8–1.2% was used. When the results were examined, it was able to create an increase of up to 25% in the compressive strength results and 50% in the flexural strength results. Also, although the properties of geopolymers produced with recycled aggregate were inferior to the others, the results obtained were quite acceptable values for the application. With the basalt sand effect, higher results were obtained by obtaining a harder and more dense structure than others.

Bacterial crack healing in metakaolin based geopolymer mortars

Geopolymer mortars have emerged as an environmentally friendly binding alternative to conventional cement. However, the gaps in their structure and early cracks reduce their service life. There are various bacterial healing studies in the literature that have been conducted on cemented mortar cracks. Nevertheless, the present study focuses on the healing of metakaolin based geopolymer mortar cracks.For this purpose, mortars with different molarities were prepared with two different endospore forming methods that did not require encapsulation or mixed chemical processes for the bacterial crack repairs on the geopolymer mortars. These mortars were then left in different healing environments with crack damage. The samples were tested using physical, optical and internal structure analyses. As a result, the cracks were improved by reducing the amount of gaps in the samples. The results showed that it is possible to bacterially repair early cracks in new generation ceramic materials with alkali-activated mortars and reduce the amount of gaps.

Mechanical Properties and Microstructure of a Metakaolin-Based Inorganic Polymer Mortar Reinforced with Quartz Sand

The synthesis, mechanical behaviour, and microstructure of metakaolin-based geopolymer mortar reinforced with quartz sand are presented in this investigation. Fine sand (quartz sand) aggregate were added in different proportions of 50, 60, 70 or 80 wt.% to prepare the fresh metakaolin-based geopolymer paste. The geopolymer mortar was achieved by a mixture of geopolymer paste and river sand in the ratio 2:5 by mass. The mixture of sodium hydroxide solution (10 M) and sodium silicate solution (Na2SiO3) in a volume ratio of 2:3 was used as an alkaline activator. All the specimens were cured at room temperature and tested after 28 days of curing. The investigations on microstructure and physical properties indicated a significant reduction of the open pores and interconnection of micro and meso cracks in the structure network with increase in the amount of quartz sand. As a consequence, the samples show a good mechanical strength principally with the addition of 60 wt.% of quartz sand to binder with values of 8 and 66 MPa for flexural strength and compressive strength, respectively.

Compositional Dependence of Pore Structure, Strengthand Freezing-Thawing Resistance of Metakaolin-Based Geopolymers.

The understanding of the composition dependent properties and freezing-thawing (F-T) resistance of geopolymer materials is vital to their applications in cold regions. In this study, metakaolin-based geopolymer (MKG) mortars were fabricated by controlling the Si/Al ratio and the Na/Al ratio. The pore structure and strength were measured by mercury intrusion porosimetry and compression tests, respectively, which both showed obvious correlations with the material composition. Mass loss, strength loss, visual rate, and microscopic observation were adopted to assess the changes of the material properties and microstructure caused by F-T loads. The results showed that the strength-porosity relationship roughly followed a linear plot. Increases of the Si/Al ratio increased the capillary pore volume, but decreased the gel pore volume and the F-T resistance. Increases of the Na/Al ratio decreased the gel pore, but roughly enhanced the F-T resistance. The MKG mortar at the Na/Al ratio of 1.26 showed the lowest total pore volume and the best F-T resistance. The mechanisms of our experimental observations were that the abundantly distributed air voids connected by the capillary pores facilitated the relaxation of hydraulic pressures induced by the freezing of the pore liquid. The findings of this work help better clarify the compositional dependence of the pore structure, strength, and freezing-thawing resistance of MKG materials and provide fundamental bases for their engineering applications in cold regions.

Chemical–physical–mechanical stability of MKG mortars under sulfate attacks

Geopolymer-based materials have attracted increasing interest in construction and infrastructure engineering due to their high mechanical properties but low carbon dioxide emissions. However, it remains unclear whether this type of material can work in a stable way in harsh environments. This study investigated the chemical–physical–mechanical stability of metakaolin-based geopolymer (MKG) mortars under the immersing actions of sodium sulfate and magnesium sulfate solutions up to 180 days by testing their visual appearance, mass, dimensions, strength, microstructure and chemical components. Results show that the immersion actions do not cause obvious changes in mass, dimensions or compressive and flexural strengths of the MKG mortars. A white deposit, consisting of magnesium hydroxide, magnesium sulfate, magnesium carbonate and magnesium silicate aluminate hydrate, appeared on the surfaces of the high-alkali MKG specimens immersed in magnesium sulfate solution for 30 days, and disappeared at later ages due to the continual consumption of magnesium hydroxide. Mg–Na exchanges occurred for the MKG mortars immersed in magnesium sulfate solution. Overall, the chemical–physical–mechanical stability of geopolymer-based materials depends on the material components and interactions with the environment. Caution should be used before any engineering applications, and relevant issues deserve further rigorous investigation in the future.

Evaluation of the 12–24 mm basalt fibers and boron waste on reinforced metakaolin-based geopolymer

Concerning sustainability and recycling considerations, geopolymers have recently raised as one of the most active alternatives to the current cement-based composites, in addition to that, the existence of fibers in any binding matrix yields a significant improvement in the behavior of the matrix. On the other hand, using waste materials in the binding matrices fulfills one of the main aims of sustainability, namely reusing wastes. In this concern, the previous research attempts focused on the effect of using fibers and wastes separately and hence using them together in one matrix is not clearly highlighted. The main objective of this study was to examine the engineering properties of metakaolin-based geopolymer mortars in the case of colemanite substitution up to 30% and basalt fiber of different lengths. In the 10 series produced, firstly 7th day and 28th day compressive and flexural strengths, UPV, abrasion resistance test, porosity, unit weight, and water absorption results were examined. As the durability tests, 90 cycles of freezing-thawing between −20 and +20 °C, high-temperature tests of 250, 500 and 750 °C were applied. Also, geopolymer samples were exposed to 10% Hydrosulfuric Acid (H2SO4) for 3 months. Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR) and Thermogravimetric/Differential Thermal Analysis (TGA-DTA) analyses were performed at the end of the durability tests. The results showed that in the case of 10% colemanite substitution, it increased the compressive strength results and lowered it at higher rates. With a colemanite substitution of 10%, the compressive strength results of 28 days increased by 1.71%, and in the case of 20% and 30% colemanite substitution, there was a decrease in the compressive strength of 13.64% and 26.99%, respectively. The compressive strength results showed that 24 mm long basalt fiber reinforced samples had better results than 12 mm long basalt fiber reinforced samples. It was seen that geopolymer specimens maintain stability in freezing-thawing, elevated temperatures, and sulfuric acid effects.

Optimization approach of granulated blast furnace slag and metakaolin based geopolymer mortars

This paper aims to find the optimum formulation among 24 geopolymer mortars (GMs) based on granulated blast furnace slag (GBFS), metakaolin (MK) and alkaline solution which is characterized by its molar ratio SiO2/Na2O (MR) and its Activator representing the dry extract. (GBFS + MK)/Activator, GBFS/MK, and MR are the variables used to evaluate the GMs performances. Their effect on the rheology, setting time, porosity, mechanical properties and stability to efflorescence was investigated and the results were compared with those of Portland cement mortar (PCM). The outcomes show that GMs and PCM exhibit a Binghamian behavior where the rheological parameters depend on the MK/GBFS ratio and the MR. The relationship between compressive and flexural strengths is independent of the raw materials while it is not the case for the relationship relating dynamic modulus with the compressive strength. The optimal performances are achieved with: GBFS/MK = 1, GBFS + MK/Activator = 3 and MR between 1.6 and 1.8. The optimum GMs have shown better or comparable properties than those of PCM.

High-temperature behavior and mechanical characteristics of boron waste additive metakaolin based geopolymer composites reinforced with synthetic fibers

This paper aims to examine high-temperature behavior and some mechanical and microstructural characteristics of boron waste additive metakaolin based geopolymer mortar composites reinforced with four various fiber types. Synthetic fibers (polyolefin, basalt, modified polyamide and polyvinyl alcohol) were used to evaluate their addition effect on the geopolymer composites behavior. Some experimental tests were performed such as compressive strength, flexural strength, toughness, abrasion resistance and high-temperature exposure to determine the influence of the fibers to be added on the properties of metakaolin based geopolymer mortars. Geopolymer mixtures were prepared by a combination of boron waste and metakaolin activated by sodium hydroxide and sodium silicate solutions and then heat cure was done. Experimental test results indicated that the addition of various types of fibers improved the strength characteristics of the geopolymer composites, for instance, PVA fiber samples yielded a flexural toughness improvement of 415.58% with respect to control sample. Concerning abrasion resistance, all of the manufactured samples showed a length change of less than 2 mm and a weight loss of less than 15 g. The use of synthetic fibers positively affected high-temperature behavior of metakaolin based geopolymer composites in terms of strength loss, weight loss and UPV loss. The microstructural analysis revealed an acceptable degree of bonding between synthetic fibers and geopolymeric matrix.

Metakaolin-based geopolymer mortars with different alkaline activators (Na+ and K+)

In this study, the influence of different alkaline activators (Na+ and K+) on the mechanical, thermal, and microstructural behaviors of metakaolin-based geopolymer mortars was investigated. Four types of mortars were manufactured, and the only difference between them was the alkaline-activating solution. Potassium and alkaline sodium silicates and potassium and sodium hydroxides were used in combination as the alkaline-activating solutions. Compressive strengths, indirect tensile strengths, and longitudinal moduli of elasticity of the mortars were evaluated along with thermal property analysis (thermogravimetric analysis and differential thermal analysis) and scanning electron microscopy (SEM). All the mortars exhibited similar compressive strength, >80 MPa, at 28 days. It was possible to observe a high gain in the mechanical strength in the early ages, which is characteristic of geopolymers. The increase in mechanical strength ranged approximately from 72% to 81% after 3 days and from 86% to 93% after 7 days with respect to the strength at 28 days. Mortars produced with alkaline sodium silicate exhibited a heterogeneous microstructure with a large number of pores, while those containing potassium silicate were denser and exhibited low porosity. Such different morphologies of the mortars are a result of the type of the alkaline cations used; their effect could also be perceived in the thermal performance of the mortars, with better performance induced by potassium silicate and potassium hydroxide.

Effect of using colemanite waste and silica fume as partial replacement on the performance of metakaolin-based geopolymer mortars

In this work, metakaolin was partially replaced with two waste materials and the effect of adding those materials on the behavior of the resulted geopolymer composites was investigated regarding physical properties. A comparison was then made between non-fibrous samples and polypropylene fiber reinforced ones in terms of ultrasonic pulse velocity, abrasion resistance, flexural and compressive strength tests together with scanning electron microscopy (SEM) and XRD analyses. In general, results revealed the fact that replacement materials proved to be beneficial. When compared to control sample, colemanite and silica fume samples yielded an improvement of 14.61% and 29.44% in flexural strength, 2.02% and 11.48 in compressive strength, 10.59% and 20% in abrasion resistance. Addition of polypropylene fibers generally helped in improving flexural strength and abrasion resistance of the samples. Microstructural analysis showed a good degree of geopolymerization of the resulted matrix. Colemanite represents a potential usable material in the geopolymer technology especially in Turkey which possesses the world’s largest boron reserves.

Effect of Using Colemanite and Basalt Fiber on the Mechanical Properties of Metakaolin-Based Geopolymer Mortars

In the current studies which focus on the impact of CO2 emissions that released from the cement industry, showed the importance of finding alternative binders other than cement like geopolymer. This paper presented metakaolin based geopolymer which was manufactured by partially replacing metakaolin with colemanite with different percentages (10%, 20%, and 30%), to investigate the effect of colemanite on the mechanical properties of geopolymer. Then a comparison was carried out between non-fibrous specimens and basalt fiber reinforced ones to evaluate the improvement in terms of flexural and compressive strength of metakaolin-based geopolymer. The results showed that replacing metakaolin with 10% colemanite and adding 1% basalt fiber achieved 28.17% improvement in flexural strength at age 7 days compared to the reference sample.

Alkali–silica reaction in metakaolin-based geopolymer mortar

This paper assesses the behaviour of metakaolin-based geopolymers in the presence of alkali-reactive aggregates. Geopolymer mortars were made with flash metakaolin and sodium waterglass solution in presence of six different aggregates, each having different levels of reactivity to alkali–silica reaction (ASR). The behaviour of geopolymer in presence of such reactive aggregates was investigated by monitoring the dimension changes and the dynamic modulus of the specimens placed in accelerated conditions at 60 °C and 95 % RH for up to 250 days. SEM and EDX analyses were also performed at 170 days to visualize the newly formed products. A comparison with an ordinary Portland cement (OPC) was made for all aggregates in order to evaluate how strongly geopolymers were affected by ASR, if at all. Results showed that geopolymers, although they contained high concentrations of alkalis, were more able to resist ASR than OPC was, and no characteristic swelling or any significant loss of rigidity was observed for the geopolymer specimens. An exception was nevertheless noted for the mortar made with crushed glass sand, where a significant drop in rigidity was found, with the presence of a gel layer at the surface of the glass grains but without any swelling.

Effect of Parawood Ash on Drying Shrinkage, Compressive Strength and Microstructural Characterization of Metakaolin-Based Geopolymer Mortar

Drying shrinkage, compressive strength and microstructural analysis of metakaolin based geopolymers partial replacement with Parawood ash was investigated. It was involved different SiO2/Al2O3 and CaO/SiO2 ratios. Characterization of geopolymer mortar was determined on drying shrinkage, compressive strength, mineral phases and microstructure was analysed by X-ray diffraction and scanning electron microscopy techniques. Test result of highest compressive strength was about 71 MPa at 6-h (4-h in oven at 80oC and 2-h ambient temperature). Voids-cement ratio is the most effect on the unconfined compressive strength of this metakaolin geopolymer mortar.

Preparation and Characterization of New Geopolymer-Epoxy Resin Hybrid Mortars.

The preparation and characterization of metakaolin-based geopolymer mortars containing an organic epoxy resin are presented here for the first time. The specimens have been prepared by means of an innovative in situ co-reticulation process, in mild conditions, of commercial epoxy based organic resins and geopolymeric slurry. In this way, geopolymer based hybrid mortars characterized by a different content of normalized sand (up to 66% in weight) and by a homogeneous dispersion of the organic resin have been obtained. Once hardened, these new materials show improved compressive strength and toughness in respect to both the neat geopolymer and the hybrid pastes since the organic polymer provides a more cohesive microstructure, with a reduced amount of microcracks. The microstructural characterization allows to point out the presence of an Interfacial Transition Zone similar to that observed in cement based mortars and concretes. A correlation between microstructural features and mechanical properties has been studied too.