Alkali-activated Matrices Research Articles

A multi-performance comparison between lime, cementitious and alkali-activated TRM systems: Mechanical, environmental and energy perspectives

The combined need to propose new solutions for the structural reinforcement of existing buildings and for the reduction of CO2 emissions is leading to the development of more sustainable composite materials, such as those based on alkali-activated mortars (AAM). In this study, different formulations of AAM, based on metakaolin or fly ash, have been evaluated as possible matrices for Textile Reinforced Mortar (TRM) systems. Two different bidirectional textiles, made of AR glass or basalt fibers, were used as internal reinforcement. The physical-mechanical properties of TRM systems based on AAM were evaluated and compared with those of commercial systems with cementitious or lime-based matrices. Direct tensile tests on TRM coupons and shear bond tests on clay brick substrates were carried out. Then, their energy and environmental-related performance have been compared. Results showed that alkali-activated matrices can be very promising and eco-friendly alternative solutions to traditional mortars in TRM systems.

Influence of Activation Parameters on the Mechanical and Microstructure Properties of an Alkali-Activated BOF Steel Slag

Steel slag (SS) is a secondary material from steelmaking production with little commercial value. Its volumetric expansion and low reactivity limit the use of SS in Portland cement (PC)-based materials. This study investigated the potential use of basic oxygen furnace (BOF) slag as a single precursor in alkali-activated matrices (AAMs). Six AAM pastes were assessed by changing the silica modulus (0.75, 1.50 and 2.22) and the sodium concentration (4% or 6% Na2O—wt. SS). The early hydration was assessed using isothermal calorimetry (IC), followed by the assessment of the mechanical performance (compressive strength), apparent porosity, and structure and microstructure characterization (X-ray diffraction, thermogravimetric analysis and scanning electron microscopy). The results indicated that although the BOF slag may be considered a low-reactivity material, the alkaline environment effectively dissolved important crystalline phases to produce hydrates (reaction products). An optimized combination of activator sources was achieved with 4% Na2O and a silica modulus of 1.50–2.22, with a compressive strength up to 20 MPa, a significant amount of reaction products (C-S-H/C-A-S-H gels), and low initial and cumulative heat release. Those properties will help to promote SS recycling use in future engineering projects that do not require high-strength materials.

The Development of Composites Based on Alkali-Activated Matrices Resistant to Extreme Temperatures

The article presents the results of research focused on the behavior of composites with a matrix based on alkali-activated materials when exposed to extreme temperatures (up to 1200°C). The behavior of the material based on alkali-activated fly ash, blast furnace slag, metakaolin, their mutual combinations respectively was analyzed in detail. The effect of thermal exposure on the properties of the developed materials was assessed by a complex of physical, mechanical and chemical methods. A specific area of research was the examination of the rheological properties of the developed mixtures in the fresh state.

Advanced Composites with Alkali-Activated Matrices for Strengthening of Concrete Structures: Review Study

Old and seismically prone buildings are in need of strengthening in order to comply with the latest building codes and to prolong their service life. For over two decades fiber-reinforced polymers (FRP) have been successfully used for this purpose. However, the poor performance in high temperatures of organic matrices has led researchers to investigate the use of inorganic matrices. Consequently, textile-reinforced mortars (TRM) have been opted for strengthening, since they incorporate textiles impregnated in inorganic cementitious matrices. Lately, in order to promote sustainability and lower the high carbon emissions of cement, alkali-activated mortars, also called geopolymers, have been investigated as an alternative. Their high performance and fireproof properties have made them excellent candidates as matrices in advanced composites for strengthening. This study aims to provide an overview of research in the field of advanced composites with alkali-activated matrices used for strengthening of concrete members. Systems implementing either fiber sheets or meshes have been used so far to strengthen reinforced concrete members, indicating promising results of the new advanced composite.

Experimental Investigation of the TRM-to-Masonry Bond after Exposure to Elevated Temperatures: Cementitious and Alkali-Activated Matrices of Various Densities.

Limited research has focused on the effect of high temperatures on the textile-reinforced mortar (TRM)-to-masonry bond. In this study, masonry prisms that were furnished with double-layered TRM strips were tested under shear bond conditions after their exposure to 200 °C and 400 °C for 1 h using the single-lap/single-prism setup. A total of four TRM systems were applied sharing the same type of textile –a dry AR glass fiber one– and different matrices: two cementitious matrices, namely a normal-weight (TRCNM) and a lightweight (TRCLM) one, and two counterpart alkali-activated matrices (TRAANM and TRAALM) based on metakaolin and fly ash. Specimens’ exposure to elevated temperatures did not alter their failure mode which was due to the sleeve fibers’ rupture along with core fibers’ slippage from the mortar. The residual bond capacity of the TRM systems decreases almost linearly with increasing exposure temperature. The alkali-activated textile reinforced mortars outperformed their cement-based counterparts in terms of bond strength at every temperature. All systems retained close to 50% of their original shear bond strength after heating at 400 °C. Per the type of binder, lightweight matrices resulted in either comparable (cement-based systems) or better (alkali-activated systems) heat protection at the TRM/masonry interface.

Improved Reactivity of Volcanic Ash using Municipal Solid Incinerator Fly Ash for Alkali-Activated Cement Synthesis

In this work, municipal solid waste incinerator fly ash (MSWI-FA) was used as precursor to alter the reactivity of low reactive volcanic ash (VA) for alkali-activated cement synthesised. The effects of various proportions of MSWI-FA (0, 5, 10, 15 and 20%) on the final properties of the inorganic polymer are reported. The increase in MSWI-FA content increases the heat of reaction in MSWI-FA as the result of the reaction between the amorphous alumina-silica and the Ca-rich precursor. Isothermal conduction calorimetry, FTIR, XRD SEM/EDS, and physico-mechanical analyses were used to follow up the different formulations. New crystalline phases, namely Margarite (CaAl2 (Al2Si2)O10(OH)2, PDF#04-013-3004), Mordernite-(Ca) ((Ca, Na2, K2)(Al2 Si10 O24)·7H2O, PDF#00-011-0155) and Thernadite (Na2SO4, PDF#01-074-2036) confirm the good reactivity. The SEM/EDS exhibited the coexistence of C–S(-N-A)–H and N-A-S-H gels in binary system alkali-activated. The compressive strength and bulk density ranged from 9.38 to 21.07 MPa and from 1.56 to 1.77 g/cm3, respectively. These results indicated the possibility to use VA and MSWI-FA to synthesize room temperature alkali-activated matrices applicable in the manufacture of structural and functional materials. MSWI-FA appears energy-efficient and sustainable solution for the improvement of the volcanic ash based alkaline activated materials.

Immobilization of Sr<sup>2+</sup>, Bi<sup>3+</sup> and Zn<sup>2+</sup> in Alkali-Activated Materials Based on Blast Furnace Slag and Fly Ash

It is well known, that alkali-activated materials (AAMs) are suitable for immobilization of heavy metals and other hazardous materials. This study is focused on the characterization of inhibition of three metals – Sr2+, Bi3+and Zn2+in alkali-activated matrices. Two type of matrices were prepared – alkali-activated blast furnace slag (BFS) and alkali-activated fly ash (FA). Sodium water glass was used as alkaline activator. The ability of these matrices to fix the metals were proved by leaching tests. Compressive strength was measured to characterize mechanical properties of the matrices. Scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) was used to examine distribution and chemical state of metals within the matrices. The observed metals mainly formed the insoluble compounds after alkali activation.

Radioactivity and Pb and Ni immobilization in SCM-bearing alkali-activated matrices

Partial or total replacement of Portland cement clinker by SCMs (Supplementary Cementitious Materials) is a priority for the cement industry in its pursuit of global sustainable development and eco-friendly binder manufacture. The most widely used SCMs include industrial by-products such as blast furnace slag, fly ash and red mud. Alkali-activated cements manufactured with SCMs may reduce the need for Portland clinker by up to 90 wt%–100 wt% with no significant decline in material strength. The trade-off, however, is the risk of higher than legally allowable levels of radioactivity and unbound heavy metals (Cd, Hg, Ni, Pb, Cr), which may leach into the soil with the concomitant adverse implications for human health and the environment.This study assessed the mechanical strength, leachability and natural radioactivity of alkali-activated cement paste containing industrial waste-based SCMs (blast furnace slag, fly ash and red mud) and Pb and Ni compounds.Strength was highest in alkali-activated slag and slag/fly ash pastes and lowest in the red mud-containing materials. The addition of Pb or Ni sulphates had no adverse effect on this parameter. Alkaline and OPC pastes showed a high level of immobilization of both lead and nickel ions.According to the radiological findings, the Activity Concentration Index (I) was higher in red mud than in OPC, blast furnace slag or fly ash. With (I) values lower than 1, however, all the hydrated/activated materials studied would be EU directive-compliant. Nonetheless, the use of these new materials will depend not only on the activity concentration index, but also on their physical and chemical properties and the quality tests that must be passed to conform to legal requirements.

Effect of SiO2/Al2O3 molar ratio on mechanical behavior and capillary sorption of MK-based alkali-activated composites reinforced with PVA fibers

This paper investigates the effect of SiO2/Al2O3 molar ratio on the mechanical behavior of poly-vinyl alcohol (PVA) fiber-reinforced alkali-activated matrices (PVA-AAM) based on the activation of metakaolin (MK) as a source material. Different volume fractions (0, 1 and 2%) of PVA fibers were used to reinforce MK-based matrices with different compositions (SiO2/Al2O3 molar ratios, i.e. 3.0, 3.4 and 3.8). The amount of activation solution was adjusted to maintain same workability for all fresh mixes and account for the fiber addition. The mechanical properties assessed were compressive and flexural strengths as well as toughness, followed by optical microscopy to observe the fractured surfaces. Capillary sorption test was conducted to estimate the durability of the composites. Test results show that the matrices of PVA-AAM can be designed for the development of high performance composites, with significantly improved deformation and toughness, as it is the case for matrices with higher SiO2/Al2O3 molar ratio and 2% PVA fiber. Adjustments in the content of the activation solution may reduce the mechanical strength. However, as the amount of activation solution increases in the mixes, so does the toughness of the PVA fiber-reinforced composites, which indicates a good correlation between the amount of soluble silica in the fresh AAM and toughness in bending.

Analysis of the Long-term Service Life of Coatings Based on Alkali-activated Matrices Exposed to Chemically Aggressive Environments

The paper deals with the possibility of using materials with an alkali-activated matrix for the secondary protection of reinforced concrete structures. Attention is primarily paid to the resistance of this type of coating to the effects of chemically aggressive environments. The long-term effect of selected types of aggressive environments on the coating based on alkali-activated substances was monitored using a set of physico-mechanical and physico-chemical tests. For comparison, a commercially produced epoxy coating with an increased resistance to the effects of chemically aggressive environments, as declared by its manufacturer, was subject to an analogous set of tests.

The Leachability of Heavy Metals from Alkali-Activated Fly Ash and Blast Furnace Slag Matrices

The ability of alkali-activated materials (AAMs) to fix and immobilize heavy metals was investigated. Two raw materials were used to prepare alkali-activated matrices – high-temperature fly ash and blast furnace slag (BFS). NaOH served as an alkaline activator. Two heavy metals (Mn, Ni) were added in different amounts to find out the influence of dosage of heavy metal on the mechanical properties of the matrices and the leachability. Leachability was measured as concentration of heavy metals in leachates (ČSN EN 12457-4) by inductively coupled plasma/optical emission spectrometry (ICP/OES). Structure of prepared matrices was characterized by scanning electron microscopy (SEM). Increasing of addition of heavy metals led to decrease of mechanical properties of matrices. The leaching tests showed, that both matrices can immobilize Mn and Ni in dosages of 0.1 – 2,5%. Higher dosages caused deterioration of the matrices and increased the leachability. After alkali activation both heavy metals were transformed into the form of insoluble salts.

Strength development of alkali-activated fly ash produced with combined NaOH and Ca(OH)2 activators

Alkali-activated matrices can attain compressive strengths of the order of 30–120MPa, primarily depending on the mix formulation. The objective of this study is to evaluate compressive strength development by testing, at different ages, fly-ash-based mortars alkali-activated with activating solutions containing varying proportions of a combination of NaOH and Ca(OH)2. The NaO/SiO2 ratio was constant for all samples (N/S=0.3). Three different CaO/SiO2 molar ratios of the total quantity of CaO to the total quantity of SiO2 present in the mix were studied; specifically: 0.05 (C/N=0.033) – M5; 0.15 (C/N=0.370) – M15; and 0.25 (C/N=0.700) – M25. The M25 mix attained compressive strength of 30MPa at 7days. However, after periods of 28 and 91days, M25 compressive strength had decreased to 22MPa and 16MPa respectively. The M15 matrices exhibited similar compressive strength results. In contrast, the M5 mix exhibited increasing compressive strength over time. The SEM micrographs of M5 and M25 matrices showed the presence of two different aluminosilicate gels: the M5 sample developed a massive aluminosilicate gel over time, while the M25 sample began to exhibit a spongy gel at 28days, resulting in a weaker material. Therefore, the reduction in compressive strength appears to be related to increasing amounts of CaO (higher C/S and C/N) for the alkali-activated matrices tested in this study.