Durability Of Alkali-activated Materials Research Articles

Study on the mechanical properties, gelling products and alkalization process of alkali-activated metakaolin: From experiment to molecular dynamics simulation

Alkali-activated metakaolin (AAMK), which is rich in aluminosilicate, is considered a green material that can replace ordinary Portland cement with high energy consumption. The exceptional durability of alkali-activated materials (AAMs) is widely recognized, but the research on the molecular-scale alkalization mechanism remains largely unexplored. Here, the mechanical properties, the composition of cementitious materials, and molecular dynamics (MD) simulations of the AAMK are studied to verify the consistency and reliability of the experiment and simulation. Research shows that the flexural strength and compressive strength of AAMK reached 80% and 65.8% respectively after 3 days of hydration. Laumontite and Gismondine with a macromolecular structure similar to zeolite were formed. Moreover, AAMK consumes a large amount of Ca (OH)2 and CaCO3. Molecular insights into the alkalization process of MK have been provided through MD simulations. A systematic study was conducted on the structure, kinetics, and alkalization process of the molecules. The results show that silicate is dissolved in sodium hydroxide (NaOH) solution while the aluminate structure remains stable. Furthermore, the observed result of MK alkalization can be regarded as a precursor to the emergence of AAMs.

Enhancing alkali-activated materials with low quality fly ash: A novel mixing approach for robust construction materials

It is highly desirable to use industrial waste materials such as fly ash with high impurities as precursors to produce alkali-activated materials (AAMs) and create sustainable construction materials. A novel, simple and efficient mixing method is introduced in this study for producing robust AAMs from precursors with high impurities. The primary precursor utilized for AAM samples was low-quality fly ash with large particle sizes and high levels of impuri-ties such as unburned carbon and sulfur oxides. In the new mixing procedure, the initial step was to dissolve the precursor in sodium hydroxide solution, followed by introducing sodium silicate solution as the final step. To examine the microstructures and phase changes of the precursor, advanced analytical techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), and Raman spectroscopy (RS) were utilized. Using RS to monitor the dissolution phase was a novel and particularly beneficial technique discovered in this study, as it helped determine the ideal mixing time based on the results. The new mixing procedure has the potential to considerably enhance various properties of AAMs, including fresh and hardened properties, as well as the durability of AAMs.

Chloride Transport and Related Influencing Factors of Alkali-Activated Materials: A Review.

Chloride transport is a vital issue in the research on the durability of alkali-activated materials (AAMs). Nevertheless, due to its miscellaneous types, complex mix proportions, and limitations in testing methods, the reports of different studies are numerous and vary greatly. Therefore, in order to promote the application and development of AAMs in chloride environments, this work systematically reviews the chloride transport behavior and mechanism, solidification of chloride, influencing factors, and test method of chloride transport of AAMs, along with conclusions regarding instructive insights to the chloride transport problem of AAMs in future work.

Formulation and characterization of cleaner one-part novel fly ash/lime-based alkali-activated material

With the advanced development in sustainable cementitious materials, cleaner one-part alkali-activated materials (AAM) have shown a significant effect on increasing compressive strength and reduction in CO2 emissions. This study is aimed to prepare one-part AAM based on fly ash (FA) and hydrated lime (LM) as main precursors. The impact of LM content as replacement of FA and activator dosage was evaluated on fresh and hardened properties of one-part FA/LM-based alkali-activated mortar. Fresh characteristics of AAM were evaluated through workability (setting time, flow table) and rheology, while hardened properties were evaluated by compressive strength, flexural strength, water absorption, durability test, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) tests. The test results revealed that the addition of FA/LM-based AAM increases the reactivity leading to earlier hardening of AAM. FA/LM-based AAM has shown higher compressive strength of 41.5 MPa. The rheology parameters show the increment in yield stress and plastic viscosity by increasing the lime content in the AAM. The activator dosage of sodium hydroxide increases the setting time and durability of AAM. The enhancement in strength and rheological parameters can be assigned to the enhanced geopolymerization reactions, leading to the formation of denser and compacted gels.

Frost Resistance of Alkali-Activated Concrete—An Important Pillar of Their Sustainability

Sustainable development of concrete construction requires sustainable materials or sustainable binders. Specifically, alkali-activated materials (AAMs) are an interesting and wide group of materials. They have good strengths and are considered environmentally friendly materials because secondary materials are consumed during the preparation of AAMs. The durability of AAMs is also excellent. One of the most important parts of durability is frost resistance. The frost resistance of alkali-activated materials is usually very good. However, some studies showed opposite properties and poor frost resistance. The reason for this may be a different composition of the activator. The content of alkalis is often considered the main characteristic of alkali-activated materials. However, SiO2 content can play an important role too. This paper discusses the different results for the mechanical properties and frost resistance of different compositions of alkali activators made of sodium water glass with a silicate modulus modified with potassium hydroxide. The role of the activator content and the water-to-cement ratio in this phenomenon is discussed. The results of this article show that the strengths of AAMs are significantly affected by the curing method. Water curing reduced some of the strength of the specimens compared to foil-covered specimens. Frost resistance depends on the method of curing and on the composition of the activator; some concretes with high strengths showed very low frost resistance.

The Durability of Alkali-Activated Materials in Comparison with Ordinary Portland Cements and Concretes: A Review

Abstract China is the largest producer and user of ordinary Portland cement (OPC), and the rapid growth of infrastructure development demands more sustainable building materials for concrete structures. Alkali-activated materials (AAMs) are a new type of energy-saving and environmentally friendly building material with a wide range of potential applications. This paper compares the durability of AAMs and OPC-based materials under sulfate attack, acid corrosion, carbonation, and chloride penetration. Different AAMs have shown distinct durability properties due to different compositions being formed when different raw materials are used. According to the calcium (Ca) concentration of the raw materials, this paper interprets the deterioration mechanisms of three categories of AAMs: calcium-free, low-calcium, and calcium-rich. Conflicts found in the most recent research are highlighted, as they raise concerns regarding the consistence and long-term properties of AAMs. Nevertheless, AAMs show better durability performances than OPC-based materials in general.

Microstructure and durability of alkali-activated materials as key parameters for standardization

Alkali-activated concrete (AAC) has been commercialized as a low-CO2 construction material, but its adoption still faces several challenges, including standardization, lack of a dedicated supply chain, limited service track record, and the question of whether laboratory durability testing can predict service life. This paper outlines how using different precursors leads to the formation of different AAC phase assemblages, and how AAC can be recognized in standards using a performance-based approach independent of binder chemistry. Microstructural assessment of pastes, strength development, water permeability, and chloride migration of two AACs (100% slag and 1:1 slag:fly ash) are presented, and compared to Portland cement concrete. Manipulation of binder chemistry leads to differences in the properties of the AACs; however, both AACs assessed exhibited technical benefits in a performance-based comparison. AACs can meet the requirements of the equivalent performance concept, independent of the binder chemistry, supporting their scale-up, regulatory acceptance, and wider adoption.