Recycling of calcined clay as an alternative precursor in geopolymers: A study of durability

Composites that use fly ash and slag as alkali-activated materials instead of cement can overcome the defects and negative effects of alkali-activated cementitious materials prepared with the use of an alkali-activated material. In this study, fly ash and slag were used as raw materials to prepare alkali-activated composite cementitious materials. Experimental studies were carried out on the effects of the slag content, activator concentration and curing age on the compressive strength of the composite cementitious materials. The microstructure was characterized using hydration heat, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM), and its intrinsic influence mechanism was revealed. The results show that increasing the curing age improves the degree of polymerization reaction and the composite reaches 77~86% of its 7-day compressive strength after 3 days. Except for the composites with 10% and 30% slag content, which reach 33% and 64%, respectively, of their 28-day compressive strength at 7 days, the remaining composites reach more than 95%. This result indicates that the alkali-activated fly ash-slag composite cementitious material has a rapid hydration reaction in the early stage and a slow hydration reaction in the later stage. The amount of slag is the main influencing factor of the compressive strength of alkali-activated cementitious materials. The compressive strength shows a trend of continuous increase when increasing slag content from 10% to 90%, and the maximum compressive strength reaches 80.26 MPa. The increase in the slag content introduces more Ca2+ into the system, which increases the hydration reaction rate, promotes the formation of more hydration products, refines the pore size distribution of the structure, reduces the porosity, and forms a denser microstructure. Therefore, it improves the mechanical properties of the cementitious material. The compressive strength shows a trend of first increasing and then decreasing when the activator concentration increases from 0.20 to 0.40, and the maximum compressive strength is 61.68 MPa (obtained at 0.30). The increase in the activator concentration improves the alkaline environment of the solution, optimizes the level of the hydration reaction, promotes the formation of more hydration products, and makes the microstructure denser. However, an activator concentration that is too large or too small hinders the hydration reaction and affects the strength development of the cementitious material.

Cooperative action and compatibility between Portland cement and MSWI bottom ash alkali-activated double gel system materials

The use of copper slag instead of part of the slag to prepare alkali-activated cementitious materials can achieve the resource utilisation of copper slag, but alkali-activated copper slag-slag composite cementitious materials have the disadvantages of high shrinkage and low flexural strength, which limit their application in practical engineering. In order to improve its performance, this study used glass fiber, polypropylene fiber, steel fiber at 0.2%, 0.4%, 0.6%, 0.8%, 1.0% by volume admixture into alkali-activated copper slag-slag composite cementitious materials, respectively, to explore the effect of fiber on the drying shrinkage and mechanical properties of alkali-activated copper slag-slag composite cementitious materials. The test results show that the addition of fibers can compensate the drying shrinkage of AAMs and improve their mechanical properties. 1.0% of GF and SF can compensate the drying shrinkage of 17.5% and 27.3%, and PPF can also inhibit the drying shrinkage of the material, but the drying shrinkage will be exacerbated when the dosage is more than 0.8%. The addition of fibers does not have a significant effect on the compressive strength, but significantly improves the flexural strength of the material. When the dosage of GF, PPF and SF reaches 0.6%, 0.8% and 1.0% respectively, the flexural strength of the specimen at 28d is increased by 7.9%, 19.86% and 61.66% compared with that of the blank control group, respectively. Through microstructural analysis, it was found that the fibers formed a mesh structure in the material, which effectively inhibited the development of cracks and improved the toughness of the material. This study provides a theoretical basis and practical guidance for the optimization of the properties of alkali-activated copper slag-slag composite cementitious materials.

Alkali-activated materials have gained increasing popularity in the field of soil barrier materials due to their high strength and low environmental impact. However, barrier materials made from alkali-activated materials still suffer from long setting times and poor barrier performance in acidic, alkaline, and saline environments, which hinders the sustainable development of green alkali-activated materials. Herein, coconut shell biochar, sodium silicate-based adhesives, and polyether polyol/polypropylene polymers were used for multi-stage material modification. The modified materials were evaluated for barrier performance, rapid formation, and resistance to acidic, alkaline, and saline environments, using metrics such as compressive strength, permeability, mass loss, and VOC diffusion efficiency. The results indicated that adhesive modification reduced the material’s setting time from 72 to 12 h. Polymer modification improved resistance to corrosion by 15–20%. The biochar-containing multi-stage modified materials achieved VOC diffusion barrier efficiency of over 99% in both normal and corrosive conditions. These improvements are attributed to the adhesive accelerating calcium silicate hydration and forming strength-enhancing compounds, the polymer providing corrosion resistance, and biochar enhancing the volatile organic compounds (VOC) barrier properties. The combined modification yielded a highly effective multi-stage green barrier material suitable for rapid barrier formation and corrosion protection. These findings contribute to evaluating multi-level modified barrier materials’ effectiveness and potential benefits in this field and provide new insights for the development of modified, green, and efficient alkali-activated barrier materials, promoting the green and sustainable development of soil pollution control technologies.

Sustainable application of waste eggshell as fillers in alkali-activated solid waste-based materials: Varying treated methods and particle sizes

By reutilizing industrial byproducts, inorganic cementitious alkali-activated materials (AAMs) contribute to reduced energy consumption and carbon dioxide (CO2) emissions. In this study, coal gangue (CG) blended with ground granulated blast furnace slag (GGBFS) was used to prepare AAMs. The research focused on analyzing the effects of the GGBFS content and alkali activator (i.e., Na2O mass ratio and alkali modulus [SiO2/Na2O]) on the mechanical properties and microstructures of the AAMs. Through a series of spectroscopic and microscopic tests, the results showed that the GGBFS content had a significant influence on AAM compressive strength and paste fluidity; the optimal replacement of CG by GGBFS was 40-50%, and the optimal Na2O mass ratio and alkali modulus were 7% and 1.3, respectively. AAMs with a 50% GGBFS content exhibited a compact microstructure with a 28 d compressive strength of 54.59 MPa. Increasing the Na2O mass ratio from 6% to 8% promoted the hardening process and facilitated the formation of AAM gels; however, a 9% Na2O mass ratio inhibited the condensation of SiO4 and AlO4 ions, which decreased the compressive strength. Increasing the alkali modulus facilitated geopolymerization, which increased the compressive strength. Microscopic analysis showed that pore size and volume increased due to lower Na2O concentrations or alkali modulus. The results provide an experimental and theoretical basis for the large-scale utilization of AAMs in construction.

Recent advances in the reuse of steel slags and future perspectives as binder and aggregate for alkali-activated materials

This paper presents research into the feasibility of using stone sawing sludge-based Alkali Activated Materials (AAMs) for conservation of Cultural Heritage. Sawing sludges are a stone processing waste product resulting from the mixing of rock powder with the water used to cool down the cutting blades. The chemical composition of the sawing sludges, when aluminosilicatic, is suitable for acting as a precursor to produce AAMs. AAMs are known for their low environmental impact and versatility since their existence is drawn from recycling waste materials. One of their possible applications is in the conservation of Cultural Heritage objects. This work presents a preliminary investigation into three sawing sludge-based AAMs with different mineralogical compositions and contributes to formulating guidelines for applying them as fillers on modern and archaeological ceramic pottery based on the evaluation of their workability, appearance and physical properties over time from the moment of application and up to 30 days. Dynamic Vapor Sorption and X-Ray Diffraction results provided an overview of the structural and mineralogical changes under high RH conditions, where the tested AAMs showed a type II isotherm curve, as expected for concrete-like materials, as well as disappearance of thermonatrite after one isothermal cycle. Ultrasonic Pulse Velocity test demonstrated the general homogeneity of the AAMs despite the lower velocity exhibited by one of the formulations, probably due to its internal pore distribution and possible presence of microstratification. The Oddy tests, application tests and colourimetric measurements evidenced the advantages and weaknesses of the AAMs, with overall encouraging results ensuing investment in further in-depth studies of these innovative conservation materials in view of their future use in the field of conservation of Cultural Heritage as a result of a circular economy model.

To address the limitations in determining the amount of activator and optimizing the mix proportion during the preparation of bauxite tailings (BX)-based alkali-activated materials (AAMs), as well as the insufficient research on the interactions of multiple factors, this study aims to synthesize and optimize composite cementitious materials with BX and granulated blast furnace slag (GGBFS) as precursors via response surface methodology and central composite design (RSM-CCD). The optimal alkali activator proportion and slag content for alkali-activated, bauxite tailing, powder slag cementitious materials were investigated. A series of tests, including XRD, FTIR, TG-DSC, and SEM–EDS, were used for analysis to further investigate the effects of the alkali activator dosage on the mechanical properties and the influence of the slag content on the hydration products and microstructure. The results show that the optimal composition of alkali-activated bauxite tailings-based cementitious material is 35% slag content, 4% alkali content, a water glass modulus of 1.3, and a water–solid ratio of 0.32. The relationship model between the activator parameters and compressive strength fits well, with model determination coefficients of 0.9803 for f3c and 0.9789 for f28c. The identified hydration products were mainly C-S-H and C-(N)-A-S-H gels in the form of SiQ3 and SiQ4 tetrahedra. The SEM–EDS results show that the incorporation of slag changes the silicon–aluminum ratio of the system, promoting an increase in the content of hydration products and increasing the complexity and density of the structure. GGBFS also has a micro-aggregate filling effect and a nucleation effect, which improve the distribution of hydration products. This study demonstrates the significant potential of BX in the preparation of cementitious materials, which contributes to the sustainable development of the construction industry.

Formulation and characterization of blended alkali-activated materials based on flash-calcined metakaolin, fly ash and GGBS

Alkali-activated materials, as a low-carbon cementitious material, are widely known for their excellent durability and mechanical properties. In recent years, the modification of alkali-activated materials using biochar has gradually attracted attention. Fibrous biochar has a highly porous structure and large specific surface area, which can effectively adsorb alkaline ions in alkali-activated materials, thereby improving their pore structure and density. Additionally, the surface of the biochar contains abundant functional groups and chemically reactive sites. These can interact with the active components in alkali-activated materials, forming stable composite phases. This interaction further enhances the material’s mechanical strength and durability. Moreover, the incorporation of biochar endows alkali-activated materials with special adsorption capabilities and environmental remediation functions. For instance, they can adsorb heavy metal ions and organic pollutants from water, offering significant environmental benefits. However, research on biochar-modified alkali-activated materials is still in the exploratory phase. There are several challenges, such as the unclear mechanisms of how biochar preparation conditions and performance parameters affect the modification outcomes, and the need for further investigation into the compatibility and long-term stability of biochar with alkali-activated materials. Future research should focus on these issues to promote the widespread application of biochar-modified alkali-activated materials.

Corncob ash (CA), a byproduct of agricultural production, is a potential resource for biomaterial applications. This study investigated the changes in the mechanical properties, linear shrinkage, setting time and microstructure of alkali-activated materials after incorporating CA. Results indicated that blending CA with alkali-activated mortar (AAM) substantially enhanced its mechanical and shrinkage properties. The 28 d compressive and flexural strengths reached 60.1 and 8.9 MPa, respectively. Furthermore, the AAM exhibited a 14.5% and 13.4% reduction in shrinkage at 56 and 60 d, respectively, compared with the control group. With a 4.0% CA content, the initial and final setting times of the blended alkali-activated cement with a 3.0% alkali concentration were 138 and 221 min, respectively. Mechanical strength and linear shrinkage of the AAM increased with higher alkali activator concentrations, whereas the setting time of the alkali-activated cement decreased. CA, characterized by a porous structure, does not generate new substances within the alkali-activated cementitious system and its inherent carbon sequestration capacity can substantially reduce carbon emissions associated with AAM. This study provides a preliminary investigation into the effects of CA on alkali-activated materials, contributing to the advancement of research and applications of these materials and biomaterials in green building construction.

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.

Dispersion, properties, and mechanisms of nanotechnology-modified alkali-activated materials: A review

Research focusing on waste management and CO2 mineralization simultaneously has been a popular topic in the mining community, and a common approach is to mineralize CO2 with coal-based solid waste (CSW, e.g., gangue (CG), fly ash (FA), coal gasification slag (CGS)) produced by mining activities. Despite the understanding of CO2 mineralization by cementitious materials, the mineralization capacity of alkali-activated CSWs remains unknown. Therefore, the mineral composition evolution and mineralization capacity of different alkali-activated materials (prepared with CG, FA, CGS, and sodium hydroxide (which works as the alkali-activator), respectively) are investigated with the adoption of Gibbs Energy Minimization Software (GEMS). The results indicate that the abovementioned three alkali-activated CSWs are majorly composed of calcium silicate hydrate, magnesium silicate hydrate, kaolinite, sodium zeolite, and liquid. Due to the difference in the chemical composition of different CSWs, the amount of hydration products varies. Specifically, the alkali-activated CSWs made with CGS have the maximum calcium silicate hydrate (C-S-H), while those prepared with FA enjoy the lowest porosity. In addition, the CO2 mineralization process will result in the formulation of carbonate and, theoretically, the maximum quantity of mineralized CO2 is less than 20% of the binder used. Furthermore, compared with CG and CGS, FA is characterized with the highest mineralization capacity. The findings in this study contribute to the understanding of CO2 mineralization with alkali-activated CSWs.