The effect of matrix composition and calcium content on the sulfate durability of metakaolin and metakaolin/slag alkali-activated mortars

The work in hand presents the results of an experimental research on the effect of different precursors (binders) used in alkali-activated materials (AMM) and its composition (i.e. SiO₂/Al₂O₃ molar ratio) on their sulfate durability. A reference matrix is formed from the activation of metakaolin (MK); this matrix was modified by the partial replacement of MK with either 20 wt% silica fume (SF) or 20 and 40 wt% blast furnace slag (BFS), so that the SiO₂/Al₂O₃ molar ratio of calcium-free and calcium-rich AAM changed from 3.0 to 3.9. The properties assessed prior to the durability testing were: density (pycnometry), compressive strength, capillary sorption and oxygen permeability. The sulfate durability was investigated by exposing the matrices to a magnesium sulfate solution for 30, 90 and 180 days of attack, after which the residual compressive strength was determined. The reductions in strength after each period of testing were correlated with variations in the pH of the sulfate solutions and with geometry changes (expansion) measured in cylinders exposed to the durability tests. X-Ray diffraction was used to determine the minerals formed onto the surface of the samples after magnesium sulfate attack. The results show that the MK-based AAM present a higher resistance to magnesium sulfate attack. Furthermore, the partial replacement of MK with BFS is responsible for reductions in the mechanical properties after attack to sulfate. This is associated with the formation of ettringite and gypsum in the presence of calcium from BFS, besides the loss of alkalinity from the migration of alkali (Na⁺) to the solution.     

Bio-derived sodium silicate for the manufacture of alkali-activated binders: Use of bamboo leaf ash as silicate source

Alkali activated binders are a promising alternative to the use of Portland cement in the manufacture of concrete for curbing CO₂ emissions. Novel sources of silicates have been investigated in recent years for reducing cost and environmental impacts associated with the use of chemical activators. This study describes the production of solid sodium silicate (SS) activating powder from bamboo leaf ash (BLA). Bamboo leaves were calcined at 550–800°C, mixed with NaOH pellets, and heated in an oven at 300°C. The obtained silicate powder was used for activating blended fly ash/slag samples. Mechanical and microstructural properties of BLA-based samples were compared to those of samples made with commercially available chemicals. The strength of BLA-activated mortars matched the commercially-sourced activators, being 25–30 MPa at 7 days and exceeding 40 MPa at 28 days. The microstructural analysis suggested that BLA-based SS showed a lesser degree of dissolution of precursors at 7 days, but the quality of the matrix was higher than that of NaOH-activated samples. These results confirmed that the reactivity of BLA-silicate powder was similar to that of commercial SS solutions, and show the potential valorization of future biomass renewable waste in the production of low carbon, alkali-activated concretes.     

New waterborne paints with different binders and corrosion inhibition application

Waterborne paint formulations based on three different emulsion resins were prepared with different pigment volume concentration (PVC). The effect of a prepared waterborne organic corrosion inhibitor with the best formula of each binder was studied. The role of binders on the corrosion inhibition of steel in the absence and presence of the corrosion inhibitor was studied. Physical, chemical, and mechanical properties, weight loss, corrosion resistance, and water up-take tests in the absence and presence of the corrosion inhibitor revealed that the protective properties of the three binders used at their optimum PVC against corrosion could be arranged in the following order: short oil alkyd > Urabrid AC 100 > acrylic emulsion resins. The prepared corrosion inhibitor showed excellent inhibition efficiency with the three used binders.     

Alkaline activation of synthetic aluminosilicate glass

The alkali activation of aluminosilicates is a procedure for obtaining new binders with many of the same properties as traditional portland cement. The type and characteristics of the products formed, however, and consequently their mechanical performance, are conditioned by a number of parameters. The present study explored the potential reactivity of synthetic alkaline silicoaluminates alkali-activated at high temperatures by analysing parameters such as the chemical composition of the starting material and the vitreous phase content. When alkali-activated, the glass produced in the laboratory developed high mechanical strength, while the performance of the resulting binders was conditioned by the SiO₂/Al₂O₃ ratio in the starting materials. The optimal values were observed to lie between 3 and 4.     

Mechanical evaluation of cordierite precursor green bodies obtained by starch thermogelling

The mechanical behavior of green bodies (porous cordierite precursors) obtained from suspensions of kaolin, talc and alumina powders consolidated by starch thermogelling was studied. Different starches were employed as consolidator/binders: potato, cassava, corn or modified cassava.Aqueous suspensions of the powders (29.6 vol.%) with 11.7 vol.% of starches were prepared by intensive mechanical mixing, homogenization and vacuum degasification. Disks were prepared by thermogelling the suspensions for 4 h at 75–85 °C and additional drying. Green bodies were characterized by bulk density and apparent porosity measurements and microstructural analysis by SEM/EDAX.The mechanical evaluation was carried out by diametral compression in displacement control. Apparent stress–strain relations were obtained from load–displacement curves and several mechanical parameters were determined: mechanical strength, apparent Young modulus and yield stress. Crack patterns were analyzed together with fractographic analysis by SEM. Mechanical results were related to the behavior of the starches in aqueous suspension and the properties of the formed gels.     

碱激发胶凝材料及混凝土研究进展

综合评述了碱激发胶凝材料及其混凝土的研究进展,总结了影响碱激发胶凝材料性能的主要因素,着重介绍了采用碱激发胶凝材料配制的混凝土性能最新研究进展,包括新拌混凝土拌合物和易性、硬化混凝土强度和抗化学侵蚀、碱集料反应、对钢筋的保护作用等耐久性问题以及硬化混凝土变形性能等,并提出当前研究存在的问题和今后研究的发展方向.     

Effect of particle size distribution of metakaolin on hydration kinetics of tricalcium silicate

The focus of this study is to elucidate the role of particle size distribution (PSD) of metakaolin (MK) on hydration kinetics of tricalcium silicate (C₃S–T₁) pastes. Investigations were carried out utilizing both physical experiments and phase boundary nucleation and growth (pBNG) simulations. [C₃S + MK] pastes, prepared using 8%_mass or 30%_mass MK, were investigated. Three different PSDs of MK were used: fine MK, with particulate sizes <20 µm; intermediate MK, with particulate sizes between 20 and 32 µm; and coarse MK, with particulate sizes >32 µm. Results show that the correlation between specific surface area (SSA) of MK's particulates and the consequent alteration in hydration behavior of C₃S in first 72 hours is nonlinear and nonmonotonic. At low replacement of C₃S (ie, at 8% _mass), fine MK, and, to some extent, coarse MK act as fillers, and facilitate additional nucleation and growth of calcium silicate hydrate (C–S–H). When C₃S replacement increases to 30% _mass, the filler effects of both fine and coarse MK are reversed, leading to suppression of C–S–H nucleation and growth. Such reversal of filler effect is also observed in the case of intermediate MK; but unlike the other PSDs, the intermediate MK shows reversal at both low and high replacement levels. This is due to the ability of intermediate MK to dissolve rapidly—with faster kinetics compared to both coarse and fine MK—which results in faster release of aluminate [Al(OH)₄⁻] ions in the solution. The aluminate ions adsorb onto C₃S and MK particulates and suppress C₃S hydration by blocking C₃S dissolution sites and C–S–H nucleation sites on the substrates’ surfaces and suppressing the post-nucleation growth of C–S–H. Overall, the results suggest that grinding-based enhancement in SSA of MK particulates does not necessarily enhance early-age hydration of C₃S.     

Investigations about the effect of aggregates on strength and microstructure of geopolymeric mine waste mud binders

Geopolymeric binders appear to be an alternative to traditional Portland cement, due to high mechanical performances and environmental advantages. Some aspects related to the effect of aggregates in the microstructure and mechanical behaviour of geopolymeric mine waste mud (GMWM) binders are reported in the present study. Compressive and tensile strength of mine waste mud binders were analyzed. The factors investigated were the aggregate/binder ratio, the aggregate dimension and aggregate type, schist, granite and limestone.Test results showed that GMWM binders have a very high strength at early ages and also possess a very high tensile strength. It's suggested that behaviour may be due to the dissolution of quartz and alumina in the presence of alkalis enhancing bonding between paste and aggregates.The aggregate dimension showed only significant effect on tensile strength. Limestone aggregates showed a chemical bond to the alkali-activated paste but presented higher shrinkage. It was also found that no traditional porous ITZ was detected in GMWM binders.     

Corrosion behavior of steel in alkali-activated fly ash mortars in the light of their microstructural,mechanical and chemical characterization

This study concerns the corrosion behavior of steel in different room temperature cured alkali-activated fly ash mortars exposed to chloride solution. The corrosion process was monitored by polarization resistance and corrosion potential measurements and the results were interpreted in the light of a complete microstructural, mechanical and chemical characterization of the mortars. The most compact alkali-activated mortars have higher porosity and lower mechanical properties than a cement-based mortar (CEM), but the protectiveness afforded to the rebars is slightly higher than that obtained in CEM. The reason for this discrepancy is connected to a lower chloride content accumulated in the former mortar type and to a specific inhibition of the rebar corrosion afforded by the pore electrolyte in alkali-activated mortars.     

Developing a hollow glass microsphere/geopolymer thermal insulation composite for hot metal surface coating

Manufacturing inorganic thermal insulation materials with superior properties such as low thermal conductivity (k < 0.1 W/mK) and high mechanical properties in terms of adhesion strength is critical for energy efficiency in energy-intense industries. Geopolymer-based composites composing of hollow glass microspheres (HGMs), waste fly ash (FA), and metakaolin (MK) were successfully applied on hot (T～300 °C) metal surfaces via spray deposition technique. The effect of Si/Al and Na/Al mole ratios and HGM loading on geopolymer composites' physical, microstructural, thermal, and adhesion strength properties were explored. The best composite composition was obtained when Si/Al mole ratio, Na/Al mole ratio, and HGM loading were 2.5, 1.0, and 10 wt %, respectively. This composition achieved an HGM/geopolymer composite material with low thermal conductivity (k ～ 0.05 W/mK), high adhesion strength (～5.0 MPa), and high stability under immersion in water and vibration environments (particularly exposed to water). The results showed that HGM/geopolymer composites could be used as a thermal insulation material in energy-intense industries.     

Fiber-reinforced one-part alkali-activated slag/ceramic binders

In the present experimental/numerical study, a combination of ceramic waste and ground-granulated blast furnace slag were used in the preparation of one-part alkali-activated binders. Moreover, the effect of fiber type and content on hardened-state properties and shrinkage was studied under two different curing conditions. In the first stage of this study water absorption, compressive strength, and flexural strength were assessed. Subsequently, the flexural performance of fiber-reinforced binders was simulated and predicted using finite element models under concentrated and distributed flexural loading, respectively. The experimental results showed that fibers improved mechanical properties, and enhancement was governed by fiber type and curing conditions. Moreover, the numerical results indicated that the developed fiber-reinforced binders offer a flexural load-carrying capacity in the range of 10–40?kN/m² and permissible service loads were well below the ultimate capacity.     

X-ray microtomography shows pore structure and tortuosity in alkali-activated binders

Durability of alkali-activated binders is of vital importance in their commercial application, and depends strongly on microstructure and pore network characteristics. X-ray microtomography (μCT) offers, for the first time, direct insight into microstructural and pore structure characteristics in three dimensions. Here, μCT is performed on a set of sodium metasilicate-activated fly ash/slag blends, using a synchrotron beamline instrument. Segmentation of the samples into pore and solid regions is then conducted, and pore tortuosity is calculated by a random walker method. Segmented porosity and diffusion tortuosity are correlated, and vary as a function of slag content (slag addition reduces porosity and increases tortuosity), and sample age (extended curing gives lower porosity and higher tortuosity). This is particularly notable for samples with ≥ 50% slag content, where a space-filling calcium (alumino)silicate hydrate gel provides porosity reductions which are not observed for the sodium aluminosilicate (‘geopolymer’) gels which do not chemically bind water of hydration.     

Sodium silicate activated slag‐fly ash binders: Part I – Processing,microstructure, and mechanical properties

Alkali silicate activated slag and class F fly ash‐based binders are ambient curing, structural materials that are feasible replacements for ordinary Portland cement (OPC). They exhibit advantageous mechanical properties and less environmental impact than OPC. In this work, five sodium silicate activated slag‐fly ash binder mixtures were developed and their compressive and flexural strengths were studied as a function of curing temperature and time. It was found that the strongest mixture sets at ambient temperature and had a Weibull average flexural strength of 5.7 ± 1.5 MPa and Weibull average compressive strength of 60 ± 8 MPa at 28 days. While increasing the slag/fly ash ratio accelerated the strength development, the cure time was decreased due to the formation of calcium silicate hydrate (C–S–H), calcium aluminum silicate hydrate (C–A–S–H), and (Ca,Na) based geopolymer. The density, microstructure, and phase evolution of ambient‐cured, heat‐cured, and heat‐treated binders were studied using pycnometry, scanning electron microscopy, energy dispersive X‐ray spectroscopy (SEM‐EDS), and X‐ray diffraction (XRD). Heat‐cured binders were more dense than ambient‐cured binder. No new crystalline phases evolved through 28 days in ambient‐ or heat‐cured binders.     

Mechanical,microstructural and mineralogical evaluation of alkali-activated waste glass and stone wool

Mineral waste wool represents a significant part of construction and demolition waste (CDW) not yet being successfully re-utilized. In the present study, waste stone wool (SW) and glass wool (GW) in the form received, without removing the binder, were evaluated for their potential use in alkali activation technology. It was confirmed that both can be used in the preparation of alkali-activated materials (AAMs), whether cured at room temperature or at an elevated temperature in order to speed up the reaction. The results show that it is possible to obtain a compressive strength of over 50 MPa using SW or GW as a precursor. A strength of 53 MPa was obtained in AAM based on GW after curing for 3 days at 40 °C, while a similar compressive strength (58 MPa) was achieved after curing the GW mixture for 56 days at room temperature. In general, the mechanical properties of samples based on GW are better than those based on SW. The evolution of mechanical properties and recognition of influential parameters were determined by various microstructural analyses, including XRD, SEM, MIP, and FTIR. The type of activator (solely NaOH or a combination of NaOH and sodium silicate), and the SiO₂/Na₂O and liquid to solid (L/S) ratios were found to be the significant parameters. A lower SiO₂/Na₂O ratio and low L/S ratio significantly improve the mechanical strength of AAMs made from both types of mineral wool.     

Mechanical properties and prediction of fracture parameters of geopolymer/alkali-activated mortar modified with PVA fiber and nano-SiO2

Properties of fly ash (FA) and metakaolin (MK) based geopolymer/alkali-activated mortar modified with polyvinyl alcohol (PVA) fiber and nano-SiO₂, including workability, compressive strength, flexural performance, elastic modulus and fracture property were tested in this study. PVA fiber content varies from 0 to 1.2%. Nano-SiO₂ content is 0 and 1%. Adaptive neuro-fuzzy interfacial systems (ANFIS) method was used to establish the artificial intelligence (AI) model to predict the fracture parameters of geopolymer/alkali-activated mortars. The inputs of ANFIS models include PVA fiber content, nano-SiO₂ content, compressive strength, flexural strength, elastic modulus, critical crack mouth opening displacement, crack load and peak load. The outputs of ANFIS model include critical effective crack length, initiation fracture toughness, unstable fracture toughness, and fracture energy. Experiment results showed that PVA fiber addition enhanced the mechanical properties especially the compressive strength and fracture performance, but decreased the workability. 0.8%–1.0% was considered as the optimal content of PVA fiber. Addition of 1% nano-SiO₂ shows a slight improvement on both workability and mechanical properties of the mortar no matter how much fiber is added. Based on the ANFIS algorithm and 42 sets of experimental data, the trained models were proved to have high accuracy with root mean square error (RMSE) under 0.15, mean absolute error (MAE) under 0.01, and coefficient of determination (R²) over 0.85. The ANFIS model established in this study combined the fracture properties with the basic mechanical properties of geopolymer/alkali-activated composites, which can provide a new method to assess the fracture performance of geopolymer/alkali-activated mortars modified with PVA fiber and nano-SiO₂ in the future.     

On the benefits of vegetable oil addition for the pore structure and acid resistance of alkali-activated systems

The impact of high additions of vegetable oil (12 vol%) on the mechanical and microstructural properties of metakaolin-slag-based alkali-activated materials (AAMs) was studied. The addition of oil resulted in a slight decrease in initial polymerization kinetics but did not affect the final degree of reaction. AAM-oil-composite-mortars exhibited approximately ∼30% lower compressive strength primarily due to the entrainment of air voids. Newly formed soap phases significantly reduced the volume of small capillary and gel pores (pore radii <15 nm), leading to a decrease in specific inner surface area by a factor of up to 15. The porosity modification induced by the oil addition greatly enhanced the resistance of AAMs against sulfuric acid attack, shifting the dominant processes from diffusion and cracks to framework-dissolution controlled by the inherent phase stabilities. Following the immersion in sulfuric acid (pH_stat = 2) for 8 weeks, the depth of corroded layer decreased by 70% and no cracks due to expansive phases were observed. These promising findings suggest that the incorporation of vegetable oil in AAMs has the potential to address durability concerns associated with diffusion-based corrosion processes, thereby expanding the range of future applications.     

The effects of cast industry waste of scale instead of cement on the mechanical and microstructural properties

The scale is a waste coming from the metal casting industry. It is an iron oxide layer formed as a result of oxidation after annealing on steel surfaces. Casting wastes, which have been left uncontrolled for many years, have caused considerable environmental damage. In the present paper, the mechanical properties and microstructural characterization of cement mortars, prepared by using scale coming from Turkey steel manufacturer were experimentally investigated. The steel scale was used as a partial replacement of cement. Recycling of solid waste scale is considered as an environmental-friendly alternative to solve the problem of disposing of wastes. Cement mortar admixtures were prepared by variable percentages of scale from 2% to 10%. The effect of scale on the strength of cement mortar was analyzed. The compressive strength values of all the specimens were recorded for 3, 7, and 28 days and the results showed that when the amount of scale replacement was increased, the strength increased slightly. However, cement mortar containing 4% wt. additive has shown the highest strength value at 3 and 28 days. As a result an optimum additive amount of scale in the cement mortar is found as 4% wt. Microstructural observation of the samples using SEM showed that scale particles were well embedded in the c matrix.     

Ti(C,N)-based cermets with fine grains and uniformly dispersed binders: Effect of the Ni–Co based binders

Metallic binder is a key factor affecting the microstructure and mechanical properties of Ti(C,N)-based cermets. To optimize the overall performances, cermets with various weight ratios of Ni/(Co + Ni) ranging from 0 to 1 were fabricated by gas pressure sintering. Microstructure, phase formation, interface structure and related mechanical properties of the sintered cermets were investigated. With the increase of the Ni/(Co + Ni) ratios, the black cores became smaller and grains of Ti(C,N) dispersed uniformly. Compared to the pure Ni or Co, Ni–Co binders accelerated the formation of rim phases, and avoided the nonuniform dispersed binder pools. When the ratio was 0.5, the cermets showed fine grains, uniformly dispersed binders and small lattice misfit of the core-rim interface, exhibiting the optimal mechanical properties, i.e. satisfactory Vickers hardness of 1670 (HV30) Kgf/mm², bending strength of 1970 MPa and Fracture toughness of 8.94 MPa m^0.5. This work sheds light on constructing the relationship between the microstructure, mechanical performance of Ti(C,N)-based cermets and the Ni/Co-based binders.     

The effect of wetting-drying / freezing-thawing cycles on properties of non-air entrained fly ash substituted cement-based composites

The use of mineral additives, arising from industrial waste, for construction materials such as cement has environmental and economic importance. Also, there is a vital gap between research and issued standards in the wetting-drying field which is well-known as one of the most important corrosive environmental conditions. Only a few pieces of research report non-air entrained fly ash substituted composites exposed to freezing-thawing. In this study, 20% of cement was replaced with fly ash and 60 wetting-drying or freezing-thawing cycles were applied to the samples. For microstructural investigation, cement paste prisms were prepared in two groups of control and fly ash-cement mixtures with the dimension of 40 x 40 × 160 mm. Additionally, 150 x 150 × 150 mm cubes were molded to study the mechanical properties of fly ash-containing concrete subjected to the wetting-drying or freezing-thawing cycles. First series of control and fly ash-added samples after a 28-day curing period were exposed to the purposed cycles. Similarly, the second and third series after 56 and 90 days were exposed to wetting-drying or freezing-thawing cycles. Based on the obtained results, fly ash-containing samples performed better than control mixtures after a 90-day curing period with/without application of wetting-drying cycles since excessive CSH gel was produced as a prominent product of pozzolanic reaction decreased porosity of the mixtures and increased adherence between particles. After the exposure to freezing-thawing cycles, control composites displayed better performance than fly ash-containing samples at early ages. As an important achievement of this study, pozzolanic reactions required at least 90 days to be completed under normal curing conditions at the standard temperature. Due to the required long curing time, partial cement replacement with fly ash can be an efficient alternative for the production of prefabricating elements used in marine construction or other constructions exposed to cold weather.     

Effect of silicate modulus and metakaolin incorporation on the carbonation of alkali silicate-activated slags

Accelerated carbonation is induced in pastes and mortars produced from alkali silicate-activated granulated blast furnace slag (GBFS)-metakaolin (MK) blends, by exposure to CO₂-rich gas atmospheres. Uncarbonated specimens show compressive strengths of up to 63 MPa after 28 days of curing when GBFS is used as the sole binder, and this decreases by 40-50% upon complete carbonation. The final strength of carbonated samples is largely independent of the extent of metakaolin incorporation up to 20%. Increasing the metakaolin content of the binder leads to a reduction in mechanical strength, more rapid carbonation, and an increase in capillary sorptivity. A higher susceptibility to carbonation is identified when activation is carried out with a lower solution modulus (SiO₂/Na₂O ratio) in metakaolin-free samples, but this trend is reversed when metakaolin is added due to the formation of secondary aluminosilicate phases. High-energy synchrotron X-ray diffractometry of uncarbonated paste samples shows that the main reaction products in alkali-activated GBFS/MK blends are C-S-H gels, and aluminosilicates with a zeolitic (gismondine) structure. The main crystalline carbonation products are calcite in all samples and trona only in samples containing no metakaolin, with carbonation taking place in the C-S-H gels of all samples, and involving the free Na⁺ present in the pore solution of the metakaolin-free samples. Samples containing metakaolin do not appear to have the same availability of Na⁺ for carbonation, indicating that this is more effectively bound in the presence of a secondary aluminosilicate gel phase. It is clear that claims of exceptional carbonation resistance in alkali-activated binders are not universally true, but by developing a fuller mechanistic understanding of this process, it will certainly be possible to improve performance in this area.