Type Of Alkali Activator Research Articles

Limestone powder-based alkali-activated materials: Influence of activator type

Limestone powder (LP)-based cementitious materials with four types of alkali activators: NaAlO2, NaOH+Al(OH)3, NaOH, and Na2SiO3, were investigated at ambient temperature. The NaAlO2 and NaOH+Al(OH)3 groups produced crystalline monocarboaluminate (Mc, 3CaO∙Al2O3∙CaCO3∙11H2O), which was associated with the second exothermic peak. TEM observations revealed that AH3 (Al2O3·3H2O) generated in the NaAlO2 group was a microcrystalline phase. This microcrystal enabled cementing LP and lamellar-shaped Mc for forming compacted microstructure. However, unreacted gibbsite (Al(OH)3) in the NaOH+Al(OH)3 group did not have such effects. Thus, the 28-day compressive strength of the NaAlO2 group (20.90 MPa) considerably surpassed that of the NaOH+Al(OH)3 group (0.81 MPa). No strength appeared in the NaOH group due to the limited cementitious properties of the newly-formed gaylussite (CaNa2(CO3)2·5H2O) and pirssonite (CaNa2(CO3)2·2H2O). The Na2SiO3 group tended to generate calcium silicate hydrate (CSH) gels, resulting in a 28-day compressive strength of 8.20 MPa. NaAlO2 was more efficient in activating LP than other activators.

Mechanical Properties and Microstructural Characterization of Metakaolin Geopolymers Based on Orthogonal Tests

Metakaolin was used as a raw material for the preparation of geopolymers, where two types of alkali activators (Na2SiO3 + NaOH and Na2SiO3 + NaOH) were used to prepare metakaolin geopolymers at room temperature. The mechanical properties and microstructures of the metakaolin geopolymers were analyzed. A three-factor, four-level orthogonal test was designed to investigate the mechanical properties of the metakaolin geopolymer with different ratios. The compressive and flexural strength of different specimens were tested for 7 and 28 days. Both the Na-based and K-based geopolymers exhibited excellent mechanical properties, but the K-based geopolymer had better mechanical properties. The optimal compressive strength and flexural strength of the K-based geopolymer were 73.93 MPa and 9.37 MPa, respectively. The 28-day optimal compressive strength of the Na-based polymer was 65.79 MPa, and the flexural strength was 8.71 MPa. SEM, XRD, and FTIR analyses showed that the mechanical properties of the geopolymers could be greatly improved by using a higher alkaline solution concentration, proper Na2SiO3/MOH mass ratio, and proper mass ratio of alkali exciter to metakaolin. Amorphous silicoaluminate was more favorable for the dissolution of silicon–alumina raw materials, promoted the formation of an amorphous silicoaluminate gel, and caused the internal structure of the geopolymer to be more compact.

Preparation of Metakaolin Based Geopolymer Foam Using a Combination of Na and K Types of Alkali Activators

In this research, the ordinary Portland cement (OPC) was mixed with metakaolin, activator, hydrogen peroxide, and olive oil to synthesize hybrid geopolymer foam. The obtained results indicated internal heat release throughout OPC hydration in the combination. OPC was employed as a calcium source in geopolymers (Geopolymer-Portland cement (HGPF)) to explore the curing process of geopolymers at ambient temperature. The functionality of geopolymer components and (HGPF) mixture, the elemental composition, and proportion analyses have been compared. A principal aim of this research focuses on developing geopolymer foam and conducting many tests such as physical tests related to the surface area and pores size and compression of the foam to investigate the capacity of applying this foam in different applications that require good strength. Furthermore, microstructure tests using SEM and XRD techniques have been conducted to examine surface structure components. Overall, the findings presented in this research show that the materials selected to develop the geopolymer foam were compatible with each other giving high porosity with acceptable compression via optimizing the processing parameters by RSM.

Physicomechanical properties, stabilization mechanism, and antifungal activity of alkali-activated slag mixed with Cr6+ and Ni2+ rich industrial wastewater

An eco-sustainable approach applied to full disposal of Ni and Cr (VI) wastewater through the fabrication of alkali-activated slag (AAS). The impact of activator type (NaOH, Na2SiO3, and NaAlO2), heavy metal-containing-wastewater (Ni or Cr), and curing time (1-90-day) on the compressive strength, drying shrinkage, and immobilization efficiency have been addressed. Both compressive strength gain and loss were detected when Cr- and Ni-wastewater was mixed with AAS, depending on the type of alkali-activator. Nevertheless, all activated mixtures with wastewater demonstrated drying shrinkage values lower than that of the control sample (mixed with tap water). Ni and Cr have stabilized inside AAS-microstructure through the cationic exchange between Mg/Ni and Cr/Al within hydrotalcite phase or their localization on a negative charge of tetrahedral Al atom inside activated aluminosilicate skeleton. All the fabricated samples showed Ni and Cr concentrations in leachates lower than the regularity limits of toxicity characteristic leaching procedure (TCLP) and solubility threshold limit concentration (STLC) after 28-day of curing. The proposed strategy not only successfully applied for the safe disposal of heavy metals but also led to the fabrication of cementitious materials with high resistivity to detrimental sulfur oxidizing Aspergillus Niger fungal strain, which makes such cement effectively applied in microorganisms-rich-media.

Effect of alkali activators on diffusivity of metakaolin-based geopolymers

A basic investigation into whether a geopolymer can be utilized as a part of an artificial barrier during radioactive waste disposal was conducted in this study. Geopolymers are comprised primarily alumina and silica, and they exhibit negligible leaching owing to the absence of calcium. Studies on geopolymers are limited compared to those on other cementitious materials because the physical characteristics of geopolymers vary with the production conditions. In this work, metakaolin based geopolymers were prepared, and their diffusion performance was analyzed. The results indicate that the diffusivity of cesium in a geopolymer is affected by the type of alkali activator. Sodium-activated geopolymers had higher cesium adsorption capacity than potassium-activated geopolymers. The cesium adsorption capacity also had a significant effect on the diffusivity of cesium in the geopolymers. It was shown that, in addition to the pore structure and surface area, the mobility of water affects the diffusion performance of the geopolymer.

The influence of two types of alkali activators on the microstructure and performance of supersulfated cement concrete： mitigating the strength and carbonation resistance

As a low-carbon cementitious material, the supersulfated cement (SSC) is composed of a thimbleful of cement clinkers and massive industry by-products, i. e: gypsum and slag. However, the concrete made from SSC suffers low strength, poor carbonation resistance, inferior frost resistance, etc. In such a scenario, two types of alkali activators (lactates and sodium hydroxyl) were employed to improve the microstructure and mitigate the performance of supersulfated cement concrete. The results indicate that the incorporation of lactates effectively improves mechanical performance, water impermeability, and carbonation resistance of supersulfated cement concrete. However, the addition of sodium hydroxide failed to result in a performance enhancement of SSC concrete. Furthermore, the mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) were comprehensively utilized to illustrate the microstructure changes of supersulfated cement concrete. It indicates the hydration degree of supersulfated cement was promoted by the addition of lactates, which refined the pore structure of concrete. A more compact matrix conduces to overall performance enhancement. This work may shed new lights on the microstructure optimization of solid-waste-based binding materials and further promote the application of SSC concrete.

Comparative Study on Geopolymer Binders Based on Two Alkaline Solutions (NaOH and KOH)

This study specifically investigated the influence of the composition of aluminosilicate material i.e. the substitution of metakaolin by rice husk ash and the nature of alkaline activators (Na+/K+) on mineralogical, structural, physical and mechanical properties of geopolymer binders. This influence was evaluated based on X-ray diffraction (XRD), Fourier Transform InfraRed spectroscopy (FTIR) and Scanning Electron Microscope (SEM analyses, apparent density, water accessible porosity, compressive strength and thermal properties. Two types of geopolymer binder were synthesized according to the type of alkali activator used, the NaOH-based geopolymer and the KOH-based geopolymer. The results of characterization performed after 14 days of curing of geopolymer samples showed that the activation of the aluminosilicate powder using alkaline solution led to change in their microstructure. The highest compressive strength was obtained with the NaOH-based geopolymer.

Effects of the type of activator on the self-healing ability of fiber-reinforced alkali-activated slag-based composites at an early age

This paper presents an experimental investigation of the self-healing ability of fiber-reinforced slag-based composites activated by a single type of alkali activator at an early age. In order to investigate the effects of the type of alkali activator on the self-healing ability of fiber-reinforced slag-based composites, three mixtures reinforced by polyethylene fibers and with different alkali activators, in this case calcium hydroxide, sodium hydroxide, and sodium silicate, were designed and prepared. Compressive strength and uniaxial tension tests were conducted to measure the mechanical properties of each mixture. The self-healing ability in each case was evaluated by observing the crack width and by measuring the resonance frequency. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were also utilized to analyze the morphology and chemical compositions of the healing materials. The results of these tests showed that autogenous healing of alkali-activated slag-based composites is accomplishable through structural restoration, specifically by reducing the crack opening and through mechanical recovery. The dominant healing materials of the calcium activator-based alkali-activated slag composite was CaCO3, in contract to the C-(N)-A-S-H, CaCO3, and some amount of Na2CO3 which formed in the sodium activator-based alkali-activated slag composites.

Superabsorbent polymers as internal curing agents in alkali activated slag mortars

The aim of this study was to investigate the potential use of superabsorbent polymers (SAPs) as internal curing agents to mitigate the autogenous shrinkage of alkali activated slag (AAS) mortar. The autogenous shrinkage, compressive strength, hydration, and microstructural characteristics of AAS mortar with a couple of types of alkali activators with SAPs use were investigated. The test results showed that SAPs played an important role in reducing the shrinkage of AAS mortars, which has been a major limitation in their wider application. Higher reductions in porosity were observed for AAS pastes with SAPs compared with those without SAPs.

Mechanical properties of steel fibre reinforced geopolymer concretes at elevated temperatures

This paper presents the effects of two types of alkali activators (Na and K-based) on the residual mechanical properties of steel fibre reinforced geopolymer concretes (SFRGC) after exposed to various elevated temperatures and compared with those of steel fibre reinforced concrete (SFRC). Compressive strength, indirect tensile strength and elastic modulus of above three types of steel fibre reinforced concretes are measured after exposure to elevated temperatures of 200, 400, 600 and 800°C. Results show that the SFRGC containing Na-based activators exhibited much higher residual compressive and indirect tensile strength at all elevated temperatures including at ambient condition than its K-based counterpart and SFRC. However, the retention of residual compressive strengths relative to ambient is comparable in both Na- and K-based SFRGC and both SFRGCs showed original (ambient temperature) compressive strength retention capacity up to about 500°C temperature. In the case of indirect tensile strength, the K-based SFRGC showed ambient temperature strength retention capacity up to about 700°C temperature with more than 60% increase in residual indirect tensile strength at 400°C. In the case of elastic modulus the SFRC, however, showed slightly higher retention capacity than the SFRGC. Good correlations between the indirect tensile strength and the compressive strength and between the elastic modulus and the compressive strength of all three types of fibre reinforced concretes are observed. Existing model to predict the compressive and indirect tensile strengths of SFRGCs is found to underestimates the test results, however, it predicts reasonably well the elastic modulus of SFRGCs. New empirical equations to predict the compressive, indirect tensile strength and elastic modulus of SFRGCs are also proposed. Both SFRGCs also show negligible damage in terms of surface cracking after elevated temperatures heating compared to visible surface cracks in SFRC.

Resistance of Alkali-Activated Slag Concrete to Chloride-Induced Corrosion

The corrosion resistance of steel in alkali-activated slag (AAS) mortar was evaluated by a monitoring of the galvanic current and half-cell potential with time against a chloride-contaminated environment. For chloride transport, rapid chloride penetration test was performed, and chloride binding capacity of AAS was evaluated at a given chloride. The mortar/paste specimens were manufactured with ground granulated blast-furnace slag, instead of Portland cement, and alkali activators were added in mixing water, including Ca(OH)2, KOH and NaOH, to activate hydration process. As a result, it was found that the corrosion behavior was strongly dependent on the type of alkali activator: the AAS containing the Ca(OH)2activator was the most passive in monitoring of the galvanic corrosion and half-cell potential, while KOH, and NaOH activators indicated a similar level of corrosion to Portland cement mortar (control). Despite a lower binding of chloride ions in the paste, the AAS had quite a higher resistance to chloride transport in rapid chloride penetration, presumably due to the lower level of capillary pores, which was ensured by the pore distribution of AAS mortar in mercury intrusion porosimetry.

양생조건이 플라이애쉬 기반 지오폴리머 강도에 미치는 영향

지오폴리머 반응은 매우 복잡하며, 플라이애쉬 화학조성, 입도분포, 자극제 농도와 종류, 양생온도, 양생시간 등이 지오폴리머 물성에 많은 영향을 미치고 있는 것으로 알려져 있다. 이 연구에서는 양생조건이 플라이애쉬 기반 지오폴리머 강도에 미치는 영향을 실험하기 위하여, 양생온도, 고온양생 전 전치시간, 고온에서의 양생시간 등을 변화시켜 양생조건 변화에 따른 지오폴리머 페이스트의 압축강도, SEM, 공극특성 등에 대하여 분석하였다. 실험 결과 양생온도가 높을수록 지오폴리머의 강도는 증가하였으며, 전양생시간이 길어질수록 지오폴리머 강도는 증가되었으나, 고온양생에서의 양생시간이 길어지면 압축강도가 저하현상이 관찰되었다. 고온에서의 양생시간이 길어지면 공극구조의 변화에 따라 강도 저하 현상이 관찰되었다. 따라서 양생온도와 양생시간은 지오폴리머 강도 및 미세구조에 큰 영향을 미치고 있는 것을 확인할 수 있었다.

Properties and sustainability of alkali-activated slag foamed concrete

Abstract In the present study, 15 concrete mixes with dry density between 300 and 500 kg/m3 were tested for developing reliable mixing proportions, and establishing the significance of sustainable application of alkali-activated (AA) foamed concrete as a thermal insulation material in floor heating systems of buildings. Ground granulated blast-furnace slag (GGBS) was activated using the following three types of alkali activators: 10% Ca(OH)2 and 4% Mg(NO3)2, 5% Ca(OH)2 and 6.5% Na2SiO3, and 2.5% Ca(OH)2 and 6.5% Na2SiO3. The unit binder content varied in the concrete mixes corresponding to each type of activator. All mixes were produced using the pre-formed foam procedure with no aggregate or filler. The test results revealed that AA GGBS foamed concrete has considerable potential for practical applications when the unit binder content is close to 400 kg/m3, achieving the minimum quality requirements specified in the KS F 4039 as well as ensuring economic efficiency. Furthermore, the tested AA GGBS foamed concrete developed higher compressive strength than ordinary Portland cement (OPC) foamed concrete with the same dry density. In particular, a lifecycle assessment demonstrated a reduction in the environmental impact profiles of all specimens relative to typical OPC foamed concrete as follows: 99% for photochemical oxidation potential, 85–93% for global warming potential, 73–85% for abiotic depletion, and 68–85% for both acidification potential and human toxicity. Overall, the developed AA GGBS foamed concrete can be enlarged to sustainable varieties of cast-in-place structural fill and precast masonry materials.

Tests on Alkali-Activated Slag Foamed Concrete with Various Water-Binder Ratios and Substitution Levels of Fly Ash

To provide basic data for the reasonable mixing design of the alkali-activated (AA) foamed concrete as a thermal insulation material for a floor heating system, 9 concrete mixes with a targeted dry density less than 400 kg/m3 were tested. Ground granulated blast-furnace slag (GGBS) as a source material was activated by the following two types of alkali activators: 10% Ca(OH)2 and 4% Mg(NO3)2, and 2.5% Ca(OH)2 and 6.5% Na2SiO3. The main test parameters were water-to-binder (W/B) ratio and the substitution level (RFA) of fly ash (FA) for GGBS. Test results revealed that the dry density of AA GGBS foamed concrete was independent of the W/B ratio an RFA, whereas the compressive strength increased with the decrease in W/B ratio and with the increase in RFA up to 15%, beyond which it decreased. With the increase in the W/B ratio, the amount of macro capillaries and artificial air pores increased, which resulted in the decrease of compressive strength. The magnitude of the environmental loads of the AA GGBS foamed concrete is independent of the W/B ratio and RFA. The largest reduction percentage was found in the photochemical oxidation potential, being more than 99%. The reduction percentage was 87% - 93% for the global warming potential, 81% - 84% for abiotic depletion, 79% - 84% for acidification potential, 77% - 85% for eutrophication potential, and 73% - 83% for human toxicity potential. Ultimately, this study proved that the developed AA GGBS foamed concrete has a considerable promise as a sustainable construction material for nonstructural element.

Alkali activation of blended cements containing oil shale ash

The influence of using different types of alkali activators namely: sodium sulphate, sodium silicate, sodium carbonate and sodium hydroxide on the hydration characteristics and physic-mechanical properties of special blended cement systems containing oil shale ash were investigated. Compressive strength values of the blended cement pastes containing 10% oil shale ash I and 30% oil shale ash II in absence of activators about 83–80% of Portland cement paste respectively. The addition of various alkali-activators to the same two mixes improves the mechanical strength of both cement mixes. The various cement alkali-activators used in this study followed this order (major to minor) with respect to the mechanical strengths Na2SO4>Na2SiO3>Na2CO3>NaOH. The results showed that SAII would have better pozzolanic properties than SAI referring to chemical composition and the degree of crystalinity and these findings were confirmed by DTA, XRD and SEM.

Propiedades y aplicaciones de los cementos alcalinos

In this paper are presented some of the technologic properties of cementitious material elaborated by alkali activation of aluminosilicates are presented. More specifically it is about the properties of alkali activated fly ash concrete and mortar (without Portland cement). So depending on the type of alkali activator that is used and after a previous thermal curing, the resulting material will show an interesting list of properties and features that includes: high initial mechanical strengths (under flexure and compression), low drying shrinkage, and a very good matrix-steel bonding, as well as an excellent strength to acid attack and an excellent behaviour when is exposed to fire. That is the reason why these new cements, due to their magnificent and durable technologic features as well as its easy adaptability to the existing installations in the precast industry, can be used in a variety of applications, for example: elaboration of railroad cross ties, blocks for building or paving, and so others.

Effectiveness of new silica fume alkali activator

The effectiveness of a new type of alkali activator is studied. The activator is a product of silica fume. The results obtained showed the silica fume activator as a highly effective substance for the alkali activation of the combinations of Portland cement, silica fume and blast furnace slag, and slag alone. The positive effect of activator is based on the intensification of the production of calcium silicate hydrates and the densifying of the forming pore structure of the activated binder.