Alkali-activated Blast Furnace Slag Research Articles

Durability of Alkali Activated Blast Furnace Slag

The alkali activation of blast furnace slag has the potential to reduce the environmental impact of cementitious materials and to be applied in geographic zones where weather is a factor that negatively affects performance of materials based on Ordinary Portland Cement. The scientific literature provides many examples of alkali activated slag with high compressive strengths; however research into the durability and resistance to aggressive environments is still necessary for applications in harsh weather conditions. In this study two design mixes of blast furnace slag with mine tailings were activated with a potassium based solution. The design mixes were characterized by scanning electron microscopy, BET analysis and compressive strength testing. Freeze-thaw testing up to 100 freeze-thaw cycles was performed in 10% road salt solution. Our findings included compressive strength of up to 100 MPa after 28 days of curing and 120 MPa after freeze-thaw testing. The relationship between pore size, compressive strength, and compressive strength after freeze-thaw was explored.

알칼리 활성화된 고로슬래그 페이스트의 물리화학적 특성 및 이산화탄소 흡수능 평가

PURPOSES: In this study, alkali-activated blast-furnace slag (AABFS) was investigated to determine its capacity to absorb carbon dioxide and to demonstrate the feasibility of its use as an alternative to ordinary Portland cement (OPC). In addition, this study was performed to evaluate the influence of the alkali-activator concentration on the absorption capacity and physicochemical characteristics. METHODS: To determine the characteristics of the AABFS as a function of the activator concentration, blast-furnace slag was activated by using calcium hydroxide at mass ratios ranging from 6 to 24%. The AABFS pastes were used to evaluate the carbon dioxide absorption capacity and rate, while the OPC paste was tested under the same conditions for comparison. The changes in the surface morphology and chemical composition before and after the carbon dioxide absorption were analyzed by using SEM and XRF. RESULTS: At an activator concentration of 24%, the AABFS absorbed approximately 42g of carbon dioxide per mass of paste. Meanwhile, the amount of carbon dioxide absorbed onto the OPC was minimal at the same activator concentration, indicating that the AABFS actively absorbed carbon dioxide as a result of the carbonation reaction on its surface. However, the carbon dioxide absorption capacity and rate decreased as the activator concentration increased, because a high concentration of the activator promoted a hydration reaction and formed a dense internal structure, which was confirmed by SEM analysis. The results of the XRF analyses showed that the CaO ratio increased after the carbon dioxide absorption. CONCLUSIONS : The experimental results confirmed that the AABFS was capable of absorbing large amounts of carbon dioxide, suggesting that it can be used as a dry absorbent for carbon capture and sequestration and as a feasible alternative to OPC. In the formation of AABFS, the activator concentration affected the hydration reaction and changed the surface and internal structure, resulting in changes to the carbon dioxide absorption capacity and rate. Accordingly, the activator ratio should be carefully selected to enhance not only the carbon capture capacity but also the physicochemical characteristics of the geopolymer.

Alkali activated slag cements using waste glass as alternative activators. Rheological behaviour

The purpose of this study is to investigate news activators in the preparation of alkali-activated materials (AAMs) alternative to Portland cements by reusing waste glass. Alkali-activated blast furnace slag (AAS) constitutes an alternative to Portland cement due to high energy and environmental pollution associated with industrial Portland cement. Moreover, alkali activated materials offer a series of higher properties than ordinary Portland cement (OPC), such as better strength and durability behaviour. However, the rheology of these materials has been much less intensely researched.The present study aimed to study the effect of waste glass as activator and as replacement of blast furnace slag on the rheological behaviour of AAS pastes, with a comparison between the rheological parameters and fluidity of these pastes to the same parameters in standard cements (CEM I and CEM III/B).The findings show that AAS paste behaviour of rheology when the activator was a commercial waterglass solution or NaOH/Na2CO3 with waste glass was similar, fit the Herschel-Bulkley model. The formation of primary C-S-H gel in both cases were confirmed. However, the rheological behaviour in standard cements fit the Bingham model. The use of the waste glass may be feasible from a rheological point of view in pastes can be used.

Effect of Si/Al ratio on engineering properties of alkali-activated GGBS pastes

The effect of synthesis parameter, silicon/aluminum molar ratio, on the engineering properties of alkali-activated blast-furnace paste has been investigated. The article presents the study of the compressive strength and microstructure of alkali-activated blast-furnace slag paste specimens prepared using sodium silicate and potassium hydroxide solution as activators. The experimental results have indicated that the compressive strength, bulk density, apparent porosity, water absorption and microstructure of the specimens are significantly affected by the silicon/aluminum molar ratio of alkali-activated ground granulated blast-furnace slag (AAS) paste. The compressive strength at 28 d of 44·90–46·30 MPa was obtained for silicon/aluminum ratios of 2·03–2·26. Increasing silicon/aluminum ratio beyond 2·26 resulted in a decrease in compressive strength of AAS paste due to delayed precipitation of calcium silicate hydrate products. The microstructural changes of hardened AAS paste were studied using scanning electron microscopy/energy-dispersive X-ray spectroscopy, and mineralogical characterization was studied with X-ray diffraction and Fourier transform infrared spectroscopy.

Thermal Insulating Alkali-Activated Systems

The paper deals with a laboratory research and development of alkali activated system with thermal insulating properties filled with low density materials. The experiment was focused on compressive strengths and heat conductivity of prepared materials. The purpose was to prepare durable but also isolating material based on alkali-activated blast furnace slag.

Distinctive microstructural features of aged sodium silicate-activated slag concretes

Electron microscopic characterisation of 7-year old alkali-activated blast-furnace slag concretes enabled the identification of distinct microstructural features, providing insight into the mechanisms by which these materials evolve over time. Backscattered electron images show the formation of Liesegang-type ring formations, suggesting that the reaction at advanced age is likely to follow an Oswald supersaturation–nucleation–depletion cycle. Segregation of Ca-rich veins, related to the formation of Ca(OH)2, is observed in microcracked regions due to the ongoing reaction between the pore solution and available calcium from remnant slag grains. A highly dense and uniform interfacial transition zone is identified between siliceous aggregate particles and the alkali activated slag binders, across the concretes assessed. Alkali-activated slag concretes retain a highly dense and stable microstructure at advanced ages, where any microcracks induced at early ages seem to be partially closing, and the remnant slag grains continue reacting.

Effect of Silicate Content on the Properties of Alkali-Activated Blast Furnace Slag Paste

The effect of silicate content (SiO2/Na2O) of an activator on physical and mechanical properties of alkali-activated blast furnace slag paste has been investigated. The paste was produced by activating blast furnace slag with sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) solution. The SiO2/Na2O ratio varied from 0.2 to 1.2. The test specimens were cast and cured in water (fully immersed condition) at room temperature and the direct compressive strength at the age of 3, 7, 14, 21, 28 days were obtained. It has been observed that the compressive strength and ultrasonic pulse velocity of test specimen increases with the increase in silicate content up to a silicate ratio of 0.8. Compressive strength is found to be a maximum 44.53 MPa at 28 days. It is noticed further that the compressive strength increases with age. It is also observed that the silicate ratio has a significant influence on porosity, water absorption and water sorptivity. The mineralogical and micro-structural changes were studied using XRD and SEM/EDX, while porosity, total pore volume, pore-size distribution, etc., were studied using mercury intrusion porosimetry.

Sorptivity Ratio and Compressive Strength of Alkali-Activated Blast Furnace Slag Paste

Abstract The initial/secondary absorption and compressive strength of alkali-activated slag (AAS) paste has been investigated. Emphasis has been given to secondary absorption of AAS paste based on the method described in ASTM C1585-04 [Standard Test Method for Measurement of Rate of Absorption of Water by Hydraulic Cement Concretes, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, 2004, pp. 1–4]. The paste was produced by activating blast furnace slag with sodium hydroxide/potassium hydroxide and sodium silicate solution. The major parameters studied were alkali content, silicate content, and the type of activator. Experimental investigation revealed that initial rate of absorption (Si) and secondary rate of absorption (Ss) decreases with increase in alkali and silicate content up to a certain limit with an increase in compressive strength. The maximum compressive strength was found to be 50.40 MPa for the specimens having lowest sorptivity. It was also found that the sorptivity ratio (Si/Ss) plays a significant role on compressive strength in turn on the durability of AAS composites. This paper is an attempt to introduce the simple method of measuring initial and secondary sorptivity described in ASTM C1585-04 to study the strength and durability of AAS. The microstructure study was carried out using SEM/EDAX.

Self-Sensing Properties of Alkali Activated Blast Furnace Slag (BFS) Composites Reinforced with Carbon Fibers.

In recent years, several researchers have shown the good performance of alkali activated slag cement and concretes. Besides their good mechanical properties and durability, this type of cement is a good alternative to Portland cements if sustainability is considered. Moreover, multifunctional cement composites have been developed in the last decades for their functional applications (self-sensing, EMI shielding, self-heating, etc.). In this study, the strain and damage sensing possible application of carbon fiber reinforced alkali activated slag pastes has been evaluated. Cement pastes with 0, 0.29 and 0.58 vol % carbon fiber addition were prepared. Both carbon fiber dosages showed sensing properties. For strain sensing, function gage factors of up to 661 were calculated for compressive cycles. Furthermore, all composites with carbon fibers suffered a sudden increase in their resistivity when internal damages began, prior to any external signal of damage. Hence, this material may be suitable as strain or damage sensor.

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.

Improvement of the performance of alkali activated blast furnace slag mortars with very finely ground pumice

This work investigates the strength and durability of alkali activated blast furnace slag mortars (AAS) with very finely ground pumice (P) at certain rates. BFS was used instead of cement at the rate of 60% and 80%. Sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) alkalis were added as solution into the mixture. First stage Na2O 6, 7 and 8wt% of the BFS was added. Better results were obtained from the strength values of the sample containing 8% Na2O. Because of this result, in the second stage, pumice was used instead of 5% and 10% of the BFS. However, silicate modules (MS=SiO2/Na2O) in both experimental studies were calculated as 0.5, 0.75 and 1.00. The durability values of the AAS+P samples were better than those of the reference samples.

The effect of elevated temperature on bond performance of alkali-activated GGBFS paste

The main reaction products were investigated by analysis of microstructure of alkali-activated ground granulated blast furnace slag (GGBFS) paste. An experimental research was performed on bond performance of alkali-activated GGBFS paste as a construction adhesive after exposure to 20–500 °C. Through XRD analysis, a few calcium silicate hydrate, hydrotalcite and tetracalcium aluminate hydrate were determined as end products, and they were filled and packed each other at room temperature. In addition, akermanite dramatically increased at 800 °C and above. The two key parameters, the ultimate load Pu.T and effective bond length Le, were determined using test data of carbon fiber-reinforced polymer (CFRP)-to-concrete bonded joints at elevated temperature. The experimental results indicate that the ultimate load Pu.T remains relatively stable initially and then decreases with increasing temperature. The effective bond length Le increases with increasing temperature except at 300 °C. The proposed temperature-dependent effective bond length formula is shown to closely represent the test data.

Mortars of alkali-activated blast furnace slag with high aggregate:binder ratios

Mortars with high aggregate:binder ratios of 10:1 and 8:1 were investigated with binders of blast furnace slag activated with 2.5%, 3.4%, 4.5%, and 6.5% Na2O relative to the mass of slag and using sodium silicate solutions with moduli SiO2/Na2O of 1.3, 1.5, 1.7, 2.0 and 2.2. The mortars were formulated at low water/binder ratio in semi-dry mixes to produce bricks. The modulus of 1.7 was chosen as most adequate and the effects of the % Na2O and temperature, on strength development, were then investigated. Various formulations showed compressive strength useful for construction, where mortars with aggregate:binder of 8:1, activated with 3.5% Na2O and cured at 60°C registered 20MPa at 28days, while mortars with 8:1 and 10:1 with 6% Na2O cured at 20°C reached 20 and 13.3MPa, respectively.

Workability and Setting Time of Alkali Activated Blast Furnace Slag Paste

Abstract This paper reports the results of an experimental investigation of blast furnace slag paste activated by potassium hydroxide and sodium silicate. Reasonable workability and setting times were maintained. The workability was measured in terms of flow diameter. The loss of flow with time was also studied. The major parameters studied were water/binder ratio, alkali content, silicate content, slag/activator ratio, silicate modulus, and sodium silicate/potassium hydroxide ratio, and their effects on workability and setting time are presented. It was found that the workability and setting time of alkali activated ground granulated blast furnace slag paste are dependent basically on the alkali content, silicate content, and silicate modulus. Locally available blast furnace slag has been utilized, and its potential as source material has been verified. This information will be useful to manufacturers and researchers.

Unrestrained Short-Term Shrinkage of Calcium-Hydroxide- Based Alkali-Activated Slag Concrete

A total of 34 concrete mixtures were tested to examine the unrestrained shrinkage behavior of calcium-hydroxide (Ca(OH)2)-based alkali-activated (AA) ground-granulated blast-furnace slag (GGBS) concrete. The main parameters investigated were water-binder ratio (w/b), unit water content (Wc), and fine-aggregate-to-total-aggregate ratio (S/A). From the regression analysis of test results, the comprehensive basic design equations were formulated to predict the 28-day shrinkage strain and time function, which can be expanded to long-term shrinkage strain or ultimate results of Ca(OH)2-based AA-GGBS concrete. The test results showed that the increasing rate of shrinkage strain against age and the amount of 28-day shrinkage strain of Ca(OH)2-based AA-GGBS concrete were significantly affected by the w/b, Wc, and the type of auxiliary activators, while they were minimally influenced by the S/A. When the w/b was higher than 0.35, a higher 28-day shrinkage strain was observed in Ca(OH)2- and Na2CO3 activated GGBS concrete than in Ca(OH)2- and Na2SiO3-activated GGBS concrete. The proposed equations showed a significantly better accuracy in the test results than the existing empirical models of which agreement was dependent on the mixing proportions of the concrete tested.

The use of alkali activated waste binders in enhancing the mechanical properties and durability of soft alluvial soils

This paper presents recent work in utilising industrial by-products as sustainable binders for use in deep soil mixing, to enhance the geotechnical properties of soft soils. The study has used geotechnical and mineralogical tests to determine the performance of the binders when incorporated into an artificial silty sand soil. Comparisons with the strength and durability of untreated and stabilised soils have been made. The study indicates that from the by-products tested, soils stabilised with alkali activated blast furnace slag resulted in the greatest strength and durability improvements; with other materials tested showing smaller improvements. The addition of alkali activators has been observed to allow pozzolanic reactions to occur, which has led to improved mechanical properties; primarily strength, which increased with time. The durability of the soil was improved by the additions of by-products, though alkali activation did not cause significant additional increase in durability.

Properties of alkali-activated systems with stone powder sludge

This article investigates the effects of stone powder sludge on the microstructure and strength development of alkali-activated fly ash and blast furnace slag mixes. Stone powder sludge produced from a crushed aggregate factory was used to replace fly ash and granulated blast furnace slag at replacement ratios of 0%, 10%, 20%, and 30% by mass. The unit weight and compressive strength of the samples were measured, and scanning electron microscopy/energy dispersive spectroscopy and X-ray diffraction (XRD) analyses were performed. The test results indicated that the compressive strength of alkali-activated blast furnace slag mixes using stone powder sludge was higher than that of the alkali-activated blast furnace slag control mix, but the compressive strength of alkali-activated fly ash mixes decreased with increasing replacement ratio of stone powder sludge. Microscopy results indicated that for alkaliactivated blast furnace slag samples, broken surfaces were more evident than for the alkali-activated fly ash samples. For all XRD diagrams, broad and diffuse peaks were observed around 2θ = 35° (d = 2.96–3.03 A), implying amorphous or short-ordering structure phases.

Influence of Si:Al ratio on the microstructural and mechanical properties of a fine-limestone aggregate alkali-activated slag concrete

Three series of fine limestone aggregate, alkali-activated blast furnace slag (AAS) concretes were fabricated and tested; two through activation with waterglass/NaOH solution, of which one included NaCl as a retarding agent, and one activated by Na2CO3. Each of these series was made up of three formulae containing different amounts of Al2O3. The compressive strengths of the series activated by waterglass/NaOH after 28 days were ≈65 ± 5.3 MPa, a 22% increase compared to previously reported formulae containing no additional Al2O3. Increasing the amount of Al2O3 did not further increase strength, however. The Na2CO3-activated formulae had strengths of ≈35 ± 3 MPa after 28 days, representing no increase in strength over formulae not containing Al2O3 previously reported. X-ray diffraction showed the main binding phase to be calcium silicate hydrate (C–S–H) gel, as is commonly found in ordinary Portland cement (OPC). Fourier transform infrared spectroscopy showed little difference from the previously reported results for formulae not containing Al2O3 and strongly resemble the spectra reported elsewhere for C–S–H. Electron microscopy, coupled with energy dispersive spectroscopy, showed the cementing phase to be a single homogenous phase—not a mixed system of geopolymer and C–S–H gel—with a lower volume fraction of unreacted slag than formulae without Al2O3. The reason for the increase in strength of Al2O3-containing formulae is unclear, but is unlikely to be ascribed to the formation of large amounts of ‘geopolymers’ and may be related to a possible increase in reaction temperature of between 2 and 5°C, depending on amount of additive.

Increasing Coal Fly Ash Use in Cement and Concrete Through Chemical Activation of Reactivity of Fly Ash

Nearly 60% of the electricity in the USA is produced by coal-fired plants, which results in the production of a large quantity of coal combustion residues, among which coal fly ash is the major component. Many applications have been developed for coal fly ash. The use of coal fly ash as a cement replacement in concrete is the most attractive one because of its high volume utilization and widespread construction. However, the replacement of cement with coal fly ash in Portland cement concrete usually increases the setting time and decreases the early strength of concrete. The addition of chemical activators such as Na2SO4 or CaCl2 accelerates pozzolanic reactions and changes pozzlanic reaction mechanisms between fly ash and lime, which results in decreased setting time, accelerated strength development, and increased strength of materials containing fly ash, especially with a high percentage of fly ash. The other promising cements include alkali-activated fly ash and alkali-activated fly ash-ground blast furnace slag cements, which can exhibit very high strength in the presence of proper alkaline activators. Thus chemical activation of reactivity of fly ash is an effective method to increase the use of fly ash in concrete.

Corrosion resistance of alkali-activated slag cement

This paper examines the corrosion of alkali-activated blast furnace slag cement in pH 3 nitric acid, pH 3 acetic acid, 30% CaCl2 solution and 15% CO2 atmosphere at relative humidity of 52%. ASTM Type I Portland cement is used as a reference. Experimental results indicate that alkali-activated slag pastes are corroded more slowly than Portland cement pastes in these acid and salt solutions. Alkali-activated slag pastes are carbonated very quickly during an accelerated carbonation test because of shrinkage cracking, while no carbonation can be detected on Portland cement pastes after 75 days of exposure to 15% CO2 at a relative humidity of 52%. Detailed analyses indicate that all these differences can be attributed to the difference in hydration products in the two types of hardened cement pastes.