Alkali-activated Blast Furnace Slag Research Articles

Possibility of using fly ash after denitrification by SNCR as admixture in alkali-activated materials

The paper deals with the possibility of using fly ash including fly ash after SNCR denitrification from power plant Třebovice as an admixture for composites based on alkali-activated blast furnace slag in order to improve selected physical and mechanical properties. Furthermore, the effect of fly ash on hydration and structure influence in thermal analysis was studied. Higher values of flexural strength were achieved in 10% of slag replacement by fly ash. The highest compressive strength values were achieved with blast furnace slag (VGS).

MECHANICAL PROPERTIES OF CEMENT COMPOSITE CONTAINING RECYCLED CONCRETE AND ALKALI ACTIVATED BLAST FURNACE SLAG

Recycling of materials is very popular and very important in these days. Finding the new ways to process and use these materials is a key to get rid of a lot of construction waste. The amount of landfilling needs to be reduced. This study observes mechanical properties of the cement composites containing recycled concrete powder and alkali-activated blast furnace slag processed on a high- peed mill as a potential binder replacement up to the 60 wt. %. These materials have a positive effect on hydration process, increase flexural strength. It can reduce compressive strength loss when an amount of cement in the mixture is reduced.

Composite foamed alkali-activated concrete with slag and steel slag

Thermally expandable foaming microspheres (FMS) are used as a foaming agent in many different fields due to their excellent dimensional stability and elastic properties. In this study, lightweight thermal insulation materials were prepared by adding FMS to foamed concrete containing alkali-activated steel slag and blast-furnace slag. The dry bulk density, cube strength, thermal conductivity and drying shrinkage of samples with different compositions were investigated and their microstructure was analysed using scanning electron microscopy and X-ray diffraction. With the addition of FMS, foamed materials with a dry bulk density of 625 kg/m3 and 40 mm cube strength of 3·6 MPa could be obtained. The 28 d drying shrinkage was 0·8 mm/m and the thermal conductivity was 0·102 W/m.K (thermal inertia index of 4·66). The results showed that FMS can be used as an organic foaming agent to prepare lightweight thermal insulation materials with improved insulation performance and mechanical properties.

Thermal Properties of Alkali Activated Slag Plaster for Wooden Structures

Currently, the production of green building materials grows up. Alkali-activated materials (AAMs) based plaster have better fire resistance properties compared to Portland cement-based concrete and plasters. Compared to Portland cement-based systems AAMs retain a significant level of structural stability after exposure to fire events. AAM based concrete doesn’t have at all or has an insignificant amount of calcium hydroxide in the binder structure which exposed to high-temperature changes to calcium oxide. This weakens Portland cement structural properties and allows cracks to appear under high-temperature conditions. This study shows that AAM based plaster that consisted of alkali-activated ground granulated blast furnace slag (slag) with the addition of Phosphogypsum (PG), sand and polypropylene fibre filling exposed to 1000 °C temperature shows up to 2% longitudinal dimension shrinkage. After exposure of elevated temperature these fibers melted leaving a network of channels that allow water vapour vaporize and inner pressure in the material decreased. The start of the wood surface charring process tch is 10 minutes after the start of heating. Using an AAM binder as fire-resistant plaster coating on a wooden structure delays the start of the char layer forming on the wood surface. This allows using AAMs base plaster for fire-resistant coatings on combustible materials as the barrier layer in order to increase the passive safety of wooden structures in heritage buildings.

Immobilization of cesium with alkali-activated blast furnace slag

Alkali-activated binders (AABs), as a promising alternative to Portland cement, are now being used on a commercial scale in various applications around the world, including hazardous and radioactive waste immobilization. In this paper, the leaching resistance, strength, and nanostructural alteration of alkali-activated blast furnace slag (AABFS) doped with 2 % and 5 % cesium were investigated. The addition of cesium caused a significant increase in the compressive strength of AABFS, followed by mild strength reduction after leaching. AABFS can be considered a potentially efficient matrix for cesium immobilization, since the mean leachability index in both cases (2 % and 5 % of Cs added) was above the threshold value of 6. Both doping with Cs and leaching caused the transformation of the AABFS nanostructure. The majority of the aluminum that was released from the C-A-S-H gel due to leaching remained within the AABFS matrix, initiating gel reconstruction: the C-A-S-H gel was converted to C-S-H gel, and an additional N-(C)-A-S-H gel was also formed. Cesium was preferentially associated with the N-(C)-A-S-H gel rather than with the C-A-S-H gel. The results of this research seem to be in good agreement with the Cross-linked Substituted Tobermorite Model (CSTM).

Influence of sodium silicate powder silica modulus for mechanical and chemical properties of dry-mix alkali-activated slag mortar

Sodium silicate powders with different SiO2/Na2O (silica modulus) were characterized by solubility rate, pH, and chemical structure (by 29Si MAS-NMR) and compared in the preparation of one-part (or dry-mix) alkali-activated blast furnace slag mortar. The low SiO2/Na2O indicated the beneficial presence of less-polymerized silica (Q1 and Q2 Si environments) and thus faster dissolution. Consequently, using sodium silicates with SiO2/Na2O of 0.9, 2.1, and 3.4 resulted 28 d compressive strengths of 103, 80, and 2 MPa, respectively, with increasing setting time and decreasing heat release in isothermal calorimetry. Adjustment of activator SiO2/Na2O from 2.1 or 3.4 to 0.9 by adding NaOH powder resulted increased or decreased mechanical properties of mortar, respectively, depending on the initial silica modulus. These properties were not, however, similar to those obtained with sodium silicate having SiO2/Na2O of 0.9 originally. Depending on the case, the added NaOH can be consumed for dissolving sodium silicate activator, slag, or the forming (C,N)-(A)-S-H gel.

Effects of shrinkage-reducing admixtures on autogenous shrinkage in alkali-activated materials

This paper deals with possible solutions of chemical and autogenous shrinkage of alkali-activated blast furnace slag (AAS). Shrinkage is the main reason why AAS is not used in practice. One of the solutions how to reduce the shrinkage is the utilization of special chemical admixtures. In concrete technology, the use of chemical compounds based on hydrocarbon alcohols is well known, however the efficiency in AAS is still very poorly elucidated. Therefore, isopropanol was used in this study. The main aim of the work was to recognize and study the relations between chemical processes in the early stages of hydration. The effect of isopropanol on alkali activation was determined by calorimetric measurements and compared with the initial and final setting time of binder. It was found out that the presence of isopropanol additive had only insignificant effect on the setting time. On the other hand, the results indicated that an increasing amount of isopropanol deaccelerates the hydration process which was represented by smaller amount of primary CSH gel formation and delayed time of secondary binder phase formation.

Utilization of basalt fabrics as reinforcement for alkali-activated blast furnace slag systems

Alkali-activated materials based on the alkali-activated blast furnace slag (AABFS) seem to be a suitable alternative construction material due to the environmental and economic aspects. More widespread application potential could be supported by the utilization of fabric reinforcement into the matrix to form AABFS composite with improved mechanical properties. The basalt fibres or fabrics (BFs) represent a reinforcing material suitable for the AABFS due to its favourable price/properties ratio. However, the question of fibres stability under the alkaline conditions should be taken into account. The effect of BFs implementation in one or more layers into various types of matrices to the mechanical properties was studied with the regard to the compressive and flexural strengths. The used matrices were based on the alkali-activation of the BFS with various activators. The fabric/matrix adhesion same as the transition zone properties were studied using the SEM analysis and pull-out tests. Indication of appreciable improvement of mechanical properties utilizing one layer of BFs for some of the mixtures activated using sodium water glass and sodium carbonate was found out.

Suitability of commercial superplasticizers for one-part alkali-activated blast-furnace slag mortar

Alkali-activated materials are a low-CO2 alternative for Portland cement in construction. However, one major issue in their use is the poor or varying functionality of the currently available commercial superplasticizers. Especially for one-part (‘just add water’) alkali-activated materials, the number of studies is limited. In this study, one-part alkali-activated mortar was prepared from blast furnace slag by using solid sodium hydroxide as an activator and microsilica as an additional silica source. Comparison of commonly used superplasticizer types revealed that lignosulfonate, melamine, and naphthalene-based superplasticizers are more efficient than the currently most used polyacrylate and polycarboxylate-superplasticizers. Lignosulfonate-based superplasticizer was overall best-performing: it improved significantly the workability (+41% spread, −51% yield stress, −27% viscosity), setting time (+70%), and compressive strength (+19%) at a 0.5 wt% dose. When the amount of water and superplasticizer were optimized, compressive strength of mortar could be doubled (from 19 to 40 MPa at 28 d).

Mechanical and microstructural properties of alkali-activated slag and slag + fly ash mortars exposed to high temperature

In this study, changes in the mechanical and microstructural properties of alkali-activated blast furnace slag (BFS) and BFS + fly ash (FA) blended mortars exposed to 25, 400, 600 and 800 °C temperatures are investigated. The alkali-activated mortars using the replacement ratios of 0, 25 and 50% FA by weight of BFS together with the activators are produced in addition to the control mortars produced by only cement. Three different optimum Ms (SiO2/Na2O ratio of activators) values of 0.50, 0.75 and 1.00 are determined for the alkali-activated mortars. The FA substitution by BFS increases the high temperature resistance of alkali-activated BFS mortars.

Investigating the mechanical property and reaction mechanism of geopolymers cement with red Pisha Sandstone

Alkali-activated geopolymers cement based on natural red Pisha Sandstone (RPS), which causes severe soil erosion, was examined. The compressive strength and softening coefficient of the alkali-activated red Pisha Sandstone cement (AAPC) were investigated at different blast furnace slag replacement levels, NaOH dosages, and curing durations. This paper also investigated the strength and reaction mechanism of the AAPC samples with 40 wt% blast furnace slag at different NaOH dosages by replacing RPS with ISO Standard Sand (referred to as G group samples). Scanning electron microscopy/energy dispersive X-ray, Fourier transform infrared spectroscopy, and X-ray diffraction were used to analyze the hydration products and reaction mechanism of the samples at the curing ages of 28 days after the strength test. The results showed that the compressive strength and softening coefficient of the cement improved significantly when blast furnace slag was used as the mineral additive. When 1.5 wt% NaOH was used as the activator, the compressive strength and softening coefficient of the AAPC sample curing for 90 days increased from 6.9 to 56.2 MPa and 0.4 to 0.95, respectively, with an increase in the slag dosages from 0 to 40 wt%. The reaction mechanism of AAPC was essentially different from that of the alkali-activated blast furnace slag cement. The blast furnace slag significantly promoted the polymerization reaction of Si–Al gels. The Si–Al and C-S-H gels strengthened the AAPC system. However, the C-S-H gels and reinhardbraunsite, which are formed by the hydration of blast furnace slag, strengthened the alkali-activated blast furnace slag cement. In the AAPC samples with blast furnace slag as the mineral additive, the hydration reaction of the slag and the polymerization reaction of the Si–Al gels promoted each other.

Modification of Alkali Activated Blast Furnace Slag for Pothole Repairs

Potholes denote small, typically sharp edged holes in the pavement. The aim of this research was to study the usability of alkali activated (AA) blast furnace slag based material in the repair of paved roads, especially during the cold winter and spring seasons when such repairs are needed most and the use of hot asphalt is not possible. The objective was to a find material which is both more cost-efficient and durable than plain cold asphalt. Properties like rapid strength development, good bonding with old paving material, weather resistance, abrasion resistance, and low shrinkage were required. The influence of the chosen factors on the performance of the material was studied applying the multi-attribute optimization method. The impact of different additives, such as Portland cement, fibers and crushed tire rubber were studied. The results indicated that the AA slag based materials studied can be improved by suitable additives to make them reach desired performance. According to the tests, adding Portland cement increased compressive strength threefold after 3 hours and reduced shrinkage by 34% but should be a negative impact on higher levels related to freeze-thaw resistance. In addition, crushed rubber was indicated to have a positive impact related to all the studied performance properties.

Geopolymers as active capping materials for in situ remediation of metal(loid)-contaminated lake sediments

Metal(loid) contamination in sediments is a widespread environmental issue. Sediments act normally as metal(loid) sinks, but if chemical conditions (such as pH or redox potential) change, they can become sources of secondary pollution. Consequently, various strategies for both in and ex situ remediation of contaminated sediments have been developed. One promising method is active capping, which involves the injection of adsorbents as a layer on the sediment surface or the mixing of adsorbents within the sediment. Adsorbents decrease the bioavailability of metal(loid)s. In the present work, the suitability of alkali-activated blast-furnace-slag, metakaolin geopolymer, and exfoliated vermiculite were evaluated for in situ stabilization of two metal(loid)-contaminated lake sediments through laboratory-scale experiments. The results indicated that adsorbent amendments had metal(loid)-specific performance: alkali-activated blast-furnace slag was suitable for Al, Cu, Fe, and Ni; metakaolin geopolymer for Cu, Cr (total), and Fe; and vermiculite for Al and Zn. None of the materials could stabilize Ba, Sr, or Ti. Furthermore, the amendments performed differently in two different lake sediments, implying that the effectiveness of the amendments needs to be confirmed on a case-by-case basis.

Factors affecting the mechanical properties of alkali activated ground granulated blast furnace slag concrete

This research work aims to study the factors affecting the mechanical properties of alkali activated blast furnace slag concrete. The mechanical properties studied herein were, compressive strength, tensile strength and modulus of elasticity.Five factors each at three levels were considered throughout this research. The effect of sodium hydroxide molarity (10, 12 and 14 M), alkaline solution/slag content (0.40, 0.45 and 0.50), curing temperature (30 °C, 60 °C, 90 °C), curing time (1, 2 and 3 days) and sodium hydroxide: sodium silicate mass ratio (1:1.75, 1:2.50 and 1:3.25) were considered throughout the research program. The fractional factorial method of experimentation was used through the experimental program to reduce the number of mixes.The results generally showed that the increase in sodium hydroxide molarity and sodium hydroxide to sodium silicate enhanced all of the mechanical properties, while the increase in alkaline solution/slag, curing temperature has adversely affected the alkali activated slag concrete. The best results of curing time has been achieved when the curing period is 2 days.

Cement Kiln By-Pass Dust: An Effective Alkaline Activator for Pozzolanic Materials.

Cement kiln by-pass dust (CKD) is a fine-grained by-product of Portland clinker manufacturing. Its chemical composition is not suitable for returning back into feedstock and, therefore, it has to be discharged. Such an increasing waste production contributes to the high environmental impact of the cement industry. A possible solution for the ecological processing of CKD is its incorporation into alkali-activated blast furnace slag binders. Thanks to high alkaline content, CKD serves as an effective accelerator for latent hydraulic substances which positively affect their mechanical properties. It was found out that CKD in combination with sodium carbonate creates sodium hydroxide in situ which together with sodium water glass content increases the dissolution of blast furnace slag particles and subsequently binder phase formation resulting in better flexural and compressive strength development compared to the sample without it. At the same time, the addition of CKD compensates the autogenous shrinkage of alkali-activated materials reducing the risk of material cracking. On the other hand, this type of inorganic admixture accelerates the hydration process causing rapid loss of workability.

Evaluation of alkali-activated blast furnace ferronickel slag as a cementitious material: Reaction mechanism, engineering properties and leaching behaviors

Blast furnace ferronickel slag (BFFS) is a kind of heavy metal-bearing slag with moderate calcium content that is obtained from the production of ferronickel in a blast furnace. The reaction mechanism, engineering properties, and leaching behaviors of alkali-activated BFFS materials were investigated. The results show that the main crystalline phase of the reaction products is strätlingite, and the gel phase is a kind of C-A-S-H with Ca/Si and Al/Si ratios of 0.64–1.65 and 0.57–1.44, respectively. The reaction of BFFS greatly depends on the alkalinity and silicon modulus, with a high alkalinity and low silicon modulus resulting in a rapid initial exothermic rate, a high reaction degree, and a denser pore structure in the alkali-activated BFFS materials. In addition, the compressive strength of the alkali-activated BFFS mortar is comparable to that of the alkali-activated blast furnace slag (BFS); furthermore, the alkali-activated BFFS shows more continuous strength gains from 7 days to 90 days and much lower 7-day autogenous shrinkage. More importantly, the alkali-activation treatment can significantly decrease the leaching amount of Mn, Cr(VI) and Cr(III), and it converts the hazardous BFFS to non-toxic construction materials.

Preparation and characterization of road alkali-activated blast furnace slag paste

Alkali-activated Materials (AAM) has been applied in road engineering as an environmentally friendly material in recent years. This study prepared road alkali-activated blast furnace slag (AABFS) paste in different mixture ratios based on an orthogonal test scheme. The fluidity, setting time, mechanical strengths, and other properties were systematically evaluated to determine the optimum mixture ratio of paste. Finally, the microstructural characteristics and reaction mechanism of AABFS paste were investigated. The results showed that changes in the mass ratio of sodium hydroxide and blast furnace slag (mSH/mSL) have great influence on the performance parameters, including the fluidity, setting time, flexural strength, and compressive strength. Results at the microstructural level indicated that the structure of the AABFS paste specimen prepared at the optimum ratio is more compact and uniform, and the AABFS paste can be applied effectively in road-grouting reinforcement.

Alkali Activated Slag Mortars Provide High Resistance to Chloride-Induced Corrosion of Steel

The pore solutions of alkali-activated slag cements and Portland-based cements are very different in terms of their chemical and redox characteristics, particularly due to the high alkalinity and high sulfide content of alkali-activated slag cement. Therefore, differences in corrosion mechanisms of steel elements embedded in these cements could be expected, with important implications for the durability of reinforced concrete elements. This study assesses the corrosion behaviour of steel embedded in alkali-activated blast furnace slag (BFS) mortars exposed to alkaline solution, alkaline chloride-rich solution, water, and standard laboratory conditions, using electrochemical techniques. White Portland cement (WPC) mortars and blended cement mortars (white Portland cement and blast furnace slag) were also tested for comparative purposes. The steel elements embedded in immersed alkali-activated slag mortars presented very negative redox potentials and high apparent corrosion current values; the presence of sulfide reduced the redox potential, and the oxidation of the reduced sulfur-containing species within the cement itself gave an electrochemical signal that classical electrochemical tests for reinforced concrete durability would interpret as being due to steel corrosion processes. However, the actual observed resistance to chloride-induced corrosion was very high, as measured by extraction and characterisation of the steel at the end of a 9-month exposure period, whereas the steel embedded in white Portland cement mortars was significantly damaged under the same conditions.

Influence of alkali ions on the efficiency of shrinkage reduction by polypropylene glycol in alkali activated systems

Alkali-activated materials, especially when activated by water glass, exhibit substantial drying shrinkage that hinders their broader industrial application. The effect of shrinkage-reducing admixtures (SRA), based on polypropylene glycol, on drying shrinkage of alkali-activated blast furnace slag (BFS) mortars was examined. The determination of SRA efficiency and the influence of potassium alkali activators with varying silicate modulus on drying shrinkage characteristics were studied. It was observed that a high amount of alkalis positively affected the effect of SRA. The higher the amount of alkalis was, the lower was the drying shrinkage. The paper further discusses and underlines the role of the amount of alkali ions on the properties of alkali-activated BFS systems.

The contribution of life-cycle assessment to environmentally preferable concrete mix selection for breakwater applications

Abstract Life cycle assessment (LCA) provides a comprehensive framework for positioning low energy and global warming potential alternatives regarding Portland cement and concrete. Published LCA work on alkali-activated cements is, however, relatively limited. In this paper, we illustrate how LCA critically supports concrete technological studies in the search for low impact concrete mixes. Previous research on breakwater applications explored replacing a low-clinker Portland cement and natural aggregates with seven different alkali-activated blast furnace slag (bfs) binder systems and with coarse and granulated bfs aggregates. Its outcome suggested a sodium silicate-activated bfs formulation as the best match between concrete properties and environmental regulation compliance. To validate this outcome through LCA, our cradle to gate assessments followed ISO 14044 (INTERNATIONAL…, 2006b) and used Ecoinvent v.2.2 and CML baseline 2001 v.2.05. We adopted the ‘net avoided burden approach’ to handle multifunctionality intrinsic to by-product-based AAC. Whilst sodium silicate-activated mixes rivaled the reference regarding GWP, impacts in several categories were increased. LCA highlighted the implications of driving mix selection by focusing on a single environmental impact category.