Alkali-activated Slag Mortars Research Articles

Effect of Steel Slag on the Properties of Alkali-Activated Slag Material: A Comparative Study with Fly Ash

Slag and fly ash (FA) are mostly used as precursors for the production of alkali-activated materials (AAMs). FA is the waste discharged by power plants, while slag and steel slag (SS) both belong to the iron and steel industry. The effects of SS and FA on the strength, microstructure, and volume stability of alkali-activated slag (AAS) materials with different water glass modulus (Ms) values were comparatively investigated. The results show that adding SS or FA decreases the compressive strength of AAS mortar, and the reduction effect of SS is more obvious at high Ms. SS or FA reduce the non-evaporable water content (Wn) of AAS paste. However, SS increases the long-term Wn of AAS paste at low Ms. The cumulative pore volume and porosity increase after adding SS or FA, especially after adding FA. The hydration products are mainly reticular C-(A)-S-H gels. Adding SS increases the Ca/Si ratio of C-(A)-S-H gel but decreases the Al/Si ratio. However, by mixing FA, the Ca/Si ratio is reduced and the Al/Si ratio is almost unchanged. The incorporation of SS or FA reduces the drying shrinkage of AAS mortar, especially when SS is added. Increasing Ms increases the compressive strength and improves the pore structure, and it significantly increases the drying shrinkage of all samples. This study provides theoretical guidance for the application of steel slag in the alkali-activated slag material.

Investigation of the Effect of Blended Aggregate on the Strength and Drying Shrinkage Characteristics of Alkali-Activated Slag Mortar.

To reduce drying shrinkage of AASC mortar (AASM), mixed aggregate mixed with river sand (RS) and silica sand in three sizes was used to investigate the effect of the physical properties of mixed aggregate on shrinkage reduction. A mixture of river sand (0.2-0.8 mm), S1 (2.5-5.0 mm), S2 (1.6-2.5 mm), and S3 (1.21-160 mm) had river sand-silica sand mean diameter ratios (dr) of 7.68 (S1/RS), 3.75 (S2/RS), and 3.02 (S3/RS). The compressive strength and drying shrinkage characteristics of mixed aggregates according to fineness modulus, surface area, bulk density, and pore space were investigated. It had the highest bulk density and lowest porosity at a substitution ratio of 50%, but the highest strength was measured at a substitution ratio of 50% or less. High mechanical properties were shown when the fineness modulus of the mixed aggregate was in the range of 2.25-3.75 and the surface area was in the range of 2.25-4.25 m2/kg. As the substitution rate of silica sand increased, drying shrinkage decreased. In particular, the drying shrinkage of RS + S1 mixed aggregate mixed with S1 silica sand, which had the largest particle size, was the smallest. When silica sand or river sand was used alone, the drying shrinkage of the sample manufactured only with S1, which has the largest particle size of silica sand, was the smallest among all mixes. Compared to RS, at a 5% activator concentration, drying shrinkage was reduced by approximately 40% for S1, 27% for S2, and 19% for S3. At a 10% concentration, S1 showed a reduction effect of 39%, S2 by 28%, and S3 by 13%. As a result of this study, it was confirmed that the drying shrinkage of AASM could be reduced simply by controlling the physical properties of the aggregate mixed with two types of aggregate. This is believed to have a synergistic effect in reducing drying shrinkage when combined with various reduction methods published in previous studies on AASM shrinkage reduction. However, additional research is needed to analyze the correlation and influencing factors between the strength, pore structure, and drying shrinkage of AASM using mixed aggregate.

How does carbonation of alkali-activated slag and Portland cement systems impact steel corrosion differently?

Carbonation of cementitious materials, associated with neutralization of pore solution, is one of the prevalent reasons for inducing corrosion of steel in concrete. Compared with ordinary Portland cement (OPC) concrete, the poorer carbonation resistance of alkali-activated slag (AAS) concrete leads to serious durability concerns of a higher risk of steel corrosion. This work investigates the corrosion behaviors of steel embedded in intact and carbonated AAS and OPC mortars exposed to various relative humidity conditions, with the aim of fostering a better understanding of how carbonation impacts steel corrosion differently in these two cementitious systems. The results show that uncarbonated AAS mortars have better protective ability against corrosion than OPC counterparts; however, poorer performance after complete carbonation. It is mainly attributed to the combined effect of severe deterioration and coarsening of pore structure and increased ratio of chloride ion concentration to hydroxide ion concentration ([Cl−]/[OH−]) in pore solution, resulted from the consumption of hydroxide ions and release of bound chloride after carbonation. Besides, the different shifts of R-C (material resistivity versus corrosion rate) relationship lines in AAS and OPC mortars after carbonation are observed, which is ascribed to the different compositional and microstructural evolutions induced by carbonation in these two cementitious systems.

Investigate the mechanical behavior of alkali-activated slag mortar using sustainable waste sodium silicate-bonded sand as an alkali activator and carbon free aggregate

Various products generate different types of industrial by-products based on their production characteristics and procedures. Therefore, an accurate classification of these characteristics would contribute significantly to achieving the goals of resource recycling and environmental protection. This study focused on Waste Alkali-Activated Slag Mortar (W-AASM). In the past, inorganic aggregation technology was successfully adopted to replace conventional concrete binding materials and reduce cement usage. Besides utilizing inorganic aggregation technology, the study has introduced a novel industrial by-product known as Waste Sodium Silicate-Bonded Sand (WSSS). WSSS not only serves as a substitute for sodium silicate in alkali-activated agents but also acts as a complete replacement for fine aggregates in mortar. The experimental findings showed that, in addition to fulfilling the original strength requirements of concrete, it results in a significant reduction in cement consumption, minimizing the extraction of natural resources, and decreasing the overall cost of Alkali-activated concrete production. The study highlighted the advantage of WSSS and showed a new dimension of waste foundry sand application in the concrete industry. This method leads to the recycling of waste materials and their optimal use, and as a result, leads to a sustainable ecosystem. The results of this study can be applied to secondary structures in civil engineering and construction projects in the future, such as precast concrete products, concrete pavements, or roads.

Alkali-silica reaction of ferronickel slag fine aggregate in Portland cement and alkali-activated slag mortars: Pessimum effect investigation

Ferronickel slag (FNS) with dense and hard nature exhibits great potential to be utilized as recycled aggregate in concrete production. This study investigated the alkali-silica reaction (ASR) and the pessimum proportion of FNS fine aggregate in Portland cement (PC) and alkali-activated ground granulated bast furnace slag (AAS) mortars as a substitute for river sand. Over a 120-day period, the accelerated mortar bar test revealed that the addition of FNS fine aggregate in both cases resulted in harmless ASR expansion. In PC mortars, the ASR expansion consistently decreased from 0.569 % to 0.114 % as the FNS content increased. In comparison, AAS mortars with higher pore solution alkalinity had the pessimum proportion of 30 % FNS. The ASR expansion values initially increased from 0.472 % to 0.554 % with an increase in FNS content from 0 % to 30 %, but then decreased to 0.146 % as the FNS content was further raised to 100 %. The amorphous ASR gel was identified as a hydrated alkali silicate gel and present within the existing veins in the river sand grain. The ASR reaction of FNS fine aggregate exhibited a self-mitigating effect on the deleterious expansion through its pessimum behavior. Additionally, the addition of FNS fine aggregate yielded a beneficial outcome by reducing the cumulative pore volume in the mortars, and thus enhanced the compressive strengths.

Effect of borax-modified activator on mechanical properties and drying shrinkage of alkali-activated slag/metakaolin mortar

Alkali-activated materials (AAMs) possess several advantages, such as high strengths and low carbon emissions. However, their application is hindered due to their significant shrinkage. This study explored the effect of borax-modified sodium silicate activator and metakaolin (MK) on the mechanical properties and drying shrinkage (DS) of alkali-activated slag (AAS) and AAS/MK (AASM) mortars. X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy were used to characterize the hydration products. The results showed that the DS reduction of the AAS mortar was related to decreased Na2O content, a reduction in the proportion of mesopores, and the formation of moisture-retaining borate compounds. The DS reduction of the AASM mortar was attributed to the ultra-fine differential effect induced by MK, reducing the connected pores. The modified activator combined with MK increased the chemically bound water content in the matrix. Additionally, the B–O bond and highly active MK improved compactness of the AASM mortar. The use of borax-modified activators and MK provides a new solution to address the significant shrinkage issue in AAMs. This sets the stage for AAMs to potentially replace OPC, contributing to low-carbon emissions and promoting environmental protection.

Effects of recycled carbon fibers on mechanical and piezoresistive properties and environmental impact in alkali-activated cementitious materials

The development of alkali-activated cementitious materials (AAM) reinforced with recycled carbon fiber (rCF) is of remarkable significance due to its cost-effectiveness compared to virgin carbon fiber, along with the sustainable reuse of solid waste serving as AAM. This study explores the integration of rCF and solid waste Ground Granulated Blast Furnace Slag (GGBS), aiming for cleaner construction practices. A novel approach involving a high-viscosity one-part GGBS slurry ensured homogeneous rCF dispersion, resulting in alkali-activated slag (AAS) mortars with outstanding overall performance at minimal dosage. At a minimum dosage of 0.1 vol%, rCF significantly enhanced flexural strength by 53.42% and reduced drying shrinkage by over 20%. The uniformly dispersed rCF acts as nucleation sites, boosting early compressive strength without compromising long-term performance. Furthermore, the connected rCF network established an exceptionally low percolation threshold (0.125 vol%) and demonstrated promising piezoelectric properties under cyclic loading, illustrating its potential as self-sensing concrete elements. Additionally, life cycle analysis (LCA) underscored the environmental benefits of rCF and GGBS, revealing their potential to remarkably cut carbon emissions (75.36%) and reduce costs (36.5%). These findings provide valuable insights into the development of rCF-reinforced self-sensing mortar with superior mechanical properties, showcasing the substantial potential of rCF in driving the construction industry toward carbon neutrality.

Effect of magnetized water on alkali-activated slag mortars incorporating raw and calcined marble powder

This study investigates the impact of magnetized water (MW) on the fresh and hardened properties of alkali-activated slag (AAS) mortars incorporating raw marble powder (RMP) and calcined marble powder (CMP). In the initial stage of the study, control specimens were manufactured to ascertain the optimal parameters for molarity, curing temperature, and curing time. The optimal parameters were determined based on the highest strength results: a molarity of 10, curing time of 24 hours, and curing temperature of 110 ℃. In the second stage of the study, while maintaining these optimal parameters, RMP and CMP were substituted in place of blast furnace slag (BFS) at rates of 10%, 20%, 30%, and 40%. In this study stage, tap water (TW) and MW were employed as the mixing water. A mini-slump test was conducted to assess the fresh state properties of the prepared AAS mortars. Subsequently, ultrasonic pulse velocity (Upv), flexural strength (ff), and compressive strength (fc) tests were carried out to evaluate the hardened state properties of the specimens. Finally, scanning electron microscopy (SEM) analysis was performed to examine microstructural properties. According to the results, the ff and fc values of the mortars produced with CMP substitution using TW showed an increase of up to 20.6% compared to the mortars based on RMP and produced withTW. Additionally, utilizing MW as the mixing water enhanced the workability of AAS mortars. Consequently, incorporating CMP and MW in AAS mortars further improved fresh and hardened state properties compared to RMP and TW.

Optimizing pore structure of perforated cenospheres for effective internal curing of alkali-activated slag mortars

Based on the controllability of the pore structure of perforated cenospheres (PCs), this study utilized PCs with different etching concentrations for the internal curing of alkali-activated slag (AAS), aiming to achieve the optimal etching parameters for efficient internal curing. Three etching concentrations (0.3 M, 0.7 M, 1.0 M) were set to investigate the effects of water-filled PCs on the performance of internal curing of AAS mortars. The results indicated that the water released from the PCs delayed the hydration of AAS and maintained high internal relative humidity. The chemical shrinkage of AAS could be reduced. Water-filled PCs internal curing promoted the generation of hydration products around the PCs and increased their packing density, leading to a reduction in mesopores and a 12.7—27.7% increase in elastic modulus at 28 d. With the optimal etching concentration of 0.7 M, water-filled PCs significantly mitigate the autogenous shrinkage by 75.8% while enhancing the compressive strength of AAS mortars.

Malzemelerin Isıtılarak Kullanılması ile Alkali-Aktive Cürufların Erken-dönem Dayanım Gelişimlerinin İyileştirilmesi

Alkali-activated cements have gained attention as sustainable alternatives to Portland cement-based concretes. This study aims to enhance the early strength development of alkali-activated slag (AAS) mortars by preheating the alkali activator, ground granulated blast furnace slag, and aggregates to temperatures reaching up to 65°C before their utilization. Preheating at 50°C enhances both early and ultimate strength of AAS, compared to specimens that were not subjected to preheating. Similarly, preheating at 65°C results in a substantial increase in early strength but leads to some strength reduction after three days. SEM and FTIR analyses reveal that preheating at 50°C improves the interfacial transition zone between paste and aggregate, reduces microcracks, and promotes alkali activation. Furthermore, FTIR analysis confirms the occurrence of hydration reactions in AAS upon contact with water.

Accelerated aging resistance, abrasion resistance and other properties of alkali-activated slag mortars containing limestone powder

The Earth's crust is widely recognized for its abundance of limestone (LS). Limestone powder (LSP) has a wide range of potential properties that make it suitable for various applications and fields. It has found extensive usage in Portland cement-based materials. Recently, the incorporation of LSP into geopolymers is becoming more of a concern than before and has become a hot topic. In this article, high range of LSP (2.5–50 wt%) was introduced into alkali-activated slag (AAS) mortars activated with NaOH and sodium silicate solution as a substitution of slag. For the first time, the effect of LSP on the accelerated aging resistance and abrasion resistance of AAS mortars was examined. In addition, the typical traditional routine properties such as flowability, setting time, mechanical strength, water absorption, and total porosity were studied. Sophisticated equipment was utilized to observe the changes in crystalline phases, hydration products, and microstructures of selected samples. The results showed higher flowability and longer setting time with including LSP. As the amount of LSP increased as the flowability and setting time increased. The introduction of 2.5–25% LSP positively affected the mechanical strength, aging resistance, abrasion resistance and transport properties. In contrast, the introduction of 35% did not show an essential change, whilst the introduction of 50% LSP negatively affected the mentioned properties.

Optimization of Mechanical Properties and Embodied Energy of Polyvinyl Alcohol Fiber-Reinforced High-Strength Alkali-Activated Slag Mortars: Comparative Study

Optimization of Mechanical Properties and Embodied Energy of Polyvinyl Alcohol Fiber-Reinforced High-Strength Alkali-Activated Slag Mortars: Comparative Study

Thermal insulation and durability of alkali-activated lightweight slag mortar modified with silica fume and fly ash

Green building indices prioritize energy efficiency, which in turn promotes sustainable development. One of the most energy-consuming sectors globally is the building sector. Developing green insulating materials based on byproducts has environmental benefits and profits in reducing energy consumption in the building sector and other sectors. This study aimed to develop thermal insulating materials based on byproducts for binders and expanded perlite (EP) for fine aggregate cured at room temperature. Alkali-activated lightweight slag mortars with/without foaming agent were prepared. To increase the efficiency of the thermal insulation, the slag was partially replaced with a fixed ratio of silica fume (SF) (15%) and different ratios of fly ash (FA) (15%, 40% and 65%). After room temperature curing, the bulk density, mechanical strength, accelerated aging resistance, water absorption, total porosity and thermal conductivity were measured. The selected samples were examined using sophisticated tools. The results showed that the selected byproducts can be efficiently employed as insulation materials, of which the obtained results of thermal conductivity, bulk density, total porosity, water absorption, compressive strength and flexural strength were in the range of 0.172–0.082 W/mK, 1191.6–450 kg/m3, 33.2–48.7%, 28–45.1%, 12.48–1.5 MPa and 3.43–0.46 MPa, respectively. All the studied specimens exhibited strength reduction after accelerating aging, but the reduction rate was mitigated by increasing the FA content. Increasing FA content has a benefit in increasing the efficiency of thermal insulation.

Relationship of electrical resistivity and corrosion rate in alkali-activated mortars with varying alkalinity and nitrite inhibitor

The quantitative relationship between electrical resistivity and corrosion rate of steel (noted as R-C relationships) is empirical but informative in indicating corrosion risk of reinforced concrete in complex environments. However, it remains unclear how the R-C relationship line is affected by the chemistry and microstructure of cementitious binders. In this work, the corrosion behaviors of steel embedded in alkali-activated slag (AAS) mortars with varying alkali dosages were investigated to uncover the role of hydroxide ion concentration (pH) of pore solution in R-C relationship lines. Additionally, the effectiveness of sodium nitrite as a corrosion inhibitor in AAS was evaluated under high pH and chloride-rich conditions, as well as its effect on the R-C relationships. The results show that the intercept of R-C relationship lines increases with the decrease of alkali dosage in activators, which is mainly attributed to the pH drop in cementitious materials. Moreover, the slope of R-C relationship lines is a parameter related to pore structure and has a linear correlation with the average pore diameter measured by mercury intrusion porosimetry. Additionally, the efficacy of sodium nitrite as a corrosion inhibitor in AAS is insignificant, likely due to its detrimental effect on pore structure and enhanced water absorption.

Interactions between alkali-activated and cement-based mortars and biofilm and their influence on microbial induced mortar deterioration

Alkali-activated fly ash (AAF) and alkali-activated slag (AAS) mortars were exposed to the sewage environment with acidophilic sulfur oxidizing bacteria and high concentration H2S to study their interaction. It was found that when they suffered the biogenic sulfuric acid attack, both the AAF and calcium aluminate cement (CAC) mortars had a strong inhibition effect on the growth and activity of microorganisms. There are more aluminum ions dissolved from AAF mortar into sewage, and its structure was characterized by the increase in 100-1000 nm pores. Still, a hard high silicon skeleton was formed to keep the apparent intact. However, CAC mortar had a higher killing rate for the bacteria in the attached biofilm. For AAS mortar, the formation of excessive gypsum increased the macropores content and was more conducive to the growth of biofilm, thereby accelerating the penetration of corrosive medium and the deterioration of mortar.

Monitoring early age elastic and viscoelastic properties of alkali-activated slag mortar by means of repeated minute-long loadings

This study investigates the development of elastic and creep properties of sodium hydroxide-activated blast furnace slag mortar, utilizing three different molarities of the activator solution and two solution-to-binder ratio and a reference OPC-based mixture, since the earliest age. The experimental phase involves a series of hourly-repeated 5-min long creep tests on the aging material. This approach enables continuous monitoring, facilitating the characterization of early-age elastic stiffness and creep properties. Linear regression is employed to calculate the tangent unloading (elastic) modulus and Poisson's ratio. To model short-term creep behaviour, a power-law creep function is utilized. The integration of these findings with calorimetry-derived evolutions of cumulative heat release establishes linear correlation between (compressive) strength and heat release, along with a power function relationship between unloading (elastic) modulus and heat release. An optimal alkali dosage (Na2O content) appears to be vital for long-term strength development. Additionally, the creep parameters, namely amplitude (A) and kinetic (K), demonstrate a gradual decrease, although they maintain values higher than those of the corresponding OPC mixture, as the heat release progresses.

Preparing hydrophobic alkali-activated slag mortar with lotus-leaf-like microstructure by adding polydimethylsiloxane (PDMS)

In this study, polydimethylsiloxane (PDMS) was applied for overall hydrophobic modification of alkali-activated slag (AAS) mortars aiming to reduce its high water sorptivity. The effects of the amount and viscosity of PDMS on the water absorption characteristics and mechanical properties of AAS mortars were investigated. Meanwhile, MIP, the surface contact angle of AAS mortars and SEM-EDS, Inverse gas chromatography (IGC), FT-IR, XRD of hardened AAS pastes were performed for analyzing the modification mechanisms. The results indicated that the water sorptivity of PDMS-modified AAS mortar decreased by 90 %, and the contact angle of the inner surface increased from 14° to 128° when the dosage of PDMS reached 8 % (by weight of binder). The main modification mechanisms of PDMS in AAS system can be summarized as: Firstly, the formation of hydrophobic coatings on the surface due to the cross-linking reaction between C-S-H gel and PDMS reduces the surface energy and the hydrophilicity of hardened AAS pastes, achieving chemical modification; Secondly, the hierarchical structures formed by micro papillary increases the surface roughness, thus enhancing the hydrophobic effect further through physical modification. Moreover, the coarsening of capillary pores also reduces the capillary force during water absorption process. Meanwhile, the addition of PMDS maintains comparable strength development of AAS mortars, which is beneficial for the practical application.

Effect of Modified Cow Dung Fibers on Strength and Autogenous Shrinkage of Alkali-Activated Slag Mortar.

Alkali-activated slag (AAS) presents a promising alternative to ordinary Portland cement due to its cost effectiveness, environmental friendliness, and satisfactory durability characteristics. In this paper, cow dung waste was recycled as a renewable natural cellulose fiber, modified with alkali, and then added to AAS mortar. The physico-chemical characteristics of raw and modified cow dung fibers were determined through Fourier transform infrared (FTIR), X-ray diffraction (XRD), and Scanning electron microscope (SEM). Investigations were conducted on the dispersion of cow dung fibers in the AAS matrix, as well as the flowability, strength, and autogenous shrinkage of AAS mortar with varying cow dung fiber contents. The results indicated that modified fiber has higher crystallinity and surface roughness. The ultrasonic method showed superior effectiveness compared to pre-mixing and after-mixing methods. Compared with raw cow dung fibers, modified fibers led to an increase of 11.3% and 36.3% of the 28 d flexural strength and compressive strength of the AAS mortar, respectively. The modified cow dung fibers had a more significant inhibition on autogenous shrinkage, and the addition of 2 wt% cow dung fibers reduced the 7 d autogenous shrinkage of the AAS paste by 52.8% due to the "internal curing effect." This study provides an alternative value-added recycling option for cow dung fibers as a potential environmentally friendly and sustainable reinforcing raw material for cementitious materials, which can be used to develop low autogenous shrinkage green composites.

Impact of magnesia and titania nanoparticles on characteristics of alkali-activated slag mortars exposed to sulfuric acid solution and elevated temperatures

Nanotechnology is being employed as potential solution to alleviate limitations of alkali-activated slag mortars (AASMs). The current study investigated the impact of prepared magnesia (NMg) and titania nanoparticles (NT) at various dosages (0, 1, 3 and 5 wt%) on mechanical and durability characteristics of AASMs. Specimens were either fully immersed in an aqueous solution of sulfuric acid (1 wt% concentration) or subjected to elevated temperatures (250, 500 and 750℃). Microstructural development and densification were analyzed by XRD, FTIR, and SEM with EDX technique to determine the primary improvement mechanism imparted as a result of employing nanoparticles to AASMs systems. Optimum performance prior to and after exposures was achieved for specimens containing 3% NMg and 1% NT in terms of compressive strength, water absorption for unexposed specimens, and weight loss and compressive strength retention for specimens exposed to sulfuric acid solution and elevated temperature. Nanoparticles produced more densified mixtures imparted by their ability to accelerate geopolymerization processes, filling effect and formation of voluminous compounds that further block available spaces in the microstructure.

Alkali-silica reaction of high-magnesium nickel slag fine aggregate in alkali-activated ground granulated blast-furnace slag mortar

The construction industry is presently encountering a significant challenge regarding the shortage supply of natural sands. The use of metallurgical slags as fine aggregates in building materials presents a solution that not only mitigates the over-exploitation of natural sands, but also provides a sustainable and valuable approach for managing industrial solid wastes instead of traditional landfill disposal. This study investigated the alkali-silica reaction (ASR) of air-cooled and water-quenched high-magnesium nickel slag (HMNS) fine aggregates in alkali-activated ground granulated blast-furnace slag (AAS) mortars. The mortars exhibited satisfactory volume stability and structural integrity throughout the 120 days of accelerated mortar bar test. The lab-simulated dissolution test revealed the formation of a thin precipitate layer of sodium silicate hydrate on the surface of HMNS fine aggregates, which contained a significant amount of Mg with Mg/Si ratios ranging from 0.16 to 0.18. The divalent Mg cations were released from the HMNS fine aggregates and served as the pseudo network former to crosslink the ASR gel instead of Ca. This reduced the moisture adsorption capacity and mitigated the expansion of ASR gel.