And Alkali-activated Slag Research Articles

The driving force of water absorption in cementitious materials: An analysis of surface-free energies and pore structure

Upon the initial contact of hydrophilic cementitious materials with water, capillary absorption becomes the dominant phenomenon leading to durability deterioration of structures. The water absorption characteristics of such materials are governed by the interplay between pore structure and surface-free energies, with the latter being a factor often overlooked. In this study, advanced inverse gas chromatography (IGC) measurement techniques were employed to investigate the one-dimensional capillary water absorption of ordinary Portland cement (OPC) and alkali-activated slag (AAS). Results revealed water sorptivity values for OPC, AAS(NaOH), and AAS(Waterglass) as 0.0112, 0.0137, and 0.0187 mm/s0.5, respectively. This phenomenon can be attributed to the higher total surface-free energies observed at the liquid-solid interface in AAS materials, measuring 100.10 and 119.56 mJ/m2, compared with 74.25 mJ/m2 for OPC, consequently increasing capillary pressures. Moreover, it is observed that OPC exhibited a more complex pore tortuosity in comparison to AAS. This study underscores the dual role played by both capillary pressures and pore tortuosity in determining the water absorption characteristics of cementitious materials.

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.

Revealing the difference between creep behavior of hydration products of Portland cement and alkali-activated slag paste at early age

This study investigates the early-age creep behavior of various gels in Portland cement (PC) and alkali-activated slag (AAS) pastes. The objective is to reveal the correlation between the composition and creep behavior of hydration products in these two systems. The results indicate that the creep modulus of C-A-S-H in AAS is significantly lower than that of outer products (Op) and inner products (Ip) in PC paste, even though Op exhibits higher elastic modulus than C-A-S-H. The intermixed CH has a more pronounced effect on the creep modulus of Op compared to its elastic modulus, which should be mainly attributed to the pinning effect of CH during the deformation of Op during the creep process. The lack of enhancement by high stiffness crystal phases in the matrix of AAS leads to the relatively low creep modulus of C-A-S-H.

The Effect of Structural Stability of Chemical Admixtures on the NaOH Alkali-Activated Slag Properties

The current study aims to construct a synthetic polymer (or family of polymers) with superior plasticizing properties in a highly alkaline medium. An invented laboratory-prepared superplasticizer (SP), namely phenol-formaldehyde sulfonilate (PFS), was synthesized, and its performance was measured against two types of commercial superplasticizers [naphthalene-based (Nb) and polycarboxylate-based (PCb)]. Fourier transform infrared (FTIR) and gel permeation chromatography (GPC) tests were performed to verify the admixture’s stability in a highly alkaline medium. The interaction between different SPs and alkali-activated slag (AAS) surface was investigated via zeta potential and adsorption measurements. The flowability, setting time, and compressive strength developments were evaluated for AAS mixes. The hydration products’ morphologies and phase compositions were studied using scanning electron microscope (SEM) and X-ray diffraction (XRD). The results reveal that the prepared PFS SP has a high stability against the highly alkaline medium. The dispersing efficiency of PFS SP resulted from the highly negative zeta potential value (−49.8 mV) compared with Nb (−41.5 mV) and PCb (−15.8 mV), as well as its high adsorption percentage as PFS, Nb, and PCb adsorbed by 18.5%, 9.2%, and 3.7%, respectively. The PFS SP enhanced the physicomechanical properties for AAS because it combined several advantages at the same time compared to commercial superplasticizers; for instance, AAS pastes admixed with PFS SP have superior workability, acceptable setting time, and high early strength.

Mechanical Performance and Durability of Mortar Based on Slag Cement and NaOH-Activated Slag

This paper investigates the valorization of slag in cement production in order to obtain a sustainable mortar and participate in protecting the environment. The study evaluated the setting time, hydration heat, mechanical strengths, drying shrinkage, sulfuric acid and sulfate attack of mortars. These mortars are based on Portland cement (PC), slag cements containing 10%, 30% and 50% slag and alkali-activated slag (AAS) using 6% and 9% of sodium hydroxide (NaOH). The results show that the increase in slag replacement rate increases the setting time accompanied by a drop in initial mechanical strength such that the compressive strength decreased by 30% at two days for a 50% slag substitution; also, it considerably reduces the shrinkage and hydration heat. The resistance to sulfate and sulfuric acid attack increases with the slag replacement rate. NaOH-activated slag mortar is the most resistant binder to sulfate attack and sulfuric acid, but it develops a lower mechanical strength and a more significant shrinkage than PC mortar. X-ray diffraction (XRD) analysis carried out on binder paste shows the formation of the same main hydration products in PC and slag cement with a small amount of portlandite in the last binder. Calcium silicate hydrate (CSH) and Hydrotalcite are the main hydration products of AAS.

Mineralogical and microstructural changes in alkali-activated and hybrid materials exposed to accelerated leaching

Alkali-activated materials (AAM) and hybrid cements (HC) have become sustainable alternatives to Portland cement (PC) due to their low carbon footprint. The main differences between AAM and HC lie in their content of clinker (none in the AAM and usually lower than 30% for HCs) and the type of activator used (strong alkaline solutions for AAM and small amounts of solid alkalis for HC). Durability problems related with microstructural changes due to decalcification and leaching of the cementitious paste have been well researched for PC pastes, but it is still not well known for AAM and HC. The present work aims to study the leaching process for cement pastes of both types of sustainable pastes.Blast furnace slag (BFS) was selected as a precursor to manufacture hybrid slag (HS) pastes and alkali-activated slag (AAS) pastes. A commercial CEM IV was selected as reference material. A 6 M NH4NO3 solution was used to accelerate leaching kinetics. After 28 days of immersion, the mineralogical and microstructural changes were evaluated.Results show that AAS pastes exhibited the highest leaching resistance of all the pastes under study, due to the absence of portlandite and the high level of polymerization of silicate chains. In HS pastes, the presence of portlandite (due to PC in the material) and gypsum (due to the activator) explains their intermediate performance, in between CEM IV and AAS.

The Volume Stability of Alkali-Activated Electric Arc Furnace Ladle Slag Mortar and Its Performance at High Temperatures

In this study, the engineering properties of Ordinary Portland Cement (OPC) and alkali-activated slag (AAS) mortar with electric arc furnace ladle slag (EAFLS) were investigated to reveal the effects of EAFLS on the expansion of cementitious mortars. Additionally, the effects of these two types of mortar were explored based on their compressive strength, especially at high temperatures. EAFLS in OPC mortars significantly reduced the compressive strength and caused serious soundness problems in the mortars after autoclaving due to the presence of free-CaO and free-MgO in the EAFLS slag. On the other hand, the AAS mortars produced with EAFLS had compressive strength comparable to ordinary OPC mortars and maintained soundness after autoclaving. During a 550 °C heat treatment, the OPC mortar cracked and lost residual strength, but the AAS mortar retained more than 90% of its residual strength. Even after an 800 °C heat treatment, the AAS mortar maintained 14% of its residual strength (about 4 MPa), sufficient to prevent the collapse of the specimen structure. The main reason is that alkali-activated technology can accelerate the hydration process and solve the delayed hydration problem. The results of this study indicated that EAFLS is suitable to partially replace the binder used in the production of AAS mortars, and the resulting AAS mortars have high volume stability, high compression strength, and good high temperature resistance.

Design, dynamic performance and ecological efficiency of fiber-reinforced mortars with different binder systems: Ordinary Portland cement, limestone calcined clay cement and alkali-activated slag

The dynamic mechanical properties and ecological efficiency of cement-based composites are of vital importance to the development of green building materials and dynamic loadings resistance. In this study, the dynamic mechanical properties, ecological and economic efficiency of fiber-reinforced mortars (FRMs) made with different binder systems, including ordinary Portland cement (OPC), limestone calcined clay cement (LC3), and alkali-activated slag (AAS), were compared. In addition, different rheological parameters were designed to evaluate the dynamic mechanical properties of FRMs. The fiber pull-out testing, Barrett-Joyner-Halenda (BJH) testing, and scanning electron microscope (SEM) were used to reveal the difference in the dynamic mechanical properties of FRMs made with different binder systems. The results showed that the loss factor of FRM made with LC3 was the highest, 70% and 150% higher than that made with OPC and AAS as the plastic viscosity of mixtures was at the same range. In addition, the loss factor of FRMs made with OPC and LC3 improved by 25% and 8.6% respectively as the plastic viscosity was improved to a higher level and caused more uniform fiber distribution. The fiber pull-out and microstructure testing indicated that FRM made with LC3 showed appropriate pore size distributions and good fiber-matrix interfacial properties. The ecological evaluation and cost analysis showed that the EE, ECO2e, and the cost of unit loss factor of FRM made with LC3 were the lowest among three kinds of mortars, showing 48369.1 MJ/m3, 6894.4 kgCO2/m3, and 20288.0 RMB/m3 respectively. Compared with FRM made with OPC, the above parameters were reduced by 55.8%, 66.9% and 46.0% respectively. Therefore, the FRM made with LC3 shows higher potential for designing green building materials with great dynamic loadings resistance, compared with FRMs made with OPC and AAS.

Glass powder as a partial precursor in Portland cement and alkali-activated slag mortar: A comprehensive comparative study

Prior studies were conducted in an isolated manner for the use of glass powder (GP) either as ordinary Portland cement (OPC) replacement material in blended cement or as a precursor in AAM material. There was no reported literature that made a direct comparison on the properties of blended cement and AAM containing GP with an equivalent mix composition. The present study was aimed to evaluate and compare the effects of GP as a partial precursor on properties of both OPC and alkali-activated slag (AAS) mortars. For this purpose, cement and ground granulated blast furnace slag (GGBFS) was partially replaced with 10%, 20%, 30%, and 40% (by weight) GP in OPC and AAS mortars, respectively. The mortar mixes were prepared for a fixed water-to-solid (W/S) ratio to make them equivalent. The evaluation was carried out on the workability, mechanical strength, water absorption, alkali-silica reaction (ASR) response and ecological analysis of the mortar. According to the test results, the flow of the fresh mortar increased monotonically with GP content, and AAS mortars showed a higher flow value than OPC mortars. Based on the mechanical properties, the optimum GP content in OPC and AAS mortars was found to be 20% and 30% by the total weight of the binder, respectively. In general, the inclusion of GP was more effective in enhancing the strength properties of AAS mortars as compared to OPC counterparts. Furthermore, the use of GP led to a systematic increase in the water absorption of mixes due to moisture instability of the sodium silicate gel produced by GP activation. According to the results of SEM analysis, AAS mortars showed a higher degree of micro-cracking, which resulted in higher water absorption and a lower flexural strength as compared to OPC mortars. Also, the incorporation of GP densified the pore system of the paste. The potential of ASR was evaluated via the accelerated mortar bar test. The OPC mortars showed up to a 55% reduction in the ASR expansion at 40% GP content. The AAS mortars, on the other hand, showed shrinkage during the first days and then started to show expansion with increasing GP content, which was beneficial as it compensated the shrinkage. The ecological analysis of mixes revealed that the carbon footprint of AAS mortars was about 60% lower than that of OPC counterpart mixes, and the use of GP reduced the CO2 emissions and energy demand of mixes.

Using in-situ observation to understand the leaching behavior of Portland cement and alkali-activated slag pastes

This paper presents the leaching behavior of hydration products and unhydrated particles in Portland cement (PC) and alkali-activated slag (AAS) systems with a new in-situ observation method. After leaching in NH4Cl solution, obvious decalcification can be found in unhydrated C3S and C2S in PC, while the unhydrated slag keeps stable in AAS. The Ca in the gel of AAS is more vulnerable to leaching than that in PC due to the lack of phases with buffering capability like portlandite. In addition, the Mg–Al layered double hydroxide, which mainly exists in the dark rim of AAS, is not stable in NH4Cl solution. The in-situ observation method proposed in this study provides a new methodology to investigate the leaching behavior of different phases directly and helps to understand the effect of leaching on cementitious materials from a new perspective, it can therefore be used to instruct the design of durable cementitious systems.

Alkali-silica reactivity of alkali-activated concrete subjected to ASTM C 1293 and 1567 alkali-silica reactivity tests

This paper presents an experimental investigation of the alkali-silica reactivity of alkali-activated concrete (AAC). Some researchers suggest that the high alkalinity of the pore solution could make AAC more susceptible to alkali-silica reaction (ASR) than comparable ordinary Portland cement (OPC) concrete, perhaps even with aggregates that are normally considered non-reactive. Many have further questioned the suitability of standardized ASR test methods for use with AAC. In an attempt to resolve this controversy, the authors subjected OPC, alkali-activated fly ash (AAF), and alkali-activated slag (AAS) concrete made with both reactive and non-reactive aggregates to accelerated and long-term ASR test protocols (ASTM C1567 and C1293). Paradoxically, some AAS mixtures with non-reactive aggregates showed significant expansion while AAF mixtures with reactive aggregates expanded minimally. AAS mixtures with reactive aggregates underwent extremely high expansions. Microstructural evaluation using scanning electron microscopy revealed significant cracking but did not identify ASR gel in the majority of specimens.

Investigation of engineering properties of normal and high strength fly ash based geopolymer and alkali-activated slag concrete compared to ordinary Portland cement concrete

Fly ash-based geopolymer (FAGP) and alkali-activated slag (AAS) concrete are produced by mixing alkaline solutions with aluminosilicate materials. As the FAGP and AAS concrete are free of Portland cement, they have a low carbon footprint and consume low energy during the production process. This paper compares the engineering properties of normal strength and high strength FAGP and AAS concrete with OPC concrete. The engineering properties considered in this study included workability, dry density, ultrasonic pulse velocity (UPV), compressive strength, indirect tensile strength, flexural strength, direct tensile strength, and stress-strain behaviour in compression and direct tension. Microstructural observations using scanning electronic microscopy (SEM) are also presented. It was found that the dry density and UPV of FAGP and AAS concrete were lower than those of OPC concrete of similar compressive strength. The tensile strength of FAGP and AAS concrete was comparable to the tensile strength of OPC concrete when the compressive strength of the concrete was about 35 MPa (normal strength concrete). However, the tensile strength of FAGP and AAS concrete was higher than the tensile strength of OPC concrete when the compressive strength of concrete was about 65 MPa (high strength concrete). The modulus of elasticity of FAGP and AAS concrete in compression and direct tension was lower than the modulus of elasticity of OPC concrete of similar compressive strength. The SEM results indicated that the microstructures of FAGP and AAS concrete were more compact and homogeneous than the microstructures of OPC concrete at 7 days, but less compact and homogeneous than the microstructures of OPC concrete at 28 days for the concrete of similar compressive strength.

Electrochemical behaviour of a novel alloy steel in alkali-activated slag mortars

The present paper aims to investigate the passivation capability and accelerated chloride-induced corrosion behaviour of a novel alloy steel (00Cr10MoV) and a conventional low-carbon steel (20MnSiV) in ordinary Portland cement (OPC) and alkali-activated slag (AAS) mortars. Both steels were embedded in mortars with intact mill scale. Compared with OPC mortar, AAS mortar resulted in the formation of less protective passive film for both 20MnSiV and 00Cr10MoV steels after passivation due to the presence of reducing sulphides. Despite this, the initial negative effect of AAS mortar on 20MnSiV steel can be well compensated by its denser interfacial microstructure after the occurrence of active corrosion induced by chlorides. As for 00Cr10MoV steel in AAS mortar, however, this compensating effect was less pronounced. Moreover, unexpected low passivation capability and corrosion resistance can be confirmed for 00Cr10MoV steel in both OPC and AAS mortars due to the presence of defective and Cr-depleted mill scale.

Use of basic oxygen furnace slag fines in the production of cementitious mortars and the effects on mortar expansion

The purpose of this study was to examine the properties of ordinary Portland cement (OPC) and alkali-activated slag (AAS) mortars produced with basic oxygen furnace (BOF) slag fines, and to reveal the effects of BOF slag fines on the expansion of the cementitious mortars. The use of BOF slag fines in OPC mortars significantly reduced the compressive strength and slightly increased the volume expansion of the mortars. Significant changes in mineralogical composition were observed in the OPC mortars produced with BOF slag fines after autoclaving. On the other hand, the AAS mortars produced with BOF slag fines had compressive strength comparable to normal OPC mortars, and their volume expansion was much lower than the AAS-Control and all the OPC mortars. The mineralogical compositions of the AAC mortars before and after autoclaving were almost identical. The results of this study indicate that the BOF slag fines are suitable to partially replace the binder for the production of AAS mortars, and the resulting AAS mortars have high volume stability.

Electrochemical performance of reinforcing steel in alkali-activated slag extract in the presence of chlorides

This study concerns the passivation behaviour and corrosion resistance of reinforcing steels in ordinary Portland cement (OPC) extract and alkali-activated slag (AAS) extract with chlorides. The electrochemical behaviour of steels in both extracts was monitored by various electrochemical measurements. Compared with OPC extract, less protective passive film was formed for steels in AAS extract without chlorides. However, much higher corrosion resistance was highlighted for steels exposed to AAS extract than OPC extract after the addition of 1 M NaCl. This was mainly attributed to the formation of gel-like substances in AAS extract, effectively suppressing the propagation of corrosion pits.

Pitting corrosion resistance of a novel duplex alloy steel in alkali-activated slag extract in the presence of chloride ions

In this study, two types of reinforcing steels (conventional low-carbon steel and a novel duplex alloy steel with Cr and Mo) were exposed to chloride-contaminated extract solutions (ordinary Portland cement (OPC) extract and alkali-activated slag (AAS) extract) to investigate their pitting corrosion resistance. The results confirm that the pitting corrosion resistance of the alloy steel is much higher than that of the low-carbon steel in both extract solutions with various NaCl concentrations. Moreover, for each type of steel, the AAS extract contributes to a higher pitting corrosion resistance compared with the OPC extract in the presence of chloride ions, likely because of the formation of flocculent precipitates on the steel surface.

Passivation and chloride-induced corrosion of a duplex alloy steel in alkali-activated slag extract solutions

In this study, the passivation and chloride-induced corrosion behavior of a novel duplex alloy steel (00Cr10MoV) and low-carbon (LC) steel in ordinary Portland cement (OPC) extract solution and alkali-activated slag (AAS) extract solution were evaluated by electrochemical measurements and corrosion morphology observation. Seawater corrosion-resistant 00Cr10MoV steel with the addition of alloying elements Cr and Mo was designed to be used in concrete structures exposed to the tidal zone. The results show that 00Cr10MoV steel exhibits significantly higher corrosion resistance than LC steel in OPC extract with chlorides due to the formation of a more protective passive film. However, the high corrosion resistance of 00Cr10MoV steel is less pronounced in AAS extract with chlorides. Accordingly, this duplex alloy steel is not recommended to be utilized in combination with AAS materials in marine environments.

Alkali activated materials vs geopolymers: Role of boron as an eco-friendly replacement

In this paper, the influence of boron ions on the strength development and microstructure variations of fly ash-based geopolymers and alkali-activated slag (AAS) was investigated through utilizing borax-based alkali-activators. In addition to cost-effective aspects, the substitution of boron ions in geopolymer and AAS matrixes meant to minimize the downside of the silicate compounds on the environment. The binders were produced by introducing borax to alkali-activator solutions. Granulated ground blast furnace slag (GGBS) and Class F fly ash were used to produce AAS and geopolymer specimens respectively. Results showed a decline in the strength of geopolymer binder as a result of the increase in the amount of borax. However, the formation of B-O bonds in geopolymer matrices prevented an unexpected behaviour of the strength derived from an imbalance in the alkali activator solution. Substitutions of more than 60% of sodium silicate in the fly ash-based geopolymer was not reasonable. The compensatory role of boron in the strength of AAS was undeniable where even introducing more than 30% of borax led to a slight growth in the AAS strength. It was concluded that borax-based binders could be potential candidates for high volume substitution of sodium silicate in both geopolymers and AAS.

Effect of internal sulfate attack on the properties of sulfate-resisting cement and alkali-activated slag

This work aims at studying the effect of internal sulfate attack on the properties of sulfate-resisting cement (SRC) and alkali-activated slag (AAS). Blast-furnace slag was activated by 3% sodium hydroxide (NaOH) and 3% sodium silicate (Na2SiO3) by weight of slag. Magnesium sulfate (MgSO4) solution with concentration of 0, 4, 12, and 20 wt.% was mixed with AAS and SRC at water to cement ratio (W/C) of .25. The results showed that the AAS setting times decrease with MgSO4%. As the MgSO4% increases up to 12%, the SRC setting times are elongated, and then shortened at 20%. The compressive strength of SRC decreases with the increase of MgSO4%. In contrast, the AAS compressive strength increases with MgSO4% up to 12%, then decreases at 20%. Drying shrinkage reduction and increment were observed when SRC and AAS were mixed with MgSO4. These results were confirmed by X-ray diffraction, Fourier transformer infrared spectroscopy, and thermogravimetric (TG/DTG) analyses. The results proved that, the AAS can be beneficially used in cement concrete containing high sulfate concentration with better performance than SRC.

Mechanical Properties and Eco-Efficiency of Steel Fiber Reinforced Alkali-Activated Slag Concrete.

Conventional concrete production that uses ordinary Portland cement (OPC) as a binder seems unsustainable due to its high energy consumption, natural resource exhaustion and huge carbon dioxide (CO2) emissions. To transform the conventional process of concrete production to a more sustainable process, the replacement of high energy-consumptive PC with new binders such as fly ash and alkali-activated slag (AAS) from available industrial by-products has been recognized as an alternative. This paper investigates the effect of curing conditions and steel fiber inclusion on the compressive and flexural performance of AAS concrete with a specified compressive strength of 40 MPa to evaluate the feasibility of AAS concrete as an alternative to normal concrete for CO2 emission reduction in the concrete industry. Their performances are compared with reference concrete produced using OPC. The eco-efficiency of AAS use for concrete production was also evaluated by binder intensity and CO2 intensity based on the test results and literature data. Test results show that it is possible to produce AAS concrete with compressive and flexural performances comparable to conventional concrete. Wet-curing and steel fiber inclusion improve the mechanical performance of AAS concrete. Also, the utilization of AAS as a sustainable binder can lead to significant CO2 emissions reduction and resources and energy conservation in the concrete industry.