Slag Pastes Research Articles

The impacts of calcium acetate on reaction process, mechanical strength and microstructure of ordinary Portland cement paste and alkali-activated cementitious paste

The gentle corrosion of acetic acid is frequently utilized to modify recycled concrete aggregates for improving concrete performance, but there is a lack of research on the effect of its process product, calcium acetate (CA), on the properties of cementitious materials. The reason for this work is to discover the role of CA in ordinary Portland cement, alkali-activated low-calcium and high-calcium cementitious materials, and to investigate its possibility as chemical admixture. The impacts of CA on the reaction process, compressive strength, and microstructure of diverse cementitious materials were mainly studied. The results appeared that the impacts of CA on diverse cement are different. The performance of ordinary Portland cement paste depended on the content of calcium acetate. The content of 0.25 wt% CA can increase the reaction degree of cement, improve strength and microstructure of cement paste. In alkali-activated low-calcium fly ash paste, the positive impact of Ca2+ dissolved from calcium acetate was obvious. With the increment of CA content, their strength improvement and microstructure characteristics were optimized. The reaction process of alkali-activated high-calcium slag paste was hindered by calcium acetate, which resulted in the deterioration of strength, the increase of porosity and pore fractal dimension, as well as the relatively uncompacted microstructure. Based on the results, calcium acetate can be used as an admixture, but the content needs to be controlled according to the characteristics of cementitious materials.

Mechanical properties and mechanisms of alkali-activated slag paste reinforced by graphene oxide-SiO2 composite

Alkali-activated slag (AAS) binders-based composites are considered potential alternatives to conventional cement-based composites. However, the quasi-brittle mechanical behavior of AAS binders-based composites limits their widespread application. In this study, a graphene oxide-SiO2 (GO-SiO2) composite was produced to address this issue. The experimental results showed that the flexural strength of an AAS paste increased by 133.2% with 1.5 wt% GO-SiO2 composite, and its corresponding flexural-to-compressive strength ratio increased 95% i.e. from 0.063 to 0.123. To explore the strengthening mechanism, the stability of the GO-SiO2 composite in alkaline solutions and the dispersion effect of GO-SiO2 composite on the mechanical properties of AAS paste were investigated. The experimental results showed that the prepared GO-SiO2 composite had stable C–O–Si chemical bonds connected GO with hydration products, thereby improving the strength of the AAS paste. The molecular dynamics study revealed the mechanism that the high-dispersed GO-SiO2 composite could improve the mechanical properties of the AAS paste dramatically.

Effects of anionic species of activators on the rheological properties and early gel characteristics of alkali-activated slag paste

The effects of anionic species of Na2O·2SiO2-NaOH-Na2CO3 ternary activator on the rheology of alkali-activated slag (AAS) pastes are studied to obtain quantitative relationships between composition of activator and rheological parameters. The interstitial fluid chemistry, especially silicate species, OH− and CO32− ions, controls the precipitation of early gels and structural build-up by affecting the competition between dissolution and polymerization kinetics. Abundant Al3+, Ca2+ and Na+ in the interstitial solution participate into silicate oligomer structures, reducing polymerization degree of silicate species. This results in reduced concentration of Si-O-Si structure and increased Si-O-NB, while the later precipitates to form gel nano particles which are attractive, leading to yield stress-spurt, great flowability loss and rapid setting. The mixture activated by the blended of Na2O·2SiO2 and Na2CO3 tends to form C-A-S-H gel with lower Ca/Si due to the competitive effect of CO32−. This study provides essential information towards better control fresh properties of AAS mixtures.

Investigating the performance of dry chemically modified steel slag powder and pastes by formic acid

Investigating the performance of dry chemically modified steel slag powder and pastes by formic acid

Engineering and micro-properties of alkali-activated slag pastes with Bayer red mud

• Red mud alleviates the flash setting phenomenon of AASR pastes. • Red mud exerted a positive effect on the compressive strengths of AASR pastes. • A proper amount of red mud enabled promoting alkali-activation of slag. • RM-donated alkali promoted the gel products formation of AASR pastes. In this study, we produced alkali-activated slag/red mud (AASR) pastes using sodium silicate as the alkali activator. The effects of Bayer red mud (RM) on the engineering and micro-properties of alkali-activated slag pastes were investigated. It was found that the decreased fluidity and the increased setting time of the fresh AASR pastes were achieved through the delayed gel coagulation of RM. The maximum compressive strength of the hardened AASR paste arrived at 89.93 MPa with an optimal proportion (i.e., R40S60). Furthermore, the increases in hydration heat peaks of AASR pastes were observed in some mixes (e.g., R20S80 and R40S60), which verified the appropriate amounts of RM enabled promoting the alkali-activated reaction of slag. Besides, the polymerization and hydration products (C-(N)-A-S-H gels and ettringite) of AASR pastes were elaborated through spectroscopic and micro-morphologic analyses.

Deep insight into mechanical behavior and microstructure mechanism of quicklime-activated ground granulated blast-furnace slag pastes

Ground granulated blast-furnace slag (GGBS) could be used as a potential low-carbon binder when activated by a moderate activator. The use of seawater in producing construction materials has increasingly attracted attention in some coastal engineering with scarce freshwater. This study aims at studying the impact of quicklime proportions on the mechanical behavior and microstructure of the binder. The mechanical behaviors were investigated through unconfined compression tests. The hydration kinetics and microstructure characteristics were examined using XRD, TGA, FTIR, and SEM-EDX. The results indicated that the hydration heat and exothermic peaks enhanced with the quicklime proportion. The exothermic peaks were observed in the first few minutes and 4–18 h. The quicklime-GGBS pastes generated the highest UCS (>18 MPa) occurred at a quicklime proportion of 0.15. The platy-like portlandite, fibrous calcium silicate hydrate (CSH), needle-like ettringite (AFt), and Friedel's salt (Fs) were detected in XRD and SEM. The Cl− in seawater promoted the formation of Fs. The XRD peaks of CSH and AFt/Fs raised and then descended with the quicklime proportion, and reached the maximum at the quicklime content of 0.15. The mass percentage of CSH and AFt/Fs from TGA first increased and then became stable when the quicklime content was 0.1–0.15. The number of amorphous GGBS particles and pores decreased under the quicklime proportion of 0.05–0.15, while the portlandite increased as the quicklime content changed from 0.15 to 0.3. Overall, the quicklime-GGBS binder could provide a potential sustainable construction material in waste recycling and freshwater saving.

Relationship between rheological property and early age-microstructure building up of alkali-activated slag

This paper investigated the effects of temperature and activator concentration on the rheological property of slag activated by ternary activator of Na2O·2SiO2–NaOH–Na2CO3. New insight on the time-dependence features of microstructure building up of alkali-activated slag (AAS) paste was revealed via multiple characterization techniques. Three featured periods are distinguished in the first 3 h reaction. The time-dependent rheological behavior of AAS is largely controlled by the formation of gel network, where significant amount of free water is trapped in the voids or interlayers. By 1H NMR analysis, it is confirmed that a small peak of immovable water always lags behind the peak value of mobile water, indicating that the early gel agglomerates trap and adsorb more mobile water after an intensive polycondensation reaction. Furthermore, similar Al/Si and Ca/Si ratios of early gels are detected, while slightly higher Al/Si ratio is found in the gel agglomerates with higher crosslinking degree. This confirms the important role of Al, that is more Al increasing the polymerization degree of early gels, leading to stronger interactions between nano gel particles, and thereby trigging the rapid development of elasticity and stiffness of the initial matrix.

Structural build-up and breakdown of sodium hydroxide-activated slag paste assessed using oscillatory rheology

Concentrated suspensions of inorganic binders are thixotropic fluids with non-Newtonian behaviour. In addition to the physical-chemical interactions responsible for the flocculation of the particles, the rheological behaviour in time is affected by the formation of the reaction products. In this paper, this topic was tackled using oscillatory rheology, namely, by increasingly used measurements at the constant strain amplitude and frequency over time (time sweep test), by the combination of these tests with an inserted rotational step (ORO test), and by almost unexplored oscillation amplitude sweep tests, leading to structural breakdown due to increasing strain amplitude. Common sodium hydroxide-activated slag paste was used for this purpose. The results showed that the recovery of the structure is not significantly altered if the sample is remixed at least after 30 minutes of reaction. In addition, the strain sweep tests suggested that the formation of the first reaction products started after about 15 minutes of reaction and had a decisive impact on the evolution of viscoelastic moduli and the corresponding stresses during this kind of test.

Effect of Carbonation on Microstructure Evolution of Alkali-Activated Slag Pastes

In order to reveal the carbonation mechanism of alkali-activated concrete, the accelerated carbonation tests based on alkali-activated slag pastes were carried out. The evolution of microstructure and chemical composition for alkali-activated slag pastes subjected to carbonation was analyzed combining thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP) and a recently developed extended X-ray attenuation method (XRAM). The results showed that, the microstructure of alkali-activated slag pastes deteriorated gradually. Based on MIP and XRAM, the porosity of S4 (sample with a water-binder ratio of 0.4) increased by 8.24% and 11% after carbonation, and that of S6 (sample with a water-binder ratio of 0.6) increased by 7.45% and 10%, respectively. Besides, thermal analysis showed that, after carbonation, 11.45 mol / L and 19.57 mol /L CaCO3 were produced separately by S4 and S6. The main carbonation product for S6 was calcite, but for S4 vaterite and disorderly stacked calcite were also presented.

Early age reaction, rheological properties and pore solution chemistry of NaOH-activated slag mixtures

In this study, the rheology and early reaction process of sodium hydroxide-activated ground granulated blast furnace slag (GGBFS) were investigated. Test results showed a strong relationship between the Vicat initial setting time and ultrasonic pulse wave for NaOH activated slag mixtures identifying the characteristic points or inflection points in the ultrasonic curves and reaching a specific value in the ultrasonic velocity. The early reaction process of the sodium hydroxide-activated slag pastes determined by ultrasonic pulse wave and calorimeter consisted of three stages: dissolution, acceleration/condensation and deceleration stages. The yield stress and apparent viscosity of the pastes increased by the addition of NaOH, and the pastes started to show shear-thickening behavior when NaOH concentration reached 8 M. It was also revealed that Si, Al and Ca ion concentrations in the pore solution increased with an increase in NaOH concentration, and the pore solution of the pastes was dominated by Na and OH−. No significant influence of NaOH concentrations upon 4 M on the compressive strength of the mortar samples was observed.

The effect of sodium citrate on NaOH-activated BFS cement: Hydration, mechanical property, and micro/nanostructure

This paper presents a study on the effect of sodium citrate solution (0.25M, 0.5M, 0.75M) on the hydration process and microstructure evolution of NaOH-activated blast furnace slag (BFS) paste. The setting behavior, flowability, mechanical strength, hydration kinetics, and micro/nanostructure characteristics were investigated through multiple tests—such as Vicat test, mini-slump test, x-ray diffraction, spectroscopy analysis, nuclear magnetic resonance, and mercury intrusion porosimetry—to reveal the hydration mechanism of NaOH-activated BFS paste in presence of sodium citrate. The results indicate that the citrate ions retarded the hydration process and contributed significantly to slag dissolution and precipitation of hydration products during the alkali-activated reaction process. The microstructure build-up from the early stage (3 days) to the mature stage (28 days) underwent a different process, resulting in a more densified and less porous matrix with higher compressive strength. However, no significant changes were observed in the type of hydration products and their nanostructural characteristics.

Evaluating the role of feldspar on the resistivity of alkali-activated slag towards seawater attack.

In this study, the effect of K-feldspar on the behavior of alkali-activated slag pastes (AAS) before and after exposure to seawater was evaluated by measuring the compressive strength and the microstructure. To achieve this objective, the mixtures were prepared upon partial replacement of ground granulated-blast furnace slag (GGBFS) with calcined and un-calcined feldspar fine particles (termed as FS and CFS) at levels ranging from 5 to 20%, by mass. Under processing conditions, the solidified specimens were immersed for 1, 7, 14, and 28 days in 100% RH at 40 ± 2 °C. After 28 days, a group of samples having the same composition were soaked in seawater solution for 1, 3, and 6 months. The new phases were evaluated using X-ray diffraction (XRD), thermogravimetric analysis (TGA/DTG), and scanning electron microscopy (SEM). The inclusion of FS and/or CFS in AAS enhances the performance of the pastes in normal and harsh conditions in relation to their reference counterparts. Among all the mixtures that were tested, alkali-activated composite comprising 15% CFS indicated more durable properties in terms of increased compressive strength and dense morphological structure after exposure to an aggressive environment.

Influence of steel slag fineness on the hydration of cement-steel slag composite pastes

Steel slag has the potential to be used as a mineral admixture, but its wide utilization is restricted by the low activity. Grinding may increase the slag's hydration activity. However, little is known about the role of superfine steel slag in the composite cement paste. In this work, the hydration, microstructure, and macroscopic performances of the samples containing ground steel slags were investigated. The results show that most pastes containing superfine steel slag have longer setting time. In addition, the compressive strengths of most mortars decrease, rather than increase, when they contain superfine steel slags. The further investigation reveals that with the fineness increasing, the steel slag paste generates more hydration heat and higher content of Ca(OH)2, while the composite paste shows reverse trends. It indicates that although the finer steel slags themselves present higher reactivity, they inhibit the hydration of cement in composite pastes. This inhibition plays a dominant role in most samples, while the filling effect and the increased slag activity seem insignificant in the composite pastes.

Understanding the workability of alkali-activated phosphorus slag pastes: Effects of alkali dose and silicate modulus on early-age hydration reactions

Understanding the workability of alkali-activated phosphorus slag (AAPS) pastes is fundamental for their efficient application. This study revealed the novel mutual inhibition mechanism through which the alkali dose and silicate modulus affect the workability of AAPS pastes based on the hydration reaction. The slump loss of NaOH-activated phosphorus slag (NAPS) paste was slow, and that of waterglass-activated phosphorus slag (WAPS) paste was faster because the soluble Si in waterglass accelerated the dissolution of the phosphorus slag (PS). At a moderate waterglass modulus, the slump losses of the WAPS pastes were slow because the moderate [Al]/{[Ca]+[Mg]} ratio in the pore solution minimized the difference in the magnitude of mutual inhibition between Ca, Mg and Al, causing all hydrates to form slowly. However, at higher and lower waterglass moduli, the WAPS pastes exhibited fast slump losses because hydrates with excess elements formed rapidly due to the excessively high or low [Al]/{[Ca]+[Mg]} ratio.

Chemical and Microstructural Properties of Fly Ash and Fly Ash/Slag Activated by Waste Glass-Derived Sodium Silicate

Sodium silicate is commonly used for activating alumina silicates to produce alkali-activated binders that can compete with conventional Portland cement in concrete. However, the cost and emissions related to activators can hinder the use of alkali-activated materials in the industry. The novel, waste-based activators have been developed in the last years, using Si-rich waste streams. Processing waste glass cullet not only reduces the glass landfill disposal but also allows the production of sodium silicate for alkali activation. In this article, the chemical and microstructural properties of neat fly ash and blended 60 fly ash/40 slag pastes activated by sodium silicate produced from glass cullet were studied and compared to equivalent ones activated by commercially available sodium silicate and sodium hydroxide solutions. Fourier transform infrared (FTIR) spectroscopy, X-ray powder diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX) were used to determine the microstructure and composition of the gel phase. Findings have confirmed that pastes activated by the processed waste glass showed chemical and microstructural properties comparable to pastes produced with commercially available activators.

Effects of sodium chloride on the mechanical strength of alkali activated volcanic ash and slag pastes under room and elevated temperatures

This study investigated effects of the sodium chloride on the microstructural and mechanical properties of alkali activated volcanic ash (VA) and ground granulated blast furnace slag as well as Portland cement pastes. To this end, unconfined compressive strength (UCS), scanning electron microscopy (SEM)-EDS-Mapping, FTIR, and XRD tests were conducted. Furthermore, effects of curing temperature on the binding capacity of chloride in alkali activated cements were examined in both elevated (HT) and room (RT) temperature conditions. The VA was replaced by slag at 0, 50, and 100 wt%. Based on the results, samples containing 100 wt% slag showed the highest mechanical strength in both curing conditions. Besides, addition of sodium chloride from 0 to 10 wt% did not significantly affect the strength of samples containing 100 wt% volcanic ash in both curing conditions. On the other hand, in HT condition, mechanical strength of samples containing 50 and 100 wt% slag, as well as Portland cement pastes increased with increasing sodium chloride from 0 to 2.5 wt%, and further addition of sodium chloride by up to 10 wt% led to a reduction in their strength. However, compressive strength of samples containing 50 and 100 wt% slag, as well as Portland cement samples, decreased with the addition of sodium chloride from 0 to 10 wt% in RT condition. Microstructural investigations were conducted, aiming to find the mechanism controlling the reactions. It was found that (N,C)-A-S-H and C-S-H gels were the dominant factor in the solidification and encapsulation of chloride ions in slag-based samples.

Electrochemical performance of low-alloy steel and low-carbon steel immersed in the simulated pore solutions of alkali-activated slag/steel slag pastes in the presence of chlorides

The corrosion behaviour of different steels immersed in the simulated pore solutions was investigated using electrochemical measurements, XPS, and SEM/EDS. The results showed that the incorporation of granulated blast furnace slag increased the content of HS− which inhibited the passivation through forming the Fe-S complex. The presence of HS− increased the film thickness but the corrosion resistance was still low due to the porous structure of Fe-S complex on the surface. Severe active corrosion occurred on low-carbon steels in the presence of chlorides while scanty pitting was observed for low-alloy steels attributed to the existence of Cr inside pits.

Influence of the type and concentration of the activator on the microstructure of alkali activated SiMn slag pastes

The influence of the type and concentration of the activator on the composition of the products formed and the microstructural development of alkali activated SiMn slag pastes and their evolution with curing time are studied. About 15–17 million tons of SiMn slag is produced per year, so valorising this residue is interesting for the economic and environmental scope. For this purpose, NaOH and WG with different concentrations of Na2O have been used as activators. The hydration products have been studied by X-ray diffraction analysis (XRD) and thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDX) has been used to study the microstructure and elemental composition of hydrated phases. The advance of the degree of hydration with time and with the activator concentration is consistent with the mechanical behaviour of these pastes observed in previous studies. All the pastes show high microcracking due to the stresses caused by the shrinkage, as it has been observed in previous studies. Elemental composition data do not show significant differences regarding the type of activator with respect to the main elements and their relationship in the reaction products except for Mn. The main reaction product formed in the alkaline activation of SiMn slag is C-S-H with a low Ca/Si ratio, this being higher for pastes activated with NaOH compared to those with WG. In addition, the modification of the C-S-H structure is demonstrated by the presence of small amounts of Al and Mn and that Mg is not incorporated into it. The presence of hydrotalcite is not detected by XRD, but small amounts have been confirmed by TGA/DTG and SEM/EDX.

Relaxation characteristics and state evolution of water during the early-age reaction of alkali-activated slag as monitored by low field nuclear magnetic resonance

Low field nuclear magnetic resonance (LF-NMR) was used to monitor the evolution characteristics of transverse relaxation time (T2) of the water in sodium hydroxide (SH) and sodium silicate (SS) activated slag pastes. According to the T2 weighted average value evolution curve, the early-age reaction process of SH-activated system could be divided into three typical stages: initial setting stage (I), acceleration stage (II) and deceleration stage (III). However, another two distinguishable stages including the second acceleration stage (Ⅳ) and the second deceleration stage (Ⅴ) could be identified in SS-activated system. The essential reactions in different reaction stages of the two types of alkali-activated pastes were discussed. The obtained relaxation signals were employed to determine the chemically bound water content in different alkali-activated slag (AAS) pastes at specific reaction stages, as well as the dynamic change of the relative contents of gel water, capillary water and free water with reaction time. The influence mechanism of the initial active SiO44− oligomer in activator solution on the early-age reaction of AAS was revealed. A distinct water release behavior was observed in the geopolymerization process at the acceleration stages of each AAS paste.

Effect of Predrying Temperature on Carbonation of Alkali-Activated Slag Pastes

Effect of Predrying Temperature on Carbonation of Alkali-Activated Slag Pastes