Influence of activator type on hydration kinetics, hydrate assemblage and microstructural development of alkali activated blast-furnace slags

The hydration of two slags with different Al₂O₃ contents activated with sodium hydroxide and hydrous sodium metasilicate (commonly named water glass) is studied using a multi-method approach. In all systems, C-S-H incorporating aluminium and a hydrotalcite-like phase with Mg/Al ratio ~ 2 are the main hydration products. The C-S-H gels present in NaOH activated pastes are more crystalline and contain less water; a calcium silicate hydrate (C-S-H) and a sodium rich C-N-S-H with a similar Ca content are observed at longer hydration times. The activation using NaOH results in high early strength, but strength at 7 days and longer is lower than for the sodium metasilicate systems. The drastic difference in C-S-H structure leads to a coarser capillary porosity and to lower compressive strength for the NaOH activated than for the sodium metasilicate activated slags at the same degree of slag reaction.     

Influence of nucleation seeding on the hydration kinetics and compressive strength of alkali activated slag paste

Addition of pure calcium silicate hydrate (C–S–H) to alkali-activated slag (AAS) paste resulted in an earlier and larger hydration rate peak measured with isothermal calorimetry and a much higher compressive strength after 1 d of curing. This is attributed to a nucleation seeding effect, as was previously established for Portland cement and tricalcium silicate pastes. The acceleration of AAS hydration by seeding indicates that the early hydration rate is controlled by nucleation and growth. For the experiments reported here, the effect of C–S–H seed on the strength development of AAS paste between 1 d and 14 d of curing depended strongly on the curing method. With sealed curing the strength continued to increase, but with underwater curing the strength decreased due to cracking. This cracking is attributed to differential stresses arising from chemical and autogenous shrinkage. Similar experiments were also performed on Portland cement paste.     

Properties and durability of alkali-activated slag pastes immersed in sea water

This paper represents the experimental trials to activate blast-furnace slag to produce cementless binding materials. The aims of the work is to study the properties of activated slag mixed with sodium hydroxide and sodium silicate liquid 6 wt% of granulated slag. Also, studying the effect of mixing water (tap and sea water) on the kinetic of activation. The rate of activation of the alkali activated slag (AAS) has been studied by FTIR, TGA, DTG and SEM techniques. The results revealed that the increase of NaOH content and mixing with sea water increase the combined water up to 90 days. On the other hand, the bulk density and compressive strength was increased by increasing Na₂SiO₃ content in presence of NaOH. The activated granulated slag showed good durability in sea water, i.e., the compressive strength increased gradually with immersing time up to 12 months. Whereas, the strength of sulfate resisting cement (SRC) pastes immersed in sea water increases up to 6 months then decreases up to one year. Therefore, it can be concluded that alkali activated slag are more durable in sea water than SRC pastes.     

Structural role of titanium on slag properties

The presence of titanium in ground granulated blast-furnace slags (GGBS) has been suspected to modify cement properties. This study provides the first evidence of a relation between the TiO₂ content of slags and the mechanical properties of mortars based on slag cements. It is observed that only the slags containing less than 1%TiO₂ show a compressive strength at 28 days that remains within the 52.5 MPa norm with CEM III cements complying with the European Standard NF EN 197-1. The structural origin of this chemical dependence of the performance of cements is investigated by determining directly the titanium speciation in various European slags by spectroscopic methods. Electron paramagnetic resonance indicates that about 76% of Ti in slag occurs as Ti⁴⁺. The atomic structure around Ti was determined by Ti K-edge X-ray absorption near edge structure, which shows that Ti is mainly five-fold coordinated in square-based pyramid geometry. Five coordinated Ti acts as network-stabilizer of the silicate network as it increases the polymerization. Requiring Ca²⁺ for charge-compensation of the titanyl bond, it reduces the availability of Ca²⁺ during glass alteration in a modified random model of glass structure, where Ca²⁺ atoms are clustered in percolating cationic domains. As a consequence, the presence of five-coordinated Ti results in a slower dissolution of the slag. These peculiar structural properties of titanium may explain the detrimental role of Ti above a 1% concentration, for many physical and chemical slag properties. This work provides a scientific ground for the technological acceptability of the upper limit of the Ti-content of GGBS.     

Effect of substitution of granulated slag by air-cooled slag on the properties of alkali activated slag

This article assesses the mechanical and durability performance of replacement of GBFS by ACS activated by 3:3 NaOH:Na₂SiO₃ (3:3 SH:SSL) wt% (at optimum value 6 wt%) mixed with sea water (SW) and cured at 100% R.H. at room temperature. The kinetic behavior of activated GBFS-ACS mixes was measured by determination of setting time, combined water, bulk density and compressive strength up to 90 days. The rate of activation of the AAS has been studied from some selected samples by FT-IR, TGA, DTG analysis and SEM techniques. The compressive strength of dried activated GBFS-ACS pastes in comparison with saturated GBFS-ACS pastes up to 90 days was determined. The results revealed that the blended pastes of 80% GBFS+20% ACS gives the higher combined water, bulk density and compressive strength than those of 40/60 and 60/40% GBFS/ACS and lower than the 100% GBFS up to 90 days. Also, the compressive strength of dried samples at 105 °C for 24 h activated by (3:3 SH:SSL) mixed with SW and cured in 100% R.H. at room temperature up to 90 days is greater than saturated samples cured at the same conditions. On increasing the amount of ACS up to 40%, the setting time decreases then increases at 60% but still shorter than 100% GBFS. Finally, ACS can be used as partial substitution of GBFS in AAS.     

Effect of TiO2 and 11 minor elements on the reactivity of ground-granulated blast-furnace slag in blended cements

Ground-granulated blast-furnace slags (GGBS) are glasses (>99%) of the CaO-Al₂O₃-SiO₂ compositional system and are widely used as supplementary cementitious materials. Differences in reactivity of GGBS were screened by modifying the content of 12 minor elements (namely Ba, Ce, Cs, Cr, K, Mn, P, Sn, Sr, Ti, V, and Zr). Scanning electron microscopy observations showed that most elements entered the silicate glass matrix, only Sn was reduced to its metallic form and P accumulated in minor minerals. Mortar strength tests showed that 2 day compressive strength was reduced by >50% for a TiO₂ content of 2.5 wt% in the slag. At 28 days the loss in compressive strength was still >40%. Calorimetric tests on other element additions showed that the content of network modifiers (Ba, Cs, K and Sr) and GGBS reactivity are positively correlated, whereas Ce, Cr, V, and Zr significantly decreased reactivity. Finally, it is shown that these effects can be estimated by the concentration and the weighted field strength of the added element.     

Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part II: Effect of Al2O3

The hydration and microstructural evolution of three alkali activated slags (AAS) with Al₂O₃ contents between 7 and 17% wt.% have been investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na₂SiO₃·5H₂O. The formation of C(A)–S–H and hydrotalcite was observed in all samples by X-ray diffraction, thermal analysis and scanning electron microscopy. Higher Al₂O₃ content of the slag decreased the Mg/Al ratio of hydrotalcite, increased the Al incorporation in the C(A)-S-H and led to the formation of strätlingite. Increasing Al₂O₃ content of the slag slowed down the early hydration and a lower compressive strength during the first days was observed. At 28 days and longer, no significant effects of slag Al₂O₃ content on the degree of hydration, the volume of the hydrates, the coarse porosity or on the compressive strengths were observed.     

Properties and hydration of blended cements with steelmaking slag

The present research study investigates the properties and hydration of blended cements with steelmaking slag, a by-product of the conversion process of iron to steel. For this purpose, a reference sample and three cements containing up to 45% w/w steel slag were tested. The steel slag fraction used was the “0-5 mm”, due to its high content in calcium silicate phases. Initial and final setting time, standard consistency, flow of normal mortar, autoclave expansion and compressive strength at 2, 7, 28 and 90 days were measured. The hydrated products were identified by X-ray diffraction while the non-evaporable water was determined by TGA. The microstructure of the hardened cement pastes and their morphological characteristics were examined by scanning electron microscopy. It is concluded that slag can be used in the production of composite cements of the strength classes 42.5 and 32.5 of EN 197-1. In addition, the slag cements present satisfactory physical properties. The steel slag slows down the hydration of the blended cements, due to the morphology of contained C₂S and its low content in calcium silicates.     

Influence of mineralogy on the hydraulic properties of ladle slag

The present study is aimed at investigating the hydraulic characteristics of ladle furnace slag (LFS), under the pretence of using LFS as a cement substitute in certain applications. Furthermore, LFS has been considered as a possible activator in a blend containing 50% LFS, and 50% ground granulated blast furnace slag (GGBFS). Phases detected in LFS were quantified using Rietveld analysis. Calorimetric studies were performed at 20, 25 and 30 °C in order to calculate the apparent activation energy of hydration and thereby to suggest a kinetic model for the tested compositions within this temperature interval. In addition, compressive strength tests were performed on mortar prisms made with LFS, and LFS/GGBFS which had hydrated for 2, 7 and 28 days. Both compositions reached acceptable early strengths, (e.g. LFS, 33.1 MPa, and LFS/GGBFS, 17.9 MPa, after 2 days), but after 28 days hydration the blend was superior to neat LFS. Related apparent activation energies were determined using an Avrami–Erofeev model and gave E_a = 58 kJ/mol for neat LFS and E_a = 63 kJ/mol for the blend. The results imply that LFS or a LFS/GGBFS blend can be favourably used as supplement in binder applications such as binder in by-product metallurgical briquettes, which are used as recycle to the blast furnace or basic oxygen furnace depending on the specific briquette composition.     

The role of alumina on performance of alkali-activated slag paste exposed to 50 °C

The strength and microstructural evolution of two alkali-activated slags, with distinct alumina content, exposed to 50 °C have been investigated. These two slags are ground-granulated blast furnace slag (containing 13% (wt.) alumina) and phosphorous slag (containing 3% (wt.) alumina). They were hydrated in the presence of a combination of sodium hydroxide and sodium silicate solution at different ratios. The microstructure of the resultant slag pastes was assessed by X-ray diffraction, differential thermogravimetric analysis, and scanning electron microscopy. The results obtained from these techniques reveal the presence of hexagonal hydrates: CAH₁₀ and C₄AH₁₃ in all alkali-activated ground-granulated blast-furnace slag pastes (AAGBS). These hydrates are not observed in pastes formed by alkali-activated ground phosphorous slag (AAGPS). Upon exposure to 50 °C, the aforementioned hydration products of AAGBS pastes convert to C₃AH₆, leading to a rapid deterioration in the strength of the paste. In contrast, no strength loss was detected in AAGPS pastes following exposure to 50 °C.     

A new steel slag for cement manufacture: Mineralogy and hydraulicity

This study presents a method of developing useful hydraulic properties in Basic Oxygen Steel Slags, by the addition of a synthetic slag forming flux during the normal steel making process. The transformation of a waste slag into a by-product at least equivalent to the best quality Blast Furnace Slags is achieved without change of technology or sacrifice in steel quality. The flux contains CaO - Al₂O₃ - MgO - Fe₂O₃ in proportions calculated from phase diagrams. Theoretical predictions are confirmed by laboratory experiments and by semi-industrial trials and show that the introduction of alumina in this manner leads to the formation of hydraulic calcium-alumino-ferrites and homogeneous slags containing < 4% free lime. The mineralogy of the slags is described and the results of calorimetry, X-ray diffraction, optical microscopy, electron microprobe analysis, and strength tests on non quenched ground slags are given to demonstrate hydraulicity.     

Effect of blended inorganic salts containing negative hydration ions on hydration and properties of alkali-activated slag cement

Water glass (WG) is generally considered to be the most effective activator to prepare alkali-activated slag (AAS) cement in terms of strength and durability. However, the rapid setting and hardening of WG activated slag results in rapid loss of fluidity of AAS concrete mixture, which limits its engineering application. In the paper, the effect of blended inorganic salts containing negative hydration ions on the fluidity, setting time and mechanical strength of AAS cement was studied. The hydration process and hydration products were used to explore the action mechanism. Ba(NO₃)₂ greatly delayed the hydration of AAS cement. The four inorganic salts (KCl, KNO₃, KBr and NaCl) blended with a small amount of Ba(NO₃)₂ can improve both the initial fluidity and fluidity retention, and a wide setting time range can be obtained to meet engineering requirements. The compressive strength decreased with the increase of inorganic salts. The incorporation of inorganic salt did not change the composition of the main hydration products. Considering the fluidity required by construction, mechanical properties and the durability of structure, it is recommended to add 4%–5% KBr or KNO₃ blended with no more than 0.2% Ba(NO₃)₂ into AAS cement.     

Effect of mechanical activation on the hydraulic properties of stainless steel slags

This work aims to assess the possibility of using ladle metallurgy and argon oxygen decarburization stainless steel slag as a hydraulic binder after mechanical activation. Prolonged milling in ethanol suspension resulted in 10-fold increase of the surface area and increase of the amorphous phase. Calorimetric analysis of slags mixed with water indicated the occurrence of exothermic reactions. XRD results revealed that periclase, merwinite, γ-C₂S and bredigite, decreased with hydration time. Thermogravimetric analyses indicated that the main hydration products are most probably C–S–H, CH and MH. The hydrated products in both slags were similar to C–S–H gel. WDS analysis demonstrated Ca and Si to be widespread in the structure. Formation of M–S–H gel or incorporation of Mg in the C–S–H gel remains uncertain. The 90 days compressive strength of mortars prepared from slags reached approximately 20% for LM and 10% for AOD of the compressive strength of mortars prepared from OPC.     

Study on modification of the high-strength slag cement material

The influence of the slag powder's fineness, the amounts of activator, type and contents of modification addition on the dry-shrinkage and strength of the high-strength slag cement material was investigated. The experimental data showed that adding 9% Na₂SiO₃ activator and 10% Portland cement (PC) made the ratios of drying-shrinkage of high-strength slag cement material similar to the ratios of Portland cement and the compressive strengths as higher. The main hydration products are calcium alumina-silicate gels and a little CH; the gel ratio of CaO/SiO₂ is close to 1 and includes a little Na₂O and MgO for high-strength slag cement material, as shown by means of scanning electron microscope (SEM) and energy-dispersive X-ray analyzer (EDXA).     

Effect of binder content on the performance of alkali-activated slag concretes

This paper assesses the mechanical and durability performance of concretes produced using alkali silicate-activated ground granulated blast furnace slag as sole binder. Alkali-activated concretes are formulated with 300, 400 and 500 kg slag per m³ of fresh concrete, and their performance is compared with reference concretes produced using Portland cement (OPCC). Regardless of the binder content, the alkali-activated slag concretes (AASC) develop higher compressive strength than the comparable reference concretes. A higher binder content leads to increased strength in both AASC and OPCC at 28 days. However, at 90 days, the performance penalty for low binder content is more significant in the OPCC than AASC samples. Permeability, water sorption and carbonation resistance properties are also improved at higher binder contents. By controlling mix design parameters, it is possible to produce AASC with mechanical strength and durability comparable to conventional Portland cement concretes.     

Evolution of strength, microstructure and mineralogical composition of a CKD–GGBFS binder

Characterization of a nontraditional binding material containing cement kiln dust (CKD) and ground granulated blast furnace slag (GGBFS) is discussed in this paper. Significant compressive strength was obtained for a CKD–GGBFS blend with 70% CKD and 30% GGBFS at a water-to-binder ratio of 0.40 after 2 days of curing at elevated temperature. Similar strength was also obtained for the samples subjected to normal moisture curing over a period of 28 days. The compressive strength increased with additional moist curing in both the cases. The microstructural and the mineralogical examinations show that the strength development was mainly due to the formation of calcium silicate hydrate (C-S-H). In addition to normal C-S-H, aluminum and magnesium incorporated C-S-H phases were also present in the CKD–GGBFS blends. The formation of ettringite appears to be a contributing factor in early age strength development of CKD–GGBFS binder.     

The gasification reactivity of unburned carbon present in gasification slag from entrained-flow gasifier

CO₂-gasification reactivity of unburned carbon in both coarse and fine slags was studied in a pressurized thermogravimetric analyzer (TGA) and compared with a char obtained from a drop tube furnace (DTF) at 1400 °C from the same original coal. Results show that the reactivity of the unburned carbon in the coarse slag is always higher than that in the fine slag, around 1.11 to 1.88 times. The DTF char is less reactive than the unburned carbon in both fine and coarse slags. In order to understand the effects of minerals on the CO₂-gasification of unburned carbon, reactivity of the unburned carbon in demineralized slag was also investigated. Results show that the minerals in the coarse slag catalyze the CO₂-gasification of unburned carbon, but that in the fine slag inhibit the CO₂-gasification. An elemental analyzer was adopted to analyze C, H, N and S in the original coal, the DTF char and the slags. The main reasons for the differences in the gasification reactivity of unburned carbon in the slags are the morphology of slags, the degree of graphitization of unburned carbon and the components of crystalline phase, which have been investigated by SEM/EDX and XRD. It is found that a higher ordering of carbon layers and a lower content of catalytic components are the main reasons for the lower reactivity of the unburned carbon in the fine slag compared to that in the coarse slag.     

The reaction between the magnesia–chrome brick and the molten slag of MgO–Al2O3–SiO2–CaO–FetO and the resulting microstructure

The reaction and microstructure at the interface of MgO–Cr₂O₃ brick and the molten slag of MgO–Al₂O₃–SiO₂–CaO–Fe_tO after static slag corrosion at 1823–1923 K for various times and the resulting microstructure were investigated and characterized. After the static slag corrosion at 1923 K for 4 h, the XRD results show the major phases of periclase MgO, MgCr₂O₄ spinel, and CaMgSiO₄ as the minor phase. MgCr₂O₄ phase causes MgO to form a discontinuous phase in MgO–Cr₂O₄ brick. After static slag corrosion at 1923 K for 4 h, SEM micrographs show that brick interior cracks, MgO and dissolved MgO. MgO dissolved due to the molten plag penetrated into the brick interior and reaction with it, leading to a localized dissolution of brick slag. TEM micrographs and ED patterns demonstrate that the minor phase of (Mg, Fe)(Al, Cr)₂O₄ precipitates in the MgCr₂O₄ matrix.     

Degradation of MgO–C refractories by MnO-rich stainless steel slags

In order to identify the influence of MnO on the wear rate of MgO–C bricks for the production of high Mn stainless steel, MgO–C refractory samples were exposed to slags containing different MnO levels (up to 26 wt.%). Although the investigations of the worn brick microstructures revealed the presence of numerous Mn-rich metal particles and the formation of a (Mg,Mn)O solid solution at the slag/refractory interface, no clear evidence of wear rate enhancement was observed due to high MnO concentration in the slags. With respect to refractory wear, the MgO content in the slag is the dominant factor. The degradation processes are discussed by combining experimental results and thermodynamic calculations.     

Strengthening effect and mechanism of titanium extraction slag on the mechanical property of calcium aluminate cement

In heavy oil recovery, calcium aluminate cement (CAC) is in the working environment of “low-temperature hardening and ultrahigh temperature service.” However, the formation of C₃AH₆ under low temperatures results in a decrease in strength and reduce the cementing quality. In this study, titanium extraction slag (TES) was used to inhibit CAC strength deterioration. TES, characterized by a high Ti content, presents challenges in terms of utilization and poses significant ecological risks owing to its large accumulation. Cementite hydration with 0%, 20%, 30%, and 40% TES relative to CAC was examined at 30 °C for 28 d. The high C₃AH₆ content of the pure CAC increased the strength deterioration, pore size, and cementite carbonation. With 20% TES, a dilution effect was observed without strength improvement. Furthermore, 30% TES generated layered double hydroxides and converted C–S–H into C–A–S–H, thereby increasing compressive strength. By-products were generated with 40% TES, which inhibited the strength development while generating C–A–S–H to maintain the compressive strength. Therefore, TES can inhibit the strength decline of CAC, and the byproducts of the LDH structure can improve corrosion resistance.