Ground Granulated Blast Furnace Slag Pastes Research Articles

A novel alkali-slag cemented tailings backfill: Recycling of soda residue and calcium carbide slag

In order to address the safety and environmental concerns associated with high disposal cost and low disposal rate associated with mine tailings, low-cost cemented paste backfills (CPBs) were prepared from solid wastes such as calcium carbide slag (CS), soda residue (SR), ground granulated blast-furnace slag (GGBS) and iron tailings (TA) in this study. The effects of mass concentration of filling slurry, GGBS content, activator dosage, and SR proportion on the mechanical properties of CPBs were studied. The results showed that a flow of 235.8 mm and 28-d compressive strength of 2.3 MPa for the freshly mixed slurry could be attained with 75 % mass concentration, 14 % GGBS content, an activator-to-binder ratio of 0.4, and a CS-to-SR ratio of 3:7. The strength of CPBs first increased and then decreased with the increasing of activator dosage or SR proportion contents, and reached the optimal value for the activator-to-binder ratio of 0.4 and SR of 70 % – 75 %. Microstructural analyses by TG-DTG, FTIR, SEM-EDS, and XRD revealed that mainly C-(A)-S-H gel, hydrotalcite (HT), and Friedel's salt (FS) were formed as the hydration products in CS-SR synergistically-activated GGBS paste. The Ca2+ dissolved from CS facilitated GGBS hydration to generate the C-(A)-S-H gels and further reacted with Cl− in SR and [Al(OH)6]3− dissolved from GGBS to generate FS. The combined effect of C-(A)-S-H gel and FS enhances the strength of CPBs. When the percentage of SR is in the range of 70–75 %, the synergistic excitation of slag by alkali slag and calcium carbide slag is superior, and the matrix structure is denser.

Effect of Phaeodactylum Tricornutum in Seawater on the Hydration of Blended Cement Pastes

Seawater can be used as mixing water for concrete with no steel reinforcement in some areas with difficult access to fresh water. Diatoms such as Phaeodactylum tricornutum are among the most abundant micro-organisms living in seawater, and they could be unavoidable when collecting seawater. In fact, diatoms can provide bio-SiO2 and bio-CaCO3 sources, namely amorphous nano-SiO2 and crystallised nano-CaCO3, which could be beneficial to cement hydration. Thus, the effects of different Phaeodactylum tricornutum concentrations (0%, 2.5% and 5% by weight of suspension of seawater and diatoms) in seawater on cement hydration in ordinary Portland cement (OPC) mixes (100% OPC) and ground granulated blast-furnace slag (GGBS) mixes (70% OPC + 30% GGBS) were investigated through tests of compressive strength, XRD, DTG–DTA and SEM. The results show that diatoms accelerated cement hydration by providing the nucleus for C-S-H structure and contributed pozzolanic reactions by amorphous nano-SiO2 and nano-CaCO3. The accelerated cement hydration was also confirmed by the fact that more Ca(OH)2 was formed in cement pastes with diatoms. However, it has also been found that diatoms decreased the compressive strength of cement pastes by leaving more weak bonds between the C-S-H structure, which was considered to be caused by the organic parts and the micron gap formed in diatoms. When comparing an OPC paste mix with 5% diatoms to a blank OPC paste, the reduction in compressive strength at 28 days can reach a maximum of 50.1%. The ability to provide bridging effects between C-S-H particles in GGBS paste was discovered to depend on the development of additional ettringite. This resulted in a 7.6% loss in compressive strength after 28 days in a GGBS paste with 5% diatoms.

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.

Influence of supplementary cementitious materials on factors controlling the fresh state of hydraulic binders

It is obvious that SCMs (supplementary cementitious materials) have diverse effects on the physical and chemical properties that can contribute in the development of the mechanical performance of cementitious materials. Moreover, the key factor to better use SCMs in blended cements is to control their fresh state properties. However, limited literature is available on the influence of SCMs on the rheological behavior of fresh pastes. In light of the increasing interest concerning the fresh state properties of hydraulic binders based SCMs, this contribution reports the rheological behavior of various hydraulic binders based GGBS (ground granulated blast-furnace slag), FA (fly ash), limestone or calcined clays, and explore the factors influencing the particle interactions during the structural build-up/breakdown process when stress is applied to the system.Yield stress and elastic modulus are considered to be the main indicators to quantify the structural changes of sheared fresh pastes. By introducing functionalized (–COO-) nanocellulose fibrils, the ability of water retention due to the hydrophilic character has reduced the mixing water in the fresh GGBS paste leading to the increase in the yield stress and viscosity. The use of limestone has increased the yield stress and elastic modulus of the cement paste. The angular shape of limestone particles and the filler effect can increase the friction and compact the cement suspensions leading to the loss of workability. However, the combination of limestone and FA can promote the fluidity of the paste due to the spherical shape of FA particles. On the other hand, when it is combined with calcined clays, limestone has a dilution effect leading to the reduction of the yield stress and elastic modulus of calcined clays blended cement at an early age while, the chemical equilibrium of limestone and calcined clays can provide strong structure network of the system and an increase in the yield stress and elastic modulus of the paste with time. However, calcined clays can reduce the mixing water in the slurry that can prevent the workability for extended durations.

Effect of Soda Residue Addition and Its Chemical Composition on Physical Properties and Hydration Products of Soda Residue-Activated Slag Cementitious Materials.

Soda residue (SR), the solid waste of Na2CO3 produced by ammonia soda process, pollutes water and soil, increasing environmental pressure. SR has high alkalinity, and its main components are Ca(OH)2, NaCl, CaCl2, CaSO4, and CaCO3, which accords with the requirements of being an alkali activator. The aim of this research is to investigate the best proportion of SR addition and the contribution of individual chemical components in SR to SR- activated ground granulated blast furnace slag (GGBS) cementitious materials. In this paper, GGBS pastes activated by SR, Ca(OH)2, Ca(OH)2 + NaCl, Ca(OH)2 + CaCl2, Ca(OH)2 + CaSO4, and Ca(OH)2 + CaCO3 were studied regarding setting time, compressive strength (1 d, 3 d, 7 d, 14 d, 28 d), hydration products, and microstructure. The results demonstrate that SR (24%)-activated GGBS pastes possess acceptable setting time and compressive strength (29.6 MPa, 28 d), and its hydration products are calcium silicate hydrate (CSH) gel, calcium aluminum silicate hydrates (CASH) gel and Friedel’s salt. CaCl2 in SR plays a main role in hydration products generation and high compressive strength of SR- activated GGBS pastes.

Effect of tartaric acid on the early hydration of NaOH-activated slag paste

This study investigated the influence of tartaric acid (TA) on the early hydration of NaOH-activated ground-granulated blast-furnace slag (GGBS) paste. Early hydration process of the paste with or without TA was characterized by the hydration heat using an isothermal calorimeter. In addition, inductively coupled plasma spectroscopy, X-ray diffraction, Fourier transform IR spectroscopy and scanning electron microscopy analyses were used as auxiliary tests. The results reveal that TA has a significant retarding effect on the hydration of NaOH-activated GGBS paste, finally leading to the initial setting time increasing with increasing TA. The complexation reaction of Al rather than Ca is the main reason for the observed retardation effect. The Al-complexed carboxylate salts formed by the complexation reaction of Al inhibit the depolymerization of the Si–Al phase, which leads to extend the time for SiO 3 2− and Al3+ to reach the saturation point in pore solution. And it subsequently delays the crystallization and nucleation of the main hydration products. However, TA has a little effect on the hydration products of NaOH-activated GGBS paste. The application of TA maintains the variety of main crystal products and the constant structure of gel products. The gel product still remains C–(A)–S–H with a primarily Q2 structure. Therefore, TA is an effective retarder for the NaOH-activated GGBS system.

Experimental study on the pH for activating ground granulated blast-furnace slag activity at different temperatures

In order to explore the initial pH values of activator solution needed for early activation of slag at different temperatures, the effects of curing temperatures (5, 20 and 35°C) and pH (12.10, 12.55, 13.02 and 13.58) on the activation and hydration characteristics of ground granulated blast-furnace slag (GGBFS) were investigated. Sodium hydroxide (NaOH) was used as the alkaline activator. The compressive strength and non-evaporable water content of GGBFS paste cured for 1, 3, 7, 14 days were determined. The hydration characteristics of slag pastes at different temperatures and pH were analyzed by XRD and SEM. The results showed that the pH value required for slag activation decreased with increasing temperature. Based on the 3 days strength of slag paste, the pH values required for activation of slag activity at 5, 20 and 35°C were 13.58, 13.02 and 12.10, respectively. With the high temperature and high pH of solution, a dense calcium-rich product formed on the surface of slag particles, which suppressed the further slag reaction. With the low temperature and high pH of solution, a layer of network product was found on the surface of the slag particles, and the later strength developed rapidly as the further hydration was not prevented. This study provides a reference for the application of alkali-activated slag.

Behavior of high performance concrete pastes with different mineral admixtures in simulated seawater environment

With the blooming of ocean construction engineering, the influence of seawater on concrete durability draws increasing attentions. This paper investigates the performances of five high performance concrete (HPC) pastes, including pure cement paste, fly ash (FA) paste, limestone powder (LP) paste, ground granulated blast-furnace slag (GGBS) paste and silica fume (SF) paste, soaked in simulated seawater. The results indicate that the addition of FA would improve the strength of paste soaked in simulated seawater compared to pure cement paste from 90 d to 360 d and the order of strength in 360 d was, FA paste > Pure cement paste > LP paste > GGBS paste > SF paste. All mineral admixtures except LP can inhibit the expansion of paste soaked in simulated seawater at later age and the order of expansion rate in 360 d was, LP paste > Pure cement paste > FA paste > GGBS paste > SF paste. The internal structure of pastes soaked in simulated seawater at 360 d mainly contains Ca(OH)2, AFt, gypsum, Mg(OH)2, C3S and C2S while the external structure of pastes mainly contains Mg(OH)2, CaCO3 and AFt through the XRD spectra. SEM results proved that the addition of mineral admixtures can make the structure of the pastes denser.

Composition and structure of an 18-year-old 5M KOH-activated ground granulated blast-furnace slag paste

The main hydration products were C-A-S-H(I), present in both inner (Ip) and outer product (Op), and a Mg-Al layered double hydroxide (LDH), present only in the Ip. The composition of C-A-S-H(I) was the same in Op and Ip. Reduced scatter in the data with age suggests a tendency towards compositional homogeneity. The mean length of the aluminosilicate anions in the C-A-S-H(I) increased with age. The layer spacings of the C-A-S-H(I) and Mg-Al LDH had not changed significantly with age. The Mg/Al ratio of the LDH was about 2.6 and had not changed between 1 and 18 years.

Self-hydration characteristics of ground granulated blast-furnace slag (GGBFS) by wet-grinding treatment

The aim of this paper is to evaluate the self-hydration characteristic of GGBFS activated by wet-grinding treatment (WGT). Three slag slurries with different fineness were prepared through milling with varying duration (20 min, 40 min and 60 min, abbreviated as S2, S4 and S6, respectively). The reaction kinetics, mechanical properties, hydration products and microstructure of hardened GGBFS pastes were analyzed to assess the effect of WGT on the self-hydration process of GGBFS. The results indicated that self-hydration was achieved after grinding without chemical activation and the rate of self-hydration was improved with the increased grinding duration. The setting times were dramatically shortened and the compressive strength was notably increased when prolonging the grinding time. The hydration process of GGBFS slurry after 20 min milling included incubation period, but the others did not. The electrical resistivity development of samples S2 and S4 decreased during the initial period, followed by setting, acceleration and deceleration stage. However, sample S6 exhibited an absence of the initial stage. The self-hydration products analyses indicated that the main products were calcium silicate hydrate, calcium aluminosilicate hydrate (C-A-S-H), hydrotalcite, akermanite and calcite.

Impact of chemical variability of ground granulated blast-furnace slag on the phase formation in alkali-activated slag pastes

The influence of ground granulated blast-furnace slag (GGBS) chemical variability on phase formation in sodium hydroxide-activated GGBS pastes has been investigated using X-ray total scattering and subsequent pair distribution function (PDF) analysis. Crystalline phase identification based on reciprocal space analysis reveals that despite large chemical variations in the neat GGBSs the secondary reaction products are quite similar, with the majority of pastes containing a hydrotalcite-like phase. However, PDF analysis reveals considerable differences in short range atomic ordering of the main calcium-sodium aluminosilicate hydrate (C-(N)-A-S-H) gel phase in the pastes. Quantitative analysis of these local structural differences in conjunction with published PDF data identifies the important role calcium plays in dictating the atomic structure of disordered silicate-rich phases in cementitious materials. This study serves as a crucial step forward in linking GGBS chemistry with phase formation in alkali-activated GGBS pastes, revealing key information on the local structure of highly-disordered cementitious materials.

Chloride binding ability and the onset corrosion threat on alkali-activated GGBFS and binary blend pastes

This paper presents an experimental study to investigate the chloride binding ability of alkali-activated ground granulated blast furnace slag (GGBFS) and binary blend pastes. Free chloride and total chloride were measured to assess the chloride binding ability. pH measurement was performed to obtain [Cl−/OH−] ratio to assess the onset corrosion threat. The results showed that bound chloride significantly increased in GGBFS pastes and it gradually increased with GGBFS content in the binary blends. GGBFS paste mixed with deionised water and binary blend pastes showed corrosion risks although their free chloride contents were significantly low. The results further demonstrated that Ordinary Portland cement may be a more appropriate option than NaOH for activating GGBFS in terms of corrosion resistance.

The effect of d-gluconic acid as a retarder of ground granulated blast-furnace slag pastes

Ground granulated blast-furnace slag (GGBFS) is a latent hydraulic binder, which has also the ability to hydrate autonomously after contact with water. The rate of hydration strongly depends on the composition of the slag and the pH-value of the mixing water or activator. The retarded GGBFS suspension and an alkali activator can be used in a special two-component binder system for variable applications. d-gluconic acid (C6H12O7) was found to be a suitable retarder for GGBFS, which retarded the hydration at least 28days. The retarder extends the dormant period of the hydration process and prevents mainly the formation of strength-giving phases. The interaction of GGBFS and retarder was investigated by heat flow calorimetry, measurement of non-purgeable organic carbon (NPOC), inductively coupled plasma optical emission spectrometry (ICP-OES), thermogravimetry (TG) and scanning electron microscopy (SEM). The basis for this study was a constant binder content, various concentrations of the retarder and different durations for the retarding effect. The concentrations were 0.05, 0.25 and 1.00% by weight of the mixing water. The samples were investigated between 0 and 28days. At the end, one mixture was optimized for a shelf time of at least 28days without separation.

Chloride binding capacity of hydrotalcite and the competition with carbonates in ground granulated blast furnace slag concrete

This paper further investigates the hypothesis and recent preliminary findings that hydrotalcite is responsible for the remarkable improvement of chloride binding in concretes containing ground granulated blast furnace slag (GGBFS). In this paper, the chloride binding capacity of hydrotalcite has been examined in a wide range of GGBFS and GGBFS-OPC (ordinary Portland cement) blends. X-ray diffraction (XRD) was conducted to identify hydrotalcite in GGBFS pastes and to distinguish it from Friedel’s salt. Using Rietica software for quantifying crystalline minerals, this paper presents quantified XRD analysis of the relative proportion of hydrotalcite formed in GGBFS and binary blend pastes. XRD revealed that hydrotalcite was formed in all pastes containing GGBFS. The results clearly demonstrated that hydrotalcite made up a significant proportion of the crystalline phase in GGBFS pastes. The results further showed that the ability of hydrotalcite to bind chlorides was not significantly impaired by the competitive adsorption of carbonates.

Strength and hydration properties of reactive MgO-activated ground granulated blastfurnace slag paste

Ground granulated blastfurnace slag (GGBS) is widely used as a partial replacement for Portland cement or as the major component in the alkali-activated cement to give a clinker-free binder. In this study, reactive MgO is investigated as a potentially more practical and greener alternative as a GGBS activator. This paper focuses on of the hydration of GGBS, activated by two commercial reactive MgOs, with contents ranging from 2.5% to 20% up to 90days. The hydration kinetics and products of MgO–GGBS blends were investigated by selective dissolution, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy techniques. It was found that reactive MgO was more effective than hydrated lime in activating the GGBS based on unconfined compressive strength and the efficiency increased with the reactivity and the content of the MgO. It is hence proposed that reactive MgO has the potential to serve as an effective and economical activator for GGBS.

Mechanical and hydration properties of ground granulated blastfurnace slag pastes activated with MgO–CaO mixtures

Since alkali-activated slag using conventional activators suffers from economical and technical problems, other alternative activators should be explored. This paper reports the results of an investigation into the activation of ground granulated blastfurnace slag by using 10% (by weight) reactive MgO, CaO and their mixtures with various ratios. The mechanical and hydration properties of pastes were examined up to 90days. It was found that the strength of slag pastes activated with MgO–CaO mixtures decreased with the increasing ratio of MgO to CaO in the early age while a much steeper strength gain was observed in pastes with MgO/CaO higher than 19/1 after 28days and longer. The addition of small amount of CaO in MgO can greatly accelerate the hydration of slag in the early age by increasing the pH of pore solution. However, pastes showed small difference in strength development at each period when MgO/CaO was less than 1. The main hydration products, analysed by XRD, TGA and SEM/EDS, were C–S–H and hydrotalcite-like phases. While CaO accelerated the formation of C–S–H in the early age, MgO induced more amount of hydrotalcite-like phases, which notably enhanced the strength of slag pastes with high MgO content after 28days and longer. Calcite, portlandite and residual MgO were also observed, depending on the MgO/CaO ratio and the hydration time. This work indicated that the replacement of MgO by CaO can make the application of reactive MgO in slag activation more economical.

Effects of Different Reactive MgOs on the Hydration of MgO-Activated GGBS Paste

Reactive MgO has recently emerged as a potential activator for ground-granulated blast-furnace slag (GGBS), which is one of the most widely used by-products in the cement industry. However, it is known that the characteristics of reactive MgO vary significantly, which may affect the activation process and hence the performance of MgO-GGBS blends. In this study, seven commercially available reactive MgOs, whose characteristics vary widely, were chosen to activate GGBS. The unconfined compressive strength (UCS) of MgO-GGBS pastes up to 90 days was measured, and the hydration products were studied by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The primary hydration products were identified as calcium silicate hydrate (C-S-H) and hydrotalcitelike phases (Ht). Minor hydration products included magnesium silicate hydrate (M-S-H) and ettringite. The reactivity and calcium oxide content of the MgO samples were found to be the two major factors affecting the hydration process of the MgO-GGBS blend. Higher reactivity did not change the hydration products, but resulted in more hydration products in the same time. Sufficient CaO content increased the pH of the system, which enhanced the slag dissolution degree and was beneficial to the strength development.

The effect of elevated temperature on bond performance of alkali-activated GGBFS paste

The main reaction products were investigated by analysis of microstructure of alkali-activated ground granulated blast furnace slag (GGBFS) paste. An experimental research was performed on bond performance of alkali-activated GGBFS paste as a construction adhesive after exposure to 20–500 °C. Through XRD analysis, a few calcium silicate hydrate, hydrotalcite and tetracalcium aluminate hydrate were determined as end products, and they were filled and packed each other at room temperature. In addition, akermanite dramatically increased at 800 °C and above. The two key parameters, the ultimate load Pu.T and effective bond length Le, were determined using test data of carbon fiber-reinforced polymer (CFRP)-to-concrete bonded joints at elevated temperature. The experimental results indicate that the ultimate load Pu.T remains relatively stable initially and then decreases with increasing temperature. The effective bond length Le increases with increasing temperature except at 300 °C. The proposed temperature-dependent effective bond length formula is shown to closely represent the test data.

Effect of simulative pore solution on the hydration kinetics of GGBFS

To characterise the hydraulic activity of ground granulated blast-furnace slag (GGBFS) in a low alkaline solution, the hydration degree, heat evolution and hydration products of GGBFS–water (de-ionised water) paste, GGBFS–5%CaO–water paste and GGBFS–5%CaO–simulative pore solution (SPS) pastes were investigated. The Ca(OH)2 content and composition of pore solution of GGBFS pastes were also determined. The experimental results show that the dissolution rate of the vitreous phase of GGBFS increases significantly with increases in alkalinity of the SPS. The hydraulic activity of GGBFS is relatively low and hydration of GGBFS mainly takes place slowly after 7 days in GGBFS–5%CaO–water paste. In contrast, the hydration rate increases dramatically in the first 3 days. Meanwhile, large amounts of hydration products (mainly C–S–H gel and C2ASH8) are generated, indicating the enhancement of hydraulic activity of GGBFS even in low alkaline solution.

EFFECT OF ALKALINE SOLUTIONS ON ENGINEERING PROPERTIES OF ALKALI-ACTIVATED GGBFS PASTE

In this study, two kinds of alkaline solutions, sodium hydroxide (NaOH) and sodium silicate (Na2SiO3), with three dosages of 0.5, 1.0 and 1.5% by weight were used to produce the alkali-activated ground granulated blast furnace slag (GGBFS) paste at three different liquid-solid ratios by weight of 0.45, 0.5 and 0.55. Most engineering properties of the paste were examined at age of 3, 7, 14, 28 and 56 days. Experimental results show that a mixed alkaline solution of sodium hydroxide and sodium silicate is necessary to produce an alkaliactivated GGBFS paste with adequate workability and strength. Both the liquid-solid ratio and the amount of the alkaline solution affect the compressive strength in the range of 30.38 to 166.22 MPa at age of 28 days. After curing 28 days, alkali-activated GGBFS paste has the coefficient of thermal conductivity and volumetric heat capacity in range of 0.585-0.791 W/m⋅K and 1.583-1.8 J/m3 ⋅K, respectively, which are lower than/close to those of normal weight concrete and thus can be used as a proper material for heat insulation. The microstructural porosity of alkali-activated GGBFS paste is closely related to its engineering and thermal properties.