Controlling the reaction kinetics of sodium carbonate-activated slag cements using calcined layered double hydroxides

In this study, Na₂CO₃-activated slag cements were produced from four different blast furnace slags, each blended with a calcined layered double hydroxide (CLDH) derived from thermally treated hydrotalcite. The aim was to expedite the reaction kinetics of these cements, which would otherwise react and harden very slowly. The inclusion of CLDH in these Na₂CO₃-activated cements accelerates the reaction, and promotes hardening within 24 h. The MgO content of the slag also defines the reaction kinetics, associated with the formation of hydrotalcite-type LDH as a reaction product. The effectiveness of the CLDH is associated with removal of dissolved CO₃^2 − from the fresh cement, yielding a significant rise in the pH, and also potential seeding effects. The key factor controlling the reaction kinetics of Na₂CO₃-activated slag cements is the activator functional group, and therefore these cements can be designed to react more rapidly by controlling the slag chemistry and/or including reactive additives.     

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.     

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.     

Pore solution in alkali-activated slag cement pastes. Relation to the composition and structure of calcium silicate hydrate

In this work, the relationship between the composition of pore solution in alkali-activated slag cement (AAS) pastes activated with different alkaline activator, and the composition and structure of the main reaction products, has been studied. Pore solution was extracted from hardened AAS pastes. The analysis of the liquids was performed through different techniques: Na, Mg and Al by atomic absorption (AA), Ca ions by ionic chromatography (IC) and Si by colorimetry; pH was also determined. The solid phases were analysed by XRD, FTIR, solid-state ²⁹Si and ²⁷Al NMR and BSE/EDX.The most significant changes in the ionic composition of the pore solution of the AAS pastes activated with waterglass take place between 3 and 24 h of reaction. These changes are due to the decrease of the Na content and mainly to the Si content. Results of ²⁹Si MAS NMR and FTIR confirm that the activation process takes place with more intensity after 3 h (although at this age, Q² units already exist). The pore solution of the AAS pastes activated with NaOH shows a different evolution to this of pastes activated with waterglass. The decrease of Na and Si contents progresses with time.The nature of the alkaline activator influences the structure and composition of the calcium silicate hydrate formed as a consequence of the alkaline activation of the slag. The characteristic of calcium silicate hydrate in AAS pastes activated with waterglass is characterised by a low structural order with a low Ca/Si ratio. Besides, in this paste, Q³ units are detected. The calcium silicate hydrate formed in the pastes activated with NaOH has a higher structural order (higher crystallinity) and contains more Al in its structure and a higher Ca/Si ratio than those obtained with waterglass.     

C4A3Š hydration in different alkaline media

This study explored the behaviour of laboratory-synthesised calcium sulphoaluminate (C₄A₃Š) in alkaline media. C₄A₃Š was hydrated in three liquid media: water, 8-M NaOH and 4 (wt.%) Na₂CO₃ added to the C₄A₃Š + water mix. Hydration kinetics were studied via isothermal conduction calorimetry and 2- and 28-day mechanical strength values were found. The reaction products were characterised with XRD and FTIR. The findings showed that whilst C₄A₃Š hydration kinetics were accelerated in the presence of alkalis, the resulting pastes had lower mechanical strength than the pastes hydrated with water and exhibited severe decay in some cases. An analysis of the hydration products revealed the presence of ettringite in the water-hydrated C₄A₃Š pastes, whereas under alkaline conditions the main calcium sulphoaluminate hydrate detected was U phase.     

Mechanical performance and microstructure of the calcium carbonate binders produced by carbonating steel slag paste under CO2 curing

Calcium carbonate binders were prepared via carbonating the paste specimens cast with steel slag alone or the steel slag blends incorporating 20% of Portland cement (PC) under CO₂ curing (0.1 MPa gas pressure) for up to 14 d. The carbonate products, mechanical strengths, and microstructures were quantitatively investigated. Results showed that, after accelerated carbonation, the compressive strengths of both steel slag pastes and slag-PC pastes were increased remarkably, being 44.1 and 72.0 MPa respectively after 14 d of CO₂ curing. The longer carbonation duration, the greater quantity of calcium carbonates formed and hence the higher compressive strength gained. The mechanical strength augments were mainly attributed to the formation of calcium carbonate, which caused microstructure densification associated with reducing pore size and pore volume in the carbonated pastes. In addition, the aggregated calcium carbonates exhibited good micromechanical properties with a mean nanoindentation modulus of 38.9 GPa and a mean hardness of 1.79 GPa.     

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 superplasticizer and shrinkage-reducing admixtures on alkali-activated slag pastes and mortars

This paper shows how several superplasticizers (polycarboxylates, vinyl copolymers, melamine and naphthalene-based) and shrinkage-reducing (polypropylenglycol derivatives) admixtures affect the mechanical and rheological properties and setting times of alkali-activated slag pastes and mortars. Two activator solutions, waterglass and NaOH, were used, along with two concentrations—4% and 5% of Na₂O by mass of slag. All admixtures, with the exception of the naphthalene-based product, lost their fluidifying properties in mortars activated with NaOH as a result of the changes in their chemical structures in high alkaline media. The difference in the behaviour of these admixtures when ordinary Portland cement is used as a binder is also discussed in this paper.     

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.     

Fast microwave syntheses of fly ash based porous geopolymers in the presence of high alkali concentration

Microwave synthesis of porous fly ash geopolymers was achieved using a household microwave oven. Fly ash paste containing SiO₂ and Al₂O₃ component was mixed with sodium silicate (Na₂SiO₃) solutions at different concentrations of sodium hydroxide (NaOH) of 2, 5, 10, and 15 M, which were used as NaOH activators of geopolymerization. The mass ratio of Na₂SiO₃/NaOH was fixed at 2.5 with SiO₂/Al₂O₃ at 2.69. After the fly ash and alkali activators were mixed for 1 min until homogeneous, the geopolymer paste was cured for 1 min using household microwave oven at different output powers of 200, 500, 700, and 850 W. Porous geopolymers were formed immediately. Micro X-ray CT and SEM results showed that the porous structure of the geopolymers was developed at higher NaOH concentrations when using 850 W power of the microwave oven. These results derive from the immediate increase of the temperature in the geopolymer paste at higher NaOH concentrations, meaning that aluminosilicate bonds formed easily in the geopolymers within 1 min.     

Preparation of porous material from waste bottle glass by hydrothermal treatment

Porous materials were prepared from colored waste glass by hydrothermal treatment with Na₂CO₃ aqueous solution. The resultant specific surface area was approximately 140 m²/g at maximum. Specific surface area increased at first, reached a maximum, and decreased gradually to be constant at approximately 60 m²/g depending on the period of hydrothermal treatment, irrespective of the concentration of Na₂CO₃ aqueous solution. However, the period at which the specific surface area reached maximum shortened with an increase in Na₂CO₃ concentration. On the other hand, the mass of the sample decreased and eventually saturated at approximately 30 mass% of the initial weight during the hydrothermal treatment. Both the dissolution of the mother glass and the formation of crystalline deposits, which were identified as calcite, zeolite-P and analcime, affected the porous structure of the treated samples.     

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

The hydration and the microstructure of three alkali activated slags (AAS) with MgO contents between 8 and 13 wt.% are investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na₂SiO₃·5H₂O (WG). Higher MgO content of the slag resulted in a faster reaction and higher compressive strengths during the first days. The formation of C(− A)–S–H and of a hydrotalcite-like phase was observed in all samples by X-ray diffraction (XRD), thermal analysis (TGA) and scanning electron microscopy (SEM) techniques. Increasing the MgO content of the slag from 8 to 13% increased the amount of hydrotalcite and lowered the Al uptake by C–S–H resulting in 9% higher volume of the hydrates and a 50 to 80% increase of the compressive strength after 28 days and longer for WG activated slag pastes. For NaOH activated slags only a slight increase of the compressive strength was measured.     

Preparation of spherical foamed body with function of media for waste water treatment by using waste LCD glass

Using waste LCD glass as a base material helped developed the manufacturing process of the spherical foamed body and its varied functionality. Also, the manufactured spherical foamed body showed great performance as a water treatment media. By mixing 90 wt% of waste LCD glass, 100 parts by weight of glass mixture that has 10 wt% kaolinite as a shaping agent, 1.0 part by weight of carbon foaming agent, and mixture of each 1.5 parts by weight of Na₂CO₃, CaCO₃ and Na₂SO₄ as foaming agents and the MgO as a parting agent for 10 min of foaming calcination in the rotary kiln at 970–1000 °C, the spherical foamed body can be manufactured effectively. The manufactured spherical foamed body performed as a great water treatment media by showing 70.5% of SS removal efficiency, 56.1% of BOD removal efficiency, 57.5% of COD removal efficiency, 28.6% of denitrification and 49.8% of phosphorous removal.     

Clay reactivity: Production of alkali activated cements

This study has assessed the suitability of dehydroxylated (5 h at 750 °C) red, white and ball clays for the use as prime materials in the production of alkaline cements. The analytical methodology applied to quantify their potentially reactive phases included selective chemical attack, which was also used in conjunction with subsequent ICP analysis of the resulting leachate to determine their reactive SiO₂/Al₂O₃ ratios. These results were compared with compressive strength values of the respective pastes activated with an 8-M NaOH solution and cured at 85 °C and 90% RH for 20 h. It was observed that when the reactive phase content was above 50%, the reactive SiO₂/Al₂O₃ ratio in the starting materials had a larger impact than the amount of reactive phase on the developed strength of the cement material. In this context, fly ash was used as the reference material. Finally, to verify the accuracy of the results, a binder consisting of 70 wt.% fly ash and 30 wt.% dehydroxylated clay was activated with an 8-M NaOH solution. The reactivity of this cement was determined by chemical attack with 1:20 HCl (v/v) and the reaction products were characterised by powder X-ray diffraction and ²⁹Si MAS NMR spectroscopy.     

Aluminosilicate coatings with enhanced heat- and corrosion resistance

This paper summarizes the results of development of heat- and acid resistant mineral coatings intended for protection of concrete structures and brickwork from exposure of acid media and elevated temperatures. The protective coatings are produced basing on the alkaline aluminosilicate bonds, in which the alkaline compounds are strongly bound into water insoluble hydroaluminosilicate complexes. On the contrary to the known-in-the-art mineral coatings based on cements containing slag and slag/ash, the coatings based on the alkaline aluminosilicates advantageously explore a correlation between corrosion resistance and phase composition due to the synthesis in the structure of the alkaline aluminosilicate matrix of high-silica zeolite-like phases (faujasite, chabazite and mordenite). The introduction into the alkaline aluminosilicate matrix of the formula Na₂O·Al₂O₃·6SiO₂·14.5H₂O of the acid resistant fillers, selected from mica, aluminosilicate microspheres and ground quartz sand taken in quantities of 3, 8 and 12 wt.% enabled to increase the acid resistance of the coatings by 1.25–1.5 fold compared to the known-in-the-art mineral analogs. Heat resistance of the coatings under study, in the conditions of alternate exposure of temperature (473 K) and vapors of oleum, may reach 100 cycles. The developed formulations of the anti-corrosion alkaline aluminosilicate coatings were successfully tried in 2007 in industrial conditions in repairing the deteriorated brickwork of the flue. After 5 years no sign of deterioration of the protective coating was reported.     

Methanation of carbon dioxide over mesoporous Ni–Fe–Al2O3 catalysts prepared by a coprecipitation method: Effect of precipitation agent

Mesoporous nickel (30 wt%)–iron (5 wt%)–alumina (denoted as NiFeAl–X) catalysts were prepared by a coprecipitation method with a variation of precipitation agent (X = (NH₄)₂CO₃, Na₂CO₃, NH₄OH, and NaOH), and they were applied to the methane production from CO₂ and H₂. Metal particle size of reduced NiFeAl–X catalysts decreased in the order of NiFeAl–NaOH > NiFeAl–NH₄OH > NiFeAl–Na₂CO₃ > NiFeAl–(NH₄)₂CO₃. In the methanation of CO₂, yield for CH₄ increased in the order of NiFeAl–NaOH < NiFeAl–NH₄OH < NiFeAl–Na₂CO₃ < NiFeAl–(NH₄)₂CO₃. This indicates that the catalytic performance in the methanation of CO₂ was strongly influenced by the identity of precipitation agent.     

Alternative prime materials for developing new cements: Alkaline activation of alkali aluminosilicate glasses

This study explored the use of alkali aluminosilicate glass obtained by melting and quenching common clay, feldspar and fluxing oxides as a possible starting material for alkaline activation. The glass was synthesized at two different temperatures (1250 and 1400 °C), using Na₂O (8 wt%) as a fluxing oxide. The effect of including small amounts of calcium in the starting mix (5.6 wt% of CaO) was likewise studied. The alkaline activation of such glasses yielded cements with high compressive strength (65 MPa after 20 h at 85 °C). The presence of calcium in the vitreous network favored the formation of a very compact cementitious matrix. TEM-EDX analyses showed that the gel forming in these systems was compositionally (and presumably structurally) non-uniform, with some local high-calcium, high Si/Al ratio (CASH-type gel) areas co-existing with low calcium and low Si/Al ratio clusters (NASH-type gel).     

Comparison of methods for distinguishing sodium carbonate activated from natural sodium bentonites

A lot of natural Ca/Mg-bentonites are turned into Na-bentonites. By adding sodium carbonate to a Ca/Mg-bentonite in the presence of some water Na⁺ enters the interlayer and Ca/Mg-carbonates precipitate outside. Natural Na-bentonites and activated Na-bentonites are rather similar. Therefore, in the present study methods are tested to distinguish both. This is relevant not only for customs but also for research and development. For activation different amounts of sodium carbonate are added. The dosage ranges from a few % of the CEC to slightly above the CEC corresponding to 1–5 mass% Na₂CO₃. Also the water content may vary from the dried state at which the actual activation (cation exchange) does not take place up to the presence of excess water leading to a complete reaction. Altogether four cases had to be considered separately (Na₂CO₃ above CEC + excess water or dry and Na₂CO₃ much below the CEC + excess water or dry). If water was absent (cation exchange was not complete) the sodium carbonate phases could be detected by XRD, IR, or with STA–MS measurements. This result was expected but surprisingly, STA–MS–CO₂ measurements were found to be applicable even in the most difficult case (sodium carbonate addition below CEC and excess of water = reaction complete). In the case of some samples activated with 2 mass% sodium carbonate only, a weak STA–MS–CO₂-peak was observed at about 100 °C. Unprocessed materials are free of any carbonates which decompose around 100 °C. Therefore, this 100 °C peak indicates alkaline activation. This method was applied to five real products with unknown activation and two of which were found to be activated. The pH of the activated materials was only slightly higher than that of a natural Na-bentonite. The measured difference of 0.3 pH units is not considered to be sufficient to unambiguously conclude alkaline activation.     

The effect of RO oxides on microstructure and chemical durability of borosilicate glasses opacified by P2O5

The microstructure, devitrification and chemical durability of borosilicate glass specimens opacified by P₂O₅, with the general composition SiO₂ 70, B₂O₃ 12, Al₂O₃ 2, P₂O₅ 2, Na₂O (13 − X), RO X (wt.%) (R = Ca, Mg, Ba, Zn) were investigated after being subjected to various heat treatment conditions, using DTA, XRD and SEM. It was shown that while heat treatments at 1073 K and >1123 K were generally detrimental for the hydrolytic resistance of glasses, due to the enhanced phase separation or formation of excessive amounts of cristobalite, heating at 1123 K for 1 h usually improved the resistance due to the partial crystallization or microstructural changes of specimens. It was also found that a progressive decrease in hydrolytic and alkaline resistance occurred during prolonged heat treatment at 1123 K due to the formation of exessive amounts of cristobalite. It was also revealed that ZnO and MgO had the worst effect on chemical durabilities of specimens containing 7 and 5.5 wt.% Na₂O, respectively.     

Indirect mineral carbonation of titanium-bearing blast furnace slag coupled with recovery of TiO2 and Al2O3

Large quantities of CO_2 and blast furnace slag are discharged in the iron and steel industry. Mineral carbonation of blast furnace slag can offer substantial CO_2 emission reduction and comprehensive utilization of the solid waste.This paper describes a novel route for indirect mineral carbonation of titanium-bearing blast furnace(TBBF) slag,in which the TBBF slag is roasted with recyclable(NH_4)_2SO_4(AS) at low temperatures and converted into the sulphates of various valuable metals, including calcium, magnesium, aluminium and titanium. High value added Ti-and Al-rich products can be obtained through stepwise precipitation of the leaching solution from the roasted slag. The NH_3 produced during the roasting is used to capture CO_2 from flue gases. The NH_4HCO_3 and(NH_4)_2CO_3 thus obtained are used to carbonate the CaSO_4-containing leaching residue and MgSO_4-rich leaching solution, respectively. In this study, the process parameters and efficiency for the roasting, carbonation and Ti and Al recovery were investigated in detail. The results showed that the sulfation ratios of calcium,magnesium, titanium and aluminium reached 92.6%, 87% and 84.4%, respectively, after roasting at an AS-to-TBBF slag mass ratio of 2:1 and 350 °C for 2 h. The leaching solution was subjected to hydrolysis at 102 °C for 4 h with a Ti hydrolysis ratio of 95.7% and the purity of TiO_2 in the calcined hydrolysate reached 98 wt%.99.7% of aluminium in the Ti-depleted leaching solution was precipitated by using NH_3. The carbonation products of Ca and Mg were CaCO_3 and(NH_4)_2 Mg(CO_3)_2·4H_2O, respectively. The latter can be decomposed into MgCO_3 at 100–200 °C with simultaneous recovery of the NH_3 for reuse. In this process, approximately 82.1% of Ca and 84.2%of Mg in the TBBF slag were transformed into stable carbonates and the total CO_2 sequestration capacity per ton of TBBF slag reached up to 239.7 kg. The TiO_2 obtained can be used directly as an end product, while the Al-rich precipitate and the two carbonation products can act, respectively, as raw materials for electrolytic aluminium,cement and light magnesium carbonate production for the replacement of natural resources.