Structural evolution of an alkali sulfate activated slag cement

In this study, the effect of sodium sulfate content and curing duration (from fresh paste up to 18 months) on the binder structure of sodium sulfate activated slag cements was evaluated. Isothermal calorimetry results showed an induction period spanning the first three days after mixing, followed by an acceleration-deceleration peak corresponding to the formation of bulk reaction products. Ettringite, a calcium aluminium silicate hydrate (C-A-S-H) phase, and a hydrotalcite-like Mg-Al layered double hydroxide have been identified as the main reaction products, independent of the Na2SO4 dose. No changes in the phase assemblage were detected in the samples with curing from 1 month up to 18 months, indicating a stable binder structure. The most significant changes upon curing at advanced ages observed were growth of the AFt phase and an increase in silicate chain length in the C-A-S-H, resulting in higher strength

Alkali-activated materials – cementing a sustainable future

This paper presents an overview examining the microstructural and macrostructural properties of alkali-activated binders based on granulated blast furnace slags, metakaolin and their blends, developed by the Composite Materials Group of Universidad del Valle over the past decade. Durability results of activated binders when exposed to aggressive agents such as chlorides, and carbon dioxide are reported. The results of this research have elucidated the great feasibility of adopting alkali-activation technology in Colombia for producing high strength concretes based on industrial by-products, with a wide range of properties that can be suitable for different civil infrastructure applications, and contribute to the valorization of low-cost industrial by products through production of more environmentally friendly building materials. Our research highlights the fact that a deep understanding of the chemistry of these systems allows the manipulation of the microstructure and therefore the performance of the final products, toward the production of sustainable and versatile materials

Water content modifies the structural development of sodium metasilicate-activated slag binders

The effect of modifying the water content of an alkali - activated slag binder was assessed, in terms of the kinetics of reaction and the structural development of the material. There is not a s ystematic correlation between the water content of the mix and the rate of reaction, indicating that there is an optimal value that favours dissolution of the slag and precipitation of reaction products. A h igher water content reduce d the crystallinity and density of the reaction products, especially at advanced age. Small changes in the water content can have a significant impact on the compressive strength development of alkali - silicate activated slag mortars, suggesting that when producing materials base d on alkali - activated binders , it is essential to carefully control the water content

Synthesis of geopolymer from spent FCC: Effect of SiO2/Al2O3 and Na2O/SiO2 molar ratios

This paper assesses the feasibility of using a spent fluid catalytic cracking catalyst (SFCC) as precursor for the production of geopolymers. The mechanical and structural characterization of alkali-activated SFCC binders formulated with different overall (activator + solid precursor) SiO2/Al2O3 and Na2O/SiO2 molar ratios are reported. Formation of an aluminosilicate ‘geopolymer’ gel is observed under all conditions of activation used, along with formation of zeolites. Increased SiO2/Al2O3 induces the formation of geopolymers with reduced mechanical strength, for all the Na2O/SiO2 ratios assessed, which is associated with excess silicate species supplied by the activator. This is least significant at increased alkalinity conditions (higher Na2O/SiO2 ratios), as larger extents of reaction of the spent catalyst are achieved. SiO2/Al2O3 and Na2O/SiO2 ratios of 2.4 and 0.25, respectively, promote the highest compressive strength (67 MPa). This study elucidates the great potential of using SFCC as precursor to produce sustainable ceramic-like materials via alkali-activation

Characterisation of Ba(OH)(2)-Na2SO4-blast furnace slag cement-like composites for the immobilisation of sulfate bearing nuclear wastes

Soluble sulfate ions in nuclear waste can have detrimental effects on cementitious wasteforms and disposal facilities based on Portland cement. As an alternative, Ba(OH)2–Na2SO4–blast furnace slag composites are studied for immobilisation of sulfate-bearing nuclear wastes. Calcium aluminosilicate hydrate (C–A–S–H) with some barium substitution is the main binder phase, with barium also present in the low solubility salts BaSO4 and BaCO3, along with Ba-substituted calcium sulfoaluminate hydrates, and a hydrotalcite-type layered double hydroxide. This reaction product assemblage indicates that Ba(OH)2 and Na2SO4 act as alkaline activators and control the reaction of the slag in addition to forming insoluble BaSO4, and this restricts sulfate availability for further reaction as long as sufficient Ba(OH)2 is added. An increased content of Ba(OH)2 promotes a higher degree of reaction, and the formation of a highly cross-linked C–A–S–H gel. These Ba(OH)2–Na2SO4–blast furnace slag composite binders could be effective in the immobilisation of sulfate-bearing nuclear wastes

High-temperature performance of mortars and concretes based on alkali-activated slag/metakaolin blends

This paper assesses the performance of mortars and concretes based on alkali activated granulated blastfurnace slag (GBFS)/metakaolin (MK) blends when exposed to high temperatures. High stability of mortars with contents of MK up to 60 wt.% when exposed to 600 °C is identified, with residual strengths of 20 MPa following exposure to this temperature. On the other hand, exposure to higher temperatures leads to cracking of the concretes, as a consequence of the high shrinkage of the binder matrix and the restraining effects of the aggregate, especially in those specimens with binders containing high MK content. A significant difference is identified between the water absorption properties of mortars and concretes, and this is able to be correlated with divergences in their performance after exposure to high temperatures. This indicates that the performance at high temperatures of alkali-activated mortars is not completely transferable to concrete, because the systems differ in permeability. The differences in the thermal expansion coefficients between the binder matrix and the coarse aggregates contribute to the macrocracking of the material, and the consequent reduction of mechanical properties

Role of carbonates in the chemical evolution of sodium carbonate-activated slag binders

Multi-technique characterisation of sodium carbonate-activated blast furnace slag binders was conducted in order to determine the influence of the carbonate groups on the structural and chemical evolution of these materials. At early age (<4 days) there is a preferential reaction of Ca2+ with the CO3 2− from the activator, forming calcium carbonates and gaylussite, while the aluminosilicate component of the slag reacts separately with the sodium from the activator to form zeolite NaA. These phases do not give the high degree of cohesion necessary for development of high early mechanical strength, and the reaction is relatively gradual due to the slow dissolution of the slag under the moderate pH conditions introduced by the Na2CO3 as activator. Once the CO3 2− is exhausted, the activation reaction proceeds in similar way to an NaOH-activated slag binder, forming the typical binder phases calcium aluminium silicate hydrate and hydrotalcite, along with Ca-heulandite as a further (Ca,Al)-rich product. This is consistent with the significant gain in compressive strength and reduced porosity observed after 3 days of curing. The high mechanical strength and reduced permeability developed in these materials beyond 4 days of curing elucidate that Na2CO3-activated slag can develop desirable properties for use as a building material, although the slow early strength development is likely to be an issue in some applications. These results suggest that the inclusion of additions which could control the preferential consumption of Ca2+ by the CO3 2− might accelerate the reaction kinetics of Na2CO3-activated slag at early times of curing, enhancing the use of these materials in engineering applications

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

Phase Formation and Evolution in Mg(OH)(2)-Zeolite Cements

The mineralogy and structure of cements in the system Mg(OH)2–NaAlO2–SiO2–H2O are investigated, with a view toward potential application in the immobilization of Mg(OH)2-rich Magnox sludges resulting from historic United Kingdom nuclear operations. The reaction process leading to the formation of these aluminosilicate binders is strongly exothermic, initially forming zeolite NaA (LTA structure), which is metastable in low SiO2/Al2O3 binders, slowly evolving into the more stable sodalite and faujasite framework types. Notable chemical reaction of Mg(OH)2 was only identified in the formulation with SiO2/Al2O3 = 1.3 (the lowest molar ratio among those tested) after extended curing times. In this case, some of the Mg(OH)2 reacted to form an Mg–Al–OH layered double hydroxide. These results demonstrate that encapsulation of Magnox sludge waste streams could be carried out in these alternative binders but that the binders would encapsulate rather than chemically incorporate the Mg(OH)2 into the wasteform unless low SiO2/Al2O3 ratios are used

Blast furnace slag-Mg(OH)(2) cements activated by sodium carbonate

The structural evolution of a sodium carbonate activated slag cement blended with varying quantities of Mg(OH)2 was assessed. The main reaction products of these blended cements were a calcium-sodium aluminosilicate hydrate type gel, an Mg-Al layered double hydroxide with a hydrotalcite type structure, calcite, and a hydrous calcium aluminate phase (tentatively identified as a carbonate-containing AFm structure), in proportions which varied with Na2O/slag ratios. Particles of Mg(OH)2 do not chemically react within these cements. Instead, Mg(OH)2 acts as a filler accelerating the hardening of sodium carbonate activated slags. Although increased Mg(OH)2 replacement reduced the compressive strength of these cements, pastes with 50 wt% Mg(OH)2 still reached strengths of ∼21 MPa. The chemical and mechanical characteristics of sodium carbonate activated slag/Mg(OH)2 cements makes them a potentially suitable matrix for encapsulation of high loadings of Mg(OH)2-bearing wastes such as Magnox sludge