Alkali-activated materials Provis, John L.
Cement and concrete research,
12/2018, Letnik:
114
Journal Article
Recenzirano
Odprti dostop
This paper, which forms part of the UNEP White Papers series on Eco-Efficient Cements, provides a brief discussion of the class of cementing materials known as ‘alkali-activated binders’, which are ...identified to have potential for utilization as a key component of a sustainable future global construction materials industry. These cements are not expected to offer a like-for-like replacement of Portland cement across its full range of applications, for reasons related to supply chain limitations, practical challenges in some modes of application, and the need for careful control of formulation and curing. However, when produced using locally-available raw materials, with well-formulated mix designs (including in particular consideration of the environmental footprint of the alkaline activator) and production under adequate levels of quality control, alkali-activated binders are potentially an important and cost-effective component of the future toolkit of sustainable construction materials.
This paper presents a review of alkali-activation technology, moving from the atomic scale and chemical reaction path modelling, towards macroscopic observables such as strength and durability of ...alkali-activated concretes. These properties and length scales are intrinsically interlinked, and so the chemistry of both low-calcium (‘geopolymer’) and high-calcium (blast furnace slag-derived) alkali-activated binders can be used as a starting point from which certain engineering properties may be discussed and explained. These types of materials differ in chemistry, binder properties, chemical structure and microstructure, and this leads to the specific material properties of each type of binder. The secondary binder products formed during alkali-activation (zeolites in low-Ca systems, mostly layered double hydroxides in alkali-activated slags) are of significant importance in determining the final properties of the materials, particularly in the context of durability. The production of highly durable concretes must remain the fundamental aim of research and development in the area of alkali-activation. However, to enable the term ‘highly durable’ to be defined in a satisfactory way, the underlying mechanisms of degradation—which are not always the same for alkali-activated binders as for Portland cement-based binders, and cannot always be tested in precisely the same ways—need to be further analysed and understood. The process of reviewing a topic such as this will inevitably raise just as many questions as answers, and it is the intention of this paper to present both, in appropriate context.
The development of low-carbon binders has been recognized as a means of reducing the carbon footprint of the Portland cement industry, in response to growing global concerns over CO2 emissions from ...the construction sector. This paper reviews recent progress in the three most attractive low-carbon binders: alkali-activated, carbonate, and belite-ye'elimite-based binders. Alkali-activated binders/materials were reviewed at the past two ICCC congresses, so this paper focuses on some key developments of alkali-activated binders/materials since the last keynote paper was published in 2015. Recent progress on carbonate and belite-ye'elimite-based binders are also reviewed and discussed, as they are attracting more and more attention as essential alternative low-carbon cementitious materials. These classes of binders have a clear role to play in providing a sustainable future for global construction, as part of the available toolkit of cements.
The development of new, sustainable, low-CO
2
construction materials is essential if the global construction industry is to reduce the environmental footprint of its activities, which is incurred ...particularly through the production of Portland cement. One type of non-Portland cement that is attracting particular attention is based on alkali-aluminosilicate chemistry, including the class of binders that have become known as geopolymers. These materials offer technical properties comparable to those of Portland cement, but with a much lower CO
2
footprint and with the potential for performance advantages over traditional cements in certain niche applications. This review discusses the synthesis of alkali-activated binders from blast furnace slag, calcined clay (metakaolin), and fly ash, including analysis of the chemical reaction mechanisms and binder phase assemblages that control the early-age and hardened properties of these materials, in particular initial setting and long-term durability. Perspectives for future research developments are also explored.
Alkali activation is a highly active and rapidly developing field of activity in the global research and development community. Commercial-scale deployment of alkali-activated cements and concretes ...is now proceeding rapidly in multiple nations. This paper reviews the key developments in alkali-activated materials since 2011, with a particular focus on advances in characterisation techniques and structural understanding, binder precursors and activation approaches, durability testing and design, processing, and sustainability. The scientific and engineering developments described in this paper have underpinned the on-going scale-up activities. We also identify important needs for future research and development to support the optimal and appropriate utilisation of alkali activated materials as a component of a sustainable future construction materials industry.
This review examines the detailed chemical insights that have been generated through 150 years of work worldwide on magnesium-based inorganic cements, with a focus on both scientific and patent ...literature. Magnesium carbonate, phosphate, silicate-hydrate, and oxysalt (both chloride and sulfate) cements are all assessed. Many such cements are ideally suited to specialist applications in precast construction, road repair, and other fields including nuclear waste immobilization. The majority of MgO-based cements are more costly to produce than Portland cement because of the relatively high cost of reactive sources of MgO and do not have a sufficiently high internal pH to passivate mild steel reinforcing bars. This precludes MgO-based cements from providing a large-scale replacement for Portland cement in the production of steel-reinforced concretes for civil engineering applications, despite the potential for CO2 emissions reductions offered by some such systems. Nonetheless, in uses that do not require steel reinforcement, and in locations where the MgO can be sourced at a competitive price, a detailed understanding of these systems enables their specification, design, and selection as advanced engineering materials with a strongly defined chemical basis.
The passivation and chloride-induced depassivation of steel rebars immersed in varying alkaline environments (0.80M, 1.12M and 1.36M NaOH solutions), simulating the pore solutions of low-Ca ...alkali-activated concretes, were investigated using a range of electrochemical techniques. The passive film on the steel rebars was complex in chemical makeup, composed of Fe–hydroxides, oxy-hydroxides and oxides. An increased degree of passivation of the rebars was observed when exposed to solutions with higher hydroxide concentrations. The critical chloride level (Cl−/OH− ratio) required to induce depassivation of steel was strongly dependent on the alkalinity of the pore solution, and was found to be 0.90, 1.70 and 2.40 for 0.80M, 1.12M and 1.36M NaOH solutions, respectively. These values all correspond to a constant value of Cl−/OH−3=1.25, which is a novel relationship to predict the onset of pitting, interlinking chloride concentration and the solubility of the passive film.
Chloride ingress and carbonation are major causes of degradation of reinforced concrete. To enable prediction of chloride ingress, and thus to improve the durability of structural alkali-activated ...slag cement (AAS) based concretes, it is necessary to understand the ionic interactions taking place between chlorides, carbonates, and the individual solid phases which comprise AAS. This study focused on two layered double hydroxides (LDH) representing those typically identified as reaction products in AAS: an Mg-Al hydrotalcite-like phase, and an AFm structure (strätlingite), in simulated AAS pore solutions. Surface adsorption and interlayer ion-exchange of chlorides occurred in both LDH phases; however, chloride uptake in hydrotalcite-group structures is governed by surface adsorption, while strätlingite shows the formation of a hydrocalumite-like phase and ion exchange. For both Ca-Al and Mg-Al LDHs, decreased chloride uptakes were observed from solutions with increased CO32−/OH− ratios, due to the formation of carbonate-containing hydrotalcite and decomposition of AFm phases, respectively.
•ICC is confirmed again to be useful in studying the thermodynamics of geopolymerization.•Na-silicate activation of metakaolin at 25°C processes an extent of ∼0.20, far from the final ...structure.•Na/Al has a more pronounced influence on the reaction extent of metakaolin than do temperature and Si/Al ratio.•ICC is expected to be a powerful technique in quantifying the reactivity of various aluminosilicate precursors.
The paper describes the outcomes of a study using isothermal conduction calorimetry (ICC) to characterize the geopolymerization kinetics of metakaolin activated with sodium silicate. Two exothermic peaks are observed in the calorimetric curves for all systems reacting within the temperature range 20–40°C. The peaks are assigned to the dissolution of metakaolin, and the formation of geopolymeric gels with disordered structure, respectively. Compared with the use of NaOH solution to activate metakaolin, the presence of soluble silicate in the activator hinders the reorganization of the local structure of geopolymeric gels and also suppresses the formation of zeolites or zeolite precursors. The ICC data are used via a thermochemical model to quantify the reaction kinetics of geopolymerization, by assuming that the geopolymeric gels have an analcime-like local structure and taking into account the speciation of the silicate monomers and dimers in the activator. Decreasing the modulus from 1.6 to 1.0 increases the fractional reaction extent from 0.12 to 0.26 after 72h at 25°C. When the modulus is 1.2, increasing the reaction temperature from 20°C to 35°C results in an improved reaction extent from 0.24 to 0.35. The rapid polymerization that occurs at 40°C appears to hinder the further reaction of MK and consequently results in a lower reaction extent than at 35°C. Combined with the findings from previous analysis of systems where NaOH was used to activate MK, the concentration of available Na+ appears to have a more pronounced influence on the extent of geopolymerization than temperature and the concentration of soluble Si. The higher reaction extent of the solid precursor particles with the soluble Si from the activator results in binders with more compact microstructure and higher mechanical strength. Considering the longer-term utilization of geopolymers, the addition of soluble silicate to the activator will delay the transformation of amorphous raw materials to locally ordered materials, potentially providing advantages in terms of microstructural stability in service.
The pore fluid within many concretes is highly alkaline and rich in reduced sulfur species, but the influence of such alkaline-sulfide solutions on the surface film formed on steel reinforcement is ...poorly understood. This study investigates the critical role of HS
−
in defining mild steel passivation chemistry. The surface film formed on the steel in alkaline-sulfide solutions contains Fe(OH)
2
and Fe–S complexes, and the critical chloride concentration to induce corrosion increases at high sulfide concentration. However, this behavior is dependent on the duration of exposure of the steel to the electrolyte, and the nature of the sulfidic surface layer.
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