Editorial: Editors’ showcase: structural materials

Copyright © 2023 Provis and Zhou. We are very pleased to present this Research Topic that highlights some of the outstanding work of the editorial team of the Structural Materials section of Frontiers in Materials. The breadth and depth of expertise featured in our editorial board is clearly evident in this Research Topic of articles, which range in topics from rock mechanics to cement chemistry, and from metallurgy to materials synthesis. Looking back to the opening of the Structural Materials section and the featured “Grand Challenges” article published in 2015 (Provis, 2015), it is striking that the topics featured in this Showcase Research Topic are very well aligned with the Grand Challenges of the section: bottom-up materials design, sustainable materials supply, and durability under extreme conditions. These areas—And with additional focus now on digitalisation—Continue to set the agenda for research in the field of structural materials

Structural Ordering of Aged and Hydrothermally Cured Metakaolin Based Potassium Geopolymers

This study evaluates the potential correlation between natural aging and hydrothermal curing (accelerated aging), related to the crystallisation of zeolites in potassium-based metakaolin geopolymer binders. 7-year old sealed-cured specimens, formulated with varying silicate contents, were evaluated. The effect of different accelerated aging durations on the mineralogy of these potassium-based geopolymers was also assessed. The results show that although zeolite formation is favoured under both natural and accelerated aging in potassium-based geopolymers, different types of zeolites are formed depending on the silicate content added to the mix, and the curing conditions of the specimens

Layered Double Hydroxides Modify the Reaction of Sodium Silicate-Activated Slag Cements

The impact of adding two types of layered double hydroxides (LDHs), commercial hydrotalcite (HT) and its thermally treated form (CLDH), on the reaction kinetics and phase assemblage development of a sodium silicate-activated slag cement was investigated. The reaction kinetics of LDH-modified cements was assessed by isothermal calorimetry, and the results were correlated with in situ attenuated total reflection Fourier transform infrared spectroscopy results collected over the first days of reaction, to identify the structural evolution of the main binding phase forming in these cements: a sodium-containing calcium aluminosilicate hydrate (C-(N)-A-S-H)-type gel. The addition of either HT or CLDH into sodium silicate-activated slag paste accelerates the precipitation of reaction products and increases the formation of HT in these cements, without causing significant changes to the C-(N)-A-S-H binding phase. This is extremely relevant in terms of the durability of alkali-activated slag cements, as a higher content of the HT-like phase has the potential to reduce their chloride permeability and enhance carbonation resistance

Chloride-induced corrosion of steel rebars in simulated pore solutions of alkali-activated concretes

The passivation and chloride-induced depassivation of steel rebars immersed in varying alkaline environments (0.80 M, 1.12 M and 1.36 M 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 0.80 M, 1.12 M and 1.36 M 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

Influence of sulfide on the onset of chloride-induced corrosion of steel reinforcement in alkali-activated slags

In alkali-activated slags (AAS), where ground granulated blast furnace slag is used as the main precursor, the presence of reduced sulfur species yields a highly reducing pore solution environment. This study investigates the influence of sulfide on steel passivation in alkali-activated slag mortars immersed in alkaline (1 M NaOH) and alkaline chloride-rich (1 M NaOH with 5 M NaCl) solutions, and the dependency of chloride-induced pitting on the immersion time and the concentration of sulfide in simulated alkali-activated slag pore solutions (containing 0.80 M OH- and 0 M, 0.01 M and 0.45 M HS-) by means of electrochemical techniques. Surfacesensitive X-ray photoelectron spectroscopic (XPS) and Raman spectroscopic analysis of the corrosion products formed on the steel surfaces, both in alkali-activated slag mortars and in simulated pore solutions, shows that sulfide has a very important influence in altering – but not necessarily in a deleterious way – the characteristics of the steel surface under these conditions

Slag-Based Cements That Resist Damage Induced by Carbon Dioxide

The use of sodium carbonate as an activator to prepare alkali-activated cements from blast furnace slag and calcined hydrotalcite offers many attractive performance and environmental benefits. However, the understanding of the long-term performance of these cements is limited. In this study, the resistance of sodium carbonate-activated slag cements to carbonation attack was determined under natural (0.04%) and elevated (1.0%) CO₂ concentrations. Two calcium carbonate polymorphs, calcite and vaterite, were formed as carbonation products at a longer time of CO₂ exposure. A cross-linked alkali aluminosilicate gel and a Ca-deficient calcium (alumino)silicate hydrate gel were identified to form by decalcification of the main binding phases initially present in these cements. However, despite these carbonation-induced mineralogical changes, the mechanical strength after carbonation was comparable to that of noncarbonated specimens, which is contrary to previous observations of strength loss due to carbonation of slag-rich cements. The high carbonation resistance of sodium carbonate-activated slag cement indicates these materials have the potential to resist attack by atmospheric CO₂ in service with sustained mechanical performance

Chloride binding and mobility in sodium carbonate-activated slag pastes and mortars

This study evaluates the chloride binding capacity and the migration of chloride in sodium carbonate-activated slag cements and mortars. The effect on chloride mobility and binding of adding a calcined layered double hydroxide (CLDH) to the binder mix was also assessed. Significantly improved durability characteristics can be achieved for sodium carbonate-activated slag mortars by the addition of small fractions of CLDH, as a consequence of a higher degree of reaction, higher chloride binding capacity, and the refined pore structures present in these modified materials, in comparison with alkali-activated cements produced without CLDH. The addition of CLDH enables the production of sodium carbonate-activated slag cements with notably reduced chloride ingress compared to silicate activated slag cements

Bubble stabilisation improves strength of lightweight mortars

Lightweight foamed mortars are produced through the addition of foaming agents into the cement blend, so that voids of different sizes are formed within the matrix, reducing the density of the material and therefore also its weight. However, the increased porosity of these materials usually compromises their mechanical strength, limiting their application as a structural material. Modern infrastructure demands high-strength lightweight concrete formulations that can be adjusted to develop more ambitious projects, both in design and application. In this study, lightweight pastes and mortars were produced using Portland cement blended with fly ash and silica fume, with varying water contents, and foamed using aluminium metal powder. To stabilise the bubbles produced through oxidation of the aluminium metal, polyethylene glycol was added to the mixes, and proved effective in yielding more uniform bubbles than were observed in the samples with no added stabiliser. This led to improvements in both the bulk density and compressive strength of the materials produced according to this new methodology