Quantification of the influences of aggregate shape and sampling method on the overestimation of ITZ thickness in cementitious materials

The microstructure of the interfacial transition zone (ITZ) surrounding the aggregate in a cementitious composite is quite different from that of the bulk matrix, because of its distinct physical nature including relatively high porosity and low rigidity. The thickness and volume fraction of the ITZ play a major role in determining the transport and mechanical behavior of cementitious composites. However, the ITZ thickness may be overestimated when undertaking sectional plane analysis of these composites. Analysis of Platonic particles has previously shown that the sphericity of the particle is an important parameter in determining the overestimation of the ITZ thickness, but this raises the question of whether sphericity is sufficient to uniquely characterize the influence of aggregate shape. This paper investigates the influence of particle shape on overestimation of ITZ thickness for aggregate shapes which have the same sphericity values as Platonic particles; specifically, spheroids of differing geometries. A normal line sampling algorithm, which is designed to replicate the practical experimental process used in ITZ determination, is employed to obtain the apparent ITZ thickness. The influences of particle shape, sampling method and particle size distribution are investigated in terms of the overestimation of the ITZ volume fraction, and the effective diffusivity within three-phase composites, using the differential effective medium approximation

Effect of drying procedures on pore structure and phase evolution of alkali-activated cements

This study reports the effects of different drying procedures on the pore determination and phase evolution of alkali-activated cements based on metakaolin (AAMK), fly ash (AAFA) and slag (AAS), as characterized by N2 adsorption and XRD and FTIR analysis, in comparison with ordinary Portland cement (OPC) paste. The selected drying methods are: (1) 65 °C/24 h vacuum drying, (2) 105 °C/24 h oven drying, (3) solvent-exchange with ethanol for 3 days then 50 °C/24 h oven drying, and (4) freeze-drying with liquid nitrogen. The pore structures of AAMK and AAFA, with zeolite-like sodium aluminosilicate gel phases and little bound water, are less sensitive to drying conditions than are AAS and OPC, which consist of calcium (alumino)silicate hydrates. The drying procedures have less impact on the phase compositions of alkali-activated cements than OPC in general. Nonetheless, caution must be applied in selection of appropriate drying procedures to obtain reproducible and meaningful information regarding the pore and phase structure of alkali-activated cements

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

On the sustainable development of cement

Cement is the most manufactured product on earth. Unfortunately, the manufacture of cement is accompanied by the emission of carbon dioxide gas. Among all manufacturing industry sectors in the UK, the cement industry is the largest CO2 emitter and these emissions are damaging our planet. The sustainable development of cement will allow future generations to develop without being compromised by the cement industry. This work identifies some of the routes to reducing the environmental burden of the cement industry

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

Thermodynamics of calcined clays used in cementitious binders: origin to service life considerations

The use of calcined clays in construction materials has attracted significant attention in the last few years. Based on the continued need for sustainable construction to meet global development challenges, the green transition of the cement industry is an urgent necessity. The use of clay-blended cements will keep increasing to meet the need for mass quantities of materials and the prospect of reducing their embodied CO2, as traditional supplementary cementitious materials are expected to decline in availability. To enable the necessary rapid increase in the fraction of clays that can be used in cements, the use of modeling tools which provide insights into the clays and their reactivity in cementitious systems is of increased interest. The aim is to predict the properties of the calcined clays based on the original rock and calcination conditions, the phase evolution, material properties, and durability of construction materials. This is crucial to reduce the time needed for development and commercialisation, whereas extensive empirical work has been used in the past to achieve material transition in the construction sector, which can be extremely time consuming. This review article therefore aims to provide an overview of available thermodynamic data, issues with database integration, modelling of process parameters, and properties prediction for cementitious materials

Decarbonisation of calcium carbonate at atmospheric temperatures and pressures, with simultaneous CO2 capture, through production of sodium carbonate

The calcination of calcium carbonate (CaCO3) is a major contributor to carbon dioxide (CO2) emissions that are changing our climate. Moreover, the calcination process requires high temperatures (~900°C). A novel low-temperature process for the decarbonisation of CaCO3 is tested whereby the CO2 is directly sequestered/mineralised in sodium carbonate. CaCO3 is reacted with an aqueous sodium hydroxide solution by mixing under atmospheric temperatures and pressures. The reaction products are calcium hydroxide (hydrated lime; Ca(OH)2) and sodium carbonate (soda ash; Na2CO3). For the first time, the extent of this reaction at ambient conditions is studied along with the NaOH requirements. Conceptual process designs, which include procedures to separate and recover material, as well as energy calculations, are also presented to demonstrate technical/industrial feasibility of the process. The technology is also successfully tested on industrially sourced limestone chalk, and the silica impurity remains inert throughout the process. This technology will enable industrial symbiosis by combining the high-temperature lime and sodium carbonate manufacturing processes into a single low-temperature process and greatly reduce the chemical (raw material) CO2 emissions associated with the production of cement and lime

Atomistic simulations of geopolymer models: the impact of disorder on structure and mechanics

Geopolymers are hydrated aluminosilicates with excellent binding properties. Geopolymers appeal to the construction sector as a more sustainable alternative to traditional cements, but their exploitation is limited by a poor understanding of the linkage between chemical composition and macroscopic properties. Molecular simulations can help clarify this linkage, but existing models based on amorphous or crystalline aluminosilicate structures provide only a partial explanation of experimental data on the nanoscale. This paper presents a new model for the molecular structure of geopolymers, in particular for nanoscale interfacial zones between crystalline and amorphous nanodomains, which are crucial for the overall mechanical properties of the material. For a range of Si–Al molar ratios and water contents, the proposed structures are analyzed in terms of skeletal density, ring structure, pore structure, bond-angle distribution, bond length distribution, X-ray diffraction, X-ray pair distribution function, elastic moduli, and large-strain mechanics. Results are compared with experimental data and with other simulation results for amorphous and crystalline molecular models, showing that the newly proposed structures better capture important structural features with an impact on mechanical properties. This offers a new starting point for the multiscale modeling of geopolymers