Sodium acetate can be used as an activator to produce blast furnace slag binders with reduced permeability; however, acetate-activated slag cements present delayed setting times compared with NaOH-activated slag cements. As a potential solution to accelerate the reaction kinetics of these promising binders, the effects of partially replacing sodium acetate with sodium hydroxide at different ratios (ranging from 0 % to 100 %) on the phase assemblage evolution, compressive strength and wettability of these cements were investigated. A higher content of sodium acetate resulted in a lower activating solution pH, slower reaction kinetics and generally lower cumulative heat up to 28 days. Aluminium substituted calcium silicate hydrate (C-(A)-S-H) type gel, along with Mg-Al layer double hydroxide phases, were identified as the main reaction products in all systems. Decreasing sodium hydroxide content in the activating solution induced only slight changes in composition of the C-(A)-S-H and the Mg-Al layer double hydroxide phase. The compressive strength increased at higher replacement of sodium acetate by sodium hydroxide, with a lower wettability observable in the 50/50 acetate/hydroxide activated binders. These findings demonstrate the potential to tailor reaction kinetics and hardened properties of alkali-activated slag cements by substituting sodium acetate with varying amounts of sodium hydroxide. Characteristics of the binders produced with the combined sodium hydroxide and sodium acetate activators demonstrate potential for applications as coating materials.

This study evaluated the mechanical and durability performance of CEM I and CEM I plus limestone blended concrete produced with calcined clays (CC) with a varying meta-kaolinite content (70, 50 or 20 wt%). Results revealed that concrete with >45 MPa can be produced with a CC with only 20 wt% meta-kaolinite. Increased compressive and flexural strengths were obtained using higher meta-kaolinite content CC. Limestone addition did not significantly change the concretes' transport or durability properties when compared to binary mixes, despite the reduced clinker factor. CC-containing concretes exhibited excellent chloride resistance, but reduced carbonation performance compared with CEM I. Using a CC with higher meta-kaolinite content enhanced the concrete's carbonation resistance, when evaluated for 650 days of natural exposure. This suggests that generalising the impact of CC addition on concrete performance can be misleading, as bespoke concrete, compliant with specific exposure class requirements, can be produced by appropriate clay selection.

Carbonation and chloride-induced deterioration of reinforced concrete can cause infrastructure damage and potential collapse. This study evaluated the impact of carbonation on compressive strength, dimensional stability, water and chloride permeability of concretes made with ternary slag cement containing 10 or 20 wt.% limestone, compared to ground granulated blast furnace slag (GGBFS) blended cement or CEM I. The carbonation rates of binary and ternary concretes were higher than those of CEM I concrete. The existing equation correlating natural and accelerated carbonation coefficients holds for the concretes evaluated and the selected carbonation exposure condition studied. The carbonation depths estimated adopting this correlation are within the limits of the cover depths recommended by the BS 8500-1:2023 for concretes for a 50 years' service life, when exposed to exposure classes XC3/XC4. Despite the higher carbonation rates, water and chloride permeability of the carbonated ternary and binary slag cement concretes were significantly lower than those of a CEM I equivalent. No clear correlation was identified between compressive strength, porosity, bulk conductivity, water sorption coefficient and carbonation rate. Each of these properties alone did not give a good indication of the overall durability performance of binary or ternary concretes. The results demonstrate that 10 % limestone addition has no adverse effect on carbonation resistance of composite cement concrete. Therefore, it is demonstrated that partial replacement of GGBFS by limestone is a practical and technically sound solution for producing concrete with a reduced clinker content and comparable durability to CEM I or binary GGBFS concretes.

The widespread use of ternary cement produced from Portland cement (CEM I), granulated blast furnace slag (GGBFS) and limestone powder in structural concrete can be anticipated given the recent standardisation of such ternary cements in EN 197-5. However, understanding of these cements is limited to paste or mortar-scale studies, and comprehensive studies on concretes produced from such cements are needed urgently. In this study, fresh and hardened properties including strength, elastic modulus, water absorption and chloride permeability of ternary cement concrete comprising 50 wt% CEM I blended with GGBFS and 10 or 20 wt% limestone are investigated, as a function of the curing duration. Reducing the GGBFS content, by replacing it with 10-20 wt% limestone powder, leads to satisfactory mechanical and durability properties relative to binary and CEM I concrete. It was established that the ternary concretes evaluated exhibit a progressive increase in compressive strength, reaching values comparable to those of the reference CEM I concrete by 365 days of curing. The flexural strength, static and dynamic elastic moduli of the GGBFS-containing concretes, with or without limestone, are comparable to CEM I concrete, at the investigated curing ages. The chloride permeability and water sorption studies demonstrated that concrete made of composite cements containing up to 20 wt% limestone has no detrimental implications on the resistance to chlorides and water ingress. This study also demonstrates that the mechanical properties of concretes with high replacement of blast furnace slag in Portland blended cements, with or without limestone, are compliant with ACI, fib and Eurocode 2 for structural concrete applications.

The impact of carbonation, induced at different CO2 concentrations (0.04 or 1 %), in the phase assemblages and compressive strength of Na2SO4-activated slag materials was determined. Carbonation led to Ca-bearing phases' decalcification (mainly C-(A)-S-H type gel and ettringite) forming different CaCO3 polymorphs, independent of the slag composition or carbonation conditions adopted. In specimens exposed to 0.04 % CO2, a negligible carbonation front was observed, along with a continued phase assemblage evolution and compressive strength gain after 500 days of exposure. Conversely, exposure to 1 % CO2 led to complete carbonation after 28 days, and a significant compressive strength reduction. Accelerated carbonation does not lead to the development of comparable microstructures to those observed in naturally carbonated pastes. The accelerated carbonation rates were similar to 33 times higher than those determined under natural carbonation exposure. Therefore, accelerated tests are considered unsuitable for predicting the long-term carbonation performance of Na2SO4-activated slag cements.

This study introduces a novel reactive transport framework tailored for hardened cementitious materials, applied to a clinker-free sodium sulfate-activated slag cement as a case study. The modelling tool enables the prediction of CO2-induced alterations in the phase assemblage, transport properties (e.g. diffusivity, permeability, saturation, liquid volume fraction, and capillary pressure) in cementitious materials, pore solution pH, the composition of aluminium-substituted calcium silicate hydrate gel (C-A-S-H), and CO2 sequestration profiles as a function of cover depth. The framework was applied to simulate one year of accelerated carbonation (1 % CO2 v/v) and ten years of natural carbonation (0.04 % CO2 v/v) under controlled temperature and humidity conditions, showing excellent alignment with experimental data. This framework represents a significant step forward in the state-ofthe-art for predicting carbonation performance of cementitious materials, enabling calculation and estimation of CO2 uptake capacity of cement systems, including those containing supplementary cementitious materials (SCMs) or based on alkali-activated binders.

The effects of CO2 exposure on sodium sulfate-activated blast furnace slag cement paste have been characterised by X-ray (attenuation) computed tomography revealing changes in micron-scale pore structure, and X-ray diffraction computed tomography (XRD-CT) elucidating changes in the spatial distribution of crystalline and semi-crystalline phases. Accelerated carbonation reduced ettringite volumes and induced formation of hydrotalcite, demonstrating the critical role of Mg-Al-SO4-layered double hydroxide phases in the CO2 uptake of these cements. These changes yield a refinement of small pores and increase the overall porosity, reaching values comparable to those of blended Portland cements. Formation factor values were determined considering the pore solution electrical resistivity, calculated from thermodynamic modelling, and the porosity. A correlation between simulated tortuosity and porosity is proposed to estimate the diffusion tortuosity and formation factor of sodium sulfate-activated slag pastes. This approach represents a significant step forward for assessing carbonation resistance and CO2 uptake capacity of cementitious pastes.

This study investigated the effect of the co-calcination temperature (600, 700, and 800 degrees C) and the blending ratio of green liquor dreg (GLD) and kaolinitic clay (2:1, 1:1, 1:2) to produce a limestone calcined clay (LC2)-type supplementary cementitious material (SCM). The phase assemblage and hydration process of composite cements comprising CEM I and varying replacement ratios (15 and 30 wt%) of the produced SCM were evaluated. An effective cocalcination temperature of 700 degrees C was identified considering the chemical reactivity of the LC2-type SCM determined by the rapid, relevant, and reliable (R3) testing. X-ray diffraction and scanning electron microscopy showed variations in the phase assemblage of the composite cements and the reference CEM I. Calcium carbonate from GLD and metakaolinite from the calcined clay contributed to the formation of carboaluminate in the composite cements, lowering its porosity. The 7- and 28-day compressive strengths of the mortars produced by replacing 15-30 wt % of the cement with this SCM, were comparable to those of the CEM I reference mortar. These findings demonstrate the industrial symbiosis potential between the paper and cement industries via applying co-calcination for resolving challenges for utilization of GLD, while producing a suitable kaolinitic clay based SCM.

The stability of cementitious materials under the harsh environment they will experience when used for radioactive waste disposal is incredibly important. Therefore, understanding the irradiation resistance of geopolymer cement, a potential alternative binder for the treatment of nuclear waste, is of the utmost importance when trying to develop a safety case for these materials. The study presented here addresses the structural and chemical changes of metakaolin-based geopolymers, designed with different water contents, and exposed to a total cumulative dose of 1 MGy of gamma radiation. The range of formulations that were tested showed a significant loss of free water related to the irradiation process, which has led to an increase in the porosity. Analysis of the chemical structure has shown minimal changes in the main binding type-gel phase, demonstrating high microstructural stability. Results showed that in samples cured for longer than 20 h, the bound/gel structure water remained in the sample when the water content was kept low enough. As the porosity and water content increase, more gel water is removed due to radiation exposure. However, the degree to which the water is removed from the gel structure is very small, and minimal changes can be seen across the geopolymers tested. Overall, metakaolin-based geopolymers appear resistant to irradiation up to 1 MGy, which offers a potentially viable alternative for the immobilization of problematic intermediate-level waste.

The pore structures of hardened Portland/slag cement pastes (>75 wt% slag content), and the initial capillary absorption of moisture through these pores, were monitored using ex situ synchrotron X-ray computerised microtomography and in situ quantitative neutron radiography. The pore structure becomes more constricted as the cement hydrates and its microstructure develops. This mechanism was effective even at a slag content as high as 90 wt% in the cementitious blend, where the lowest total porosity and a significant pore refinement were identified at extended curing ages (360 d). By combining this information with neutron radiographic imaging, and directly quantifying both depth and mass of water uptake, it was observed that 90 wt% slag cement outperformed the 75 wt% slag blend at 90 days in terms of resistance to capillary water uptake, although the higher-slag blend had not yet developed such a refined microstructure at 28 days of curing. The assumptions associated with the "sharp front model" for water ingress do not hold true for highly substituted slag cement pastes. Testing transport properties at 28 days may not give a true indication of the performance of these materials in service in the long term.

Low-purity calcined clays are becoming increasingly popular as supplementary cementitious materials (SCMs) due to their wide availability, and potential ability to reduce the carbon footprint associated with concrete production. To ensure the longevity of concrete structures, it is crucial to understand the mechanisms governing long-term durability when using new SCM-containing cement formulations. Understanding of the carbonation resistance of cements containing calcined clay is limited, and this remains a concern. This research is part of the collaborative USA-UK project “Response to CO2 exposure of concrete with natural supplementary cementitious materials” (RENACEM), aiming to understand the connections among the properties of natural clays, activation treatments to enhance their chemical reactivity, and the response to CO2 exposure of cements, mortars and concretes produced with them. The current study presents the carbonation resistance results of binary and ternary materials containing calcined clays upon exposure to natural CO2 concentrations under controlled relative humidity (57

By replacing a large portion of Portland cement with calcined clay, carbon emissions associated with concrete production can be significantly reduced, facilitatingnet-zero targets in construction projects. Vast amounts of soil waste generated from major infrastructure projects could serve as a valuable resource for producing cement replacements or supplementary cementitious materials (SCMs); however, it is largely unknown whether low-purity, low-kaolinite content clays are suitable for producing resilient concrete. This study investigated the use of low-purity calcined clay, derived from excavation operations in the greater London area, as an SCM in the production of concrete. Specifically, the effects of replacing CEM I with calcined excavated London Clay (30 wt.

Mechano-chemical activation enhances early age (<12 hours) pozzolanic reactivity of clays by transforming kaolinite to meta-kaolinite, reducing particle size, and potentially increasing the edge : basal surface area ratio of meta-kaolinite.

Hybrid organic-inorganic binders based on blast furnace slag were produced using sodium (NaAc) or potassium (KAc) acetate as the sole activator, and their properties were compared with those of sodium- or potassium hydroxide-activated slag pastes. The acetate-activated binders showed significantly lower cumulative heat release and extended setting time (∼230 h) than the hydroxide-activated binders. The main reaction products forming in all binders were calcium aluminosilicate hydrate-type gels and a hydrotalcite-like phase, independently of the activator type used. Compressive strengths of the acetate-activated pastes (∼40 MPa at 180 days) were lower than those of the hydroxide-activated binders (∼80 MPa at 180 days). However, the acetate-based binders exhibited superior impermeability and reduced wettability at 28 days, likely due to hydrophobic acetate groups. It is hypothesized that acetates dissociate in water, forming calcium acetate and alkali silicates via a reaction with species dissolving from the slag. This study demonstrates alkali acetates are effective activators for creating hybrid slag-based binders with reduced permeability.

The dimensional stability in stressed states of concrete made with ternary Portland cements containing limestone is not well understood, particularly when the concrete undergoes carbonation. In this study, the dimensional changes of blended slag cements, with and without limestone, under simultaneous compressive stress and accelerated carbonation exposure are reported. Four concretes made with CEM I, CEM I + slag or ternary slag–limestone cements were cured for 7 days before subjecting them to a 30

Measuring the time-dependent deformations of concrete is essential to ensure the serviceability and durability of reinforced concrete (RC) structures. Until recently, however, it remains largely unknown the time-dependent performance of concrete with limestone calcined clay cement (LC3) during their service life. To address this research gap, this research study aims at investigating the shrinkage and creep behaviour of LC3 concrete with or without the moisture exchange in the surrounding environment. LC3 prismatic concrete specimens were prepared and used to measure the total and autogenous shrinkage as well as the total and basic creep behaviour of the material. The experimental results show that the total shrinkage strain of LC3 concrete was lower than that shown in the literature for concrete with other binders. Different curing ages showed limited influences on the evolution of shrinkage of LC3 concrete with time. LC3 concrete creep specimens subjected to compressive load had high elastic strain of 309 µε. Under the sustained load for 56 days, the creep coefficient obtained from the creep test on LC3 concrete was 20.7

This study investigates the potential of using steel slag blended grouts for 3D concrete printing applications. Steel slag, a by-product of the steel-making industry, is considered chemically less reactive compared to other supplementary cementitious materials. However, finely ground steel slag can influence the fresh-state properties of Portland cement mortars used for 3D printing. The effect of steel slag addition on the flow and static yield strength of mortars made with different Portland cement to steel slag ratios was evaluated. Effectiveness of commercial chemical admixtures enhancing the extrusion properties of Portland cement–steel slag blended mortar at the mixing stage, and further for the on-set of mortar after extrusion from the printing nozzle was determined for one selected mix design. Results indicate that adding steel slag reduces the static yield strength of the mix design required for structure build-up. Nevertheless, the accelerating admixture at the printing nozzle of the 3D printer (bi-component, 2K) helps gain the static yield strength required for buildability. Findings reveal that accelerator admixture has no impact on the mechanical performance of mortar containing 50 wt.

Developing a greater understanding of kaolinite dehydroxylation upon calcination is crucial for several industrial applications, including cements. Aluminium coordination in meta-kaolinite indicates the extent of its dehydroxylation and its potential chemical reactivity, and it is typically determined using 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. This technique however presents limitations for Fe-rich materials, given the magnetic properties of Fe ions and minerals containing Fe. In this study, the effect of calcination on Al coordination was assessed in a low-Fe clay used as a reference system, and a Fe-rich clay. Al coordination in the low-Fe clay was quantified via 27Al MAS NMR spectra deconvolution, using data collected at 9.4 T and 11.7 T. Energy dispersive X-ray spectroscopy (EDX) maps and electron energy loss spectroscopy (EELS) measurements were carried out in a scanning transmission electron microscope (STEM) on both clays. Al K-edge EEL spectra showed distinguishable 4/5-fold Al and 6-fold Al sites in both clay types. Differences in line-profile indicated a higher proportion of 4/5-fold Al in kaolinite in the Fe-rich clay compared to the low-Fe clay. Conversely, the Fe-rich clay contained a lower proportion of 4/5-fold Al in meta-kaolinite after calcination, relative to the low-Fe clay. These differences are consistent with the greater structural disorder of the meta-kaolinite identified in the Fe-rich clay by X-ray diffraction and the geological origins of both clays. Overall, this study demonstrates the potential of EELS to provide information about Al coordination for individual kaolinite and meta-kaolinite particles.

The chemical reaction between CO2 and a blended Portland cement concrete, referred to as carbonation, can lead to reduced performance, particularly when concrete is exposed to elevated levels of CO2 (i.e., accelerated carbonation conditions). When slight changes in concrete mix designs or testing conditions are adopted, conflicting carbonation results are often reported. The RILEM TC 281-CCC ‘Carbonation of Concrete with Supplementary Cementitious Materials’ has conducted a critical analysis of the standardised testing methodologies that are currently applied to determine carbonation resistance of concrete in different regions. There are at least 17 different standards or recommendations being actively used for this purpose, with significant differences in sample curing, pre-conditioning, carbonation exposure conditions, and methods used for determination of carbonation depth after exposure. These differences strongly influence the carbonation depths recorded and the carbonation coefficient values calculated. Considering the importance of accurately determining carbonation potential of concrete, not just for predicting their durability performance, but also for determining the amount of CO2 that concrete can re-absorb during or after its service life, it is imperative to recognise the applicability and limitations of the results obtained from different tests. This will enable researchers and practitioners to adopt the most appropriate testing methodologies to evaluate carbonation resistance, depending on the purpose of the conclusions derived from such testing (e. g. materials selection, service life prediction, CO2 capture potential).