Effects of temperature and curing duration on the stability of slag cements in combined chloride-sulphate environments

This experimental study investigates the effects of temperature and curing duration on the stability of slag blended cement systems exposed at 20 °C and 38 °C to combined sodium chloride (30 g/L)-sodium sulphate (3 g/L) solutions. Two slags, designated as slag 1 and 2, having CaO/SiO2 ratios of 1.05 and 0.94, were respectively blended with Portland cement CEM I 52.5R at 30 wt.% replacement level. Mortar prisms and cubes with w/b ratio of 0.5 and binder/aggregate ratio of 1:3 were then prepared for length and mass changes. The samples were cured in lime water for either 7 or 28 days before ponding for a total exposure period of 544 days. Analogous paste samples were also prepared to follow changes in the hydration products using X-ray diffraction (XRD). The results showed that curing at 38°C resulted in less expansion and prolonged curing generally reduced expansion except for slag 1 blend at 20 °C. Also, mass-change was minimal at 38 °C compared to 20 °C, and curing up to 28 days further improved mass stability. There was a positive correlation between mass change and length change for the period of investigation

Development of calcium sulfoaluminate cement composites for nuclear waste encapsulation

In the UK, nuclear wastes are usually ‘cemented’ before disposal so that harmful radionuclides can be physically and chemically contained. In this process, conventional Portland cement is blended with high levels of relatively inert mineral additions, mostly to reduce the high heat evolution in large pours. Calcium sulfoaluminate cement (CSAC) has recently attracted interest in various applications due to its lower pH and ability to bind significant quantities of water compared with conventional Portland cement. Such qualities are particularly suited to the encapsulation of legacy wastes such as aluminium and uranium, which would otherwise corrode if embedded within a Portland cement environment. While some early trials have demonstrated 111 good potential of CSAC, the rapid reaction rate (and associated heat generation) is still restricting its use. In this paper, common mineral additions such as ground granulated blastfurnace slag (GGBS), pulverised fuel ash (PFA) and limestone powder (LSP) were incorporated at very high replacement levels (up to 75%) in an attempt to dilute the cement matrix and hence reduce the heat of hydration. Studies of compressive strength, heat of hydration and aluminium corrosion revealed that these CSAC composites demonstrate excellent potential for aluminium waste encapsulation. Keywords: Calcium sulfoaluminate cement, composites, nuclear waste encapsulation, corrosion, aluminiu

Influence of chloride ions on progress of carbonation in concretes

Our infrastructure and environment face unprecedented challenges in addressing a low carbon future with limited natural resources, expanding population, increased pollution and climatic uncertainties. Adaptation and innovations must therefore play a vital role in addressing the anticipated wide ranging complex scenarios ahead. The environment in which construction materials will need to function will become far more complex and aggressive and hence a fundamental revaluation of the most appropriate materials for future infrastructure and environment will be required in order to tackle those challenges. This paper focuses on a class of construction materials, both old and new, based on magnesia (MgO). They include a wide range of materials from those that contain MgO as a small additive to those which solely consist of MgO. They include concrete with MgO as an expansive additive, pervious concrete, alkali-activated cements, magnesium phosphate cements, carbonated products, stabilising additives for ground improvement, self-healing additives, carbon capture and storage materials and binders for waste 105 and contaminated land remediation. Those materials and products offer a range of technical and sustainability benefits for a range of structural, geotechnical and environmental applications. The paper highlights the applications and benefits that would be achieved with magnesia-bearing construction materials

Effect of calcium sulfates on the early hydration of calcium sulfoaluminate cement and the stability of embedded aluminium

Conventional Portland cement-based systems have been considered un-suitable for immobilising nuclear wastes containing reactive metals, such as alu-minium, due to the high pH of the pore solution (usually around 12.5) and free moisture. On the contrary, calcium sulfoaluminate cement (CSA) produces a low-er pH (10.5-12) environment and has an excellent water binding capability as a re-sult of the formation of its main hydration product, ettringite. Therefore, it offers a good potential to immobilise aluminium. However, the pore solution pH and ettringite formation depend largely on the raw materials used to formulate the CSA, which is usually a blend of 75%-85% of CSA clinker and 15-25% of calci-um sulfate (in the form of gypsum or anhydrite). In this paper, it was found that, compared to anhydrite, gypsum (15%wt of the blend) demonstrated the highest reduction in the corrosion of embedded Al, possibly due to its lower initial pH (around 10.5) and self-desiccating nature at the early stage of hydration. Whilst the CSA/anhydrite had a higher Al corrosion rate, the initial set was more ac-ceptable than CSA/gypsum. Nonetheless, overall, it was concluded that CSA with gypsum (15%wt) should be considered as a base formulation for the encapsulation of Al waste. The unfavorable rapid set and high heat generation, however, demon-strated that modifications are required, potentially by using mineral additions

Monitoring the cementitious materials subjected to sulfate attack with optical fiber excitation Raman spectroscopy

Formation of ettringite and gypsum from sulfate attack together with carbonation and chloride ingress have been considered as the most serious deterioration mechanisms of concrete structures. Although electrical resistance sensors and fiber optic chemical sensors could be used to monitor the latter two mechanisms on site, currently there is no system for monitoring the deterioration mechanisms of sulfate attack. In this paper, a preliminary study was carried out to investigate the feasibility of monitoring sulfate attack with optical fiber excitation Raman spectroscopy through characterizing the ettringite and gypsum formed in deteriorated cementitious materials under an optical fiber excitation + objective collection configuration. Bench-mounted Raman spectroscopy analysis was also conducted to validate the spectrum obtained from the fiber-objective configuration. The results showed that the expected Raman bands of ettringite and gypsum in the sulfate-attacked cement paste can be clearly identified by the optical fiber excitation Raman spectrometer and are in good agreement with those identified from bench-mounted Raman spectrometer. Therefore, based on these preliminary results, it is considered that there is a good potential for developing an optical fiber-based Raman system to monitor the deterioration mechanisms of concrete subjected to sulfate attack in the future

The Influence of Different European Cements on the Transport and Early-age Properties of Concrete in the Cover-zone

The use of in situ tests for performance based specification would require demonstration of their suitability to distinguish the quality of concrete. With the introduction of new European Standards for cements, this would mean concretes produced with these new cements should be classified for their quality using the performance tests. It is generally believed that transport properties of concrete are related to their durability and hence the measurement of these properties can form the basis of performance based specifications. This paper reports data indicating that transport properties measured at 28-days for concretes manufactured with different European cements and water-binder ratios can form the basis of classifying concrete for their durability. The results also demonstrated how the different cements specified in European Standards influence the transport properties and other early-age properties

Chloride ingress through alkali activated slag concretes

Alkali activated slag (AAS) is a credible alternative to Portland cement (PC) based binder systems. The superior strength gain and low embodied carbon make it a potential binder for next generation concretes. However there is little known about the long term durability of AAS systems, especially the chloride transport and subsequent corrosion of reinforcing steel. In this study, chloride transport through 12 AAS concretes with different alkali concentrations (Na2O% of mass of slag) and different modulus (Ms) of sodium silicate solution activator was investigated. A non-steady state chloride diffusion test was used for this study due to its similarity to the real exposure environment in terms of chloride transport through concrete. The results showed that the chloride concentration at the surface (Cs) of AAS concretes was higher than that for PC concrete. However, lower non-steady state chloride diffusion coefficient (Dnssd) was obtained for the AAS concretes. The Dnssd of the AAS concretes decreased with the increase of Na2O% and Ms of 1.50 gave the lowest Dnssd. The results are encouraging and it can be concluded that AAS concrete offers a superior performance in terms of chloride transport

Establishing the Carbonation Profile with Raman Spectroscopy: Effects of Fly Ash and Ground Granulated Blast Furnace Slag

Establishing the carbonation profile is of great significance to the prediction of the service life of reinforced concrete structures. In our previous work, Raman spectroscopy was shown to be an efficient tool for characterizing calcium carbonate (CaCO3) polymorphs and their profile in plain Portland cement (PC) matrices. However, as supplementary cementitious materials (SCMs), particularly fly ash (FA) and ground granulated blast furnace slag (GGBS), are widely used in concrete, establishing the carbonation profile without considering the possible effects of these SCMs could be of little significance to the real world. This paper, thus, investigated the effects of FA and GGBS on the working capacity and reliability of Raman spectroscopy for establishing the carbonation profile in PC blends containing SCMs. The thermogravimetry (TG) analysis was also conducted to verify the results from Raman spectroscopy. The results show that Raman spectroscopy demonstrated a good capacity for differentiating the variation of CaCO3 contents in FA or GGBS blends. However, the incorporation of FA and GGBS into the PC system caused some adverse effects on the quantification of CaCO3 by Raman spectroscopy, which could be attributed to the darker color and weak scatter nature of FA and the high content of glassy phases in GGBS

A Raman spectroscopy based optical fibre system for detecting carbonation profile of cementitious materials

Sensors demonstrate huge potential in civil engineering for monitoring the health condition and performance of concrete structures. Amongst various chemical deterioration mechanisms causing inadequate durability of concrete structures, carbonation is one of the most severe mechanisms. It occurs from the chemical reactions between intruded CO2 and calcium-bearing phases, hence is accompanied by the formation of calcium carbonate (CaCO3) and the decrease of the alkalinity of concrete pore solution, causing corrosion of rebar in concrete. Thus, detecting carbonation process, especially, determining the carbonation profile (i.e. the content of carbonation products formed against the depth into concrete structure), is of great importance to the diagnosis of the health condition of concrete structures and the prediction of service life. Unfortunately, existing sensors for health monitoring systems suffer from various limitations. Optical fibre Raman technology offers a unique opportunity for developing a novel chemical sensor system capable of monitoring the service-condition of concrete in situ. In the current work, a bespoke ‘coaxial’ optical fibre sensing platform based on Raman spectroscopy was successfully established with a 514.5 nm laser. All the optics were tailored for efficiently exciting and receiving signals from cementitious materials, and their diameters were restricted within 0.5 in. in order to explore the feasibility of developing an embeddable miniature sensor system in the future. This sensing system was then employed to detect the carbonation mechanism of a plain Portland cement (PC) paste. The calcium carbonate polymorphs as well as the carbonation profile in the PC paste was successfully recognised and established with the results being verified favourably by bench-mounted Raman, X-ray Diffraction (XRD) and Thermogravimetry (TG) analyses. Our results demonstrate a good potential for developing a novel Raman spectroscopy based optical fibre sensor system for monitoring the health condition and the performance of concrete structures in future