On the use of the Jander equation in cement hydration modelling

The equation of Jander [W. Jander, Z. Anorg. Allg. Chem. (1927) 163: 1-30] is often used to describe the kinetics of dissolution of solid cement grains, as a component of mathematical descriptions of the broader cement hydration process. The Jander equation can be presented as kt/R2 =[1-(1-α) (1/3) ]2 where k is a constant, t is time, R is the initial radius of a solid reactant particle, and α is the fractional degree of reaction. This equation is attractive for its simplicity and apparently straightforward derivation. However, the derivation of the Jander equation involves an approximation related to neglect of particle surface curvature which means that it is strictly not correct for anything beyond a very small extent of reaction. This is well documented in the broader literature, but this information has not been effectively propagated to the field of cement science, which means that researchers are continuing to base models on this erroneous equation. It is recommended that if the assumptions of diffusion control and unchanging overall particle size which lead to the selection of the Jander equation are to be retained, it is preferable to instead use the Ginstling-Brounshtein equation [A.M. Ginstling, B.I. Brounshtein, J. Appl. Chem. USSR (1950) 23: 1327-1338], which does correctly account for particle surface curvature without significant extra mathematical complication. Otherwise, it is possible (and likely desirable) to move to more advanced descriptions of particle-fluid reactions to account for factors such as dimensional changes during reaction, and the possibility of rate controlling influences other than diffusion

Alkali-activated cements and concretes – Durability testing to underpin standardisation

Alkali-activated cements, including \u27geopolymer\u27 materials, are now reaching commercial uptake in the UK and elsewhere, providing the opportunity to produce concretes of good performance and with reduced environmental footprint compared to established technologies. The development of performance-based specifications for alkali-activated cements and concretes is ongoing in many parts of the world, including in the UK where the world-first British Standards Institute (BSI) Publicly Available Specification PAS8820:2016 has been published to describe these materials and their utilisation. However, the technical rigour, and thus practical value, of a performance-based approach to specification of novel cements and concretes will always depend on the availability of appropriate and reliable performance tests. This paper will briefly outline the requirements of PAS8820, and discuss the activities of RILEM Technical Committee 247-DTA in working to validate durability testing standards for alkali-activated materials, bringing scientific insight into the development of appropriate specifications for these materials

Alkali-activated materials

© 2017 Elsevier Ltd.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

670 nm light mitigates oxygen-induced degeneration in C57BL/6J mouse retina

BACKGROUND Irradiation with light wavelengths from the far red (FR) to the near infrared (NIR) spectrum (600 nm -1000 nm) has been shown to have beneficial effects in several disease models. In this study, we aim to examine whether 670 nm red light pretreatment can provide protection against hyperoxia-induced damage in the C57BL/6J mouse retina. Adult mice (90-110 days) were pretreated with 9 J/cm2 of 670 nm light once daily for 5 consecutive days prior to being placed in hyperoxic environment (75% oxygen). Control groups were exposed to hyperoxia, but received no 670 nm light pretreatment. Retinas were collected after 0, 3, 7, 10 or 14 days of hyperoxia exposure (n = 12/group) and prepared either for histological analysis, or RNA extraction and quantitative polymerase chain reaction (qPCR). Photoreceptor damage and loss were quantified by counting photoreceptors undergoing cell death and measuring photoreceptor layer thickness. Localization of acrolein, and cytochrome c oxidase subunit Va (Cox Va) were identified through immunohistochemistry. Expression of heme oxygenase-1 (Hmox-1), complement component 3 (C3) and fibroblast growth factor 2 (Fgf-2) genes were quantified using qPCR. RESULTS The hyperoxia-induced photoreceptor loss was accompanied by reduction of metabolic marker, Cox Va, and increased expression of oxidative stress indicator, acrolein and Hmox-1. Pretreatment with 670 nm red light reduced expression of markers of oxidative stress and C3, and slowed, but did not prevent, photoreceptor loss over the time course of hyperoxia exposure. CONCLUSION The damaging effects of hyperoxia on photoreceptors were ameliorated following pretreatment with 670 nm light in hyperoxic mouse retinas. These results suggest that pretreatment with 670 nm light may provide stability to photoreceptors in conditions of oxidative stress.This work was supported by the Australian Research Council Centre of Excellence in Vision Science

Alkali activated slag mortars provide high resistance to chloride-induced corrosion of steel

The pore solutions of alkali-activated slag cements and Portland-based cements are very different in terms of their chemical and redox characteristics, particularly due to the high alkalinity and high sulfide content of alkali-activated slag cement. Therefore, differences in corrosion mechanisms of steel elements embedded in these cements could be expected, with important implications for the durability of reinforced concrete elements. This study assesses the corrosion behavior of steel embedded in alkali-activated blast furnace slag (BFS) mortars exposed to alkaline solution, alkaline chloride-rich solution, water, and standard laboratory conditions, using electrochemical techniques. White Portland cement (WPC) mortars and blended cement mortars (WPC and BFS) were also tested for comparative purposes. The steel elements embedded in immersed alkali-activated slag mortars presented very negative redox potentials and high apparent corrosion current values; the presence of sulfide reduced the redox potential, and the oxidation of the reduced sulfur-containing species within the cement itself gave an electrochemical signal that classical electrochemical tests for reinforced concrete durability would interpret as being due to steel corrosion processes. However, the actual observed resistance to chloride-induced corrosion was very high, as measured by extraction and characterization of the steel at the end of a 9-month exposure period, whereas the steel embedded in WPC mortars was significantly damaged under the same conditions

A discussion of the papers "Impact of hydrated magnesium carbonate additives on the carbonation of reactive MgO cements" and "Enhancing the carbonation of MgO cement porous blocks through improved curing conditions", by C. Unluer & A. Al-Tabbaa

This paper is a discussion of two recent papers by Unluer & Al-Tabbaa which analysed accelerated carbonation of reactive MgO blocks. We suggest that the authors have incorrectly analysed key data, leading to overstated claims of MgO carbonation. Based on the reassignment of their X-ray diffraction data, it is proposed that little MgO carbonation occurred in the samples discussed in those papers, with CaCO3 instead forming during accelerated carbonation. We also draw attention to the thermodynamic instability of nesquehonite under ambient conditions, which calls into question the long-term stability of these binders

Performance of sodium carbonate/ silicate activated slag materials

Alkali-activated slag (AAS) materials are acknowledged as environmentally friendly due to the reduced embodied energy associated with their production. However, the use of highly alkaline solutions such as sodium silicate to promote the chemical reactions that lead to their hardening, poses potential human and environmental hazards that might constrain their utilization beyond specialized applications. It is possible to use less alkaline solution based on near-neutral salts as activators, such as sodium carbonate, to produce alkali-activated slag binders with desirable properties. However, to achieve this, the ‘right match’ between slag chemistry and activation conditions is required. The use of sodium carbonate presents several advantages compared with using sodium silicate when producing AAS, including reducing alkalinity to values comparable to that of Portland cement, and extending the setting time and improving workability, which facilitates the casting of these materials. Sodium carbonate-activated slag binders do not always meet the setting time and strength requirements for on-site concreting, which has limited the application of these materials. A recent study in pastes demonstrated that the addition of sodium silicate in these binders significantly improves the compressive strength development, while effectively controlling the kinetics of reaction, which makes AAS binders produced with a blend of activators an attractive candidate for producing concretes. In this study we report compressive strength, water absorption and durability properties of AAS concretes produced with a blended sodium carbonate/silicate activator. Shrinkage microcracking of these materials was also studied, by drying the specimens for 8 weeks at 65% relative humidity (RH) and 23ºC. The results obtained are compared with concretes produced solely using sodium silicate as alkali activator

Advances in understanding alkali-activated materials

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

Electrochemical cell design and impedance spectroscopy of cement hydration

Understanding the complexity of the chemical and microstructural evolution of cement during hydration remains a controversial subject, and although numerous techniques have been used to assess this process, further insight is still needed. Alternating current impedance spectroscopy has been demonstrated to be a sensitive and powerful technique for cement characterisation in both fresh and hardened states; however, it has also shown certain experimental limitations (e.g. data interpretation, electrode, and parasitic effects) that prevent its wider acceptance. This study assesses electrochemical cell design and the impedance response during cement hydration. The results show that a significant decrease in the parasitic effects at high frequencies (caused mainly by leads and electrode effects) can be achieved through an optimal cell design and impedance measurements correction, enabling correlation of impedance measurements to particular aspects of the cement hydration process. However, due the limited solid phase microstructural development and the high conductivity of cement paste at low degrees of hydration, the parasitic effects could not be fully eliminated for fresh or early-age cement pastes