Alkali Activation of Common Clay Deposits: Evaluation of the Suitability by an IR Spectroscopic Method

In the context of a sustainable use of resources with the aim of the reduction of the CO2 footprint, the development of alternative concrete materials has attracted a great deal of attention. In this context, geopolymers, obtained from common clay deposits, are found to be interesting construction materials with very versatile properties. In this paper, a completely novel approach for the evaluation of the suitability of clays for the geopolymer formation is investigated. The method is based on simple and easy-to-handle IR spectroscopic measurements, through which the surface area under the OH stretching band in the IR spectrum of the clay can directly be correlated to the amount of reactive clay components. These reactive components are required for the success of the alkali activation of the clays in order to access geopolymers. Based on the theoretical reaction pathway of the geopolymer formation, the linear relationship between the OH stretching band area and the reactive components can be used for the estimation of the required activator amount for the alkali activation of calcined clays and predict the quality of the casted geopolymer mortar in terms of strength. This new method not only gives an insight into the suitability of a common clay for the geopolymer formation, but also facilitates a straightforward alkali activation procedure without tedious preliminary testing of the required activator amount

Microstructure and Mechanical Properties of Slag Activated with Sodium Silicate

The study focuses on developing a detailed understanding of microstructure and mechanical behaviour of alkali-activated slag with sodium silicate. The slag used in this study was provided by Ecocem (Netherlands) and consists of 38.8 % CaO, 36.3 % SiO$_2$, 12.8 % Al$_2$O$_3$ and 8.0 % MgO. It was activated with two sodium silicate solutions. The two mixes were studied on a paste level. They have the same water/slag ratio w/s = 0.4 kg/kg and the same sodium content n = 5.0 g Na$_2$O / 100 g slag. Two silicate ratios were investigated: Ms = 0.5 and Ms = 2.2 moles of SiO$_2$ / moles of Na$_2$O. Early-age mechanisms and reaction products of the activation of the slag were analysed with isothermal calorimetry, SEM images and chemical characterization. In addition, compressive and flexural strength were determined. Despite different reaction steps in comparison to ordinary Portland cement, the obtained mechanical properties seem to be sufficient for structural application

On the relation between the mean compressive strength and the characteristic one

Since the beginning of the construction of structures with reinforced concrete, it has been known that concrete presents a variability that should be taken into account. In modern codes, this variability implies the use of a characteristic strength corresponding to a 5% fractile of the distribution of strength. The actual relation in Eurocode 2 between the characteristic and the mean strength fck = fcm ? 8 MPa was introducedseveral years ago by Rüsch and is integrated in CEB or fib model codes since 1978. In this article, it is presented how the relation was obtained and it is discussed if this relation is still valid considering the fact that the range of concrete strengths is now larger. Considering the scatter of the SD, the relation proposed by Rüsch could still be used but engineers should keep in mind that the SD on site could be very different from the one predicted by means of the relation between fck and fcm

Experimental study on basic and drying creep for an alkali‐activated slag concrete and comparison with existing creep models

Slag is a by-product of the steel industry that can be activated using alkali solutions to form concrete. This study presents new experimental results of basic and drying creep behavior of alkali-activated slag (AAS) concrete. Different parameters affecting creep such as loading age, sample size and creep stress-strength ratio were varied for experimental studies. The results show that the basic creep of AAS concrete is higher than that of ordinary Portland cement (OPC) concrete. The drying creep of AAS is lower than for OPC and this could be explained by a higher internal drying during the activation of slag. The experimental results were used to check the applicability of two existing engineering models, the fib MC 2010 and the B4s model, for AAS concrete. It was found that both models could be extended to predict the basic creep of AAS concrete. For drying creep, the B4s could better capture the creep behavior. For the fib MC 2010, a new formulation for drying creep would be required

Extension of the fib MC 2010 for basic and drying shrinkage of alkali‐activated slag concretes

Alkali-activated slag is an alternative to ordinary Portland cement that has been studied for the past 20 years. One of the main challenges for its practical use is understanding and controlling its shrinkage behavior. In this study, new experimental results for two alkali-activated slag concrete mixes are presented under both sealed and unsealed conditions. The results show that basic shrinkage increases with increased sodium silicate ratio. Under unsealed conditions, the age to exposure to drying has a most significant impact on the final drying shrinkage. Finally, the mechanisms explaining shrinkage of such materials are discussed and thefib Model Code 2010 is extended for alkali-activated slag concrete using the new experimental results. The extended model consists of four parameters influencing the final values and the speed of both basic shrinkage and drying shrinkage. It is extensively compared with experimental datasets from the literature and improves significantly predictions compared with the original models for both basic and drying shrinkage. This demonstrates clearly the feasibility to extend it for predicting shrinkage of alkali-activated slag concrete

State-of-the-Art Report - Plastic Concrete for Cut-Off Walls

Plastic Concrete plays a key role in the remediation of earthen dams using cut-off walls to counter dam seepage. However, Plastic Concrete has yet to be thoroughly understood, since little attention has been given to this material in literature. The principal objective of this report is to set out the fundamental material science parameters, which describe Plastic Concrete’s mechanical and hydraulic behaviour as well as describing the mix design and application of Plastic Concrete. For this, an extensive and comprehensive literature review was carried out. The results show that Plastic Concrete can hereby be considered to be a low-strength, low-stiffness impervious concrete with a high deformation capacity under load and the capability of sustaining larger strains than normal concrete. This study further identifies reference values, which may be used in cut-off wall design. All in all, the research results represent a further step towards the understanding of Plastic Concrete material behaviour

Alkalinity and Its Consequences for the Performance of Steel-Reinforced Geopolymer Materials

This paper investigates the development of the alkalinity and its impact on carbon steel reinforcement embedded in alkali-activated fly ashes (AAFA) and alkali-activated fly ashes with ten percentage mass (wt%) of blast furnace slag (AAFAS)-based materials (geopolymer–GP). The pH analysis of eluates indicates a remarkable decrease of alkalinity in AAFA and AAFAS in the first hours of the geopolymerization process. Phenolphthalein solution and pore solution tests on concretes also show a sharp decrease of alkalinity with increased Ca content in the binder due to carbonation. Micro X-ray computer tomography (µXCT) and electrochemical techniques indicate that the changed pH in the GP systems was accompanied by a decrease in the corrosion rates of steel reinforcement when compared to ordinary Portland cement (OPC) systems. In contrast to calcite and vaterite, which were detected in OPC and AAFAS after a carbonation process, only sodium carbonate natron was determined at lower levels in AAFA by X-ray diffraction (XRD)

Effect of steel fiber on the crack permeability evolution and crack surface topography of concrete subjected to freeze-thaw damage

This paper describes the steel fiber effect on the crack permeability and crack surface topography of concrete subjected to freeze-thaw damage. The sequential crack permeability of steel fiber reinforced concrete are investigated by a vacuum permeability set-up. The topographical analysis is applied on the crack surface by an invented 3-D laser scanning equipment. The results show that the crack permeability of concrete is less than the value predicted by the Poiseuille flow model and their difference decreases gradually with the crack widening. With increment of steel fiber dosage and freeze-thaw damage level, the effect of steel fiber on reducing the crack permeability becomes strong. Topographical analysis illustrates that both steel fiber and freeze-thaw damage enhance the roughness of concrete crack surface. The relationship between roughness number of crack surface and material permeability parameter α follows an exponential function, which can be employed to quickly estimate the crack permeability of concrete.National Natural Science Foundation of China (Grant: 527 51578109