Bahrami, Hossein; (2017) Early age shrinkage of alkali activated slag paste. Doctoral thesis (Ph.D), UCL (University College London).

Abstract

In recent years, alkali-activated slag (AAS) as an alternative low-carbon emission and high strength cementitious material has received increased attention. However, high shrinkage and crack tendency of AAS limit its wider industrial applications. Although considerable researches have already been carried out on the early age shrinkage of Portland cement based systems, the information and understanding on the early age shrinkage of AAS is still limited. On the other hand, early age shrinkage occurs mainly due to a high rate of water evaporation from the surface of fresh concrete. Therefore, identifying the relationship between evaporation and shrinkage is important to understand the mechanism of early age shrinkage. However, in the literature, there is no available experiment that monitors both the evaporation and the shrinkage at the same time under a controlled environmental condition. This research aims to a) understand fundamental concept behind higher early age shrinkage of AAS paste than that of the PC paste based on moisture loss and shrinkage behaviour by developing new experimental technique, b) Identify influence of different parameters on early-age shrinkage of AAS which is vital to enable competent concrete mix design and long-term performance; and to assist engineers in designing AAS mixes with minimal shrinkage, c) Simulate the experiments to predict the early age shrinkage and crack width at the surface of paste specimens. In this research work, a drying technique based on low-pressure condition was developed so that both the weight loss and the early age shrinkage could be measured under controlled environment in the early drying period from placement to 24 hours. Lateral and vertical shrinkage were measured using two laser displacement sensors. The tests can be performed at different levels of humidity and temperatures which simulate a dry atmospheric condition. Using this system, a comprehensive study can be carried out to compare the early age shrinkage behaviour between sodium silicate activated slag and PC paste. The evaporation and the shrinkage curves of both systems show that the early-age drying under the constant external condition occurs at four distinct stages. Stage I is a period of constant evaporation rate with the rate of the lateral shrinkage of all the samples increased. Stage II is a period with reduced evaporation rate in AAS, in which the lateral shrinkage rate increases until it reaches the critical value which start to fall. The evaporation rate of the PC at stage II is raising due to hydration reactions. At stage III, for both PC and AAS, the evaporation rate is falling with low rate. At stage IV, volumetric contraction stops and the evaporation rate reduces to very low as the water is drawn from the inside of the specimen. The amount of early age shrinkage of AAS is about two times more than that of the PC paste at stage I and II. However, the amount of moisture loss in AAS is about half of the PC paste owing to higher surface tension and viscosity of sodium silicate solution than water. Since the amount of early age shrinkage in stages I and II contributes to about 80% of the total shrinkage, it means that drying at these stages plays an important role in the high early age shrinkage behaviour of the AAS. In another part of this study seeks to explore the potential of three typical mineral additions, namely, pulverised fuel ash (PFA), metakaolin (MK) and limestone powder (LSP), on improving the early age shrinkage of AAS. The slag was replaced by different additions at 5% or 15% by the mass of slag. The mini-slump, setting time, rheology (yield stress & plastic viscosity), early age shrinkage and drying shrinkage were measured. The most significant findings to emerge from this study is that, whilst all the additions showed some potential in reducing the early age shrinkage of AAS, MK also demonstrated its superb performance in diminishing shrinkage, which is rather encouraging. On the other hand, it is interesting to note that PFA also demonstrated some possibilities for improving the rheological characteristics of AAS. The effect of w/b ratio, Na2O content and waterglass modulus was studied on moisture loss and shrinkage of AAS. Parameters including waterglass modulus and Na2O content have significant effects on shrinkage results of sodium silicate activated slag. AAS paste with moduli of 0.75 and activator content of 4% showed lower amount of early age shrinkage. Moreover, the particle size, and specimen’s geometry influence significantly on early age shrinkage and water loss. The cumulative moisture loss is not affected by increase of particle size in PC and AAS. However, the shrinkage is increasing with the reduction median particle size for both AAS and PC pastes. With reduction of sample thickness, the evaporated water and early age shrinkage decreased. However, for both AAS and PC pastes, the increase in surface area showed a lower early age shrinkage. A micromechanical model for autogenous and plastic shrinkage predictions of AAS paste at early-age is developed. The model is based on the capillary tension created in capillary pores in paste and uses the interior humidity (RH) that can relate to the moisture loss in paste directly as driving parameter for shrinkage predictions. The developed model can be used for shrinkage prediction of AAS paste, whatever the shrinkage is caused by self-desiccation or by dying. In addition, because the model can reflect the effect on moisture loss of age and position from one parameter, RH, the model can predict shrinkage strain in paste structures not only for different time but also for different positions. The model predictions on the development of shrinkage strain are compared with experimental results and a good agreement between them is found.

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