Shrinkage characteristics of alkali-activated slag cements

Open Access
Author:
Cartwright, Christopher Paul
Graduate Program:
Civil Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 21, 2014
Committee Members:
  • Aleksandra Z Radlinska, Thesis Advisor
  • Farshad Rajabipour, Thesis Advisor
  • Dr Maria Lopez De Murphy, Thesis Advisor
Keywords:
  • drying shrinkage
  • autogenous shrinkage
  • chemical shrinkage
  • slag
  • cement
  • mortar
Abstract:
The annual production of Portland cement concrete exceeds 1010 tonnes/year. This large production volume has resulted in high energy consumption and emissions (e.g., CO2) harmful to the environment. In addition, the aging infrastructure in developed countries coupled with shrinking financial resources for repair and renewal of infrastructure has resulted in much greater need to improve the durability and extend the service life of concrete structures. A growing incentive to develop durable, highly recycled, energy efficient and environmentally friendly concrete materials has initiated research on alkali-activated concretes. Alkali-activated slag (AAS) utilizes blast furnace slag, an industrial byproduct of steel manufacturing, as a full (100%) replacement of Portland cement in concrete. AAS concrete has a low embodied energy, low CO2 footprint, and equivalent or better strengths in comparison with ordinary Portland cement (OPC) concrete. AAS also has better durability against fire, chloride-induced corrosion, and chemical (e.g., acid, sulfate) attack. On the other hand, one of the most significant conundrums impeding the implementation of AAS is their volumetric instability. Specifically, AAS concretes are prone to significant shrinkage as a result of drying, carbonation, and self-desiccation and have shown a high risk of cracking. The main objective of this research is focused on characterizing and understanding shrinkage deformations of AAS. It provides an unprecedented detailed study of the development of several AAS mortar mixtures and characterizes their susceptibility to the different types of shrinkage (drying, carbonation, and autogenous). It was found that AAS mixtures, with comparable strength to OPC, show a higher autogenous and drying shrinkage. A lower elastic stiffness, higher degree of saturation, and potentially higher chemical shrinkage contribute to the high autogenous shrinkage of AAS. A lower elastic stiffness of mixtures activated solely by NaOH leads to a larger drying shrinkage. At each relative humidity, AAS mixtures lose more mass and have larger drying shrinkage deformations than OPC mixtures, regardless of the drying rate. However, the ultimate drying shrinkage values of AAS are dependent on the drying rate as these materials shrink more when the relative humidity is decreased gradually instead of rapidly. The carbonation of AAS materials increases their drying shrinkage deformations. At high relative humidities (70% RH), creep plays a significant role in the drying shrinkage of AAS. The degree of saturation of AAS mixtures upon drying is lower than that of OPC and does not contribute to high drying shrinkage strains. Finally, after adjusting for the degree of hydration, the average chemical shrinkage of AAS paste is determined as 0.253 ml/gslag. This is 3.95 times larger than the chemical shrinkage of Portland cement, and contributes directly to the large autogenous shrinkage of AAS materials.