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SHRINKAGE CHARACTERISTICS OF ALKALI-ACTIVATED SLAG CEMENTS
SHRINKAGE CHARACTERISTICS OF ALKALI-ACTIVATED SLAG CEMENTS
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, chlorideinduced 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.
TABLE OF CONTENTS
List of Figures …………………………………………………………………………………………………………..v List of Tables …………………………………………………………………………………………………………… vi Acknowledgements …………………………………………………………………………………………………… vii Chapter 1 Objectives and Organization ……………………………………………………………………….. 1 1.1 Introduction …………………………………………………………………………………………………. 1 1.2 Research Objectives ……………………………………………………………………………………… 31.3 Research Approach ………………………………………………………………………………………. 31.4 Outline ………………………………………………………………………………………………………… 6 1.5 References …………………………………………………………………………………………………… 6 Chapter 2 Background ………………………………………………………………………………………………. 9 2.1 Motivation …………………………………………………………………………………………………… 9 2.2 Ground Granulated Blast Furnace Slag ……………………………………………………………. 10 2.3 Alkali Activated Slag ……………………………………………………………………………………. 13 2.4 Microstructure Characteristics ……………………………………………………………………….. 15 2.5 Compressive Strength …………………………………………………………………………………… 16 2.6 Fresh Properties ……………………………………………………………………………………………. 17 2.7 Time of Setting …………………………………………………………………………………………….. 17 2.8 Elastic Modulus and Creep ……………………………………………………………………………. 182.9 Durability ……………………………………………………………………………………………………. 18 2.10 Shrinkage ………………………………………………………………………………………………….. 19 2.10.1 Drying Shrinkage ……………………………………………………………………………… 19 2.10.2 Autogenous Shrinkage ………………………………………………………………………. 23 2.11 Effect of Activator Type and Dosage on AAS Properties…………………………………. 26 2.12 Shrinkage Mechanisms ……………………………………………………………………………….. 27 2.13 Carbonation ……………………………………………………………………………………………….. 28 2.14 Shrinkage Mitigation …………………………………………………………………………………… 292.15 Literature Conclusions ………………………………………………………………………………… 30 2.16 References …………………………………………………………………………………………………. 31 Chapter 3 Shrinkage characteristics of alkali-activated slag cements ………………………………. 37 3.1 Introduction …………………………………………………………………………………………………. 38 3.2 Materials……………………………………………………………………………………………………… 40 3.3 Experimental Methods ………………………………………………………………………………….. 443.4 Results ………………………………………………………………………………………………………… 483.5 Discussion …………………………………………………………………………………………………… 56 3.6 Conclusions …………………………………………………………………………………………………. 60 3.7 Acknowledgements ………………………………………………………………………………………. 61 3.8 References …………………………………………………………………………………………………… 62Chapter 4 Drying shrinkage of alkali-activated slag cements ………………………………………….. 65 4.1 Abstract ………………………………………………………………………………………………………. 654.2 Introduction …………………………………………………………………………………………………. 66 4.3 Experimental ……………………………………………………………………………………………….. 68 4.3.1 Materials ……………………………………………………………………………………………. 68 4.3.2 Test Procedures ………………………………………………………………………………….. 70 4.4 Results ………………………………………………………………………………………………………… 73 4.4.1 Drying shrinkage in nitrogen ………………………………………………………………… 73 4.4.2. Drying shrinkage in air ……………………………………………………………………….. 774.4.3. Elastic modulus …………………………………………………………………………………. 78 4.4.4. Degree of saturation …………………………………………………………………………… 79 4.5 Discussion …………………………………………………………………………………………………… 80 4.5.1. Mass loss, drying shrinkage, and relative humidity ………………………………… 804.5.2. Carbonation shrinkage ………………………………………………………………………… 82 4.5.3. Creep, stiffness, and effective stress …………………………………………………….. 83 4.6 Conclusions …………………………………………………………………………………………………. 87 4.7 Acknowledgements ………………………………………………………………………………………. 88 4.8 References …………………………………………………………………………………………………… 89 Chapter 5 Measuring the chemical shrinkage of alkali-activated slag cements using the buoyancy method ………………………………………………………………………………………………. 92 5.1 Abstract ………………………………………………………………………………………………………. 92 5.2 Introduction …………………………………………………………………………………………………. 93 5.3 Materials……………………………………………………………………………………………………… 95 5.4 Experimental Methods ………………………………………………………………………………….. 97 5.5 Results & Discussion ……………………………………………………………………………………. 98 5.6 Conclusions …………………………………………………………………………………………………. 102 5.7 Acknowledgements ………………………………………………………………………………………. 103 5.8 References …………………………………………………………………………………………………… 103 Chapter 6 Conclusions and Future Research ………………………………………………………………… 105 6.1 Conclusions …………………………………………………………………………………………………. 105 6.2 Future Research and Potential Mitigation Strategies …………………………………………. 107 Chapter 1
Objectives and Organization
1.1 Introduction
Concrete is a composite material in which aggregates (quarried) are bound together using a mixture of Portland cement and water. The mixture hardens and gains strength over time. Portland cement is produced by heating ground limestone and a source of silica (sand and/or clay) in a rotary kiln at temperatures up to 1550°C, resulting in an energy intensive process (embodied energy = 5.7MJ/kg) with a large CO2 footprint (0.95kg CO2/kg cement) (Mindess et al. 2003, Marinshaw and Wallace, 1994). The annual production of concrete exceeds 1010 tonnes/year, more than all other manmade materials combined (Ashby, 2009). This large production rate creates a large carbon footprint for Portland cement, resulting in approximately 5% of the world’s total anthropogenic green house gas (GHG) emissions (Damtoft et al. 2008). The aging infrastructure in developed countries, coupled with shrinking financial resources for repair and renewal of the current infrastructure, has resulted in a much greater need to improve the durability and extend the service life of concrete structures. This underlines the need for evaluating more environmentally friendly products to replace Portland cement in concrete. A growing incentive to develop durable, highly recycled, energy efficient and environmentally friendly concrete materials has initiated research on alkali-activated concretes. Multiple products have been researched up to date to partially or fully replace Portland cement in concrete. Research has shown (Ben Haha et al. 2011, Shi et al. 2006, Radlinska et al. 2012) that industrial waste products, e.g. blast furnace slag from steel manufacturing or fly ash from coal combustion, can be used to successfully produce a Portland cement-free concrete. Alkali-activated slag (AAS) is a promising product, proven to have cementitious properties capable of possessing compressive strengths equal to or higher than those of OPC. Certain durability aspects of AAS have been shown to be quite favorable compared to OPC. Shi et al. (2006) has shown that AAS materials have lower chloride and water permeability and a higher resistance to attack by acids, sulfates, and fires. However, one durability aspect limiting the usage of AAS is its volumetric instability. For example, many authors (Shi et al. 2006, Melo Neto et al. 2008, Douglas et al. 1992, Bakharev et al. 2000, Palacios and Puertas 2007, Cincotto et al. 2003) have shown that AAS concrete can have up to 2.5 times larger shrinkage deformations than PC mixtures. This high shrinkage poses concerns since it can result in cracking when concrete members are restrained from shrinking freely (e.g., in pavements and bridge decks). Formation of cracks allows salts and other aggressive chemicals to penetrate the concrete, corrode the reinforcing steel, and significantly reduce the service life of the structure. The shrinkage mechanisms of AAS systems remain relatively unknown as certain types of shrinkage (e.g., chemical and carbonation) have yet to be properly measured and analyzed. Many of the current shrinkage test methods are either not standardized or not directly applicable to AAS materials, thus other methods need to be explored. Furthermore, a comprehensive study that investigates every mechanism (i.e., creep, modulus of elasticity, pore size distribution, and bulk modulus) that can potentially cause shrinkage in AAS systems has not been performed. Several attempts to mitigate shrinkage in AAS have failed due to a lack of understanding of these shrinkage mechanisms. This research aims to properly measure, investigate, and mitigate the AAS shrinkage problem. Doing so can provide tremendous benefits towards producing durable AAS concretes and a viable replacement for OPC concrete.
1.2 Research Objectives
The main objective of this research is focused on measuring, understanding, and mitigating the large shrinkage deformations of AAS. The first step is to correctly measure each shrinkage type individually to determine how much AAS materials shrink. Subsequently, determining which parameters govern each type of shrinkage will provide insight on why these materials have such large deformations. Bridging such knowledge gaps will then result in developing efficient shrinkage mitigation tools for alkali-activated concretes and create a viable alternative for OPC concrete.
1.3 Research Approach
This research was conducted based on four AAS mixtures and the characterization of their mechanical and shrinkage properties. The study provides comparisons with properties of an OPC mixture of similar initial porosity. Depending on the test performed, paste or mortar was used. Four AAS mixtures and a control OPC mixture were designed using a constant volumetric liquid (water + activator) to solid (slag or cement) ratio of 1.30, yielding an initial binder porosity of 56.5%. Binder porosity was calculated assuming 100% hydration of cementitious material, 1:1 volume consumption of the liquid to solid, and evaporation of excess liquid after the entirety of the cement (or slag) powder had reacted. For the control OPC mixture, this is equivalent to a mass-based w/c = 0.414. For the AAS mixtures, the mass-based liquid to solid ratio was in the range 0.489-0.510, depending on the concentration and density of the alkali activating solutions (Table 1-1). A constant volumetric liquid to binder ratio creates equivalent initial binder porosities across all five mixtures that allows for easy comparisons of paste and mortar properties. Table 1-1. Naming system for mixtures
Mix ID (Description) | n = (SiO2/Na2O) molar-based | pH | Density (g/cc) |
AAS1 (Lower SiO2 Na-Si) | 0.41 | 14.31 | 1.09 |
AAS2 (Higher SiO2 Na-Si) | 1.22 | 13.78 | 1.13 |
AAS3 (2M NaOH) | 0 | 14.30 | 1.04 |
AAS4 (4M NaOH) | 0 | 14.60 | 1.09 |
Figure 1-1: Typical Age vs. Shrinkage graph. Source: Palacios and Puertas (2007) Table 1-1 provides a simple naming system for the mixtures throughout the entire project. The majority of the research will focus on comparing shrinkage strains and other parameters of the four AAS mixtures to OPC. Figure 1-1 provides the typical format of an age vs. shrinkage graph that will be presented throughout this report. These types of graphs will provide easy comparisons of several types of shrinkage tests of the four AAS mixtures and OPC. The following tests are performed on all mixtures:
- ASTM C1437: Standard Test Method for Flow of Hydraulic Cement Mortar
- ASTM C109: Standard Test Method for Compressive Strength of Hydraulic Cement
Mortars
- ASTM C403: Standard Test Method for Time of Setting of Concrete Mixtures by
Penetration Resistance
- ASTM C596: Standard Test Method for Drying Shrinkage of Mortar Containing Hydraulic Cement
- ASTM C1698: Standard Test Method for Autogenous Strain of Cement Paste and Mortar
- Degree of Hydration of Blast Furnace Slags: Procedure adopted from Luke and Glasser (1987)
- Chemical Shrinkage of Hydraulic Cements: Procedure adopted from Sant et al.
(2005) The first three tests listed (mortar flow, time of setting, and compressive strength) characterize the development of AAS and OPC mixtures. Shrinkage deformations were quantified by running drying shrinkage (ASTM C596), autogenous shrinkage (ASTM C1698), and chemical shrinkage (via the buoyancy method (Sant et al. 2005)) on each mixture. The degree of hydration of every AAS mixture was measured (via selective acid dissolution (Luke and Glasser, 1987) to normalize the results of the chemical shrinkage. Drying shrinkage of non-standardized “mini-bars” in environmental chambers will be measured under various relative humidities as well as compressed dry nitrogen (N2) and air purge. These results will provide insight on the role carbonation plays in the drying shrinkage of AAS materials.
1.4 Outline
This thesis is presented in five (5) remaining chapters. Chapter 2 will provide the motivation and background on the characterizing the development and shrinkage of AAS systems. Chapter 3 is a technical paper named “Shrinkage characteristics of alkali-activated slag cements” recently accepted by the ASCE Journal of Materials in Civil Engineering. This paper was initially a conference paper accepted and presented at the Third International Conference on Sustainable Construction Materials and Technologies on August 18-21, 2013. The paper was selected for recognition as an “Award Winning Paper” and sent to ASCE Journal of Materials in Civil Engineering for journal publication. Chapter 4 is another technical paper named “Drying shrinkage of alkali-activated slag cements. Part 1: Characterization of drying shrinkage under various relative humidities” recently submitted to the Cement and Concrete Research journal. Chapter 5 is a conference paper named “Measuring the chemical shrinkage of alkali-activated slag cements using the buoyancy method”. This paper was accepted and presented at the Ninth International Conference on Creep, Shrinkage, and Durability Mechanics (CONCREEP-9) on September 22-25, 2013 in Cambridge, MA. Finally, Chapter 6 will conclude the thesis proposal with research conclusions and future work. References are provided at the end of each respective chapter.
1.5 References
American Society for Testing and Materials (ASTM), “Standard Practice for Mechanical Mixing of Hydraulic Cement Pastes and Mortars of Plastic Consistency,” ASTM International, ASTM C 305-12. American Society for Testing and Materials (ASTM), “Standard Specification for Concrete Aggregates,” ASTM International, ASTM C 33M-11a American Society for Testing and Materials (ASTM), “Standard Specification for Slag Cement for Use in Concrete and Mortars.” ASTM International, ASTM C989-10. American Society for Testing and Materials (ASTM), “Standard Test Method for Compressive Strength of Hydraulic Cement Mortars,” ASTM International, ASTM C 109M-11. American Society for Testing and Materials (ASTM), “Standard Specification for Lightweight Aggregate for Internal Curing of Concrete,” ASTM International, ASTM C 1761M-12. American Society for Testing and Materials (ASTM), “Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance,” ASTM International, ASTM C403M- 08. American Society for Testing and Materials (ASTM), “Standard Test Method for Drying Shrinkage of Mortar Containing Hydraulic Cement,” ASTM International, ASTM C596. American Society for Testing and Materials (ASTM), “Standard Test Method for Flow of Hydraulic Cement Mortar,” ASTM International, ASTM C1437-07. American Society for Testing and Materials (ASTM), “Standard Test Method for Autogenous Strain of Cement Paste and Mortar,” ASTM International, ASTM 1698. Ashby, M. (2009), Materials and the Environment, Elsevier, Inc, Burlington, MA Bakharev, T., Sanjayan, J.G., Cheng, Y.B. (2000), “Effect of admixtures on properties of alkaliactivated slag concrete.” Cement and Concrete Research, 30, 1367-1374. Ben Haha, M., Le Saout, G., Winnefeld, F., Lothenbach, B. (2011), Influence of activator type on hydration kinetics, hydrate assemblage and microstructural development of alkali activated blast furnace-slags. Cement and Concrete Research, 41 301-310. Cincotto, M.A, Melo A. A., Repette, W.L. (2003), “Effect of different activators type and dosages and relation to autogenous shrinkage of activated blast furnace slag cement.” In: G. Grieve, G. Owens, Eds, Proc. 11th International Congress on the Chemistry of Cement, Durban, South Africa, (2003) 1878-1888. Damtoft, J.S., Lukasik, J., Herfort, D., Sorrentino, D., Gartner, E.M. (2008), Sustainable development and climate change initiatives, Cement and Concrete Research, 38, 115-12. Douglas, E., Bilodeau, A., Malhotra, V.M. (1992), Properties and durability of alkali-activated slag concrete, ACI Materials Journal, 89, 509-516. Luke, K., Glasser, F.P. (1987), Selective dissolution of hydrated blast furnace slag cements, Cement and Concrete Research, Vol. 17, pp. 273-282. Marinshaw, R., Wallace, D., (1994). “Emission factor documentation: Portland cement manufacturing.” United States Environmental Protection Agency (EPA), www.epa.gov/ttn/chief/ap42/ch11/bgdocs/b11s06.pdf (accessed May 15, 2013). Melo Neto, A.A, Cincotto, M.A., Repette, W. (2008) “Drying and autogenous shrinkage of pastes and mortars with activated slag cement.” Cement and Concrete Research 38(4) 565-574. Mindess, S., Young, J.F., Darwin, D. (2003), Concrete 2nd Ed., Prentice Hall, Upper Saddle River, New Jersey. Palacios, M., Puertas, F. (2007), Effect of shrinkage-reducing admixtures on the properties of alkali-activated slag mortars and pastes, Cement and Concrete Research 37, 691–702. Shi, C., Krivenko, P., Roy, D. (2006), Alkali-Activated Cements and Concrete, Taylor & Francis. Sant, G., Lura, P., Weiss, J. (2005), Measurement of Volume Change in Cementitious Materials at Early Ages, Transportation Research Board. Radlinska, A., Salera, M., Ernst, S., Yost, J., Rajabipour, F. (2012), “Linking material and structural response of alkali-activated fly ash binders.” International Congress on Durability of Concrete.