Controls on Thaumasite in Buried Concrete: Effect of Clay Composition and Cement Type
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Controls on Thaumasite in Buried Concrete: Effect of Clay Composition and Cement Type A thesis submitted for the degree of Doctor of Philosophy in The Faculty of Engineering of the University of Sheffield By Farhat Abubaker (BSc Structural Eng., MSc Concrete Eng.) Department of Civil and Structural Engineering The University of Sheffield June 2014 ACKNOWLEDGEMENTS First, I would like to express my gratitude to Dr John Cripps and Dr Cyril Lynsdale for supervising this work. My main acknowledgement is for their constant encouragement, valuable and accurate scientific advice, discussions and rising of important questions. Their enthusiasm, commitment and patience provided a good basis for presenting this thesis. Without their support this work would not have started and certainly would not have finished. This thesis surely became reality only through their support. I would like to acknowledge with gratitude the support of the Ministry of Higher Education, Libya, for funding this project. I would like to thank the technicians of the Structures Laboratory, Geotechnical Laboratory and Groundwater Protection Laboratory for their continuous and excellent help during the experimental work. Especially, I would like to thank Kieran Nash, Paul Blackbourn, Andy Fairburn, Paul Osborne and Mark Foster. I am grateful to the University of Sheffield for the opportunity to be a student of one of best UK universities and one which is highly ranked and recognised throughout the world. Finally, special thanks are due to my brothers and sisters and my wife. I would like to thank them for their valuable support and encouragement during my study and hard moments; without their support it would have been impossible to finish this work. I would like to express my deep and sincere gratitude to them. ABSTRACT Problems due to the thaumasite form of sulfate attack (TSA), which has a significant influence on the strength and durability of buried concrete, have been extensively reported in the UK and worldwide. Thaumasite forms as a result of the presence of high levels of sulfate in pore waters in the ground surrounding concrete, particularly where sulfate is formed by the oxidation of pyrite and the ground temperature is less than about 12OC. In spite of this association with pyrite-bearing ground, an extensive literature search revealed that most previous research, including studies on which current concrete design recommendations are based, was carried out by exposing test specimens to sulfate-rich solutions rather than to natural ground materials. In fact, the present study appears to be the first extensive investigation of TSA in which various concretes have been tested in simulated field conditions. The changes in chemistry of different clays and clay pore solutions were also investigated. The work includes the long-term exposure (nine years) of Portland cement (PC), Portland limestone cement (PLC), sulfate-resisting Portland cement (SRPC) and Portland cement blended with 25% pulverized - fuel ash to slightly weathered Lower Lias Clay of sulfate design class DS-2 at 5°C. Parts of the exposed concrete were coated with bitumen to test the performance of this method of protection. The study also includes an investigation into the influence of clay composition (weathered and slightly weathered Lower Lias Clay and Coal Measures mudstone) on the severity of TSA in various concretes made with CEM I, CEM I blended with 10% limestone filler (LF), CEMI - 50% PFA and CEMI - 70% GGBS; this was complemented by parallel studies which assessed the performance of specimens of the same concretes, placed in sulfate solutions equivalent to DS-2 and DS-4 and simulated pore waters at the same temperatures. The performance of the different concretes in these tests was assessed by means of visual observation, supported by X-ray diffraction (XRD), infra-red scanning (IR) and scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) to identify the deterioration products. The change in the chemistry of clay was assessed by the determination of water- and acid-soluble sulfate, total sulfur, rate of pyrite oxidation and change in carbonate content. Where applicable, the compositions of the different clays and clay pore solutions were also investigated. It was found that deterioration due to the thaumasite form of sulfate attack occurred in all four concretes exposed for nine years to slightly weathered Lower Lias Clay. PLC concrete was the worst affected, with complete loss of binding of up to 47 mm thickness of concrete, but PC- 25% PFA replacement and SRPC concretes were also badly deteriorated. The degree of attack decreased with increasing burial depth, probably as a result of reduced access to air. The bitumen coating proved to be effective at preventing deterioration in all concretes. Exposure to clay of design sulfate class DS-2 was found to cause similar or greater deterioration than that in case of exposure to DS-4 sulfate solution, so the aggressivity of clay may be under-estimated if only the total potential sulfate (TPS) value currently used for aggressivity classification is considered. X-ray diffraction analysis revealed that gypsum and thaumasite were the main products in the concrete exposed to solutions, whilst thaumasite and carbonate were formed in the samples exposed to clay, suggesting that the more complex chemistry of clay results in a different chemical interaction. Replacement of CEM I with 50% PFA and 70 % GGBS revealed a very good performance, as no deterioration was observed after two years in any of the exposure conditions, including DS-4 solution and pyritic clays. However, thaumasite solid solution was detected in both concretes exposed to pyritic clay at 5 , which suggests that even these binders may be susceptible to thaumasite formation and TSA with time. Changes to the clays confirmed that pyrite oxidation resulted in elevated sulfate levels, and the generation of sulfuric acid, which reacted with calcite and clay minerals in the clays. It is concluded from this that the carbonate content of the clay affects its aggressivity, although current standards do not take this into account. TABLE OF CONTENTS List of Figures List of Table List of Symbols and Abbreviations 1. Introduction ……………………………………………………………………... 1 1.1 Background………………………………………………………………………... 1 1.2 Aims and objectives……………………………………………………………….. 4 1.3 Structure of the thesis ……………………………………………………………... 5 2. Literature Review……………………………………………………………….. 7 2.1 Introduction……………………………………………………………………….. 7 2.2 Conventional form of sulfate attack……………………………………………….. 8 2.3 Thaumasite form of sulfate attack………………………………………………… 9 2.3.1 Formation of Thaumasite……………………………………………………... 11 2.4 Thaumasite in buried concrete exposed to pyritic ground………………………… 14 2.4.1 Sulfate and sulfide in aggressive ground……………………………………. 15 2.4.2.1 Weathering of pyrite…………………………………………………… 16 2.4.2 The role of sulfuric acid produced as a result of pyrite oxidation in TSA…... 17 2.4.3 The presence of Mobile ground water………………………………………. 20 2.5 Factors controlling thaumasite formation and TSA……………………………….. 21 2.5.1 The role of pH on formation and stability of thaumasite…………………… 21 2.5.2 The role of magnesium……………………………………………………….. 22 2.5.3 The effect of sulfate solution concentration…………………………………… 22 2.5.4 The effect of temperature…………………………………………………… 23 2.5.5 The role of chloride in thaumasite form of sulfate attack……………….. 24 2.5.6 Relative resistance of cement and cement binder…………………………... 26 I. Limestone filler ……….……………………………………………………… 26 II. Pulverised fuel ash (PFA)……………………………………..………...... 27 III. Ground Granulated Blastfurance Slag (GGBS). ………………………. ... 28 VI. Sulfate Resistant Portland cement (SRPC) 29 2.5.7Surface Coating……………………………………………………………... 30 2.6 UK guidance of the assessment of aggressive ground condition………………….. 32 I 3. Methodology ……………………………………………………………… 34 3.1 Introduction………………………………………………………………………... 34 3.2 Experimental Design and Programme…………………………………………….. 34 3.2.1 Long-term study (9 years) of concrete- Lower Lias Clay interaction…………. 35 3.2.2 The influence of clay compositions on severity of TSA………………………. 35 3.3 Experimental Details………………………………………………………………. 37 3.3.1 Long-term study (9 years) of concrete- Lower Lias Clay interaction…………. 37 3.3.1.1 Cementitious binders and concrete mixes………………………………… 37 3.3.1.2 Lower Lias Clay……………………………………………………………. 38 3.3.1.3 Specimen preparation and exposure……………………………………… 38 3.3.2 The influence of clay compositions on severity of TSA (series II)…………… 40 3.3.2.1 Materials…………………………………………………………………... 40 I.Cementitious binders…………………………………………………………. 40 II.Aggregate……………………………………………………………………. 41 III.Clay…………………………………………………………………………. 41 3.3.2.2 Water and Solutions……………………………………………………. 44 3.3.2.3 Concrete Mixes and Casting………………………………………….. 45 3.3.2.4Curing…………………………………………………………………….. 46 I.Initial curing………………………………………………………………... 46 II.Long- term exposures to solutions and pyritic clay………………………… 46 3.4Test Methods……………………………………………………………….. 48 3.4.1 Sampling………………………………………………………………………. 48 3.4.2 Concrete Cores……………………………………………………………….. 49 3.4.3 Concrete Testing.……………………………………………………………… 49 3.4.3.1 Visual observation………………………………………………….. 49 3.4.3.2 Deterioration thickness …………………………………………………... 49 3.4.3.2 Chemical Analysis………………………………………………………. 50 I.Determination of sulfate content …………………………………………… 50 II. Determination of pH………………………………………………………. 50 III.Carbonate content profile………………………………………………..... 51 3.4.3.4 Mineralogical and microstructure