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Title: Controls on thaumasite in buried concrete : effect of clay composition and cement type
Author: Abubaker, Farhat
ISNI:       0000 0004 5356 5769
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2014
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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 12°C. 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.
Supervisor: Cripps, John Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available