Thaumasite––direct, woodfordite and other possible formation routes

Two main formation routes for thaumasite exist below 15 °C. One is the direct route from C–S–H reacting with appropriate carbonate, sulfate, Ca²⁺ ions and excess water. The other route is the woodfordite route from ettringite reacting with C–S–H, carbonate, Ca²⁺ ions and excess water, in which thaumasite arises through the intermediate formation of the solid solution woodfordite. The woodfordite route for thaumasite formation appears to be relatively quicker (although still slow) than the direct route, presumably because with the former the ettringite already has the octahedral [M(OH)₆] units that can facilitate the critical change from [Al(OH)₆]³⁻ to [Si(OH)₆]²⁻ groupings. Both routes are mutually dependent on each other. The presence of magnesium salts can modify the path to thaumasite formation. High pressure might be able to stabilise [Si(OH)₆]²⁻ groupings and allow thaumasite to become formed above 15 °C. This possibility is discussed.     

The role of pH in thaumasite sulfate attack

The thaumasite form of sulfate attack (TSA) has been recognised in recent years as a distinct mechanism by which degradation of buried concrete can occur in the presence of an external source of sulfate ions. There is, however, disagreement about the role of pH. It has been proposed that attack by sulfuric acid, produced by oxidation of pyrite, is sometimes the primary cause of deterioration. Others believe that the acid is rapidly neutralised giving a higher concentration of sulfate ions in the ground, hence increasing the extent of attack. The aim of the laboratory study reported here was to understand the role of sulfuric acid in TSA by examining concrete cubes, made from three types of cement and two types of aggregate, immersed at low temperature in two solutions, one alkaline corresponding to BRE Design Sulfate Class DS-3 and the other acidic. It is concluded that the presence of acid does not promote the formation of thaumasite. Although degradation of the concrete was observed in acid conditions, the mechanism was not TSA as observed in alkaline conditions.     

Effects of thaumasite on bond strength of reinforcement in concrete

The conditions necessary for the formation of thaumasite are well known and much work is in progress to identify concrete mixes resistant to thaumasite form of sulfate attack (TSA). However, there have been no data to indicate how TSA affects the nature and strength of the bond between reinforcement steel and concrete and hence the load capacity of reinforced concrete elements.

During works to repair and strengthen the thaumasite-affected Tredington–Ashchurch Overbridge in Gloucestershire, sections of column were removed and placed in storage. These column sections presented an opportunity to perform pullout tests on full size TSA-affected structural elements and unaffected control specimens from the same structure. In total 63 pullout tests were performed on plain round reinforcement bars embedded in two unaffected and four TSA-affected reinforced concrete elements. The sections were also characterised in terms of estimated in situ cube strength and depth of softened zone.

A statistical analysis of the experimental results indicates that the bond of the plain round reinforcement bars in the unaffected concrete exceeded that of the plain round reinforcement bars in the TSA-affected concrete. TSA reduced the mean experimental bond coefficient by 24% for corner bars and 10% for other bars, representing an average reduction in mean experimental bond coefficient of 15% for all bars.     

Mechanism of thaumasite formation in concrete slabs on grade in Southern California

Thaumasite formation has been observed in residential concrete slabs on grade in Southern California. The concrete examined did not contain any carbonate bearing aggregates or fillers. Microstructural analyses showed a carbonated layer with calcite and gypsum at the bottom of the concrete. Above the carbonation layer, deposits of intermixed gypsum and thaumasite were observed. Further into the concrete towards the upper surface, deposits of thaumasite alone or in combination with ettringite were observed. Most of the thaumasite deposits were observed in air voids. SEM–EDS analysis showed deposits of ettringite, thaumasite and intermediate phases within the same air voids. The formation of thaumasite, ettringite and gypsum was caused by ingress of sulfate and carbonate ions from ground water. The presence of thaumasite, ettringite and intermediate phases in the same air void indicates that ettringite is first formed followed by thaumasite with a series of solid solutions. In this reaction process the pH of the local environment and the balance between sulfate, silicate and carbonate ions are important parameters.     

Microstructural changes in concretes with sulfate exposure

In prior papers the responses of concretes to 50,000 ppm MgSO₄ exposure depending on cement type, w/cm and the presence of slag were described. The present paper completes this analysis by examining the effects of immersion of concretes produced using slag blended cements, in solutions containing 50,000 ppm of sodium sulfate. The spatial evolution of microstructure associated with carbonation and sulfate attack show differences which can be related to the nature of the cation associated with the sulfate, the cement type, and the w/cm ratio.     

Effect of limestone filler on the deterioration of mortars and pastes exposed to sulfate solutions at ambient temperature

This paper presents data on engineering properties such as compressive strength, visual change and expansion of mortar specimens incorporating limestone filler subjected to severe sulfate attack at ambient temperature. Specimens with four replacement levels of limestone filler (0, 10, 20 and 30% of cement by mass) were immersed in sodium and magnesium sulfate solutions with 33,800 ppm of SO₄²⁻ concentration. In order to identify the products formed by sulfate attack, microstructural analyses such as XRD and SEM were also performed on the paste samples with similar replacement levels of limestone filler.The test results demonstrated that mortar and paste samples incorporating higher replacement levels of limestone filler were more susceptible to sulfate attack irrespective of types of attacking sources. However, the deterioration modes were significantly dependent on the types of sulfate solutions. Additionally, although the samples were exposed to sulfate solutions at 20 ± 1 °C, the deterioration was strongly associated with thaumasite formation in both sulfate solutions.The deterioration mechanism and resistance to sulfate attack of cement matrix incorporating limestone filler at ambient temperature is discussed in the light of the test results obtained.     

Sulphate resistance of self-compacting concrete

This article outlines a laboratory study on sulphate resistance of self-compacting concrete (SCC). For this purpose, more than 40 cylinders of concrete were subjected to a solution with sodium sulphate, sea or distilled water during 900 days. Age at start of testing was either 28 or 90 days. Weight and internal fundamental frequency (IFF) were measured. Comparison was done with the corresponding properties of vibrated concrete (VC). When cured in a solution with sodium sulphate, the results show larger loss of mass of SCC than that of VC probably due to the limestone filler content in SCC. After curing in water, sea or distilled, no such weight difference between the curing types was observe. IFF did not decrease or differ between the two types of concrete, i.e. no internal deterioration took place due to thaumasite sulphate attack (TSA) during the 900 days of exposure. The project was carried out from 1999 to 2002.     

Microstructure of 5-year-old mortars containing limestone filler damaged by thaumasite

This paper presents the findings of a long-term study on the microstructure of Portland cement mortar specimens containing 5%, 15% and 35% limestone filler, as cement replacement, after exposure to a solution of magnesium sulfate at a concentration of 1.44% SO₄, for 5 years at 5 °C. The findings are compared to results reported earlier, obtained from the same systems but after 1-year exposure. It was found that the deterioration due to thaumasite advanced with the increased exposure period and limestone content. Thaumasite solid solutions (Tss) formed as the dominant phases within the deteriorated cement matrix and at the paste-aggregate interface resulting in cracks and delamination. Thaumasite was also found as an inner product in various clinker grains. Interestingly, the control specimens, with no limestone filler, were found to exhibit cracks due to the formation of Tss, with atmospheric carbon dioxide being the most likely source for the carbonates.     

Physical and microstructural aspects of sulfate attack on ordinary and limestone blended Portland cements

The consequences of external sulfate attack were investigated by traditional test methods, i.e. length and mass change, as well as by a newly developed, surface sensitive ultrasonic method, using Leaky Rayleigh waves (1 MHz). The macroscopic changes are discussed and compared with thermodynamic calculations and microstructural findings (SEM/EDS). The results show that the main impact of limestone additions on resistance to sulfate degradation are physical — i.e. addition of a few percent in Portland cement reduces the porosity and increases the resistance of Portland cement systems to sulfate; but higher addition of 25% increase porosity and lower resistance to sulfate. The kinetics of degradation were dramatically affected by the solution concentration (4 or 44 g Na₂SO₄/l) and the higher concentration also resulted in the formation of gypsum, which did not occur at the low concentration. However the pattern of cracking was similar in both cases and it appears that gypsum precipitates opportunistically in pre-formed cracks so it is not considered as making a significant contribution to the degradation. At 8 °C limited formation of thaumasite occurred in the surface region of the samples made from cement with limestone additions. This thaumasite formation led to loss of cohesion of the paste and loss of material from the surface of the samples. However thaumasite formation was always preceded by expansion and cracking of the samples due to ettringite formation and given the very slow kinetics of thaumasite formation it was probably facilitated by the opening up of the structure due to ettringite induced cracking.The expansion of the samples showed a steady stage, followed by a rapidly accelerating stage, with destruction of the samples. The onset of the rapidly accelerating stage occurred when the thickness of the cracked surface layer reached about 1–1.5 mm–10–15% of the total specimen thickness (10 mm).     

The thaumasite form of sulfate attack in the UK

During the last 15 years, the thaumasite form of sulfate attack (TSA) has been found in over 80 UK field structures and buildings and is particularly prevalent in buried concrete. This form of sulfate attack completely destroys the cementitious binding ability of the concrete by transforming it into a mush. In 1998, the occurrence of several high profile cases in the foundations of UK motorway bridges initiated a rapid pan-industry response, culminating in a report by the UK Government’s Thaumasite Expert Group. A brief account of the Group’s findings, the diagnosis and risk factors needed for TSA and some postulated reasons for its apparent increase in recent years are discussed in this paper.

The Building Research Establishment has carried out extensive research since the early 1990s on the occurrence of TSA in the field and in laboratory concretes and mortars. Prior to this, a few other UK workers had been involved in more fundamental studies of the mineral thaumasite including its structural characterisation and its laboratory synthesis. Once the profile of TSA was raised in 1998, the number of government and industry funded research programmes increased significantly in the UK and the findings from many of these will be presented in this journal. Information gleaned from UK laboratory research is incorporated in the present paper along with the author’s thoughts on a possible reaction mechanism for TSA.