Thaumasite Form Of Sulfate Attack Research Articles

The occurrence of thaumasite as a product of seawater attack

This paper describes the mechanism of the rapid deterioration caused by seawater attack on the bedding mortar of recently constructed harbour wall steps in South Wales. Within 2 years, the mortar had suffered severe cracking and spalling. The degraded mortars were examined under optical and electron microscopes and also using X-ray diffraction analysis. The reaction products formed included thaumasite, ettringite, brucite and hydrated magnesium silicate. A further end product of deterioration was the precipitation of large, non-interlocking crystals of calcite (termed popcorn calcite) in the areas of the mortar previously occupied by cement paste. Investigation of degradation of associated structural concrete from this site also showed the early stages of the thaumasite form of sulfate attack.

Thaumasite and secondary calcite in some Norwegian concretes

Abstract Thaumasite formation (TF) and limited thaumasite form of sulfate attack (TSA) has recently been detected in several Norwegian sprayed concretes. TF and TSA is frequently associated with contemporaneous and late stage internal calcite formation by: (a) decalcification of calcium silicate hydrate (CSH): (b) decomposition of thaumasite associated with secondary liberation of SO 4 2− and occasional formation of subordinate gypsum; (c) supersaturation of fluids in voids. Popcorn calcite and other textural forms were characteristic for these reactions. Also co-precipitation of popcorn calcite + thaumasite, as well as later stage dissolution of both minerals occurred. The entire process was represented by a drop in pore fluid pH from about 13 towards 5–7. In this paper we study the TF–TSA–carbonation process in several environments: (1) three examples of 2–13 years old steel fibre reinforced sprayed concrete made with Sulfate Resisting Portland Cement (SRPC) and silica fume in contact with carbon-, calcite- and sulfide bearing Alum Shale: (2) two examples of ≈30 years old, and severely damaged, SRPC based sprayed concrete within the Alum Shale: (3) one 16 years old sprayed concrete made with Portland Cement (PC) and possibly fly ash in presence of sulfate bearing ground water and (4) one 10 years old steel fibre reinforced sprayed concrete in a sub-sea tunnel with inflow of somewhat modified seawater. This PC based concrete with silica fume had suffered localised crumbling and mush formation after less than 5 years. The critical factors for thaumasite formation are discussed together with consequences for further deterioration and timing of repair.

Long term durability of Portland-limestone cement mortars exposed to magnesium sulfate attack

Mortar prisms made with Portland-limestone cement have been stored in air and in 1.8% magnesium sulfate solution at 5 °C and have been examined over a period of 5 years. This paper is primarily concerned with the results obtained at the end of this period. The limestone content in the samples varied from 0% to 35%, but the water to cement plus limestone powder ratio was kept constant. The status of the samples after storage for 5 years is reported based on visual examination and a thorough characterisation using X-ray diffraction, infra-red spectroscopy and scanning electron microscopy. The prisms stored in magnesium sulfate solution were all showing clear signs of deterioration, increasing in intensity with limestone content. The mortar prism with 5% limestone replacement was, however, seriously degraded in comparison with the ordinary Portland cement control prism, and it is shown that this was due to the thaumasite form of sulfate attack .

Resistance of concrete containing ggbs to the thaumasite form of sulfate attack

Abstract A long-term laboratory study has investigated how cement-type, aggregate-type and curing, affect the susceptibility of concrete to the thaumasite form of sulfate attack (TSA). The cements were Portland cement (PC), sulfate-resisting Portland cement (SRPC) and a combination of 70% ground granulated blastfurnace slag (ggbs) with 30% PC. These were combined with various carbonate aggregates or a non-carbonate control. Initial curing was either in water or in air. Concrete cubes were immersed in four strengths of sulfate solution at 5 and 20 °C. This paper reports the results after up to six years of immersion in sulfate solution. Deterioration, consistent with TSA, was observed on many of the PC and SRPC concretes that had been made with carbonate aggregate and stored in sulfate solutions at 5 °C, with SRPC providing no better resistance to TSA than PC. Good quality concretes made with 70%ggbs/30%PC showed high resistance to TSA and the presence of carbonate in the mix substantially improved their general sulfate resistance. An initial air-cure, proved beneficial against both the conventional and thaumasite form of sulfate attack.

Increased sulfate resistance of ggbs concrete in the presence of carbonate

Abstract Significant occurrence of the thaumasite form of sulfate attack (TSA) has only been identified in concrete containing carbonate aggregate, leading to a perception that the presence of calcium carbonate is undesirable in concrete exposed to sulfate attack. This paper reports on how the addition of a small percentage of calcium carbonate or calcium sulfate, affects the sulfate resistance of concrete containing ground granulated blastfurnace slag (ggbs). Three test methods were employed to assess sulfate resistance (all at 20 °C): • Cubes were immersed in magnesium and sodium sulfate solutions and monitored for corner-loss and strength-loss, over six years. • Mortar was sieved from fresh concrete and used to make prisms. These prisms were immersed in magnesium and sodium sulfate solutions and their expansions monitored for up to six years. • In accordance with a draft of a European Standard for sulfate resistance, mortar prisms were prepared and monitored for expansion for one year. • It was found that both calcium carbonate and calcium sulfate additions, had a consistent beneficial effect on the resistance of ggbs concrete to conventional sulfate attack, both in respect to expansion and in respect of disintegration. The paper discusses possible mechanisms for the improved resistance.

The resistance of metakaolin (MK)–Portland cement (PC) concrete to the thaumasite-type of sulfate attack (TSA)––Programme of research and preliminary results

Abstract The thaumasite form of sulfate attack (TSA), is a deleterious physico-chemical attack of the calcium silicate hydrate (C–S–H), binding phase of concrete. Water:binder ratios (w:b), are known to control ingress of potentially deleterious ions by pore structure refinement at low (0.40) values. Equally, a physical–chemical barrier exists at about w:b ratio of 0.45 regardless of the binder type. The inclusion of ultra-fine pozzalans (e.g. metakaolin) in the binder has been shown to impart such properties through densification of the matrix and removal of calcium hydroxide. A small-scale experimental design programme to establish the potential resistance of metakaolin–Portland cement (MK–PC), blended concrete to the thaumasite-type of attack is ongoing. Results are presented for concrete incorporating dolomitic limestone aggregate and with 0% and 7% metakaolin replacement of a high-C 3 A PC binder at w:b ratios to 0.40 and 0.46. Exposure to three environments containing a sulfatic clay, sulfate solution and water has produced evidence of deleterious reactions within these samples. Visual data are quantified by a wear rating and supported by compressive strength and expansion values for up to 280 days of exposure. These results confirm the importance of low water binder ratios in the resistance of ion ingress and give an early indication of the desirable durability-enhancing properties of MK replacement of PC. Finally, the results also show that the two sulfatic environments of exposure produce marked differences in the degradation modes, which may be important in the methodologies used to determine TSA in the laboratory with reference to field observations.

The laboratory investigation of concrete affected by TSA in the UK

Abstract Procedures that have been successfully employed by Geomaterials Research Services Ltd. in the determination of the distribution of the thaumasite form of sulfate attack (TSA) in concrete samples taken from bridge and other motorway structures throughout the UK are described. Electron microprobe analysis has been used to provide confirmation of the presence of thaumasite and to investigate the distribution of sulfate compounds including gypsum and ettringite in cement paste at the surfaces of concrete affected by sulfate attack. Electron microprobe analysis has the advantage that it is capable of detecting very small quantities of thaumasite. Electron microprobe analysis used in conjunction with petrographic analysis is regarded as the most effective tool for the diagnosis of TSA and other forms of sulfate attack. The analysis of numerous cores taken from bridge foundations throughout the UK shows that the development of TSA is often accompanied by the formation of calcium carbonate in cement paste. High levels of chloride in the cement paste of TSA damaged concrete suggest the importance of run-off moisture as a contributory factor in the development of TSA.

Contribution of construction activity to aggressive ground conditions causing the thaumasite form of sulfate attack to concrete in pyritic ground

Abstract Historically, the potential for sulfate concentration to be increased as a result of disturbance of pyrite-bearing ground during construction has not been taken into account in UK guidance documents for concrete specification such as BRE Digest 363. This omission was found in 1998 to have contributed to the occurrence sulfate attack in the foundations to several bridges on the M5 motorway founded on pyrite-bearing Lower Lias Clay. The concrete for these was specified to meet the maximum Sulfate Class 2 natural sulfate levels measured during site investigation, however, Sulfate Class 3 and Class 4 levels were generated subsequently by oxidation of pyrite in backfill. In response to the M5 cases, new UK guidance has been issued for assessing pyrite-bearing ground which may be disturbed by construction activity, this being based on the newly defined parameter total potential sulfate.

Procedures for assessing thaumasite sulfate attack and adjacent ground conditions at buried concrete structures

Abstract In 1998 Halcrow were appointed by the Highways Agency to investigate the thaumasite form of sulfate attack at the buried concrete foundations of structures on major highways in Gloucestershire, UK. Detailed investigations were completed at 28 structures and the results used to apply a risk assessment procedure to a further ninety structures. Standard procedures for sampling, testing and classifying soil, groundwater and concrete were devised to ensure that rigorous statistical analyses of the data could be applied. The procedures were also designed to meet the then current UK guidance notes, BRE. Digest 363 [Building Research Establishment, Sulphate and acid resistance of concrete in the ground, BRE Digest 363, CRC Press, Boca Raton, 1996] and the Thaumasite Expert Group Report [Thaumasite Expert Group, The thaumasite form of sulfate attack: Risks, diagnosis, remedial works and guidance on new construction, DETR, 1999] and are compatible with the subsequent BRE Special Digest 1 [Building Research Establishment. Concrete in aggressive ground, Part 1: assessing the aggressive chemical environment, BRE Special Digest 1, CRC, Boca Raton, 2001]. Recommendations are made for applying many of the procedures that have been developed to future site investigations to enable consistency, compatibility and easier data transfer for future research on the thaumasite form of sulfate attack.

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.

A comparison of classification systems for aggressive ground with thaumasite sulfate attack measured at highway structures in Gloucestershire, UK

Part one of the BRE Special Digest 1 ‘Concrete in aggressive ground’ (2001) provides UK guidelines on assessing aggressive ground conditions with respect to the specification of concrete and protective measures required for concrete structures in contact with the ground. This supersedes the guidelines of BRE Digest 363 (1996) and the Thaumasite Expert Group Report (1999). All three publications rely in part on classifying the ground based on the chemical composition of the soil, backfill and groundwater at the location of the proposed structure. This paper provides an empirical assessment of the three classification schemes that was made possible following an extensive investigation into the thaumasite form of sulfate attack (TSA) in buried concrete highway structures in Gloucestershire. Ground classifications were made at 21 of the sites. The results of this study confirmed that whenever possible, the sulfate concentration in groundwater should be measured, and that as noted in SD1, the potential sulfate classification can be too conservative. The conclusions are limited geologically to the Lower Lias Clay Formation and to Recent Alluvium. The current classification system needs to be developed further by undertaking and reporting similar case studies from a variety of ground conditions.

Occurrences of thaumasite in laboratory and field concrete

Abstract The mineral thaumasite (CaSiO 3 · CaCO 3 · CaSO 4 · 15H 2 O) has been observed by the authors in a number of concrete samples including (i) an 80 year-old aqueduct in Manitoba, (ii) a 33 year-old pavement in Ontario, (iii) test samples exposed to marine (tidal) conditions, and (iv) laboratory samples exposed to wet–dry cycles in sulphate solution. The source of carbonate differed for these cases being variously derived from de-dolomitization of dolostone aggregate, carbonate ions in the seawater, or atmospheric CO 2 through the process of carbonation. In some cases the thaumasite mineral was found in close association with the mineral ettringite (3CaO · Al 2 O 3 · 3CaSO 4 · 31H 2 O). The paper discusses the results of detailed analyses using optical and electron microscopy. Although the mineral thaumasite can be readily identified by these techniques, it is possible that the thaumasite form of sulphate attack is frequently misdiagnosed as conventional sulphate attack due to the similarities in crystal structure and appearance of the minerals thaumasite and ettringite.

Parameters affecting thaumasite formation in limestone cement mortar

Abstract Concrete made from limestone cement may exhibit a lack of durability due to the formation of thaumasite. This work deals with the factors affecting thaumasite formation in cement mortars and particularly the limestone content, the curing conditions and the type of sand used. Three types of cement were examined: (i) OPC, (ii) Portland limestone cement containing 15% w/w limestone and (iii) Portland limestone cement containing 30% w/w limestone. Mortar specimens were prepared using calcareous and siliceous sand. The specimens were immersed in a 1.8% MgSO 4 solution and cured at: (i) 5 °C and (ii) 25 °C. The formation of thaumasite was checked and confirmed by XRD and TGA. In addition visual inspection, strength tests and ultrasonic pulse velocity measurements were carried out for several months. It is concluded that mortars containing limestone, either as sand or as a main constituent of the cement, suffer from the thaumasite form of sulfate attack at low temperature. At room temperature, no sulfate attack was observed after a year of exposure.

Pyrite oxidation in Lower Lias Clay at concrete highway structures affected by thaumasite, Gloucestershire, UK

The process of pyrite oxidation with the resulting formation of sulfates in disturbed pyrite-rich mudstone is recognised in the UK as a potential engineering hazard. This paper describes and quantifies the extent of pyrite oxidation in Lower Lias Clay in Gloucestershire using a large data set collected in 1998 and 1999 during the investigation of twenty-eight, approximately 30-year old highway structures affected by the thaumasite form of sulfate attack. Most of the structures have concrete spread footings founded on undisturbed and relatively unweathered Lower Lias Clay, and are surrounded by backfill of reworked Lower Lias Clay. Chemical, X-ray diffraction and X-ray fluorescence analyses are used to determine the composition of the soils. Differences are identified in the sulfides, sulfates and total sulfur concentrations between unweathered and weathered Lower Lias Clay and the backfill. The work is supported by pyrite oxidation trials in the laboratory on clay samples stored in bags and sealed tubes. The results indicate that pyrite oxidation, sulfate formation and leaching have occurred as the Lower Lias Clay weathers in situ and also during and following disturbance and reworking for highway construction. The amount and rate of pyrite oxidation are estimated, and comparison is made with the concentration of sulfate in the surrounding groundwater, and the extent of thaumasite sulfate attack measured at adjacent structures.

Effects of thaumasite on bond strength of reinforcement in concrete

Abstract 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.

Evidence from the highways agency thaumasite investigation in Gloucestershire to support or contradict postulated mechanisms of thaumasite formation (TF) and thaumasite sulfate attack (TSA)

In March 1998 the thaumasite form of sulfate attack (TSA) was identified in the reinforced concrete foundations of Tredington Ashchurch Road Bridge on the M5 Motorway in Gloucestershire, UK and the UK Government’s Highways Agency appointed Halcrow to investigate the problem. A total of 28 structures were selected for investigation to encompass sites where bridge works were planned and structures were assessed to be at greatest risk as well as to provide a representative sample of sites and structure types. Desk study work and peer reviews were continued throughout the project to establish protocols for the site work and facilitate the interpretation of the data. The latter included a dual approach of a systematic examination of the data collected to identify trends coupled with a review of postulated mechanisms for the formation of thaumasite from the desk study work against the data from the investigation. This paper summarises the findings of this review, which examines in detail four proposed stages in the development of TSA.

Stability and reactivity of thaumasite at different pH levels

Thaumasite (CaCO3·CaSO4·CaSiO3·15H2O) has been reported to form at low temperatures (below 15 °C) during sulfate attack. Reactions between calcium silicate hydrate (C-S-H) and Ca+2, CO3−2, SO4−2, CO2 and water, or between ettringite and C-S-H, CO3−2 and/or CO2 and water, result generally in the formation of thaumasite. In some instances, thaumasite may be affected by the presence of other chemicals in the surrounding environment (i.e., phosphates and ammonia in agricultural soil). There are insufficient data regarding the stability of thaumasite at different pH levels in the presence of other chemical ions. Understanding this issue might help in the detection of the thaumasite form of sulfate attack, and, therefore, in one's choice of the appropriate protection technique. This work reports the reactivity of thaumasite with phosphate, carbonate and bicarbonate ions at different pH levels ranging from 6.00 to 12.00, as well as the stability of thaumasite at high pH levels (greater than 12.00). Thaumasite was found to react with these ions at pH levels at and below 12.00; however, thaumasite was stable with minimal reactivity at pH levels greater than 12.00.

The occurrence of thaumasite in modern construction – a review

This paper provides a review of the reported cases of deterioration in modern construction, where the formation of the sulfate-bearing mineral thaumasite has been identified. The formation of thaumasite can cause varying degrees of deterioration within a wide variety of building systems including lime–gypsum plasters, exposed sulfate-bearing brickwork, concrete foundations and floor slabs exposed to sulfate-bearing ground, concrete roads and sub-bases, sewer pipes, tunnel linings and old blastfurnace slags used as landfill. In Spring 1998, the thaumasite form of sulfate attack was found in the foundation concrete of 10 motorway bridges in Gloucestershire, western England. As a result, the UK Government convened the Thaumasite Expert Group (TEG), who were commissioned to prepare a Report `The thaumasite form of sulfate attack: Risks, diagnosis, remedial works and guidance on new construction' [Thaumasite Expert Group, The thaumasite form of sulfate attack: Risks, diagnosis, remedial works and guidance on new construction, DETR, London, 1999]. This paper was one of the references used in the preparation of the TEG Report, which was published in January 1999. Deterioration as a result of the formation of thaumasite has become recognised as a separate form of sulfate attack, which has the potential to affect a wide variety of components and a range of building materials. The full extent of the problem is still unknown. There are two distinct ways in which thaumasite can precipitate as a reaction product, especially within concretes and mortars. These have been termed the thaumasite form of sulfate attack (TSA) and thaumasite formation (TF) by the Thaumasite Expert Group. TSA refers to cases where there is significant damage to the matrix of a concrete or mortar as a consequence of replacement of cement hydrates by thaumasite. TF, on the other hand, refers to incidences where thaumasite can be found in pre-existing voids and cracks without necessarily causing deterioration of the host concrete or mortar. In relation to the current paper, it was not always easy to identify retrospectively whether the occurrence of thaumasite in the reported field cases was associated with TSA or TF. Distinctions were therefore only made when it was possible to do so. The references reviewed in this paper span a time period of over thirty years and some of these, especially the more recent UK cases, prompted the TEG to provide new interim recommendations in order to minimise the effect of TSA in new construction. However, in many of the remaining cases reported in the following paper, deterioration could have been avoided simply by following good practice of the period, either by the use of more suitable materials for given service conditions or by better workmanship on site.

The thaumasite form of sulfate attack in Portland-limestone cement mortars stored in magnesium sulfate solution

Mortar prisms (40×40×160 mm 3) made at a water:solids ratio of 0.5 and a cement:aggregate ratio of 1:2.5 from Portland cement with 0%, 5%, 15% and 35% limestone additions were cured in water at 20 °C for 28 days. They were subsequently stored in air and submerged in 1.8% magnesium sulfate solution at 5 and 20 °C for a year. The prisms were inspected visually every 28 days, the solution was changed every 84 days, and selected samples were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) after specified intervals. The thaumasite form of sulfate attack was readily identified in the ordinary Portland cement (OPC) – 35% limestone mortar after 126 days storage in magnesium sulfate solution. The surface layer of the prism had spalled and was mushy, while the core was still solid and sound. Gypsum, thaumasite and brucite were identified in the surface layer. Secondary electron images of polished sections of deteriorated Portland-limestone cement mortars revealed the microstructure of the cement to be suffering from the thaumasite form of sulfate attack. The extent of this attack was greater at 5 °C than at 20 °C, although some thaumasite was formed even at the higher temperature.

Study of thaumasite and ettringite phases formed in sulfate/blast furnace slag slurries using XRD full pattern fitting

The thaumasite form of sulfate attack (TSA) has been investigated using a method known to accelerate the formation of the sulfate minerals thaumasite, ettringite and gypsum. Mixes containing different cements and aggregates in magnesium sulfate solution were prepared at different water:solid ratios. The work concentrated, in particular, on the role of blast furnace slag as a cementitious material in preventing the formation of thaumasite. The formation of sulfate minerals in the mixes was followed for a period of two years by X-ray powder diffraction. Full pattern fitting, a computer-based XRD data analysis technique, was used to identify the nature of the thaumasite/ettringite solid solutions produced. Thaumasite was formed only in mixes containing carbonate-bearing aggregates at higher water:solid ratios. In most of the mixtures containing blast furnace slag cement (70% blast furnace slag cement, 30% Portland cement), an ettringite-based solid solution was the main sulfate-bearing phase produced. Only one of the mixes containing blast furnace slag cement was found to produce thaumasite. Some samples, containing only blast furnace slag as the cementitious component, produced gypsum and no thaumasite/ettringite. The variation in the exact nature of thaumasite/ettringite produced with different aggregates, cements and water:solid ratios is discussed.