THAUMASITE SULFATE ATTACK: CASE STUDIES AND IMPLICATIONS

Hugh (Xiaoqiang) Hou, Laura J. Powers, John Lawler, Koray Tureyen

Wiss, Janney, Elstner Associates, Inc.

330 Pfingsten Road, Northbrook, Illinois 60062

ABSTRACT

The thaumasite form of sulfate attack (TSA) is a unique distress mechanism in portland cement

concrete in which thaumasite formation (TF) alters the primary binder, calcium silicate hydrate

(CSH), in addition to calcium hydroxide and calcium aluminate hydrates. TSA in concrete may

cause loss of paste-aggregate bond, strength, coherence, and eventually serviceability. The time

frame of TSA can be as short as a couple of years. Reported TSA cases have mostly involved

sulfate from external sources. This paper presents two less common TSA cases, in which sulfate

was determined to be from an internal source: in one case, from dolostone coarse aggregate and in

the second case from the cementing material. Characteristics of TSA distress and the composition

and texture of the concrete are discussed, and these cases are compared with other reported internal

and external TSA cases. It is concluded that petrographic examination of the concrete, particularly

using thin-sections, provides the most definitive diagnosis of TSA. Significant implications of

findings from the two case studies regarding mechanisms of thaumasite formation and potential

propensity of portland-limestone cement for TSA are also discussed.

KEYWORDS: Thaumasite, sulfate attack, TSA, concrete, durability, deterioration, limestone, dolomite,

petrography, gypsum

INTRODUCTION

Thaumasite is a rare mineral in nature and had only been scholarly or intellectually interesting

before it was identified in concrete in the 1960s (1, 2). Thaumasite is one of few minerals with a

six-coordinated silicon octahedral structural unit (3, 4), as expressed in its repeating structural unit

formula Ca3Si(OH)6(CO3)(SO4)·12H2O, or stoichiometrically CaSiO3·CaCO3·CaSO4·15H2O.

Six-coordinated silicate minerals such as stishovite typically form at high pressure and high

temperature. However, thaumasite in concrete was generally believed to form in a moist, cool

environment, favorably near 5oC (5).

Thaumasite drew the attention of the concrete industry because it involves a unique form of sulfate

attack, in which the formation of thaumasite alters the primary binder and causes concrete to

progressively lose paste-aggregate bond, strength, coherence and eventually serviceability.

Reactions involved in TSA are not limited by the amount of aluminate in cement (as is delayed

ettringite formation, DEF) nor suppressed by the consumption or absence of portlandite. Internal

sulfate TSA can develop progressively and aggressively once it occurs, and the affected concrete

may become “mushy” or even fluid as described in the technical literature (5, 6). Thus, there are

sufficient reasons to call thaumasite the “true concrete cancer.”

TSA was initially identified in concrete in the 1960s (1, 2), and had been occasionally reported

thereafter in the next thirty years. More than ten high-profile cases of deteriorated bridge

foundations and columns due to TSA found in the UK had prompted the British authorities to

convene a Thaumasite Expert Group (TEG) in 1998 to investigate the issue and provide

recommendations to mitigate or prevent TSA in new construction (6, 7). An interim report by the

TEG, which summarized risks, diagnosis, remedial works and guidance, was published in January

1999. The significance of this unique distress mechanism has since been increasingly recognized,

thanks largely to the work by the TEG. Following this interim report, an International Conference

on Thaumasite in Cementitious Materials was held at the British Research Establishment (BRE)

in Garston, UK, in June 2002. Approximately sixty papers presented at the conference were

published in a special issue of Cement & Concrete Composites (Volume 25 No. 8 2003). Since

then, the number of TSA-related papers has increased steadily.

Reported TSA cases mainly involved sulfate from external sources associated with soils,

groundwater, sulfate-bearing clay bricks, and gypsum plaster, and mortars containing gypsum or

sulfides (5-8). This paper presents two internal TSA cases, in which sulfate was determined to be

from the cementitious material in the floor slab deterioration of the first case, and from the

dolostone coarse aggregate in the foundation wall deterioration of the second case.

CASE 1 - FLOOR SLAB DETERIORATION

The first case involved concrete floor slab deterioration at a meat processing facility located in

Pleasant Prairie, Wisconsin. Floor repair concrete in the production area of the facility reportedly

exhibited spalls within five years, compelling the investigation. The floor and production

equipment were exposed to a rigorous cleaning process on a daily basis. The temperature of the

space where the deterioration had occurred was maintained at approximately 45°F (7oC), which is

considered nearly optimal for thaumasite formation based on the technical literature (5, 6, 7).

Constituents and General Characteristics of The Concrete: The appearance of saw-cut and

lapped surfaces of a representative core from a deteriorated region is shown in Figure 1. The

concrete is composed of pea-gravel coarse aggregate (top size of 3/8 inch) and natural sand fine

aggregate dispersed in a non-air entrained hardened paste of portland cement (and possibly minor

calcium aluminate cement). The pea gravel mainly consisted of dolomite with small amounts of

sandstone and cherty dolostone. The sand is siliceous and composed mainly of quartz.

Microscopical observations and physical characteristics of the paste are indicative of a moderate

water-to-cementitious materials ratio. The aggregates did not contain gypsum and generally

appeared to be sound.

Distress Characteristics and Thaumasite: The concrete contains many discontinuous transverse

cracks throughout the three-inch depth of the core (Figures 1, 2 and 3). Concrete at greater depth

reportedly deteriorated into rubble and only small fragments were recovered. The transverse cracks

extended mainly around aggregate particles; however, a few cracks went through the aggregate

particles. Rims or halos around aggregate or portions of aggregate particles are abundant. These

rims usually occurred above, below, or both above and below aggregate particles; they occur less

commonly surrounding whole particles. They may also connect to the transverse cracks. These

rims or halos were often filled or partially filled with white secondary deposits. The secondary

deposits were soft and powdery. Paste-aggregate bond was weak or absent. Gravel particles readily

came off from cored or saw-cut surfaces.

A polarized-light microscopical examination revealed that the white deposits in the rims or halos

are predominantly thaumasite (Figures 4 and 5), as characterized by the feather or needle-like

crystals, refractive indexes less than 1.54, and yellow to blue birefringence colors. The

birefringence colors are distinctively different from that of ettringite, which typically exhibits gray

color. Thaumasite in plane-polarized light is colorless, light brown, or light hay-yellow. In the

thaumasite halos, residual cement and fine aggregate particles were observed. The abundances of

residual cement particles and sand increased towards the paste side and decreased towards the

aggregate side within the thaumasite halos. Overall, the abundances were lower than in a general

area of paste, suggesting some cement particles may have dissolved and participated reactions,

been forced apart by expansion, or both of these processes. We believe that in a halo, thaumasite

adjacent to the aggregate forms later than the thaumasite on the paste side of the halo. A dark, less

transparent paste band surrounding a thaumasite halo (Figure 4) was often observed. The band

appeared to contain more abundant residual cement particles.

Some rims or halos were partially open or unfilled, typically at the aggregate side, exhibiting an

appearance of adhesion cracks or plastic gaps (Figures 1 through 6). Microscopical evidence,

however, indicates they were not plastic adhesion cracks. Small broken portions of aggregate were

attached to the thaumasite side, indicating the separation occurred after the precipitation of

thaumasite (Figure 5). The features may also suggest multiple, intermittent expansion events.

Localized drying shrinkage during sample preparation may have contributed to the gapping but

was considered to be a minor factor based on observations of freshly sawed surfaces and the

general shapes and textures of the separations.

Thaumasite also fills or partially fills cracks and voids (Figures 1, 2 and 3). Paste away from the

peripheral regions also appeared to be substantially altered and often replaced by thaumasite

(Figure 4).

Source of Sulfate: Cement-sized crystalline phases, identified as mainly anhydrite and gypsum,

were observed (Figures 7 and 8). These sulfates were estimated at 2 to 4 percent by volume among

the paste. The crystals were either angular and tightly embedded in the paste, or rounded and

floated in “voids” that appeared to have been created from the dissolution or percolating of the

sulfate phases. These crystals did not appear to be secondary deposits, and it is therefore assumed

that they are from the original mix. Hydration-rims and dissolution voids associated with

conversion or dissolution of the sulfate phases were also observed (Figures 7 and 8). The hydration

textures provide compelling evidence of sulfate mineral dissolution and involvement in thaumasite

formation.

The sulfate content of the core was determined. Sulfate content was found to be 3.0% (as SO3 by

mass of concrete) at depth interval 0.1 to 0.5-inch and 3.1% at depth interval 2.5 to 3.0-inches.

These values are substantially greater than the sulfate that can be accounted for by typical portland

cements alone. The sulfate contents at the two depth intervals are considered essentially identical

and are consistent with an internal source and the petrographic interpretation.

The presence of excessive sulfates and possibly minor calcium aluminate cement may suggest the

concrete mixture is a patch material. Patch materials with excessively high sulfate additions are

not appropriate for environments that will be exposed to moisture and low temperature, such as

this floor.

CASE 2 - CRACKING OF FOOTING AND FOUNDATION WALLS

This TSA case involved distress in the footings and foundation walls of a building located in

Canton Township, Michigan. The building was approximately 10 years old when distress was first

noticed. Distress manifested as map cracks on the structure (Figure 9). The present investigation

represents an extension of a previous study of the case (9). The previous work concluded that the

subject concrete distress was due to thaumasite-related sulfate attack. However, the study failed to

identify the source of excessively high sulfate in the concrete (>1.0% throughout concrete depth),

because of limited information and lack of detailed thin-section examination. This extended

investigation reveals that the sulfate feeding thaumasite formation was from the dolostone coarse

aggregate. The characteristics of the concrete, the problematic dolostone aggregate, and the

distress mechanism are found to be very similar to those reported in another case of internal

thaumasite sulfate attack in Michigan by one of the study participants (10), although the two

concrete mixtures had very different ages and were from different regions of Michigan.

Constituents and General Characteristics: The concrete is composed of crushed dolostone

coarse aggregate (top size of 3/4 inch) and natural sand fine aggregate dispersed in a non-air

entrained hardened paste of portland cement and fly ash (Figure 10). Microscopical and physical

characteristics of the paste are indicative of a moderate to moderately high water-to-cementitious

materials ratio.

Distress Characteristics and Thaumasite: General distress characteristics can be found in the

list of references (9, 10). In summary, these include: 1) thaumasite secondary deposits surrounding

or partially surrounding aggregate particles (Figures 10 and 11); 2) frequent cracks in both paste

and aggregates; 3) paste being significantly softened locally and paste-aggregate bond being weak;

and 4) thaumasite filling voids and cracks and replacing a portion of paste.

Source of Sulfate: Sulfate feeding the thaumasite formation was determined to have been derived

from the dolostone coarse aggregate. The problematic dolostone is somewhat porous, composed

of equi-granular dolomite rhombs or irregular crystals several tens of microns in size. The rock

often contained large patches of gypsum (Figures 11 and 12) or interstitial gypsum and small

amounts of anhydrite (Figure 13 and 14). Voids suggesting dissolution of gypsum or anhydrite

were observed. Initial formation of small amounts of thaumasite immediately adjacent gypsum in

dolostone particles was also observed (Figure 12). A previous SEM-EDX study (9) had also

identified the presence of gypsum but gypsum was interpreted as a void-filling secondary deposit,

rather than properly as an inherent portion of the aggregate (Figure 15, and the Figure 8 in reference

9). When these gypsum-containing particles are crushed, a portion of the gypsum embedded in the

particles is liberated and readily accessible to prime thaumasite formation. The total sulfate

content, expressed as percent SO3 by mass of the concrete, ranged from 1.0 to 1.6 percent at various

depths up to 17 inches (core interior end), greater than can be accounted for by the portland cement

alone. The total sulfate content of a local soil sample was low (<0.01% or 40 ppm).

SUMMARY AND DISCUSSION

1. Petrographic Examination

Petrographic examination, particularly using polarized-light microscopy, is the most effective

method to identify TSA distress. When TSA deterioration is suspected, the first technique to utilize

for investigators should be petrography. Petrographic examination consists of a systematic

sequence of observations carried out at increasingly higher levels of magnification from visual and

low-magnification (0 to ~50X) examinations of as-received, lapped, and freshly broken samples,

to high-magnification (50 to 500X) petrographic microscope examination of thin sections and

polished sections, and to higher magnifications (100X to greater than 5,000X) with the scanning

electron microscope (SEM) when needed.

The petrographic microscope examination of thin sections is critical in differentiating thaumasite

from ettringite. Thaumasite has a much higher birefringent color than ettringite due to a higher

birefringence (0.036 vs. 0.06). XRD analysis would be the most definitive technique to positively

identify thaumasite. However, XRD patterns of thaumasite and ettringite overlap to a certain extent

(Figure 16), and if thaumasite is small in quantity, dilution due to aggregate and paste may make

the positive identification difficult.

2. Carbonate Rock

Carbonate rock, principally limestone and dolostone, are the most widely used crushed rock types

for aggregate production for portland cement concrete (PCC), due to its abundance, ease of

processing, and typically satisfactory performance. However, cases of concrete deterioration have

frequently occurred in our concrete troubleshooting practice, including alkali-carbonate reaction

(ACR), alkali-silica reaction (ASR), D-cracking, popouts, general freeze-thaw damage, surface

dusting, and thaumasite sulfate attack (TSA) as reported here, all due to differences in the quality

of the carbonate aggregate.

Geologically, the formation of carbonate rock involves complicated physical, chemical, and

biological processes, resulting in substantial variability in mineralogical composition and

petrological fabric that affect carbonate aggregate performance. More often than not, mined

carbonate aggregate (including natural carbonate gravel) contains many rock types transitioning

between ideal limestone (all calcite) and dolostone (all dolomite). Many other rock types,

commonly including shale, clay/mudstone/ironstone, sandstone, and chert are present, and there is

a wide range in porosity, induration, and other characteristics. Sulfide and sulfate minerals,

carbonaceous organic matter, coal, and other impurities are also common. The compositional and

textural variability of carbonate and associated rocks has been shown to significantly affect their

performance as aggregate. Petrographic examination, combined with other analyses when

necessary, may effectively evaluate the abundance of gypsum, sulfides, and other potentially

deleterious components in aggregate. To date, published studies appear to have emphasized the

role of carbonate aggregates providing carbonate in TSA but have often ignored the potential role

of aggregate supplying sulfate. The second case study reveals the general texture of a dolostone in

Michigan that contains excessively high gypsum. Petrographic examination of the aggregate

would have prevented the use of the aggregate, or at least recommended additional chemical tests.

3. Portland-Limestone Cement (PLC)

Limestone is also being increasingly used in portland-limestone cement (PLC). Greater scrutiny

of the mineralogical, chemical, and physical complexity of this widely-used construction material

and the effects of their variability on PLC performance is needed in practice and in research. The

PLC might be potentially vulnerable to TSA, although conflicting results have been reported

regarding the durability of PLC concrete (11, 12) depending on the replacement rate and many

other factors. This paper demonstrates the unique capabilities and usefulness of aggregate and

concrete petrography, a method that is strongly recommended to contractors and manufacturers

before a “limestone” is mixed in concrete mixtures or blended in clinker/cement and causing

problems.

4. External vs. Internal TSA

Internal TSA is considered more deleterious and little can be done once it initiates. The required

components are all conveniently available or within readily accessible distance, generally

throughout the concrete, in contrast to external TSA. Supplementary cementitious materials such

as fly ash may not be helpful preventing the reactions; effectiveness of low w/c may also be limited.

Fly ash is present in both of the Michigan cases and fly ash apparently did not prevent TSA.

Internal TSA causes the concrete to deteriorate fairly uniformly throughout and at a fast pace. The

reported external TSA in England motorway bridges exhibited a deteriorated depth of

approximately 1.5 inch in 30 years (13). Concrete in these two internal TSA cases has exhibited

deterioration throughout the depth of the cores up to a foot in five or ten years. A zoning texture,

increasingly severe damage toward the exterior surface as reported for external TSA (5, 6), is

typically not observed for internal TSA. In addition, TSA may be more widespread than generally

perceived because of buried structures and misidentification of thaumasite as ettringite, as

acknowledged by other workers. It appears that TSA is easy to duplicate in laboratory experiments

provided that several parameters including sulfate levels, pH levels, and low storage temperature

are met. We strongly believe that other similar internal TSA cases exist in the state of Michigan

and probably in nearby Ohio and Indiana. Inspection is recommended for buried structures under

similar conditions in the state. However, it should also be noted that the formation of thaumasite

does not necessarily cause distress. We have observed thaumasite formation in some historic

mortars and concrete without significant associated deterioration (14).

5. Topochemical vs. Through-Solution Mechanism

Two major mechanisms of thaumasite formation, a topochemical mechanism and a through-

solution mechanism, have been proposed (5, 6). These two mechanisms seem to correlate with an

indirect route and a direct route (15). Observation of residual cement and sand within a thaumasite

halo may indicate that the formation of thaumasite was not simply the filling of an empty space

(at least at certain stage) but was likely a result of replacement of existing in-situ paste. This feature

suggests the hypothesis of topochemical formation of thaumasite (6) (under the broad definition

and in a less strict sense (8)) is relevant. Features associated with a thaumasite halo also appear to

be different from ettringite surrounding aggregate particles in DEF-affected concrete. In our

experience, ettringite lining the peripheral gaps or cracks often appear to be pure and free of

residual cement and aggregate. In addition, ettringite tended to surround various aggregate

particles indiscriminately whereas thaumasite appeared to occur more often around carbonate

aggregate particles. Thaumasite rims were absent or far less developed around siliceous aggregate

(either black or white gravel in this concrete) than dolomitic ones (brown, yellow or gray, Figures

1 and 6), even though the aggregate particles were of similar sizes. The observation may imply

roles of carbonate or even magnesium (5) from the dolomite. The observed dark band surrounding

a thaumasite halo, however, appeared to be more supportive of a through-solution mechanism, in

which residual cement particles freed in solution were accumulating into the band. Nevertheless,

thaumasite occurring in voids must have formed through solution and precipitation processes.

ACKNOWLEDGEMENT

The authors greatly appreciate the clients who provided opportunities for us to work on these

interesting projects and help them troubleshoot their concrete problems. The authors thank

Margaret Reed and Susanne Papas for their assistance with petrographic and chemical analyses

and Brookelynn Schmeck for her exhaustive efforts in editing and formatting this manuscript.

REFERENCES

1. Erlin, B., and Stark, D. C. (1966). Identification and occurrence of thaumasite in concrete.

Highway Research Record, No. 113, pp 108–113. Publisher: Highway Research Board.

2. Stark, David C. (2003). Occurrence of thaumasite in deteriorated concrete. Cement &

Concrete Composites, 25(8), pp 1119-1121.

3. Edge, R. A., and Taylor, H. F. W. (1969). Crystal structure of thaumasite, a mineral

containing [Si(OH)6]2− groups. Nature, 224, pp 363-364.

4. Martucci, A., and Cruciani, G. (2006). In situ time resolved synchrotron powder diffraction

study of thaumasite. Phys. Chem. Minerals, 33, pp 723-731.

5. Sharp, J. H. (2006). Surely we know all about cement – don't we? Advances in Applied

Ceramics, 105 (4), pp 162-174.

6. Crammond, N. J. (2003). The thaumasite form of sulfate attack in the UK. Cement &

Concrete Composites, 25(8), 809–818.

7. TEG Report Department of Environment, Transport and the Regions (1999). The

thaumasite form of sulfate attack: Risks, diagnosis, remedial works and guidance on new

construction. Report of the Thaumasite Expert Group. DETR, January 1999. London.

8. Sulfate Attack on Concrete, Jan Skalny, Jacques Marchand and Ivan Odler ed. 2002, pp.

53-54.

9. Marusin S. L., Reed, M. H. (2007). Thaumasite in concrete - a case study. Proceedings of

the Twenty-Nine International Conference on Cement Microscopy, Quebec City, PQ,

Canada, May 20 -24, pp 70-90.

10. Hou, H., and Daugherty A. (2011). Petrographic study of concrete: two case studies

involving internal and external sulfate attacks. Proceedings of the Thirty-Third

International Conference on Cement Microscopy, April 17 – 20, 2011. San Francisco,

California, U.S.A.

11. Ramezanianpour, A. M., and Hooton, R. (2013). Thaumasite sulfate attack in portland and

portland-limestone cement mortars exposed to sulfate solution. Construction and Building

Materials 40, pp 162–173.

12. Gaze, M. E. (1997). The effects of varying gypsum content on thaumasite formation in a

cement: lime: sand mortar at 5 °C. Cem. Conc. Res. 27 (2) pp 259-265.

13. Slater, D., Floyd, M., and Wimpenny, D.E. (2003). A summary of the Highways Agency

Thaumasite Investigation in Gloucestershire: the scope of work and main findings. Cement

& Concrete Composites 25, pp 1067-1076.

14. Powers, L.J., and Walsh, J. (2005). A new look at an old cement. Proceedings of the 27th

International Conference on Cement Microscopy, p 118-131, April 24-28, 2005, Victoria,

British Columbia, Canada.

15. Rahman, M.M., and Bassuoni, M.T. (2014). Thaumasite sulfate attack on concrete:

mechanisms, influential factors and mitigation, Construction and Building Materials 73,

pp 652–662.

Figure 1. The pea gravel concrete and its topping. Abundant narrow,

discontinuous, transverse cracks and white halos surround aggregate

particles. Black and white siliceous particles tend less frequently to

exhibit white halos or secondary deposits than gray and yellow

carbonate aggregates.

Figure 2. Close-up view of thaumasite rims (arrows).

Distinct rims or halos above or below aggregate

particles. Thaumsite rims in upper photo absorbed epoxy

from sample preparation and exhibited darkened color.

Figure 3. Close-up view of cracks in paste.

Figure 4. Thin-section photomicrographs illustrating thaumasite

surrounding aggregate particles and locally replacing paste. A few

residual cement particles were present in the thaumasite halos. A

dark band presented at the outer edge of the halo (arrows). Upper

photo: plane-polarized light; lower photo: cross-polarized light.

Figure 5. Thin-section photomicrographs illustrating opened rims

and thaumasite surrounding aggregate particles. Arrows show small

detached portions of the aggregate particle, suggesting the gap was

probably further opened after the thaumasite had formed and

therefore was not new or recent. Upper photo: plane-polarized light;

lower photo: cross-polarized light.

Figure 6. Thin-section photomicrographs illustrating thaumasite

surrounding a dolostone coarse aggregate particle (upper left) but

not a siliceous particle of similar size (lower right). Upper photo:

plane-polarized light; lower photo: cross-polarized light.

Figure 7. Thin-section photomicrographs illustrating rounded

sulfate, likely anhydrate (arrows). The texture is consistent with

dissolution of preexisting material rather than secondary deposits.

Upper photo: plane-polarized light; lower photo: cross-polarized

light.

Figure 8. Thin-section photomicrographs illustrating rounded sulfate

(arrows). Again, the texture is consistent with dissolution of

preexisting material instead of secondary precipitation. Upper:

plane-polarized light; lower: cross-polarized light.

Figure 9. Footing/foundation wall exhibited cracks.

Figure 10. Saw-cut and lapped section of a

segment of a concrete core subjected to

thaumasite form sulfate attack.

Figure 11. Thin-section photomicrographs illustrating thaumasite

rims surrounding aggregate particles. Arrows show gypsum and

anhydrite (top) within a dolostone aggregate particle. Upper: plane-

polarized light; lower: cross-polarized light.

Figure 12. Thin-section photomicrograph illustrating gypsum and

anhydrite at edge of a dolostone aggregate particle, appearing to

exhibit localized dissolution. Minor amounts of thaumasite formed

surrounding the particle (arrows). Cross-polarized light.

Figure 13. Thin-section photomicrograph illustrating gypsum and

anhydrite within a dolostone aggregate particle, appearing to exhibit

localized dissolution. Cross-polarized light.

Figure 14. Thin-section photomicrograph illustrating

anhydrite within a dolostone aggregate particle. Cross-

polarized light.

Figure 15. Unpublished data from ref [9]. Gypsum within a

dolostone aggregate particle (lower left) surrounded by

thaumasite. Similar data was reported in Figure 8 of the

referenced paper. The round portion at lower left corner of the

left photo appears also to be gypsum.

Figure 16. XRD powder pattern of secondary deposits from aggregate sockets in TSA-affected concrete.

Upper: full scanned 2Ɵ range; Lower: expanded 2Ɵ ranges from 8 to 10.5 degrees and 15 to 17 degrees

illustrating that diffraction peaks are bettered resolved at higher 2Ɵ range 15 to 17 degrees. The third

order diffractions are even better resolved (arrow). Data presented at ICMA.10

5 15 25 35 45 55

15 15.5 16 16.5 178 8.5 9 9.5 10 10.5

Ettringite

Thaumasite

2Ɵ KαCu 2Ɵ KαCu

Ettringite

Th.+Ett. Dol

Th.+Ett.