Ettringite is one of the main reaction products formed during external sulfate attack on concrete. During this process, the microstructure of the original cement paste is altered. The formation of excessive amounts of ettringite may, under some conditions, have deleterious effects leading to expansion and cracking of concrete. Depending on the severity of attack, local environmental conditions, and availability of sulfate, gypsum may be formed in the paste and at the paste aggregate interfaces. Sometimes the presence of a third mineral, thaumasite (CaSiO3·CaCO3·CaSO4·15H2O) is overlooked due to its morphological similarity to that of ettringite. Examination of concrete obtained from various structural locations of residential homes of Southern California by scanning electron microscopy (SEM) and optical microscopy revealed the presence of thaumasite in association with ettringite and/or gypsum. Morphologically, both thaumasite and ettringite have very similar structure. Analysis of the chemical composition using energy dispersive X-ray spectroscopy (EDS) shows the presence of calcium, silicon, sulfur and oxygen in thaumasite and calcium, aluminum, sulfur and oxygen in ettringite as the major components. Thaumasite deposits are usually associated with gypsum deposits in highly carbonated pastes.

Evidence of thaumasite formation in Southern California concrete

Identification of thaumasite (CaSiO3·CaO3·CaSO4·15H2O) in concrete undergoing external sulfate attack by X-ray powder diffraction or by microscopic techniques is difficult due to its crystallographic and morphological similarity with ettringite. Widefield Raman chemical imaging via liquid crystal tunable filter (LCTF) technology has been used in a preliminary study to determine the presence of thaumasite in association with ettringite (3CaO·Al2O3·3CaSO4·32H2O) and gypsum (CaSO4·2H2O). Raman chemical imaging combines Raman spectroscopy with optical microscopy and digital imaging to provide images with molecular-based contrast. Thaumasite has three major peaks at 658, 990, 1076 cm−1 and three minor peaks at 417, 453, 479 cm−1. Ettringite has major peaks at 990, 1088 cm−1. Gypsum has a major peak at 1009 cm−1 and minor peaks at 417, 496, 621, 673, 1137 cm−1. When these minerals are presented together, Raman chemical imaging provides an excellent way to determine their molecular composition and spatial distribution within the sample.

Identification of thaumasite in concrete by Raman chemical imaging

The reported studies made on field elements affected by thaumasite sulfate attack are discussed, together with the reported laboratory performance of concrete and mortars immersed in sulfate solutions maintained at 5 °C. It is concluded that magnesium ions ingressing from groundwater or resulting from dedolomitization of an aggregate containing dolomite play a major role in increasing the risk of thaumasite attack and that attack can also arise from exposure to sulfuric acid bearing groundwaters, or sulfuric acid resulting from oxidation of pyrites within aggregates. Guidance is given on concrete qualities and materials to resist sulfate attack and thaumasite sulfate attack resulting from salts present in groundwater.

Thaumasite sulfate attack in field and laboratory concretes: implications for specifications

The objective of the research was to accelerate the thaumasite form of sulfate attack (TSA) under laboratory conditions in order to identify its effects on skin friction at the soil/concrete interface. The experimental programme was organised into five series which investigated the formation of TSA under unrestrained and restrained conditions whereby the acceleration of TSA was observed at unrestrained conditions depending on water-cement ratio, cement content, casting face and aggressive solution. Restrained conditions simulated soil/concrete interface interactions and were applied to identify changes of the skin friction affected by the formation of thaumasite. TSA was successfully accelerated and a linear deterioration progress was monitored using a developed needle test method. Using clay-restrained conditions thaumasite formed attached to the concrete and favoured a more severe deterioration culminating in thaumasite layers of up to 25mm depending on interface pH and applied pressure. Thaumasite at the interface did not decrease the shear strength including skin friction and cohesion. Therefore it was concluded that TSA occurring at piles or foundation bases does not affect the stability of the superstructure regarding loss of friction and settlements, however, continuous loss of concrete can increase the slenderness and cause premature corrosion.

Accelerating the thaumasite form of sulfate attack and an investigation of its effects on skin friction

This paper reports an experimental study on the damage mechanism and resistance of Type I portland cement mortar and paste samples exposed to 5% sodium sulfate solution with different solution temperatures; namely, 4 C, 10 C and 20 C. The resis- tance of mortar samples was evaluated using expansion, compressive strength and flexural strength measurements. Some microstructural observations such as x-ray diffraction, differential scanning calorimetry and scanning electron microscopy were also introduced to elu- cidate reactants formed by sulfate attack, especially in a low temperature condition. From the results, it was found that the degree of damage in the mortar samples was significantly associated with the temperature of sulfate solution. Low temperature of the sulfate solu- tion led to the formation of thaumasite in mortar and paste samples, and subsequently a poor resistance to sulfate attack. Thus, it is noted that when concrete structures are exposed to sulfate media in the condition of a cold region or whether, special care should be taken.

Sulfate Attack Resistance and Microstructural Observations of Cement Matrix Exposed to a Low Temperature Condition

Some different degrees of alteration have been found in a concrete tunnel lining affected by dissolved calcium sulphate in water due to the oxidation and the solubility of the pyrites contained in the rock: in some cases the damage has been of such an extent that the concrete has changed into a plastic mass. Measurements taken inside the tunnel over many years showed a range of temperature from 4 deg to 6 deg c. The chemical analysis of the concrete, the diffractometric analysis and the scanning electron microscope analysis performed on some alteration products all clearly showed the presence of thaumasite and calcite, while no ettringite was found. The recorded thermal conditions, the presence of silica due to feldspar alteration and the free carbon dioxide in the water lead to the conclusion that the ettringite has been transformed completely into thaumasite. In order to understand this transformation, some laboratory tests were carried out on mortar immersed in a water mixture containing calcium sulphate, calcium carbonate, calcium hydroxide and amorphous silica in an environment rich in carbon dioxide, at the temperature of 5 deg and 20 deg c respectively. (Author/TRRL)

Thaumasite in deteriorated concretes in the presence of sulfates. concrete durability. katherine and bryant mather international conference, held at atlanta, georgia, usa, 27 april-may 1987

G Baronio

Papers

Thaumasite in deteriorated concretes in the presence of sulfates. concrete durability. katherine and bryant mather international conference, held at atlanta, georgia, usa, 27 april-may 1987