Page 151 - Handbook of Thermal Analysis of Construction Materials

Section 9.0 - Durability Aspects 135

formation of Friedel’s salt. Lime and Friedel’s salt were detected by peaks
at 460–570° and 280°C, respectively.
Several complex reactions occur when concrete is exposed to sea
water. Compounds such as aragonite, calcium bicarbonate, calcium
monosulfate hydrate, ettringite, Ca-Mg silicate hydrates, magnesium sili-
cate, thaumasite, etc., have been identified by the application of thermal
analysis in conjunction with other techniques. Thus, the factors leading to
the deterioration of concrete can be established.
Several cases of distress of concrete railway ties have been re-
ported. Examination of cracks in such concretes has revealed that they were
filled with secondary products. Ettringite was a prominent constituent,
accompanied by CH, CaCO , and alkali-silica gel. DTA data indicated the
3
presence of a substantial amount of secondary ettringite in many samples
but petrographic and XRD did not reveal any ettringite. [91] Thermograms
were used to quantitatively estimate the amounts of ettringite formed in
failed samples.
Thaumasite, is a mineral of relevance in concrete technology. In a
pure form it has the composition Ca [Si(OH) ] (CO ) (SO )•24H O. This
2
4
3 2
6
6 2
compound may form through a combination of sulfate attack and carbon-
ation. It can cause damage to concrete by decomposing the C-S-H phase. Its
formation can be rapid in the presence of finely divided CaCO . DTA and
3
TG techniques have been adopted to identify thaumasite in concrete. [92] In
DTA the decomposition starts at 110°C, with a peak at 150°C. A small
exothermic peak at 710°C has been attributed to a disorder-order type
transition, akin to devitrification. TGA shows decomposition of mineral
thaumasite starting at 110°C. A small loss of CO occurs simultaneously
2
with water up to 550°C. Most CO is lost between 950 and 980°C.
2
9.7 Aged Concrete

Many studies have been carried out on old concretes to determine
the reactions that could be responsible for deterioration. Sarkar, et al., [93]
examined a seventy-five year old stone building containing mortar that had
shown signs of distress. The presence of gypsum (endothermic effect at
133°C), quartz (endothermal peak 573°C), calcium carbonate (endother-
mal effect at 900°C), and tharndite (endotherm at 880°C) could be identi-
fied. It was concluded that one of the main causes of deterioration was the
interaction of SO from the atmosphere with mortar and sandstone. In
2
another study, [94] a fifty year old concrete was subjected to examination