Endothermic effect mortars

Many studies have been carried out on old concretes to determine the reactions that could be responsible for deterioration. Sarkar, et al.,1 examined a seventy-five year old stone building containing mortar that had shown signs of distress. The presence of g5q)sum (endothermic effect at 133°C), quartz (endothermal peak 573°C), calcium carbonate (endother-mal effect at 900°C), and thamdite (endotherm at 880°C) could be identified. It was concluded that one of the main causes of deterioration was the interaction of SO2 from the atmosphere with mortar and sandstone. In another study,a fifty year old concrete was subjected to examination... 

Thermal analytical techniques have been applied to investigate the causes leading to the deterioration of concrete subjected to various environmental factors. A mortar nearly two thousand years old obtained from El Tajan near Mexico City was analyzed by TG and electron microscopy. TG analysis was done on samples taken from different areas and depths. The loss of water below 100°Cwas caused by the adsorbed water from the volcanic tuff, while the endothermal effect at >700°C corresponded to the carbonated lime and carbonated silicates and aluminates derived from the pozzolan. The extent of the reaction of lime with pozzolan was computed to be 6.91%. 

In mortars originally formed with both ADP and STPP, there is a small endotherm around 70°C. At early times, e.g., 5 minutes, the superposition of exothermic and endothermic effects (at about 107°C) results in a double endotherm indicating the presence of schertalite. A small exothermic peak above 8 00°C may be due to the formation of Mg(P04)2 coincident with an endothermal effect due to the melting of Na4P20y formed from STPP. 

An addition of 0.1% CLS may extend the initial and final setting times of cement mortar by two and three hours, respectively. The influence of 0.3% CLS on the hydration of cement is shown in Fig. 9. Thermograms indicate that the reference cement containing no admixture exhibits a broad endothermal peak below 200°C, representing the formation of both ettringite and C-S-H phase. These peaks increase in intensity as the hydration period is increased. The effect between 450 and 500°C is caused by the dehydration of Ca(OH)2 and its intensity indicates the extent to which the hydration of the C3S component has progressed. The cement hydration, in the presence of lignosulfonate, is retarded as seen by the lower intensity of the Ca(OH)2 decomposition peak. The low temperature effect below 300°C in the presence of CLS is not sharp as that obtained in the reference sample. 

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