| Abstract |
Corrosion was documented in two concrete structures in the Leyte Geothermal Production Field (LGPF). Deterioration of the cement matrix to a weak and powdery state left the rock aggregates protruding out of the concrete surfaces of the rock muffler in South Sambaloran and of spray tower 0posts in the Upper Mahiao Zero Effluent Disposal System (UM ZEDS). These field observations were initially noted in September 2003 in the rock muffler and during the October 2004 annual Preventive Maintenance Service (PMS) of UM ZEDS. Various laboratory analyses such as phenolphthalein test, X-ray diffractometry (XRD) and scanning electron microscopy (SEM-EDAX) indicate that deterioration of the rock muffler structure was mainly caused by attack of sulfuric acid (H2SO4) formed from H2S in the condensed steam. Sulfuric acid reacts with portlandite in the cement matrix, which becomes weak and powdery, leaving the aggregates exposed as the matrix is gradually removed from the exposed surface of the concrete. On the other hand, the soft friable nature of the UM ZEDS spray tower posts and the development of secondary calcite in the cement matrix suggest that he concrete was attacked by carbonic acid formed by dissolved CO2 in the power plant effluents. Apart from decomposing the cement paste, carbonation also lowers the pH of concrete hastening corrosion of reinforcing steel bars in the spray tower posts. To minimize acid attack on other concrete structures in LGPF using the South Sambaloran rock muffler as a case study, it was recommended to modify concrete composition to reduce content of chemicals vulnerable to aggressive ions. Portlandite content must be kept low and blended cements with high amounts of fly-ash, pozzolan, or blast furnace slag must be used. For the UM ZEDS, the spray tower posts were resurfaced with special cement mix of high density blended with fly-ash. In addition, the posts and the basin were jacketed with molded high density polyethylene plastic that is both durable and resistant to corrosion attack. |