Unlike conventional hydrothennal geothermal technology that utilizes hot water as the energy conversion resources tapped from natural hydrothermal reservoir located at {approx}10 km below the ground surface, Enhanced Geothermal System (EGS) must create a hydrothermal reservoir in a hot rock stratum at temperatures {ge}200 C, present in {approx}5 km deep underground by employing hydraulic fracturing. This is the process of initiating and propagating a fracture as well as opening pre-existing fractures in a rock layer. In this operation, a considerable attention is paid to the pre-existing fractures and pressure-generated ones made in the underground foundation during drilling and logging. These fractures in terms of lost circulation zones often cause the wastage of a substantial amount of the circulated water-based drilling fluid or mud. Thus, such lost circulation zones must be plugged by sealing materials, so that the drilling operation can resume and continue. Next, one important consideration is the fact that the sealers must be disintegrated by highly pressured water to reopen the plugged fractures and to promote the propagation of reopened fractures. In response to this need, the objective of this phase I project in FYs 2009-2011 was to develop temporary cementitious fracture sealing materials possessing self-degradable properties generating more » when {ge} 200 C-heated scalers came in contact with water. At BNL, we formulated two types of non-Portland cementitious systems using inexpensive industrial by-products with pozzolanic properties, such as granulated blast-furnace slag from the steel industries, and fly ashes from coal-combustion power plants. These byproducts were activated by sodium silicate to initiate their pozzolanic reactions, and to create a cemetitious structure. One developed system was sodium silicate alkali-activated slag/Class C fly ash (AASC); the other was sodium silicate alkali-activated slag/Class F fly ash (AASF) as the binder of temper-try sealers. Two specific additives without sodium silicate as alkaline additive were developed in this project: One additive was the sodium carboxymethyl cellulose (CMC) as self-degradation promoting additive; the other was the hard-burned magnesium oxide (MgO) made from calcinating at 1,000-1,500 C as an expansive additive. The AASC and AASF cementitious sealers made by incorporating an appropriate amount of these additives met the following six criteria: 1) One dry mix component product; 2) plastic viscosity, 20 to 70 cp at 300 rpm; 3) maintenance of pumpability for at least 1 hour at 85 C; 4) compressive strength >2000 psi; 5) self-degradable by injection with water at a certain pressure; and 6) expandable and swelling properties; {ge}0.5% of total volume of the sealer. « less