The Performance Of Novel MgO-Expansive Agent For Longtan Rrc Dam

Number items of hydraulic power stations have been built up in our countryrecently to meet an increasing demand for electricity. However, the thermal stressproblem of mass concrete is still not thoroughly resolved. The current commonly-usedmeasures to solve this problem is to lower temperature of cements and aggregates, toadd ice into concrete mixtures, to lay water-cooling pipes or to cast thin sections andto postpone casting time of the next section etc. These solutions are sometimesdifficult to practice. They are also expansive and time-consuming.For solving the problem effectively, China had carried out many researches onusing delayed gentle expansion magnesia concrete to prevent cracks of dams andfasten dam construction in late 1980s, which has been successfully utilized in Qinxidam and Dongfeng dam etc. This technology makes it possible to construct damscontinuously whether environmental temperature is high or low. It can simplify damconstruction, reduce the duration of dam construction, and therefore brings aboutgreat economic and social benefit.However, there are some technical factors to hide the use of this technology. Themain one is that no qualified MgO-bearing expansive agent is available due to simpleand old-fashioned technologies of production. The another one is thenarrowly-distributed magnesite for producing MgO. There is a long distance betweenthe sites of production and application of MgO. This may cause loss of reactivity ofMgO expansive agents.This paper tried to utilize widely-distributed materials such as dolostone andother MgO-bearing minerals to produce novel magnesia-bearing expansive agents.Three expansive agents designated as A, B and C are designed and prepared by usinga mixture of dolostone and magnesite. MgO marked as D and produced by calciningmagnesite is also prepared and used as a comparison. All the four expansive agentswere calcined at 1150℃, 1200℃ or 1250℃ for 1h. The expansive performances ofcement pastes incorporated with the agents were tested by autoclaving in 2 MPa IIImoisture or curing in 20℃, 30℃, 40℃ or 50℃ water. The results reveal thatexpansion increases with the boosting of calcining temperature. Expansion increaseswith increase of quantity of expansive agents added and grow with prolonging ofcuring time. Development of expansion will be gentle until 150 days. Fly ash mayrestrain expansion, and this phenomenon is more obvious with the increasingproportion of fly ash. The novel magnesia expansive agent is better adapt to damconcrete incorporated with a great deal of fly ash than expansive agent from thecalcined magnesite only.Hydration of novel magnesia expansive agents is analysed. The products ofhydration are brucite, portlandite and CSH. The existing of C2S and f-CaO in agentsmay obstacle the hydration of MgO at early age, restrain migration of brucite formedand promote pH value of local pore solutions near the agents. These will ensure moreMgO to hydrate at late age promote the expandability of brucite. As a result, thespecial MgO-bearing agents may occupy a better expandability than MgO obtained bycalcining magnesite only.On the base of results in lab, the novel magnesia expansive agents have beenproduced industrially. The analysis of component and the expansion performance of itwere also conducted. The middle-stage experimental of the agents is now carried outat the cofferdam of Longtan hydraulic power station.Expansion curves of the special MgO-bearing agents in pastes cured in 30-40℃water are similar to curves of temperature stress caused by cooling of concrete.Therefore, expansion of the special MgO-bearing agents may effectively compensateshrinkage brought about by cooling of concrete.