Setting and Strengthening Mechanisms of Cementitious Materials
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SETTING AND STRENGTHENING MECHANISMS OF CEMENTITIOUS MATERIALS W. JIANG AND D.M. ROY Materials Research Laboratory, Pennsylvania State University, University Park, PA16802
ABSTRACT This paper highlights the physico-chemical characteristics of cementitious materials responsible for or involved in setting, and how these are related to the strengthening and mechanical properties. Five main mechanisms which can be used to increase cementitious materials strength were discussed. The substantially improved cement matrix materials can be obtained by deliberately using one or more above mechanisms. Experiments have been performed on ordinary portland cements and phosphate cements used as construction materials, whose setting time is several hours, and dental cement, whose setting time is several minutes. INTRUDCTION Setting Mechanism When cement is first mixed with water, it exists in a fluid-like state prior to becoming a rigid, load-bearing material. Cement hydration reactions begin immediately upon mixing with water, there is a period of time-several hours(before the setting reaction precludes further handling) in which the pastes are fluid or viscous. Setting is the term used to describe the stiffening of the cement paste, although the definition of the paste which is considered is some what arbitrary. Generally speaking, setting refers to a change from a fluid to a rigid state. Although during setting the paste acquires some strength, for practical purposes it is convenient to distinguish setting from hardening, which refers to gain of strength of a set cement paste. In another word setting is stiffening without significant development of compressive strength, and typically occurs within a few hours. Hardening is significant development of compressive strength, and normally a slower precess. Basically, two main types of cements can be identified, a) Hydraulic cements, set by hydration reactions when they react with water. b) Reaction cements, all dental cements are based on the hardening reaction between a powdered solid and a viscous hydrogen-bonded liquid. These cement themselves fall within Wygant's definition of reaction cement, a term applied to cement other than those which set by hydraulic action. Dental cements are more closely defined as acid-base reaction cements. The cement-forming liquids are acidic, viscous hydrogen-bonded liquids capable of donating protons, exemplified by phosphoric acid. The powdeiz are zinc oxide or aluminosilicate glass, which act as proton acceptors. The cement-forming reaction is essentially an acid-base interaction between these two components, the plastic pastes formed by mixing these two components set rapidly to hard salt-like gels which formed in Mat. Res. Soc. Symp. Proc. Vol. 245. 91992 Materials Research Society
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the liquid phase, acting as a binding matrix. The cement-forming reaction is one where hydrogen bridges in the liquid phase are progressively replaced by more rigid metal ion bridges, a process which causes the liquid to gel and gel to harden. Strengthening
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