Abstract
Nowadays, workability is absolutely necessary for piping concrete, which is influenced by characteristics of structure of fresh cement paste. In this paper, the movement of particles in electric field and flocculent units at different W/C were observed, and rheologic characteristics of fresh cement paste at different W/C and the pastes with naphthalene sulphonate (NS) and poly carboxylate (PC) superplasticizers were investigated using rotational viscometer (NXS-11A). It is found that at initial hydration age cement particles carry different electric quantity and electrical property; increment of W/C induces flocculent units more separated, less hysteresis loop area and lower crush point; application of superplasticizers brings much less loop area and indistinct crush point. It is concluded that 1) fresh cement paste is filled with flocculation units of different strength; 2) different units are connected with one another loosely; 3) tunnels for bleeding form in the weakest connecting area. Water can disperse flocculent units to some extent, especially for superplasticizers. However, the latter can’t separate flocculation of cement particles completely.
Highlights
Workability of fresh concrete is required in the process of placing concrete, especially for high-rises[1]
Rheologic characteristics of fresh cement paste are relevant to its inner structure, which is associated with the nature of particles in it
Naphthalene sulfate and poly carboxylate superplasticizers are anionic surfactants, which tends to adsorb on positive cement particles, and enlarge repulsive force between them through increasing static-electric or steric repulsive force, which brings forth the destruction of some flocculent units
Summary
Workability of fresh concrete is required in the process of placing concrete, especially for high-rises[1]. Superplasticizer, can change the structure of cement paste in fresh concrete, and enables concrete to be plasticized.[5][6][7]. Surface of cement particles is changed, and a type of paste with yields stress, named weak solid by Heimuth [9], establishes. Rheologic characteristics of fresh cement paste are relevant to its inner structure, which is associated with the nature of particles in it. Shear stress and structure of fresh cement paste. Zhang [11] draw a conclusion from the viewpoint of correlation between viscosity and fresh paste structure. This paper would illustrate the formulation of fresh cement paste through rheological and optical method, which is important for design of piping or self-compacting concrete
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