Cement paste very high strength

Kendall el al. (K64) considered that MDF cements owed their high strengths primarily to the absence of large flaws, which they held to be the reason for the relative weakness of normal cement pastes. This view has been challenged (B115,K65,E4). Beaudoin and Feldman (B115) discussed ways in which stress concentrations were reduced in very strong cement pastes, and both they and Eden and Bailey (E4) considered that in MDF pastes the polymer plays a major role. 

In eighties of twentieth century the two new technologies of very high strength cement pastes were developed the DSP— Densifted Systems containing homogeneously arranged ultra-fine Particles (in Denmark), and the MDF— Macro Defects Free Cements. 

Roy, D.M., Gouda, G.R., and Brabovski, A. (1972) Very high strength cement paste prepared by pressing and other high pressure techniques. Cement and Concrete Research 2,349-366. 

Kendall, K., Birchall, J. G. (1985) Porosity and its relationship to the strength of hydraulic cement pastes, in Mat. Res. Soc. Symp. Very High Strength Concrete-Based Materials, 42 153-8. 

In another approach a cement paste produced with a normal water/cement ratio is exposed to uniaxial pressure after the paste has achieved its initial set. In this way a portion of the mixing water is removed, and the water/solid ratio may be reduced to about 0.1. The residual water allows a continuation of the hydration process, in which a very low-porosity, high-strength material, called pore-reduced cement (PRC), is produced (Macphee, 1990 Macphee et al., 1992 Geslin, 1995). Compressive and tensile strength improvements of up to three and six times respectively may be achieved by this approach. 

Very high compressive strength of cement matrices was obtained over 70 years ago. In the laboratory of the Lone Star Cement Corporation in the USA under the direction of D. A. Abrams, the compressive strength of cement paste equal to 276 MPa at an age of 28 days was achieved in 1930 (Powers 1947). The paste was of very low wic ratio equal to 0.08. This record achieved in a laboratory had no particular influence on contemporary practice and up to the 1970s the strength of concretes rarely exceeded 30 MPa or 40 MPa. 

Step 1—Roman cement pastes harden within a few minutes after the initial set. Six-hour strength values of up to 4 MPa are obtained. This high initial strength makes the cements suitable for casting architectural details since it permits very early removal of the objects from their molds. 

Properties of CBCs lie between ceramics and cements. These materials are formed at room temperature like cements, or may be synthesized at slightly elevated temperatures, but their structure is highly crystalline or glass-crystalline composite. The particles in CBCs are bonded by a paste formed by chemical reaction, as in cements, but the particles themselves are mostly crystalline. Their strengths are higher than those of cements but fall short of sintered ceramics. Their corrosion resistance is close to ceramics, but at the same time, they may be vulnerable to erosion like cements. The ease of formation of these ceramics, their rapid setting behavior and low cost make them very attractive for the various applications discussed in this book. 

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