Macro-defect-free Cement Research Articles

Advanced materials from hydraulic cements

Hydraulic cements are energy-cheap relative to other common materials and this was the driving force that led to the development of macrodefect-free (MDF) cements in 1981. In this paper the scientific principles of such materials in terms of porosity, particle packing and rheology are demonstrated and the use of a range of different cement types will be reviewed. The role of the polymeric rheological aid is emphasized and distinguished from that of so-called super-plasticizers used with conventional cements. The distinction between MDF cements and polymer concretes of various types is highlighted and the similarity of MDF cements and chemically bonded ceramics (CBCS) when the polymer is removed is noted. Various properties of MDF cements are given with emphasis on their high degree of mechanical reliability. These properties place MDF cements in a unique position in the materials field, a position that makes the replacement of hitherto more advanced engineering materials, such as metals, polymers and ceramics, a viable proposition. Areas of use are discussed.

The effect of latex on macrodefect-free cement

The effect of adding latex to macrodefect-free cement was studied. The porosity of the modified pastes was smaller than the normal macrodefect system but the pore size was not significantly changed. The traditional relationship between strength and flaw size in the cement system was found to be inadequate to explain the observed mechanical behaviour of the latex-modified systems. Scanning electron microscopy of the fracture surfaces of the normal and latex-modified pastes revealed that the use of different polymers resulted in different fracture paths through the cement composite. This was due to the difference in chemical nature of the polymer. The results demonstrated that the modulus (E) and fracture energy (R) terms in the traditional Griffith equation become limiting factors for strength determination in low-porosity cement systems.

Fibre toughening of MDF cement

Fibre reinforced cements are of considerable industrial importance, the production of asbestos cement amounting to about 5 m tonne per annum. The search for fibres other than asbestos has led to the investigation of glass, polypropylene, cellulose (and other fibrous materials) in a cement matrix. Whilst there has been much work on the fibrous component of cement-fibre composites little attention has been paid to the effect on the final composite properties following improvements in the cement matrix. The preparation of macro-defect-free (MDF) cements has recently been reported and the work presented here describes the preparation and the properties of fibre-reinforced MDF cement composites. The Young's modulus of elasticity and the strength of MDF cements are greatly improved and do not need fibrous reinforcement to show benefit. The main purpose of reinforcing MDF cement with fibres is to enhance toughness and impact performance. It is shown that small volume fractions of fibre (∼2%) in the MDF cement matrix produce composites with excellent toughness and impact properties.

A study of ultrafine porosity in hydrated cements using small angle neutron scattering

Small angle neutron scattering makes use of the neutron contrast due to differences in the scattering power between small, particulate regions and the general background medium, and has only very recently been applied to study the porosity in hydrated cement systems. The technique is applied to pore sizes below approximately 30 nm and produces data on pore size distribution, pore volume and pore shape without recourse to drying techniques and the potential structural degradation which may occur. Results indicate a bi-modal pore size distribution at approximately 5 and 10 nm diameter, with a total volume accounting for some several percent of the total cement block. The best estimate of the 5 nm pore shape is considered to be curved-faced tetrahedra. The pores appear to be relatively unaffected by changes in the water-to-cement ratio or accelerating admixture investigated, but macro defect free cement does show significant pore structure alteration.

The Properties and Potential Applications of Macro-Defect-Free Cement

AbstractThe properties of Macro-Defect-Free (MDF) cements are reviewed. Combinations of these properties place MDF cements in a unique position in the materials field, a position which makes direct replacement of current engineering materials such as metals plastics and ceramics a viable proposition. Perhaps of more importance is the invention of devices and components hitherto unavailable because of the property-restricted nature of available materials. Examples are given of unusual uses for cement. These include applications in the fields of cryogenics, ballistic protection, electromagnetic radiation screening and acoustics.