Flexural strength and porosity of cements

A curious feature of hydraulic cements, such as those based on calcium silicate, calcium aluminate and calcium sulphate, is that they exhibit similarly low flexural strengths, typically between 3 and 10 MPa, despite their differing chemical composition, varying degrees of hydration and contrasting setting mechanisms^1-3. Because of these low strength values, unreinforced cements are never used in flexure or tension, and studies of cement strength are usually confined to compression. Those few studies which have considered flexural or tensile failure have concluded that hydraulic cements have an intrinsic maximum tensile strength of about 20 MPa^4,5. Here we demonstrate that the commonly observed flexural weakness of cement is due to the presence of large voids which are largely undetected by conventional methods of pore analysis such as gas adsorption and mercury porosimetry. The removal of such macro-defects results in flex strengths up to 70 MPa, despite the large volume of gel pores remaining in the material. These strength figures, comparable with those of sintered ceramics, have been achieved without the use of elevated pressures or temperatures, and without fibrous reinforcement.