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Several hydrated portland cement systems have been studied at DBR in a wide range of porosities. These systems include room-temperature hydrated paste, autoclaved paste, autoclaved with addition of sulfur and silica, hot-pressed samples and compacts of synthetic 14A tobermorite. Measurements included compressive strength, Young's modulus, product density, porosity and helium inflow, by which the various systems were characterized. It was concluded that, at a given porosity, an optimum proportion of higher density crystalline material and poorly aligned and ill-crystallized material yields the best strength but the quantity of poorly-crystallized material required decreases with porosity.

Compressive strength, modulus of elasticity, and microhardness measurements were made on a variety of autoclaved cement-silica preparations covering a wide range of porosity. A spectrum of linear log mechanical property — porosity functions was observed. Analysis demonstrated that the slopes of these functions were dependent on the preparation silica content and density of the hydrated product. The slopes of the lines for each mechanical property bore a constant ratio with slopes calculated from the corresponding lines of the other two mechanical properties. This resulted in expressions interrelating mechanical properties which were independent of porosity and valid for all preparations studied. Compressive strength and microhardness for preparations studied were shown to be directly related.

A study has been made of the strength of 3CaO.Al[2]O[3] and 3CaO.Al[2]O[3] + gypsum pastes as well as of the influence of the admixture CaCl[2] on these pastes. The methods employed were microhardness indentation and cone penetration. With 16 per cent CaCl[2] added to the 3CaO.Al[ 2]O[3] paste a significant gain in strength was obtained after 3 months, while an 8 per cent CaCl[2] addition to the 3CaO.Al[2]O[3] + gypsum system gave the highest strength at the same age. Morphological changes in the pastes were found to be related to changes in hardness. From a comparison of the two methods it was concluded that cone penetration measurements are useful up to a certain hardness level.

Results of an investigation of the use of high-aspect-ratio mica flakes in hardened portland cement matrices are reported. Increases in flexural strength by a factor of 2 and in fracture toughness by a factor of 4, depending on matrix porosity, are obtained by the addition of small amounts of mica. The dependence of these properties on matrix porosity is determined and the effect of mica flake addition on matrix characteristics discussed, e.g., porosity, surface area, non-evaporable water content.

Cement matrices were prepared by autoclaving portland cement-silica mixtures having various percentages by weight of silica, with water-to-cement ratios of from 0.22 to 0.45. Normal cement paste samples were also made for comparison. Samples were heated to 100°C in vacuum for 24 h before impregnation with methyl methacrylate monomer (MMA), which was then polymerized; impregnation was repeated for complete impregnation. Young's modulus and microhardness were measured as a function of volume fraction of PMMA and results compared with those obtained from similar samples conditioned to 11% RH. Autoclaved impregnated and unimpregnated specimens and room-temperature-cured cement paste specimens at both conditions were exposed to 100% RH and their length changes measured for up to four years. Permeation to water vapour in the impregnated specimens, although decreased, still occurs and impregnation increased long-term expansion on exposure. Expansion on exposure to 100% RH is much larger for D-dried than for 11% RH-conditioned matrices; the latter produced stronger bodies on impregnation.

Fracture toughness measurements made at 11% RH on autoclaved cement/silica mixtures having a wide range of silica contents and porosities yielded a series of curves for K c versus porosity and K c versus silica content. Those for preparations having low porosity and silica content were unique. Apparently α C 2 S-hydrate, unreacted silica and pores having specific size distribution can act as crack arrestors. K c versus hardness curves were similar to those for K c versus porosity. Simple microhardness tests may be useful in predicting fracture toughness of autoclaved cementitious systems.

To determine how the surface modification of mica influences flexural strength and fracture toughness of mica-reinforced cement composites, mica was treated with HF acid and three coupling agents: two silanes (CVBS, γMPS) and a titanate (IDT). Significant increases in mechanical properties resulted, depending on type of treatment, concentration of treating solution, reaction time, and w/c ratio.

Mechanical properties of Ca(OH)[2] are compared with those of Portland cement paste at different porosities. Intrinsic values of modulus of elasticity (E) and microhardness (H) for Ca(OH)[2] compacts were obtained by extrapolating to zero porosity log E and log H versus porosity curves. The values for the Ca(OH)[2] and Portland cement paste systems are of similar magnitude. Flexural strength values for both systems were also determined to be of similar magnitude for the porosity range studied. Fracture terms (K[c], J[c], G[ c], gamma [T]) were determined. K[c] and gamma [T] versus porosity plots for both systems revealed that these fracture terms are of similar magnitude in the porosity range common to both.