Portland Cement Systems Research Articles

Studies of composites made by impregnation of porous bodies. 2. Polymethyl methacrylate in portland cement systems

Porous bodies formed by autoclaving portland cement-silica mixtures and by normally curing portland cement were characterized by measuring Young's modulus, microhardness and porosity. These bodies were impregnated with methyl methacrylate and irradiated, the procedure being carried out twice. The bodies were almost completely impregnated. Increases in mechanical properties were greater for microhardness but less for Young's modulus when compared to sulphur impregnation. It was concluded that polymethyl methacrylate forms a stronger bond with the matrices studied than does sulphur.

Impregnation as a technique for measuring zero porosity modulus of porous materials

The validity of the equation E = E 01 exp(-bP) is tested. This relates Young's modulus of a porous body to its porosity; E 01 represents the modulus of the zero porosity material and can be determined by extrapolation. Impregnation of the porous body with sulphur, and use of the Reuss equation ▪ provides an alternate method for determining E 01. Both methods are applied for seven different portland cement systems. The results differ from each other by 5 to 23 per cent.

Studies of composites made by impregnation of porous bodies. 1. Sulphur impregnant in portland cement systems

Porous bodies formed by autoclaving portland cement-silica mixtures and by normally curing portland cement were characterized by measuring Young's modulus, microhardness and porosity. These bodies were impregnated almost completely with molten sulphur. The bodies were characterized again. It was found that the mechanical properties of the composites could be described by a form of Reuss' mixing law. Equations relating the improvement of the mechanical properties of the composite to the properties of the porous body were derived for both Young's modulus and microhardness.

Microstructure and strength of hydrated cement

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 14Å 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.

Strength development in magnesium oxychloride and other cements

Measurements of compressive strength, modulus of elasticity, and microhardness were made on magnesium oxychloride cement systems having a wide range of porosity. Analysis of the plots of log mechanical property versus porosity gave expressions interrelating mechanical properties which were independent of porosity. Comparison of these results with those of the portland cement system show that magnesium oxychloride cement paste has a higher value of modulus of elasticity and microhardness over the porosity range studied. Microhardness of magnesium oxychloride cement paste is also greater than that for all gypsum plaster preparations in the porosity range studied. Mechanical behaviour of magnesium oxychloride cement compacts is improved when heated in water at 85°C. It appears that this heat treatment results in formation of a porous body consisting mainly of Mg(OH) 2. Among the systems magnesium oxychloride, portland cement, gypsum and magnesium hydroxide, it appears that magnesium hydroxide forms the strongest bodies.

Helium flow characteristics of rewetted specimens of dried hydrated portland cement paste

In an effort to determine how and when water re-enters interlayer spaces the helium flow technique is applied to the dried hydrated portland cement system during re-exposure to water vapour. Two sets of samples at four water-cement ratios were exposed consecutively, to 11, 32, 42, 66, 84 and 100 per cent RH. After each exposure they were reconditioned to 11 per cent RH before measurements were made. Measurements were also taken following second drying. Results show that water re-enters the interlayer structure after exposure to the lowest humidity. Measurements following second drying show that rewetting regenerates the process that occurred on first drying.

Griffith Fracture Criterion and Concrete

The original Griffith fracture criteria was developed to describe the rapid extension of a crack in a homogeneous elastic body. To check the applicability of the Griffith theory to portland cement systems which are neither elastic nor homogeneous, specimens of hardened cement paste, mortar and concrete made with normal and lightweight aggregates, and with notches of varying lengths were tested in flexure and in tension. While the paste specimens were notch-sensitive, mortar and concrete strengths were independent of notch length. The notch-insensitivity of mortar and concrete appears to be due to their composite nature. Similar behavior occurs in glass-Al²O³, fiberglass-reinforced epoxy and tungsten-reinforced copper composites. It appears that for mortar and concrete the direct application of Griffith criteria is not valid up to a certain length of cracks. This critical length depends on the size, volume, and type of aggregate.

Gas works engineering

One approach to decarbonising the cement and construction industry is to replace Portland cement systems with lower carbon alternatives that have suitable properties. We show that seven cementitious binders comprised of metakaolin, silica fume and nano-silica have improved thermal performance compared with Portland cement and we calculate the full CO2 emissions associated with manufacture and transport of each binder for the first time. Due to their high porosity, the thermal conductivity of these novel cements is 58–90% lower than Portland cement, and we show that a thin layer (20 mm), up to 80% thinner than standard insulating materials, is enough to bring energy emissions in domestic construction into line with the UK 2013 Building Regulations. Carbon emissions in domestic construction can be reduced by c. 20–50% and these cementitious binders are able to be recycled, unlike traditional insulation materials.