R.F. Feldman

Hydration reactions in portland cement-silica fume blends

Early hydration reactions of portland cement-silica fume blends were followed by conduction calorimetry, Ca(OH)2 estimation, and later reactions by Ca(OH)2 and non-evaporable water contents. Silica fume accelerated both C3S and C3A hydration in the first few hours. At longer periods pastes of blends hydrated to a greater extent than mortars, possibly because sand acted as a Ca(OH)2 sink in the mortars.

Interaction of chloride and CSH

Abstract The chloride binding properties of synthetic CSH preparations having a wide range of C/S and H/S ratios are examined. Bound chloride is separated into two types, alcohol-insoluble and tightly held. Dependence of so-called chemisorbed chloride on C S ratio, C S ratio and surface area is observed. A mechanism for chloride interaction compatible with the Feldman and Taylor models for CSH is suggested. Implications with respect to corrosion of steel in concrete are also discussed.

Influence of silica fume on the microstructural development in cement mortars

Abstract Cement mortars containing 0, 10 and 30 percent silica fume were prepared at water/cement + silica fume ratios of 0.45 and 0.60. Compressive strength, Ca(OH) 2 and non-evaporable water contents and pore-size distribution were monitored up to 180 days. Silica fume reacts with most of the Ca(OH) 2 formed during hydration within 28 days and improves the compressive strength of the mortar. In addition it affects the pore-size distribution of mortars by reacting with Ca(OH) 2 formed around the sand grains and also with that dispersed throughout the cement paste.

Pretreatment of hardened hydrated cement pastes for mercury intrusion measurements

Abstract Porosity is one of the major factors controlling durability and strength of hydrated cement products. A measure of pore size distribution of these materials is more definitive and can lead to a basic understanding of many phenomena occuring within the material. An accurate measurement of this is, however, difficult to obtain. Hg intrusion porosimetry to 414 MPa was used in this work to measure the pore size distribution of cement pastes prepared at water/cement ratio of 0.8, 0.6 and 0.45. Specimens were predried before intrusion measurements by several techniques including solvent replacement with methanol or isopropanol, evacuation and/or heating for various periods and conditioning at 11% RH. Second Hg intrusions were also performed to investigate the effects of first intrusion. It was concluded that it is not possible to obtain an actual pore size distribution of cement paste by Hg intrusion because of its sensitivity to stress.

Microstructure and strength of hydrated cement

Abstract 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, Youngs 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.

Properties of portland cement-silica fume pastes I. Porosity and surface properties

Abstract Properties such as porosity, pore-size distribution, surface area and drying shrinkage are determined for silica fume-portland cement blends (0–30% silica fume) cured from 1 to 180 days at w/ (c+sf) of 0.25 and 0.45. Porosity was measured by helium, mercury and water techniques. It is shown that in paste blends a discontinuous pore structure is formed after as little as seven days of curing due to reaction of Ca(OH) 2 with silica fume. The pores are not so large as those in mortars owing to the lack of interface effects. Total porosities of the blends can only be measured by a water technique, whereas for plain cement pastes other techniques are applicable.

MECHANISM OF CREEP OF HYDRATED PORTLAND CEMENT PASTE

Abstract Understanding of the mechanism of creep requires direct measurement of the physical changes that occur in the micro-units of the solid and in the solid body itself as a result of change in exposure conditions. Results of rigidly controlled water sorption experiments are presented, giving measurements of change in length of specimen, weight, solid volume, and helium flow characteristics as a function of change in conditions of exposure. These are correlated with creep results in the literature. It is concluded that creep is a manifestation of the gradual crystallization or aging process of the layered material, resulting in further layering. Water movement, although occurring, is not a major mechanism. Other processes such as slippage and micro-cracking are also present.

Properties of portland cement-silica fume pastes II. Mechanical properties

Compressive strength, Youngs modulus, and microhardness are measured as a function of porosity for pastes of silica fume-cement blends made with silica fume contents of 0, 10 and 30 per cent at w/(c+sf) ratios of 0.25 and 0.45. Porosities are lower for pastes without silica fume addition, but strengths are greater at w/ (c+sf) of 0.25 at 180 days curing. At a w/(c+sf) of 0.45, however, strengths are marginally greater with large additions of silica fume. Modulus of elasticity is greater without addition. It is shown that silica fume addition at a w/(c+sf) of 0.45 results in a relatively low Eo but high So. This is due to a combination of low-density dispersed hydrate of low CaO/SiO2 and low Ca(OH)2 content, leading to a relatively homogeneous composite. Correlations between log mechanical property and non-evaporable water content are good for all specimens, with and without silica fume.

A study of mechanical properties of autoclaved calcium silicate systems

Abstract 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.

THE FLOW OF HELIUM INTO THE INTERLAYER SPACES OF HYDRATED PORTLAND CEMENT PASTE

Abstract A helium flow technique has been worked out whereby the changes occurring to hydrated portland cement as a result of the removal of interlayer water can be measured. Results were obtained at water-cement ratios of 0.4, 0.6, 0.8 and 1.0 “Solid volume” measurements were also made. It was concluded that the spaces into which helium flowed were interlayer spaces and not fixed-dimension, narrow-necked pores. This work supported the mechanism discussed in the new model of hydrated portland cement on the sequence of water removal from the interlayer spaces and subsequent collapse of the spaces. An estimate of the density of the first 5.35% of the water removed from the 11% R.H. condition was made; this was 1.27 ± 0.08 gm/cc.