Hardened portland cement pastes of low porosity VI. Mechanism of the hydration process

The hydration of low-porosity portland cement pastes may be divided into three stages. The first stage starts with a fast hydration until 10 to 15% of the cement is hydrated (pre-dormant period), which is followed by a very slow hydration, caused by the formation of a coating on the cement grains (dormant period). After 15 to 20% of the cement is hydrated, the coating is ruptured, and a fast reaction starts, which lasts until about 30% of the cement is hydrated. This is the second stage of the reaction. In the third stage, the hydration slows down, due to retardation by the accumulating hydration products. The mechanism of the third stage is treated quantitatively. The diffusion through the very narrow pores between the hydration products is activated diffusion, and the apparent energy of activation of the diffusion is calculated.     

Hardened portland cement pastes of low porosity IV. Surface area and pore structure

This paper deals with the total surface areas, total pore volumes, and the distribution of pore surface and pore volume in pores of different sizes of 25 low-porosity pastes. A Type II clinker was ground with two grinding aids: diethyl carbonate and Reax 70. The water-cement ratios were 0.2 and 0.3. The hydration temperatures were 5°, 25°, and 50°C. Nitrogen adsorption isotherms at -196°C and water vapor adsorption isotherms at 25°C were used for the surface area and pore structure determinations. The methods used in the analyses of micropores and wider pores were those developed by Brunauer and his coworkers.     

Hardened portland cement pastes of low porosity: VII. Further remarks about early hydration. Composition and surface area of tobermorite gel. Summary

The first part of the paper discusses certain aspects of the earliest stage of the hydration process. Mechanisms are proposed for the actions of calcium hydroxide, potassium carbonate and calcium lignosulfonate. The second part deals with the composition and specific surface area of the calcium silicate hydrate, tobermorite gel, the most important constituent of the cement paste. The method of calculation of these quantities and the assumptions involved in the calculations are presented. The third part gives a summary of the most important results obtained in the investigation of low-porosity cement pastes.     

Hardened Portland cement pastes of low porosity V. Compressive strength

The compressive strengths of low-porosity pastes made from a Type I and a Type II clinker were investigated. Besides the effect of the type of cement, the effects of different grinding aids, surface of the cement, water-cement ratio (0.2 and 0.3), temperature of hydration (5°, 25°, and 50°), and the age of the paste on compressive strength were determined. By and large, the strength results correlated well with degree of hydration and total porosity, but there was definite indication of at least one other factor. The distribution of pore volume in pores different sizes is suggested as a third factor influencing compressive strength.     

Hardened portland blast-furnace slag cement pastes III. Corrosion of steel reinforcement versus pore structure of the paste matrix

The pore structure of the slag cement paste matrix seems to affect to a sensible extent the corrosion behavior of embedded steel. For both additive free, and additive containing slag cement pastes, the pore structure data were discussed in paper I of this series, whereas the corrosion behavior of embedded steel was discussed in paper II. In this paper III, the correlation between papers I and II is established, and the concluding remarks presented.     

Degree of hydration of portland cement pastes with low water/cement ratios as influenced by curing conditions

Measurements of chemically combined water contents were made on Portland cement pastes made with water/cement ratios of 0.20 to 0.30 by weight. In addition to the factors involving water/cement ratio, curing medium, curing history and curing period, the degree of hydration of cement pastes cured at room temperature was significantly influenced by the availability of adequate water supply during the first 24 hours of the curing process.     

Hardened alite pastes of various porosities. I. Degree of hydration,compressive strength and total porosity

The alite used in this investigation was synthesised from the stoichiometric mixture at 1550°C. The hardened alite pastes were made using initial water/alite ratios of 0.20, 0.30, 0.45 and 0.60. Degree of hydration, compressive strength and total porosity were estimated at various hydration time intervals of 0.5 h, 2 h, 6 h, 1 day, 3 days, 7 days and 28 days. A meaningful relation between compressive strength and water/alite ratio was established at constant values of degree of hydration, total porosity and Powers' gel-space ratio.     

Hardened portland cement pastes,modelisation of the micro-structure and evolution laws of mechanical properties III — Elastic modulus

This paper is the last one of a series of three (1)(2). It proves that the difference in the evolution laws of the compressive strength R_C and of the elastic modulus E is just apparent. Indeed, on the one hand the micro-structure model developed regarding R_C evolution allows to account for E evolution, its validity is thus strengthened, on the other hand R_C and E both obey laws of the same type, R_C = R_Oe^?An and E = E_Oe^?A′nWe show that this dual behaviour is due to the fact that E_O and A′ are much more sensitive than R_O and A to the hardening time or the hydration degree. This allows us to forsee the possibility of a selective altering of E and R_C.     

Investigations on the structure of fully hydrated portland cement and tricalcium silicate pastes. II. Total porosity and pore size distribution

On a series of OPC pastes hydrated at 20 °C and 90 °C and Ca₃SiO₅ pastes hydrated at 20 °C the total porosity and pore size distribution were measured. Data obtained by mercury porosimetry and nitrogen adsorption did not agree well as the volumina of pores filled with Hg and N₂ differed. Regardless on the method employed the found pore size distribution depended on the starting water-solid ratio; as equal porosity, it was different in different hydrated materials.     

Low-temperature scanning electron microscope analysis of the portland cement paste early hydration

The use of the frozen hydrated scanning electron microscopy (FHSEM) in the study of cement paste is described. This technique permits analysis of the fractured surface of cement paste in a fully hydrated state with water present as ice in a low temperature scanning electron microscope. At 110 K the paste has a substantial increase in mechanical strength, because water is converted from liquid to a solid state, and this permits the use of bulk specimens at very early hydration. Some preliminary results for 1 hour hydration are presented and future applications of this technique are discussed.     

Chemical shrinkage of portland cement pastes

Chemical shrinkage of normal Portland cement pastes (0.4 ≤ w/c < 0.8) has been measured at 20°C and of pastes with w/c = 0.5 furthermore at 35, and 50°C by means of measuring the volume change of samples of cement paste during the hydration. A small increase in the chemical shrinkage at “infinite time” was found at increasing water-cement ratio. The influence of the temperature was found to be twofold: Increasing temperature caused an increasing rate of the development of chemical shrinkage and a decrease of the chemical shrinkage at “infinite time”.Earlier studies of chemical shrinkage of Portland cement paste are also reported.     

Hardened slag-cement pastes of various porosities I. Compressive strength,degree of hydration and total porosity

Hardened blast-furnace slag-cement pastes were prepared from cements of different Blaine areas, and mixed with various water/cement ratios in the range 0.20–0.70. The pastes were cured for various periods ranging from 1 to 365 days, and the degree of hydration, total porosities, and compressive strengths were determined. It is recommended in this investigation that the compressive strength values be compared at either constant total porosities or constant degree of hydration. The results obtained could indicate that the total porosity plays a more dominant role in affecting the strength than the degree of hydration.     

Investigations on the relationship between porosity,structure and strength of hydrated portland cement pastes I. Effect of porosity

In a series of cement paste specimens made with different water-cement ratios and hydrated for different times the relationship between porosity and strength was determined. For a range of porosities between 5 and 28 per cent this relationship can be best expressed in the form of a linear plot. At equal porosities strengths of specimens obtained by pressing lie distinctly below those obtained by casting.     

Investigations on the relationship between porosity structure and strength of hydrated portland cement pastes III. Effect of clinker composition and gypsum addition

In as series of cements made out of clinkers with variable C₃A/C₄AF ratios and containing different amounts of gypsum, the strength development and the composition of the hydrated material were studied. For a single clinker composition the obtained strength appears to be just a function of porosity. Variations in the C₃A/C₄AF content affected both the structure and intrinsic properties of the formed hydration products and thus the position of the pertinent strength porosity plots.     

Hardened portland blast-furnace slag cement pastes: I. Effects of additives on surface area and on pore structure

Portland blast-furnace slag cement pastes were prepared with various water/cement ratios. Specific surface areas and pore structures of the hardened pastes were investigated by nitrogen adsorption. The “accessibility” of the nitrogen molecules to the pore structure is discussed in terms of degree of hydration and total porosities of the pastes. Effect of presence of CaCl₂, a typical steel reinforcement corrosive agent, was also studied, and results indicated that it alters the area and pore structure extensively, to a more “open structure,” thus facilitating its own accessibility. Lime and gypsum addition was also studied in presence and in absence of CaCl₂, and the effect of the Blaine surface area of the unhydrated cement is particularly emphasized in this investigation.     

Effect of some liquefying agents on properties and hydration of portland cement and tricalcium silicate pastes

The effect of a melamine sulfonate resin, a naphthalene sulfonate resin and a sulfonated lignin on the rheological properties and the hydration of portland cement and tricalcium silicate pastes was studied. In addition to improving the flow properties of the pastes all three substances retarded the hydration of C₃S and altered the stoichiometric composition of the CSH-phase formed. The rate of ettringite formation was altered by the agents differently in two different cements studied.     

Drying shrinkage of portland cement pastes I. Microcracking during drying

The occurrence of microcracking of portland cement pastes during drying has been studied by comparing the effects of specimen thickness on shrinkage and cracking using light microscopy. Increases in specimen thickness tended to impede drying and wetting, but there were only slight changes (less than experimental errors) in total and reversible shrinkage once equilibrium was attained. Although microcracking occurred at the beginning of drying whenever the thin specimen (thickness <2mm) was suddenly exposed to low relative humidity (～50%), the cracks eventually closed up. It was concluded that no matter whether or not this microcracking happened, the shrinkage of the specimen after reaching equilibrium was unrestrained.     

Hardened portland cement pastes,modelisation of the micro-structure and evolution laws of mechanical properties II- compressive strength law

The test results reported here confirm the validity and the generality of the compressive strength law and of the model of the microstructure proposed for hardened Portland cement pastes in the first paper of this series (1).The compressive strength obeys the law $\text{R}{}_{\mathrm{c}}\text{=}\text{R}{}_{\mathrm{o}}\text{e}{}^{\mathrm{<mtext>?</mtext><mtext>An</mtext>}}\text{or}\text{R}{}_{\mathrm{c}}\text{=}\text{R}{}_{\mathrm{o}}{}_{\mathrm{e}}\text{A}\text{e}{}^{\mathrm{AK\Gamma }}$.It depends on the capillary porosity n, linked to the product KΓ and to the hydration degree by a chain of reciprocal relations.The main features of the model so confirmed are : the growth mode of the hydrated grains, the charactéristic patterns of the evolution of the capillary porosity and of the hydration degree for pastes of the first or the second group.     

Air-entraining actions of anionic surfactants in portland cement pastes

The air-entraining actions of a number of anionic surfactants in cement pastes were studied in order to determine whether precipitated calcium surfactants influence the air-entrainment process. Air contents of pastes containing both soluble and precipitated calcium surfactants were higher than those containing only soluble calcium surfactants, thus indicating that precipitated material contributed towards air-entrainment. It was shown further that the precipitated calcium surfactant could re-dissolve appreciably during mixing and supply additional surfactant ions in solution to replace those adsorbed on cement. It was concluded that these additional ions contributed to air-entrainment by further adsorption on cement particles and by augmenting the foam capacity of the mixing water.     

Variation in density of portland cement hydration products

The average density of the Portland cement hydration products was studied on compacts having the initial porosity ranging from 28.4% to 34.1%, and made of cement which contained no gypsum and had the fineness (Blaine) of 2,030, 3,290 and 4,570 sq.cm per g. Results indicated that the average density increased with the increase in the degree of hydration and the decrease in the initial porosity. This was attributed to the increase in the relative amount of the products formed as a result of a diffusion mechanism. It was suggested that hydration due to such mechanism, taking place in a confined space, is associated with pressure build-up and results, therefore, in denser products.