Compressive strength versus residual porosity in polymer impregnated cement pastes

Impregnated cement pastes were prepared from fine cement of Blaine area 600 m²/kg using various initial W/C ratios in the range 0.25–0.70, cured for various durations, and by using styrene or methyl methacrylate, which were polymerized $\text{in situ}$ by thermal treatment. The polymer effect on compressive strength was found to be variable and depends on pore size. Upon impregnation, low porosity samples show measurable reduction in zero strength porosity, whereas high porosity samples show measurable reduction in the strength at zero porosity. Enhancement of compressive strength takes place at an optimum range of porosity and gel/space ratio.     

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 III. Degree of hydration. Expansion of paste. Total porosity

The degree of hydration, the expansion during hydration, and the total porosity of low-porosity portland cement pastes were investigated at hydration times ranging from 1 hour to 180 days. The effects of the type of cement (Type I and II), the grinding aid, the surface of the cement, the water-cement ratio (0.2 and 0.3), and the temperature of hydration (5°, 25° and 50°C) were determined.     

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.     

Hardened Portland blast-furnace slag cement pastes II. The corrosion behavior of steel reinforcement

The corrosion behavior of reinforcing steel embedded in various slag cement pastes was studied using the galvanostatic polarization technique. The corrosion resistance is appreciably affected by the degree of fineness of the dry slag cement. In pastes produced from high Blaine area cement, the behavior of embedded steel was very close to that in normal or type I portland cement paste, and is much better than a low Blaine area cement. W/C ratios of 0.25 and 0.40 provided a better passivating medium as compared with W/C ratios of 0.18 and 0.70. Effects of lime or gypsum addition were also investigated and comparatively studied for their action on the corrosion of embedded steel. The results obtained were supported by corrosion rates obtained using the linear polarization technique.     

Microcrystalline calcium hydroxide in Portland cement pastes of low water/cement ratio

A microcrystalline form of calcium hydroxide has been observed in Portland cement pastes of low water/cement ratio by transmission electron microscopy of sections thinned from bulk, hardened pastes. The typical morphology is of clusters of microcrystals in the form of lamellae 10nm thick, parallel to the basal plane. The lamellae tend to be aligned although the perfection of the alignment is variable. The clusters are embedded in the hydrate gel.     

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 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.     

Consistency,setting, and strength gain characteristics of a “low porosity” Portland cement paste

The mixing characteristics, consistency, setting time, and strength gain characteristics of a family of “low porosity” Portland cement pastes are investigated. These pastes are prepared at w:c ratios of 0.22 to 0.24 from a moderately finely ground ASTM Type I clinker not interground with gypsum but regulated with an admixture consisting of a sodium form of sulfonated lignin and sodium bicarbonate. Freely-flowing pastes of complex rheological character are produced which are self-compacting, which set after a reasonable period but more suddenly than conventional Portland cement pastes, and which rapidly develop high strengths (9,500 psi by 1 day, 17,000 psi by 7 days).     

Hardened Portland cement pastes,modelisation of the micro-structure and evolution laws of mechanical properties I- Basic results

This paper is the first of a series of three. For Portland cement pastes, it presents a model of the micro-structure and the associated compressive strength law which will be the base of the two following papers.Model and strength law involve that the most important parameter influencing the strength is the capillary porosity. They allow to foresee the characteristic properties of the pastes of the first and of the second group. At last they give informations about the structure of the gel.     

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. II. Effect of pore structure and of degree of hydration

Cement pastes made with different water-cement ratios were hydrated at different temperatures for different times. The main factor influencing the strength properties of the obtained samples was found to be their porosity, however pores with radii of less than 10 nm affected the resultant strength only negligibly. At equal porosites the strength increased with increasing amount of hydrate phases present. Specific surface area and no significant effect on strength.     

Hydration of low porosity slag-lime pastes

Slag-lime pastes of low porosity (water/solid ratio of 0.20) were hydrated from 6 hours to 180 days at 20°C. The kinetics and mechanisms of the hydration process were studied from the results obtained in this investigation. The depth of the hydrated layer on the slag particles is found to be thin indicating that the hydration reaction is very slow. The molar compositions of the formed hydrates could also be calculated from the free lime, nonevaporable water and uncombined slag contents. A high lime product (molar C/S+A ratio of 2.5–2.6) is formed during the early stage of the hydration process, then the molar C/S+A ratio drops to a value of 1.5 and finally rises to a value of 1.7 at 180 days. The surface areas and pore volumes of hydrates were determined from water and nitrogen adsorption measurements. For water vapor adsorption, the water molecules in the adsorbed phase seem to be highly oriented in an ordered array. This effect might be associated with the polar character of water molecule, when adsorbed on an ionic surface like high lime hydrate. The results of x-ray diffraction and SEM observations indicate only the formation of ill-cyrstallised hydration products.     

Characterization of the hydration state of Portland cement pastes

In a hardened Portland cement paste an important part of aluminate hydrates (mainly the sulfoaluminates) have physico-chemical properties and formation kinetics very much different from those of the remainder of the paste. Consequently a unique and global hydration degree cannot satisfactorily characterize the hydration state. The error induced by the reference to different global hydration degrees is illustrated on a series of figures. The importance, often neglected, of the dehydration o of some hydrates when extracting evaporable water is pointed out as well. The hydration state must be characterized by at least two partial hydration degrees me and ms. For moist cured pastes older than a few days, me = 1 and a formula is proposed to calculate ms from the measured non evaporable water.     

Optimization of strength in cement pastes

It has been demonstrated that porosity is by far the dominant controlling factor limiting strength of hydrated cement paste. Mechanical means have been employed in the present study to minimize this porosity, “hot pressing” under rather modest temperatures and pressures, producing materials having very low porosity and unusually high strength. A new relationship to describe the interrelation of strength and porosity is given, and the effect of maturity of specimens, composition and microstructure are illustrated. Though theoretical density has not yet been achieved, the cement pastes have compressive strengths (as well as tensile and shear strengths) an order of magnitude higher than in normally hydrated cements, are stable and durable, and have dense interpenetrating microstructures.     

高强低钙硅酸盐水泥制备关键技术研究*

基于节能减排和应对气候变化的新要求,研究开发低钙水泥有多方面的重要意义。试验分别采用化学试剂和工业原料,运用化学分析、X-射线衍射、岩相分析等测试手段,初步探讨了高强低钙硅酸盐水泥制备关键技术。结果表明:工业原料中的微量元素能够解决高强低钙硅酸盐水泥的粉化问题;在试验设计的矿物组成和煅烧制度下,w(C2S)设计值在40%~45%,煅烧温度1400~1450℃时熟料性能最优。     

High strength generation in cement pastes

Unusually high strengths have been generated in materials produced by employing “hot-pressing” techniques, and intermediate ranges of strengths have been achieved by applying high pressures at room temperature, to portland cement pastes. By pressing at ca. 250°C and 50, 000 psi strengths are as high as 95, 000 psi (compressive), and 9250 psi (indirect tensile). The hot-pressed materials are volume stable when immersed in water and subsequently evacuated. The microstructures of such materials are very compact, consisting of an intergrowth of dense hydrated cement “gel” surrounding residual unhydrated cement grain cores. The lowest porosity of the materials measured was approximately 1.8%, by far the closest approach to zero porosity or theoretical density yet achieved in cement pastes. The effect of microstructure and porosity are discussed, and high pressure techniques are compared with other methods of strength generation.     

Effects of aluminate and sulfate contents on the hydration and strength of Portland cement pastes and mortars

Evaluation of effects of C₃A and SO₃ contents in Portland cement on its compressive strength, hydration rate and products. Three cement samples varying in C₃A content and one varying in SO₃ content were used, hydrated initially at three temperatures. Compressive strength, bound water content, free lime content and Differential Scanning Calorimeter curves were determined during the progression of hydration. In interpretation bound water was used as a measure of quantity of binding material and Free Lime to Bound Water Ratio (FLWR) - of chemical constitution and quality.     

Physical properties of high strength cement pastes

Recent developments in methods of processing ordinary Portland cement have shown that above average strengths (in flexure) are easily obtained without expending energy on high pressure compaction techniques. This paper reports strengths and other physical properties of both ordinary pastes and modified (macro defect free) pastes and attempts to compare and contrast the two types. The apparently greater notch sensitivity of the modified pastes is explained in terms of a reduced inherent flaw size. Optical microscopy shows that large pores are absent in the modified paste but transmission electron microscopy (TEM) shows that the fine scale microstructures of ordinary pastes and high strength pastes are very similar.