Non-evaporable Water Research Articles

Residual mechanical properties of heated concrete incorporating different pozzolanic materials

This paper aims at showing the effect of high temperatures on mechanical properties of concretes in which Portland cement (PC) has been partially replaced by pozzolanic materials. Three types of pozzolanic material, one natural pozzolana and two lignite fly ashes (one of low and one of high lime content) were used for the replacement. Concrete specimens were tested at four temperature levels of 200, 400, 600 and 800°C without any imposed load. The specimens for each of the chosen temperatures were heated under the same heating regime, so a comparison of their behaviour during heat exposure concerning their mechanical properties was possible. Tests of compressive strength, splitting tensile strength and modulus of elasticity were carried out on specimens cooled slowly to room temperature 1 day, 7 days, 1 month and 3 months after heating. Based on the results of this experimental work it can be said that concretes with pozzolanic materials added to the mixer in partial replacement of PC are more sensitive to exposure to fire than conventional concretes. A relatively greater drop in the strength of the concretes with pozzolanic materials was found especially in the temperature area of 200°C when they were compared with conventional concrete made from PC of Greek type (10% insoluble residue). This particular sensitivity was attributed to heat deformations of hardened pastes, as all the other testing parameters concerning consistency and rate of heating remained the same for all specimens. It is suggested that this is caused by the escape of the non-evaporable water that exists as combined water in hardened pastes of pozzolanic binding agents. These pastes are richer in compounds such as calcium aluminates, calcium aluminate sulfates and tobermorite gel (C-S-H) which are decomposed between 110 and 150°C.

Monitoring Portland cement hydration: Comparison of methods

Various experimental methods of monitoring Portland cement hydration are compared. The methods are quantitative X-ray diffraction analysis (QXRD), measurement of non-evaporable water as loss on ignition and by thermogravimetry, conduction calorimetry and measurement of chemical shrinkage. Correlations between these different measures of hydration are obtained using samples of paste for three different Portland cements. The correlation between non-evaporable water and QXRD data was dependent on the chemical composition of the cement and shows that less water was chemically bound at early ages, for a given amount of cement reacted. A near-linear relationship was found between chemical shrinkage and QXRD data. The correlation between heat of hydration and QXRD data was approximately linear and not greatly affected by cement type.

Preparation and characterization of high and low CaO/SiO2 ratio “pure” C–S–H for chemically bonded ceramics

A type of solidified “pure” C–S–H (calcium silicate hydrate) has been prepared by hydrating mixtures of synthesized Ca3SiO5(C3S) and silica fume at very low water/solid (w/s) ratios (< 0.15). The mechanical properties (splitting tensile strength), stoichiometry, and microstructure of hydration products have been investigated. Results are reported here in which 12.03%−41.56% (by weight) silica fume was added to C3S. It has been found that the strength of the materials with CaO/SiO2(C/S) molar ratio of 1.5 is comparatively higher than that of the others, after curing from 1 day to 28 days. The homogeneity and rheology are of direct importance to the formation of the calcium silicate bonding, which influences greatly the mechanical properties. The hydrates are primarily C–S–H with only small amounts of Ca(OH)2. The compositions of the solidified C–S–H are (1.33–1.37) CaO·SiO2. (1.02–1.21) H2O and (1.40–1.19)CaO·SiO2.(0.68–0.91)H2O for mixtures of C/S = 1.5 and 0.83, respectively. The characterization of the chemically bonded materials by Wn (nonevaporable water), density, x-ray diffraction, and scanning electron microscopy (SEM) studies has been carried out. The relationships between these characteristics and the strength are discussed.

Pozzolanic and cementitious reactions of fly ash in blended cement pastes

Results of thermogravimetric tests on hardened cement pastes containing fly ash are reported. Ashes derived from subbituminous and lignite coal were used at replacement levels of 30% and 50% by weight. Thermal analysis was performed on samples which were water cured for various ages from 3 days to one year. An analytical technique was used to split the non-evaporable water content of cement pastes into two components: water held by calcium hydroxide, and water held in other reaction products. The technique is used to identify and monitor the progress of reaction of the different types of fly ash. There are distinct differences in the manner in which the various ash/cement pastes gydrate. The subbituminous ash relies more upon the “pozzolanic reaction” between calcium silicate and calcium hydroxide to provide the strength-giving hydration product. Conversely, the lignite ash produces a substantial amount of hydrate by direct reaction between compounds of the ash and water. At early and intermediate ages the hydrate produced from the reaction of fly ash may combine substantially more water per unit weight than the hydrates normally produced through the reaction of cement.

Evidence for Minimal Pozzolanic Reaction in a Fly Ash Cement during the Period of Major Strength Development

AbstractStrengths developed in fly ash concretes usually equal or exceed that of similar plain concrete after a few months, with much of the response usually attributed to “pozzolanic” reaction between ash and secondary calcium hydroxide (CH). The CH contents of pastes made with five different fly ashes were determined by DTA for periods up to six months. The CH contents found did not decrease notably over the period, and were substantially identical to that expected for plain cement pastes diluted with the same amount of inert material as the amount of fly ash used. Scanning electron microscope examination of the pastes showed only minimal evidence of reaction even up to 1 year of age, although many fly ash grains were in intimate contact with CH. Non-evaporable water contents of the fly ash pastes were substantially higher than expected at each age, suggesting that the fly ash promoted more complete cement hydration or that the hydration products formed bound substantially greater amounts of water than plain cement paste ordinarily does.

Evidence for Minimal Pozzolanic Reaction in a Fly Ash Cement During the Period of Major Strength Development

ABSTRACTStrengths developed in fly ash concretes usually equal or exceed that of similar plain concrete after a few months, with much of the response usually attributed to “pozzolanic” reaction between ash and secondary calcium hydroxide (CH). The CH contents of pastes made with five different fly ashes were determined by DTA for periods up to six months. The CH contents found did not decrease notably over the period, and were substantially identical to that expected for plain cement pastes diluted with the same amount of inert material as the amount of fly ash used. Scanning electron microscope examination of the pastes showed only minimal evidence of reaction even up to 1 year of age, although many fly ash grains were in intimate contact with CH. Non-evaporable water contents of the fly ash pastes were substantially higher than expected at each age, suggesting that the fly ash promoted more complete cement hydration or that the hydration products formed bound substantially greater amounts of water than plain cement paste ordinarily does.

Lattice defects and the effect of Melment on the hydration of alite

The hydration of alite has been studied in the presence of different concentrations of the superplasticizer Melment by using various experimental techniques. Heat of hydration and non-evaporable water content determinations show that Melment retards the hydration; pH measurements indicate that in presence of Melment, Ca2+ ion dissolution is reduced. Zeta potential measurements give definite proof that Melment molecules are adsorbed at alite surfaces and the retardation of hydration may be due to adsorption. Thermoelectric power measurements prove that the material is an n-type semiconductor, whereas electrical conductivity measurements of solid pellets of C3S (C = CaO, S = SiO2) show that the material is an intrinsic semiconductor above 746 K and an extrinsic semiconductor below 746 K. Extrinsic semiconductivity may be due to the presence of defects or impurities in the crystal lattice. The results also show that oxide ion vacancies are created in the crystal lattice and the reactivity of C3S is related to the defects. Probably Melment molecules are adsorbed at the sites of defects and oxide ion vacancies and hence retard the hydration.

Bond strength development between latex-modified cement paste and steel

Bond strength development between latex-modified cement paste and steel was determined using a new technique, the overhanging beam method. Six latexes were studied. In general bond strength is improved with latex addition. Factors affecting bond strength include non-evaporable water content, heat of hydration, and CH content. The role of the interface zone on bond strength has been evaluated.

Hydration versus strength in a portland cement developed from domestic mineral wastes — a comparative study

Early age hydration studies on fresh pastes of a Type I cement produced from local mineral tailings have been conducted by using thermal techniques such as DTA and TGA and degrees of hydration at varying time intervals have been estimated in terms of nonevaporable water and free calcium hydroxide incorporated in the hydration products. The results are subjected to an earlier proposed expression which gives a direct correlation between strength and degree of hydration for a given hydrating cement paste except for the period of low reactivity, commonly referred to as the dormant period exhibiting very low heat evolution in a calorimetric plot, when cement remains plastic while still undergoing a certain degree of hydration. It is noticed that the critical degree of hydration at which strength starts developing is a function of hydration time and theoretically corresponds to the termination of the acceleration period when tricalcium silicates and dicalcium silicates in the cement have hydrated to produce enough hydration products to intermesh and initiate early strength. The experimental data are compared with those of a commercially available Type I cement and are interpreted in terms of strength development as a function of hydration.

Investigation of relations between porosity, pore structure, and C1 − diffusion of fly ash and blended cement pastes

By-product fly ash from coal combustion influences the properties of cement pastes significantly. A study has been carried out to contribute to understanding the behavior and function of fly ash and other substituents in cement pastes. Comparisons are made with the properties of pastes of as-received portland cement and preliminary comparisons made with granulated blast furnace slag cement and silica fume-cement. Relations between the median pore size, porosity and Cl − diffusion of fly ash-cement pastes, and the implications of observations for diffusion mechanisms and the probable pore structure of the blended cement pastes are discussed. Measurements include non-evaporable water (Wn), porosity and pore structure by mercury porosimetry, Cl − transport by a rapid method under the influence of an applied electric field, and the permeability to water. X-ray diffraction phase characterization has been made before and after Cl − diffusion.

Effect of mica addition of the microstructure of portland cement paste

In assessing the effect of mica reinforcement on cement paste microstructure, modifications to pore-size distribution, porosity, non-evaporable water content, and CH particle size distribution were observed for samples hydrated for up to five years. Microstructural changes were dependent on water-cement ratio and amount of mica added.

Restricted Hydration of Mass-Cured Concrete Containing Fly Ash

ABSTRACTCement paste and concrete specimens containing three different mixtures of portland cement (PC) and high-lime fly ash (FA) were subjected to various curing conditions, and the strengths, non-evaporable water, and porosities compared with control mixes containing portland cement only. Strength and porosity data indicated that the cementing action of the mixtures was in all cases inferior to portland cement. For each mixture the strength loss resulting from imperfect curing, i.e. sealed or exposed to 50% relative humidity, was greater than for portland cement. In the case of concrete drying from one surface, strength differences were found, but these were not as clearly defined as those obtained in the paste specimens.

Restricted Hydration of Mass-Cured Concrete Containing Fly Ash

ABSTRACTCement paste and concrete specimens containing three different mixtures of portland cement (PC) and high-lime fly ash (FA) were subjected to various curing conditions, and the strengths, non-evaporable water, and porosities were compared with control mixes containing portland cement only. Strength and porosity data indicated that the cementing action of the mixtures was, in all cases, inferior to portland cement. For each mixture the strength loss resulting from imperfect curing, i.e. sealed or exposed to 50% relative humidity, was greater than for portland cement. In the case of concrete drying from one surface, strength differences were found, but these were not as clearly defined as those obtained in the paste specimens.

Bound Water in Cement Pastes and its Significance for Pore Solution Compositions

ABSTRACTThe problem of defining bound water in a cement paste is discussed; a reasonable definition is one that includes interlayer water in C-S-H and AFm phases, structural water in ettringite, and adsorbed water, but not water in micropores or in larger pores. On this basis, structural considerations indicate a value of around 32% on the ignited weight for a fully hydrated paste. ‘Non-evaporable’ water, typically around 22% on the ignited weight at full hydration, cannot be identified with bound water, because dehydration to the state in which only non-evaporable water remains causes major loss of interlayer water and destruction of ettringite. In the interpretation of pore solution data, the definition of bound water, and the value assumed for this quantity, are important, because the ionic concentrations in the pore solution are greatly affected by the volume of free water available to dissolve them. If cement is partially replaced by low calcium fly ash, the quantity of bound water at any given age is substantially reduced. This effect contributes to the relatively low concentrations of alkali metal and hydroxyl ions that are observed in the pore solutions of many portland-fly ash cement pastes.

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

Compressive strength, Young's 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 E o but high S o. This is due to a combination of low-density dispersed hydrate of low CaO/SiO 2 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.

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 C 3S and C 3A 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.

Resistance of mortars containing silica fume to attack by a solution containing chlorides

Mortars prepared at water/cement + silica fume ratios, w/ (c+sf), of 0.45 and 0.60 were exposed to a solution containing a mixture of Mg-, Ca- and Na- chlorides to investigate their resistance to attack. Mixes contained 0, 10 and 30% silica fume, and sand-binder ratio was 2.25. Stiffness, pore-size distribution, Ca(OH) 2 content and non-evaporable water were measured before and after exposure to salt solution. Inclusion of silica fume resulted in increased durability of mortar, especially in specimens prepared at w/ (c+sf) of 0.45 and cured for 28 days. Specimens prepared at the same w/ (c+sf) with 30% silica fume were also durable even after seven days of curing. Calcium hydroxide in mortars was reduced to zero by exposure to the salt solution.

Influence of silica fume on the microstructural development in cement mortars

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.

Changes in the structure of hardened cement paste due to high temperature

The investigation performed was aimed at explaining explaining experimentally the changes occurring in the microstructure and phase composition of cement pastes during their exposure to heat in the temperature range 20–800°C. The investigation was performed by means of the following methods; thermal analysis, X-ray diffraction analysis, infrared spectroscopy analysis and mercury porosimetry. During the investigation observations were made regarding the behaviour of the following phases in pastes exposed to heat: Ca(OH)2, CaCO3, C−S−H, non-evaporable water and micropores. From an analysis of the experimental results, temperature ranges of the following changes were determined in the structure of the investigated paste: additional hydration of unhydrared cement grains, recrystallization and carbonization of Ca(OH)2, deformation and transformation of C−S−H phases, non-linear changes in the distribution of pore diameters and total porosity.

Effect of calcium formate on the hydration of tricalcium silicate

The effect of 0, 0.5, 1.0, 2.0, 4.0 and 6.0% of calcium formate on the hydration of C 3S has been studied. Free lime determinations, non-evaporable water content, pH of the liquid phase, zeta potential, thermal analysis and infrared spectral studies have been made for understanding the mechanism of action of calcium formate. Results indicate that calcium formate acts as an accelerator up to 2%. Above this concentration, the excess of it has practically no effect.