Showing papers in "Cement and Concrete Research in 1988"

Relation between resistivity and corrosion rate of reinforcements in carbonated mortar made with several cement types

Abstract: Many years ago Stratfull and colleagues suggested the measurement of concrete resistivity as an indirect indication for active corrosion of the reinforcements. Their results were obtained from potential mapping measurements in chloride contaminated concrete. In the present paper results of corrosion rate and simultaneous electrical resistance values of rebars in mortar fabricated with six different types of cements are given and the relation between those values is presented. The specimens were carbonated in order to promote active corrosion of the rebars. They were held in chambers with different relative humidities. The results of this study indicate a strong relation-ship between the corrosion rate measured on the rebars (by means of Polarization Resistance method) and the electrical resistance of the concrete, which suggests a method of estimating the rebar corrosion rate from the simple measurement of the electrical resistance when rebars are depassivated. The relation between icorr and Rohm obtained is quite similar in all the cases and suggests that the concrete electrical resistivity may act as a “controlling” factor of the corrosion rate.

Pozzolanic and cementitious reactions of fly ash in blended cement pastes

Abstract: 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.

The chemical composition of mortars made from magnesia-phosphate cement

Abstract: Mortars made from magnesia-phosphate cement were observed to set within 15 minutes at 22°C and to harden within 1 hour. The major hydrate formed was struvite, NH4MgPO4·6H2O, usually accompanied by schertelite, (NH42Mg(HPO4)2·4H2O, at least initially. Some hydration products also contained dittmarite, NH4MgPO4·H2O, and/or stercorite, NaNH4HPO4·4H2O, but these were present only as minor constituents.

29Si MAS NMR studies of portland cement components and effects of microsilica on the hydration reaction

Abstract: 29Si MAS NMR is shown to be a valuable tool for quantitative analysis of synthetic and natural cement minerals and for following the hydration of white Portland cement Computer deconvolution of the alite/belite 29Si MAS NMR spectrum for white Portland cement allows the C3S and β-C2S content to be obtained with high accuracy This composition differs significantly from that of a Bogue calculation In the hydration of white Portland cement addition of microsilica has the effect of accelerating the reaction of C3S Microsilica itself is consumed during the hydration, and an increased amount of polymer calcium silicate hydrates is formed Preliminary high-speed 27Al MAS NMR investigations applied to the low aluminate content in white cement show great potential for this technique in studies of cement hydration reactions An approximate value for the 27Al nuclear quadrupole coupling constant in hydrated white cement is reported

The performance of cement paste and concrete subjected to sulphuric acid attack

Abstract: Different cement pastes and concrete mixes were prepared using ordinary Portland cement and subjected to sulphuric acid attack. The main parameters investigated included w/c ratio (and cement content) and age of the cementitious materials. 102 mm cubes were immersed in a channel containing an approximately 2% solution of continously flowing sulphuric acid. The changes in weight with time for each cube were determined continously up to a maximum exposure period of 50 days. The results indicated that the deterioration of the cubes for this high acid concentration decreased with a decrease in the cement content. The effect of age was slightly more significant for cement paste than for concrete cubes.

Internal and external sources of sulfate ions in portland cement mortar: two types of chemical attack

Abstract: This paper compares the effects of chemical attack on portland cement mortar from internal and external sulfate sources. Factors such as cement type, cement content, water to cement ratio, and addition of fly ash are studied with respect to expansion and strength development and it is found that substantial differences exist between the two types of sulfate attack. Expansion values used to evaluate both external and internal attack are discussed and a low bound of maximum expansion corresponding to the beginning of strength loss for internal sulfate attack is proposed. This threshold is a function of cement content rather than a constant.

Measurement of porosity in blended cement pastes

Abstract: Assessment of porosity was made by several methods on plain Portland cement pastes and on those containing silica fume or fly ash. Water/solid ratio and proportion of pozzolan were varied to provide a wide range of porosities and pore-structures. Measurements were made on specimens that had been cured at 20°C for 7 days, 28 days and 3 months, and at 35°C for 3 months. Porosity was assessed by mercury intrusion porosimetry; oven-drying; water, methanol and propanol resaturation; and methanol and propanol exchange. Selected pastes were also analyzed by nitrogen sorption. The different methods give varying views of the pore structures of the materials examined. Conclusions about the pore structure can be made by comparing and contrasting results from the different methods. There are fundamental differences in the nature of the porosity in plain and blended cement pastes. Simpler, less expensive, methods than mercury porosimetry are available which produce useful estimates of porosity.

Palm kernel shell as a lightweight aggregate for concrete

Abstract: These concrete mixes of widely differing water/cement ratios were made using palm kernel shell as course aggregate. The properties tested include the physical properties of the shell, the compressive, flexural and tensile splitting strengths of the concrete. These properties were compared with those of similar concrete specimens made with crushed granite as course aggregate. Results of the tests suggest that palm kernel shell cannot produce concrete with compressive strength above 30 MPa. However, for concrete grade 25 and below, the material was found to compare favourably with other conventional aggregate such as crushed granite.

The rheological behavior of fresh cement pastes

Abstract: The rheology of fresh cement pastes is difficult to obtain and data is often difficult to reproduce. Factors involving mixing, testing techniques, test geometries, etc contribute to the measurement problems. This review addresses these problems and comments on the analysis of experimental data.

Reactions of some calcium silicates with metal cations

Abstract: The reactions of Pb 2+ , Cd 2+ , Mn 2+ , Zn 2+ , Cu 2+ , Mg 2+ , Co 2+ , or Ni 2+ with calcium silicates such as tobermorite, xonotlite and wollastonite were investigated. Among these cations, Pb 2+ , Cd 2+ , Mn 2+ , Zn 2+ and Cu 2+ appear to replace surface Ca 2+ but it is difficult to delineate the extent of this reaction from that of precipitation as hydroxides, hydroxy carbonates or carbonates and double decomposition reaction with calcite which is present as an impurity. Two unsubstituted tobermorites exhibited an Mg 2+ exchange of only 0.4 meq/g which is due to exchange with surface Ca 2+ . Almost all of the Ca 2+ in tobermorites and xonotlite was irreversibly replaced by Co 2+ or Ni 2+ which led to their amorphization. The mechanism of Co 2+ and Ni 2+ replacement of Ca 2+ is by the breakdown of structural CaO bonds and appears to take place from edge to core in these minerals. The replacement of Ca 2+ by Co 2+ and Ni 2+ in these unsubstituted calcium silicate minerals is irreversible and lead to their amorphization and hence their reactions are not strictly analogous to cation exchange in clays and zeolites.

Effect of damage in reinforced concrete on carbonation or chloride penetration

Abstract: The influence of mechanical loading effects on carbonation or chloride penetration is studied. Specimens are reinforced concrete beams of 3 m long which are stored in a mechanical loading state and in aggressive environments. Carbonated cover depths and chloride contents are measured. The results show that damage at grain/paste interfaces in tensile zones leads to an increase of penetration of aggressives ions.

Properties of concrete reinforced with fibrillated polypropylene fibres under impact loading

Abstract: Concrete beams containing fibrillated polypropylene fibres were tested under impact loading. An instrumented drop-weight impact machine was used, dropping a 345 kg mass from a height of 0.5 m. The specimens had dimensions (1×w×d) of 1200×100×125 mm, and were simply supported on a clear span of 960 mm. The 19.1 mm fibres were added in volume concentrations from 0.1% to 0.5%. The addition of the fibrillated polypropylene fibres increased both the fracture energy and the impact strength of the concrete.

Alkali-silica reaction products and their development

Abstract: The work described was undertaken to investigate whether the NBRI accelerated test in fact mimics the natural alkali-silica reaction. The test specimens showed all the characteristics of the alkali-silica reaction and the reaction products proved very similar to those found in structures affected by the reaction in the field. These products can be grouped into (a) exuded and surface gels (amorphous material and a tobermorite gel) and (b) white reaction products generated within the concrete and mortar specimen. The NBRI test, therefore, appears to accelerate the reaction and not to modify the naturally occuring process. This in turn indicates that the test can be used to investigate the mechanism of the alkali-silica reaction. The alkalis Na and K in most of the reaction products that form during the test can be removed by soaking the concrete and mortar specimens in water. In some cases the Na and K are replaced by Ca. This suggests that when sufficient water is available, the reaction products are able to interact with a Ca-bearing phase in the cement matrix. If this phase is Ca(OH) 2 , as expected, then it is very likely that alkali hydroxides are by-products of the interaction, and consequently that the alkali-silica reaction is self-perpetuating, since the alkali hydroxide would be continuously regenerated.

Concrete carbonation as influenced by curing regime

Abstract: The paper discusses the results of accelerated tests carried out to investigate the effects of using different curing regimes on the rate of concrete carbonation. Concrete prisms were made and cured in water and/or air for different periods of time. The prisms were then placed in a chamber filled continuously with carbon dioxide gas. Slices broken off prisms and sprayed with phenolphthalein solution were used to determine the depth of carbonation. The results indicated that the rate of concrete carbonation decreased when the water curing period was increased. The carbonation rate of prisms cured in water for 28 days was only 17% of that for prisms cured in air for the same period of time.

Modification of cement pore fluid compositions by pozzolanic additives

Abstract: The impact of blending agents on the internal environment of cement systems is assessed by chemical analysis of the pore fluid in cured blends The short-term behaviour of PFA blends is complicated by the presence of soluble alkali In general, pore fluid sodium levels are not much altered by PFA while potassium is reduced SiO 2 fume has a more immediate impact on the pore fluid chemistry: 10–20% additions can lead to order-of-magnitude reductions, especially in caesium content The potential of cement to uptake chloride anions is assessed: the correlation with the sum of tricalcium aluminate (C 3 A) and ferrite phases is more significant than with C 3 A alone Blast furnace slags can markedly affect the internal redox potential, which decreased from +100mV in Portland cement to minus 200–225mV in slag-rich blends

The effect of superplasticizer molecular weight on its adsorption on, and dispersion of, cement

Abstract: The effect of molecular weight of a superplasticizer on the adsorption on cement was investigated by microelectrophoresis and UV-absorption techniques. The purpose of the experiments was to investigate any differences in the magnitude of the Zeta Potential (ZP) and in the amount of superplasticizer adsorbed by using sulfonated polystyrenes of different molecular weight, MW = 4,000 g/mole, MW = 16,000 g/mole, MW = 31,000 g/mole, and MW = 70,000 g/mole. The results show that the superplasticizer with the largest molecular weight gives the largest negative ZP, and would therefore be assumed to have a higher dispersing capability. UV-absorption results have shown that the polymer of MW = 16,000 g/mole is the most adsorbed of the four superplasticizers, whereas the polymers of MW = 4,000 g/mole, MW = 31,000 g/mole, and MW = 70,000 g/mole are less adsorbed, with MW = 70,000 g/mole being the least adsorbed. That the polymer which is the least adsorbed gives the highest negative ZP is explained by the diffuse double-layer theory.

Corrosion resistance of steel fibres in concrete under marine exposure

Abstract: This is the final paper of a series (1, 2, 3) which have reported different aspects of a long term study on the marine durability of steel fibre reinforced concrete (sfrc) Two mixes, one with and one without pfa were reinforced with three types of steel fibres The cement content of the mixes was 430 and 590 kg/m3 respectively Prism specimens of these mixes were cured under marine exposure, both in the laboratory and at Aberdeen beach, for up to 2000 wet-dry cycles (1200 days) The state of corrosion of the steel fibres was investigated visually and by electrochemical analysis of fibres exposed at fractured surfaces of specimens after flexural testing The results show that the generally accepted activation level of 04% Cl− by weight of cement does not apply to sfrc Similarly, the threshold value of 061 for the (Cl−)(OH−) ratio, as proposed by Hausemann for initiation of corrosion, is not valid to steel fibre reinforced concrete No corrosic of fibres embedded in concrete was evident at Cl− and (Cl−)(OH−) levels greatly exceeding the above values

Compressive strength of low cement/high fly ash concrete

Abstract: The effect of replacing 35 to 50 percent of cement by fly ash, on the compressive strength of lean concrete mixtures (350 lbs/yd3; 210 kg/m3) was investigated, using two ASTM Class F and two ASTM Class C fly ashes. The test results showed that, particularly in lean concrete, fly ash has significant effect on the properties of fresh and hardened concrete. It was presented in the previous paper(1) that the workability of the fly ash concretes was found to be much better, and the water requirement lower; the rate and volume of the bleeding water were either higher or about the same, and setting was slower, depending on the type and properties of the fly ash and on the mix proportions. This paper evaluates the effect of fly ash on the compressive strength development of the hardened concrete. Test results show that, compared with the Reference Mix which reached the required strength (2000 psi; 14 MPa) at 28 days, the concretes with the above fly ash replacement levels reached the strength at 35 to 170 days, depending on the replacement percentage and the properties of fly ash. From the standpoint of concrete behavior, it was concluded that in lean concrete, the use of a large amount of fly ash, as replacement for both cement and sand, is highly beneficial.

The effect of drying on cement pastes pore structure as determined by mercury porosimetry

Abstract: The effects of three drying procedures, namely oven-drying, sublimation and sublimation followed by oven drying, on the pore structure of cement pastes were studied. Cement pastes were prepared at water-cement ratios of 0.60, 0.40 and 0.20 and tested at 14d. It was observed that oven-drying may affect the cement pore structure by opening pores in the range of 0.02 and 0.1 micrometers.

The hydration of monocalcium aluminate at different temperatures

Abstract: The chemistry of hydration of monocalcium aluminate, CA, has been studied at several temperatures using conduction calorimetry, X-ray diffraction and other techniques. At 4 °C, hydration to the decahydrate CAH 10 occurs about 15 hours after mixing; this hydration time increases with increasing temperature up to 30 °C, and C 2 AH 8 appears as a hydration product. At 40 °C rapid hydration to C 2 AH 8 is followed over a period of weeks by the ‘conversion’ reaction producing C 3 AH 6 . The reaction of CA to form crystalline hydrates was monitored by X-ray diffraction analysis; the results indicate that hydration also produces significant amounts of noncrystalline material. The enthalpies of the reactions involved in hydration and ‘conversion’ were measured by conduction calorimetry.

Hydrochloric acid attack on cement mortar — An analytical study

Abstract: An analytical study of hydrochloric acid attack on cement mortar prisms is done in this work. 4×4×16 cm morter prism specimens (cement sand ratio 1:3) were made. They were put in hydrochloric acid solution. After a certain period they were taken out and were then quite porous on the surface. They were carefully cut. Very distinct layers were visible on the cut surface. The colour of the specimens varied from yellow on the outer surface to steel grey in the middle of the specimens (undamaged). Just after the undamaged part a brown ring was noticed. Different layers were analysed by atomic absorption, SEM and EDAX. The prisms specimens were impregnated with epoxy and thin sections were made which were studied under SEM-EDS and electron microscope. X-ray line analysis were done on these samples and back scattered electron image micrographs were taken. It was noticed that the undamaged mortar was surrounded by a ring of iron. This was rather hard and protected the mortar from further damage. The calcium concentration decreased successively from the middle of the outer surface. The concentration of silica increased in the layers outside the brown ring. After this the amorphous phase (gel) starts. Two gel phases are observed here, an iron rich amorphous phase (brown ring) followed by a lighter ash colour amorphous phase, it is most probably a silica dominating phase. It is also hypothesized that besides dissolution, there is a possibility of calcium chloride aluminate hydrates SiAl, CaAlSi and SiFe complexes formation. This gives rise to the expansion cracks in the specimens. Thus the hydrochloric acid attack is twofold. This study has given a little better understanding of the process of hydrochloric attack on the cement mortar.

Internal chemical evolution of the constitution of blended cements

Abstract: Blends comprising 30% of either fly ash or blast furnace slag and 70% ordinary Portland cement have been aged up to two years at 25°, 40° and 55° in CO2-free conditions. Changes in the mineralogy were determined by selective dissolution, TG, XRD, analytical electron microscopy and Ca(OH)2 determinations on both whole samples and dissolution residues. Portland cements converge on a steady-state but blended cements, especially those with slags, show much evidence of persistent internal inhomogeneity. Higher cure temperatures accelerate reaction but not necessarily in the same direction as at 25°C.

Contribution of PFA to concrete workability and strength development

Abstract: An assessment of the nature of the components of pulverized-fuel ash (pfa) has shown a dominance of hollow, glassy spheres with a composite shell. Total of partial coating by magnetite can be present. The form, size and physical nature of the particles are shown to be the main influences affecting the workability and water demand of fresh pfa concretes. The relationship between progressive pfa particle reaction and strength gain with time has been observed for standard cured concretes up to two years old. The early pfa contribution to strength development is largely a beneficial interaction with the normal hydration of Portland cement while the pozzolanic reaction becomes significant after 28 days. The potential performance of pfa can be predicted from a consideration of its fineness, as measured by 45 μm sieve residue.

Solidification theory for aging creep

Abstract: The paper presents a new theory for the basic creep of concrete which takes aging into account in a manner which is better justified physically, better agrees with test results, and is more efficient computationally. The aging is treated as a consequence of volume growth of the load-bearing solidified matter (hydrated cement) whose properties are nonaging and are described by a Kelvin chain with age-independent moduli and viscosities. The thermodynamic restrictions on the aging creep law are satisfied, and the condition of nondivergence of creep curves is met. There are only four free material parameters, and they can be identified from the given test data by linear regression. Deviations from the principle of superposition are also correctly modeled by the theory. Good agreement with typical test data on creep and creep recovery is demonstrated.

The sulphur cycle in cement kilns: Vapour pressures and solid-phase stability of the sulphate phases

Abstract: The relative vapour pressures of sulphates important to cement clinkering have been measured using the Knudsen effusion cell technique. With this and additional information on the stability of the solid phases — especially Ca5Si2(SO4)O8 — on melting relationships and with the aid of a simplified model of circulation in the kiln, some features of the oxidized sulphur cycle are interpreted.

Freeze-thaw durability and deicer salt scaling resistance of a 0,25 water-cement ratio concrete

Abstract: Specimens of 0,25 water-cement ratio concrete with various air void spacing factors were tested for freezethaw durability in accordance with ASTM Standard C 666 (procedure A) and for deicer salt scaling resistance in accordance with ASTM Standard C 672 All specimens were moist cured 14 days Few specimens showed signs of deterioration after 300 and 800 cycles of freezing and thawing and a critical air void spacing factor of approximately 750 μm was determined All specimens that underwent scaling tests showed a very good resistance to deicer salts and, even after 150 cycles, a maximum of only 06 kg/m 2 of scaled off particles was measured

The hydration of 12CaO.7A12O3 at different temperatures

Abstract: The hydration behavior of C2A7, a component of many calcium aluminate cements, has been studied over a range of temperature using conduction calorimetry, X-ray and thermal analysis techniques. At 4°C, hydration occurs about 2 hours after mixing; hydration is slower at 20°C but at 40°C a very rapid reaction is observed. The initial hydration products are C2AH3 together with CAH10 or alumina gel; considerable amounts of amorphous material are always also formed. At 20°C and above these initial products slowly convert to C3AH6; the conversion becomes faster with increasing temperature. The enthalpies of the hydration and conversion reactions were measured by conduction calorimetry.

Kinetics of hydration of tricalcium aluminate in the presence of gypsum

Abstract: Based on a conceptual model for the stages of hydration of tricalcium aluminate in the presence of gypsum, a mathematical model was developed for the kinetics of hydration. The model was solved, illustrated and compared to experimental heat release data. Analytical equations were presented for the first two stages, while the last stages of hydration reduced to a previously developed model for the hydration of tricalcium aluminate in the absence of gypsum. Results suggested that pore diffusion through a thickening ettringite barrier layer controlled the first stage of hydration, while pore diffusion through a thinning ettringite barrier layer controlled the second stage. After the ettringite layer disappeared, the reaction of the remaining tricalcium aluminate proceeded more rapidly, but by a similar mechanism, than in the absence of sulfate containing species.

Infrared study on the refinement of phosphogypsum for cements

Abstract: Phosphogypsum, the by-product of phosphoric acid manufacture, is considered as a gypsum for the control of the setting time of Portland cement and trass cement. However, it is well known that impurities, usually included in phosphogypsum, injuriously affect the hydration of cement. The object of this paper is to clarify the characters of impurities in phosphogypsum produced in Turkey and to obtain refined phosphogypsum using various methods following IR technique.

Automatic quantification of microcracks network by stereological method of total projections in mortars and concretes

Abstract: The stereological method of total projection was practically not applied yet because of the difficulty to use it manually. Yet, determinist, it is more precise than the oriented secants technique which is statistic. The author shows how it may be applied to the study of microcracking in concrete, providing its computerization.