Superplasticized portland cement: production and compressive strength of mortars and concrete

This paper deals with the effect of intergrinding different percentages of a naphthalene-based superplasticizer with Portland cement clinker and gypsum on the fineness of the product, and on the water requirement and the compressive strength of the mortars made with the superplasticized cement. The properties of the fresh and hardened concrete made with the superplasticized cements were also investigated. The results showed that the intergrinding of a given amount of a naphthalene-based superplasticizer with Portland clinker and gypsum reduced the grinding time required for obtaining the same Blaine fineness as that of the control Portland cement without the superplasticizer. The water requirement of the mortars made with the superplasticized cements was similar to that of the mortars made with the control Portland cements when the same amount of the superplasticizer was added at the mortar mixer; for a given grinding time and a Blaine fineness of 4500 cm²/g, the mortars made with the superplasticized cement had higher compressive strength than those made with the control Portland cement. For a given grinding time or Blaine fineness of cement ≥5000 cm²/g, the slump loss, air content stability, bleeding, autogenous temperature rise, setting times, and compressive strength of the concrete made with the superplasticized cements were generally comparable to those of the concrete made with the control Portland cements when the superplasticizer was added at the concrete mixer.     

Effect of large amounts of natural pozzolan addition on properties of blended cements

In this study, the effects of 35, 45, and 55 wt.% natural pozzolan addition on the properties of blended cement pastes and mortars were investigated. Blended cements with 450 m²/kg Blaine fineness were produced from a Turkish volcanic tuff in a laboratory mill by intergrinding portland cement clinker, natural pozzolan, and gypsum. The cements were tested for particle size distribution, setting time, heat of hydration, compressive strength, alkali-silica activity, and sulfate resistance. Cement pastes were tested by TGA for Ca(OH)₂ content and by XRD for the crystalline hydration products. The compressive strength of the mortars made with blended cements containing large amounts of natural pozzolan was lower than that of the portland cement at all tested ages up to 91 days. Blended cements containing large amounts of pozzolan exhibited much less expansion with respect to portland cement in accelerated alkali-silica test and in a 36-week sulfate immersion test.     

Gypsum Dehydration During Comminution and Its Effect on Cement Properties

A mix containing fixed amounts of gypsum and ordinary portland cement clinker was ground in plant and laboratory ball mills to examine the effect of grinding in different mills on the cement properties. Mineralogical examination showed that in the plant mill (1200 cm×260 cm), because of the rising temperature during grinding, gypsum was dehydrated to hemihydrate, while there was no dehydration in the laboratory mill (45 cm×45 cm) grinding. Study of hydration of the cement pastes showed that the presence of hemihydrate increased the rate of ettringite formation. Hemihydrate also was rehydrated to gypsum and in the process retarded the setting times and reduced the strength development of the cement. This has been further confirmed by examination of cements prepared with gypsum, hemihydrate, and anhydrite.     

Difference in autoclave expansions of plant and laboratory ground cements

In a cement plant, it was found that the cement ground in the laboratory mill showed higher autoclave expansion than the cement produced in the plant mill, although both of them were of the same specific surface (Blaine) area. DTA of the laboratory ground (autoclaved) cement showed only one endothermic peak corresponding to one dissociation temperature of Mg(OH)₂, while that of the plant cement showed two dissociation temperatures. It has been found that a wide range of particle size of periclase is responsible for two dissociation temperatures of Mg(OH)₂ produced in the autoclaved cement samples. It has also been established that periclase of narrower particle size range causes higher autoclave expansion than an equal amount of periclase of wider particle size range, although the latter may contain some coarser periclase than the former. Thus the difference in autoclave expansions of plant and laboratory ground cements is attributed to the difference in particle size ranges of the periclase present in the cement samples.     

Influence of the fineness of sewage sludge ash on the mortar properties

Sewage sludge ash (SSA) is a recycled material and can be used in cement mortar as pozzolan. To improve the mortar properties, this research utilized mechanical grinding to adjust the fineness of SSA. Finely ground SSA with Blaine fineness of 500-1000 m²/kg was added to mortar to replace 20% of portland cement. The initial and final setting times of SSA-cement paste simultaneously prolonged when SSA fineness increased. Because of the lubricant effect and morphology improvement, the workability of SSA mortar increased when fineness increased. In addition, the pozzolanic activity of SSA and the compressive strength of mortar increased when SSA fineness did. The strength activity index (SAI) value approximately increased 5% when SSA fineness increased per 100 m²/kg. According to the results, the application of mechanical grinding to adjust SSA fineness was an effective modification to improve SSA mortar properties including workability and compressive strength.     

Influence of the fineness of pozzolan on the strength of lime natural-pozzolan cement pastes

The fineness of a cement is an important factor affecting the rate of strength development. This paper examines the importance of the fineness of natural pozzolans to the strength development of lime-pozzolan cements (LPCs). A natural pozzolan from Bolivia, which is typical of natural pozzolans found in South and Central America, was ground to various finenesses, blended with lime with a mass ratio of 80:20, and the resulting LPC was used to make hardened cement paste cylinders. In some pastes 4% sodium sulphate or 4% sodium chloride activator was used to enhance strength development. Strength of the cylinders were measured at ages from 3 to 90 days during continuous moist curing at 50°C. The experimental results show that there is a good linear correlation between the Blaine fineness of the natural pozzolan and compressive strength at all ages and for all pastes. The fineness of the natural pozzolan has its most significant effect on early strength gain. The addition of chemical activator increases both the rate of strength gain and the sensitivity of strength gain to fineness.     

Effect of clinker grinding aids on flow of cement-based materials

Grinding aids (GAs) are increasingly used during cement production to reduce energy consumption and/or optimize clinker factor. This paper seeks to assess the effect of such additions on variations in flow of cement pastes, including static yield stress (τ₀) and viscosity (η). Grinding tests were performed at fixed specific energy consumption (Ec) or Blaine cement fineness. For fixed Ec, tests have showed that the increase in cement fineness resulting from the addition of higher GA concentration leads to reduced flow and increased τ₀ and η values. Conversely, cement ground for fixed Blaine fineness exhibited an improvement in flowability together with reduction in τ₀ and η values. This was related to a dispersion effect of cement agglomerates in the presence of GA molecules. Special emphasis is placed throughout this paper regarding the effect of GAs on ASTM C465 requirements.     

The effect of particle size distribution on the properties of blended cements incorporating GGBFS and natural pozzolan (NP)

This paper investigates the effect of particle size distribution on the properties of blended cements incorporating ground granulated blast-furnace slag (GGBFS) and natural pozzolan (NP). Pure Portland cement (PPC), NP and GGBFS were used to obtain blended cements that contain 10, 20, 30% additives. The cements were produced by intergrinding and separate grinding and then blending. Each group had two different Blaine fineness of 280 m²/g and 480 m²/g. According to the particle size distribution (PSD) curves, 46% of the coarser specimens and 69% of the finer specimens passed through the 20 μm sieve. It was observed that the separately ground specimens were relatively finer than the interground ones and had higher compressive strength and sulfate resistance. The separately ground coarser specimens had the lowest heat of hydration. The separately ground finer specimens, which had the highest compressive strength and sulfate resistance, had the highest percent passing for each sieve size. For these specimens 34, 69, 81 and 99% passed through 5, 20, 30 and 55 μm sieves, respectively. For the interground specimens, which had the same fineness, the respective values for the same sieves were 32, 68, 75 and 94%.     

Characteristics and pozzolanic reactivity of glass powders

This paper deals with the morphology, fineness and pozzolanic activity of four glass powders: one (GP-fine) from the screening of crushed waste glasses, one (GP-dust) from a dust collector for the glass crushing process and two (GP-4000 and GP-6000) from further grinding of the powder from the dust collector in a ball mill. GP-fine and GP-dust consist mainly of large flaky particles, while GP-4000 and GP-6000 consist mainly of small angular particles. The finenesses of these glass powders are measured by particle size distribution and Blaine fineness method. For a similar particle size distribution, ground glass powder has a higher Blaine specific surface area than Portland cement due to the angular morphology of glass particles. Finely ground glass powders exhibited very high pozzolanic activity. The finer the glass powder is, the higher its pozzolanic reactivity is. An increase in curing temperature accelerates the activation of pozzolanic reactivity of both glass powder and coal fly ash in terms of strength development rate. Mortar cube strength results (ASTM C109) indicated that curing temperature has a greater influence on the glass powder than on fly ash. The rapid mortar bar expansion test (ASTM C1260) results indicate that the replacement of Portland cement with ground glass powder also reduces the expansion due to alkali-aggregate reactions, although it is not as effective as coal fly ash.     

A comparison of the performance of various synthetic gypsums in plant trials during the manufacturing of OPC clinker

This paper compares the plant performances of various synthetic gypsums used as set regulators in cement. The decision about the suitability of a specific gypsum was based on measurements and comparisons of the specific areas (Blaine), initial setting time (min), final setting time (h), SO₃ content and compressive strength of the OPC clinker mixed with it. The results from the trials run with synthetic gypsum that was wet milled and treated with milk of lime showed no significant difference in compressive strengths at all ages, but a delay of approximately 35% in initial and 50% in final setting times occurred. Gypsum from a Tioxide plant can be successfully used as set retarders for OPC cement if the handling problems can be solved. In this case, no significant difference in either initial or final setting times or the compressive strengths, were observed. The ultrasonically treated phosphogypsum displayed a large variability of 50% or more in final and initial setting times, as well as compressive strengths at all ages, and is unsuccessful in rendering the gypsum usable as a set retarder for OPC cement.     

Properties of alite cements

For carrying out a comprehensive investigation on physical and mechanical properties of alite mortars and concretes, large quantities of monoclinic alite were produced at the University of California at Berkeley. Laboratory-size specimens were employed to determine strength, drying shrinkage, and sulfate resisting characteristics of mortars and concretes made with alite cements. Small amounts of gypsum (3%) addition accelerated the setting and hardening of the alite cements, however, large amounts (6%) resulted in strength deterioration. Drying shrinkage of alite concretes was significantly lower than portland cement concretes of the same fineness. Long term sulfate immersion of concrete specimens made with an alite cement caused serious spalling and strength loss.     

Ultrafine grinding of sugar cane bagasse ash for application as pozzolanic admixture in concrete

Sugar cane bagasse ash, a byproduct of sugar and alcohol production, is a potential pozzolanic material. However, its effective application in mortar and concrete requires first the controlled use of grinding and classification processes to allow it to achieve the fineness and homogeneity that are required to meet industry standards. The present paper investigates the role of mill type and grinding circuit configuration in grinding in laboratory- and pilot plant-scale on the particle size, specific surface area and pozzolanic activity of the produced ashes. It was observed that, although different size distributions were produced by the different mills and milling configurations, the pozzolanic activity of the ground ash was directly correlated to its fineness, characterized by its 80% passing size or Blaine specific surface area. From a low pozzolanic activity of less than 50% of the as-received ash, values above 100% could be reached after prolonged grinding times. Electric power requirements to reach the minimum pozzolanic activity were estimated to be in the order of 42 kWh/t in an industrial ball mill. Incorporation of an ultrafinely-ground ash in a high-performance concrete in partial replacement of Portland cement (10, 15 and 20% by mass) resulted in no measurable change in mechanical behavior, but improved rheology and resistance to penetration of chloride ions.     

Modeling the coupled effects of temperature and fineness of Portland cement on the hydration kinetics in cement paste

This paper revisits the coupled impacts of fineness and temperature on the kinetics of Portland cement hydration. The approach consists in i) modeling the impact of fineness on cement dissolution through the hypothesis that the surface dissolution rate of cement particles is independent of their size, in order to, in a second step, ii) model the impact of temperature on the kinetics of cement dissolution. The analysis of the experimental results shows that the effect of cement fineness on the hydration kinetics can be captured by a simple hypothesis: for any age, the reacted thickness of cement grains can be considered independent of the initial cement particle size. In addition, the analysis of the results at different temperatures shows that a constant activation energy can account for the effect of temperature on the hydration kinetics, with an Arrhenius equation applied to the kinetics of surface dissolution. The results from the model give a good agreement with the experimental results in a significant number of combinations (different Portland cements, water/cement ratio from 0.5 to 0.6, cement Blaine fineness from 3500 to 6600 cm²/g, temperature histories between 20 and 60 °C).     

Thermal analysis of borogypsum and its effects on the physical properties of Portland cement

Borogypsum, which consists mainly of gypsum crystals, B₂O₃ and some impurities, is formed during the production of boric acid from colemanite, which is an important borate ore. In this study, the effect of borogypsum and calcined borogypsum on the physical properties of ordinary Portland cement (OPC) has been investigated. The calcination temperature and transformations in the structures of borogypsum and natural gypsum were determined by differential thermal analysis (DTA), thermogravimetric analysis (TGA) and X-ray diffraction (XRD) techniques. Thermal experiments were carried out between ambient temperature and 500 °C in an air atmosphere at a heating rate of 10 °C min⁻¹. After calculation of enthalpy and determination of conversion temperatures, borogypsum (5% and 7%), hemihydrate borogypsum (5%) and natural gypsum (5%) were added separately to Portland cement clinker and cements were ground in the laboratory. The final products were tested for chemical analysis, compressive strength, setting time, Le Chatelier expansion and fineness properties according to the European Standard (EN 196). The results show that increasing the borogypsum level in Portland cement from 5% to 7% caused an increase in setting time and a decrease in soundness expansion and compressive strength. The cement prepared with borogypsum (5%) was found to have similar strength properties to those obtained with natural gypsum, whereas a mixture containing 5% of hemihydrate borogypsum was found to develop 25% higher compressive strength than the OPC control mixtures at 28 days. For this reason, utilization of calcined borogypsum in cement applications is expected to give better results than untreated borogypsum. It is concluded that hemihydrate borogypsum could be used as a retarder for Portland cement as an industrial side. This would play an important role in reducing environmental pollution.     

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.     

Reliable specific surface area measurements on anhydrous cements

The specific surface area (SSA) of anhydrous cement has a direct impact on its reactivity and on the properties of fresh and hardened concrete. However, measuring this physical parameter is non-trivial.The choice of the degassing conditions (temperature, pressure and time) is essential for reliable SSA measurements by nitrogen adsorption because of the dehydration of the gypsum it contains. Such dehydration involves a significant increase of the SSA_BET, whereas Blaine test is poorly sensitive to the hydration state and structural modification of the calcium sulfate carrier. Because of this, SSA_BET values may be more variable than Blaine fineness. More consistent SSA_BET results could be obtained by degassing anhydrous cement samples at 40 °C under N₂ flow for 16 h, as at these conditions the cement composition is preserved.     

Mechanical behaviour of various mortars made by combined fly ash and limestone in Moroccan Portland cement

Physico-chemical properties and mechanical behaviour of ternary cements made by Portland cement, fly ash and limestone are studied. The mixtures at various compositions of clinker, gypsum fly ash and limestone are intimately ground and compared to other compositions without fly ash. Blended fly ash cements are also studied. The results show that fly ash acts as grinding agent by reducing the required time to obtain the same percentage of particles retained on a 80-μm sieve as the standard cement. Fly ash cements lead to an important extension of setting time than limestone cements. The replacement of clinker by limestone gives better mechanical strengths than the mixtures containing fly ash at early days; after 28 days, the cements prepared by incorporation of fly ash gain an important strength. From mechanical point of view, an optima dosage was obtained at 77% clinker, 2% gypsum, 7.5% fly ash and 13% limestone composition.     

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.     

The influence of grinding technique on the liberation of clinker minerals and cement properties

This paper describes a study of the relationship between the physical, chemical and mineralogical parameters of cement products obtained by different grinding mechanisms namely high pressure grinding rolls (HPGR) and ball milling, and their effects upon the properties of cements prepared from the ground clinker. Samples were prepared as narrow size fractions and also as distribution samples. Characterization parameters were ascertained by using XRF, laser sizing, Blaine and BET surface area and image analysis methods. HPGR grinding resulted in higher degrees of liberation of clinker phases arising from the intergranular breakage along the grain boundaries compared to ball mill grinding. As for service properties, water demand of HPGR products was higher than ball mill products resulting from high micro fissured structure. Despite high liberation of particularly alite mineral in HPGR grinding, the compressive strength of ball mill products was slightly higher than HPGR products for narrow size samples. Finally, particle size distribution effect on strength was more obvious for distribution samples; generally ball milling gave higher strength values.