Studies on blended Portland cements containing Santorin earth

Although pozzolans, such as the Santorin earth, have been in use for over two thousand years for making cementitious products, the mechanism by which pozzolanic reactions contribute to the strength and chemical durability of mortars and concretes is not fully understood. Reported here are the results of an investigation in which portland pozzolan cements containing 10, 20, or 30 weight percent Santorin earth were used. Performance of the cements was evaluated with respect to strength development, drying shrinkage, sulfate resistance, and alkali-silica activity. The cement containing 20 percent pozzolan showed the highest compressive strength at 1 year, and the cements containing 20 or 30 percent pozzolan showed the least permeability and best resistance to sulfate attack. Microstructural investigations involving scanning electron microscopy, X-ray diffraction analysis, determination of free Ca(OH)₂ present, and pore-size distribution were conducted on hydrated cement pastes for the purpose of understanding the factors responsible for the observed behavior of the cements. From the results, it is concluded that the process of pore refinement associated with pozzolanic reactions plays an important part in enhancing the strength and chemical durability of portland pozzolan cements. It is suggested that the rate at which pore refinement occurs in a hydrating pozzolan cements is not only useful as a measure of the activity of the pozzolan presen, but also for a more reliable prediction of the performance characteristics of the cement.     

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.     

Application of Powers’ model to modern portland and portland limestone cement pastes

Powers’ model is a simple approach for estimating the relative volumes of hydration products, porosity, and chemical shrinkage present in portland cement paste as a function of its starting water‐to‐cement ratio (w/c) and current degree of hydration. It forms an important link between cement composition, microstructure, and performance, necessary for modeling cement‐based systems. Previous researchers have adapted Powers’ model for inert fillers to illustrate their effects on the hydration, porosity, and chemical shrinkage of blended cements; however, it is well‐documented that limestone is not, in fact, an inert filler, but rather participates in cement hydration through both chemical and physical processes. This research experimentally investigates the applicability of Powers’ model to modern portland cements containing up to 15% by mass finely divided limestone. The results demonstrate that the modified Powers’ model is insufficient for predicting the influence of finely divided limestone additions on the chemical shrinkage of both ordinary portland cement pastes and portland limestone cement pastes. Possible explanations for the discrepancy are discussed and a plausible source is proposed.     

Estimation of the degree of hydration of blended cement pastes by a scanning electron microscope point-counting procedure

A scanning electron microscope (SEM) point-counting technique was employed to study the hydration of plain portland and blended cement pastes containing fly ash or slag. For plain portland cement pastes, the results for the degree of cement hydration obtained by the SEM point-counting technique were consistent with the results from the traditional loss-on-ignition (LOI) of nonevaporable water-content measurements; agreement was within ±10%. The standard deviation in the determination of the degree of cement hydration via point counting ranged from ±1.5% to ±1.8% (one operator, one sample). For the blended cement pastes, it is the first time that the degree of hydration of cement in blended systems has been studied directly. The standard deviation for the degree of hydration of cement in the blended cement pastes ranged from ±1.4% to ±2.2%. Additionally, the degrees of reaction of the mineral admixtures (MAs) were also measured. The standard deviation for the degree of fly ash reaction was ±4.6% to ±5.0% and ±3.6% to ±4.3% for slag. All of the analyses suggest that the SEM point-counting technique can be a reliable and effective analysis tool for use in studies of the hydration of blended cement pastes.     

Investigation of blended cement hydration by isothermal calorimetry and thermal analysis

Hydration of portland cement pastes containing three types of mineral additive; fly ash, ground-granulated slag, and silica fume was investigated using differential thermal analysis, thermogravimetric analysis (DTA/TGA) and isothermal calorimetry. It was shown that the chemically bound water obtained using DTA/TGA was proportional to heat of hydration and could be used as a measure of hydration. The weight loss due to Ca(OH)₂ decomposition of hydration products by DTA/TGA could be used to quantify the pozzolan reaction. A new method based on the composition of a hydrating cement was proposed and used to determine the degree of hydration of blended cements and the degree of pozzolan reaction. The results obtained suggested that the reactions of blended cements were slower than portland cement, and that silica fume reacted earlier than fly ash and slag.     

Cement Composition Effects on the Rheological Property Evolution in Concentrated Cement–Polyelectrolyte Suspensions

We have studied the rheological property evolution and hydration behavior of white and ordinary portland cement (type I) pastes and concentrated cement–polyelectrolyte suspensions. Cement composition had a marked effect on the elastic property evolution ( G '( t )) and hydration behavior of these suspensions in the presence of poly(acrylic acid)/poly(ethylene oxide) copolymer (PAA/PEO), even though their affinity to adsorb such species was nearly identical. Both white and ordinary portland cement pastes exhibited G '₀ values of ∼10⁴ Pa and fully reversible G '( t ) behavior until the onset of the acceleratory period ( t = 2 h), where the pastes stiffened irreversibly. In contrast, cement–PAA/PEO suspensions exhibited G '₀ values of ∼1 Pa and G '( t ) behavior comprised of both reversible and irreversible features. Interestingly, ordinary portland cement–PAA/PEO suspensions experienced a gel-to-fluid transition on high shear mixing at short hydration times (<1 h), and the particle network did not rebuild until ∼24 h of hydration. In sharp contrast, white portland cement–PAA/PEO suspensions remained weakly gelled throughout the initial stage of hydration even after high shear mixing. At longer hydration times (>1 h), both cement–PAA/PEO suspensions exhibited G ' _i ( t ) ∼ exp( t /τ_c) with τ_c values of 5.6 and 1.3 h for ordinary and white portland cement, respectively. Our observations suggest that hydration phenomena impact interparticle forces during early stage hydration and, ultimately, lead to initial setting through the formation of solid bridges at the contact points between particles within the gelled network.     

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.     

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.     

The influence of silica fume on the mono/di silicate anion ratio during the hydration of CSF-containing cement paste

The influence of silica fume addition on silicate polymerization during hydration of portland cement pastes was studied. Trimethylsilyl derivatives were identified by gas-liquid chromatography. DTA/TGA method was applied for determination of the free lime content in hyrated paste samples. The obtained results point to the conclusion that pozzolanic reaction causes the shift towards Si-anion dimerization during hydration of csf-blended cement pastes.     

掺煅烧石膏水泥早期水化过程的研究

利用DTA，XRD，IR测定水泥水化浆体的化学结合水和Ca(OH)2的生成量，研究了煅烧石膏，二水石膏对硅酸盐水泥早期水化过程的影响。结果表明：在水化龄期相同时，掺煅烧石膏水泥浆体中水化产物同掺二水石膏相比，Ca(OH)2生成量大；在1d前无钙钒石（AFt)生成，结合水量在1d前，前者高于后者，而1d后则相反。指出了煅烧石膏加快水泥水化产物形成的机理在于：由于它的溶解度较低，在水泥水化初期（1d前），存在于水泥中的铝酸盐相不能形成AFt，从而减缓了AFt对水泥水化的延缓作用，加速了整个熟料矿物相的水化。     

Hydration of fly ash-portland cements

Hydration products of fly ash-portland cements were studied with x-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) as part of a continuing research effort to understand the pozzolanic activity of fly ashes. It was found that the amount of calcium hydroxide crystals in the cement pastes is diminished due to the addition of fly ash to the cement. Ettringite was produced in the early age, and the consumption of sulfate by the formation of ettringite was accelerated by the addition of fly ash. A partial conversion of ettringite to monosulfate within the first 7 days of hydration in the fly ash-portland cement pastes, but the formation of ettringite continued to form up to at least 28 days of hydration in the pastes without fly ash. Examination of the fly ash bearing pastes showed, in all cases, varying amounts of calcium hydroxide and unreacted portland cement, with minor quartz and gehlenite hydrate. It appears that hydration reactions actually occur in the fly ash cement pastes more or less on a particle-by-particle basis.     

The Pore Structure of Hydrated Cementitious Compounds of Different Chemical Composition

The pore structure ofβ-C₂S, C₃S, and portland cement pastes was investigated using mercury porosimetry and H₂O and N₂ adsorption. The β-C₂S had more total macro- and mesoporosities than C₃S and portland cement pastes of a similar degree of hydration. C₃S and portland cement pastes had similar total porosities but differed in the porosity size distribution. In the mesopore range, the various test methods gave different results. These differences are discussed on the basis of the various models proposed for cement paste. It is shown that shrinkage could be correlated with the volume of pores <0.03 μm, but not with total porosity.     

A soft X-ray microscope investigation into the effects of calcium chloride on tricalcium silicate hydration

Calcium chloride (CaCl₂) is one of the most recognized and effective accelerators of hydration, setting, and early strength development in portland cement and tricalcium silicate (C₃S) pastes. The mechanisms responsible for this acceleration, as well as the microstructural consequences, are poorly understood. Soft X-ray transmission microscopy has recently been applied to the study of cementitious materials and allows the observation of hydration in situ over time. This technique was applied to the examination of tricalcium silicates hydrating in a solution containing CaCl₂. It appears that CaCl₂ accelerates the formation of “inner product” calcium silicate hydrate (C-S-H) with a low-density microstructure.     

Mössbauer and X-ray investigations of some portland cements

The Mössbauer spectroscopic and x-ray diffraction investigations have been carried out on a variety of ordinary portland, portland pozzolanic, portland slag and sulphate resisting portland cements, using dry as well as hydrated samples. The discussion of the Mössbauer parameters shows that Fe atoms occupying distorted octahedral and tetrahedral sites in the dry cements are hydrated to form ferrite monosulphate without producing Fe(OH)₃ and its gel; hydration of the slag cement proceeds much faster than other cements; and that the composition of the iron-bearing phase in the sulphate resisting portland cement, studied in detail, is close to C₄AF.     

Cement pastes of low water to solid ratio: An investigation of the polymerization of silicate anions in the presence of a superplasticizer and silica fume

Polymerization of silicate anions was investigated by the molybdate method in cement pastes with water to solid ratio of 0.28. Two series of samples - with and without superplasticizer admixture - were prepared using ordinary portland cement (OPC) in which 0, 5, 10, and 15 percent by weight was replaced by condensed silica fume (CSF). During the hydration ranging from 4 hours up to 50 days, the free lime content and insoluble residue were also determined. The 28 day compressive strengths of the hardened composites were between 79 and 108 MPa. The results from the molybdate complex confirm that the presence of CSF in hydrating blends and its pozzolanic activity influences the size dispersity of silica anions by increasing the proportion of polymers. It is also suggested that in the cement pastes of w/s ratio of 0.28, the conversion level of CSF by pozzolanic reaction decreased. Finally, a comparison is made between the polymerization characteristics of the 0.28 w/s-ratio pastes and pastes of w/s = 0.68 which have been hydrated for 6.3 years.     

Possibilities of quantitative determination of the AFt-(ettringite) and AFm-(monosulphate) phases in hydrated cement pastes

The possibilities to determine AFt-(ettringite) and AFm-(monosulphate) in hydrated cement pastes were studied by measuring the intensities of the pertinent DTA and XRD peaks on 4 synthetic AFt preparations, 5 synthetic AFm preparations and on a series of portland cement pastes of different composition and hydration time. For AFt determination the DTA method appears to be most suitable. AFm may be determined with any method at best semiquantitatively.     

Effect of calcium chloride on the volume contraction,non-evaporable water content and bound chloride content of hydrated portland cement and blast-furnace slag

The change in volume during hydration, Δv, and the water retained at 105°C at ultimate hydration, w_n^o, have been measured for cement-calcium chloride-water systems in which the cements consisted of portland cement, blastfurnace slag and mixtures of portland cement and blastfurnace slag in equal parts by mass.The results showed that the quantity of chloride bound in the solid phases increased with the initial concentration of chloride in the aqueous phase but the parameters Δv and w_n^o for a particular system were independent of the initial chloride concentration in the aqueous phase.     

Alternative calcium sulfate-bearing materials as cement retarders: Part I. Anhydrite

The hydration process of cement pastes is of great importance to the physicomechanical properties of the hardened material. Thus, substances that regulate the setting of cement, such as natural anhydrite, have attracted significant scientific interest during the past years. This paper briefly describes the results of utilization of natural anhydrite in cement pastes of CEM-I and CEM-II types. The aim of the study has been to inquire the extent of natural gypsum replacement by natural anhydrite. The result of the hydration process has been expressed by the setting time and the compressive strength development, with respect to the SO₃⁻² content of each mixture. The experimental data conclude that natural anhydrite can be a very efficient retarder of the setting of cement, with no significant change in the physicomechanical properties of the hardened pastes.     

减水剂的加入时间对新拌水泥浆体流变性能的影响

研究了三聚氰胺甲醛磺酸盐(MFS)减水剂的掺加时间对普通硅酸盐水泥浆体在初始120 min的水化时间内流变性能的影响,研究中MFS的后掺时间为0 min、5 min、10 min、15 min、20 min和25 min。检测了在不同减切速率(3～147 s-1)下水泥浆体水化30 min和120 min时的剪切应力和表观粘度。测定了水化120 min后的水泥浆体的Ca2+浓度和化学结合水。结果表明:推迟减水剂的后掺时间降低了水泥浆体在120 min内的屈服应力和表观粘度,减水剂MFS的最佳后掺时间为10～15 min。