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.     

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.     

Effect of Temperatures up to 90°C on the Early Hydration of Portland–Blastfurnace Slag Cements

Isothermal conduction calorimetry has been used to monitor the early hydration of Portland–blastfurnace slag (BFS)-blended cements. Portland:BFS composite cements with ordinary Portland cement replacements from 0 to 90 wt% were studied at curing temperatures from 12° to 90°C. Peak II, principally associated with alite (Ca₃SiO₅) hydration, was accelerated with increasing temperature for all blends. Peak S, associated with BFS hydration, was particularly noticeable at 40° and 60°C. At higher curing temperatures, peak S merged with peak II, indicating thermal activation of BFS. Novel plots of total heat output against percentage replacement show that BFS contributes to the heat of hydration, even at temperatures below its thermal activation.     

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.     

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.     

DTA-TG study on the Ca(OH)2 - pozzolan reaction in cement pastes hydrated up to three years

In order to study the way in which Santorin earth (pozzolan) acts during its hydration with portland cements and specifically, the rates of its action and its optimum content, the amount of Ca(OH)₂ derived during the hydration has been quantitatively determined, by means of thermogravimetry. Thus, cement pastes have been prepared with mixtures of portland cement containing proportions of Santorin earth up to 40% of various finenesses. These pastes were cured in water up to three years.     

Industrially interesting approaches to “low-CO2” cements

This article discusses the practicality of replacing portland cements with alternative hydraulic cements that could result in lower total CO₂ emissions per unit volume of concrete of equivalent performance. Currently, the cement industry is responding rapidly to the perceived societal need for reduced CO₂ emissions by increasing the production of blended portland cements using supplementary cementitious materials that are principally derived from industrial by-products, such as blast-furnace slags and coal combustion fly ashes. However, the supplies of such by-products of suitable quality are limited. An alternative solution is to use natural pozzolans, although they must still be activated either by portland cement or lime or by alkali silicates or hydroxides, the production of all of which still involves significant CO₂ emissions. Moreover, concretes based on activated pozzolans often require curing at elevated temperatures, which significantly limits their field of application.The most promising alternative cementing systems for general concrete applications at ambient temperatures currently appear to be those based at least in part on calcium sulfates, the availability of which is increasing due to the widespread implementation of sulfur dioxide emission controls. These include calcium sulfoaluminate-belite-ferrite cements of the type developed in China under the generic name “Third Cement Series” (TCS) and other similar systems that make good use of the potential synergies among calcium sulfate, calcium silicate and calcium aluminate hydrates. However, a great deal more research is required to solve significant unresolved processing and reactivity questions and to establish the durability of concretes made from such cements. If we are to use these potentially more CO₂-efficient technologies on a large enough scale to have a significant global impact, we will also have to develop the performance data needed to justify changes to construction codes and standards.     

Early hydration of portland cement with crystalline mineral additions

This research presents the effects of finely divided crystalline mineral additions (quartz and limestone), commonly known as filler, on the early hydration of portland cements with very different mineralogical composition. The used techniques to study the early hydration of blended cements were conduction calorimeter, hydraulicity (Fratini's test), non-evaporable water and X-ray diffraction. Results showed that the stimulation and the dilution effects increase when the percentage of crystalline mineral additions used is increased. Depending on the replacement proportion, the mineralogical cement composition and the type of crystalline addition, at 2 days, the prevalence of the dilution effect or the stimulation effect shows that crystalline mineral additions could act as sites of heat dissipation or heat stimulation, respectively.     

Improvement of the durability of glass fiber reinforced cement using blended cement matrix

Blended cements prepared with two fly ashes were used as matrices in glass fiber reinforced cement (GRC) composites in an attempt to improve their durability. The hydrated matrices from the two blended cements investigated here had similar strength and composition. Both fly ashes reduced the Ca(OH)₂ content to the same extent but in both cases the pH level was only slightly reduced compared to the portland cement matrix. In spite of these similarities, the GRC prepared with one fly ash showed considerable improvement in durability while the other one had only a small positive effect. SEM observations indicated that the improved durability in one case was associated with modification in the microstructure of the hydration products deposited in between the glass filaments, resulting in a much more open structure compared to that of portland cement matrix or the other blended cement. It is therefore suggested that the potential of the blended cement matrix to improve the durability of GRC is associated with its ability to modify the microstructure of the paste at the glass interface. This characteristic is not necessarily related to the overall composition of the blended cement matrix and to the reactivity of fly ash with Ca(OH)₂.     

Supersulphated cement from blastfurnace slag and chemical gypsum available in the Netherlands and neighbouring countries

The present report covers research into the mechanical strength development and the surface hardness of supersulphated cement from blastfurnace slag, chemical gypsum and portland clinker. The tests were carried out on sand/cement mortar as well as on concrete. A comparison was made with a type of supersulphated cement which was in the market until recently, and with portland and blastfurnace cement. A review has been made of the effects of the hardening temperature, the relative humidity, the degree of grinding of the cement, the use of a water-reducing additive, and treatment with a curing compound. The conclusion is drawn that one of the cements produced, consisting of 83% m/m Dutch blastfurnace slag, 15% m/m fluorogypsum (anhydrite) and 2% m/m portland clinker ground to the relatively high specific surface of 500 m²/kg, is not inferior to blastfurnace cement as regards the properties examined.     

Relationship between free chloride and total chloride contents in concrete

Linear relationships between free chloride and total chloride contents in concrete are proposed based on the results of several long-term exposure tests under marine environment for various cements, such as ordinary portland cement (OPC), high early strength portland cement (HES), moderate heat portland cement (MH), calcium aluminate cement (AL), slag cements of Types A (SCA) and B (SCB), and fly ash cement of Type B (FACB). A high chloride-binding ability is found for AL as compared to the other cements. Replacing the OPC with slag reduces the chloride-binding ability. The proposed linear relationships show reasonably good agreement with field data obtained from the wharf structures.     

Model for the quantitative description of the kinetics of hardening of portland cements

A new mathematical form of a cement model is introduced for the prediction of concrete strengths obtained at various curing temperatures from the properties of the cement used. This model considers the hydrations of C₃S and C₂S as first order reactions in which the C₃A acts as catalyst. Not only does this model reproduce the strengths of various portland cements, but also it provides quantitative information about several important characteristics of the kinetics of hydration as a function of curing temperature. These characteristics are: the time of beginning of the hardening; rates of hardening; the time when the diffusion control of hydration starts; how much this strength is; and, the final strength potential of various portland cements.It is shown that the new model is well supported by experimentally obtained strength results.     

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.     

Physical performances of alkali‐activated portland cement‐glass‐limestone blends

The potential of calcium aluminosilicate (CAS) glasses as supplementary cementitious materials is studied in terms of the development of compressive strength for mortars containing a mixture of portland cement, CAS glass, and limestone. In addition, the impact of internal and external alkali activation of the cementitious systems on the mortar performances is investigated. Internal alkali activation is obtained by adding alkali oxides to the CAS glass system, whereas external alkali activation is realized by hydration of the blended cements containing alkali‐free CAS glasses using alkaline solutions. For the internally alkali‐activated systems and the alkali‐free mortars, higher strengths are achieved in comparison to the reference mortar prepared from plain ordinary portland cement. In contrast, the externally alkali‐activated mortars exhibit lower compressive strengths, implying the importance of both the immediate availability of alkali ions in the cementitious system and the increased dissolution rate of the glass particles caused by the network depolymerization. The glasses are also studied by thermal analysis and the results are used to calculate the theoretical CO₂ emissions. The lowest embodied CO₂ emission is estimated for the blends containing alkali‐activated CAS glasses.     

Early stage hydration of slag-cement

The early hydration characteristics of slag cements (blends of separately ground granulated blast furnace slags with portland cement) have been examined. Isothermal calorimetry, chemical shrinkage and compressive strength measurements were made. The kinetics of hydration have been treated; apparent activation energies determined for a slag cement were ～49 KJ/mole compared with ～44 KJ/mole for the portland cement used in the blends.     

Studies on blended cements containing a high volume of natural pozzolans

This paper presents the results of an investigation on the characteristics of laboratory-produced blended portland cements containing 55% by weight volcanic tuffs from Turkey. Volcanic tuffs from two different resources were used. Using different grinding times, particle size distribution, setting time, compressive strength, and alkali-silica activity of the blended cements were investigated and compared with reference portland cements ground for the same time period. For the compressive strength test, a superplasticizer was used to obtain mortar mixtures of adequate workability at a constant water-to-cement (w/c) ratio of 0.45. Compared to portland cement, the blended cements containing 55% pozzolan showed somewhat lower strengths up to 91 days when the grinding time was 90 min. However, at 91 days, blended cements and portland cements ground for 120 min showed similar strength. Moreover, blended cements containing 55% natural pozzolans showed excellent ability to reduce the alkali-silica expansion.     

Improvement of in vitro osteogenesis and antimicrobial activity of injectable brushite for bone repair by incorporating with Se-loaded calcium phosphate

The main objective of this research is to study the effect of selenium (Se) on injectable brushite cement (Bru) derived from Se-loaded cement starting calcium phosphate powder (SP) to reveal whether injectable Se-loaded Bru has potential for bone repair. Se was incorporated into Bru by cement SP with a Se/P molar ratio of 0.05, 0.10, and 0.15, respectively. The results show that although the cement SP changes from β-calcium phosphate to hydroxyapatite as the Se content increases, brushite is still the dominant crystalline phase of the cements. The increase of Se concentration prolongs the cement setting time and promote the cement injectability, however, excellent anti-washout ability and degradability is observed for all the cements. Moreover, the cements with higher content of Se release out Se faster when soaking in phosphate buffer saline. The cell experimental results show that cements with a Se/P of 0.05 and 0.10 can not only be beneficial for osteoblastic MG63 cells adhesion and proliferation but also enhance cell mineralization property, while the cement with a Se/P ratio of 0.15 shows cytotoxicity. Furthermore, both the agar plate tests and the broth antimicrobial tests reveal that Se-loaded Bru can significantly inhibit the growth of E. coil, S. aureus, and P. aeruginosa. The antibacterial activity increases with the increase of Se concentration in the cement. Therefore, the biological performance of injectable Se-loaded brushite cement is dose-dependent and brushite cement with an appropriate dose of Se has potential for bone repair application.     

高掺量混合材复合水泥的水化性能

通过水化微量热、化学结合水测定和X射线衍射、热重-差热分析、扫描电镜等测试方法研究了3种高掺量矿渣、粉煤灰、石灰石复合水泥的水化性能,并与硅酸盐水泥的水化进行了对比。结果表明：高掺混合材复合水泥的水化放热特征与硅酸盐水泥有明显不同,早期水化反应速度低于硅酸盐水泥,但后期由于矿渣、粉煤灰的二次水化反应使其水化速度增长较快。主要的水化产物亦为水化硅酸钙凝胶、钙钒石和Ca(OH)2晶体,但Ca(OH)2含量明显低于硅酸盐水泥浆体中的Ca(OH)2含量。     

Durability of Alkali‐Activated Materials: Progress and Perspectives

In the last decade, there has been rapid growth in interest in alternative binders, as part of the toolkit of cement technologies needed to mitigate the carbon footprint associated with the construction industry. Alkali‐activated materials (AAMs), including geopolymer binders and other related systems, have been identified as a key component of this move to lower CO₂ cements and concretes. These are clinker‐free cements which can exhibit comparable performance to conventional portland/blended cements, when they are adequately formulated and cured. However, AAMs have a somewhat limited record of durability in service, and this is one of the main limitations facing their commercial adoption at present. To provide the best possible answers to the question of long‐term durability within an experimentally accessible timeframe, standardized accelerated degradation testing methods have been widely adopted, in an attempt to simulate natural processes. It has been identified that the interactions between material and environment, which take place on microstructural and nanostructural levels, have a very significant influence on the outcomes of the durability tests. Here, we present an overview of the results obtained when AAMs are exposed to aggressive testing conditions such as elevated concentrations of CO₂, sulfates or chlorides. The key outcome of this article is a broader synthesis of the available data regarding the interactions between these new materials and their surrounding environment, which is then available to be used in the design, development, and implementation of environmentally sustainable, high‐performance cements and concretes for the 21st century.     

Understanding expansion in calcium sulfoaluminate–belite cements

Calcium sulfoaluminate–belite (CSAB) cements are promoted as sustainable alternatives to portland cement because of their lower energy and CO₂ emissions during production and comparable performance. However, the formation of ettringite, the main hydration product in CSAB cements, can be expansive, sometimes resulting in cracking. The factors controlling expansive behavior in CSAB cements have not been completely elucidated. In this study, three CSAB cements synthesized from reagent-grade chemicals with varied phase compositions were examined for dimensional stability in water and sulfate solutions. The interdependent effects of C₄A₃? (Ye'elimite) content, calcium sulfate content, water-to-cement ratio, and particle fineness on CSAB cement expansion were evaluated. The results show that the expansive behavior can be controlled by altering chemical and physical factors in CSAB clinker, cement, and paste.