Influence of alkalis on porosity percolation in hydrating cement pastes

Loss-on-ignition (LOI) measurements and low temperature calorimetry (LTC) are used to study the properties of hydrating cement pastes with various quantities of alkalis. In addition to the well-known acceleration of early age hydration and “retardation” of later age hydration, the alkalis are observed to have a significant effect on the percolation of the porosity in the hydrating systems, as assessed using the LTC technique. At equivalent degrees of hydration, the capillary pores in cement pastes with sufficient added alkalis may depercolate while those in lower alkali cement pastes remain percolated. A simple dissolution/precipitation three-dimensional microstructural model is applied to examine the potential effects of hydration product morphology (random, needles, and plates or laths) on pore space percolation. The model suggests that the observed experimental results could be consistent with the higher alkali levels modifying the morphology of the C–S–H gel to produce more lath-like hydration products, as has been observed by others previously using electron microscopy. Potential implications for the transport properties and durability of these materials are discussed.     

Blended cements containing high volume of natural zeolites: Properties, hydration and paste microstructure

In this study, properties and hydration characteristics as well as paste microstructure of blended cements containing 55% by weight zeolitic tuff composed mainly of clinoptilolite mineral were investigated. Free Ca(OH)₂ content, crystalline hydration products and decomposition of zeolite crystal structure, pore size distribution and microstructural architecture of hydrated cement pastes were examined. Superplasticizer requirement and compressive strength development of blended cement mortars were also determined. The blended cements containing high volume of natural zeolites were characterized with the following properties; (i) no free Ca(OH)₂ in hardened pastes at the end of 28 days of hydration, (ii) less proportion of the pores larger than 50 nm when compared to portland cement paste, (iii) complete decomposition of crystal structure of zeolite at the end of 28 days of hydration, (iv) presence of tetra calcium aluminate hydrate as a crystalline product of pozzolanic reaction, (v) more compatibility with the melamine-based superplasticizer when compared to the naphthalene based product, and (vi) similar 28 days compressive strength of mortars to that of reference portland cement.     

Influence of cellulose ether on hydration and carbonation kinetics of mortars

The hydration process of Portland cement and retarding effect of cellulose ether (CE) on hydration and carbonation were studied. The degree of CE-substitution is a major parameter which plays an important role in terms of retardation of both hydration and carbonation. For the hydration process, this CE-effect was highlighted through the results of an experimental campaign based on thermogravimetric analysis (TGA) performed on mortar samples conserved in an ambient air in which the atmospheric CO₂ was absorbed by whitewash solution. This type of conservation is chosen in order to make precise the measurement of dehydration rate by TGA tests. While for the carbonation mechanism, the CE-effect was identified by the measurement of carbonation depth with phenolphthalein spraying.This paper aims to determinate a coefficient of retardation of hydration according to the CE-rate used in the manufacturing of mortars. This coefficient may be taken into account in the calculation of the reaction rate of anhydrous constituents of cement in order to determine a precise hydration degree of mortars. Consequently, this delay in cement hydration delays the carbonation processes because of the lack of hydrates to react with CO₂.     

Effect of nano clay particles on mechanical, thermal and physical behaviours of waste-glass cement mortars

Worldwide, around 2.6 billion tons of cement is produced annually. This huge size of production consumes large amounts of energy and is one of the largest contributors to carbon dioxide (CO₂) release. Accordingly, there is a pressing demand to minimise the quantity of cement used in the concrete industry. The main challenge to this is to get durable concrete with less cement and within reasonable cost. The economic, environmental and engineering benefits of reusing ground waste-glass powder (WGP) as a partial cement replacement has been established, but low glass reactivity and the possible alkali-silica reaction (ASR) are a drawback. Recent advances in nano-technology have revealed that nano-sized particles such as nano clay (NC) have a high surface area to volume ratio that provides the potential for tremendous chemical reactivity, accelerating pozzolanic activity and hindering ASR. This paper presents a laboratory study of the properties of NC/WGP cement composites. The microstructure, ASR, fracture energy, compressive and flexural properties of cement mortars containing WGP as a cement replacement with and without NC are investigated and compared with plain matrix. In addition, the hydration of cement compounds was followed by differential thermal analysis (DTA), thermogravimetric analysis (TGA), and also X-ray diffraction (XRD). The results showed that incorporation of glass powder has a positive effect on the mechanical properties of cement mortars after 28 days of hydration. Also, the results revealed that the mechanical properties of the cement mortars with a hybrid combination of glass powder and NC were all higher than those of plain mortar and with glass powder after 28 days of hydration. In addition, the DTA/TGA results and XRD analysis showed a reduction in the calcium hydroxide (CH) content in mortars with glass powder and with a hybrid combination of glass powder and NC, which confirms the improvements of mechanical properties and occurrence of pozzolanic reaction after 28 days of hydration.     

Effects of the molecular architecture of comb-shaped superplasticizers on their performance in cementitious systems

The objective of this study is to link the molecular structure of polycarboxylate-ether-type superplasticizers with the performance of cementitious systems in order to develop new products with enhanced properties, e.g. improved water reduction with a wide range of cements or a reduced retardation of cement hydration. Different experimental superplasticizers have been synthesized varying length and density of the polyether chains as well as the molecular weight of the polymer. The influence of these polymers on the properties of cement pastes and mortars was determined using various characterization methods like mortar flow, rheological and calorimetric measurements, adsorption measurements and mortar compressive strength.Characteristic connections between molecular structure of the polycarboxylate-type water reducers, adsorption behaviour, workability and retarding effect have been determined allowing the synthesis of new superplasticizers with improved performance.     

The effect of polymer addition on the electrical behavior of ordinary and pulverized fly ash modified cement mortars

The electricla behavior of polymer-modified cement mortars has been evaluated as a function of the polymer and the evaporable water contents. The effects have been correlated to microstructural changes induced by the addition of an acrylic polymer and an epoxy resin purposely formulated for cement-based composites. Moreover, the effect of the addition of pulverized fly ash, which has proved to be beneficial on many of the cement composite properties, has also been studied. Polymer modified cement mortar composites exhibit a tendency towards insulating behavior which is greatly enhanced by water losses: the electrical properties must thus be carefully evaluated when static charge accumulation is not desired. The addition of pulverized fly ash to ordinary Portland mortars slightly influences the electrical behaviour, but strongly enhances the insulating behaviour of polymer modified cement mortars.     

Comportement au gel interne de bétons sous contraintes mécaniques

The literature relative to the freezing of concrete is based on freezing-thawing cycle data obtained from specimens which are not subjected to mechanical stresses. In the present study, the initial tensile stresses of concrete are taken into account. Data were developed on control mortars, containing a smaller amount of cement than standard ones in order to represent the actual content of cement in concrete. Two binders, both with and without an air-entraining agent, were utilized: ordinary Portland cement and metakaolin-blended cement, in which the ordinary Portland ciment/metakaolin ratio was 80/20. Prismatic specimens of mortars (40×40×160 mm³) were subjected, in a novel manner, to freezing-thawing cycles in the presence of variable flexural stress rates: 0 to 85% of the rupture load. In order to separate the freezing effect from the load effect, measurements were conducted both in a cold-heat chamber and in a room where the temperature was maintained at 20°C. The strength evolution was analysed and some microstructural properties were assessed (porosity, hydrates). The results obtained show that this new method of testing allows the acceleration of damage when the rate of flexural stress reaches about 60% of the rupture load. The mortar containing metakaolin performs as well as mortars containing air-entraining agents (ordinary Portland ciment-air-entraining agent; metakaolin-air entraining agent). The microstructural analyses explain the behaviour of the different mortars.     

Mechanical properties of cement pastes and mortars at early ages: Evolution with time and degree of hydration

To better understand the relations between the hydration reactions and the evolution of the mechanical properties of concrete, we developed an integrated approach. Ultrasonic, calorimetric, and conductometric techniques were applied simultaneously to the same batches of cement pastes and mortars. Young's modulus of elasticity and Poisson's ratio were determined acoustically and studied as functions of time and of degree of hydration. The amount of hydration needed to reach the beginning of set (percolation threshold of solid phase) was determined using our techniques. Although conditioned by the hydration rate, the mechanical behavior is also related to the microstructure evolution. Two main mechanisms have been evidenced in the development of mechanical properties. The first is the connection of cement particles, for which a percolation model is applied. This concept gives good insight of the setting time. The second mechanism corresponds to the filling of capillary pores by the hydrates. The evolutions of elastic properties and compressive strength have been compared. This study provides a new way for assessing the hydration models and for optimization of high performance concrete formulations.     

Assessment of Pb-slag, MSWI bottom ash and boiler and fly ash for using as a fine aggregate in cement mortar

Three types of wastes, metallurgical slag from Pb production (SLG), the sand-sized (0.1-2 mm) fraction of MSWI bottom ash from a grate furnace (SF), and boiler and fly ash from a fluidised bed incinerator (BFA), were characterized and used to replace the fine aggregate during preparation of cement mortar. The chemical and mineralogical behaviour of these wastes along with the reactivities of the wastes with lime and the hydration behaviour of ordinary Portland cement paste with and without these wastes added were evaluated by various chemical and instrumental techniques. The compressive strengths of the cement mortars containing waste as a partial substitution of fine aggregates were also assessed. Finally, leaching studies of the wastes and waste containing cement mortars were conducted. SLG addition does not show any adverse affect during the hydration of cement, or on the compressive strengths behaviours of mortars. Formation of expansive products like ettringite, aluminium hydroxide and H2 gas due to the reaction of some constituents of BFA and SF with alkali creates some cracks in the paste as well as in the cement mortars, which lower the compressive strength of the cement mortars. However, utilization of all materials in cement-based application significantly improves the leaching behaviour of the majority of the toxic elements compared to the waste as such.     

Impedance spectra of hydrating cement pastes

Complex impedance spectra were obtained over the frequency range 5 Hz to 13 MHz on Portland cement pastes with water/cement ratios of 0.3, 0.35, and 0.4 at various hydration times from 6 h to 24 days. Features of the spectra which could be associated with the bulk material and which could be separated from the electrode arc, were studied. The overall bulk resistance of each paste was thereby determined as a function of hydration time. Bulk features evolved from a simple high-frequency intercept to an intercept with a single arc, then an intercept with two overlapping arcs, and back to an intercept with a single arc. A plausible equivalent circuit was developed involving an electrodeR/C network and constant phase element in series with one or two bulkR/C networks and a bulk resistor. Possible physical interpretation is discussed but assignment of equivalent circuit elements to microstructural features and/or processes will require microstructural characterization and a knowledge of pore-phase chemistry and properties.     

Fine limestone additions to regulate setting in high volume fly ash mixtures

High volume fly ash (HVFA) concrete mixtures are being considered more frequently due to their cost and sustainability advantages. While the long term performance of these HVFA concretes typically meets or exceeds that of conventional concretes, their early age performance is often characterized by excessive retardation of the hydration reactions, delayed setting times, and low strengths. Extending an HVFA mixture to a ternary blend that incorporates a fine limestone powder may provide a viable solution to these deficiencies, particularly the hydration retardation and setting issues. In this paper, a nano-limestone powder and two other limestone fillers of increasing median particle size (4.4 μm and 16.4 μm) are investigated for their propensity to accelerate early age reactions and reduce setting times in a Class C fly ash/cement blend. The fineness of the limestone has measurable effects on its efficacy in accelerating hydration and decreasing setting times. Companion specimens prepared with a fine silica powder suggest that the fine limestone may act favorably through both a physical and a chemical mechanism. Isothermal calorimetry and Vicat needle penetration measurements on pastes are accompanied by strength measurements on mortars, to verify that the limestone powder substitutions are not negatively impacting strength development. A linear relationship with a reasonable correlation is found to exist between 1 d and 7 d compressive strengths of mortars and their accompanying cumulative heat release values as determined using isothermal calorimetry.     

Application of thermally activated municipal solid waste incineration (MSWI) bottom ash fines as binder substitute

Untreated municipal solid waste incineration (MSWI) bottom ash fines (0–2 mm) have poor pozzolanic properties, and contain substances which can pose an environmental risk (e.g. heavy metals and salts). This study investigates combined treatments applied on bottom ash fines (BAF) to increase their reactivity. The treated BAF is compared with both untreated BAF and cement, and its contribution to cement hydration is investigated. Additionally, the utilization of the treated BAF in mortar as cement replacement is tested. Finally, the leaching properties of mortars containing treated and untreated BAF are estimated. According to the results obtained, the 28-day compressive and flexural strengths of mortar with 30% treated bottom ash are about 16% and 6% lower than the reference mortar, respectively. The leaching of contaminants from the crushed mortars with BAF are well under the limit values imposed by Dutch legislation.     

Hydration and properties of nano-TiO2 blended cement composites

Two types of nano-TiO₂ particles were blended into cement pastes and mortars. Their effects on the hydration and properties of the hydrated cement pastes were investigated. The addition of nano-TiO₂ powders significantly accelerated the hydration rate and promoted the hydration degree of the cementitious materials at early ages. It was demonstrated that TiO₂ was inert and stable during the cement hydration process. The total porosity of the cement pastes decreased and the pore size distribution were also altered. The acceleration of hydration rate and the change of microstructure also affected the physical and mechanical properties of the cement-based materials. The initial and final setting time was shortened and more water was required to maintain a standard consistence due to the addition of the nano-TiO₂. The compressive strength of the mortar was enhanced, practically at early ages. It is concluded that the nano-TiO₂ acted as a catalyst in the cement hydration reactions.     

Efficiency of internal curing by superabsorbent polymers (SAP) in PC-GGBS mortars

This paper evaluates the effect of superabsorbent polymers (SAP) on hydration and microstructure of PC-GGBS mortars. Development of autogenous shrinkage, microstructural characteristics (MIP/SEM) and compressive strength were analysed during the first 90 days. Four levels of Portland cement (PC) replacement by GGBS (0%, 25%, 50% and 75%) and two types of SAP with different water absorption capacities were considered. The results proved the efficiency of internal curing by SAPs in PC-GGBS systems due to significant reduction in autogenous shrinkage, especially for higher contents of GGBS. SAP facilitates GGBS hydration activated by portlandite; its products can be deposited into the nano pores leading to a small relative expansion of the hardened bulk volume. This process is initiated during the second week and it lasts until the sixth week. Despite increased total porosity, compressive strength of SAPs modified mortars is comparable to the reference samples for low GGBS contents in advanced ages.     

Micro-mechanical properties of calcium sulfoaluminate cement and the correlation with microstructures

To investigate the micro-mechanical properties of calcium sulfoaluminate cement and the correlation with the microstructures, we apply a variety of advanced techniques of microstructural and micro-mechanical characterization, including scanning electron microscopy with backscattered electron and energy-dispersive X-ray spectroscopy detectors, X-ray fluorescence, X-ray diffraction and nanoindentation. For the first time, the micro-mechanical properties of material microstructures present in a calcium sulfoaluminate cement are estimated. In the calcium sulfoaluminate cement used in this research, two type of hydration product microstructures with the differentiable microstructural morphologies, compositions and micro-mechanical properties are identified and investigated. The correlation of the micro-mechanical properties with the microstructures shows that the hydration product microstructure containing more ettringite has lower indentation modulus and hardness than that containing more aluminum hydroxide.     

Influence of emulsifier content on cement hydration and mechanical performance of bitumen emulsion mortar

The objective of this research was to investigate the influence of a cationic emulsifier on the kinetics of cement hydration and on the overall mechanical performance of bitumen emulsion mortar, as an essential structural component of cold asphalt mixtures, within a 28-day curing period. This influence was investigated as a function of emulsifier content and bitumen grade, selected to cover the whole range of their applicability. The hydration kinetics was evaluated by heat release of cement pastes and bitumen emulsion mortars. The residual w/c was determined at characteristic points during curing. The emulsifier showed a very powerful effect on the complex relation between hydration kinetics, emulsion rheology, and water binding. The presence of the emulsion initially accelerated the hydration, with no main peaks and dormant period observed, but had almost no effect on the ultimate hydration degree. The rate of hydration during the rest of the curing period was critically dependent on the residual w/c which was decisively influenced by the content of emulsifier. The cement could be expected to reach a very high degree of hydration only in mixtures with the highest emulsifier content, considering the specimens as a whole. Together with emulsion breaking at early stage, cement hydration later on was directly reflected on indirect tensile properties of the mortars, which performed from very ductile to very brittle. In conclusion, the emulsifier was recognised as a key factor for a fundamental understanding of the mechanical performance of cold asphalt mixtures.     

Sulfate resistance and carbonation of plain and blended cements

Results of an experimental investigation on the sulfate resistance and carbonation of plain and blended cement mortars are reported in this paper. In the sulfate resistance test all the specimens were immersed in a 5% Na₂SO₄ solution for 24 months. Two different types of lignite fly ashes and two natural pozzolans were used for the production of 13 blended cements. An ordinary portland cement and a commercially available blended cement were also used for reference. The effect of mineral admixtures on the carbonation depth of mortars was also investigated. Results show that the addition of pozzolanic admixtures in most cases had a positive effect on the sulfate resistance. The carbonation depth in all blended mortars was greater than that in portland cement mortar. However the rate of carbonation of blended mortars was reduced as hydration progressed.     

Improvement of the early-age reactivity of fly ash and blast furnace slag cementitious systems using limestone filler

This article analyzes the effects of the addition of limestone filler on the hydration rate, setting times and early-age mechanical properties of binary and ternary-binder mortars containing Portland cement, blast furnace slag (BFS) and fly ash (FA), with various substitution rates of cement with mineral additions going up to 50%. Vicat needle penetration tests and measurements of heat flow of reaction, compressive strength and dynamic Young’s modulus were carried out on 14 mortars prepared with binary and ternary binders, at 20°C. The results obtained on the mortars containing binary binders, show that their loss of mechanical strength at early age is not caused by a deceleration of the reactions of cement in the presence of mineral additions, but is mainly explained by the dilution effect related to the reduction in cement content. A moderate addition of limestone filler (8–17%) makes it possible to obtain ternary binders with early-age reactivity equal or even higher than that of Portland cement, and with 28-days mechanical resistance close to those of the binary-binder mortars. This accelerating effect of limestone filler is particularly sensitive in the case of mortars containing FA.     

Fractographic studies of microstructural development in hydrated Portland cement

A comprehensive study of early hydration and morphological development in freeze-dried Portland cement paste has been carried out using scanning electron microscopy in conjunction with energy dispersive X-ray analysis. The microstructure develops from its early appearance as a porous C-S-H spherulite structure containing hexagonal CH crystals, with Hadley grains and large inherent pores being significant features, to one of large CH dispersions in massive C-S-H. Some aspects of microstructural influence on mechanical properties are discussed.     

Effect of EVA on the fresh properties of cement paste

The improved workability effect of latex in dry mortars has not been fully clarified. The purpose of this research was to investigate the influence of the EVA copolymer on the cement hydration and on the rheological properties of cement pastes. The results pointed to a minor influence of EVA on cement hydration and to a ball-bearing effect. In fact, the shear thinning behavior of reference paste at 15 min after mixing changed to shear thickening owing to the EVA addition. This behavior could be explained by the decrease in the interparticle separation distance as consequence of the solid content increase in case of shearing detachment of weakly adhered EVA particles from the cement particles surfaces. The expected EVA plasticizing effect was observed at 60 min. Such behavior points to the stabilization of EVA on the cement particles surfaces, thus resulting in a steric barrier effect.