Porosity and specific surface area of Roman cement pastes

Mercury porosimetry, water vapour and nitrogen adsorption were used to follow the hydration of Roman cements — belite cements calcined at low temperature. Generally, unimodal distribution of pore sizes was observed, with the threshold pore width decreasing considerably with increasing curing time. An open porous structure with the threshold pore diameter between 0.2 and 0.8 μm and the specific surface area not exceeding 20 m²/g was produced at early ages when quick growth of the C–A–H phases is observed. The surface area reached up to 120 m²/g and the threshold pore width shifted to around 0.02 μm when the subsequent formation of C–S–H gel filled the larger pores. Both mercury porosimetry and water vapour adsorption were found to be capable of following the progress of hydration of the Roman cements with high reliability at least for a comparative evaluation of historic Roman cement mortars and repair materials used in restoration projects.     

Properties of cement pastes and mortars cured under different experimental conditions

The effects of water-to-cement ratio, time, and temperature of dry-hot-air treatment on the mechanical properties of cement pastes and mortars, immediately after demolding and after additional 7 and 28-day water curing at 20°C, is discussed. The results obtained are compared to those obtained on samples treated for 1, 7 and 28 days under normal curing conditions at 20°C. The samples were tested for density, compressive strength, porosity, and loss on ignition in the range 100–1000°C.     

Diffusion of chloride ions in limestone filler blended cement pastes and mortars

The diffusion of chloride ions in neat cement pastes and mortars made with and without a limestone filler was investigated. In order to study the effect of the limestone filler, all mixtures were prepared at a fixed water/cement ratio (0,55). Test results indicate that the addition of a limestone filler reduces the diffusion coefficient of chloride ions. This reduction is attributed to the effect of the limestone filler particles on the tortuosity of the system. Results also indicate that the presence of interfaces has only a limited effect on the diffusion coefficient of chloride ions.     

Investigations on the performance of silica fume-incorporated cement pastes and mortars

Studies on the performance of cementitious products with silica fume (SF) are very important, as it is one of the inevitable additives to produce high-performance concrete (HPC). In this study, some experimental investigations on the influence of SF on various preliminary properties of cement pastes and mortars are reported. The properties included specific gravity and normal consistency (NC) of cement and air content and workability of mortar with different SF contents. Pozzolanic and chemical reactions of SF have been studied on setting times, soundness and shrinkage of cement pastes. Further, strength developments in compression and tension in cement mortars have also been studied at various SF contents. SF was varied from 0% to 30% at a constant increment 2.5/5% by weight of cement. Test results show that the SF changes the behavior of cement pastes and mortars significantly. It has been observed that the water-binder (w/b) (cement+SF) ratio seemed to play an important role for the performance of the products with higher SF contents. NC, soundness and drying shrinkage of cement pastes and the strength of mortar increase as the SF content increases, while the initial setting times of cement pastes and the air content and workability of mortar decrease as the SF content increases. However, hardly any influence has been observed on the final setting times of cement pastes. The early age hydration reactions of C₃A and C₃S increase with the addition of SF. The optimum SF content ranges between 15% and 22%.     

Expression and analysis of pore fluids from hardened cement pastes and mortars

A device is described that has been used for several years for expression pf pore solution from hardened portland cement pastes and mortars. Particulars with respect to the design, fabrication, and operation of such equipment are given, and methods for the analysis of the resulting small volumes of pore solutions are briefly discussed. It is believed that the compositions of the pore solutions obtained are representative of that of the bulk of the pore solution within the paste or mortar from which the solutions have been obtained.     

Drying and autogenous shrinkage of pastes and mortars with activated slag cement

Activated slag cement (ASC) shows significantly higher shrinkage than ordinary Portland cement agglomerates. Cracking generated by shrinkage is one of the most critical drawbacks for broader applications of this promising alternative binder. This article investigates the relationship between ASC hydration, unrestrained drying and autogenous shrinkage of mortar specimens. The chemical and microstructure evolution due to hydration were determined on pastes by thermogravimetric analysis, conduction calorimetry and mercury porosimetry. Samples were prepared with ground blast furnace slag (BFS) activated with sodium silicate (silica modulus of 1.7) with 2.5, 3.5 and 4.5% of Na₂O, by slag mass. The amount of activator is the primary influence on drying and autogenous shrinkage, and early hydration makes a considerable contribution to the total result, which increases with the amount of silica. Drying shrinkage occurred in two stages, the first caused by extensive water loss when the samples were exposed to the environment, and the second was associated with the hydration process and less water loss. Due to the refinement of ASC porous system, autogenous shrinkage is responsible for a significant amount of the total shrinkage.     

Water desorption and shrinkage in mortars and cement pastes: Experimental study and poromechanical model

The aim of the present research was to study moisture changes and strains induced by smooth water desorption of several cement based materials. The main advantage of this small-steps drying is to dramatically limit the structural effect within tested samples, by lowering moisture gradients and therefore cracking due to differential shrinkage. Resulting data are of importance as they allow water retention curves, porosity distribution and desiccation shrinkage to be determined versus a large range of relative humidity. Experiments were conducted on ordinary mortars and cement pastes with water-to-cement ratio of 0.5 and 0.8. The role of the cementitious matrix and of aggregates over water-related behaviour of these materials can also be studied. Finally, a simple numerical model, based on experimental poromechanical results, was proposed to predict the shrinkage when the material is submitted to drying.     

Methods of obtaining pore solution from cement pastes and mortars for chloride analysis

Two techniques for the recovery of pore solution from cement mortars are examined: pore solution expression and miscible displacement using a high pressure permeameter. In the former, the pore solution is expressed from the mortar by crushing; in the latter, it is eluted from the mortar over 30 min by miscible displacement with water. Experimental results are presented for a range of cement pastes and mortars into which known amounts of chloride ion have been incorporated by using sodium chloride solution as the mix water. The results show that both eluted and expressed solutions exhibit a decrease in chloride ion concentration as the cement matrix ages, with the elution method showing a greater sensitivity to mix composition. Both methods show a decrease in chloride concentration as the water: cement ratio of the mix is increased. Overall, the high pressure elution method is capable of recovering a significantly higher proportion of the incorporated chloride. The application of these techniques to pore solution analysis is discussed.     

Influence of mix proportions on microstructure and gas permeability of cement pastes and mortars

This paper presents a study on the influence of mix proportions of cementitious materials on their transfer properties, namely porosity and gas permeability. These latter are known as durability indicators. The work is performed on a wide range of cement pastes and mortars (24 compositions). These compositions are defined by mix proportion parameters (water/cement ratio, limestone filler/cement ratio, and amount of superplasticizer and volume fraction of paste). To characterize these materials, an experimental campaign was carried out, including different types of test (water porosimetry, mercury intrusion porosimetry, desorption isotherms and gas permeability). The influence of the composition parameters on the studied durability indicators is highlighted and correlation between gas permeability and microstructural properties (total porosity and critical pore diameter) is established. Finally, a method to predict materials permeability from that of the cement paste is proposed.     

Effect of nanosilica and microsilica on microstructure and hardened properties of cement pastes and mortars

Abstract

This paper reports the effects of nanosilica (nS), microsilica (silica fume, SF) and their simultaneous use (nS+SF) on “both” the microstructure of cement pastes and the mechanical properties of mortars. After water curing at 21°C for 7, 28 and 90 days, samples with water/binder w/b ratio of 0·35 were characterised by thermogravimetric analysis, X-ray diffraction, scanning electron microscopy and compressive strength test. Single or mixed mineral additions did not generate “any” distinct hydration phases compared to the reference material without additives. A decrease in the calcium hydroxide contents in later curing ages indicated a pozzolanic effect of nS and SF. The chemical action promoted by nS together with the physical effect due to the small particle size distribution given by SF result in higher compressive strength and better hardened properties, suggesting the synergistic action of nS+SF compared with single additions.     

Estimating the permeability of cement pastes and mortars using image analysis and effective medium theory

A method to estimate permeability of cement-based materials using pore areas and perimeters from SEM images is presented. The pore structure is idealised as a cubic lattice having pores of arbitrary size. The hydraulic conductance of each pore is calculated using the hydraulic radius approximation, and a stereological factor is applied to account for the random orientation of the image plane. A ‘constriction factor’ is applied to account for variations in pore radius along the pore axis. Kirkpatrick's effective medium equation is then used to obtain an effective pore conductance, from which the macroscopic permeability is derived. The method was tested on forty-six pastes and mortars with different w/c ratio, cement, age and sand content. The permeabilities ranged from 3 × 10^? 18 m² to 5.8 × 10^? 16 m². It was found that 76% of the permeabilities were predicted to within a factor of ± 2, and 98% within a factor of ± 5 from measured values.     

The effect of waste oil-cracking catalyst on the compressive strength of cement pastes and mortars

Epcat, one of the spent fluid catalytic cracking (FCC) catalysts from oil-cracking refineries, shows pozzolanic activity. In this study, pastes and mortars with Epcat were prepared and cured, and their compressive strengths after 3, 7 and 28 curing days were measured. The water/binder (W/B) ratios were 0.2, 0.25 and 0.3, and the replacement levels of cement by Epcat were 0, 5, 10 and 15 wt.%. Proper amount of superplasticizer was added into each mix to ensure similar workability.The results indicate that the presence of Epcat would increase the compressive strength of mortars substantially, but increase the compressive strength of the related pastes only slightly. Epcat mortars with W/B=0.25 show more strength-enhancing effect than those with W/B=0.3, and this effect increases with the catalyst content. Therefore, the mix (W/B=0.25) incorporated 15% Epcat exhibits the greatest compressive strength (92.3 MPa). For mortars with W/B=0.2, the strength-enhancing effect occurs only for those containing 5% catalyst; this effect becomes unclear when mixes containing 10% Epcat or more because high dosage of superplasticizer was added in obtaining proper workability and that affects the strength development. The improvement in the mechanical properties of mortars is attributed to the increase in the hydrated cement paste itself and, more importantly, improved bonds between the cement paste and aggregate.     

Effects of aluminate and sulfate contents on the hydration and strength of Portland cement pastes and mortars

Evaluation of effects of C₃A and SO₃ contents in Portland cement on its compressive strength, hydration rate and products. Three cement samples varying in C₃A content and one varying in SO₃ content were used, hydrated initially at three temperatures. Compressive strength, bound water content, free lime content and Differential Scanning Calorimeter curves were determined during the progression of hydration. In interpretation bound water was used as a measure of quantity of binding material and Free Lime to Bound Water Ratio (FLWR) - of chemical constitution and quality.     

The effect of cement particle size distribution upon properties of pastes and mortars with low water-to-cement ratio

Technological properties of pastes and mortars (viscosity, time development of strengths) with low water-to-cement ratio (the clinker-sodium lignosulfonate-alkali carbonate or bicarbonate-H₂O system) depend upon clinker particle size distribution. These properties cannot be described explicitly as a function of specific surface measured according to Blaine. The granulometric composition of clinker particles according to idealised curves (Fuller, EMPA recommendation) represent a specific optimum state, permitting minimum viscosities to be reached at given water volume as well as maximum short-term strength values.     

The influence of fly ash cenospheres on the details of cracking in flyash-bearing cement pastes

Compact tension specimens were prepared from a cement-flyash paste with an especially-high content of cenospheres, to permit examination of the details of the influence of cenospheres on the cracking pattern obtained on loading such specimens in the SEM. It was found that even after extensive aging, the advancing crack typically went around the cenosphere-paste interface rather than cleaving through the cenosphere itself. Branching of the crack on its path around the cenosphere perimeter was commonly observed. Thus, cenospheres in flyash-cement systems appear to act as energy-dissipating inclusions in fracture and do not necessarily weaken the system.     

Polymer-Impregnated portland cement mortars

Cured precast portland cement mortars were impregnated with the monomer-initiator mixture (mmA or styrene or both and AIBN or benzoyl peroxide), polymerized, and cured and the change in compressive strength, and, in some cases, the change in tensile strength were studied. The effect of impregnation on the compressive strength and environmental stress (freezing and thawing, acid resistance, weathering effect, and sea water resistance) were studied. It was found that the incorporation of polymer into the pores of the already-set cement increased the compressive strength even after freezing and thawing, acid resistance, and sea water resistance and weathering. Among the monomers used. mmA was found to give the best properties to the mortar. Fly ash, when added in small amounts, increased the compressive strength. © 1996 John Wiley & Sons, Inc.     

Cement pastes and mortars with low water-to-cement ratio I

Clinkers of nominal Czechoslovak production were ground in presence of grinding aid (derivative of lignin) to specific surfaces 400 – 2700 m²/kg and were processed in presence of sodium lignosulfonate and alkali carbonate (resp. bicarbonate) into pastes and mortars with low water-to-cement ratio. The clinker lignosulfonate-carbonate-H₂O system (further under “A”) is qualitatively different to the clinker-gypsum-H₂O system in rheological properties and in reaching short and long-term strengths. The volume changes (A) are similar to volume changes in Portland cements. The properties of system A are affected by concentration and type of carbonate and lignosulfonate.     

Autoclaved compressed cement pastes

Cement pastes, prepared by high-pressure compaction, were autoclaved under several experimental conditions. The results obtained are compared to those for water-cured compacted pastes.     

Analysis of triethanolamine in cement pastes

Triethanolamine (TEA) in hardened cement paste has been analyzed by infrared spectrophotometry. The technique is sensitive for the qualitative detection of small amounts of TEA. However, the percent recovery is very much concentration dependent and, until the reproducibility of the method has been established, reliable quantitative data from this technique cannot be expected.     

Optimization of strength in cement pastes

It has been demonstrated that porosity is by far the dominant controlling factor limiting strength of hydrated cement paste. Mechanical means have been employed in the present study to minimize this porosity, “hot pressing” under rather modest temperatures and pressures, producing materials having very low porosity and unusually high strength. A new relationship to describe the interrelation of strength and porosity is given, and the effect of maturity of specimens, composition and microstructure are illustrated. Though theoretical density has not yet been achieved, the cement pastes have compressive strengths (as well as tensile and shear strengths) an order of magnitude higher than in normally hydrated cements, are stable and durable, and have dense interpenetrating microstructures.