Pore structure and mechanical properties of cement-lime mortars

Studies focusing on materials used in Cultural Heritage conservation projects are becoming increasingly important. In this paper, the pore structure and mechanical properties of lime-cement mortars are evaluated in order to analyze their potential use, because this kind of mortar could reduce the disadvantages presented by both lime-based mortars and cement-based mortars. The microstructure of these blended mortars is studied taking into account porosity, pore size distribution and surface fractal dimension. Compressive and flexural strengths are discussed as a function of several parameters: curing time, binder composition and B/Ag (Binder/aggregate) ratio. The mechanical strength versus the deformation of the material is also evaluated, by analysis of Young's modulus, as well as the elastic and plastic zones. Unlike cement-based mortars, blended mortars with a high percentage of lime present a large plastic zone, which could be useful in the service-life of these mortars as a result of their ability to absorb strains caused by wall movements.     

Implications of unconventional setting conditions on the mechanical strength of synthetic bone grafts produced with self-hardening calcium phosphate pastes

This work presents the effects of several factors on the mechanical strength of a calcium phosphate cement (CPC) based on alpha tricalcium phosphate and correlates the results with the microstructure and percentage of conversion to hydroxyapatite. Conversion rate increased by raising the setting temperature in the studied range (4–90 °C), but the strength exhibited an increasing-decreasing trend due to changes in the morphology of hydrated crystals. Plate-like crystals were formed in the range of 22–60 °C, mechanically reinforcing the material, whereas the formation and refinement of needle-like crystals at higher setting temperature decreased the strength. Moreover, cements with dissimilar particle sizes had different optimal hydrolysis temperatures that resulted in the maximum strength. The finest powder led to higher strength at lower setting temperature due to the formation of a more compact crystal network and higher conversion. Therefore, optimization of powder particle size may allow to achieve the highest possible strength at room temperature, being beneficial for the production of the strongest pre-set CPC-based implants without the use of energy. Furthermore, the particle size can be also engineered to produce formulations that develop the highest strength at physiological temperature, with application as injectable bone grafts. The incorporation and crosslinking of gelatine further increased the mechanical strength of pre-set cements by bridging the hydroxyapatite crystals, the setting temperature showing a similar effect to that of gelatine-free cements. In contrast, moisture decreased the strength and reduced the brittleness by solvating intramolecular association between hydroxyapatite crystals and between gelatine molecules. Moreover, large cement bodies were slightly weaker than small ones, but the size effect was not statistically significant.     

Yield stress during setting of cement pastes from penetration tests

Measurements of highly visco-elastic media such as cementitious materials during hardening and setting are difficult with standard rheometers. That is why the transition between liquid and solid state of cement based materials is currently measured with standardized penetration tools such as Vicat needles, ball indentation, penetrometers and Hilti nail guns. The obtained results however depend on the measuring device and only give information in arbitrary units. Moreover, no existing theory correlates these tests together although empirical correlations between the Vicat, the Proctor or the Hilti nail gun measurements and more classical rheology can be found in literature. In the present paper, an overview of this type of test is presented. By examining in detail experimental results, elasto-plastic finite elements simulations and visco-plastic fluid dynamic simulations in the specific case of penetrometer test, it is demonstrated that there exists a systematic correlation between these test results and the yield stress of the tested material. Finally more general analytical relations for several penetration tests between yield stress of the tested material and experimental data are proposed.     

Autoclaved slag-clinker-sand pastes surface area,pore structure and compressive strength

Surface areas, total pore volumes and pore size distributions were measured from the adsorption of both nitrogen and water vapour on hydrothermally hardened slag-clinker-sand pastes. The compressive strength of the hardened specimens could be related to the pore structure and pore size distribution curves, and the interesting result obtained is that mesopores affect the compressive strength of the autoclaved pastes more than micropores, thus providing another factor to be considered in addition to the total porosity of the paste.     

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.     

High strength generation in cement pastes

Unusually high strengths have been generated in materials produced by employing “hot-pressing” techniques, and intermediate ranges of strengths have been achieved by applying high pressures at room temperature, to portland cement pastes. By pressing at ca. 250°C and 50, 000 psi strengths are as high as 95, 000 psi (compressive), and 9250 psi (indirect tensile). The hot-pressed materials are volume stable when immersed in water and subsequently evacuated. The microstructures of such materials are very compact, consisting of an intergrowth of dense hydrated cement “gel” surrounding residual unhydrated cement grain cores. The lowest porosity of the materials measured was approximately 1.8%, by far the closest approach to zero porosity or theoretical density yet achieved in cement pastes. The effect of microstructure and porosity are discussed, and high pressure techniques are compared with other methods of strength generation.     

Enhancing mechanical strength of hydrogels via IPN structure

In this investigation, polyacrylamide (PAAm) as the flexible network is introduced to enhance the mechanical strength of hyaluronic acid–gelatin (HA–Gel) hydrogels by interpenetrating polymer network (IPN). The structure, mechanical property, and rheology property of the IPN hydrogels are investigated. It is found that the compressive strength of the HA–Gel/PAAm IPN hydrogels has increased five times higher than that of HA–Gel hydrogels. Rheological test demonstrates that elastic moduli (G′) and viscous moduli (G″) of HA–Gel/PAAm IPN hydrogels increase 100 times higher than those of HA–Gel hydrogels. Moreover, the HA–Gel hydrogels are fractured under the low compressive stress, whereas HA–Gel/PAAm IPN hydrogels are not broken under the high compressive stress. It is envisioned that the IPN hydrogels will be an effective approach to enhance the mechanical strength and broaden the range of hydrogels' applications. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44503.     

Rheological properties of cement pastes: A discussion of structure formation and mechanical property development

The objective of this paper is to examine the evolution of rheological properties (e.g. yield stress) and to evaluate the use of these properties as a method to monitor structure formation and mechanical property development (i.e. setting) in cementitious materials. The authors utilize the stress growth technique to assess the development of a solid structure in cement pastes. An increase in the yield strength of the system due to cement hydration is identified to occur near the end of the dormant period as identified by chemical shrinkage. The transition from a fluid to a solid state and the development of elastic properties in the material are both noted to occur prior to the time of initial set as identified by the Vicat needle.     

Variation with solid-phase concentration of composition,structure and strength of cement pastes at high age

Portland and white cement pastes of widely-variable water/cement ratios were studied after six years' hydration by means of quantitative X-ray diffractometric analysis of the CH and C-S-H gel contents. The results obtained have led to certain conclusions on the crystallinity and possible crystal structure of the gel. It was also observed that the flexural strength was considerably reduced for the cement pastes of lowest water/cement ratios.     

Physical properties of high strength cement pastes

Recent developments in methods of processing ordinary Portland cement have shown that above average strengths (in flexure) are easily obtained without expending energy on high pressure compaction techniques. This paper reports strengths and other physical properties of both ordinary pastes and modified (macro defect free) pastes and attempts to compare and contrast the two types. The apparently greater notch sensitivity of the modified pastes is explained in terms of a reduced inherent flaw size. Optical microscopy shows that large pores are absent in the modified paste but transmission electron microscopy (TEM) shows that the fine scale microstructures of ordinary pastes and high strength pastes are very similar.     

Air entrainment in portland blast furnace slag cement pastes: Effects on strength and pore structure

The compressive strength of air entrained blast furnace slag cement pastes were compared with the neat cement pastes of the same water/cement ratios. Air entrainment leads in all cases investigated to a decrease in the degree of hydration and this effect besides the increase in the porosity could lead to a decrease in compressive strength. A third factor, namely the changes taking place in the pore size distribution of micro- and mesopores, and their bearing on the strength results at different water/cement ratios is discussed.     

Hardened portland cement pastes,modelisation of the micro-structure and evolution laws of mechanical properties II- compressive strength law

The test results reported here confirm the validity and the generality of the compressive strength law and of the model of the microstructure proposed for hardened Portland cement pastes in the first paper of this series (1).The compressive strength obeys the law $\text{R}{}_{\mathrm{c}}\text{=}\text{R}{}_{\mathrm{o}}\text{e}{}^{\mathrm{<mtext>?</mtext><mtext>An</mtext>}}\text{or}\text{R}{}_{\mathrm{c}}\text{=}\text{R}{}_{\mathrm{o}}{}_{\mathrm{e}}\text{A}\text{e}{}^{\mathrm{AK\Gamma }}$.It depends on the capillary porosity n, linked to the product KΓ and to the hydration degree by a chain of reciprocal relations.The main features of the model so confirmed are : the growth mode of the hydrated grains, the charactéristic patterns of the evolution of the capillary porosity and of the hydration degree for pastes of the first or the second group.     

Roughness of fracture surfaces and compressive strength of hydrated cement pastes

A new type of roughness number Rn_o is formulated as a possible analytical tool for surface studies using confocal microscopy. The formulation accommodates fractal dimension and both of the boundary length scales limiting the fractal region of the fracture surface under investigation. Besides the number Rn_o other roughness characteristics are discussed and their effectiveness in the field of cementitious materials is tested. 3D-surface profile SP and surface roughness SR parameters designed for surface 3D-analysis were calculated for fracture surfaces of hydrated Portland cement pastes with different values of water-to-cement ratio. The surface profile SP parameters monotonically increased with water-to-cement ratio and monotonically decreased with compressive strength. A short discussion of possible reasons for such a behavior is presented.     

Network structure and compositional effects on tensile mechanical properties of hydrophobic association hydrogels with high mechanical strength

Hydrophobic association hydrogels (HA-gels) were successfully prepared through micellar copolymerization of acrylamide (AM) and a small amount of octylphenol polyoxyethylene acrylate (OP-4-AC) in an aqueous solution containing sodium dodecyl sulfate (SDS). HA-gels exhibited excellent mechanical properties and transparency. Especially, HA-gels possessed the capability of re-forming, such as self-healing and molding. From Fourier transform infrared, swelling behavior and re-forming capability of HA-gels, the network structure was established. On the basis of the micellar copolymerization theory, the statistical molecular theory of rubber elastic, and using uniaxial stretching data, the length of the hydrophobic microblocks, the effective network chain density and the molecular weight of the chain length between cross-linking points were evaluated for all HA-gels; furthermore, they were also evaluated for the region of medium deformation by the Mooney-Rivlin theory. For HA-gels, we investigated in detail the effects of the content of compositions in the initial solution, OP-4-AC, SDS and AM, on their tensile mechanical properties on the basis of the proposed network structure. The results clearly indicate their tensile strength, fracture energy, elastic modulus, and elongation strongly depended on their composition content.     

A study on the setting characteristics of sodium silicate-activated slag pastes

In this paper, we investigate which factor for a sodium silicate-based activator influences the setting time of alkali-activated slag (AAS) paste. Several factors, such as the pH value of the activator, the alkali modulus (AM) and the alkali activator dosage, were evaluated when the liquid/slag ratio was kept constant. It was found that the pH value and the AM have a strong relationship, however, they have no distinct relationship with the setting time. The activator dosage, which is defined as the sum of the SiO₂ and Na₂O concentrations, shows a significant trend with the setting time. It was also found that the influence of SiO₂ on the setting time is more apparent than that of Na₂O. Increasing amounts of SiO₂ decrease the pH value of the activator and increasing amounts of Na₂O increase the pH value of the activator. Phosphoric acid, used as a retarder, was found to have a strong retarding effect.     

Microstructure and strength development of beta-dicalcium silicate pastes with and without admixtures

Microstructural developmentin hydrating $\text{beta}$-dicalcium silicate pastes was studied up to 188 days using scanning electron microscopy. The overall development is similar to that observed previously for tricalcium silicate pastes, but some unusual features were observed and are described. The calcium silicate hydrate, Type I, is larger and better developed than that which is formed in tricalcium silicate pastes; and, moreover, calcium hydroxide also forms larger crystals. Tensile strength development follows the same relationship to gel-space ratio found for tricalcium silicate pastes, and is unaffected by the presence of admixtures. Chloride and carbonate salts promote the formation of Type II calcium silicate hydrate.     

Investigations on the relationship between porosity,structure and strength of hydrated portland cement pastes I. Effect of porosity

In a series of cement paste specimens made with different water-cement ratios and hydrated for different times the relationship between porosity and strength was determined. For a range of porosities between 5 and 28 per cent this relationship can be best expressed in the form of a linear plot. At equal porosities strengths of specimens obtained by pressing lie distinctly below those obtained by casting.