Thausamine in failed cement mortars and renders from exposed brickwork

The mineral thaumasite occurs within certain cement-based building materials as a direct result of sulphate attack. It readily forms within certain types of brickwork and not only contributes to expansive cracking of the brickwork but is also accompanied by severe softening of the cement matrix. Samples were taken of both brickwork mortars and renders, some of which were still relatively sound whereas others had deteriorated into a paste. The mortar samples were investigated using a quantitative Xray diffraction technique in order to determine the amounts of thaumasite, ettringite and gypsum present. Results show that if conditions are favourable then thaumasite formation can proceed rapidly and can even result in the complete breakdown of very strong renders.     

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.     

Fracture toughness of cement paste and mortars

The effective fracture toughness of a range of cement pastes and mortars have been measured using both a notched beam and a doublt cantilever beam method. The influence of size and quality of aggregate on the effective toughness has been studied as the cracks advance. The observed values of fracture toughness are dependent on crack velocity.     

Shrinkage cracking in Roman cement pastes and mortars

Roman cements were key materials used in the architecture of the nineteenth and early twentieth centuries. Fine cracks, caused by restrained shrinkage during drying, are a distinct characteristic of all Roman cement stuccoes. Today, cracking has become an important barrier preventing broader acceptance of Roman cement as a material by the restoration and construction sector. Drying shrinkage and tensile properties of Roman cement pastes and mortars submitted to various curing and drying regimes were determined as key parameters controlling cracking. A higher volume of aggregate in the mortar mix and a moderate curing time produce optimum Roman cement mortars from the standpoint of reducing the risk of cracking. Fast drying produced significant microcracking due to moisture gradients and differential shrinkage across the specimens. Stress relaxation observed during the long-time loading of the materials reduced their vulnerability to cracking.     

Modification of cement mortars by polymer latex

The polymer systems of vinyl latex, epoxy latex/resin, and phenol formaldehyde were used to modify sand-cement mortar at room temperature. The compressive strength increased with an increase of the latex/resin concentration, with the addition of CaCl₂ or CaCO₃, and with the addition of superplasticizer. Tensile and flexural strengths also increased with polymer incorporation. The porosity of the modified mortar decreased with the addition of resin. The percentage of water absorption and acid solubility were found to decrease for the latex/resin modified samples. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1251–1257, 1997     

Hospital waste ashes in Portland cement mortars

Nowadays, most concretes incorporate mineral additions such as pozzolans, fly ash, silica fume, blast furnace slag, and calcareous filler among others. Although the technological and economical benefits were the main reasons for the use of mineral additions, the prevention of environmental contamination by means of proper waste disposal becomes a priority. The chance of incorporating hospital waste ashes in Portland cement-based materials is presented here. Ash characterization was performed by chemical analysis, X-ray diffraction, radioactive material detection, and fineness and density tests. Conduction calorimetry and setting time tests were developed on pastes including ash contents from 0% to 100%. Mortars were prepared including ash contents up to 50% of cement. The results of setting time, temperature development, flexural and compressive strengths, water absorption, density, and leachability are analyzed. Results indicate that Portland cement systems could become an alternative for the disposal of this type of ashes.     

Properties of latex blends and its modified cement mortars

In this paper, the mechanical properties of three latex blends and the mechanical properties and chloride diffusivity of the latex-modified mortars are studied. The relationships between the properties of polymer films formed from latex blends and the properties of the latex blend-modified mortars are illustrated. The test results showed that the modified mortar with the blend of styrene-acrylic ester (SAE) and styrene-butadiene rubber (SBR) showed synergistic effect; especially the flexural strength of the SAE/SBR blend-modified mortars was about 20-40% higher than that of monolatex-modified mortars. However, the vinyl chloride-vinylidene chloride copolymer (PVDC)/SBR and PVDC/SAE blends-modified mortars showed antisynergistic effect. The compressive strength of the modified mortars increased with the increasing of the tensile strength of the latex films, while the flexural strength of the modified mortars did not depend on the tensile strength of the latex films. When PVDC with the mass fraction of 0.2 or SAE copolymer emulsion with mass fraction of 0.4 was blended into SBR latex, the latex blend-modified mortars showed lower chloride diffusivity. The chloride diffusivity of the modified mortars increased approximately linear with the tensile strength of the latex blend films, and decreased with increase of the elongation at rupture of the latex blend films. When the elongation at rupture of the latex blend films increased from 200-300% to more than 800%, the chloride diffusivity of the modified mortars decreased from 10-15×10⁻¹² to 3-4×10⁻¹² m²/s.     

Effect of waste aluminosilicate material on cement hydration and properties of cement mortars

The effect of waste material (catalyst used previously in catalytic cracking of petroleum in fluidized bed—fluidized bed cracking catalyst denoted as FBCC) on cement hydration kinetics was investigated in terms of fineness of this admixture. The compressive strength and microstructure of cement mortars were also examined. Variable percentage of this aluminosilicate admixture, originating from batches of quite different grain size composition, was introduced to cement pastes. Further on, cement mortars were produced using the material of higher activity, as it has been found in admixtured cement investigations. The waste was added as cement replacement or, partially, as sand replacement. The activity of waste catalyst was strongly related to the fineness—finer grains indicate better activity. In the presence of a FBCC admixture, the Ca(OH)₂ content decrease in cement pastes due to the pozzolanic reaction is observed. The surface area of hydrated paste becomes higher and, simultaneously, the mean pore diameter decreases, as compared to reference sample, without admixture. The strength improvement is observed particularly when the aluminosilicate material is introduced as partial sand replacement.     

Differentiating seawater and groundwater sulfate attack in Portland cement mortars

The study reported in this article deals with understanding the physical, chemical and microstructural differences in sulfate attack from seawater and groundwater. Portland cement mortars were completely immersed in solutions of seawater and groundwater. Physical properties such as length, mass, and compressive strength were monitored periodically. Thermal analysis was used to study the relative amounts of phases such as ettringite, gypsum, and calcium hydroxide, and microstructural studies were conducted by scanning electron microscopy. Portland cement mortars performed better in seawater solution compared to groundwater solution. The difference in performance could be attributed to the reduction in the quantity of the expansive attack products (gypsum and ettringite). The high Cl concentration of seawater could have played an important role by binding the C₃A to form chloroaluminate compounds, such as Friedel's salt (detected in the microstructural studies), and also by lowering the expansive potential of ettringite. Furthermore, the thicker layer of brucite forming on the specimens in seawater could have afforded better protection against ingress of the solution than in groundwater.     

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.     

Comparison of natural and manufactured fine aggregates in cement mortars

The performance of cement mortars using manufactured fine aggregates produced by cone crushing or impact crushing has been compared to that of mortars prepared from a natural sand control-sample. Samples from both crusher products have been additionally subjected to classification for partial removal of fines, being also used in preparing mortars. Particle shape analyses indicated that material produced by impact crushing presented intermediate sphericity and aspect ratio, between those found in natural fine aggregate and cone-crushed material, and that the aspect ratio of the cone-crushed material increased for finer particle sizes. The unclassified impact crusher product presented the highest packing density, and mortars produced from it had comparatively low porosity and low absorptivity and the highest unconfined compressive strength. The classified product from cone crushing presented low packing density and mortars were characterized by the highest porosities, absorptivities and lowest unconfined compressive strength, probably mostly due to its poor particle shapes. Modeling of the stress-strain response with scalar damage mechanics showed that manufactured aggregate produced from classification of the cone crusher yielded a mortar with highly inelastic deformation response, whereas mortars produced from unclassified product of impact crushing showed more elastic deformation response. Results were also analyzed in light of de Larrard's Compressible Packing Model.     

The effect of fillers on strength of cement mortars

The effect of three fillers (ground limestone, dolomite and basalt) on the strength of cement mortars was studied on 1:2.75 mixes having a w/c ratio of 0.70. The filler content ranged from 10 to 40% of the cement weight and their fineness (specific surface) from 1,150 to 11,200 sq. cm per g. Results confirmed earlier conclusions that fillers effect on strength is primarily an accelerating effect on the cement hydration. The improvement in strength is essentially the same for all non-pozzolonic fillers increasing with both filler content and fineness. In part, this improvement in strength is attributable to the increase in the density of the mix (i.e. a lower air content) associated with the use of fillers. There is some indirect evidence that monocalcium carboaluminate is formed when the finest limestone filler (10,300 sq. cm per g) is used. This formation, however, apparently does not affect strength.     

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%.     

Physical and mechanical properties of styrene-butadiene rubber emulsion modified cement mortars

Polymer-modified cement mortars were prepared by varying polymer/cement mass ratio (P/C) with a constant water/cement mass ratio of 0.4. The effect of styrene-butadiene rubber (SBR) emulsion on the physical and mechanical properties of cement mortars is studied. With P/C below 10%, the toughness of the modified mortars enhances with the increase of P/C. A relationship between the physical and mechanical properties of the modified cement mortars at P/C below 10% is found; that is, the compressive strength and flexural strength of the modified mortars are directly proportional to the apparent bulk density. But when P/C is above 10%, the mechanical properties are not highly dependent on the apparent bulk density, and the flexural and compressive strength of the mortars are not improved further with more polymer. Two curing methods [wet cure: 2, 6 or 27 days immersed in 20 °C water; mixed cure: 6 days immersed in 20 °C water followed by 21 days at 20 °C and 70% relative humidity (RH)] were also evaluated in this paper. The results have shown that the mixed cure is more beneficial to the improvement of the mortar properties. A possible mechanism for polymer modification and the relationship between the physical and mechanical properties is proposed based on SEM and IR analyses. The interpenetrating structure between the polymeric phase and cement hydrates forms at a P/C of 8%, and fully develops at a P/C of 10%. The properties of the polymer-modified mortars are influenced by the polymer film, cement hydrates and the combined structure between the organic and inorganic phases.     

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.     

Sorel's cement mortars: Decay susceptibility and effect on Pentelic marble

To evaluate the decay susceptibility of mortars of magnesium oxychloride cement used for the restoration of blocks of Pentelic marble of the Athens Acropolis monuments, mineralogical and physicochemical analyses were performed on samples from different exposure conditions. The decay of mortars depends mainly on environmental influences. The observed decay forms and the responsible processes are: (1) expansion of mortars leading to break-up of the marble as a result of the carbonation of oxychloride phases; (2) disintegration of mortars exposed to washing and staining of marble due to the release of magnesium chloride; and (3) in mortars sheltered from rain action, efflorescence due to the sulfation of mortar constituents that yields hydrated sulfate salts of magnesium and calcium with different water molecules; their hydration, crystallization, and rehydration during microclimatic shifts imply volume changes and the release of corrosive solutions leading to formation of cracks and staining.     

Effects of organo-modified montmorillonite on strengths and permeability of cement mortars

Organo-modified montmorillonites (OMMT) which have been widely used in polymer/clay nano-composites are employed here as fillers and reinforcements in cement mortars. The ratio of quartz sand and cement is 2.75 while the water/cement ratios of 0.40, 0.485 and 0.55 are considered for the cement mortars we studied. Experimental results indicate that the coefficients of permeability of cement mortars could be 100 times lower if a lower dosage of OMMT micro-particles is added. At the same time, the compressive and flexural strengths of cement mortars can be even increased up to 40% and 10%, respectively. It is also found that the optimal dosage of OMMT micro-particles to give higher compressive and flexural strengths and a lower coefficient of permeability for cement mortars is less than 1%. Meanwhile, the microstructure of cement mortars is characterized by using SEM, EDS and MIP to evaluate the effects of OMMT micro-particles on the improvements of strengths and permeability of cement mortars.     

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.     

Changes of pore structure of cement mortars due to temperature

Changes of pore structure of cement mortars caused by high temperatures (up to 900 °C) as well as by extremely low temperatures (down to ? 170 °C) could be proved by means of mercury porosimetry. While high temperatures lead to an increase of the total volume of pores $\text{> 40}\text{A}\text{?}$ low temperatures do not. But in both cases a coarsening of the pore structure occurs. The usefulnes of the test method for detecting structural defects caused by a temperature treatment was shown exemplarily by establishing an experimental relation between the pore volume and the residual strength after low temperature cycles.     

Influence of inorganic pigments on the fluidity of cement mortars

Among the admixtures used for cement composites, an inorganic pigment, which contributes color to the final product, enhances the esthetic value of a building. It can be reasonably assumed that the use of inorganic pigments will increase, given the recent trend to make cities more beautiful with color. The aim of this study was to investigate the effects of inorganic pigments on the fluidity of cement mortar. For this purpose, a flow test was carried out on cement mortar mixed with inorganic pigments by changing the proportion of cement mortar, water-cement ratio, and ratio of pigment. When red and yellow pigment mortars were used, the fluidity rapidly decreased with increasing ratio of pigment. To secure an acceptable workability, the amount of mixing water had to be increased or a superplasticizer employed. When a green pigment mortar was used, however, the fluidity of the mortar recorded −2.4-6.9%, indicating almost no change in flow. When a black pigment mortar was used, the pigment had no effect on fluidity.