Polymer modified mortars

Summary Aqueous polymer dispersions admixed to cement concretes or mortars affect their properties. Some research work has been made to find the optimum type and proportion of polymer dispersion admixture, and to see how concrete and mortar properties are thereby modified.

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Résumé Les émulsions de polymères dans l’eau modifient les propriétés des mortiers et bétons de ciment. On expose ici quelques recherches faites afin de déterminer la proportion optimale et le type de polymère à employer dans ces émulsions, et l’on montre comment les propriétés du mortier et du béton en sont modifiées.

     

Carbonation of calcium sulfoaluminate mortars

This study investigated potential physical and chemical parameters that could govern the carbonation rate of calcium sulfoaluminate (CSA) mortars and endeavored to elucidate the microstructural and chemical factors that govern CSA cement's carbonation rate. Experiments included: water absorption, oxygen diffusion, mercury intrusion porosimetry, quantitative X-ray diffraction, thermogravimetric analysis, accelerated carbonation, compression and flexure tests. Additionally, the carbonation process was investigated using thermodynamic modeling. The results show that CSA mortars carbonate much faster than Portland cement mortars and at approximately the same rate as calcium aluminate cement mortars. Additionally, CSA mortars carbonate slower with decreasing w/c, and the anhydrite content of the CSA mortars strongly affects the ye'elimite reaction kinetics which plays an important role in imparting carbonation resistance in CSA mortars. Finally, calcium sulfate additions to CSA clinker to produce CSA cement dilutes the clinker content and reduces the amount of CO₂ that the CSA cement can ultimately bind.     

Sulphate resistance of lime-based barium mortars

This work studies the effect of barium hydroxide on the sulphate-resistance of lime-based mortars when used as an additive material to the lime binder. The overall aim of the work is to study the potential of barium hydroxide in producing a mixed binder with lime, which is able to fix the sulphate ions, block the diffusion of sulphate solutions and therefore, reduce the degradation rate of mixtures.The durability of different mixtures was studied through sulphate salt crystallisation and acid rain simulation tests, along with the characterisation of their pore-space properties. X-ray diffraction (XRD), scanning electron microscopy (SEM/EDX) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) were used for materials characterisation. Experimental results proved that barium hydroxide increases the durability of hardened mixtures against sulphate attack, without affecting the microstructure characteristics.     

Formation of thaumasite in carbonated mortars

The main objective of this study was to investigate the formation of thaumasite in carbonated Portland cement mortars. Another important purpose was to study the role that ettringite (AFt) plays in the process of formation of Th and its influence in the deterioration of the mortars.

Work was carried out with mortar prisms, elaborated using two different cements: one of them with low Al₂O₃ content (exempt of C₃A) and the other one with high Al₂O₃ content. Additions of different amounts of gypsum and/or calcium carbonate were also done. After three months of curing at 21 °C and 100% RH mortars were submitted to an accelerated carbonation process until total transformation of Ca(OH)₂ into CaCO₃.

Then mortar prisms were partially immersed in distilled water and kept at temperature ranging from 0 to 5 °C for one year. A mineralogical, micro-structural and physical characterisation was carried out on samples at different ages.

The thaumasite formation rate was much lower in those mortars elaborated with the cement exempt of C₃A than in those mortars made with the cement having high proportion of C₃A when they are conserved in water. The study of transversal sections of prisms reveals the presence of a white expansive product (thaumasite) not always homogeneously distributed.     

Wet deposition studies of hydraulic mortars

Cement mortars and lime-pozzolan mortars have been exposed to wet/drying cycling of ‘acid rain’ solution, using realistic presentation rates, to simulate outdoor conditions. Cement type, water/cement ratio, and curing temperature were also examined. Form the pH of run-off solution, it is evident that reaction between the mortar and the ‘acid rain’ proceeds over the exposure period, with significant weight increases for the cement mortars and little influence of curing temperature for lime-pozzolan mortars. Exposure of the cement mortar gives to significant calcium loss to run-off and is also associated with retained souuble stalts; these results from the presence of free hydrated lime in the mortar. Conversely, lime-pozzolan mortars, associated with a clacium carbonate compounds, reveal comparatively reduced calcium loss,i.e by about one-tenth that of cement mortar.     

Fine ceramics replacing cement in mortars Partial replacement of cement with fine ceramics in rendering mortars

The main objective of the use of very fine red clay ceramic waste in rendering mortars is the reduction in the primary binder (cement) content made possible by the potential pozzolanic effect of this recycled material, with very clear environmental benefits in the reduction of overly-high energy consuming cement and economic benefits in the potential reduction of the cost of mortars. This paper presents an experimental program where ceramic waste crushed to very fine particles was used to partially replace cement in mortars manufacturing, acting as a secondary binder. A large number of tests of the most relevant characteristics of various mortars in which this principle was applied were performed and compared with the results of the same tests in a reference rendering mortar with no ceramic fines (and no reduction of the cement content). The results are most promising both from a performance-based and an environmental point of view.     

Carbonation process of alkali-activated slag mortars

This study analyzes the behaviour of waterglass- or NaOH-activated slag mortars after carbonation. The effect of a superplasticizer based on vinyl copolymer and shrinkage reducing polypropylenglycol derivative admixtures on that process was also examined. The same tests were run on cement mortars for reference purposes. The mortars were carbonated in a chamber ensuring CO₂ saturation for four and eight months, after which ages the samples were tested for mechanical strength; mercury porosimetry and mineralogical (XRD, FTIR) and microstructural characterization (SEM/EDX) were also conducted. The results obtained indicate that alkali-activated slag mortars were more intensely and deeply carbonated than Portland cement mortars. Carbonation took place directly on the gel, causing decalcification. When waterglass was the alkaline activator used, carbonation caused a loss of cohesion in the matrix and an important increase in porosity and decrease in mechanical strength. When a NaOH solution was used as the alkali activator, carbonation enhanced mortar compaction and increased mechanical strength. Finally, in waterglass-activated slag mortars, the inclusion of organic admixtures had no effect either on their behaviour after carbonation or the nature of the reaction products.     

Early age fracture properties of microstructurally-designed mortars

This paper compares the fracture properties as well as crack initiation and propagation of real and equivalent mortars. The development of the elastic modulus, tensile strength, and fracture energy at different hydration stages were determined by inverse analysis of load-displacement curves obtained by the compact tension test (CTT). Further, the impact of the moisture content on the aforementioned material properties was also tested on oven-dried equivalent mortars. Digital image correlation (DIC) was used to follow the crack initiation and propagation.The elastic modulus, tensile strength, and fracture energy support the validity of the equivalent mortars approach. The load-displacement curves obtained by the CTT were also compared to those simulated by finite element method showing excellent correlations. DIC revealed the formation of similar crack patterns at comparable load levels between the two mortars. At early age, the moisture content has a considerable influence on the tensile strength and the fracture energy.     

Microstructure of autoclaved refined wood-fibre cement mortars

Autoclaved cement mortars reinforced with refined wood-pulp fibres have been tested in flexure and the fracture surfaces examined by scanning electron microscopy. The observations indicate that failure occurs by the dual mechanism of fibre fracture and fibre pull-out, and that the interfacial bonds between the fibres and the cement matrix are stronger than had been previously considered.     

Study of cement mortars modified by emulsifier-free latexes

Cationic copolymer latex of vinyl acetate (VA) with methacryloxyethyl trimethylammonium chloride (MTC) and anionic copolymer latex of VA with sodium acrylate (SA) were prepared via emulsifier-free emulsion copolymerization. Modifications of cement mortars by these copolymer latexes were studied, and the results were compared with mortars modified by homopolymer of VA (PVA) as well as with blank mortars. The experimental results showed that flow of the mortars modified by PVA homopolymer and P(VA–MTC) copolymer latexes were increased compared with the blank, whereas a decrease was observed for that modified by P(VA–SA) anionic copolymer latex; a volume expansion was detected in polymer modified mortars by PVA and P(VA–MTC), while no volume expansion was observed in the anionic P(VA–SA) modified mortars; the flexural strength of mortars modified by P(VA–MTC) increased by 79.5% compared to unmodified mortars after 5 day water curing plus 21 day air curing. This value reached 95% when the curing process was followed by a supplementary 24 h vacuum curing. These polymer modified mortars were then subjected to hydration analysis through X-Ray diffraction (XRD). Mechanisms of the mortar modification are discussed based on the characterization results.     

Properties of mortars produced with reactivated cementitious materials

The production of reactivated cementitious materials is an option for the recycling of hydrated-cement-rich fines discarded during recycled aggregate production. Reactivation is based on a thermal process where calcium silicate hydrates present in the fines decompose forming new hydraulic compounds. In the reported study, materials reactivated at temperatures between 660 °C and 940 °C were characterized using X-Ray diffraction and particle size analysis, and evaluated as binders using a central composite experiment to model the effects of reactivation temperature and reactivated material substitution level on the flowability, compressive strength and expansion of mortar mixtures. Reactivation temperature effects correlated with the relative concentration of reactive phases, particularly a stabilized form of alpha'-C₂S identified in the materials. Substitution effects depended on the supplementary material tested, and lacked significant interaction with reactivation temperature. In the region explored, mortars based on materials produced at 800 °C, 40% substituted by silica fume, achieved highest strength but lowest flowability.     

Air-cured woodpulp,fibre/cement mortars

The mechanical properties of cement mortars reinforced with kraft wood-pulp fibres have been studied. The composites were air-cured for up to 450 days. Although the fibre mortars have lower flexural strength values than either autoclaved fibre mortars or air-cured fibre cements, they do have improved values of fracture toughness of approximately 3kJ m⁻² at fibre loadings of 8% fibre by mass. This would suggest there may be a use for such products in areas where the materials are subjected to impact, eg, renders on walls or products which are handled frequently such as outdoor gardenware.     

Impact response of lightweight mortars containing expanded perlite

This paper describes the mechanical response of lightweight mortars subjected to impact loading in flexure. Expanded perlite aggregate with a bulk density of 64 kg/m³ was used at between 0 and 8 times by volume of Portland cement to yield a range of mortars with density between 1000 and 2000 kg/m³. Some specimens were reinforced with a polypropylene microfibre at 0.1% volume fraction and the dynamic fracture toughness was evaluated by means of an instrumented drop-weight impact system. Companion tests were carried out in compression under quasi-static loading to standardise the mixes. The compressive strength and elastic modulus scale as the cube of the relative density, defined as the ratio of the density of the mortar to that of Portland cement paste. Whereas the flexural strength and fracture toughness were both linearly proportional to the relative density of the mortar under quasi-static loading, there was an increase in their sensitivity to relative density at higher loading rates. Contrary to what is seen in regular concrete, fibre reinforcement led to an increase in the stress-rate sensitivity of flexural strength in lightweight mortars. For the same impact velocity, the stress-rates experienced by a specimen was strongly influenced by its density. While the stress-rate sensitivity of flexural strength dropped with a decrease in the mix density, that of the fracture toughness was consistently higher for the lighter mixes.     

About drying effects and poro-mechanical behaviour of mortars

The drying of mortar and concrete generally affects their elastic properties and strength. Such property variations are of interest, since they affect the durability of structures designed with these materials. Previous studies, conducted on two mortars with different W/C ratio, showed that drying such material led to decreases in their elastic modulus and their Poisson ratio. Although generally attributed to micro-cracking, these decreases are nevertheless consistent with simple poro-mechanical effects i.e. transition from undrained behaviour toward drained behaviour. The main objective of this experimental study was therefore to state whether or not pore fluid pressure played a direct role in the variation of elastic properties measured on saturated or dried material. Poro-mechanical measurements of drained and undrained bulk moduli, Biot and Skempton coefficients were carried out. They actually revealed that poro-mechanical coupling effects could explain a part of the differences observed between elastic properties. Ethanol was chosen as the pore fluid to avoid possible unexpected chemical effects on the materials tested (often observed with water). To conclude about the saturation effects, a complementary experimental procedure was conducted in two steps: compression tests were performed on previously dried samples, which were then re-saturated with ethanol before being mechanically tested again. The results obtained are unambiguous and showed that poro-mechanical effects are not involved in the change in elastic properties. On an other hand, the major roles played by microcracks induced by drying and by capillary suction are highlighted: (a) The decrease in elastic properties, observed as drying occurs, is shown to be due to induced micro-cracking; (b) the increase in capillary pressure, brought about by drying, leads to an increase in the mortar failure strength.     

Impact of water-soluble cellulose ethers on polymer-modified mortars

Cellulose ethers (CEs) are employed in many polymer-modified mortars, such as cement renders, masonry mortars, tile adhesives, repair mortars, skim coats, and self-levelling mortars. The addition of CEs to mortars causes the retardation of cement hydration and modifies the microstructural characteristics and the properties of these mortars. The present work attempts to provide a comprehensive review of the current state of knowledge on the effects of CEs and critically identifies gaps in the knowledge. A fundamental scientific understanding concerning the chemistry and hydration of cement, chemical natures, and relevant properties of CEs are discussed. The behaviours and mechanisms of CE adsorption on cement are assessed. The influences of CEs on the kinetics of cement hydration, mechanisms of retardation, and microstructural evolution of the mortars also are reviewed. Finally, the impact of CEs on the properties of fresh and hardened mortars as well as the approaches used to mitigate the negative impacts of CEs are discussed.     

Quantitative analysis of historical mortars using optical microscopy

As a part of the activities of the RILEM TC 167-COM committee two methods of quantitative microscopy have been developed for analysing historical mortars. This paper gives background information for the application of these methods. The methods concern the determination of mix proportions and aggregate size distribution using microscopical methods. A method is also described for correcting determinations of mix proportions based on chemical analysis of acid-soluble calcium oxide for the presence of carbonate in the aggregate.

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Résumé Dans le cadre des activités de la Commission RILEM TC 167-COM, deux méthodes de microscopie quantitative ont été développées pour l’analyse des mortiers historiques. Ce document donne des informations sur les antécédents de l’application de ces méthodes. Celles-ci concernent la détermination des proportions de mélanges et la répartition des tailles des appareils en utilisant la méthode microscopique. Une méthode est également décrite pour la correction des déterminations des proportions de mélanges, à partir de l’analyse chimique de l’oxyde de calcium soluble dans l’acide pour déterminer la présence de carbonate dans l’appareil.

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TC MEMBERSHIP: Chairman: Caspar Groot, the Netherlands.Secretary: Geoff Ashall, UK.Members: Giulia Baronio, Italy; Peter Bartos, UK; Luigia Binda, Italy; Kristof Callebaut, Jan Elsen, Belgium; Rob van Hees, the Netherlands; John Hughes, UK; Loek van der Klugt, the Netherlands; Jan Erik Lindqvist, Sweden; Elisabeth Marie-Victorie, France; Bernhard Middendorf, Germany; Ioanna Papayianni, Greece; Margaret Thomson, USA; Eleni-Eva Toumbakari, Greece; Alf Waldum, Norway.