When concrete is exposed to sea water, it has been observed that the composition of the outer most millimeters of the concrete is considerably altered compared to the composition of the bulk ...concrete. The limited size of this zone complicates the investigation of the phases formed. This paper presents a new experimental set-up in which hydrated cement paste is exposed to sea water allowing a detailed investigation of the phase changes observed in that zone on a bulk material. The paste was characterized before and after sea water exposure by XRD, DTA/TG, ICP-MS and SEM-EDS. In the exposed sample, calcium carbonate and calcium sulfate had formed and calcium hydroxide was depleted. Two types of agglomerates of hydrated cement paste were observed. One type consisted of decalcified C–S–H and a combination of ettringite and Cl-AFm phases. The other type consisted mainly of M–S–H. These findings are in line with the observations on long term marine exposed samples in which the formation of a magnesium rich phase at the concrete surface is detected, followed by a sulfate and chloride enriched zone. The knowledge of these phase changes are important to obtain a better understanding of concrete deterioration in marine environment.
The effect of minor additions of limestone powder on the properties of fly ash blended cements was investigated in this study using isothermal calorimetry, thermogravimetry (TGA), X-ray diffraction ...(XRD), scanning electron microscopy (SEM) techniques, and pore solution analysis. The presence of limestone powder led to the formation of hemi- and monocarbonate and to a stabilisation of ettringite compared to the limestone-free cements, where a part of the ettringite converted to monosulphate. Thus, the presence of 5% of limestone led to an increase of the volume of the hydrates, as visible in the increase in chemical shrinkage, and an increase in compressive strength. This effect was amplified for the fly ash/limestone blended cements due to the additional alumina provided by the fly ash reaction.
The microstructural changes of paste, mortar and concrete based on Portland and fly ash containing Portland composite cements caused by carbonation have been studied. The objective was to find out ...why fly ash containing mortars carbonate with double rate of the neat Portland cement mortars. The reason is partly because they contain less calcium containing species prone to carbonation, but mainly because of their different hydrate assemblage: 1) Less calcium hydroxide that gives a volume increase upon carbonation. 2) More C-S-H with lower Ca/Si that might give an overall shrinkage upon carbonation. 3) More AFt and AFm phases that yield a substantial volume decrease per mole upon carbonation since their crystal water goes back to liquid form. The third microstructure difference is thought to be the dominating reason for coarser pores in the carbonated zone of CEM II/B-V compared to CEM I resulting in a faster carbonation rate.
Well hydrated cement paste was exposed to MgCl2, CaCl2 and NaCl solutions at 20°C. The chloride binding isotherms for free chloride concentrations ranging up to 1.5mol/l were determined ...experimentally. More chlorides were found to be bound when the associated cation was Mg2+ or Ca2+ compared to Na+. The chloride binding capacity of the paste appeared to be related to the pH of the exposure solution. In order to explain the cation dependency of the chloride binding a selection of samples was investigated in detail using experimental techniques such as TG, XRD and SEM–EDS to identify the phases binding the chlorides. The experimentally obtained data were compared with the calculations of a thermodynamic model, GEMS. It was concluded that the measured change in chloride binding depending on the cation was mainly governed by the pH of the exposure solution and thereby the binding capacity of the C-S-H.
The interaction between limestone powder and fly ash in ternary composite cement is investigated. Limestone powder interacts with the AFm and AFt hydration phases, leading to the formation of ...carboaluminates at the expense of monosulphate and thereby stabilizing the ettringite. The effect of limestone powder on OPC may be restricted due to the limited amount of aluminate hydrates formed by the hydration of OPC. The additional aluminates brought into the system by fly ash during its pozzolanic reaction amplify the mentioned effect of limestone powder. This synergistic effect between limestone powder and fly ash in ternary cements is confirmed in this study and it translates to improved mechanical properties that persist over time.
The effect of a softwood calcium lignosulfonate, LSs, on the ettringite formed in cement paste was investigated. Two Portland cements, mainly differing in surface area and C3A content, were used. The ...effect of LSs addition time was studied, by adding either the LSs immediately with the mixing water or after 10 min of hydration. After 30 min of hydration of both cement pastes, the immediate addition of LSs caused the formation of numerous small ettringite crystals. The ettringite crystals had similar shape in pastes with and without LSs addition: cubic or cuboidal shape with length between 0.1 and 0.4 μm. These small particles caused an increase in surface area, which in turn increased the LSs adsorption by the cement paste. This could potentially lead to incompatibility issues between cement and plasticizer.
The effect of softwood calcium lignosulfonate, LSs, on the rheology and setting time of cement paste has been investigated. Two Portland cements with different surface area and C3A content were used. ...The lignosulfonate was added either immediately with the mixing water or delayed after 10min of hydration. The cement pastes were characterized in terms of specific surface, rheology and heat of hydration. Extracted pore solutions were analysed for free lignosulfonate concentration and for changes in elemental composition. Immediate addition of LSs increased the specific surface, but not delayed addition. Correlations were found between rheology and surface coverage by LSs, as determined by adsorption isotherms, and between the setting time and the amount of free LSs in the pore solution. An increased setting retardation upon delayed addition related to an increased concentration of Al in the pore solution.
The aim of this paper is to assess the mechanisms of consumption of softwood calcium lignosulfonate (LSs) by cement paste. The LSs consumption by two different cements (CX and ANL) and two reference ...materials (CaCO3 and Ca(OH)2) was investigated, either by adding the LSs immediately with the mixing water (IA) or after 10min of hydration (DA). For IA, the increase in LSs dosage caused additional ettringite formation and an increase in particle surface area. This was not observed for DA. Since no AFm phase could be detected, intercalation in AFm seemed not to occur for the investigated materials. The main mechanism of LSs consumption for CX cement (both for IA and DA) and for ANL cement (only for DA) appeared to be monolayer adsorption. For IA, the amount of consumed LSs could not be ascribed exclusively to monolayer surface adsorption and other LSs consumption mechanisms might play a role.
The effect of the curing temperature (5, 20 and 40°C) on the degree of hydration, amount of bound water and calcium hydroxide, porosity and the development of mechanical properties was investigated ...on pastes and mortars prepared with fly ash (FA)–limestone (L) Portland composite cements. Increasing the curing temperature for ordinary Portland cement (OPC) leads to a more inhomogeneous distribution of hydration products, resulting in an increased coarse porosity and therefore a lower compressive strength after 7 days and longer. In contrast, the FA containing mortars showed higher compressive strength with increasing curing temperature up to 90 days. The reaction of the FA is increased at 40°C and strongly retarded at 5°C. At 20 and 40°C, FA reduces the porosity at later ages. The replacement of 5% of the OPC or FA by L powder did not impair the strength at 5 and 20°C, but lowered strength slightly at 40°C for the FA blended cements. The porosity appears to be the dominating factor regarding the compressive strength, independent of whether part of the OPC is replaced by FA and L powder or not.
The dispersing effectiveness of five commercial plasticizers; lignosulfonate (LS), naphthalene sulphonate–formaldehyde polycondensate (NSF) and three polycarboxylate ethers (PCEs) were quantitatively ...investigated in blended cements where ordinary portland cement (OPC) was partly replaced by fly ash (FA) up to 60%. The capacity of the plasticizers in FA blended cements mimics that in the OPC system. At low replacement amounts, FA acts mainly as filler and does not impart any significant effect on the plasticizers. From 40% FA loading, PCEs showed decreased performance compared to NSF and LS relative to that in OPC systems, attributing to higher affinity of the latter polymers for cement clinkers. Retardation by plasticizers is more pronounced at higher FA contents due to lower adsorption on FA than on cement grains. Performance of plasticizers in industrial FA blended cements mimics that in OPC due to its relative higher surface area to compensate otherwise lower early strength.
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