Showing papers on "Ettringite published in 2014"

Influence of limestone and anhydrite on the hydration of Portland cements

Abstract: The addition of CaCO 3 and CaSO 4 to Portland cement clinker influences the hydration and the strength development An increase of the CaSO 4 content accelerates alite reaction during the first days and results in the formation of more ettringite, thus in a higher early compressive strength The late compressive strength is decreased in Portland cements containing higher quantities of CaSO 4 The reduced late compressive strength seems to be related to an increase of the S/Si and Ca/Si content in the C–S–H The presence of calcite leads to the formation of hemicarbonate and monocarbonate thus indirectly to more ettringite Only a relatively small quantity of calcite reacts to form monocarbonate or hemicarbonate in Portland cement Although hemicarbonate is thermodynamically less stable than monocarbonate, hemicarbonate formation is kinetically favored Monocarbonate is present only after 1 week and longer independent of the quantity of calcite available and the content of sulphate in the cement

Resistance of concrete and mortar against combined attack of chloride and sodium sulphate

Abstract: Marine environments are typically aggressive to concrete structures, since sea water contains high concentrations of chlorides and sulphates. To improve predictions of concrete durability within such environments, it is important to understand the attack mechanisms of these ions in combination. In this research, the reciprocal influence of Cl− and SO42− was investigated for four mixtures, namely with Ordinary Portland Cement, High Sulphate Resistant cement, and with Blast-Furnace Slag (50% and 70% cement replacement). Chloride penetration depths and diffusion coefficients were measured to investigate the influence of SO42− on Cl− attack. Besides, length and mass change measurements were performed to examine the influence of Cl− on SO42− attack. Since the formation of ettringite, gypsum and Friedel’s salt plays an important role, XRD-analyses were done additionally. It can be concluded that chloride penetration increases when the sulphate content increases at short immersion periods, except for HSR concrete. Concerning the sulphate attack, the presence of chlorides has a mitigating effect.

A hydration study of various calcium sulfoaluminate cements

Abstract: The present work studies the hydration process and microstructural features of five calcium sulfoaluminate (CSA) cements and a ternary mixture including also ordinary Portland cement (OPC). The pastes were studied with simultaneous differential thermal-thermogravimetric (DTA-TG) analysis, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and expansion/shrinkage tests. The DTA-TG analysis confirmed the role of the hydration reactions involving the main CSA clinker constituent, tetracalcium trialuminate sulfate, which produced (i) ettringite when combined with lime and calcium sulfate, (ii) ettringite and aluminum hydroxide in the presence of calcium sulfate alone, and (iii) monosulfate and aluminum hydroxide in the absence of both lime and calcium sulfate. The MIP and SEM were able to discriminate between expansive (ternary mixture and CSA cement containing 50% gypsum) and non-expansive cements. Expansive cement pastes had (i) a nearly unimodal pore size distribution shifted toward higher radii and (ii) ettringite crystals smaller in size during the first day of curing. In a SEM image of a hardened paste of the CSA cement containing 50% gypsum, a stellate ettringite cluster was observed.

Changes in the phase assemblage of concrete exposed to sea water

Abstract: In the present study the phase changes in an ordinary concrete standing for 10 years in a tidal zone were investigated with a range of techniques. From the exposed surface and inwards different zones had formed. SEM–EDS analysis of a polished section of the surface near region, showed a Mg rich layer with a thickness of 10–20 μm, as well as the filling of cracks leading from the surface with a Mg rich phase, most likely brucite. In the outermost 2 mm, an increase in the calcium carbonate content was identified by XRD and TGA. In the same zone SEM–EDS analyses indicated enrichment in ettringite and thaumasite. In the first 20 mm calcium hydroxide leaching was observed using XRD and TGA. Chlorides appeared to have penetrated up to approx. 70 mm from the surface. Part of the chlorides were found to be bound in alumina containing phases and in the C–S–H by SEM–EDS. These experimentally observed phase changes generally agreed with the predictions of a thermodynamic model.

In situ interaction between different concretes and Opalinus Clay

Abstract: Interactions between cementitious materials and claystone are driven by chemical gradients in pore water and might lead to mineralogical modifications in both materials. In the context of a radioactive waste repository, this alteration might influence safety-relevant clay properties like swelling pressure, permeability, or specific retention. In this study, interfaces of Opalinus Clay, a potential host-rock in Switzerland, and three concrete formulations emplaced in the Cement–Clay Interaction (CI) Experiment at the Mont Terri Underground Laboratory (St. Ursanne, Switzerland) were analysed after 2.2 years of interaction. Sampling techniques with interface stabilisation followed by inclined intersection drilling were developed. Element distribution maps of the concrete–clay interfaces show complex zonations like sulphur enrichment, zones depleted in Ca but enriched in Mg, strong Mg enrichment adjacent to the interface, or carbonation. Consistently, the carbonated zone shows a reduced porosity. Properties of the complex zonation strongly depend on cement properties like water content and pH (ordinary Portland cement vs. low-pH cement). An increased Ca or Mg content in the first 100 μm next to the interface is observed in Opalinus Clay. The cation occupancy of clay exchanger phases next to the ordinary Portland cement interface is depleted in Mg, but enriched in Na, whereas porosity shows no changes at all. The current data suggests migration of CO 2 / HCO 3 - , SO 4 2 - , and Mg species from clay into cement. pH decrease in the cement next to the interface leads to instability of ettringite, and the sulphate liberated diffuses towards higher pH regions (away from the interface), where additional ettringite can form.

Hydration of a silica fume blended low-alkali shotcrete cement

Abstract: Ettringite and C–S–H are the main hydrates formed during the hydration of the low-alkali cement “ESDRED” consisting of 60% CEM I, 40% microsilica and 4.8% set accelerator. Small quantities of portlandite and hemicarbonate present as intermediate phases destabilise within a few weeks. The use of a set accelerator leads to massive ettringite precipitation, a moderate decalcification of C–S–H and reduction of pH due to presence of dissolved formate. The slow reaction of the silica fume during hydration decalcifies the C–S–H and decreases the alkali concentration to 30 mM and the pH value of the pore solution to 11.5 after 1 year and longer. The further reaction of the silica fume is expected to be slow and to result in a decrease of pH to 11. Further, the destabilisation of ettringite to thaumasite is expected. The long-term stability of C–S–H and the pH of approximately 11 make ESDRED a good candidate for usage in contact with the clay-based barriers of a repository for radioactive waste.

Coupled chemo-transport-mechanical modelling and numerical simulation of external sulfate attack in mortar

Abstract: We develop and apply in this study a chemo-transport-mechanical model for simulating the external sulfate attacks in Portland (CEM I) cement pastes and mortars. Basically, this degradation consists in the simultaneous decalcification of the hydrated phases resulting from leaching processes, and the migration of sulfate ions within the material and its subsequent interactions with these phases. The sulfate uptake leads generally to ettringite precipitation mainly from monosulfate, which in turn may produce intense macroscopic expansions and cracking. In our approach, crystallization pressures arising from the restrained growth of monosulfate crystals due to the confinement of the surrounding C–S–H matrix are assumed to initiate the observed macroscopic expansions. A macroscopic strain tensor evaluated from the volume fraction of supplementary precipitated ettringite is further introduced in the mechanical behavior law for explicitly reproducing the macroscopic expansions. Analytical homogenization schemes are applied to estimate both mechanical and diffusive properties from the local volume fraction of solid phases. The numerical platform Alliances is then used for solving both reactive transport and mechanical coupled problems, and is applied to the simulation of laboratory tests consisting in prismatic mortar specimens immersed in solutions containing sodium sulfate and subjected to free expansions. Comparison of the numerical results with experimental ones in terms of phase assemblage profiles, evolutions of mass changes and expansions shows a correct agreement. Finally, the extension of the model towards cases of restrained displacement conditions is discussed and some modifications regarding the kinetics of ettringite precipitation are proposed for such situations.

Gypsum Induced Strength Behaviour of Fly Ash-Lime Stabilized Expansive Soil

Abstract: Physical and engineering properties of soil are improved with various binders and binder combinations. Fly ash and lime are commonly used to improve the properties of expansive soils. An attempt has been made, in this paper, to examine the role of gypsum on the physical and strength behaviour of fly ash-lime stabilized soil. The change in strength behaviour is studied at different curing periods up to 90 days, and the mechanism is elucidated through pH, mineralogical, microstructural and chemical composition study. The strength of soil-fly ash mixture has improved marginally with the addition of lime up to 4 % lime and with curing period for 28 day. Significant increase in strength has been observed with 6 % lime and enhanced significantly after curing for 90 days. The variations in the strength of soil with curing period is due to cation exchange and flocculation initially, and binding of particles with cementitious compounds formed after curing. With addition of 1 % gypsum to soil-fly ash-lime, the strength gain is accelerated as seen at 14 day curing. The accelerated strength early is due to formation of compacted structure with growth of ettringite needles within voids. However, strength at curing for 28 day has been declined due to annoyance of clay matrix with the increase in size of ettringite needle; and again increased after curing for 90 days. The rearrangement of clay matrix and suppression of sulphate effects with formation of cementitious compounds are observed and found to be the main responsible factors for strength recovered.

Resistance of MgO–GGBS and CS–GGBS stabilised marine soft clays to sodium sulfate attack

Abstract: Reactive magnesia (MgO) and carbide slag (CS) were used to activate ground granulated blastfurnace slag (GGBS) to stabilise a marine soft clay, and the stabilised clays were subjected to accelerated sodium sulfate attack. The results indicated MgO–GGBS stabilised clay was nearly inert to this sodium sulfate solution. The resistance of CS–GGBS stabilised clay to sodium sulfate attack was lower than that of MgO–GGBS stabilised clay, but higher than that of Portland cement stabilised clay. Unlike CS–GGBS stabilised clay, there was no calcium aluminate hydrate or alumino-ferrite monosulfate produced in MgO–GGBS stabilised clay, and hence no ettringite, which would cause deterioration, formed when exposed to sodium sulfate.

Investigation of properties of fluorogypsum-slag composite binders – Hydration, strength and microstructure

Abstract: A composite binder of high strength and low water absorption has been developed using industrial by-products fluorogypsum, granulated blast furnace slag and Portland cement. The development of strength in the binder at an early age is attributed to the conversion of anhydrite into gypsum and at later age is due to the formation of ettringite and tobermorite, as a reaction of slag with lime produced during the hydration of cement. These cementitious phases fill in pores and voids of the hydrating gypsum crystals to form a dense and compact structure of low porosity and low pore volume. The reaction products formed during the hydration period were confirmed by scanning electron microscopy and X-ray diffraction. The reduction in porosity and low pore volume of binders, as studied by mercury intrusion porosimetry, are responsible for attainment of high strength and better stability towards water in composite binders than the conventional gypsum plaster.

The Early Age Hydration Reactions of a Hybrid Cement Containing a Very High Content of Coal Bottom Ash

Abstract: A study examining the complex hydration chemistry of a hybrid alkaline cement containing a high content of coal bottom ash (BA) (>70%) and a low content of portland cement clinker in the presence of an alkaline activator is presented. The use of a water reducing additive was found to significantly delay the overall hydration process, allowing an opportunity to more clearly distinguish the hydration reactions that take place. The results presented showed that both the cement clinker phases and the ash glassy phases are highly reactive for the first 3 d of hydration. In situ formed reaction products portlandite and gypsum were shown to be metastable and had disappeared within 3 d of hydration. Ettringite stability was limited in the hybrid system but unlike gypsum and portlandite, remained detectable for the first 3 d of hydration at least. SEM-EDX and subtracted Fourier transform infrared evidence suggest the development of three different gel bond environments, tentatively attributed to C–(A)–S–H, C–A–S–H, and (N,C)–A–S–H type gels.