OPC Paste Research Articles

Evolution of HTO and 36Cl− diffusion through a reacting cement-clay interface (OPC paste-Na montmorillonite) over a time of six years

Cement and clays are proposed as sealing materials in underground repositories for radioactive waste. When cement and clay come into contact, chemical gradients between their very different porewater compositions lead to diffusive exchange of solutes, which can result in mineral transformations and alterations of transport properties at the materials interface. Small samples of cement-clay interfaces were prepared and let react over a period of six years. During this time, the changes in transport properties of the samples were periodically monitored by means of through-diffusion experiments. This technique allows studying the evolution of the diffusive flux across the reacting interface and characterizing the variation of the corresponding effective diffusion coefficient (De) over time. All experiments were performed on samples consisting of a hardened high porosity OPC paste and Na-montmorillonite. These model materials were chosen in order to simplify the mineralogy of the system. The experiments allowed obtaining relevant information regarding the development of the diffusive properties and the reactivity of such a cement-clay interface system. HTO and 36Cl− were used as tracers to study the evolution of both the total and the anion accessible porosity. After six years of reaction a considerable reduction of the flux for both HTO and 36Cl− was observed. The flux of HTO did, however, not approach zero, which means that connected porosity for diffusive transport of water is still available. The periodic monitoring of the sample evolution showed a strong reduction of the effective diffusion coefficient De of the samples within the first 1.5 years of the experiment and a less prominent decrease in the period between 1.5 and 6 years. The De of 36Cl− showed a stronger reduction compared to that of HTO; for some cells no chloride flux at all could be measured anymore at t > ~4 a. Using additional information on the extension of porosity changes from a neutron imaging study performed in parallel on the same samples, the diffusive properties of each component of the interface, or of a clay zone with reduced porosity could be estimated. For HTO the relation between the evolution of De and of the porosity in the clay part can be well described with Archie's empirical law. For chloride large uncertainties regarding the accessible porosity do not allow a precise correlation. Whether a complete porosity clogging will take place or some fraction of connected pore space will persist in the sample over reaction times ≫ than 6 years remains an open question.

Resistance to thaumasite sulfate attack in BaCO3-bearing cement pastes and mortars

Recent studies on the harmful effects of sulfates on OPC have shown that these salts can be immobilised by conversion to insoluble BaSO4. The response of blended OPC-BaCO3 cement to thaumasite sulfate attack is still poorly understood, however. This study explored the effect of BaCO3 on sulfate resistance in fairly high C3A (>10%) OPC mortars and pastes in the presence and absence of CaCO3. Further to the findings, due to low BaCO3 solubility at 8 °C, the temperature that favours thaumasite precipitation, neat OPC sulfate resistance was only slightly improved. Specimens, particularly mortars with 12.5% BaCO3 and no added CaCO3 (12.5B), deteriorated due to gypsum, ettringite and thaumasite precipitation, although less intensely than the samples bearing other blends. None of the BaCO3 or BaCO3 + CaCO3 blends performed better than the commercial sulfate-resistant cement used as the reference.

Influence of Drying on the Microstructure of Hardened Cement Paste: A Mercury Intrusion Porosimetry, Nitrogen Sorption and SAXS Study

Since the durability of concrete depends on the paste matrix, a sound knowledge of its pore structure is required. The most common methods to investigate the microstructure like mercury intrusion porosimetry (MIP) and gas sorption require the removal of free water in the pore space. However, the different drying methods used for this task either do not remove all free water or remove also chemically bound water and thus alter the microstructure. Small-angle X-ray scattering (SAXS) is one of the rare methods which allows to investigate the microstructure without complete water removal. Therefore, in order to compare the effect of different drying methods on the pore structure, hardened OPC paste samples (w/c = 0.5) were investigated with MIP, nitrogen sorption and for the first time with SAXS, after different drying was carried out. Since specific surfaces obtained for SAXS showed a similar trend as for MIP and nitrogen sorption, it can be concluded that SAXS can be utilized to predict the influence of drying on these methods as well. Solvent exchange appears to be a drying method that preserves the microstructure well, since for this the largest specific surface was obtained.

Use of biomass-fired power plant bottom ash as an addition in new blended cements: Effect on the structure of the C-S-H gel formed during hydration

The identification of new supplementary cementitious materials (SCMs) for use in blended cements is an area of major interest to the scientific community. The industry is equally keen for, given the vast amounts of raw materials and emission of large volumes of greenhouse gases involved in cement manufacture, it is under pressure to reduce its environmental footprint. This study explored hydration in blended cements bearing 20 wt% bottom ash (BBA) from three biomass-fired power plants. The results showed that the OPC-BBA blended and the neat OPC pastes exhibited similar degrees of hydration (~90%) and their C-S-H gels similar Al/Si ratios (0.08–0.09), while the mean chain length of the C-S-H gels was slightly higher in the former (5.15–5.30 for blended cement vs. 4.71 for OPC). The presence of pozzolanic activity in the BBA pastes, in turn, was confirmed by their lower CH and higher C-S-H gel content than would be expected on the grounds of the dilution of the amount of cement used.

RETRACTED: Influence of nano-CaCO3 addition on the compressive strength and microstructure of high volume slag and high volume slag-fly ash blended pastes

Abstract This paper presents the effects of various nano-CaCO3 (NC) contents on 28 days compressive strength and microstructure of high volume slag (HVS) pastes and high volume slag-fly ash (HVSFA) blended pastes. The results show that addition of 1–4% NC improved the compressive strengths of HVS pastes containing 70%, 80% and 90% slag, respectively by 8–24%, 1–16% and 2–20% compared to their reference pastes. The compressive strength of HVSFA paste containing combined slag and fly ash content of 70% is also increased significantly by 16–24% than the reference control paste due to the addition of 1–4% NC and maintained higher compressive strength than OPC paste after 28 days of curing. The addition of NC reduced the large capillary pores and cumulative pore volume of HVS pastes and HVSFA paste. Significant reduction of calcium hydroxide (CH) in HVSFA paste containing combined slag and fly ash content of 70% is found due to the addition of NC by thermogravimetric analysis (TGA) and a slight increase of CH is noticed in all HVS paste. A denser microstructure with fewer internal micro-cracks and more hydrated products are revealed in scanning electron microscopic (SEM) images and EDS trace analysis in all HVS and HVSFA pastes due to the addition of NC. HVS and HVSFA paste with significantly lower carbon footprint and higher compressive strength than control cement paste can be achieved with the inclusion of 1% NC.

Effects of calcium bicarbonate on the properties of ordinary Portland cement paste

Bicarbonates can be used as accelerators to enhance the early age properties of cementitious materials. Bicarbonates like KHCO3 and NaHCO3 could form strong alkali (KOH, NaOH) and cause potential risk of alkali-silica reaction. Ca(HCO3)2 could serve as a better accelerator than these bicarbonates because of formation of CaCO3 and Ca(OH)2, which are usually found in cementitious system. The effects of Ca(HCO3)2 on the hydration and physical properties of OPC paste were studied through heat of hydration, setting time, extension degree, compressive strength, pore size distribution, TG-DTA, XRD and SEM tests. The results showed that Ca(HCO3)2 can accelerate the formation of ettringite and CO2-AFm at the initial stage, and increase the early age strength of OPC paste. Besides, Ca(HCO3)2 was found to accelerate the hydration of C3S and C2S at a later stage and modify the pore structure of OPC paste. The reaction between Ca(HCO3)2 and portlandite formed CaCO3, which could fill the harmful pores in cement paste. The optimum content of Ca(HCO3)2 was found to be 1% by weight of cement, and further increase of Ca(HCO3)2 could decrease the compressive strength by increasing the harmful pores.

Effect of rice husk ash addition on the compressive strength and thermal stability of metakaolin based geopolymer

The influences of rice husk ash incorporated as partial replacement (by up to 40%) of metakaolin on the thermal stability and microstructures of metakaolin based geopolymer were investigated in this paper. The results indicate that the aluminosilicate fractions of RHA were associated with the acid-base reaction of geopolymerization, promoting the more formation of gels. The pores of geopolymer were refined by the filling effects of rice husk ash and enrichment of gel phases, which were primary reason for the optimization of geopolymer thermal stability. The geopolymer added with RHA maintained excellent comprehensive performance in comparison to OPC paste after high temperature exposure. This is attributed to the different microstructure between geopolymer and OPC. Addition of 30% RHA would make the best contribution to the strength of geopolymer before or after temperature exposure significantly.

Comparison of Effects of Sodium Bicarbonate and Sodium Carbonate on the Hydration and Properties of Portland Cement Paste.

Carbonates and bicarbonates are two groups of accelerators which can be used in sprayed concrete. In this study, the effects of the two accelerators sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) (0%, 1%, 2%, 3%, and 4% by weight of ordinary Portland cement OPC) on the properties of OPC paste were compared. The results show that both of them could accelerate the initial and final setting time of OPC paste, but the effect of the two accelerators on the compressive strength were different. After 1 day, sodium bicarbonate at 3% had the highest strength while sodium carbonate at 1% had the highest strength. After 7 days, both of the two accelerators at 1% had the highest compressive strength. After 28 days, the compressive strength decreased with the increase of the two. The improved strength at 1 and 7 days was caused by the accelerated formation of ettringite and the formation of CaCO3 through the reactions between the two with portlandite. The decrease of strength was caused by the Na+ could reduce the adhesion between C-S-H gel by replacing the Ca2+. NaHCO3 was found be a better accelerator than Na2CO3.

Suitability study of soybeans husk ash as a mixing material to OPC: Effect of calcination time-preliminary investigation

Soybeans husk was collected from a dump site, dried de-carbonated, separated into six samples and calcined at a temperature of 600 °C, for 1, 2, 3, 4, 5 and 6 hours respectively. Samples were taken for X- ray Florescence (XRF) analysis, Standard Electron Microscopy (SEM) imaging, setting time, standard water of consistency determination and specific gravity test. The result of XRF analysis revealed that the SHA contain less than 70 % SiO2+Al2O3+Fe2O3 stipulated by ASTM C618 for pozzolanas, but has very high CaO content. The SiO2 +Al2O3+Fe2O3 and CaO content though varies slightly with the calcination time of soybean husk. The specific gravity of SHA varies with calcination time of SH and ranges between 2.3 and 2.7 and is less than that of OPC irrespective of the calcination time. Setting times of OPC/SHA pastes at 10 % replacement of OPC with SHA increases with calcination time and is greater than that for OPC paste. The SEM analysis shows that the crystal structure of the SHA changes with calcination temperature. The compressive strength of mortar using OPC/SHA as a binder was determined after 7, 14 and 28 days curing and was found to be less than that for OPC mortars at all ages. However, the percentage reduction in strength decreased with curing age, with SHA calcined at 4 hours having the greater strength (i.e. 87.4 % of OPC strength at 28 days). The percentage gain in strength was more pronounced from 7 to 14 days. In all, SH calcined at 600 °C for 4 hours optimizes the pozzolanic potential of SHA.Kathmandu University Journal of Science, Engineering and Technology Vol. 12, No. I, June, 2016, Page: 13-22

Comparative study of two PCE superplasticizers with varied charge density in Portland cement and sulfoaluminate cement systems

Two PCEs with acid to ether monomer ratio of 4 and 22, marked as PCE4 and PCE22 respectively were prepared via radical polymerization in order to study their impacts on fluidity and early hydration in Portland cement (OPC) and Sulfoaluminate cement (SAC) systems. Dynamic light scattering measurement reveals that hydrodynamic radius of PCE22 is more sensitive to Ca2+ from being crosslinked. Fluidity tests show that PCE22 exhibits lower initial fluidizing effect than the lower charged PCE4 in OPC paste because of the inter-polymer crosslinking by Ca2+ bridging effect while the opposite comparison is observed in SAC paste due to the lower [Ca2+] in SAC paste than in OPC paste. PCE22 is found to significantly depress the early formation of AFt crystal in SAC paste, indicated by the results of XRD, TGA, BET and calorimetric tests. Thus, PCE22 behaves better in fluidity retention than PCE4 in SAC paste.

Microstructure and composition of fly ash and ground granulated blast furnace slag cement pastes in 42-month cured samples

Microstructure and composition of 42-month cured pastes using Portland cement (OPC), fly-ash cement (FAC), and ground-granulated-blast-furnace-slag cement (GGBFSC) were investigated. The granular structure of C-A-S-H in outer products of GGBFSC and FAC pastes was finer than in OPC paste. Fly ash offered higher capacity in increasing the Si/Ca ratio of C-A-S-H than slag in the pastes. However, their effects on the chain length were opposite. The incorporation of aluminum into silicate anions in GGBFSC paste was greater compared to others. AlO45− played an essential role in bridging the shorter silicate anions together, forming a longer chain of aluminosilicate anions in the pastes. Mg-Al layered-double hydroxide (LDH) was abundant in the inner products of slag particles with the interbedding of TAH, leading to a low ratio of Mg/Al ratio on hydrotalcite-like phase.

Recycled cement

Cement recycling can reduce the environmental impact caused by the landfills of demolished concrete and the demand of raw materials for producing cement because the waste cement paste in demolished concrete could be used to produce recycled cement. Recycled cement was produced through burning a 2-year-old waste cement paste at 4 different temperatures (120 °C, 450 °C, 750 °C and 1150 °C) and then grinding into powder. The selected temperatures were the temperatures where there was a significant weight loss based on the thermogravimetric analysis (TGA) result of the old cement paste. The results show that the highest compressive strength happened at 450 °C and the recycled cement paste had a similar strength as OPC paste but with a poor workability which was even beyond the measurement range of a rheometer. Increasing the particle size of recycled cement powder and partially replacing the cement powder with ground-granulated blast-furnace slag (GGBS) were found to be able to improve the workability of recycled cement paste effectively and at same time without reducing the compressive strength.

Hydration-drying interactions in OPC pastes blended with recycled OPC paste fine powder: Critical curing relative humidity hampering hydration

The paper presents an experimental study performed on OPC pastes blended with recycled OPC paste fine powder. Pastes with different OPC contents: 25, 50, 70 and 100% were dried, immediately after demolding at one day, in controlled environmental conditions with different relative humidities: 57, 76, 82, 88, 95 and 98% and temperatures: 5, 10, 20 and 30 °C. During the drying of the pastes, their hydration was monitored for 90 days by thermogravimetric analysis (TGA). The results showed that the critical curing relative humidity hampering hydration depends on properties intrinsic to the material, as hydration level, total porosity and microstructure fineness.

The effect of ageing and drying on laser scabbling of concrete

Laser scabbling of concrete is a process by which the surface layer of concrete is removed through the use of a high power (low power density) laser beam. In order to understand how the age and treatment of structures may affect the laser scabbling process, the aim of the research presented in this paper was to establish a relationship between laser interaction time, surface temperature and volume removal for cementitious materials of different ages and different degrees of saturation. The investigation focussed on (i) the effect of age on saturated specimens and (ii) the effect of prolonged drying. The results show that drying of specimens had the largest effect on scabbling. The effect of age on saturated specimens was small for PFA + OPC pastes, mortars and concretes, but significant for OPC pastes, where the volume of scabbling dramatically reduced with age.

Rheology of activated phosphorus slag with lime and alkaline salts

The present study aimed to determine the rheological behavior of activated phosphorus slag (AAPS) with a combination of Ca(OH)2 and Na2CO3 (CNC activator) or Na2SO4 (CNS activator) compared to OPC paste. The findings showed that the rheology of AAPS pastes fit with the Herschel-Bulkley model and behaved like a shear-thinning fluid. The AAPS pastes rheology is affected by concentration and nature of activator. The shear stress in AAPS pastes was lower than OPC paste, and increased with increasing the activator concentration. In early ages, reaction products of PS/CNC pastes included calcite-type compounds with the formed gel while gypsum compounds formed in the PS/CNS pastes. This resulted in higher shear stress and lower workability for PS/CNC pastes compared to PS/CNS pastes. The formed gel due to interaction between Ca2+ ions of CNC activator and Si4+ ions of Phosphorus slag caused an increase in the shear stress for PS/CNC during the time.

CHARACTERIZATION OF VARIOUS KINDS OF REFUSED DERIVED FUEL AND THEIR EFFECTS ON CEMENT PROPERTIES

Recently, there is an increasing demand for using refused derived fuels (RDF) as replacement for traditional fossil fuel in energy intensive industries like cement industry. However, using such sources will affect the properties and quality of the produced cement since RDF ash was mixed with OPC clinker during production. In the present study, four solid refused derived fuels namely; rubber waste, tree trimmings, rice straw and municipal solid wastes MSW are characterized and tested as RDF. Also, the effect of their ash on the setting and hardening of the hardened OPC pastes was investigated. Results showed that rubber waste has the highest calorific values followed by MSW. OPC pastes mixed with RDF ash showed reduced setting times. Besides, OPC admixed with rubber wastes ash offered the highest compressive strength values between all the studied RDF sources. Phase composition of the formed hydrates is studied by X-ray diffraction analysis and results showed that the ash within OPC matrix does not change the hydration mechanisms since the same hydration products are formed.

Effect of Temperature vis-à-vis curing time on the hydration products of OPC paste

Effect of Temperature vis-à-vis curing time on the hydration products of OPC paste

Hybrid Alkaline Cements: Bentonite-Opc Binders

Moderately alkaline activators can be used to formulate cementitious binders with a high Supplemetary Cementitious Materials (SCMs) and a low portland cement content (hybrid alkaline cements). This study aimed to prepare hybrid alkaline cements containing large percentages of dehydroxylated bentonite (BT) and small Portland cement (OPC) fractions, with 5% Na2SO4 as a solid alkaline activator. The hydration kinetics of the pastes hydrated in water in the presence and absence of the solid activator were assessed by isothermal conduction calorimetry, whilst the reaction products were characterised with X-Ray Powder Diffraction (XRD) and Fourier-transform Infrared Spectroscopy (FTIR). The presence of the alkaline activator hastened OPC and BT/OPC hydration: more heat of hydration was released, favouring greater initial bentonite reactivity. The portlandite forming during cement hydration reacted readily with the Na2SO4, raising medium alkalinity and enhancing bentonite dissolution and with it reaction product precipitation (primarily (N,C)-A-S-H-like gels that co-exist with C-S-H- or C-A-S-H-like gels). The presence of sulfate ions favoured the formation of AFm-like phases. Preceding aspects accelerated the hydration reactions, with the formation of more reaction product and matrix densification. As a result, the 28 days Na2SO4 activated systems developed greater mechanical strength than the water-hydrated systems, with the 60% BT/40% OPC blends exhibiting higher compressive strength than the 100% OPC pastes.

Biodeterioration mechanisms and kinetics of SCM and aluminate based cements and AAM in the liquid phase of an anaerobic digestion

In biogas structures, concrete faces aggressive media during anaerobic digestion. Biological activities allow the conversion of organic matter into biogas, leading to a medium characterized by a variability of composition in time and space. In order to ensure the sustainability of this expanding industry, solutions for increasing concrete durability are needed. This study aims to analyse the deterioration mechanisms of different binders focusing on the impact of the binder nature on the medium (biochemical composition) during the digestion. Binders with favourable composition to chemically aggressive media were tested: slag cement (CEM III/B), calcium aluminate cement (CAC) and metakaolin-based alkaliactivated material (MKAA), and a reference binder: OPC (CEM I). They were exposed to three anaerobic digestion cycles in liquid phase in laboratory bioreactors. The organic acids and ammonium concentrations of the liquid phase were monitored by GC and HPIC. For OPC and slag cement pastes, the chemical and mineralogical changes were characterized by SEM/EDS and XRD. Locally, the presence of binder materials has an impact on the kinetics of the digestion reaction, and therefore on the quantities of gas produced. Ammonium concentrations were above the XA3 class range. Under the conditions explored, biodeterioration mainly led to the carbonation of cement pastes.

Metakaolin as an Active Pozzolan for Cement That Improves Its Properties and Reduces Its Pollution Hazard

Prepared metakaolin produced by calcination of a kaolin sample at 750&degC after soaking for 5 hrs was tested as an active pozzolan for locally produced cement. Blended pastes of partially replaced ordinary Portland cement with different metakaolin amounts 5%, 10% and 15% were prepared, then they were hydrated with water for various time intervals of 1, 3, 7, 14 and 28 days. At each time interval, the hydrated paste specimens were tested for compressive strength, hydration kinetics and followed up using differential scanning calorimetry, X-ray diffractometry analyses and scanning electron microscopy techniques. Results showed that the blended pastes OPC-MK10 recorded the highest compressive strength values at all the hydration times reaching 33.10, 86.40, 101.20, 112, and 122 MPa with increasing the age of hydration as compared with the neat samples which recorded 31.70, 65.20, 72.6, 82 and 101.30 MPa respectively. Meanwhile, the OPC-MK10 blend showed a decrease in the free CaO content reaching 4.07%, 4.27%, 4.23%, 4.19%, and 4.11% CaO with increasing age of hydration compared with the neat samples which recorded 4.27%, 5.15%, 5.42%, 5.61%, and 5.96% CaO respectively. The DSC thermograms results for the hardened neat and OPC-Mk10 pastes at the 14 and 28 hydration days showed the formation of hydrated materials mainly calcium silicate hydrates, calcium sulphoaluminate hydrates and calcium aluminates hydrates. The X-ray diffractometry analyses of both hardened neat and OPC- MK10 showed that, the intensity of calcium hydroxide peaks of OPC-MK10 was lower than in case of neat OPC, while the peak of the hydration products of calcium silicate hydrates and calcium aluminates silicate hydrates of OPC- MK10 samples were of higher intensity than in case of neat OPC. Scanning electron microscopy micrographs indicated the formation of denser micro- structure for the hardened OPC-MK10 paste as compared to neat OPC pastes after the 28 days age of hydration.