Hardened Portland Cement Paste Research Articles

Properties of Portland cement paste incorporated with loamy clay

Metakaolin which is calcined kaolin clay is being widely applied as supplementary cementitious materials for Portland cement. Due to their scarcity and high cost, calcined ubiquitous polymineral clays are actively studied as alternative to metakaolin. This article presents the study results of influence of calcined ground loamy clay on the properties of Portland cement paste. About 5–15% of loamy clay calcined at 400–600 °C and ground to 250–500 m2/kg were found to be more effective compared to metakaolin of specific surface area of 1200 m2/kg for improvement of compressive strength, water resistance, and increase in density of hardened Portland cement paste.

Influence of the addition of dispersed fine polymineral calcined clays on the properties of Portland cement paste

The addition of certain types of polymineral clays containing 40–62% or 0% without kaolinite, calcined at 400–800°C and ground to 250–800 m2/kg, was found to increase the strength, density and water resistance of hardened Portland cement paste relative to the addition of metakaolin and silica fume. Increased research in this field could provide a scientific and technological basis for the standardised manufacture of effective pozzolans based on polymineral clays in different countries and regions. Effective pozzolanic additives based on calcined polymineral clays with high pozzolanic activity can be produced by calcination at lower temperatures (400–600°C) and with a lower fineness than metakaolin.

Enhancing the thermal resistance and mechanical properties of hardened Portland cement pastes by using pumice and Al2O3

This study aimed to evaluate the suitability of using unfired and fired pumice as cement replacement materials as well as their effect on the thermal resistance of hardened ordinary Portland cement (OPC) pastes. Different OPC–pumice (unfired and fired) blends were prepared by partial replacement of OPC by 0, 10 and 20 of pumice (mass%). The effect of the addition of 1 and 5 % of active alumina on the mechanical properties and thermal resistance of different OPC–pumice (unfired) blends was investigated. The fire resistance test was done by exposing the hardened blended cement cubes to elevated temperatures of 200, 400, 600 and 800 °C for 3 h and allowed them to cool down to room temperature before testing for their mechanical properties. The phase composition and thermal analysis of some selected specimens were investigated by XRD, DSC and DTA/TG techniques. The obtained results indicated that replacing OPC by 10 and 20 % by pumice (unfired and fired) improved its thermal stability at different firing temperatures. The cement blend prepared by replacement of OPC with 10 % pumice showed the highest fire resistance. The addition of 1 and 5 % of alumina (A) to OPC–pumice blends causes a notable improve in their mechanical properties and thermal resistance.

Effect of carbonation in supercritical CO2 on the properties of hardened cement paste of different alkalinity

It is well-known that the strength of cementitious building materials is increased by carbonation. This process was accelerated using supercritical carbon dioxide scCO2 at 150bar. Hardened Portland cement pastes with different alkali contents and w/c ratios of 0.5 and 0.6 were treated for four hours in scCO2 after preliminary storage at different relative humidities. Strengths were measured and the microstructure investigated by X-ray diffraction, scanning electron microscopy and mercury intrusion porosimetry. Carbonation in scCO2 produces a substantial increase in strength which is favoured by low moisture contents and low-alkali cements. The retarding effect of alkalis on the carbonation reaction was confirmed by thermodynamic calculations.

레드머드 대체율에 따른 폴리머 혼입 알칼리활성화 슬래그-레드머드 시멘트모르타르의 강도 및 기공특성

The alkali-slag-red mud(ASRC) cement belongs to clinker free cementitious material, which is made from alkali activator, blast-furnace slag(BFS) and red mud in designed proportion. This study is to investigate strength and pore characteristics of alkali-activated slag cement(NC), clinker free cementitious material, and ordinary portland cement(C) mortars using polymer according to red mud content. The results showed that the hardened alkali-activated slag-red mud cement paste was mostly consisted of C-S-H gel, being very fine in size and extremely irregular in its shape. So the hardened ASRC cement paste has lower total porosity, less portion of larger pore and more portion of smaller pore, as compared with those of hardened portland cement paste, and has higher strength within containing 10 wt.(%) of alkali-activated slag cement(NC) substituted by red mud.

Prediction of water vapour sorption isotherms and microstructure of hardened Portland cement pastes

Water vapour sorption isotherms of cementitious materials reflect the multi-scale physical microstructure through its interaction with moisture. Our ability to understand and predict adsorption and desorption behaviour is essential in the application of modern performance-based approaches to durability analysis, along with many other areas of hygro-mechanical and hygro-chemo-mechanical behaviour. In this paper, a new physically based model for predicting water vapour sorption isotherms of arbitrary hardened Portland cement pastes is presented. Established thermodynamic principles, applied to a microstructure model that develops with hydration, provide a rational basis for predictions. Closed-form differentiable equations, along with a rational consideration of hysteresis and scanning phenomena, makes the model suitable for use in numerical moisture simulations. The microstructure model is reconciled with recently published 1H NMR and mercury intrusion porosimetry results.

고온 환경에 노출된 시멘트 경화체의 공극 구조 변화

Hardened cement-based materials exposed to the high temperatures of a fire are known to experience change in the pore structure as well as microstructural changes that affect their mechanical properties and tend to reduce their durability. In this experimental investigation, hardened Portland cement pastes were exposed to elevated temperatures of 200, 400, 600, 800, and 1000℃ for 60 minutes, and the resulting damage was studied by thermogravimetry (TG), mercury intrusion porosimetry (MIP) and density measurements. These results revealed that the residual compressive strength is increased at temperatures greater than 400℃ due to a small pore size of 3 nm and/or rehydration of the dehydrated cement paste. However, a loss of the residual strength occurs at temperatures exceeding 500 and 600℃. This can be attributed to the decomposition of hydrates such as portlandite and to an increase in the total porosity.

Effect of curing regime on degree of Al3+ substituting for Si4+ in C-S-H gels of hardened Portland cement pastes

The effect of curing regime on degree of Al3+ substituting for Si4+ (Al/Si ratio) in C-S-H gels of hardened Portland cement pastes was investigated by 29Si magic angel spinning (MAS) nuclear magnetic resonance (NMR) with deconvolution technique. The curing regimes included the constant temperature (20, 40, 60 and 80 °C) and variable temperature (simulated internal temperature of mass concrete with 60 °C peak). The results indicate that constant temperature of 20 °C is beneficial to substitution of Al3+ for Si4+, and Al/Si ratio changes to be steady after 180 d. The increase of Al/Si ratio at 40 °C is less than that at 20 °C for 28 d. The other three regimes of high temperature increase Al/Si ratio only before 3 d, on the contrary to that from 3 to 28 d. However, the 20 °C curing stage from 28 to 180 d at variable temperature regime, is beneficial to the increase of Al/Si ratio which is still lower than that at constant temperature regime of 20 °C for the same age. A nonlinear relation exists between the Al/Si ratio and temperature variation or mean chain length (MCL) of C-S-H gels, furthermore, the amount of Al3+ which can occupy the bridging tetrahedra sites in C-S-H structure is insufficient in hardened Portland cement pastes.

Chloride adsorption by calcined layered double hydroxides in hardened Portland cement paste

This study investigated the feasibility of using calcined layered double hydroxides (CLDHs) to prevent chloride-induced deterioration in reinforced concrete. CLDHs not only adsorbed chloride ions in aqueous solution with a memory effect but also had a much higher binding capacity than the original layered double hydroxides (LDHs) in the cement matrix. We investigated this adsorption in hardened cement paste in batch cultures to determine adsorption isotherms. The measured and theoretical binding capacities (153 mg g−1 and 257 mg g−1, respectively) of the CLDHs were comparable to the theoretical capacity of Friedel's salt (2 mol mol−1 or 121 mg g−1), which belongs to the LDH family among cementitious phases. We simulated chloride adsorption by CLDHs through the cement matrix using the Fickian model and compared the simulation result to the X-ray fluorescence (XRF) chlorine map. Based on our results, it is proposed that the adsorption process is governed by the chloride transport through the cement matrix; this process differs from that in an aqueous solution. X-ray diffraction (XRD) analysis showed that the CLDH rebuilds the layered structure in a cementitious environment, thereby demonstrating the feasibility of applying CLDHs to the cement and concrete industries.

Structure and micro-nanomechanical characterization of synthetic calcium–silicate–hydrate with Poly(Vinyl Alcohol)

The principal phase of hardened Portland cement pastes is calcium silicate hydrate (C–S–H), which influences the physical and mechanical properties of construction materials. In this work calcium silicate hydrate (C–S–H) was synthesized, with the addition of Poly(Vinyl Alcohol) (PVA), for the development of C–S–H/polymer nanocomposites. Among the polymers available, PVA is indicated by the literature as one of the most viable for producing C–S–H/polymer complexes. However, no consensus exists regarding the kind of interaction. The resulting compounds were characterized by XRD, FT-IR, TGA, carbon content (CHN), TEM, SEM and elastic modulus and hardness were measured by instrumented indentation. The set of results presented do not confirm the intercalation of PVA in the interlayer space of C–S–H, but presented evidence of the potential for intercalation, since changes in the structure clearly occurred. A significant change in the micro-nanomechanical properties of C–S–H occurred in the presence of PVA.

Macro and micro aspects of the transport of chlorides in cementitious membranes

A study of the transport of chlorides through hardened Portland cement paste membranes is reported. The flux of chlorides was induced by migration (AC and DC fields) and diffusion.Two parallel studies were developed. A macroscopic one where the migration of chlorides was followed by Electrochemical Impedance Spectroscopy (EIS) and a microscopic one in which the natural diffusion was investigated using the scanning electrochemical microscopy (SECM). Both studies have revealed to be complementary. The local investigations performed with the SECM were successfully employed for the interpretation of the EIS data.The results have shown that the transport of chlorides occurs mainly through pores of sub-micrometre equivalent diameter. The study of the transition time has allowed to conclude that the diffusion coefficient approaches the usual value obtained in bulk solution. The macroscopic study has allowed presenting an explanation for the beneficial effect of the AC current in the transport of chlorides. A possible method for estimating the flux of chlorides from the percolation resistance is also discussed.

Influence of pozzolana on sulfate attack of cement stone affected by chloride ions

This study investigated the influence of natural pozzolana (opoka) additive on the hydration of Portland cement and the effects of pozzolana on sulfate attack of cement stone affected by chloride ions. In the samples, 25 % (by weight) of the Portland cement was replaced with pozzolana. The specimens were hardened for 28 days in water, and then one batch was soaked in a saturated NaCl solution and another in a 5 % Na2SO4 solution for 3 months at 20 °C. After being kept for 3 months in a saturated NaCl solution, samples were transferred to a 5 % Na2SO4 solution and kept under these conditions for 3 months. It was estimated that under normal conditions, pozzolana additive accelerated the hydration of calcium silicates and initiated the formation of CO3 2−–AFm; opoka also decreased the threshold pore diameter of hardened Portland cement paste. It was found that Cl– ions penetrate to monosulfoaluminate, form Friedel’s salt, and release SO4 2− ions, which react with unaffected monosulfoaluminate and form extra ettringite; when samples were transferred to the 5 % Na2SO4 solution, a greater quantity of new ettringite was formed. Meanwhile, pozzolana additive reduced the penetration of chloride and sulfate ions into the structure of Portland cement hydrates and inhibited sulfate attack of cement stone treated in a saturated NaCl solution.

Estimating the original cement content and water–cement ratio of Portland cement concrete and mortar using backscattered electron microscopy

In a previous study, a method was proposed to estimate the cement content, water content, free water–cement ratio (w/c) and degree of hydration of hardened Portland cement pastes that have unknown proportions. The method is based on measuring the volumetric fractions of capillary pores, hydration products and unreacted cement using backscattered electron microscopy and the volumetric increase of solids during cement hydration. It has the advantage that it is quantitative and does not require comparison with reference samples made with the same materials and cured to the same hydration degree as the unknown sample. However, the method was tested on neat cement pastes only, hence limiting its practical application. In this paper, the extension of the method to Portland cement mortars and concretes is presented, and results are provided, obtained from samples made with three cement types and a range of aggregate content (40–70% vol.), w/c ratio (0·30–0·70) and curing age (3–90 days). The study also involved sealed-cured, wet-cured and field-exposed samples. It is observed that bleeding and aggregate absorption can have a significant effect on the free w/c ratio. Overall, good agreement between the estimated and actual values is found. The error in estimating free w/c ratio using the proposed method is no more than 0·05 for all samples tested in this study.

Characteristics of two types of C-S-H gel in hardened complex binder pastes blended with slag

The characteristics of C-S-H gel in hardened Portland cement pastes and complex binder pastes with ground granulated blast furnace slag were investigated with nanoindentation. The composition of C-S-H gel was analysed with SEM-EDS. The obtained results showed that the volume fraction of LD C-S-H gradually reduced and the volume fraction of HD C-S-H increased with the prolongation of hydration age. Most of the C-S-H gel produced at later age was HD C-S-H. The volume fraction of HD C-S-H increased as the fraction of slag in complex binder pastes increased, suggesting that HD C-S-H was mainly in the hydration products of slag. The chemichal compositions of the two types of C-S-H gel were simlar, meaning that formation and transformation of the two types of C-S-H gel were not affected by their Ca/Si ratio.

Utilization of electric arc furnace dust as an admixture to Portland cement pastes

Electric arc furnace dust (EAFD) is termed as a hazardous waste due to its contamination with heavy metals. Inertization of such very fine dust can be occurred via stabilization and solidification process within the hydrated Portland cement matrix. In this paper, the effect of the addition of various ratios of EAFD on the properties of the hardened Portland cement paste was investigated. Compressive strength, chemically combine water and free lime contents were determined. In addition, phase composition using XRD; DTA analysis; as well as microstructure of the formed hydrates for some selected samples were investigated using SEM. The obtained results showed that the paste containing 1/mass% EAFD give the highest compressive strength values at most hydration ages, specially the later ages, compared to the neat Portland cement blank paste. Whileas, the pastes containing 3 and 5/mass% EAFD showed lower values of compressive strength compared to those of the blank paste.

Effect of addition of Sikament-R superplasticizer on the hydration characteristics of portland cement pastes

The effect of addition of Sikament-R superplasticizer (modified lignosulphonate base) on the hydration characteristics of hardened Portland cement pastes were studied at different curing conditions. Four mixtures were prepared using 0, 0.2, 0.4 and 0.6wt% addition of Sikament-R superplasticizer (SR) of cement. These pastes were hydrated under two different conditions; (i) normal curing at room temperature; 25°C up to 90days periods and (ii) hydrothermal curing at a pressure of 8atm. of saturated steam up to 24h. The compressive strength, combined water content, free lime content, gel/space ratio and microstructure of hardened cement pastes were studied. The results revealed that addition of SR superplasticizer promote the dispersion of cement particles and interacts with Ca(OH)2. The addition of SR superplasticizer exhibits Portland cement better workability during the preparation of pastes. In addition, amore compact structure were obtained leading to higher values of compressive strength for all the hardened hydrated pastes under both normal and hydrothermal curing. The results indicated that the addition of SR superplasticizer to Portland cement does not alter the types of hydration products formed during normal or hydrothermal conditions; only it caused a decrease in the degree of the porosity of the formed pastes.

CHANGES IN THE MORPHOLOGY OF HYDRATES WITH POZZOLANIC REACTION BETWEEN HARDENED CEMENT PASTE AND SILICA FUME OR BLAST FURNACE SLAG

セメントペースト硬化体を粉砕して得られた粒子とシリカフュームや高炉スラグ微粉末を質量比1：1で混合し、W/S＝0.8のペーストを20℃で1週間から10年間養生してポゾラン反応させた。セメント硬化体中に生成していた水酸化カルシウム結晶が減少、消失していく過程と反応中に起こる形態の変化を検討し、以下の結果を得た。セメント硬化体とシリカフューム系試料ではポゾラン反応の進行に伴って、水酸化カルシウムが消失し、水酸化カルシウム結晶の存在した場所は空隙になっていく。さらに反応が進行すると空隙に水和物が析出する。セメント硬化体とスラグ微粉末系試料では、10年養生でも水酸化カルシウムは消失せず、結晶の周囲に大きな空隙は認められない。

Thermal resistance of hardened cement pastes containing vermiculite and expanded vermiculite

The durability and thermal stability of hardened Portland cement pastes containing vermiculite (V) and expanded vermiculite (EV) exposed to high temperatures were studied. Different mixtures were prepared using 2.5, 5, and 10 wt% of both types of V. Each mixture, after 28 days of hydration, was heated at 300, 600, and 800 °C for 3 h. Two modes of cooling were used; gradual cooling in air and rapid cooling in cold water. The percentage of residual strength, chemically combined water content, change in phase composition, and the thermal stability of the heated specimens were studied. The specimens cooled in water showed greater loss in strength than the air-cooled specimens. The presence of V improved the heat resistance of ordinary type I Portland cement (OPC) pastes. 5 wt% replacement revealed the best performance at all heating temperatures. The EV showed better thermal resistance than the nonexpanded one. Addition of silica fume (SF) with V in OPC pastes lead to superior performance. This can be explained as result of the combined effects of insulation properties of V and pozzolanic reactivity of SF which accounts for the notable increase in the residual strength for these mixes.

Physical and mechanical properties of the repaired sandstone ashlars in the facing masonry of the Charles Bridge in Prague (Czech Republic) and an analytical study for the causes of its rapid decay

The facing masonry of the Gothic Charles Bridge in Prague (Czech Republic) has been largely altered during nineteenth and twentieth century repairs, due to extensive replacements of weathered sandstone ashlar blocks. Natural stone varieties used during these replacements show different responses to a variety of weathering processes which lead to their rapid decay. The decay of the newer stone has been accelerated by use of Portland cement paste as the binding and fill material, instead of the original hydraulic lime-based and cocciopesto-type mortars. The hardened Portland cement paste makes an almost impermeable barrier, with permeability three to four orders of magnitude lower, compared to the original mortars and natural stone. The low permeability of this new alien material results in the accumulation of water-soluble salts (specifically nitrates, but also sulphates and chlorides) in the facing masonry ashlars, with their crystallisation coming in the form of both sub- and efflorescence, as well as the development of various decay forms (blistering, granular disintegration and/or scaling and flaking). The source of the water-soluble salts can be traced back to previous restoration attempts. These included chemical cleaning (black crust removal) by nitric acid, ammonium hydroxide (‘ammonium water’), or ammonium hydrogen carbonate, all which can be linked to high nitrate content in the efflorescence.

Desiccation shrinkage of cementitious materials as an aging, poroviscoelastic response

This paper describes and evaluates a new model that utilizes aging poroviscoelasticity for predicting the shrinkage of cementitious materials induced by loss of moisture from the pore structure (i.e. desiccation). The new model incorporates well-accepted mechanisms for desiccation shrinkage and accounts for the effect of changing concentrations of dissolved species in the pore fluid. Additionally, the model is used to interpret viscoelastic behavior during the drying process via comparisons of model predictions with measured shrinkage of hardened portland cement paste. It was found that while a poroelastic model under predicts the measured shrinkage, the poroviscoelastic model significantly over predicts the shrinkage unless intrinsic aging of the C–S–H gel is included in the model.