Consumption Of Portlandite Research Articles

Chemico-microstructural changes in earthen building materials containing calcium carbide residue and rice husk ash

Clay earthen materials can essentially be stabilized with calcium carbide residue (CCR) and rice husk ash (RHA) to produce compressed earth block (CEBs) with improved mechanical performances for application in building construction. Nevertheless, the curing process in mixtures of these materials needs to be monitored in order to assess the maturation of the reaction among these materials and their reactivity. This study investigated the curing process in mix solutions and microstructural changes in the cured mixtures made of kaolinite-rich earthen material, portlandite-rich CCR, and silica (amorphous)-rich RHA. Dry mixtures were prepared by adding 0–25% CCR and 10–25% CCR:RHA (various ratios) to the earthen material (5 g). The mix solutions were prepared by addition of 100 mL of deionized water to the dry mixtures. Stabilized CEBs were also produced by manually compressing moisturized mixtures in 295 × 140 × 95 mm3 mold. The curing process was undertaken between 1 and 90 days at 40 ± 2 °C. Throughout the curing, the chemical changes in mix solutions were monitored by measuring the pH, electrical conductivity (EC) and concentration of unconsumed calcium ions [Ca2+]. The microstructural changes in cured mixtures were characterized using X-Ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The EC and [Ca2+] decreased over the curing time mainly due to the consumption of calcium ions through pozzolanic reaction involving the earthen material with CCR and CCR with RHA. The minimum values of EC and [Ca2+] were reached after 45 days of curing in the mix solutions containing the earthen material and CCR alone and 28 days in those containing the earthen material and CCR:RHA. This was related to the end of the reaction and occurrence of optimum maturity in the respective mixtures. The XRD analyses revealed the consumption of kaolinite and portlandite from the raw materials and formation of calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH) in the cured mixtures. The SEM micrographs showed the formation of porridge-like products forming an interlocking network which densely cemented the matrix of stabilized CEBs.

Mitigation of the potential alkali–silica reaction of FNS using ground FNS as a supplementary binder

Alkali–silica reactivity of granulated ferronickel slag (FNS) aggregate was evaluated by accelerated mortar bar tests. The test results showed noticeable expansions of the samples containing 50% FNS aggregate due to potential alkali–silica reaction (ASR). On the other hand, finely ground FNS (GFNS) exhibited pozzolanic property in the studies of compressive strength development, thermogravimetric analysis and X-ray diffraction analysis. Therefore, GFNS was used as a partial replacement of cement to study its effect on the reduction of ASR. It was found that the use of GFNS for at least 30% cement replacement reduced the 10 d and 21 d mortar bar expansions below the allowable limits. The consumption of portlandite by the pozzolanic reaction of GFNS is considered as the primary reason for the reduction of expansion. The ASR mitigation process of GFNS was further confirmed by scanning electron microscopy images and energy-dispersive X-ray spectroscopy of the reaction product.

Structure and reactivity of synthetic CaO-Al2O3-SiO2 glasses

Reactivity of glasses is predominantly linked to the degree of depolymerization of the glass structure. In this work, eight calcium aluminosilicate glasses were synthesized with compositions reflecting the glassy phases of industrial fly ashes and slags. The chosen composition ranges allow for isolation and identification of the roles of Ca and Al. The reactivity of the glasses is found to be mainly dependent on the CaO content as calcium depolymerizes the glass structure and increases the rate of reaction of the glasses. The effect of Al2O3 on glass reactivity is less pronounced and relies on chemical weakening of the glass structure. However, an increasing alumina content has a pronounced effect on the hydrate phase assemblage, bound water content, portlandite consumption and the heat of reaction. The resulting compressive strength of composite cement mortars incorporating the synthesized CaO-Al2O3-SiO2 glasses is found to be a function of both the degree of glass reaction and the actual phase assemblage.

Using isothermal calorimetry, X-ray diffraction, thermogravimetry and FTIR to monitor the hydration reaction of Portland cements associated with red mud as a supplementary material

This study is part of a global project to find some large-scale application for red mud. A previous work confirmed that the use of RM as partial substitution for cement can affect its rheological properties; however, it is possible to produce cementitious materials without lose mechanical performances at harden state. The main goal of the work was to assess the impact of red mud (RM) as replacement of different Brazilian Portland cements by monitoring the formation of hydrated products over time, by thermogravimetry, X-ray diffraction and FTIR. Results show that the presence of RM increases the induction period, the gypsum and portlandite consumption as well as the amount of ettringite and monosulfoaluminate produced. The formation of sodium silicoaluminate hydrate (NASH) was also observed, regardless the kind of cement, as a result of the reaction with aluminates and silicates from the binder.

Predicting the degree of reaction of supplementary cementitious materials in cementitious pastes using a pozzolanic test

A pozzolanic test that can correctly identify pozzolanic materials is key in the search for alternative supplementary cementitious material (SCMs). Determination of the degree of reaction of these alternative SCMs in cementitious pastes is also important, as the reactivity of certain SCMs can be quite low. Since typical methods to determine SCM degree of reaction can be complex and laborious, the current study explores whether parameters obtained from a newly developed pozzolanic test can potentially be used to develop a simple method for determining the degree of reaction of SCMs. In the pozzolanic test, (cumulative) heat release and calcium hydroxide consumption of SCMs from a mixture of SCM and calcium hydroxide (water-to-solids ratio 0.9, pH 13.5, and testing temperature of 50 °C) are measured. Corresponding values of cumulative heat release and calcium hydroxide consumption of SCMs in a cementitious paste are also measured at two different water-cementitious materials (w/cm) ratios. The ratio between the values of cumulative heat release and the calcium hydroxide consumption in the cementitious paste and the pozzolanic test are considered to be measures of degree of reaction. Four different SCMs – class F fly ash, ground granulated blast furnace slag, metakaolin, and undensified silica fume were tested in this study. The effect of temperature and w/cm on degree of reaction were assessed. Degree of reaction values obtained from this method were compared with those from obtained from a portlandite consumption method suggested in literature and from typical values suggested in literature. Good correlation (R2 = 0.87) is obtained for the degree of reaction values determined here using the calcium hydroxide ratio and the portlandite consumption method. The degree of reaction values increase as the temperature increases but do not strongly depend on w/cm. The degree of reaction values are in general agreement with the range of values obtained from literature, however, this range is rather large, and the values depend strongly on the method used to determine degree of reaction. These preliminary results are promising and suggest that this method may potentially be used to provide information about pozzolanicity and degree of reaction of various SCMs.

Contribution to thermal study of modified cement pastes based on siliceous SCMs

This study aims to highlight siliceous additive available in Algeria: ground blast furnace slag 'S' and dune sand or Quartz 'Q' compared to silica fume 'SF'. Monitoring the hydration of cement pastes modified by replacing 15% additive by thermal analysis (Differential thermal and thermo gravimetric analysis -DTA/TG) quantifies the Portlandite consumed by the additive, the combined water 'Wn' and the degree of hydration as function of time. The results of this study confirm and complement those found in previous research: compared to SF and S, quartz has a low pozzolanic activity resulted in consumption of Portlandite released during cement hydration. This study is completed by mechanical tests on ultra high performance concrete (UHPC), with a same level of replacement. A new generation of sustainable and eco-concrete is obtained based on local materials.

Durability Study of Silica Fume-mortar exposed to the Combined Sulfate and Chloride-rich Solution

The present article investigates the mechanism behind the sulfate attack of the Ordinary Portland Cement (OPC) and the Silica Fume (SF) based mortar exposed to the combined sulfate and chloride-rich solution. In this study, the control mortar sample was fabricated using OPC. Additionally, the silica fume based mortar composites were fabricated using SF replacing the 5, 10, and 15% of OPC. The 28 days cured control and silica fume based mortar samples were exposed to a sodium sulfate (5%) solution and a mixture of sodium sulfate (5%) and sodium chloride (3.5%) solution for 510 days to examine the deterioration of mortar samples by the sulfate attack. The results reveal that a less extent of deterioration takes place in the silica fume based mortar as compared to that of the OPC mortar exposed to both solutions. The retarded deterioration of silica fume based mortar is primarily governed by the formation of a less extent of expansive ettringite and gypsum due to the consumption of portlandite in producing secondary Calcium Silicate Hydrate (CSH). Based on the analytical analyses, a model has been proposed to explain the overall performances of the silica fume based cement mortar composites exposed to different aggressive environments.

Sulfate resistance of calcined clay – Limestone – Portland cements

Calcined clays emerge as a promising source of supplementary cementitious material, which can provide a significant lowering of the Portland clinker content in blended cements. This study focusses on sulfate resistance of calcined clay (CC) – limestone (L) – Portland cements for mortars exposed to a 0.11 M Na2SO4 solution at 5 and 20 °C after a hydration period of 91 days. The pozzolanicity, compressive strength, pore structure, and sulfate resistance of mortars containing laboratory-made metakaolin or calcined montmorillonite and limestone have been investigated in cements with 35 wt% replacement of a white Portland or an ordinary Portland clinker. The results show that all mortars with CC/(CC + L) ≥ 0.5 exhibit excellent sulfate resistance. The consumption of portlandite by the pozzolanic reactions of the calcined clays and the dilution of the Portland clinker lead to a lower amount of calcium available for the secondary formation of gypsum and ettringite, which is identified as the main reason for the excellent sulfate resistance of the ternary blends. The results suggest that calcined clay – limestone – Portland cements are included in standards as a new type of sulfate-resisting Portland pozzolana cement and Portland composite cement.

Durability of cement pastes exposed to external sulfate attack and leaching: Physical and chemical aspects

Sulfate ions in seawater or underground can attack the cement paste leading to expansion and strength loss. This expansion is usually related to ettringite and gypsum precipitation. This study aims to characterize the possible altered zone and to identify the mechanisms of degradation under such conditions. For this study, cement pastes manufactured using CEM I with two w/c ratios (0.45 and 0.60) were exposed to sodium sulfate solutions (semi-immersion at a controlled pH = 8.0 ± 0.1) after one year of curing in water. The analysis of the physical aspect of this phenomenon shows that two modes of transfer occur: transfer of sulfate ions to the cement matrix and leaching of calcium ions to the external solution. The analysis of the chemical composition of the affected material highlights the progressive consumption of portlandite at the surface, the decalcification of C-S-H and the formation of AFt from both Afm and aluminium incorporated in C-S-H.

Kinetic analysis and thermodynamic simulation of alkali‐silica reaction in cementitious materials

AbstractThis work aims to study the alkali‐silica reaction (ASR) with incorporated kinetic‐thermodynamic analysis. The model reactant tests were first conducted to study the silica dissolution in the simulated pore solution under different temperatures. Then, the kinetic model for silica dissolution was further improved by considering the influence of effective surface area. The improved kinetic model was also incorporated into the GEMS thermodynamic simulation. The model analysis demonstrated the silica dissolution rate is decreased mainly caused by reduced hydroxyl activity with increased saturation degree. The dissolved silica first reacted with the portlandite phase and formed the Calcium–Silicate–Hydrate (CSH) gel. The ASR gel can only be generated under high silicate ion concentration after the consumption of portlandite.

Thermal analysis of ternary gypsum-based binders stored in different environments

Although gypsum belongs to the low-energy environmentally friendly binders, its wider applications in building constructions are limited due to the negative effect of moisture on its mechanical properties. When calcined gypsum (CaSO4·1/2H2O) transforms into its hydrated form (CaSO4·2H2O), it is partially soluble in water and it has a relatively low strength. This problem can be resolved when gypsum is used as a part of binary or ternary binders. In this paper, a system consisting of calcined gypsum, lime, and silica fume is presented as a functional solution for a wider utilization of gypsum in wet environments. For this purpose, the newly designed materials were stored in different environments (laboratory conditions in air or water) up to 182 days. The effect of silica fume on the hydration process and the growth of the main products is evaluated by using differential scanning calorimetry and thermogravimetry in the temperature range from 25 to 1000 °C with a heating rate of 5 °C min−1 in an argon atmosphere. The carbonation level of studied materials is also evaluated. Besides this, the information about the thermal stability of studied materials is provided. These results are supported by evolved gas analysis, X-ray diffraction, and scanning electron microscopy. The basic physical and mechanical properties are determined to provide more detailed information about the behavior of the designed materials under various conditions at selected days of hydration. The addition of silica fume to the gypsum–lime system activates the pozzolanic reaction of the analyzed pastes, which is proved by the presence of the CSH phase and by the consumption of portlandite in the mixtures. Wet environment speeds up the hydration processes and prevents samples from carbonation.

Influence of Physicochemical Properties of Sugarcane Bagasse Ash (SCBA) in Portland Cement Hydration

Due to the concern with the environmental impacts caused by the gases emitted by the cement industry and by the inadequate disposal of wastes generated in the sugar-alcohol industry, such as sugarcane bagasse ash (SCBA), a search for the development of new technologies, which are less aggressive to the environment and that propose feasible alternatives, began in order to reuse these wastes properly. Among these alternatives is the reuse of SCBA as partial replacement to cement or as addition to cementitious matrices. In this way, the present research has the objective of analyzing the influence of SCBA obtained by the calcination of sugarcane bagasse (SCB), at 600°C, in the process of Portland cement hydration. Initially, the SCBA was characterized physically, chemically and mineralogically, and then cement pastes with 20% and 35% substitution contents were elaborated, besides the reference paste, which were analyzed through X-ray diffraction (XRD) and thermogravimetric (TG) techniques. The results obtained show that there is a consumption of portlandite as a consequence of the use of SCBA, evidencing the pozolanicity of these ashes. In the pastes with 35% substitution content, there was an intense consumption of the portlandite, indicating, in this proportion, the pozzolanic reaction was more intense.

Effect of chemical and thermal activation on the microstructural and mechanical properties of more sustainable UHPC

This paper investigates the influence of the content of blast furnace slag (BFS) on the microstructural and mechanical properties of non-activated and activated ultra-high performance concrete (UHPC). Three volume-substitution rates of cement with BFS were explored (30% for UHPC2, 50% for UHPC3 and 80% for UHPC4) and two activation methods, chemical and thermal, were tested. Results show that with 30% of BFS, heterogeneous nucleation prevailed over dilution, accelerating the hydration reaction of cement and increasing the amount of C-S-H formed. C-S-H decreased the porosity of UHPC2 by 18% and 20% respectively at 3 and 90 days. The compressive strengths of UHPC1 (without BFS) and UHPC2 were very similar. For high BFS contents, the dilution effect prevailed and there was less portlandite, which decreased the amount of hydrated products, particularly at early age. As a result, the porosity of UHPC3 and UHPC4 was more than twofold higher than that of UHPC1.To boost the hydration reaction of blended UHPC with a high BFS content, KOH was added. The use of [KOH]3 significantly increased the amount of hydrated products, reducing the porosity of UHPC4 1.6-fold at 3 days and increasing its compressive strength by 42% at the same age. However, this activation mode was not enough to ensure the required compressive strength of UHPC1. Thermal activation at 90 °C for 2 days was therefore tested. Results showed the acceleration of the reaction of solid components, which increased the consumption of portlandite and hence the development of hydrated products. This resulted in improving the packing density of blended UHPC, decreasing its porosity and enhancing its compressive strength. In comparison with reference concrete at 90 days, the compressive strength of UHPC3-T increased by 7% and that of UHPC4 was 12.5% lower.

Use of recycled glass powder to improve the performance properties of high volume fly ash-engineered cementitious composites

High-volume fly ash (HVFA) Engineered Cementitious Composites (ECC) show reduced strength and physical properties, especially at early curing ages. The goal of this study was to improve their strength characteristics by incorporating recycled glass powder (RGP) for enhanced material sustainability. Composites containing 15, 30, 45 and 60% RGP as a replacement for FA, with FA to cement ratio of 2.2, were studied in HVFA-ECC. Standard ECC mixtures with FA to Portland cement (FA/PC) ratio of 1.2, and HVFA-ECC with FA/C ratio of 2.2 without RGP were also produced as control mixtures. The experimental results confirmed that incorporating RGP into HVFA-ECC significantly improves compressive and flexural strengths, chloride ion resistance and electrical resistivity, and results in a comparable ductility to standard ECC, based on FA to cement ratio of 1.2. Furthermore, self-healing of HVFA-ECC was accelerated and final recovery rate of strength and physical properties improved. Microstructural analysis showed high portlandite consumption and the formation of low Ca/Si ratio in new C-S-H structures near C-Na-Al-S-H as the main outcome of the binary admixture in ECC.

Service life of metakaolin-based concrete exposed to carbonation: Comparison with blended cement containing fly ash, blast furnace slag and limestone filler

Carbonation of cement-based materials can lead to corrosion of the steel bars in reinforced concrete, and supplementary cementing materials (SCMs) often increase the carbonation kinetics compared to reference concretes made of cements composed of clinker only. The aim of this work was to assess the consequences, in terms of service life of structures, of a possible increase in the carbonation depth of metakaolin concretes. Experimental testing using accelerated (4% CO2) and natural carbonation conditions showed that increasing the metakaolin content tended to increase the carbonation depth of concretes, due to the consumption of portlandite by the pozzolanic reaction. However, most of the time, the carbonation was within the range of carbonation depths found in commercially available blended cements (including fly ash, GGBS or limestone filler) that had already proved their worth on the market. The combination of MK and limestone filler (in CEM II/A-LL with 15% MK) behaved very well with respect to carbonation, the carbonation depth being almost equivalent to that of CEM I samples. The modelling of CO2 ingress into the concretes showed that, although metakaolin increased the carbonation (except when associated with limestone filler), the carbonation depth did not exceed 30mm after 50years, far from the value of 50mm generally used as concrete cover to protect the steel bars. This means that the formulations including metakaolin would not have deteriorated by the end of the building's service life.

Effect of mechanical processing on sugar cane bagasse ash pozzolanicity

This work intends to contribute to the understanding about the effect of sugar cane bagasse ash (SCBA) on the hydration of cement-based pastes by focusing on the influence of different particle size (or specific surface area). Isothermal calorimetry, thermogravimetry, chemical shrinkage, and strength pozzolanic index tests were performed to compare the hydration and chemical evolution of pastes containing SCBA with different fineness and pastes with inert (quartz) and pozzolanic (rice husk ash) materials. The results showed that a clear change in the kinetics of hydration and portlandite content by comparing the SCBA with different particle sizes. In addition, the different SCBAs had a marked effect on the chemical shrinkage and portlandite, with an intermediate behavior between the pastes with quartz and rice husk ash. Finally, with increasing in specific surface area and soluble fraction of SCBA, its pozzolanic activity was progressively enhanced due to both portlandite consumption and physical effects.

Outcomes of the RILEM round robin on degree of reaction of slag and fly ash in blended cements

Working group 2 of the RILEM TC 238-SCM undertook a comparison of laboratories and techniques for the quantification of the degree of reaction of supplementary cementitious materials in blended cements. A common set of binary pastes of Portland cement with two slags, a calcareous and a siliceous fly ash was tested in seven laboratories. The results obtained by selective dissolution produced were quite scattered and seemed to underestimated the degree of reaction. The analysis of portlandite consumption was found to significantly underestimate the reaction unless additional data from XRD and electron microscopy was gathered to complete the corrections. Despite limited access to electron microscope among the participants and thus only a small data set being collected, this technique appeared as one of the most consistent. XRD-PONKCS gave considerable scatter, due mainly to a lack of a strict protocol and excessive overlap of slag and C–S–H signals. Overall, the study indicated that the precision of determination of the degree of reaction of SCMs in cement pastes is rather low and at best ±5%.

Effect of Pozzolanic Additives on the Strength Development of High Performance Concrete

The aim of this research is to estimate the effect of pozzolanic substitutes on the temperature generated by the hydration and on the final strength of concrete. Differential thermal analyses (DTA) were conducted. Ternary cementitious systems with different ratios of Portland cement, silica fume and calcined illite clay were investigated. The results showed that the rates of pozzolanic reaction and portlandite consumption in the silica fume-blended cement pastes are higher than in the illite clay-blended cement pastes.

Influence of nano-silica and nano-alumina in lime-pozzolan and lime-metakaolin binders

The purpose of this work is to study the influence of two nanoparticles (nano-silica and nano-alumina) to the properties of pozzolanic binders. Natural pozzolan and metakaolin, as artificial pozzolanic material, were used and both of them combined with aerial lime. Nanoparticles were incorporated as additives in the mixture of binders by adding nano-silica or nano-alumina in water (by mass of binder). In natural pozzolan systems amorphous nano-silica increased compressive strength up to ∼39%, when crystalline nano-alumina decreased it by ∼35% at 28 days. In metakaolin system, nano-alumina increased compressive strength by ∼85%. Also, the use of nanoparticles reduced carbonation in all hydrated systems comparing to reference specimens. SEM observation lead to C-S-H and C-A-S-H compounds identification, showing that way the benefits of microstructure when nanoparticles were incorporated in pozzolan and metakaolin respectively. Capillarity and volume change agreed to all the other measurements. DTA-TGA attributed to the positive results by quantifying calcium carbonate decrease and XRD confirmed portlandite consumption.

Evaluation of activated high volume fly ash systems using Na2SO4, lime and quicklime in mortars with high loss on ignition fly ashes

In general, concretes made with blended Portland cement containing high volumes of fly ash provide an alternative to conventional Portland cement concrete to enable carbon footprint reduction. This study evaluates the chemical activation of four fly ashes in blends with Portland cement, by assessing their effects on hydration and compressive strength. In this study, a sieving process is used to regulate the fly ash composition, which has an effect in the chemistry and reaction of the mix. The results show the importance of the amorphous content of the fly ash with respect to achieving a high compressive strength. The effect of sodium sulfate, added as an activator, is significant in terms of compressive strength at early age for two of the fly ashes studied; in this case, the parameter used to correlate with the compressive strength evolution is the amount of portlandite consumed through pozzolanic reactions. However, sodium sulfate does not have the same effect on fly ashes with a high amount of Fe2O3, in which portlandite consumption is much lower.