Hydrated Calcium Aluminosilicate Research Articles

Topological origin of phase separation in hydrated gels

HypothesisDepending on their composition, hydrated gels can be homogeneous or phase-separated, which, in turn, affects their dynamical and mechanical properties. However, the nature of the structural features, if any, that govern the propensity for a given gel to phase-separate remains largely unknown. Here, we argue that the propensity for hydrated gels to phase-separate is topological in nature. SimulationsWe employ reactive molecular dynamics simulations to model the early-age precipitation of calcium–alumino–silicate–hydrate (CASH) gels with varying compositions, i.e., (CaO)1.7(Al2O3)x(SiO2)1 –x(H2O)3.7 +x. By adopting topological constraint theory, we investigate the structural origin of phase separation in hydrated gels. FindingsWe report the existence of a homogeneous-to-phase-separated transition, wherein Si-rich (x ≤ 0.10) CASH gels are homogeneous, whereas Al-rich (x > 0.10) CASH gels tend to phase-separate. Furthermore, we demonstrate that this transition is correlated to a topological flexible-to-rigid transition within the atomic network. We reveal that the propensity for topologically-overconstrained gels to phase-separate arises from the existence of some internal stress within their atomic network, which acts as an energy penalty that drives phase separation.

Mechanical performance and micro-structure of bentonite-fly ash and bentonite-sand mixes for landfill liner application

Abstract Low permeability (10−9 m/s) and adequate unconfined compressive strength (UCS = 200 kPa) forms the basis of using compacted bentonite (B) clay in landfill liner. However, B induces cracks or swells upon excessive drying and wetting events, respectively. To overcome failure of liner layer, B is amended with cohesionless sand (S), despite S being costly and ecologically unsustainable. In the recent past, fly ash (FA) a by-product from thermal power plants, has been explored as a suitable additive material in landfill liner, in line with the adoption of cleaner usage of waste material in construction. The objective of this study was to investigate the mechanical performance of both B-FA and B–S mixes along with the related changes in microstructure and physio-chemistry. 180 sets of UCS tests were conducted on 4 B-FA mixes (including class C and F) and B–S mixes at different compaction state and additive rate. Unlike B–S mixes, all B-FA mixes showed an increase in UCS with respect to B regardless of compaction state and additive rate. Detailed microstructure and physio-chemical characterization for B–FAs revealed the relative abundance in the formation of calcium aluminosilicate hydrate, and ettringite minerals after curing as compared to B–S mixes. In fact, Class C FA exhibited the largest UCS even at high moisture content, which can be attributed to higher formation of minerals upon curing. All B-FA mixes showed UCS greater than the minimum limit for liner application regardless of the compaction state. All B–S mixes did not adhere to the minimum UCS criterion at higher moisture content. FA additive at 50% showcased the highest increase in strength improvement factor (10 out of 12 cases), while for B–S mixes, this factor decreased with an increase in additive rate. Based on the measured data, two new linear relationships of UCS with pH and electrical conductivity (both indicative of reaction) of the mixes were observed. The relationship with respect to pH was more pronounced at higher moisture content as pH indicates the existence of CaO minerals.

Influence of calcium and alumina-based pozzolanas on the strength properties of low calcium fly ash geopolymer concrete

This work presents the effect of Ground granulated blast furnace slag (GGBS), fly ash (FA) and metakaolin (MK) on the strength properties of geopolymer concrete (GPC). Geopolymer concrete made with FA produces calcium aluminosilicate hydrate (C-A-S-H) product due to presence of alumina and sodium aluminosilicate hydrate (N-A-S-H) gel as main reaction product of polymerization. Geopolymer concrete made with FA and GGBS, calcium silicate hydrate (C-S-H) also gets produced additionally with calcium aluminosilicate hydrate (C-A-S-H) gel and sodium aluminosilicate hydrate (N-A-S-H) gel due to presence of high content of CaO in GGBS. This additional product imparts more strength performance in GPC. In geopolymer concrete made with FA and MK, the more amount of calcium aluminosilicate hydrate (C-A-S-H) is produced due to presence of high amount of alumina in metakaolin along with sodium aluminosilicate hydrates (N-A-S-H) giving more strength to GPC. Metakaolin is recommended to be used for the development of GPC because it has high amount of alumina.

Long-term use of modern Portland cement concrete: The impact of Al-tobermorite formation

The distribution of compressive strength in thick concrete members exposed to various environments in a nuclear power plant undergoing decommissioning is investigated. X-ray diffraction data, scanning electron microscopy analysis, and chemical analysis data reveal that rock-forming minerals in the aggregate had reacted and portlandite had been consumed to form calcium alumino silicate hydrates (C-A-S-H) when evaporable water content was sufficient. In addition, the study confirms Al-tobermorite formation in modern concrete after 16.5 years of elevated temperature conditions ranging from 40 to 55 °C. It is concluded that an appropriate aggregate and binder combination for the concrete enhances the compressive strength of thick concrete members thanks to the reaction of rock-forming minerals with portlandite, and also strengthens the chemical stability through the formation of Al-tobermorite under elevated temperature conditions and sufficient evaporable water content.

Effect of waste glass powder as a partial precursor in ambient cured alkali activated fly ash and fly ash-GGBFS mortars

This study investigated the feasibility of using waste glass powder (GP) as a partial precursor in fly ash (FA) and ground granulated blast furnace slag (GGBFS) based alkali activated mortars cured at ambient condition. FA was replaced by 0%–40% GP to produce alkali activated FA and FA-GGBFS mortars. The experimental results showed that the use of GP reduced workability of both FA and FA-GGBFS mortars. While no strength increase was observed by use of GP in the FA mortars, 5%–11% increase of compressive strength was noted in the FA-GGBFS mortars by the use of 10%–20% GP as a replacement of FA. Similarly, porosity, sorptivity and chloride permeability decreased significantly by 10%–20% GP in the FA-GGBFS mortars. However, drying shrinkage of both FA and FA-GGBFS mortars increased with the increase of GP content. Microstructural investigations showed that the use of GP significantly improved the microstructure of FA-GGBFS mortars by increasing the calcium containing reaction products such as calcium aluminosilicate hydrate (C-A-S-H) and calcium silicate hydrate (C-S-H) gel.

Hydration and Microstructure of Steel Slag as Cementitious Material and Fine Aggregate in Mortar.

Due to the low hydration activity and poor volume stability, extensive steel slag utilization is restricted. In this paper, the hydration process and microstructure of alkali-activated materials with steel slag as a cementitious material and fine aggregate were studied. The phase composition and micro-morphology of hydration products were measured using XRD, NMR and SEM. The response relationship between microstructure and mechanical properties during hydration was revealed. The results show that the main hydration products of the alkali-activated steel slag powder-granulated blast furnace slag powder cementitious system are Ca(OH)2 and calcium aluminosilicate hydrate (C-A-S-H) gel. With the progress of hydration, the amount of calcium silicate hydrate (C-S-H) gel and the average molecular chain length increase, Al[4]/Si decreases, while C/S increases first and then decreases, and the structure of cement paste becomes much more compact. The interface between steel slag sand and cement paste is denser than that of river sand, since the hydration occurs on the surface of steel slag sand, which leads to the formation of C-A-S-H gel and Ca(OH)2. As a result, the compressive strength of concrete prepared by steel slag sand is higher than that of river sand with the same mix proportion.

Gel composition and molecular structure of alkali-activated metakaolin-limestone cements

The influence of the compositional parameters Na2O/Al2O3 = 0.60–1.08, SiO2/Al2O3 = 2.42–3.17 and of 20–60% limestone on the compressive strength, gel composition and structure of alkali-activated metakaolin-limestone cements was studied. The reference formulation of SiO2/Al2O3 = 3.17, Na2O/Al2O3 = 1.08 and 20%limestone, formed sodium aluminosilicate hydrate (N-A-S-H) intermixed with a Ca-containing N-A-S-H (N-(C)-A-S-H), both with 3D-network structure. A reduced SiO2/Al2O3 = 2.42 resulted in the formation of Zeolites and calcium-aluminosilicate-hydrate (C-A-S-H) as secondary phases that reduced the strength. A lower Na2O/Al2O3 = 0.60 reduced the reactivity of metakaolin and the Al uptake in the gels, forming silica gel and reduced the strength of mortars. Increasing the limestone load to 60% increased the Ca2+ uptake in N-(C)-A-S-H via ion exchange Na+↔Ca2+ as charge balancers, without modifying the 3D-network structure nor the strength of pastes. Limestone can be considered as a precursor; its addition reduced the alkali consumption, changed the chemical composition and the assemblage of the reaction products, while preserving their 3D-network structure.

Influences of cross-linking and Al incorporation on the intrinsic mechanical properties of tobermorite

Tobermorite is the model for calcium aluminosilicate hydrate (C-(A-)S-H), the glue in concrete, and is also the binding phase in cement-based materials under geological conditions and in ancient Roman concrete. Correlating the mechanical properties of tobermorite with atomic structures substantially improves the understanding of C-(A-)S-H. We study this correlation for tobermorite with various Al contents using Raman, nuclear magnetic resonance spectroscopy, and high-pressure X-ray diffraction. Al incorporation shortens the cross-linked tobermorite chains, leading to more charge-balancing Ca and strong H-bonding, and enhancing the incompressibility of the basal-layer of Al-tobermorite. Non-cross-linked tobermorite exhibits high incompressibility of the basal layer but low incompressibility of the interlayer due to lack of cross-linking. The substitution of stronger SiO with AlO shows no negative influence on the mechanical properties of tobermorite. The results provide fundamental information on how atomic structure influences the intrinsic properties of C-(A-)S-H and have implications in validating computational methods and parameters.

Precipitation of calcium-alumino-silicate-hydrate gels: The role of the internal stress.

Concrete gains its strength from the precipitation of a calcium-alumino-silicate-hydrate (C-A-S-H) colloidal gel, which acts as its binding phase. However, despite concrete's ubiquity in the building environment, the atomic-scale mechanism of C-A-S-H precipitation is still unclear. Here, we use reactive molecular dynamics simulations to model the early-age precipitation of a C-A-S-H gel. We find that, upon gelation, silicate and aluminate precursors condensate and polymerize to form an aluminosilicate gel network. Notably, we demonstrate that the gelation reaction is driven by the existence of a mismatch of atomic-level internal stress between Si and Al polytopes, which are initially experiencing some local tension and compression, respectively. The polymerization of Si and Al polytopes enables the release of these competitive stresses.

Thermodynamic modelling of phase evolution in alkali-activated slag cements exposed to carbon dioxide

Carbonation of cementitious materials induced by their interaction with atmospheric CO2 is one of the main degradation mechanisms threatening their durability. In this study, a novel thermodynamic model to predict the phase evolution of alkali-activated slags exposed to an accelerated carbonation environment is presented. This model predicts the phase assemblages of carbonated alkali-activated slag cements, as a function of CO2 uptake under 1 v/v % CO2 conditions, considering the bulk slag chemistry and activators used. The changes taking place during the carbonation process regarding the physicochemical properties of the main binding gel, an alkali calcium aluminosilicate hydrate (C-(N)-A-S-H), the secondary reaction products CaAl and MgAl layered double hydroxides, and amorphous aluminosilicate gels, were simulated and discussed. The predictions of the thermodynamic model are in good agreement with experimental data retrieved from the literature, demonstrating that this is a valuable tool for predicting long-term performance of alkali-activated slag cements.

Effects of Aluminum Incorporation in Tobermorite Structure on Chloride Diffusion: A Molecular Dynamics Simulation Study

In this paper, the effects of different aluminum to silicon ratios in silicate chains of calcium silicate hydrates (C-S-H) are evaluated on the diffusion coefficient of chloride ions by molecular dynamics method. Tobermorite is a crystalline phase that is used for studying C-S-H properties in nano scale, because of its analogous chemical composition to C-S-H. Aluminum incorporation in C-S-H and the formation of calcium aluminosilicate hydrates (C-A-S-H) is due to both of hydration of tricalcium aluminate (C3A) in portland cement and aluminum oxides in pozzolans. There exist different Al/Si ratios in the tetrahedral chains of C-A-S-H depending on available aluminum oxides in cementitious raw materials. In order to compare the simulation results with previously-published experimental researches on cement pastes, a novel method is introduced here to calculate Al/Si ratio in tetrahedral chains of C-A-S-H using pozzolan replacement ratio in cementitious paste. MK (metakaolin) and FC3R (Fluid Catalytic Cracking Catalyst Residue) are the pozzolans that are used to validate the obtained results in this paper. Results showed that diffusion coefficients of chloride ions in C-A-S-H decrease by Al/Si ratio increasing in the tetrahedral chains as it was observed experimentally in previous researches.

Effect of ionic solutions on the performance of electrokinetic treatment of soft soils

This study evaluates the performance of electrokinetic (EK) treatment to improve the engineering properties of soft soils. Laboratory tests were conducted using ionic solutions of calcium chloride, CaCl2 and sodium carbonate, Na2CO3. Atterberg limits, unconfined compression, electrochemical, thermal, and microscopic tests were performed in terms of treatment duration, anode-to-cathode distances, and different combinations of ionic solutions as calcium chloride-distilled water (CaCl2-DW), sodium carbonate-distilled water (Na2CO3-DW) and calcium chloride-sodium carbonate (CaCl2-Na2CO3). EK treatment caused a reduction in the specific surface area, Sa, cation exchange capacity, CEC, and plasticity resulting in 26%, 14%, 19% reduction using CaCl2-DW, Na2CO3-DW, and CaCl2-Na2CO3 ionic solutions, respectively. The formation of the new compounds, calcium aluminosilicate hydrates, CASH resulted in the strength gain of the EK treated soils and increased the strength from 21 kPa to 90 kPa, 63 kPa, and 92 kPa with the ionic solutions of CaCl2-DW, Na2CO3-DW, and CaCl2-Na2CO3, respectively.

Flexural behaviour of reinforced concrete beams with partial replacements of metakaolin and marble powder

Concrete is the most widely used and versatile building material which is generally used to resist compressive force. By addition of some pozzolanic materials as admixtures, the various properties of concrete can be improved. Many modern concrete mixes are modified with the addition of admixtures, which improve the micro structural properties as well as decrease the calcium hydroxide concentration by consuming it through a pozzolanic reaction. Metakaolin is a cementitious pozzolanic material highly efficient and can react rapidly with the excess calcium hydroxide resulting from OPC hydration by a pozzolanic reaction, to produce calcium silicate hydrate and calcium alumino silicate hydrates. Due to rapid growth of construction activity, the available sources of natural sand are getting exhausted. Therefore, it is necessary to replace natural sand in concrete by an alternate material either partially or completely without compromising the quality of concrete. Waste marble dust is one such material which can be used to replace sand as fine aggregate in concrete. The present study describes an experimental program conducted to investigate the flexural behaviour of Reinforced Concrete beams with metakaolin as partial replacement for cement and marble powder as partial replacement for river sand in preparation of concrete. The percentage replacement of cement by metakaolin considered in this experimental study are 0, 2, 4, 6 and 8% by weight of cement and sand by marble powder are 20, 15, 10, 5 and 0%. The mechanical properties such as compressive and split tensile strength are analysed for conventional and modified concretes, at the age of 7, 14 and 28 days. A total of five RC beams, with dimensions 150 × 200 × 1500 mm was cast and tested under four point loading. The observations made were initial crack load, ultimate failure load and deflection of the beams. The results show that metakaolin and marble powder can be used as alternate construction materials at lower percentage replacement levels.

Natural carbonation-induced phase and molecular evolution of alkali-activated slag: Effect of activator composition and curing temperature

In this work, the evolution of reacted phase assemblage and molecular structure of alkali-activated slag (AAS) exposed to atmospheric natural carbonation is studied. The hardened AAS pastes with various activator types (i.e., sodium hydroxide, sodium carbonate, sodium sulfate, potassium hydroxide, and potassium carbonate solutions) and cured under two temperature conditions (20 °C and 80 °C) are evaluated. The results show that natural carbonation leads to a strong decalcification and silicate polymerization of calcium-aluminosilicate-hydrate (C-A-S-H) and precipitation of various calcium carbonate polymorphs depending on activator composition. The sulfate-activated slag shows the poorest carbonation resistance likely due to its limited CO2 absorption capacity, manifested with the formation of calcium carbonate polymorphs, gypsum, gibbsite, and calcium-deficit aluminosilicate gels. While high-temperature curing tends to increase the crystallinity level of C-A-S-H, it has little effect on its carbonation resistance in terms of phase and molecular stability.

Hydration kinetics and products of MgO-activated blast furnace slag

Hydration kinetics and products of MgO-activated slag are investigated by employing multiple analytical characterization techniques and thermodynamic modelling. The main hydration products of this cement are a calcium-aluminosilicate hydrate type gel, ettringite, monosulfate, hydrotalcite, brucite, and a third aluminate hydrate, while the extent of reaction and formation of reaction products significantly varied by MgO dosages. Higher dosage of MgO increased the degree of reaction of slag, and led to a higher population of Al in the octahedral region, which can be attributed to greater competition for Al required for the formation of hydrotalcite. The experimental and simulated volume of the solid binder increased as the MgO dosage increased, showing a good correlation with the strength increase of the samples with higher MgO dosage.

Quantitative phase analysis of slag hydrating in an alkaline environment

An X-ray diffraction (XRD)-based evaluation of the crystalline and amorphous phases in slag hydrating in an alkaline environment is presented. A method is developed for the quantification of the amorphous phases present in hydrating slag in a sodium hydroxide solution. In hydrating slag, the amorphous reaction product is identified as calcium aluminosilicate hydrate. A water-soluble sodium-based amorphous reaction product is also produced. The XRD-based quantification method relies on the direct decomposition of the XRD intensity pattern of the total amorphous phase present in partially hydrated slag into the intensity patterns of the amorphous unreacted slag, the hydrate and the sodium-based product. The unreacted slag content in partially hydrated slag is also determined from the decomposition of the intensity signature of the total amorphous phase. An independent verification of the amorphous unreacted slag content in hydrating slag is obtained from measurements of blends of unhydrated and partially hydrated slag. The XRD-based phase-quantification procedure developed here provides a basis for evaluating the extent of reaction in hydrating slag.

18-month hydration of a low-pH cement for geological disposal of radioactive waste: The Cebama reference cement

Low-pH cements are candidate materials for use in the construction of geological disposal facilities for the long-term management of nuclear waste. Since these facilities will operate over long time scales, the changes in mineralogy and microstructure require evaluation as a function of time. As a first step towards this understanding, the hydration of a standardised low-pH cement paste, known as the Cebama reference cement, was investigated over an 18-month period. Characterisation was performed at 28 days of curing, at 20 °C and 40 °C, and novel synchrotron radiation X-ray diffraction experiments were performed, in-situ, from 90 min to 18 months of curing. Concurrent solid state 29Si and 27Al MAS NMR data were acquired for parallel samples to quantify the extent of cement hydration and the composition and mean chain length of the predominant calcium aluminosilicate hydrate (C-(A)-S-H) reaction product. After 18 months, cement clinker phases were still present, highlighting the slow hydration kinetics of this low-pH cement. The data presented provide a benchmark for ongoing and future studies of low-pH cements in geological disposal environments, over extended time scales.

Use of Stabilized Lateritic and Black Cotton Soils as a Base Course Replacing Conventional Granular Layer in Flexible Pavement

The present work investigates the improved properties of lateritic and black cotton soils stabilized with ground granulated blast furnace slag (GGBFS) and alkali solutions. The alkali solution includes a mixture of sodium hydroxide and sodium silicate. The lateritic and black soils are treated with 30% GGBFS and the alkali solutions consisting of 6% Na2O having silica modulus (Ms) of 0.5, 1.0 and 1.5 at a constant water binder ratio of 0.25. The treated samples were air-cured for 0 (immediately after casting), 3, 7 and 28 days at ambient temperature. The treated lateritic soil with 0.5 and 1.0 Ms is found durable after 3, 7, and 28 days curing. Whereas, the treated BC soil found durable with Ms 0.5 at modified Proctor density after 28 days curing. The formation of calcium silicate hydrate and calcium aluminosilicate hydrate structures resulted in a remarkable improvement of compressive strength, flexure and fatigue life of treated soils due to dissolved calcium ions from GGBFS, silicate and aluminium ions from alkali solutions. The microstructure image of the durable soil sample shows the crystal orientation of particles. The design of high and low volume roads is proposed by replacing the conventional granular layer with the durable stabilized soil and stress–strain analysis is carried out using pavement analysis software.

Degradation mechanisms of alkali-activated binders in sulfuric acid: The role of calcium and aluminum availability

Although alkali-activated binders (AABs) show a stronger resistance against sulfuric acid (H2SO4) attack than ordinary Portland cement (OPC), it remains unclear how the resistance capacity is affected by the compositional variability of gel products, i.e., sodium calcium aluminosilicate hydrate (N-C-A-S-H). In this work, the compositional and molecular alteration of synthetic N-C-A-S-H exposed to 1.0%, 2.5%, and 5.0% H2SO4 solutions are studied. The results show that the formation of ettringite is less favored than gypsum when the calcium availability is limited, despite sufficient aluminum sources. The calcium and aluminum dissolved from N-C-A-S-H can contribute to the ettringite formation even though the AFm phases are present, particularly for those with relatively high Ca/Si and Al/Si ratios. The aluminum content in AABs controls the AFm formation and Al/Si ratio of N-C-A-S-H, thus affecting the condition for ettringite and gypsum precipitation. The presence of calcium in the atomic structure of N-C-A-S-H makes it more susceptible to the acid-induced degradation and polymerization.

Strengthening Behavior of Cemented Paste Backfill Using Alkali-Activated Slag Binders and Bottom Ash Based on the Response Surface Method.

A new type of cemented paste backfill (CPB) was prepared by using the bottom ash (BA) from a thermal power plant as an aggregate, alkali-activated slag as a binder, and an air-entraining agent as an admixture. Based on the central composite design (CCD) response surface method, the mix ratio was optimized, and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) was performed on the optimal mix ratio. ImageJ software was utilized to determine the porosity of the experimental samples at various curing ages. The results indicate that the optimal mix ratio of the aggregate-binder ratio is 3.28, the alkali dosage is 3%, the solid content is 67.44%, and the air-entraining agent dosage is 0.1%. As the curing age increases, the porosity of CPB gradually decreases. A calcium aluminosilicate hydrate (C-A-S-H) gel is the main hydration product of alkali-activated slag. At the beginning of the hydration reaction, the slag gradually dissolves, and the C-A-S-H product binds the BA together. At 14 d, complete calcium hydroxide (CH) crystals appeared in the hydration product. Finally, the degree of C-A-S-H crystallization increased further to form a dense structure.