A microstructural approach to adherence mechanism of poly(vinyl alcohol) modified cement systems to ceramic tiles

PVA is a water soluble polymer used as cement modifier. An important modification observed by addition of PVA is the increase of the bond strength between cement paste and aggregate. The purpose of this work was to investigate the effect of PVA on the mechanism of adherence of cement pastes to ceramic tiles. Pastes with and without PVA were applied on the back side of porcelain tiles and after 56 days the microstructures of the interfaces were evaluated by SEM. The mode of rupture changed from mostly interfacial failure to a mixed-mode interfacial-cohesive failure for the paste with polymer addition in which was observed the reduction of the thickness of the porous transition zone between tile and paste bulk. Also, in plain paste the formation of a duplex film (CH plus C-S-H) in contact with tile surface was observed while in modified paste a single layer of C-S-H was identified.     

Evolution of strength, microstructure and mineralogical composition of a CKD–GGBFS binder

Characterization of a nontraditional binding material containing cement kiln dust (CKD) and ground granulated blast furnace slag (GGBFS) is discussed in this paper. Significant compressive strength was obtained for a CKD–GGBFS blend with 70% CKD and 30% GGBFS at a water-to-binder ratio of 0.40 after 2 days of curing at elevated temperature. Similar strength was also obtained for the samples subjected to normal moisture curing over a period of 28 days. The compressive strength increased with additional moist curing in both the cases. The microstructural and the mineralogical examinations show that the strength development was mainly due to the formation of calcium silicate hydrate (C-S-H). In addition to normal C-S-H, aluminum and magnesium incorporated C-S-H phases were also present in the CKD–GGBFS blends. The formation of ettringite appears to be a contributing factor in early age strength development of CKD–GGBFS binder.     

Phase Evolution during Autoclaving Process of Aerated Concrete

The reactions were investigated that occur when lime, cement, and quartz sand are mixed together and molded, then treated at 180°C under saturated steam pressures to produce autoclaved aerated concrete. The hydrothermal treatment of mixtures gives Ca-rich C-S-H with varying Ca/Si ratios as an initial product, which reacts further with silica dissolved from quartz to form 1.1-nm tobermorite with increase of curing time. During autoclaving, the composition of C-S-H and tobermorite as a binder continues to change until after 8 h, when the Ca/(Al + Si) ratio becomes constant at 0.8. As the reaction proceeds, the number of micropores increases, and the strength also increases due to the binder effect of the tobermorite. However, the total pore volume does not change, remaining constant values.     

Influence of activator type on hydration kinetics, hydrate assemblage and microstructural development of alkali activated blast-furnace slags

The hydration of two slags with different Al₂O₃ contents activated with sodium hydroxide and hydrous sodium metasilicate (commonly named water glass) is studied using a multi-method approach. In all systems, C-S-H incorporating aluminium and a hydrotalcite-like phase with Mg/Al ratio ~ 2 are the main hydration products. The C-S-H gels present in NaOH activated pastes are more crystalline and contain less water; a calcium silicate hydrate (C-S-H) and a sodium rich C-N-S-H with a similar Ca content are observed at longer hydration times. The activation using NaOH results in high early strength, but strength at 7 days and longer is lower than for the sodium metasilicate systems. The drastic difference in C-S-H structure leads to a coarser capillary porosity and to lower compressive strength for the NaOH activated than for the sodium metasilicate activated slags at the same degree of slag reaction.     

Influence of the C/S and C/A ratios of hydration products on the chloride ion binding capacity of lime-SF and lime-MK mixtures

The mechanisms of chloride ion binding in cementitious systems when supplementary cementing materials are present are not completely understood, although it is believed to relate to the alumina content of the mixture. This relationship is investigated through the use of lime-silica fume and lime-metakaolin mixtures. It was found that while the alumina content does have an important influence, the chloride binding capacity was also controlled by the calcium-to-alumina and calcium-to-silica ratios. At a high C/A ratio, the formation of monocarboaluminate is favoured, which has a high ability to form Friedel's salt, and does so at low chloride concentrations (less than 0.1 M). With a low C/A ratio, the formation of stratlingite is favoured, with little formation of monocarboaluminate. If alumina is not present, chloride is bound by the C-S-H phase. The binding capacity of the C-S-H was found to depend on its calcium-to-silica ratio, C-S-H with a higher C/S having a greater binding capacity.     

Composition and microstructure of 20-year-old ordinary Portland cement-ground granulated blast-furnace slag blends containing 0 to 100% slag

The morphology of outer-product (Op) C-S-H in 20-year-old slag-cement pastes appeared in most blends to be finer than at younger ages. The Ca/Si and Ca/(Si + Al) ratios of the Op C-S-H decreased with increasing slag content, and the Al/Si ratio increased. The Ca/Si ratio of C-S-H in the slag-containing pastes was lower at 20 years than at 14 months and the amount of Ca(OH)₂ was reduced indicating that additional slag must have reacted. The mean aluminosilicate chain length of the C-S-H was very long in all the samples and would be expected to have increased with age. The TEM-EDX and NMR data are consistent with nanostructural models for C-S-H. The Mg/Al ratio of the Mg-Al layered double hydroxide phase (LDH) was lower at 20 years than at 14 months in all cases except for the neat slag paste; aluminium hydroxide-based structure might be interstratified with those of the Mg-Al LDH.     

Influence of additions of coal fly ash and quartz on hydrothermal solidification of blast furnace slag

Blast furnace water-cooled slag (BFWS) has been solidified hydrothermally with tobermorite formation. The experimental results showed that the addition of fly ash and quartz was favorable to the formation of tobermorite, and the strength development of solidified body depended on both of the tobermorite formation and filling degree of formed tobermorite in the spaces between BFWS particles. The fly ash added appeared to have a higher reactivity than the quartz used during the initial hydrothermal processing due to the higher solubility of glassy phase in fly ash. The tobermorite formation seemed to be very sensitive to the fly ash content, e.g., the addition of fly ash 10-20 mass% was favorable to tobermorite formation, while the excessive addition of fly ash (> 20 mass%) appeared to impede the tobermorite formation. The excessive addition of quartz was also shown to exert a negative effect on the tobermorite formation, which causes strength deduction.     

Alkali binding in hydrated Portland cement paste

The alkali-binding capacity of C-S-H in hydrated Portland cement pastes is addressed in this study. The amount of bound alkalis in C-S-H is computed based on the alkali partition theories firstly proposed by Taylor (1987) and later further developed by Brouwers and Van Eijk (2003). Experimental data reported in literatures concerning thirteen different recipes are analyzed and used as references. A three-dimensional computer-based cement hydration model (CEMHYD3D) is used to simulate the hydration of Portland cement pastes. These model predictions are used as inputs for deriving the alkali-binding capacity of the hydration product C-S-H in hydrated Portland cement pastes. It is found that the relation of Na⁺ between the moles bound in C-S-H and its concentration in the pore solution is linear, while the binding of K⁺ in C-S-H complies with the Freundlich isotherm. New models are proposed for determining the alkali-binding capacities of C-S-H in hydrated Portland cement paste. An updated method for predicting the alkali concentrations in the pore solution of hydrated Portland cement pastes is developed. It is also used to investigate the effects of various factors (such as the water to cement ratio, clinker composition and alkali types) on the alkali concentrations.     

Effect of temperature on the pore solution, microstructure and hydration products of Portland cement pastes

The effect of temperature on the hydration products and the composition of the pore solution are investigated for two Portland cements from 5 to 50 °C. Increased temperature leads to an initially fast hydration and a high early compressive strength. At 40 and 50 °C, the formation of denser C-S-H, a more heterogeneous distribution of the hydration products, a coarser porosity, a decrease of the amount of ettringite as well as the formation of very short ettringite needles has been observed. At 50 °C, calcium monosulphoaluminate has formed at the expenses of ettringite. In addition, the amount of calcium monocarboaluminate present seems to decrease. The composition of the pore solution mirrors the faster progress of hydration at higher temperatures. After 150 days, however, the composition of the pore solution is similar for most elements at 5, 20 and 50 °C. Exceptions are the increased sulphate concentrations and the slightly lower Al and Fe concentrations at 50 °C.     

Structure and phase investigations on crystallization of 11 Å tobermorite in lime sand pellets

The present work examines the crystallization behaviour of 11 Å tobermorite and its dependence on the reactivity of different silica sources (quartz sand, grain-size ≤ 0.30 mm; quartz powder, grain-size ≤ 0.08 mm; inflated clay sand, grain-size ≤ 0.50 mm and raw perlite, grain-size ≤ 1 mm). The influence of different C/S ratios (calcium/silica ratio: 0.53, 0.83) was also investigated. For simulation of the industrial production process of lime sand products, a synthesis of lime sand pellets was carried out with a hydrothermal treatment (T = 200 °C, t = 40.5 h). The C-S-H phases were characterized by ESEM, EDX and X-ray powder diffraction.The investigations revealed that the grain-size, C/S ratio and porosity of the silica sources influence the formation of 11 Å tobermorite. A formation of 11 Å tobermorite using inflated clay sand with a grain-size ≤ 0.50 mm and a high porosity was only found with a C/S ratio of 0.53. This indicates a negative influence of an increase of lime content inside the synthesis mixture for tobermorite crystallization.Besides, a formation of xonotlite inside big pores of the lime sand pellet with inflated clay sand could be observed. The formation of portlandite and calcite was detected in all samples. The amount of calcite increased with the grain-size and with a higher C/S ratio.     

Impact of a 70 °C temperature on an ordinary Portland cement paste/claystone interface: An in situ experiment

Radioactive wastes in future underground disposal sites will induce a temperature increase at the interface between the cementitious materials and the host rock. To understand the evolution of Portland cement in this environment, an in situ specific device was developed in the Underground Research Laboratory in Tournemire (France). OPC cement paste was put into contact with clayey rock under water-saturated conditions at 70 °C. The initial temperature increase led to ettringite dissolution and siliceous katoite precipitation, without monosulfoaluminate formation. After 1 year of interaction, partial decalcification and diffuse carbonation (calcite precipitation) was observed over 800 μm in the cement paste. At the interface, a layer constituted of phillipsite (zeolite), tobermorite (well-crystallised C-S-H) and C-(A)-S-H had formed. Globally, porosity decreased at both sides of the interface. Geochemical modelling supports the experimental results, especially the coexistence of tobermorite and phillipsite at 70 °C, minerals never observed before in concrete/clay interface experiments.     

Structure of the hydrogen bonds and silica defects in the tetrahedral double chain of xonotlite

We use a combined experimental/theoretical approach to investigate the structure and stability of the silicate tetrahedral double chain in xonotlite. The M2a2bc polytype of xonotlite was found to be dominant in the synthetic samples. This polytype was used to predict the relative stability of silica defects in the tetrahedral double chain of xonotlite. The defects in Q³ sites were found to be substantially more abundant compared to Q² tetrahedra. Moreover, the paired substitutions in the silicate double chain at neighbouring Q³ sites are energetically more stable then isolated Q³ defects. The defects in the silicate chains of xonotlite offer structural channels accessible for diffusion of ions and water. The defect sites are potential candidates for incorporation of foreign ions in the xonotlite structure. Based on the crystal-chemical similarities with xonotlite the structure of silicate chains in tobermorite and C-S-H phases is discussed.     

The effects of nano-C-S-H with different polymer stabilizers on early cement hydration

A promising approach to accelerate cement hydration known as “seeding technology” has been discovered using nano-particles to provide additional nucleation sites for growing of C-S-H. Two different types of polymer, polycarboxylate (PCE) and polysulfonate (PSE) were used as stabilizer to synthesize nano-C-S-H via co-precipitation process. The obtained C-S-H-polymer composites were characterized by means of XRD, FTIR, thermogravimetric analysis (TGA), TEM, dynamic laser scattering (DLS), and BET. DLS measurement shows that the particle size of the obtained C-S-H-polymer suspension ranges from 82.6 to 589.9 nm. The results of DLS and BET show that the particle size of the C-S-H particles synthesized using PCE polymer as stabilizer is smaller than those synthesized with PSE polymer, and hence the specific surface area is much higher. FTIR and TGA results confirm the presence of the polymers in the obtained C-S-H composites particles. XRD results indicate that the presence of the polymers reduces the crystallinity of C-S-H due to the absence of the d002 peak at 2θ of 7°. The calorimetry results show that the main hydration peak of cement is dramatically increased by the addition of the C-S-H-polymer composites. It is interestingly found that the acceleration effect of the C-S-H-polymer composites is linearly proportional to the total surface area of the nanoparticles introduced into the cement pastes. At the same time, it is found that the secondary hydration peak, usually known as the sulfate-depletion peak, is greatly advanced by addition of the C-S-H nano-particles in comparison with the blank cement paste. The acceleration effect of the nano-C-S-H is further verified in a pure C₃S system.     

Incorporation of aluminium in calcium-silicate-hydrates

Calcium silicate hydrate (C-S-H) was synthetized at 20 °C to investigate the effect of aluminium uptake (Al/Si = 0–0.33) in the presence and absence of alkalis on the composition and the solubility of a C-S-H with a Ca/Si equal to 1.0.C-S-H incorporates aluminium readily resulting in the formation of C-A-S-H at Al/Si ≤ 0.1. At higher Al/Si ratios, in addition to C-A-S-H, katoite and/or stratlingite are present. Aluminium is mainly taken up in the bridging position of the silica dreierketten structure, which increases the chain length. The aluminium uptake in C-S-H increases with the aqueous aluminium concentrations.The presence of potassium hydroxide leads to higher pH values, to the destabilisation of stratlingite and to higher dissolved aluminium concentrations, which favours the aluminium uptake in C-S-H. Potassium replaces partially the calcium ions on the surface and interlayer, thus leading to more negative surface charge and to shortening of chain length.     

Influence of gypsum additive on the gyrolite formation process

The influence of gypsum additive on the gyrolite formation process and a sequence of intermediary compounds formation in the CaO-SiO₂·nH₂O-H₂O system was examined and explained. The synthesis has been carried out in unstirred suspensions. The molar ratios of primary mixtures were CaO/SiO₂ = 0.66. The amount of sulphate ions to be added to a raw mixture was 1-10%. The duration of isothermal curing at 200 °C was 4, 8, 16 and 72 h.It was determined that the quantity of sulphur which penetrates into the crystalline structure of gyrolite depends not only on the synthesis conditions but also on the composition of initial mixture. A larger amount of sulphate ions stimulate the formation not only of gyrolite, but also of CaSO₄. Gypsum additive has no influence on the re-crystallization temperature of C-S-H(I), Z-phase and gyrolite into wollastonite. The composition of initial mixtures is recommended to calculate according to molar ratios. In other cases, upon increasing the amount of sulphate ions, the basicity of the mixture decreases and gyrolite forms more difficult.     

Effect of TEA on fly ash solubility and early age strength of mortar

The investigations focused on the dissolution behaviour of fly ash in alkaline solution and the effect of triethanolamine (TEA) addition. TEA is known as a grinding aid in cement production and is an Al and Fe chelating agent. To determine the effect of TEA on the dissolution behaviour of fly ash constituents, fly ash was mixed with a KOH solution at pH 13 and different dosages of TEA. Samples were taken after different times and analysed by ICP-OES. The effect of TEA on the heat evolution rates of fly ash cement pastes was investigated using isothermal calorimetry. Strength tests were also conducted to investigate the effect of TEA on plain Portland cement and fly ash/cement mortars. TEA was found to increase the dissolution rate of Al, Ca and Fe from fly ash. A slight, but reproducible, effect on heat evolution rates and an increase in early age strength was observed for fly ash cements.     

Formation of 11 Å tobermorite from mixtures of lime and colloidal silica with quartz

Mixtures of lime, colloidal silica and quartz (<10 μm, 10–20 μm) were treated hydrothermally in stirred suspensions at 180°C to prepare 11 Å tobermorite with Ca/Si = 0.8. The runs made using the colloidal silica and lime quickly formed CSH; but did not convert into crystalline tobermorite even after 20 h. The runs with the mixtures of colloidal silica and quartz gave highly crystalline 11 Å tobermorite after 5 h through the reaction of Ca-rich C-S-H and quartz. The reaction of quartz was controlled by its rate of dissolution. The thermal behaviour of the tobermorites was normal, trending to mixed with increase in processing time.     

Synthesis of normal and anomalous tobermorites

Tobermorites were made from several starting materials at 105 – 180°C and Ca/(Si + A?) 0.8 – 1.0. Reaction gives in succession C-S-H, normal, mixed and anomalous tobermorites, and finally xonotlite. High C/S ratio (1.0), short time, low temperature, stirring, presence of A?, and if quartz is used, small particle size, all tend to stop it at normal tobermorite. In some cases, this effect is due to promotion of crystal growth of normal tobermorite. Low C/S ratio (0.8), long time, high temperature, no stirring, presence of A? plus alkali, and, if quartz is used, large particle size, all tend to give anomalous tobermorite. However, at 180° C this changes easily into xonotlite if C/S > 0.9.     

Nanogranular packing of C-S-H at substochiometric conditions

Herein, we present a comprehensive nanoindentation investigation of cement pastes prepared at substoichiometric water-to-cement (w/c) mass ratios between 0.15 and 0.4 with and without heat treatment. Based on a statistical indentation technique, we provide strong evidence of the existence of a statistically significant third hydrated mechanical phase in addition to the already known Low-Density (LD) and High-Density (HD) C-S-H phases. The nanomechanical properties of this third phase are found to follow similar packing density scaling relations as LD C-S-H and HD C-S-H, while being significantly greater. This third phase, whose nano-packing density is measured at 0.83 ± 0.01, is therefore termed Ultra-High-Density (UHD) phase. All three phases are present in concrete materials in different volume proportions: LD dominates cement-based materials prepared at high w/c mass ratios; HD and UHD control the microstructure of low w/c ratio materials. In addition, heat treatment favors the formation of HD and UHD. The insight thus gained into the link between composition, processing and microstructure makes it possible to monitor packing density distributions of the hydration products at the nanoscale.     

Composition, morphology and nanostructure of C-S-H in 70% white Portland cement-30% fly ash blends hydrated at 55 °C.

Outer product C-S-H had a mixture of fibrillar and foil-like morphology in a 28-day-old water-activated paste, and foil- or lath-like morphology in an alkali-activated paste. It was not possible to determine the chemical composition of C-S-H using SEM-EDX because of fine-scale intermixing with other phases; TEM-EDX was necessary. The C-S-H formed in the alkali-activated paste had a lower mean Ca/(Al + Si) ratio than that formed with water. The mean length of the aluminosilicate anions in the C-S-H was similar in both systems and increased with age; those in the Op C-S-H were likely to be shorter than those present in the Ip C-S-H with water activation, but longer (and more protonated) with alkali. The potassium in the alkali-activated paste was present either within the C-S-H structure charge balancing the substitution of Al³⁺ for Si⁴⁺, or adsorbed on the C-S-H charge balancing sulfate ions.