Paste Specimens Research Articles

Three-dimensional mesoscale modeling of foamed cement paste using peridynamics

Foamed concrete’s unique mesoscale porous structures influence its mechanical performance. This study adopted peridynamics method to build the three-dimensional (3-D) porous structure of a foamed cement paste specimen. The numerical model was verified through a comparison with the experimental results. Based on the model validation, a series of models were established to investigate the influence factors of porosity and void shape on the mechanical performance of foamed cement paste under uniaxial compression. The porosity was ranged from 10% to 70%, and the sphericity of a single void was ranged from 0.6 to 1.0. The results showed that the mechanical properties, including modulus, peak strength and fracture energy, decreased with the increase of porosity, while increased with the increase of void sphericity. Significant heterogeneity was noted among the models with the same porosity, as the void sphericity could cause a different wall thickness between voids. The internal stress could easily cause a damage to a local area with thin wall thickness, leading to weakening the mechanical performance of foamed cement paste.

A Numerical Approach to Multiscale Simulation of Cement Paste Strength

Recent experimental investigations on the nanoscale of hardened cement paste revealed that the tensile strengths of the microstructural phases present amount to several hundreds of MPa. Confrontation with macroscopic tensile strength testing, by e.g. Brazilian splitting, shows a decrease over two orders of magnitude. A computational model based on a hierarchical representation of hardened cement paste microstructure is presented in this paper, attempting to shed light on the factors affecting the scaling of strength from the nanoscopic scale up to the macroscopic scale. The model is validated on a case study featuring a Portland-limestone cement paste subjected to an external sulfate attack. Such conditions compromise the nanoscopic integrity of the C-S-H gel as a consequence of the progressive decalcification and affect the overall load-bearing capacity of the macroscopic cement paste specimen.

Carbon dioxide uptake by pure Portland and blended cement pastes

Carbon dioxide sequestration by cement-based materials is a chemical process known as carbonation. Carbon dioxide absorption by cement consists of its reaction with calcium hydroxide, among other Portland cement components. This phenomenon should be taken into account in the calculation of the net contribution of cement production to greenhouse gas emissions. Therefore, carbon dioxide uptake should be considered into future life-cycle assessment protocols.This paper presents the carbon dioxide absorption results recorded in paste specimens during almost four years by 15 different types of Portland composite cements made of siliceous coal fly ash, natural pozzolan, ground granulated blast-furnace slag and limestone. Prismatic (10 ​× ​10 ​× ​60 mm) Portland cement pastes made of several types of cement (5% and 35% by weight) and with two cement/water ratios (0.45 and 0.60) were manufactured. The specimens were cured at 95% relative humidity for 24+ ​48 ​h and afterwards they were kept under laboratory room conditions for 25 more days. After this period, a set of the specimens remained in the lab, whereas other were moved outdoors for being kept sheltered or exposed from rain.Thermogravimetric measurements were performed along the time to determine the carbon dioxide absorption by the Portland cement pastes. The results were fitted to an exponential function. The maximum carbon dioxide absorption and the absorption rate depends on the exposure conditions, the water/cement ratio, the cement type (calcium oxide contents, type and percentage of the additions in the cement paste and compressive strength) and portlandite formed at 28-days. The results of this paper show that the absorption amounts were 13% and 30% in weight. of ignited cement.

Recycling carbonated reactive magnesium cement (RMC) as a building material

This study assessed the recyclability of reactive magnesium cement (RMC). To simulate the condition of construction waste recovery, a carbonated RMC-based paste was ground and used as a replacement for fresh RMC at 20 and 40 wt%. Both paste and mortar specimens were prepared and subjected to carbonation under 20% CO2, 80% RH, and 30 °C. Calorimetry data revealed that the main hydration peaks were delayed. The cumulative heat flow values were reduced, attributable to an increase in pore solution pH and physical barrier created by amorphous phases, which were confirmed by the appearance of broad peaks coinciding with those of hydromagnesite. Under carbonation, the peaks became more apparent due to the transformation of the phases into crystalline hydromagnesite. At 20 wt% replacement, the composite incorporated with the recycled RMC attained an improved strength performance due to more formation of the amorphous phases and hydrated magnesium carbonates (HMCs) from brucite.

Hydration, Microstructure and Compressive Strength Development of Seawater-mixed Calcium Sulphoaluminate Cement-based Systems

In-depth understanding of the calcium sulphoaluminate (CSA) cement, a type of environment-friendly cement, is essential for its wide application in engineering practice. Partial replacement of CSA by alternative alumina-rich powders has significant potential in the production of cost-effective CSA cement. In this work various supplementary cementitious materials (SCMs) including slag, metakaolin and silica fume with different replacement levels were added into CSA cement, and mixing with seawater, to prepare paste and concrete specimens. The development of hydration products in the paste specimens was characterized by X-ray diffraction and Thermogravimetric analysis. The pore structures of the seawater-mixed paste specimens obtained from mercury intrusion porosimetry tests, along with the pH value and free chloride content in the pore solution, were analyzed. Results show that various SCMs have significantly different effects on the pore solution chemistry, hydration and microstructural formation of the seawater-mixed CSA cement, resulting in different mechanical behavior and durability properties. The results of this work are helpful for the development of cost-effective CSA cement-based concrete used in marine constructions.

Pore structure characteristics of MgO–SiO2binder

AbstractThis paper reports on an investigation into the pore structure characteristics of MgO–SiO2binders. Paste specimens were prepared using two MgO/SiO2ratios (1 and 1.5), two silica sources (Silica fume‐SF and Metakaolin‐MK), two water to binder ratios (0.5 and 0.6) and compared to a portland cement (PC) control mix. Mercury intrusion porosimetry was used to understand the evolution of pore structural features of the binder with hydration period. A continued reduction in porosity and refinement of pores was observed with the progress in hydration irrespective of the type of the silica source or the initial MgO/SiO2ratio. The MgO–MK mixes exhibited a finer and denser microstructure as compared to the equivalent MgO–SF mixes. The formation of hydrotalcite in addition to M‐S‐H in MgO–MK mixes was postulated to be the primary reason for the difference in microstructural characteristics, compared to MgO–SF mixes. The space‐filling capacity of hydration products of MgO–MK binder was found to be better than PC.

Hydration and microstructural development of calcined clay cement paste in the presence of calcium-silicate-hydrate (C–S–H) seed

The microstructure of Portland and calcined clay cement paste incorporating calcium-silicate-hydrate (C–S–H) seed was investigated to understand the role of nucleation seed in cement hydration, and to study the potential synergy between C–S–H seed and calcined clay. The development of compressive strength of samples with and without seed was monitored until 28 days of hydration. Microstructural analysis techniques including thermogravimetric analysis (TGA), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), and quantitative X-ray diffraction (QXRD) were used to study the phase assemblage and composition of the cement paste specimens. Image analysis (IA) of backscattered electron (BSE) micrographs was used to determine the hydration degree of cement at the age of 1 day and 28 days. The results showed that the introduction of C–S–H seed clearly accelerated the hydration of cement and enhanced the hydration degree of cement even at the later age. The presence of seed also modified the composition of the inner products (IP) formed around the anhydrous cement grains. The combination of calcined clay and C–S–H seed enabled high replacement levels (up to 40 wt%) of cement without compromising its mechanical properties.

Sulfate performance of blended cements (limestone and illite calcined clay) exposed to aggressive environment after casting

This paper evaluates how early aggressive exposure affects the sulfate resistance of blended cements containing limestone filler and/or calcined clay. Mortar and cement paste specimens were elaborated with different combinations of limestone filler and two different illitic calcined clays and exposed to a sodium sulfate solution shortly after casting. Assessment of sulfate resistance was based on expansion, mass variation, and compressive strength. Microscale evolution and distribution were examined by mercury intrusion porosimetry, X-ray diffraction, and scanning electron microscopy with energy-dispersive spectrometry measurements. Results prove superior sulfate resistance of compositions with high calcined clay content over limestone filler addition. Furthermore, the results reported suggest that the pozzolanic reaction progresses enough to reduce the sulfate ingress even at early exposure conditions. Therefore, calcined clay replacement can still provide effective pore refinement to limit sulfate penetration, increase strength, and reduce available CH to limit expansive phase formation.

Experimental study of flexural fatigue behaviour of cement paste at the microscale

This study presents an experimental investigation of fatigue properties of cement paste at the microscale. A strong size dependence is found for the flexural fatigue life of the cement paste specimen. Microscopic observations on the fractured surfaces suggest that there is a higher density of nano-scale cracks generated during the fatigue loading compared to the static fracture. However, the fatigue damage evolution is found to be very slow and small even under high stress levels. The development of residual deformation for cement paste can be explained by the combined effects of viscoelastic deformation and fatigue cracking growth.

Comparative study on mechanisms for improving mechanical properties and microstructure of cement paste modified by different types of nanomaterials

Abstract Filling and nucleation are the mechanisms of modifying cement paste with nanomaterials, as investigated by previous studies, and are difficult to reflect the different effects of nanomaterials, especially on the changes of cement clinker and hydration products in the cement hydration process. In this study, the mechanisms of modifying cement paste with nano-calcium carbonate (NC), nano-graphene oxide (NG), nano-silica (NS), and nano-titanium dioxide (NT) were investigated by determining the mechanical properties of cement paste treated with nanomaterials and analysing the changes in the cement clinker (tricalcium silicate and dicalcium silicate), hydration products (portlandite and ettringite), and microstructure through many micro-test methods. The results indicate that the incorporation of nanomaterials could improve the early strength of cement paste specimens due to more consumption of cement clinker. Meanwhile, different nanomaterials promote the formation of different hydration products at early ages. C–A–S–H gel, flower-like ettringite, and C–S–H seeds are widely distributed in the cement paste with the incorporation of NC, NG, and NS, respectively. NT exhibits insignificant nucleation effect and has inhibitory effect on portlandite precipitation. This study provides key insights into the mechanism of nanomaterials from the perspective of cement hydration, which may promote the further research and application of nanomaterials in the cement and concrete industries.

Influence of curing conditions on pumice-based alkali activated composites incorporating Portland cement

One way to promote sustainability, energy conservation and environmental protection is to produce construction materials by the activation of aluminosilicate precursors with alkalis. Alkali activated materials produced with raw materials containing low calcium such as pumice harden in aggressive conditions. Therefore, determining the optimum curing conditions are of great importance for the evaluation of these materials. The curing conditions of pumice-based alkali activated materials has been reported by a few studies to a limited extend. This paper succinctly discusses the different curing conditions of pumice-based alkali activated composites incorporating ordinary Portland cement (OPC). The precursor raw material consists of 80% pumice and 20% OPC or %100 pumice. The prepared 50 × 50 × 50 mm cubic paste specimens were subjected to different heat-curing temperatures (20 °C, 40 °C, 60 °C, 80 °C, 100 °C and 120 °C) for 24, 48, 72 and 96 h and aging effect (3-28-90 days) was studied. Compressive strength , UPV , dry density, porosity and total water absorption tests were performed after curing processes. Furthermore, SEM/EDX images of the samples were taken to investigate the effect of curing conditions. The results show that OPC addition has increased the compressive strength under all curing conditions. The highest compressive strength was obtained by curing OPC added samples at 100 °C for 72 h. However, as the heat-curing temperature increased samples gained much of strength with less curing duration. Satisfactory compressive strength results can be obtained from samples cured under ambient conditions and especially containing OPC (63 MPa). The specimens containing OPC with the highest compressive strength are also the specimens with the highest density (1.89 g/cm 3 ) due to dense geopolymeric structure. At the same time, as the compressive strength increases the water absorption and porosity values decrease due to the reduced voids. Consequently, experimental findings revealed that the curing conditions influence the mechanical and physical properties of pumice-based alkali activated composites incorporating OPC. • Curing conditions affected both the mechanical and physical properties of pumice-based alkali activated pastes. • The highest compressive strength in P-HCSs was obtained in 90-day specimens exposed to 100 °C for 72 h. • The addition of OPC enabled the P–HSCs to gain strength in ambient conditions.

Expansion strain model and damage risk control for cement-based materials with low water–binder ratios under rehydration

Unhydrated cementitious materials in cement-based materials with low water–binder ratios undergo rehydration after re-exposure to water. Long-term rehydration not only damages cement-based materials but also influences their volume stability; however, the relationship between macroscopic strength and expansion strain remains unclear. In this study, rehydration tests were conducted on cement paste to analyse the influence of water soaking temperature and water–cement ratio on the expansion strain of specimens, and an expansion strain model was established based on a hyperbolic model form. The methods of assessment and control of damage risk induced by long-term rehydration for cement-based materials were developed on the basis of the established model and the correlation between compressive strength and expansion strain. Results showed that the expansion strain of the cement paste specimen increases with increasing water soaking temperature but decreases with increasing water–cement ratio. The values predicted by the model were in good agreement with the experimental results. The control effect for the damage risk caused by rehydration became more significant as the amount of silica fume increased. It provides experimental and theoretical bases for the application and life-cycle design of high-performance concrete (HPC) and ultra-high-performance concrete (UHPC) structures over long service periods in water environments.

Phase changes under efflorescence in alkali activated materials with mixed activators

Efflorescence in alkali-activated materials is a strong function of precursor and activator composition, which dictates their engineering properties and durability. In this study, the efflorescence of naturally cured NaOH/Na2SiO3 alkali-activated fly ash and alkali-activated fly ash-slag blended binder mixes was assessed with alkali concentration of 9 wt% Na2O, and 10 to 30 wt% of slag, and compared with binder mixes with 9 wt% Na2O, and 10 to 30 wt% of slag along with 2 wt% Na2CO3. The effects of efflorescence were assessed using visual and leaching inspections, and the compressive and split tensile strengths were determined post activation. Atomic absorption spectrometry was used to quantify free alkalis in the leachate, and X-ray diffraction, and Fourier transform infrared spectroscopy, magic-angle-spinning nuclear magnetic resonance and thermo-gravimetric analysis were performed to analyse the phase changes in binder pastes after efflorescence. The increased slag content facilitated the formation of C-A-S-H gel and enhanced both chemical and mechanical properties of binder pastes. Furthermore, the inclusion of slag content also led to the reduction of the open porosity and efflorescence formation. Subsequent exposure of binder specimens to efflorescence conditions aided the formation of carbonate products, degradation of N-A-S-H and N-(C)-A-S-H gel, and a decrease in split tensile strength in the binder paste specimens.

Heating and de-icing function in conductive concrete and cement paste with the hybrid addition of carbon nanotubes and graphite products

This paper aims to study the viability of conductive cement paste and conductive concrete with the hybrid addition of carbon nanotubes (CNT) and graphite powder (GP) as a self-heating material for heating, ice formation prevention and de-icing in pavements. Different heating tests, ice-preventing tests and de-icing tests were performed with cement paste and concrete specimens. Results confirm that the conductive cement composites studied, with the addition of 1% CNT + 5% GP, exhibited heating, de-icing and ice-prevention properties, when applying constant AC/DC voltages between the two end sides of each specimen, with relatively low energy consumption. The main contribution of this work is to achieve a sufficient conductivity level for the development of the heating and de-icing function using this hybrid addition in concrete, which has not been used so far, in order to be applied in real concrete structures.

Study on Adsorption Properties of Calcined Mg-Al Hydrotalcite for Sulfate Ion and Chloride Ion in Cement Paste.

In the marine environment, sulfate ions and chloride ions are abundant. Therefore, sulfate attack and chloride ion attack are common failure forms of marine concrete. Mg–Al hydrotalcite is a layered bimetallic hydroxide, which can be used as guest molecular adsorbent. In this experiment, we synthesized Mg–Al hydrotalcite, and the crystal state, surface morphology, and composition of this adsorbent were investigated by modern micro-analysis technology. Mg–Al hydrotalcite was added into the prepared target ion solution, to explore the influence of various factors on the adsorption performance of Mg–Al hydrotalcite, and then calcined Mg–Al hydrotalcite was added into cement paste, to study the mechanical properties and durability of the paste samples. The experimental results show that the optimum conditions for adsorption of chloride ions by calcined Mg–Al hydrotalcite are an adsorption time of 4 h, temperature of 35 °C, LDO (calcined Mg-Al hydrotalcite) dosage of 3.5 g/L, and a pH of 8. The adsorption effect of sulfate ion is best when the adsorption time is 6 h, the temperature is 35 °C, the dosage of LDO is 4 g/L, and the pH = 8. The optimal adsorption conditions of calcined Mg–Al hydrotalcite for chloride ion and sulfate ion are not completely the same, and the adsorption of these two ions in mixed solution shows competitive adsorption. Compared with the common paste specimens without Mg–Al hydrotalcite, the mechanical properties and deformation properties of cement specimens can be significantly improved by adding Mg–Al hydrotalcite.

Studies on microstructural and thermo-physico properties of microencapsulated eutectic phase change material incorporated pure cement system

Application of different MPCM dosage i.e. 5, 10 and 15% has been tested on the cement paste specimens in order to elucidate its role in deciphering the thermal comfort. Effective thermo-physico parameters like compressive strength, bulk density, porosity, water absorption, thermal conductivity and specific heat capacities were investigated and results obtained revealed significant changes in these properties with the incorporation of MPCM. Incorporation of MPCM causes decrement in compressive strength, bulk density, thermal conductivity while increment in porosity, water absorption and specific heat capacity. Moreover, instrumental analysis revealed that the incorporation of MPCM leads to loosening of dense microstructure in cement paste matrix and alters the rate of cement hydration to large extent as the formation of hydrated products like C-S-H and C-H decreases with the increase in the dosage of MPCM. Based on the findings, the developed MPCM may be used in the cement paste for better thermal comfort.

Geopolymer leaching in water and acetic acid

Elemental leaching of metakaolin based geopolymers was investigated by immersing hardened paste specimens in a solution. For this, pure water and 0.1 molar acetic acid solutions were replenished ten times distributed over 56 days in total. Dissolution and diffusion of the elements through and from the geopolymer paste into the surrounding solutions was investigated on cross-sections of specimens by SEM-EDS microscopy, indentation, X-ray powder diffraction analysis and measuring the eluted elements by ICP-MS when replenishing the solution over time. The presented new methodological approach thus combines the dissolution rate kinetics obtained via wet chemistry (ICP) with the complementary solid state characterisation methods to gain new insights into the complex geopolymer dissolution mechanisms.
 Results indicated a relatively small leachability of geopolymers, limited only to the surface layer which is directly exposed to the aggressive solution, while the more inner parts of the geopolymer framework remain intact. Elemental maps revealed dissolution of aluminates that occurred across the outermost surface parts of the sections, while potassium leached out gradually but reached deeper inner parts. However, there was still a high portion of potassium being left bonded inside the geopolymer, even for the harsh acidic conditions, limited by the diffusion-reaction mechanism which took place within the geopolymer. The obtained experimental results represent a first approach towards feeding reactive transport numerical modeling approaches still to be developed for simulating leaching and degradation of geopolymer materials when exposed to water or acidic solutions.

Effects of soft water attack on Portland and natural zeolite blended cements

The durability of concrete infrastructure is related to the properties of the applied concrete and the effects of the aggressive external environment on it. When concrete is directly exposed to soft water, the leaching of calcium ions from hardened cement or cement-based pastes occurs, causing reduction in strength and further deterioration of the concrete structure. This paper presents the experimental results of soft water attack effects on phase composition and compressive strength of blended cement pastes, when cement is replaced with 0, 10, 20 and 30% of natural zeolite. In order to simulate soft water attack in laboratory conditions, paste specimens were exposed to leaching in deionised water up to 180 days. The evaluation of the changes in phase composition (ettringite, portlandite, calcium silicate hydrate gel) due to calcium ion leaching was made based on X-ray diffraction analysis, Fourier-transform infrared spectroscopy and paste compressive strength tests. The presence of portlandite and ettringite after 180 days of exposure to deionised water indicates that leaching did not influence the stability of the hydration products in blended cement pastes. Moreover, blended cement paste with 10% of natural zeolite had a higher compressive strength than the reference one.

A Novel Approach to Characterize the Quantity and Distribution of Microorganisms in Microbial Self-Healing Cementitious Materials

Microorganisms can effectively heal the cracks of cementitious materials through the formation of calcium carbonate by microbial mineralization deposition, which has a wide application prospect in cementitious materials. In order to analysis the activity change and distribution of microorganisms in cementitious materials, this paper prepared cement pastes incorporated microbial powder, and extracted microorganisms by pre-crushing, grinding and ultrasonic, at the age of 3, 7 and 28 days respectively. The relationship between the optical density and effector cells is near linear fit, reflecting the activity change and specific distribution of microorganisms in cement paste specimen of different ages. The in-depth research on the activity change and distribution of microorganisms in the microbial cementitious materials can effectively characterize the microorganisms in the cementitious materials, which has a guiding role in microbial self-healing cementitious materials.

Synthesis of intumescent materials by alkali activation of glass waste using intercalated graphite additions

Abstract Intumescent materials are defined as the ability to swell when they are subjected to fire or thermal treatment. This paper presents the preparation of geopolymer pastes from glass waste powder without/with intercalated graphite (IG) powder (0.1, 0.2 and 0.3) wt.% additions by 5 M NaOH solution alkali activation. The Intercalated Graphite (IG) was prepared by mixing graphite powder (G) with Nitric Acid and Potassium permanganate in weight ratio (1:2:1) for 3 min. A thermal treatment was subjected to the hardened pastes between 400 °C and 800 °C, for 60 min to achieve a favorable porous microstructure. The result values of compressive strength were between 10 and 25 MPa. During the thermal treatment, a mass loss for all prepared paste specimens, ranging from 5.69 − 10.2% for the TG-N5 sample and between 8.27 and 9.4%, 7.06–11.02%, 8.12–9.97% for the (TGIG1-N5, TGIG2-N5 and TGIG3-N5) samples respectively the intercalated graphite additions recorded a decrease in the initial temperature of swelling at 500 °C (i.e. determined the swelling at temperature below 100 °C) as compared with the composition without addition, lightweight and volume increase of the specimens as well as the porosity (closed pores with various shapes and sizes) due to the intumescence process. Consequently, this type of materials could be used for fire protection in buildings, providing thermal and acoustic insulations or sealing the penetrations in walls preventing fire extension to adjacent rooms.