Gel Pore Water Research Articles

Study on the effect of mineral admixture on the water stability of UHPC under long-term immersion

Ultra-high performance concrete (UHPC) is prone to re-hydration as well as generating new hydration products in high humidity environments, which causes internal expansion and surface cracking. The addition of mineral admixture has a positive effect on long-term water stability of UHPC. In this study, four groups of UHPC specimens with different mineral admixture contents were prepared. Long-term immersion experiments of up to 360 days were conducted. The physical and mechanical properties of those specimens were investigated, and the re-hydration mechanism was revealed by NMR, XRD, and SEM tests. The specimens blended with silica fume and fly ash showed the best water stability, which exhibited the best physical and mechanical properties, and the less water absorption under long-term room temperature immersion. Compared with reference specimen of pure cement, the flexural strength, compressive strength of blended specimens was increased by 12%, 3.6%, and the mass change rate was reduced by 0.95%, respectively. In the early stage of immersion, the gel pore water and capillary pore water in the interior of the specimens were consumed and converted into free water. After 180 d, the percentage of water molecules with strong free degree was increased and the free water molecules were gradually reduced. The mechanism is that the small particle size of the silica fume and fly ash makes the UHPC stacking more compact. In addition, those mineral admixtures have higher volcanic ash effect, which can react with calcium hydroxide in hydration products to generate C-S-H and resulting in denser of UHPC internal structural. The double effect of filling effect and volcanic ash effect prevents more water from entering, which makes UHPC blended with silica fume and fly ash have better water stability.

Characterization of Pore Size Distribution and Water Transport of UHPC Using Low-Field NMR and MIP

Water transport is vital for the durability of ultra-high performance concrete (UHPC) in engineering, but its absorption behavior requires further comprehension. This study investigates the impact of silica fume (SF) and metakaolin (MK) on water absorption in UHPC matrix with a high volume of limestone powder (LS) under two curing temperatures, and the variation in water transport with pore size obtained by low field nuclear magnetic resonance (LF-NMR). Relations between cumulative water absorption with other properties were discussed, and the pore size distribution (PSD) measured by Mercury intrusion porosimetry (MIP) was compared with that determined by LF-NMR. Results showed that MK outperformed SF in reducing water absorption in UHPC matrix, containing 30% LS under steam curing due to the synergistic effect between MK and LS. The incorporation of LS greatly affected the water absorption process of UHPC matrix. In samples without LS, capillary and gel pores absorbed water rapidly within the first 6 h and slowly from 6 h to 48 h simultaneously. However, in samples with 30% LS, gel pore water decreased during water absorption process due to the coarsening of gel pores. MK was able to suppress gel pore deterioration caused by the addition of a large amount of LS. Compared with PSD measured by MIP, NMR performed better in detecting micropores (<10 nm).

Water transport in calcium silicate hydrate nanoscale pores by molecular dynamics: Unsaturated steady-state diffusion and suction

Water transfer in calcium silicate hydrate (CSH) nanoscale pores is an important factor affecting the durability of cement-based materials. Using molecular dynamics, the transport behaviour of nanoscale water in unsaturated CSH gel pores can be simulated to explore the transport law and mechanism of unsaturated water in the gel pores. In this paper, two kinds of transport models were established: an unsaturated steady-state diffusion model and a suction model. The effects of pore size and saturation level on the steady-state density distribution, dipole orientation, dipole moment and diffusion coefficient of water in each region were studied. Then, the influence of temperature, pore diameter, initial saturation level and tortuosity on the water absorption rate in the suction process was analysed, and the CSH molecular formula for the saturation state was calculated. The results showed that the distribution of water in the CSH gel pore was mainly related to the saturation level and wall position, and the percolation effect was triggered when the saturation of the gel pore reached 53 %. Water molecules located in the interlayer zone, adsorption zone and free zone exhibit the laws of the free zone > adsorption zone > interlayer zone due to the differences in geometric constraints and bonding. With the increase in the saturation level of water in the gel pore, the overall diffusion coefficient of water presents a change law of first rising and then falling. A higher temperature will magnify the effect on the promotion of transport. The pore size of the gel pore is positively correlated with the diffusion capacity of water, while higher initial saturation level and tortuosity will both inhibit the absorption rate of the gel pore. The CSH formula in the saturated stable state is (CaO)1.65SiO2(H2O)2.1. For cement-based materials with a high proportion of large gel pores (above 2 nm), limiting the saturation level in the gel pores below the percolation threshold (preferably below 45 %) as far as possible can effectively inhibit the water transport capacity. The simulation results are in good agreement with the data in the literature.

Issue Information

Cover Photograph: Schematic of water‐content‐dependent lamellar microstructure of C‐S‐H backbone sheets (black curves) and distribution of interlayer pore water (grey squares) and gel pore water (blue circles) at water‐saturated state (top), unsaturated state (middle) and totally dry state (bottom). [Image credit: Prof. Chunsheng Zhou; https://doi.org/10.1111/jace.17918] image

Understanding the microstructural evolution of hypersaline cemented paste backfill with low-field NMR relaxation

Cemented paste backfill (CPB) comprising mineral tailings, binders and mixing water is an important potential support material in the mining industry. As the mechanical properties of CPB are significantly influenced by its microstructural characteristics the development of measurement tools to better understand its pore structure evolution is important for its increased utilisation. This study reports the application of low-field nuclear magnetic resonance (NMR) relaxation time measurements to characterise the microstructural evolution of CPB materials over 56 days of hydration, contrasting common tap water and hypersaline water (~22 wt% salt) as mixing water. Distinct NMR relaxation time populations were evidenced within each CPB sample, revealing the presence of both capillary (T1,2 ≈ 10 ms) and gel pore water (T1,2 ≈ 300–500 μs), with time-dependent relaxation measurements facilitating characterisation of capillary pore structure evolution over the hydration period assessed. Hypersaline samples demonstrated a time-lag in this measured capillary pore evolution, relative to those hydrated with tap water, while pore structure evolution rates were observed to increase with increased CPB binder content. Further, both T1 and T2 NMR relaxation times were found to correlate with the uniaxial compressive strength of the CPB materials investigated, facilitating the formulation of a predictive correlation function between NMR relaxation characteristics and mechanical properties.

Influence of curing temperature on cement paste microstructure measured by 1H NMR relaxometry

1H nuclear magnetic resonance (NMR) relaxometry, supported by X-Ray diffraction and thermogravimetric analysis, has been used to characterise microstructure of white cement pastes underwater cured at temperatures in the range 10 °C to 60 °C. As the temperature increases, so the C-S-H, capillary pore water and interhydrate pore water content all increase; the ettringite and gel pore water content decrease; and the Portlandite content stays constant. A non-linear increase in the C-S-H ‘solid’ and ‘bulk’ densities, that exclude and include gel pore water respectively, has been observed with the increase of temperature. This is accompanied by a decrease in C-S-H water content but no change in the Ca/(Si + Al) ratio. The increase in the C-S-H ‘solid’ density has been attributed to a decrease in the number of locally stacked C-S-H layers. The increase in the C-S-H ‘bulk’ density is additionally attributed to the decrease in the gel porosity.

Monitoring the internal swelling in cementitious mortars with single-sided 1H nuclear magnetic resonance

Moisture transport into concrete and mortar surfaces strongly influences the durability of structures. Transport processes are foremost controlled by the material's porosity. Cementitious materials, such as concrete and mortars, exhibit distinct changes in pore structure while drying and subsequent rewetting due to the internal shrinkage and swelling of the layered calcium-silicate hydrates.Mortars with different w/c-ratios and binders were dried and rewetted and the subsequent time dependent changes in water filled pore structure were monitored with single-sided 1H nuclear magnetic resonance. This technique combines a localized depiction of all pore water with a good time resolution. The NMR signal is split into the four pore species C-S-H interlayer water, gel pore water, interhydrate water, and capillary water by multi-exponential analysis. Two consecutive running processes in the swelling of cementitious mortars were identified and discussed based on recent models of the cement microstructure as well as its implications on the sorptivity.

Observation of the redistribution of nanoscale water filled porosity in cement based materials during wetting

Spatially resolved GARField NMR has been used to follow the ingress of water into previously dried Portland cement concrete and mortar samples. It is shown that the amount of capillary water in the surface layers of different samples after 1day of capillary absorption exceeds the amount found before drying but that over the subsequent 7days the amount of capillary water decreases once more, even though the external source of water is maintained. The hydrate gel pore water was additionally tracked in the mortars. It shows complementary behaviour. The data is discussed in terms of C–S–H swelling.

Change of the Coefficient of Thermal Expansion of Mortar Due to Damage by Freeze Thaw Cycles

This paper presents the experimental methods and findings in obtaining the strain behavior of mortar during freezing and thawing cycles (FTC) at the meso-scale under fully saturated condition and the coefficient of thermal expansion (CTE) and elastic modulus of mortar after FTC tests. A heat-cool cycle test comparing an oven-dried and undried mortar was also performed prior to the FTC test, confirming that oven drying at 105°C does not affect the CTE of samples, thereby ensuring the reliability of the results. During FTC with constant moisture content, a limitation in the increase in tensile strain was observed and this tensile strain decreased until contraction was observed. The contraction is attributed to the removal of gel pore water arising from negative pressures. Due primarily to the absence of available water supply, the displaced pore water cannot be refilled, which results in contraction at the end of the FTC. More importantly, the results show that after FTC, the CTE of frost damaged mortar increases while its elastic modulus decreases, primarily owing to microcracking when frost damage sets in. Microcracks act as broken bridges that can detach the aggregate from the hardened cement paste and in effect reduces the thermal restraints that each part (fine aggregate and cement paste) exerts on the other. The hardened cement paste can then expand/contract more freely under temperature variation, and thus can significantly affect (increase) the CTE of the whole composite (mortar). Further, stress transfer in the material is prevented due to microcracking resulting in elastic modulus reduction.

Water Removal by Freeze-Drying of Hardened Cement Paste

This paper investigates the water removal extent of freeze-drying (F-drying) technique on hardened cement pastes. Two cement pastes with w/c = 0.5, 0.3 were prepared, and the authors measured the hydration degree, specific surface area, porosity, and solid skeleton density of the material samples at curing ages from 7 days to 180 days. From these results, the C-S-H gel volumes of samples are predicted from Jennings model and also evaluated from drying data. The good agreement between the results of two approaches confirms that F-drying removes only partially the water in gel pores of LD C-S-H structure. At the age of 180 days, the residual porosity saturated by water after F-drying is evaluated as 28% (w/c = 0.5) and 55% (w/c = 0.3) with dried C-S-H estimated as 28% (w/c = 0.5) and 13% (w/c = 0.3).

Densification of C–S–H Measured by 1H NMR Relaxometry

The nanoscale morphology of, and pore water interactions in, calcium silicate hydrate (C–S–H), the active component of cement, remain uncertain. 1H nuclear magnetic resonance (NMR) can fully characterize the nanoporosity of C–S–H in as-prepared material without the need for damaging sample drying. We use NMR to follow the density of C–S–H in sealed cured pastes as a function of degree of hydration (α) and water to cement ratio. We show clear evidence for C–S–H densification. The C–S–H “solid” density, exclusive of gel pore water, slightly decreases from ρx = 2.73 g/cm3 at α ≈ 0.4 to 2.65 g/cm3at α ≈ 0.9 due to an increase in the number of layers in the nanocrystalline aggregates. In the same range, the C–S–H “bulk” density, including gel water, increases from around 1.8 to 2.1 g/cm3. The increase corresponds to a transition from growth of low-density product containing gel pores to higher density product devoid of gel pores. We update Powers’ classical model from 1947. In contrast to the single “hydrate” ...

Probing of wood–cement interactions during hydration of wood–cement composites by proton low-field NMR relaxometry

Proton NMR T2 relaxometry has been applied to investigate phenomena involved in wood–cement composites during hydration. The transformation of capillary pore water into hydrates and gel pore water, as well as the microstructural changes occurring in the cement matrix, was continuously monitored during the first 28 days of hydration. Water in wood and its transfer into the matrix as cement hardens were also evidenced with the method. It has been found, for example, that some of the water in the mixture is retained in wood in the form of bound or free water, depending on the initial water content. By measuring the area under the different peaks, the consumption of water during hydration can be measured and the advancement of the hydration process can be evaluated via the hydration advancement coefficient α. The cement hardening within the composite has been also studied in the presence of calcium chloride, an accelerating agent. The acceleration was clearly evidenced at the early stage of the hydration process. The influence of extractives has been evaluated by comparing the hydration behaviour of composites prepared from Eucalyptus saligna (low extractives content) and Afzelia bipendensis (high extractives content), and a new compatibility index based on NMR relaxometry measurements has been proposed.

Characterisation of intra- and inter-C–S–H gel pore water in white cement based on an analysis of NMR signal amplitudes as a function of water content

1H nuclear magnetic resonance (NMR) relaxation analysis of water in progressively dried white cement paste is used to estimate the width and relative specific area of intra-C–S–H sheet pores and inter-C–S–H particle gel pores. The measurement is based on the ratio of solid echo to free induction decay signal amplitudes and the observation that as water is removed, so the surface fraction contributing to the solid echo increases. The intra- and inter-C–S–H pores are found to be 1.5 nm and 4.1 nm thick respectively. The total specific area and volume ratio is 2.4 and 0.88 respectively. The volume ratio of readily evaporable water within the pore types is 0.63. Hence, the sheet porosity is 47% of the total or 38% if based solely on evaporable water. The method is distinct from NMR analyses based on the relaxation time. There is good agreement between the measured widths by the two methods.

Effect of moisture content of concrete on water uptake

The penetration of water and non-polar hexane in Portland cement mortar prisms with different initial moisture contents was investigated using nuclear magnetic resonance ( 1H NMR). The amount of water in gel pores strongly affects the penetration of water in much larger capillary pores. Water penetration is reduced by the self-sealing effect as characterized by non-√ t dependence of capillary uptake and penetration depth. This is explained by the ongoing redistribution of water from capillaries into gel pores which results in internal swelling and loss of continuity of the capillary pore system; a correlation was observed between the amount of redistributed water and departure from √ t behaviour. A descriptive model is used to explain the dependence of water uptake and penetration on moisture content. For increasing initial moisture contents up to a critical value equivalent to equilibrium with a relative humidity between 65 and 80%, less penetrating water is able to redistribute. Thus more penetrating water is in larger capillaries with less viscous resistance; uptake and penetration depth increase. Above the critical initial moisture content, uptake and penetration depth decrease towards zero. This is explained by (a) an overall reduction in capillary pressure because transport takes places in fewer and larger pores and (b) an increase in viscous resistance due to the connection of penetrating capillary water with pores already containing water. Less capillary pore space is available for transport. The surface region of concrete placed in contact with water is not instantaneously saturated. Water content increases with time depending on the degree of surface saturation. A new transition coefficient for capillary suction γ is defined for the calculation of surface flux.

Proton Spin–Spin Relaxation Study of the Effect of Temperature on White Cement Hydration

The chemical and microstructural changes within a white cement paste were characterized in situ using proton nuclear magnetic resonance spin–spin relaxation at 30 MHz, and X‐ray diffraction. Paste samples with a water‐to‐cement ratio of 0.42 were cured at constant temperatures of 2°, 20°, 60°, and 100°C. Proton nuclear magnetic resonance spin–spin relaxation allows tracking the evolution of the mixing water into the solid fractions of calcium silicate hydrate, calcium hydroxide, and monosulfate, and the liquid phases: the calcium silicate hydrate interlayer water, gel pore water, and capillary pore water. It is shown that the hydration process is markedly accelerated with increasing hydration temperature, and that proton nuclear magnetic resonance relaxation measurements can quantitatively determine the proportions of water phases, their magnetic resonance characteristics, as well as the setting times of the cement during the hydration process.

Determination of the Neutron Scattering Contrast of Hydrated Portland Cement Paste using H2O/D2O Exchange

Abstract Accurate measurements of the surface area of cement based materials using small-angle neutron scattering (SANS) require determination of the neutron scattering contrast between the C-S-H gel and H2O. Because the C-S-H gel has a poorly understood structure and a variable composition, calculated values of the C-S-H:H2O neutron scattering contrast based on an assumed C-S-H chemical composition and density are subject to error. The C-S-H:H2O neutron scattering contrast was determined experimentally by measuring the change in apparent surface area as the H2O in a hydrated cement specimen was replaced with D2O, resulting in a new value of 6.78 × 1028 m−4. This new contrast value increases some previously reported SANS surface area values by 21%. The results also confirm that C-S-H interlayer water should be included in the solid phase whereas gel pore water should be excluded.

Pore structure of hydrating cement paste by magnetic resonance relaxation analysis and freezing

Nuclear magnetic resonance relaxation analysis has been applied to interpret the evolution of microstructure in a cement paste during hydration. A basic understanding of the wet-dry and freeze-thaw processes of cement pastes has been developed. The pore structure evolution has been studied by the suppression of the freezing temperature of water and compared with spin-spin relaxation analysis performed at room temperature. Both methods consistently show that hydrating cement pastes have two principal components in their size distribution. The NMR relaxation times provide a measure of the characteristic pore sizes. Their interpretation is made in the context of a fast exchange model. Supercooling and thawing point depression of confined water has been studied systematically. The depression of the freezing point of liquid water confined within a pore was found to be dependent on the pore size, with capillary pore water freezing at 240 K and the remaining gel pore water freezing over a temperature range extending to as low as 160 K.A modified Gibbs-Thompson analysis was used to determine pore volume distributions from the distribution of thawing temperatures.

Environmentally Sensitive Polymers and Hydrogels

Environmentally sensitive polymers and hydrogels exhibit sharp changes in behavior in response to relatively small changes in conditions such as temperature or pH. Typical changes include precipitation of the polymer in water or collapse of a hydrogel with expulsion of a large fraction of the gel pore water. These changes are depicted in Figure 1. If such a polymer is grafted or adsorbed on another polymer surface or within the pores of a porous membrane, then one observes sharp changes in wettability or permeability, respectively, when the polymer is environmentally stimulated. These changes are usually reversible, although the recdissolution or reswelling processes are often slower than the precipitation or deswelling steps. Other examples of environmental stimuli and polymer system responses are listed in Tables I and II.A large number of polymers display sharp responses to such stimuli. Many are based on vinyl monomers, such as anionic and cationic monomers for pH, specificion, solvent, or electrically responsive systems, and N-alkyl substituted acrylamides (e.g., N-isopropyl acrylamide, or NIPAAm) for temperature-sensitive systems. Other copolymer compositions may contain the responsive moiety as a pendant group on one of the monomers. Photoresponsive groups have been incorporated in such a way.The ratio in the responsive polymer of the responsive monomeric component to the “nonresponsive” comonomer (e.g., the pH-sensitive monomer, acrylic acid, versus acrylamide or the temperature-sensitive monomer, NIPAAm versus acrylamide) will control both the sharpness and intensity of the response as well as the specific environmental conditions where it begins.

Phase transformations of water in hardened cement paste a low-temperature DSC investigation

Differential scanning calorimety (DSC) was used to investigate the properties of gel pore water in hardened cement paste (hcp). By comparing heating and cooling thermograms using a step scanning procedure evidence was found for new phase transitions due to structured water. The new transitions occur at −31 °C and −24 °C during cooling. On drying the transitions vanish near relative humidities of 70% and 88%, respectively. An approximately cylindrical shape was deduced for the gel pores. The sorption hysteresis was also investigated.

A low-temperature DSC investigation of hardened cement paste subjected to chloride action

Differential scanning calorimetry (DSC) was used to investigate the freezing behaviour (−175°C to 20°C) of hardened cement paste (hcp) subjected to NaCl and CaCl 2 solutions containing up to 5.4 mol Cl/l. Thermograms were also recorded for samples dried to relative humidities (RH) between 61 % and 96 %. The low-temperature phase transition at −38°C exhibited the freezing behaviour of an aqueous solution in gel pores of ≈ 4 nm radius. Chloride ions strongly reduce the coupling forces between gel particles and structure the gel pore water. On desorption critical behaviour of the low-temperature transition energy was observed at 0.52 mol Cl/l which is the concentration most damaging to hcp. Above 1.3 mol Cl/l freezing of supercooled bulk water in macropores was observed at more than one temperature.