CEMENT最新文献

A review is made of the decisive parameters when predicting moisture conditions in a concrete structure exposed to natural climate. The required material properties such as the desorption isotherm, the scanning curves and the moisture transport coefficient are presented and discussed. The translation of parameters describing a natural climate into boundary conditions at a concrete surface is commented upon and examples are given on the effect of including or neglecting different parameters.

This study focuses on the characterization of white deposit occurring after wet/dry cycles on the surface of cement mortars presenting large amount of AFt phase. Correlative light-electron microscopy (CLEM) was confirmed to be a powerful method in characterizing and identifying microscopic visible white deposit that causes macroscopic visible surface whitening in those cement mortars. It was discovered that the white deposit formed after wet/dry cycles was caused by solids precipitated on the surface of cement mortars during hydration and/or on drying, and those solids were composed of CaCO₃, AFt, AFm or their solid solutions. Due to the resolution limit of the human eye, there is a threshold size 100 μm for the microscopic visible white deposit or its cluster to become macroscopic visible surface whitening. Partial covering of red pigments by the newly formed solids on the surface of the cement mortars further confirmed the relationship between the size and quantity of microscopic visible white deposit and macroscopic visible surface whitening.

Alkali activated materials (AAMs), also known as geopolymers, have been proposed as lower carbon footprint alternative cementitious materials to ordinary portland cement (OPC). Geopolymers are formed from an aluminosilicate precursor, such as fly ash, mixed with an alkaline activating solution. These materials may have self-healing behavior, which makes them promising materials for applications where controlling crack widths is critical, such as in well bores or steel-reinforced concrete structures. In this study, the self-healing ability of a Class F fly ash-based geopolymer was investigated using a pressure transmission test (PTT) to measure the initial permeability of the material, the permeability after damage using freeze-thaw cycling to initiate cracks, and the permeability following post-damage curing to encourage self-healing. The geopolymer was found to have low permeability, 0.26 ± 0.09 µD at 28 days, which is comparable to reported values for OPC. The permeability increased after damage, but then decreased again after the self-healing curing period. The results show an inverse correlation between the level of initial damage and the level of permeability restoration upon self-healing. This work indicates that geopolymers can indeed self-heal cracks to reduce damage, suggesting that they are promising barrier materials for well construction and other applications.

Thermal effects in concrete are associated with a heat release from the exothermic cement hydration reactions during concrete curing under normal conditions or under severe cold conditions, or when it is subjected to cycles of freezing and thawing. These thermal effects may cause cracking, impact concrete porosity and affects its thermal, mass and hydraulic conductivities, and hence create major durability problems. This paper presets an experimental study of the thermal management ability of an aqueous waterproofing solution (the Multi-Crystallization Enhancer (MCE)) that is intermixed with water at the time of batching. The experiments were performed according to the applicable ASTM procedures for measuring the rate of heat release, temperature profiles, compressive and flexural strengths, temperature-time factor and thermal and electrical conductivities. Additionally, the impact of cycles of freezing and thawing on the percentages of mass change, length change and relative dynamic modulus were investigated. The findings indicate that the addition of the MCE at a dosage of 2% of cement weight has the potential to mitigate the thermal effects during cement hydration and during curing concrete under freezing conditions providing a solution for thermal problems of mass concrete. The findings demonstrate that the MCE can delay the exothermic heat release and can reduce its rate at the initial stage. It can also increase the resistance of concrete against cycles of freezing and thawing by achieving 92% reduction in the percentage mass change, 15% reduction in the percentage length change and 17% enhancement in the relative dynamic modulus, after 300 cycles. These thermal impacts of the MCE are also associated with 16% reduction in the thermal conductivity and 90.7% reduction in the total charge passage through concrete.

Localized corrosion developed on post-tensioned steel strand in deficient grout, relating to elevated concentrations of sulfate ions. The deficient grout can also have low-level chloride ion concentrations below threshold values originating from the base materials. Open-circuit potential, linear polarization resistance (LPR), and electrochemical noise (EN) measurements were made on steel specimens exposed in saturated calcium hydroxide solution with 0.012 M Cl⁻, 0.04 M SO₄²⁻, or combined. Results showed that the combined presence of sulfates in low-level chloride alkaline solutions elevated the corrosion rate and the extent of corrosion pitting. The EN technique was shown to provide corrosion rate estimates consistent with LPR and was able to identify pitting characteristics. The outcomes of the research provides supporting evidence that analysis of deficient grout for chlorides alone may not capture the risk for corrosion and that corrosion associated with elevated sulfate concentrations can be exacerbated in presence of low-level chlorides.

To identify the impact of drying rate on mechanisms of drying shrinkage, two hardened cement paste (hcp) samples were prepared. Mature samples were dried directly at the target relative humidity (RH), “rapid drying”, or at RH decreasing from 95% step by step till 11%, “gradual drying”. When comparing the relation of mass change versus drying shrinkage, at high RH range over 80%, gradually dried samples showed less mass change for same amount of shrinkage comparing to rapid drying samples. For the range of RH of 80%-40%, the incremental values of both mass change and drying shrinkage were same for two drying methods. The specimens were characterized by XRD, TG-DTA and water vapor sorption isotherms. By combining the results with findings in the literature, we postulated that an additional part of drying shrinkage is activated when dried at high relative humidity for a longer time and we attributed this additional part to gel pores of calcium silicate hydrates.

In this work, we show that non-superabsorbent, thermo-responsive poly(N-isopropyl acrylamide) (PNIPAM) hydrogel particles (< 250 μm) can reduce autogenous shrinkage in cement paste and improve early-age stiffening that can be caused by traditional superabsorbent polymers (SAPs). Swelling measurements in DI water and cement filtrate solution suggest that SAP-induced early-age stiffening is caused by its super-absorbency in low-ionic solutions – a behavior not exhibited by non-superabsorbent PNIPAM. Addition of PNIPAM resulted in a 29% and 60% reduction in autogenous shrinkage strain at 14 days when used alone (0.3 wt% PNIPAM) and in combination with SAP (0.15% PNIPAM, 0.15% SAP), respectively, compared to a Control with no polymer addition. Furthermore, an addition of 0.3 wt.% PNIPAM exhibited a ∼29% and ∼37% decrease in static yield stress compared to a Control and 0.3 wt% SAP-modified cement pastes, respectively. Taken together, the results provide initial evidence to suggest that the use of hydrogels as internal curing agents may not necessarily require super-absorbency to reduce autogenous shrinkage. Non-superabsorbent hydrogels, like PNIPAM, may help reduce autogenous shrinkage while alleviating the effects of SAP-induced early-age stiffening.

Increasing pore solution pH in a concrete matrix will enhance CO₂ dissolution. In this study, calcined limestone was used as a replacement of reactive magnesium oxide (MgO) cement (RMC) at 5 and 10 wt.% to increase its carbonation rate and content. Its influence on strength development, chemical evolution, and microstructure was also investigated. The calcined limestone was found to increase the pore solution pH and consequentially reduce the hydration of RMC. Aggravated by a smaller particle size of the formed brucite, the composite strength under air curing was significantly reduced. Yet, the high pH environment, smaller hydration products and microporosity enhanced carbonation and retained strength development. The carbonation products were characterized by a mixture of hydrated magnesium carbonates (HMCs), calcite, and amorphous phases. The outcome of the study opens up a possibility for using less pure sources of magnesite and calcium oxide as a brine precipitation agent to produce RMC for construction applications.

Carbonation may potentially lead to corrosion of steel bars in reinforced concrete. This concern presents a major barrier against the implementation of sustainable low-clinker cementitious materials in the design of reinforced concrete structures. Various studies have documented the relationship between different equilibrium moisture states in carbonated concrete and the corrosion rate of the embedded steel. However, limited attempts were focused on visually observing the dynamic (time-dependent) behavior of moisture penetration into concrete and the related corrosion state and rate. Moreover, there is a lack of data on the local moisture state in the cementitious matrix in the steel-concrete interfacial zone. In this study, liquid water uptake in carbonated mortar was in-situ and over time monitored by neutron imaging. The corrosion state of embedded steel was monitored by means of electrochemical measurements. This combined experiment revealed that the arrival of the waterfront at the steel surface led to a sharp decrease of the steel potential. The corrosion rate increased from negligibly low values (<1 µm/year) to about 31 µm/year within a couple of minutes. Based on the neutron images, it is concluded that the moisture ingress through the concrete cover is locally affected by the heterogeneity of projected (depth-averaged) porosity distribution, and that large obstacles such as entrapped air have an effect. These observations were further confirmed by numerical simulation results of water transport, which also showed that liquid water permeability of the studied carbonated mortar determined by the inverse analysis is much higher than reported values in the literature. Overall, this study highlights the importance of considering the dynamic and coupled corrosion and moisture transport behavior during the periods which active corrosion can occur in carbonated concrete exposed to cyclic wetting/drying conditions.

Superabsorbent polymers (SAPs) efficiently reduce total shrinkage and cracking susceptibility of fibre reinforced mortars (FRM). This paper discusses the effects of SAPs on the microstructure and mechanical properties of FRM containing fly ash (FA) and ground granulated blast-furnace slag (GGBS) during a period of 180 days. Three types of cement including CEM I, CEM II/B-V and CEM III/A and three types of SAP with different chemical compositions and particle gradings were studied. The paper argues SAP's contribution to hydration of FA and GGBS and a subsequent deposition of these products on the fibres surface and in pores below 20 nm diameter. The analysis confirmed that SAPs provide additional water for hydration (internal curing) but also a required space for later hydration products (additional refilling of collapsed SAPs), resulting in more homogenous internal microstructure. This improvement is more prominent in mortars containing finer SAP (around 80 μm), which can facilitate strength recovery of up to 50%. The strength recovering process in FRM-SCM samples is boosted after the 2nd week, and is more dominant for samples containing CEM III/A.