The comparison between sulfate salt weathering of portland cement paste and calcium sulfoaluminate cement paste

In this paper, the damage performances of sulfate salt weathering of Portland cement paste and calcium sulfoaluminate (CSA) cement paste were compared according to authors' previous studies. It was found that the evaporation zone of speciments partially immersed in 10% Na2SO4 solution were both severely deteriorated for Portland cement and CSA cement. However, the differences were more significant: (1) the CSA cement paste were damaged just after 7 days exposure compared to the 5 months exposure of Portland cement paste under the same exposure condition of RH 60% and 20°C; (2) the cement paste specimen was split into several pieces along the shrinkage cracks, and the damaged CSA cement paste consisted of a detachment of successive paste layers; (3) gypsum and ettringite were identified in the Portland cement paste and attributed to the paste failure mechanism, however sodium sulfate crystals were clearly observed in the detached paste layers. According to the comparison the so-called sulfate weathering of Portland cement concrete was discussed

Study of cement-fly ash paste exposed to sodium sulfate solutions with different concentrations at different temperatures

When concrete elements are partially exposed to sulfate environment, in the upper part of concrete elements above ground an aquiferous zone containing almost saturated and high pH value (> 12.5) sulfate pore solution will be formed. The concentration of sulfate solution is much higher than 5 %, as usually used in laboratory. It is necessary to study the performance of cement-fly ash paste in different high concentration sulfate solutions at different temperatures. In this paper, pure cement paste and cement - fly ash (25 % dosage) paste specimens were immersed in the 5 %, 15 %, 20 %, 30 %, 40 % and 50 % sodium solutions at 20°C, 30°C and 40°C respectively. After 1, 3 and 6 months immersion, the compressive strength of the specimens was measured. XRD and thermal analysis were employed to analyze the reactive products of the paste. The experimental results show that the reactive aluminum in fly ash is activated by high concentration sodium sulfate solution at different temperatures and more ettringite is generated than pure cement paste

The damage of calcium sulfoaluminate cement paste partially immersed in NA(2)CO(3) solution

In the presented paper, the tests were designed to offer indirect evidences for the physical sulfate attack on ordinary concrete. The calcium sulfoaluminate (CSA) cement paste was partially exposed to 10% Na2CO3 solution under condition of 20oC and RH 60%. The microanalysis results showed that Na2CO3 crystallization did not occur in the CSA cement paste and the Na2SO4 crystallization, the product of chemical reaction between CSA cement paste and Na2CO3, caused the cracks formation at the edge of specimens. The Na2CO3 crystallization occurred after the chemical reactions disappeared. As comparison, it can be confirmed that the physical sulfate attack or sulfate crystallization also cannot occur in the ordinary concrete due to the chemical reactions between Portland cement paste and sulfate

The effect of epoxy coating on sulfate attack on cement paste partially immersed in sulfate environment

In the present paper, cement paste and cement and fly ash paste specimens with and without epoxy coating are partially immersed in saturated sodium sulfate solution at 20°C for 6 months. The evaporation speed of solution and development of compressive strength of specimens are measured. The results show that epoxy coating cannot only improve the sulfate resistance of upper part of paste above solution but alos of lower part under solution. According to XRD analysis, chemical sulfate attack is more likely to be the mechanism resulting in the failure of concrete elements partially immersed in sulfate environment. The epoxy coating can alleviate the chemical sulfate attack and improve the sulfate resisting performance of concrete

The real world of physical attack on concrete

According to an extensive literatures review, the the real world of physical salt attack on concrete was summarized. The physical salt attack on concrete can be divided into two categories according to the physical and chemical relationship between salt and the hydration products of cement: (1) a typical physical salt attack is attributed to the damage mechanism for the evaporative concrete where the salt is inert to the hydration products of cement, such as Portland cement concrete partially immersed in Ca(NO3)2 solution and Calcium Sulfoaluminate cement concrete in Na2SO4 solution. The concrete will be damaged in a quiet short time. (2) a complicated physical and chemical salt attack combining concrete carbonation is attributed to the damage mechanism for the evaporative concrete where the salt is reactive with the hydration products of cement. The Portland cement concrete partially immersed in the sulfate environment belongs to the second category and its damage mechanism is that the chemical sulfate attack will be the only mechanism causing the evaporative concrete damage if there is no the effect of carbonation. With the progress of exposure, the outer layer of evaporative concrete will be neutralized by carbonation and become inert to sulfate, then the physical sulfate attack occurs in the neutralized concrete layer. In the inner area isolated from carbonation, the chemical sulfate attack is still the damage mechanism. The evaporative concrete damage is collectively controlled by the susceptibility to carbonation of concrete and the factors in favour of chemical sulfate attack

Mechanical performance of concrete partially exposed to sulfate solution under hydrostatic pressure

In an extended experimental program, the development of mechanical performance of concrete cylinders partially exposed to sodium sulfate solution under hydrostatic pressure was investigated. The effects of relative humidity, concrete carbonation, fly ash and surface coating on concrete performance under sulfate attack were studied. The experimental results showed that (1) the concrete cylinders showed more severe damage in a humid environment than in a dry environment, (2) concrete carbonation could promote the failure of concrete, (3) fly ash concrete was more susceptible to damage than ordinary concrete, (4) the surface coating was an effective way to prevent sulfate attack on partially exposed concrete. Finally, the mechanism of sulfate attack on partially exposed concrete was explained

Sulfate Attacks on Uncarbonated Fly Ash + Cement Pastes Partially Immersed in Na2SO4 Solution

In this study, the sulfate attack on uncarbonated cement paste partially exposed to Na2SO4 solution was experimentally investigated and compared with that on carbonated specimens with the same exposure regime and uncarbonated specimens without exposure. N2 was used to protect specimens from carbonation throughout the sulfate exposure period. The effects of the water-to-cement (w/c) ratio and the fly ash as cement replacement on the sulfate attack were evaluated. Portland cement paste specimens with different w/c ratios of 0.35, 0.45, and 0.55 or fly ash replacement rates of 10%, 20%, and 30% were prepared. These specimens were partially immersed in 5% Na2SO4 solution for 50 d and 100 d exposure periods. The micro-analysis was conducted to evaluate the effect of the partial sulfate attack on the uncarbonated cement paste using X-ray diffraction (XRD) and thermo-gravimetric (TG) techniques. The results confirmed that, for uncarbonated cement paste, the chemical attack rather than the physical attack is the deterioration mechanism and is responsible for more severe damage in the evaporation zone (dry part) compared with the immersed zone (immersed part). When the effect of carbonation is well excluded, there is an optimal w/c ratio of 0.45 for minimizing the sulfate attack, while incorporating fly ash tends to reduce the sulfate attack resistance

Comparative Research on Tensile Properties of Cement–Emulsified Asphalt–Standard Sand (CAS) Mortar and Cement–Emulsified Asphalt–Rubber Particle (CAR) Mortar

The paper compared the tensile strength and elongation at break of cement–emulsified asphalt–standard sand (CAS) mortar and cement–emulsified asphalt–rubber particle (CAR) mortar. The tensile properties of CAS and CAR mortars were investigated. Microscopic analysis was carried out by Environmental Scanning Electron Microscopy and Energy Dispersive Spectrometer. The test results showed that the tensile strength of the CAR mortar at 7 days improved by about 9.09% higher than that of the CAS mortar, and further increased to 17.76% higher at 28 days The values of elongation at break of the CAR mortars at 3 days, 7 days, and 28 days increased by about 70% higher than those of the CAS mortars. Microscopic analysis showed that in the hardened CAS mortar, an obvious bubble accumulation layer with many pores appeared at the interfacial transition zone (ITZ). In the hardened CAR mortar, asphalt wrapped both cement hydration products and rubber particles and formed an integrated structure where a relatively dense and strong ITZ was formed as a result. This paper proves that the CAR system has superior tensile properties and has a promising future in waste rubber disposal