Reduction In Compressive Strength Research Articles

Mechanical behavior and its variability analysis of fiber reinforced polymer rebars after high temperatures

This study investigated the influence of high temperatures on the mechanical properties of glass and basalt fiber reinforced polymer (GFRP and BFRP) rebars. These rebars with a wide range of diameter from 10 to 25 mm were heated up to 400 ℃ and tested under shear and compression. Based on the data from the extensive experiments, the variability of shear and compressive strength was assessed. Results indicate that the reduction of both shear and compressive strength with high temperatures did not show notable dependence on the rebar diameter and type. High temperatures, especially 350 and 400 ℃, would induce more reduction of compressive strength than that of shear strength. Using the combined probability test and the Bayesian information criterion (BIC) test, this study proposed an approach to establishing appropriate probabilistic strength models for FPR rebars after high temperatures. The Gamma distribution outstood all chosen models for capturing the variability of residual shear strength of FRP rebars, while both Gamma and Weibull distributions were needed for compressive strength. Results herein can facilitate a better understanding of high-temperature induced change in properties of GFRP and BFRP rebars, and provide useful probabilistic strength models for probability-based evaluation of FRP rebars reinforced concrete structures after a fire.

A novel binder prepared from municipal solid waste incineration fly ash and phosphogypsum

At present, the recycling technology for municipal solid waste incineration fly ash (MSWIFA) and phosphogypsum (PG) is not yet mature, and they are mostly disposed of in open piles or in situ landfills, taking up a lot of land resources and harmful to the environment. A novel binder was developed from MSWIFA and PG in this study. Firstly, the feasibility of using MSWIFA to activate slag (GGBS) and fly ash (FA) for a ternary binder MGF was investigated. Subsequently, based on the optimal mix proportions of MGF, 5%–15% PG was used to substitute GGBS for a quaternary binder MGFP. The results revealed that without PG, the alkali environment provided by MSWIFA can effectively activate GGBS and FA, achieving a compressive strength of 14.0 MPa at 3 d and 25.7 MPa at 28 d when MSWIFA: GGBS: FA = 3: 6: 1. The strength of the ternary binder MGF was further improved by the addition of 5% PG due to the fact that the SO42− from PG reacts with the intermediate product [Al(OH)6]3- generated during MSWIFA activation of GGBS-FA to form ettringite which optimizes the microstructure of the hardened matrix. However, excessive PG over 5% was found to cause reduction in compressive strength, attributed to the dilution of the hydration products and the formation of cracks induced by excessive ettringite. The developed binders presented excellent ability in stabilization and solidification of heavy metals and the heavy metal leaching results meet the Chinese Standard GB18598-2019. The results obtained in this study provide technical support for the application of MSWIFA and PG in construction and building materials.

Impact damage assessment of thin defective composite laminates manufactured by Automated Fiber Placement technique

Fiber tow-gap is a composite defect that occurs during fiber deposition using Automated Fiber Placement technique. This defect resulting from the inherent gap between the adjacent fiber tows causes local thickness consolidation. It creates relatively large resin pockets that can affect the impact response and residual compressive strength of composite structures. Although the tow-gap effect on the in-plane response of composite laminates has been widely studied, there is still a lack of knowledge in damage assessment of defective composite plates subjected to Low-Velocity Impact (LVI) loading. In this study, a series of experimental Compression After Impact (CAI) tests are carried out on the standard carbon/epoxy composite plates subjected to impact loads to investigate the combined effect of impact damage and gap defect on the residual compressive strength reduction. Effect of the curing process on the gap formation is evaluated by measuring the Induced Gap Shrinkage Factor (IGSF) using microscopic observation. This factor is used for numerical simulation of the defective composite plates. Digital Image Correlation (DIC) technique is also used to measure the in-plane strains of the specimen under compressive loading. Furthermore, a numerical investigation is carried out by implementing the Induced Defect Layer Method (IDLM) model. The numerical studies contain both LVI and CAI results, including the impact response, impact damage, and final failure of impacted specimens under compressive loading. Results show that the induced gaps play a significant role in the residual strength reduction of thin composite plates, which is significant in low-velocity impact loading. It is also shown that IDLM is a robust meso-macro damage method for damage analysis of the composite laminates with manufacturing-induced gaps.

Study on the Oil Well Cement-Based Composites to Prevent Corrosion by Carbon Dioxide and Hydrogen Sulfide at High Temperature

Complex wells with high temperature and the presence of carbon dioxide and hydrogen sulfide acid gas require the use of high-temperature and high-density anti-corrosion cement slurry for cementing operations, and conventional cement slurry does not have the advantages of high density, high-temperature resistance, or corrosion resistance. In order to avoid the severe corrosion of cement slurry by carbon dioxide and hydrogen sulfide at high temperatures, solid phase particles with different particle sizes are combined with polymer materials to form a dense, high-density, high-temperature- and corrosion-resistant cement slurry. In this paper, we consider the use of manganese ore powder weighting agent, composite high-temperature stabilizer, inorganic preservative slag and organic preservative resin to improve the corrosion resistance of cement slurry, design a high-density cement slurry that is resistant to high temperature and carbon dioxide and hydrogen sulfide corrosion, and evaluate the performances of the cement slurry at 180 °C. The results show that the manganese ore powder weighting agent effectively improves the density of the cement slurry. Using composite silica fume with different particle sizes as a high-temperature stabilizer can ensure the rheology of the cement slurry and improve the ability of the cement sample to resist high-temperature damage. The use of slag and resin as preservatives can effectively reduce the corrosion degree in cement slurry. The high-temperature corrosion-resistant cement slurry systems with different densities designed using these materials exhibit good rheological properties, with water loss of less than 50 mL and a thickening time of more than four hours. The compressive strength decreased by less than 5.8% after 28 days at high temperatures. After being corroded by hydrogen sulfide and carbon dioxide (total pressure 30 MPa, 16.7% hydrogen sulfide and 6.7% carbon dioxide) under high temperature (180 °C) for 30 days, the corrosion depth of the cement sample was less than 2 mm, the reduction of compressive strength was low, and the corrosion resistance was strong. These research results can be used for cementing operations of high-temperature oil and gas wells containing hydrogen sulfide and dioxide.

Effect of Hydrophobic Water Repellent Admixture on the Compressive Strength of Concrete in Highly Aggressive Water (Dead Sea Water as a Case Study)

The compressive strength of concrete may be affected by seepage of salt water. Dead Sea water is a hazardous environment for concrete due to high concentration of salts. The aim of this study was to investigate the effect of Dead Sea water on the compressive strength of concrete containing hydrophobic water-repellent admixture, and to determine the optimum dosage of admixture to be used to reduce this effect. Three types of cement were used (Ordinary Portland Cement, Pozzolanic Cement, and Sulphate-Resisting Cement) with various dose rate of hydrophobic water repellent admixture (0%, 2%, and 4% of cement weight). An experimental approach, in which the specimens were divided based on the type of cement and the specimen’s age, was adopted. Nine control cubic specimens were cured in plain water to ensure that the required compressive strength was reached and (54) specimens were cured in Dead Sea water. The specimens cured in Dead Sea water were tested at 7 and 28 days to investigate the effect of Dead Sea water on the early and long-term strength. The results showed that a reduction in the 28-day compressive strength occurred in all specimens for the three types of cement used, but with the addition of water repellent, this reduction was alleviated, and the optimum percentage of admixture was 2% of cement weight.

Mechanical Properties of Poly(Alkenoate) Cement Modified with Propolis as an Antiseptic.

We assessed the effect of propolis on the antibacterial, mechanical, and adhesive properties of a commercial poly(alkenoate) cement. The cement was modified with various concentrations of propolis, and antibacterial assays were performed against S. mutans by both MTT assays and agar diffusion tests. The compressive, flexural, and adhesive properties were also evaluated. the modified cement showed activity against S. mutans in both assays, although reductions in compressive (from 211.21 to 59.3 MPa) and flexural strength (from 11.1 to 6.2 MPa) were noted with the addition of propolis, while adhesive strength (shear bond strength and a novel pull-out method) showed a statistical difference (p < 0.05). the antiseptic potential of modified material against S. mutans will allow this material to be used in cases in which low mechanical resistance is required (in addition to its anti-inflammatory properties) when using atraumatic restorative techniques, especially in deep cavities.

Experimental study on the effect of fireproof coating and cooling methods on the mechanical properties of concrete exposed to high temperature

As a thermal barrier at the surface of concrete, fireproof coating can well protect concrete from direct temperature and slow down the damage caused by high temperature inside concrete. In order to study the effect of fireproof coating and cooling methods on the mechanical properties of concrete exposed to high temperature, concrete was heated to 200 °C, 400 °C, 600 °C, and 800 °C, and then the concrete was cooled to room temperature by air cooling and water cooling. The effect of fireproof coating on the temperature rise curve and mass loss of the concrete was investigated. The compressive strength and splitting tensile strength of concrete with fireproof coating after high temperature were tested, and the effects of cooling methods and fireproof coating on the mechanical properties of concrete after high temperature were analyzed. The results showed that the non-intumescent fireproof coating can reduce the mass loss of concrete exposed to high temperature. After being exposed to 200–800 °C, the compressive strength of concrete with fireproof coating increased by 10.4 %-93.1 % and the splitting tensile strength increased by 24.7 %-70.7 % compared with the concrete without fireproof coating. The compressive strength and splitting tensile strength of concrete with fireproof coating were higher than concrete without fireproof coating. At 600 °C, the compressive strength increased by 34.2 % and splitting tensile strength increased by 44.7 %. The compressive strength of water-cooled specimens was obviously lower than that of air-cooled specimens, and the reduction of compressive strength gradually increased with the increase of temperature. At 800 °C, the compressive strength of water-cooled specimens was reduced by 43.5 % compared to air-cooled specimens. The splitting tensile strength of water-cooled specimens decreased significantly with increasing temperature, and the splitting tensile strength of water-cooled specimens was lower than that of air cooling after all four target temperatures.

Evaluation of the Loss of Uniaxial Compressive Strength of Sandstones Due to Moisture

ABSTRACT The reduction in uniaxial compressive strength (UCS) was investigated for sandstones under various moisture levels. Thirty-four UK Darney sandstone samples were tested under six different moisture conditions. The time-dependent moisture gain and loss were also evaluated. For 77 sandstones identified in the literature, the loss of UCS between oven-dry and saturated conditions was up to 45%, with an average of 20%. For Darney stone, the average loss of UCS was around 20%, with UCS around 72 N/mm2 when oven-dry and 58 N/mm2 when fully saturated. During saturation, significant loss of UCS occurred soon after exposure to water with 80% of UCS being lost within the first 2.5–6 hours. For Darney stone 50% of UCS was lost at air-dry conditions. Results from the 78 sandstone types were compared with the equivalent compressive strength defined by BS EN 772–1:2011 for oven-dry, air-dry, and saturated conditions. The estimated values by BS EN 772–1:2011 for dry and saturated UCS agreed well with the available test data and indicated a lower-bound solution. For immersed conditions, BS EN 772–1:2011, however, overestimates the reduction in UCS for a large number of samples and provides an average, instead of a lower-bound solution.

Experimental Investigation on the Failure Behavior of Single-Stiffened Composite Panels under Compressive Load

Whilst the thin-walled composite structures have been widely used to build the primary components in aircraft, Low-Velocity Impact (LVI) can cause severe intra-laminar and inter-laminar damage in the composite material, reducing the mechanical performance of these components. This paper presents an experimental study on the compressive failure behavior of a Single Hat-Stiffener Panel (SHSP) with two typical flaws, i.e., impact-induced damage and skin/stiffener debonding. LVI tests were performed to introduce the damage around the radius edge of the stiffener and the center of the flat skin panel, respectively, while the skin/stiffener debonding was introduced to the SHSP by artificially embedding a PTFE film at the skin/stiffener interface during autoclave curing. The impact energy to cause Barely Visible Impact Damage (BVID) was determined to be 22 J for the thin SHSP configuration. Whilst the considerable damage area was detected by the ultrasonic scan for the thick SHSP at an energy level of 35 J, the impact-induced indentation was not visually detectable (<0.35 mm), indicating a high impact resistance of this thick SHSP. Compression After Impact (CAI) results showed a significant reduction (59.1%) in the ultimate compressive strength of the SHSP when the impact location was on the stiffener compared to the pristine specimen. However, this reduction is less significant (34.7%) when the damage resulted from the same impact energy in the flat skin panel. In this study, the presence of skin/stiffener debonding does not adversely influence the compressive strength of the thick SHSP, while this debonding flaw appears to cause a minor reduction in the compressive strength of the thin SHSP.

Effect of slag and solid activator on flowability and compressive strength of fly ash based one-part geopolymer pastes

The geopolymerization process has led to the transformation of industrial by-products into sturdy and long-lasting construction materials, such as geopolymer binders, which can be used to mitigate the massive CO2 emissions associated with the production of Ordinary Portland Cement (OPC). These binders are produced from aqueous solutions of alkali activators and alumina and silica rich industrial waste materials. Strong, caustic, and viscous aqueous solutions are used in alkali activation. Its handling, usability, and mass production are all tough, even transport and site difficulties compound these issues. The solid alumina-silica rich components, solid alkali activators, and free water are dry mixed in this work to create a unique “one-part” or “simply add water” geopolymer binder that is equivalent to OPC in its manufacture. Researchers looked at the flowability and compressive strength properties of fly ash based one-part geopolymer mixes while adding ground granulated blast furnace slag and a solid activator (anhydrous sodium metasilicate powder). At the 25 and 50% replacement levels, GGBS was used in place of fly ash. Solid activator content varied from 8 to 16% at an interval of 2% for each replacement level of GGBS. Microstructural and mineralogical alterations were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. According to the findings of the tests, the flowability and compressive strength improved with decreasing slag and solid activator concentration. It was found that activator content increments beyond 12% result in minor reduction in compressive strength, and that the highest compressive strength was measured at 50% GGBS and 12% activator content. Both flowability and compressive strength were improved by the 50% GGBS and 12% activator mixture, which also displayed symptoms of having a dense and compacted microstructure.

Influence on compressive strength of concrete by partial replacement of river sand with fly ash geopolymer sand

The growth in the building construction activity sector is increasing leaps and bounds with boast to infrastructure. The growing requirement for River Sand (RS) is having an adverse impact on the environment, necessitating the urgent need for an alternate construction resource. Finding a different substitute or replacement for natural river sand is therefore urgently needed. When silica and alumina-rich materials react with alkaline liquid, a geopolymeric material is created. In this study, fly ash geopolymer sand (FAGS ), was manufactured using fly ash and an alkaline activator solution as an alternative to river sand, and was used in M 25 grade concrete. FAGS was prepared a combination of sodium hydroxide and sodium silicate and was cured at 100 ͦ C for one hour. In comparison to RS, FAGS has a similar specific gravity (2.53), pH (11.48), water absorption (3.5%), and bulk density (1.762 kg/m3). By substituting FAGS for RS in concrete, compressive strength for 7 days, 14 days and 28 days was evaluated for varying percentages of FAGS . The replacement was done at 25%, 50%, 75% and 100% of natural sand with FAGS . The compressive strength at 7 days and 14 days with 25% and 50% replacement was 16.6 MPa and 22.6 MPa for both replacements respectively, and the 28-day strength with both these partial replacements was found to be 28.7 MPa. When a higher percentage of replacement was done beyond 50%, there was a huge reduction in compressive strength. It was found that the replacement with 75% and 100% use of FAGS, compressive strength was 7.3 MPa and 6.4 MPa at 7 days, 13.5 MPa and 12.8 MPa at 14 days and 14. MPa at 28 days respectively. The mechanical strength of the FAGS was found to be virtually identical to that of the concrete made with the river sand, indicating that FAGS might be utilized as a partial alternative material in place of river sand in concrete. In order to take the role of RS as a permanent material in building technology, more research work has to be carried out on the durability features of the concrete containing FAGS.

RBSM-based mesoscale study of frost deterioration for recycled concrete considering air-entrainment in old and new mortar

To describe or predict the frost damage of recycled aggregate concrete (RAC), one should consider the material details of both old mortar and new mortar, especially the air entrainment. In this study, a freezing pressure model with entrained air is embedded into Rigid Body Spring Model (RBSM) to simulate degradation of RAC during freezing-thawing cycles (FTCs). First, quantitative models on frost damage of RAC are developed, and basic concepts of RBSM are introduced. Then the deformation of mortar affected by air entrainment is discussed and RBSM-based model for normal concrete is verified. Furthermore, RAC models are established with the method of RBSM, and the simulated frost damage results are compared with experimental results to validate this model. Finally, the validated model is used to study more cases of RAC, providing a comprehensive picture of frost resistance of RAC with different air entrainment, water to cement ratios and replacement ratios. It is found that while an air content of 6% is enough to protect new mortar from frost damage, weak old mortar still renders RAC vulnerable to frost damage and the damage significantly becomes more severe with the increase of replacement ratio of recycled aggregate. On the contrary, replacement ratio has little influence on frost resistance of RAC with weaker new mortar than old mortar. Besides, a linear relationship between compressive strength reduction and frost-related expansion is obtained.

Experimental investigation on leaching behavior of ultra-high performance concrete submitted to a flow environment

Ultra-high performance concrete (UHPC) is a cementitious material with exceptional durability. Uncertain is, however, the leaching behavior of UHPC in a flow environment. To investigate the leaching behavior of UHPC under the influence of moving water, accelerated tests simulating various naturally flowing solution conditions were conducted. The effect of flow environment, steel fiber content, and superplasticizer content on the degradation of UHPC was investigated. There are three flow velocities: static (0 m/s), low (0.3 m/s), and high (2 m/s), as well as three leaching solutions: 6 mol/L ammonium chloride, 3 mol/L ammonium chloride, and natural groundwater. Mass loss, porosity, degradation depth, flexural strength, and compressive strength were measured. The result showed that the maximum degradation of UHPC was observed in a high-velocity flow environment containing 6 mol/L ammonium chloride. Porosity, mass loss, degradation depth, and compressive strength loss following deterioration declined as the steel fiber content rose, reaching a minimum at 2% steel fiber in the same flow environment. The loss of flexural strength decreases and subsequently increases as the steel fiber content increases. The largest reduction in compressive strength was 16.4%, and the maximum reduction in flexural strength was 48.2%. At the same degraded ratio, the compressive strength loss of UHPC is significantly less than that of ordinary concrete. A predicted model for the loss of UHPC's cubic compressive strength is established.

Effects of various additives on the crumb rubber integrated geopolymer concrete

The use of scrap tyres in construction materials has been promoted to curb the environmental exploitation caused by the open disposal of non-biodegradable waste rubber. Tyre grinds as aggregates in geopolymer concrete (GPC) would increase its sustainability value by reducing the consumption of natural aggregates. Although there is limited literature addressing the damage to GPC characteristics caused by rubber aggregates, this investigation was designed to determine the extent of possible side effects of using crumb rubber (CR) in GPC. Additionally, this investigation aims to address any resulting reduction in strength and durability using additives such as cement and fibres. Geopolymer specimens with CR replacement of fine aggregates by volume (0, 5, 10 and 15%) showed a compressive strength reduction of up to 17% when tested according to ASTM standards. Substituting the total binder by weight with Ordinary Portland cement (OPC) (0%, 5%, 10%, 15%, and 20%) improved the microstructural integrity of the rubberised geopolymer mix with the highest percentage of OPC. Despite producing new and additional binding products (CSH and CASH gels), the GPC surface readily disintegrated under acid exposure. Optimum glass fibres (GF) reinforcement (0.30%) effectively disrupted the GPC pore network, consequently reducing the acid permeability of the matrix. Further addition of steel fibres (SF) enhanced the GPC specimen's compressive and flexural strength. To analyse the cumulative effect of these additives on GPC microstructure, supporting tests such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy were carried out. Although these additives enhanced the overall performance of rubberised geopolymer, it might somewhat reduce its green aspect.

Microstructure refinement and affected zone reinforcement for internal curing cement paste by composite microgel with nano silica

The introduction of superabsorbent polymers into cement increases the number of macroscopic pores, resulting in reduction of compressive strength. These limitations can be addressed by incorporated nano silica in polymer structure. Herein, a series of composite microgels containing nano silica were prepared and incorporated into cement paste. The effect of composite microgels on the hydration degree, compressive strength of cement paste was investigated. The microstructure and affected zone of cement paste were characterized by nitrogen adsorption desorption (NAD), backscattering (BSE), thermogravimetry (TGA) and microhardness techniques. The equilibrium absorptivity and water release rate of the composite microgels can be improved by proper crosslinking density and silica content. The addition of low crosslinking density composite microgels accelerated the formation of hydration products, which encouraged to filling formed voids. More dense arch structure was observed in vicinity of formed void. These further suggested that the reduction in the compressive strength of cement paste can be offset by composite microgels with proper crosslinking density, thus providing important application prospects in internal curing.

Seawater sea-sand Engineered Geopolymer Composites (EGC) with high strength and high ductility

In this study, seawater sea-sand Engineered Geopolymer Composites (SS-EGC) were developed and investigated for the first time. The developed EGC achieved high compressive strength (over 140 MPa) and high tensile ductility (around 8%) simultaneously. Emphasis was placed on understanding the influence of seawater and sea-sand (compared to freshwater and washed sea-sand) on the matrix properties and tensile performance of EGC, with two fly ash-to-slag ratios (8:2 and 2:8) considered in the matrices. Results showed that the use of seawater hindered the reaction of EGC matrix and led to a slight reduction of compressive strength (compared to the freshwater counterpart). It was found that the content of hydrotalcite phases in SS-EGC matrix was higher than that of freshwater EGC. In addition, using seawater was found to increase the average modulus of matrix obtained from nanoindentation, leading to a higher fiber/matrix bond strength. The tensile strain capacity of SS-EGC was slightly lower than that of freshwater EGC. The developed SS-EGC showed superior crack resistance and better sustainability than the cement-based counterpart from the literature (with similar compressive strength). The findings of this study provided useful knowledge for the design and development of high-strength high-ductility SS-EGC towards sustainable and resilient marine infrastructures.

Material property and constitutive model of C120 hybrid fiber ultra-high performance concrete at elevated temperatures

This paper uses a purpose-built rigid element-assisted loading system to conduct uniaxial compression tests on C120 hybrid fiber ultra-high performance concrete (HF-UHPC) at elevated temperatures. The test results show that C120 HF-UHPC exhibits brittle failure characteristics at elevated temperatures within the range of 100 °C to 400 °C, while the plastic damage characteristics are obvious at room temperature and within the range of 400 °C to 800 °C. The compressive strength of C120 HF-UHPC increases with the temperature from 20 °C to 200 °C and decreases with the temperature within the range of 200 °C to 800 °C. Based on the experimental data, the uniaxial compression strength and elastic modulus reduction factors, as well as the peak strain of C120 HF-UHPC variation factors at elevated temperatures, are proposed as functions of the temperature. Besides, the elevated-temperature constitutive model of C120 HF-UHPC is proposed. The comparison of the proposed model, experimental data, and existing models indicates that the proposed model is more accurate and reasonable. The findings of this research can lay an essential foundation for the structural analysis of HF-UHPC structures under fire conditions.

Effect of Wet Condition and Aggregates Type Used in Cement-Based Blocks on Mechanical Behavior of Masonry

Water penetration into masonry walls can occur in the masonry structures during the rainy climate. Water penetration not only causes discoloration or efflorescence but also can be damaging to material property. Eventhough several studies from past literature focus on wet conditions effect on brick masonry, but there is limited study on cement block masonry. The present research aimed to evaluate the effect of wet conditions on the mechanical characteristic of cement block, binder mortar and masonry. Where three types of fine aggregates, namely river sand, lateritic soil and manufactured sand were used for masonry block production. For binding mortar, two mortar classes M2 and M6 according to British Standard European Norm were used. Compression, flexural bending, and splitting tensile tests for blocks and binding mortar were performed in dry and wet conditions. Also, compression, direct shear and bond test were performed on masonry prisms in dry and wet conditions. The results demonstrate that a considerable amount of strength reduction was observed in wet conditions. In wet conditions, cement-soil block masonry showed a higher reduction in compression strength and bond strength, but showed lesser shear strength reduction when compared with the other two masonry types. Overall, cement-river sand block masonry has shown better performance in wet conditions.

The role of salt dissolution on the evolution of petrophysical properties in saline-lacustrine carbonate reservoirs: Pore structure, porosity–permeability, and mechanics

The invasion of low-salinity fluids into saline-lacustrine reservoirs may induce salt dissolution, resulting in significant changes in petrophysical properties. Accurate characterization of these changes due to salt dissolution is critical for efficient drilling and development. Here, low-salinity core flooding experiments and several core analysis technologies were employed to investigate the changes in pore structure, porosity–permeability evolution, and mechanical properties. Ion compositions in outlet fluids analyzed in real-time to determine the dissolved salt mineral compositions, mainly include halite (1.66 %), anhydrite (7.32 %), and glauberite (0.98 %). Salt dissolution produces many dissolution pores, especially with sizes ranging from 0.01 to 10 μm, broadens the seepage channels, and enhances the connectivity between pores, which significantly improves rock porosity close to 1.80 times the initial one and permeability to 70 times initial one. During the salt dissolution process, the stability of clay minerals is weakened to induce dispersion, migration, and swelling behavior, leading to the reduction of porosity and permeability. A semi-empirical power law relating expressing porosity and permeability for saline-lacustrine reservoirs is proposed, allowing for salt dissolution and clay swelling during the invasion process of low-salinity fluids. Although salt dissolution could slightly relieve stress sensitivity damage by 56.54 %, it significantly weakens rock strength by reduction of elastic modulus (43.51 %) and compressive strength (61.54 %). The positive and negative effects of salt dissolution on petrophysical properties are critical to guide the development of formation damage control measures for saline-lacustrine reservoirs.

Effect of chitosan on the mechanical properties and acid resistance of metakaolin-blast furnance slag-based geopolymers.

Adding organics in the process of geopolymer synthesis can combine the functional groups of organics with the three-dimensional structure of geopolymer, thus changing the properties of geopolymer such as compressive strength, flexural strength, and acid resistance. In this work, excellent mechanical properties and acid resistance of metakaolin-blast furnance slag (Mk-BFS)-based geopolymer were synthesized by the incorporation of chitosan. The formation of sodium-alumino-silicate-hydrate (N-A-S-H) gel and calium-alumino-silicate-hydrate (C-A-S-H) gel in geopolymerization were characterized by 29Si nuclear magnetic resonance (NMR). At 5% of chitosan, the compressive strength of Mk-BFS-based geopolymer could reach to 33.7MPa. The results could be ascribed to that chitosan reacts with geopolymer to generate new C-O-Si structure meanwhile adhesion produced by the combination of positively charged cations and negatively charged ions makes the structure of geopolymer denser. After 7days of sulfuric acid immersion, the reduction of compressive strength is less than 3MPa, demonstrating its great acid resistance. The acid resistance of Mk-BFS-based geopolymer could attribute to that the free amino groups in chitosan preferentially react with acid solution and weakened the erosion of sulfuric acid. This study optimizes the compressive strength and acid resistance of geopolymer by adding appropriate amount of chitosan.