Cementitious Composite Materials Research Articles

Effect of Diethanol-Isopropanolamine and Typical Supplementary Cementitious Materials on the Hydration Mechanism of BOF Slag Cement Pastes

This study investigated the effects of lithium slag and iron tailings on the hydration mechanism of Basic Oxygen Furnace (BOF) slag cement paste with the addition of 0.06% diethanol-isopropanolamine (DEIPA). This study examined the fluidity, compressive strength, pore solution pH, and hydration products of BOF slag-based composite cementitious materials. The results showed that DEIPA facilitated the conversion from ettringite (AFt) to monosulphate (AFm) and improved the early compressive strength of the BOF slag–cement mortar. Incorporating lithium slag into the DEIPA-containing BOF slag–cement system promoted AFt formation, increased calcium-silicate-hydrate production, and enhanced the microstructure. BOF slag, lithium slag, and iron tailings exhibit synergistic effects in cement pastes. BOF slag and lithium slag provided the reactive components SiO2 and Al2O3. In the early hydration stages, the iron tailings primarily served as fillers, accelerating the system’s reactions.

Jute textiles with enhanced interfacial bonding as reinforcement for cementitious composites

In fabric-cement composites, the limited impregnation of cementitious matrix products due to thick and twisted yarns leads to premature failure due to poor bonding strength. In addition, cellulosic textile reinforcements have many challenges about durability, appearance of voids at mortar-fiber interface, and rise of microcracks. Textile performances were evaluated in different conditions: coated with micro-silica powder, pretreated, and without any treatment. This study also assessed how textile weave structure and yarn geometry configuration affect the interactions of two different jute textiles (Close Weave Jute Fabric – CJF and Open Weave Jute Fabric - OJT) when used as reinforcement in mortar matrix. Textile characterization and composite analysis (by four-point bending tests, SEM/EDS, and physical tests) were conducted to assess the different textile reinforcements, the mechanical behavior of produced composites, and visual and chemical compounds analysis of the interfacial transition zone between textile and mortar matrix after silica coating. Micro silica powder coating was deemed necessary to address limited impregnation and to avoid telescope pull-off. Weave structure determined the difference between jute fabrics to reinforce mortar matrix, being only OJF (larger interstices in the weave structure) with micro silica coating allowed a better matrix interaction and stood out from the other textiles and achieved the best specific energy of all samples, (4.28 ± 0.91) kJ.m-2. Calcium and silicon inside the yarn interstices and textile-matrix interface indicate the formation of strong bonds by calcium-silicate-hydrate products. The silica coating treatment enhanced formation of strong bonds, which demonstrated future promise for natural fiber application.

Performance improvement of nanocolloidal silica-aluminate cement composite grouting materials with organic acids

The combination of nanocolloidal silica and aluminate cement is a potential technical approach to solve the problem of mudstone grouting, but there is a lack of high-performance admixtures to improve early strength and gel time. The effects of three common organic acids, citric acid, tartaric acid and malic acid, on the properties of the composite slurry were explored. Meanwhile, the influence of citric acid on the fluidity and microstructure characteristics of the composite slurry was deeply analysed. Finally, the key engineering properties such as injectability and reinforcement of composite slurry were explored. The main conclusions were as follows: the citric acids had a significant beneficial effect on the compressive strength and gel time. After the addition of 4% citric acid, the consolidation rate of the composite slurry was 92.68%, the compressive strength was 39.22 MPa, the consolidation rate and compressive strength were at high levels, and the gel time was 90–110 min. The main reasons for the excellent strength performance were the dense microstructure, CAH10 and C2AH8 hydrates and 98.68% of pores having pore sizes less than 20 nm. The composite slurry can be injected in the rock with average pore throat diameter of 15.14 μm. The hydration products could be better combined with mudstone, the rock softening zone was smaller than that of ordinary Portland cement, and the composite slurry could reduce the softening phenomenon of mudstone. The findings of this work could provide guidelines for the development of high-performance grouting materials and solving the reinforcement problem of mudstone.

Seismic Performance Analysis of Concrete Columns Reinforced with Prestressed Wire Ropes Embedded in Polyurethane Cement Composites

In order to improve the seismic performance of reinforced concrete (RC) columns, a reinforcement technology using prestressed steel wire ropes embedded in polyurethane cement composite material is proposed. Four concrete columns reinforced with different materials were subjected to a combination of axial compression and horizontally repeated loading tests (one of which was not reinforced, while the remaining three were reinforced with prestressed steel wire rope, polyurethane cement composite material, and prestressed steel wire ropes embedded in polyurethane cement composite material). The experimental results show that the ductility and energy dissipation capacity of reinforced concrete columns after reinforcement are significantly improved. After strengthening with prestressed steel wire ropes embedded in polyurethane cement composite material, the ultimate horizontal displacement and energy dissipation capacity of reinforced concrete columns were significantly improved, which were 69% and 3.2 times higher than those of unreinforced columns, respectively.

Performance Analysis of Ferronickel Slag-Ordinary Portland Cement Pervious Concrete.

It is unknown whether Ferronickel slag (FNS)-ordinary Portland cement (OPC)-based pervious concrete (FOPC) is feasible. To this end, a feasibility study was conducted on FOPC. Firstly, a detailed microscopic examination of the FNS powder was conducted, encompassing analyses of its particle size distribution, SEM, EDS, and chemical composition. These analyses aimed to establish the suitability of a composite of FNS and OPC as a composite cementitious material. Subsequent experimentation focused on evaluating the compressive strength of the composite paste material with varying mixed proportions, revealing a slight reduction in strength as the FNS substitution rate increased. Furthermore, the study designed eighteen different mix proportions of FOPC to investigate the key physical properties, including porosity, density, compressive strength, and the coefficient of permeability. Findings indicated that increases in the cementitious material proportion correlate with enhanced concrete strength, where the ratio of cementitious to aggregate increased by 6.7% and 16.5%, and the strength of FOPC increased by 10-13% and 30-50%, respectively. Conversely, a rise in the FNS substitution rate led to a reduction in compressive strength across different mix ratios. Additionally, the ratio of paste material to aggregate was found to significantly influence the permeability coefficient. These comprehensive performance evaluations suggest that incorporating FNS into OPC for pervious concrete applications is a feasible approach, offering valuable insights for the promotion of waste reuse and the advancement of energy conservation and emissions reduction efforts.

Mechanical Properties of Aeolian Sand Concrete Made from Alkali-Treated Aeolian Sand and Zeolite Powder.

The aim of this study is to promote the application of the excited zeolite powder (ZP)with aeolian sand powder (ASP) in the field of aeolian-sand concrete (ASC) production. This study utilises NaOH to treat composite cementitious materials containing aeolian sand and zeolite powders, which were used to replace 50% of the cement in aeolian-sand concrete (ASC). Production of alkali-inspired cement-based windswept concrete(AAZC).The mechanical properties of treated ASC considerably improved, especially when the NaOH dosage was 4% by mass. After curing this sample (denoted as AAZC-4) for 28 d, its compressive strength improved by 17.2%, and its split tensile increased by 16.3%. Potassium feldspar and montmorillonite in zeolite powder and SiO2 in the sand were decomposed by OH- and combined with other elements to generate various silicate gels and A-type potassium zeolite crystals inside the concrete. Microscopic examination showed that the gels and crystals intertwined to fill the pores, decreasing (increasing) the percentage of large (small) pores, thus optimising the pore structure. This substantially improved the mechanical properties of ASC. Freeze-thaw salt-intrusion tests showed that the extent of mass loss, degree of damage and loss of compressive strength of AAZC-4 were similar to those of ordinary concrete but were reduced by 36.8%, 19% and 52.1%, respectively, compared with those of ASC. Therefore, AAZC-4 has a sustainable working performance in chloride-ion permeable environments in cold and arid areas.

A Review of Waterborne Polymer–Cementitious Composite Repair Materials for Application in Saline Soil Environments: Properties and Progress

The properties of a large number of concrete infrastructures in China are deteriorating year by year, raising the need for repairing and strengthening these infrastructures. By introducing waterborne polymers into a cement concrete system, brittle cracks and easy bonding performance defects of concrete can be compensated for to form a long-life, semi rigid, waterborne polymer-modified cementitious repair material with a promising development prospect. This paper investigates the modification effect of polymer emulsions on ordinary cement mortar. Our research mainly focused on the physical and mechanical properties, durability, microstructure and application status of waterborne polymer-modified cementitious composites. Literature studies show that with the increase in waterborne polymer content (0 wt%~20 wt%), the performance of cement mortar significantly improves, which in turn expands its application range. Compared with ordinary cement mortar, the introduction of waterborne polymers blocks some of the pores in the cement to a certain extent, thus improving its permeability, freeze–thaw resistance and durability. Finally, this paper describes the application of waterborne polymer–cementitious composites in western saline soil environments, as well as discusses the prospects of their development.

Study on microstructural and mechanical properties of cementitious materials composed of fly ash and dacite powder

The hydration reaction of mass concrete seriously endangers the structural safety. At present, the concrete production relies excessively on fly ash due to hydration reaction. In view of the problem that the demand of fly ash exceeds the supply, this paper proposes to use dacite powder to partially replace fly ash as the raw material for preparing cementitious materials. Through comprehensive tests and microstructure tests, various properties of dacite powder and fly ash composite cementitious materials are studied. The results show that: 1. The dacite powder with a specific surface area of 650 m2/kg, a fineness of 15% under laser particle size and a ball-milling time of 1.0 h has the best performance. 2. It is advisable to mix dacite powder and fly ash. The total amount of dacite powder should not exceed 30% of the cementitious material. It has the best performance when the amount of dacite powder and fly ash is the same. 3. The alkali activity of aggregate can hardly be inhibited by mixing dacite powder alone. The recommended measures to inhibit the alkali activity of aggregate are: i) mixing more than 20% fly ash alone. ii) mixed with 25% dacite powder and more than 15% fly ash.

Modelling and optimization of multiple replacement of supplementary cementitious materials for cement composite by response surface method

Supplementary cementitious materials are beneficial in improving performance and lessening the cement consumption with highly lessening CO2 emission. Many researchers used blast furnace slag, bentonite, and active limestone separately or two of them together to improve the performance of cementing materials, however, it is not well known how all react together in cement composite materials. So, the present study used modeling and optimizing the replacement of blast furnace slag, raw bentonite, and active limestone each by the doses of 0 to 20% to maximize strength and minimize the fresh bulk density of cementing materials by central composite design-response surface method (CCD-RSM). The results found, the employment of blast furnace slag, bentonite, and active limestone in the cement composite materials generally lessens the early strength compared to the control mixture. However, the replacement of blast furnace slag and active limestone by 20% significantly improves the 28-days compressive strength while employing raw bentonite by 20% reduced compressive strength by 6.45% compared to the control mixture. However, blending raw bentonite with active limestone by half improved the compressive strength. Besides these, the substitution of bentonite and active limestone reduces the fresh bulk density and flexural strength than the control mixture. Generally, the study optimized depending on the criteria of maximizing strength and minimizing fresh density and found the mix design replacement of blast furnace slag 1.01%, raw bentonite 5.30%, and active limestone 20% that improves 28 days compressive strength simultaneously reduces fresh bulk density in addition to replacing more than 54 different optimized design mix results.

Impact Analysis of Section Seismic Capacity in Potential Plastic Hinge Zone of Engineered Cementitious Composite Based on OpenSees

In this paper, OpenSees is used to analyze the moment-curvature (M-φ) curves of the bottom cross section of 4×40m continuous girder piers under the same reinforcing conditions, which are completely made of C40 concrete, partially made of Engineered Cementitious Composite material concrete(ECC) outsourced concrete, and completely made of ECC concrete, and then evaluate its seismic performance. Meanwhile, the yield curvature and ultimate curvature changes of the abutment section under different axial compression ratios are investigated. The results show that the yield curvature and ultimate curvature of ECC are larger than those of ordinary concrete, and the deformation capacity of ECC is stronger under the same axial force. The results also show that the yield moment and yield curvature of the section gradually increase with the increase of axial compression ratio, and the curvature ductility of the section deteriorates.

Preparation and hydration process of copper slag-granulated blast furnace slag-cement composites

Replacement of cement by solid wastes is an effective method to utilize secondary resources and reduce CO2 emissions. In this study, copper slag (CS) and granulated blast furnace slag (GBFS) were used to replace cement to address the low activity of CS, which limits its large-scale utilization. The dissolution characteristics, compressive strength, hydration products, and microstructure of the composite cementitious materials containing CS and GBFS were investigated. The results showed that adding GBFS into the CS-cement composites increased the compressive strength, and the 3-day and 28-day strengths could be increased by 112% and 90%, respectively. The CS acted as a diluting and heterogeneous nucleating agent, promoting the hydration of the GBFS and cement to produce more C−S−H gels. In addition, the number of pores decreased and the microstructure densification increased. This study provides a solution for the comprehensive utilization of CS, which contributes to the high-value green utilization of solid wastes.

The property, structure, and phase evolution of a binary cementitious material derived from sintering flue gas desulphurization ash and steel slag

The sintering flue gas desulphurization ash (DA) has emerged as the most significant solid waste, second only to granulated blast furnace slag and steel slag (SS), generated by steelmaking plants. However, its high CaSO3 content has rendered it challenging to recycle effectively. This study delves into the hydration characteristics of a low-carbon cementitious material composed of DA and SS, across different sulphur contents (3.14%, 6.28%, 9.42%, 12.56%) and water to binder (W/B) ratios (0.30, 0.35, 0.40, 0.45). The hydration products of the DASS composite cementitious material encompass C–S–H, C–A–S–H, Ca(OH)2, and K2Ca5(SO4)6·H2O. The setting time and fluidity exhibit a diminishing trend with increasing sulphur content. Notably, the hydration exothermic peak for pastes with 3.14% sulphur content emerges at 3 h, signifying a 2-h advancement compared to pure steel slag paste. The compressive strength of cured DASS paste gradually increases with sulphur content ranging from 3.14% to 9.42% over the initial 28 d, but experiences a decline with further sulphur content increases. A reduction in the W/B ratio significantly reduces setting time and achieves a two-fold increase in 28 d compressive strength. In DASS paste featuring a W/B ratio of 0.35 and a sulphur content of 9.42%, a substantial quantity of newly generated Ca(OH)2 is consumed through the pozzolanic reaction with steel slag after 60 d, leading to the production of more C–S–H to fill the interstitial voids between raw material particles. In conclusion, the recommended formulation for the DASS cementitious material should incorporate approximately 6%–9% sulphur supplied by DA, while greater mechanical properties can be attained by minimizing the W/B ratio. This innovative low-carbon cementitious material mitigates the impact of the massive amount of cement demand and promise an alternative to industrial waste management.

Effects of raw and different calcined bentonite on durability and mechanical properties of cement composite material

Bentonite is a natural pozzolanic material mostly composed of SiO2 and employed in cementing materials for the reduction of cement consumption and CO2 emission besides improving performance of cement composite material by consuming portlandite to form extra C-S-H gel. So, many researchers studied on partial replacement of bentonite to improve concrete performance. However, bentonite mostly exists as a consolidated form that needs treatment for activation of pozzolanic reactivity to improve performance of cement composite materials. However, it is not well known how different replacement of raw and calcined bentonite affects performance of cementing materials in different hazardous environment. Hence, the present study identified the most influential replacement of bentonite, evaluating different calcined and raw bentonite in environments of acids, salt, and elevated temperature in addition to assessing the mechanical and physical properties of cement composite material. So, the results indicate that the employment of raw and calcined bentonite reduces the fresh bulk density, improves compressive strength, and Vicker hardness at 28 days, reduces the mass loss due to 56 days immersion in 5% HCl, 10% NaCl, and 5% HNO3 compared to the control mixture. Moreover, the employment of calcined bentonite at 800 °C significantly improved the durability and mechanical properties of cement composite materials compared to the control mixture. Specifically, replacing bentonite calcined at 800 °C for 3hs by 20% has the highest compressive strength than all samples after 28 and 56 days. Besides these, the study reveals that the corrosion potential of steel bars embedded in the mortar is reduced as the proportion of raw and different calcined bentonite increases. Generally, the employment of calcined bentonite is very beneficial to get improved performance on mechanical and durability properties of cement composite materials.

Effects of Dosing and Mechanical Activation on the Performance of Reservoir Sediment-Cement Composite Cementitious Material

To solve the problem of reservoir sediment accumulation, this paper intends to study the feasibility of preparing reservoir sediment-cement composite cementitious material by partially replacing cement with reservoir sediment. Using the single factor test method, the replacement rate of reservoir sediment and mechanical activation time were selected as variables to explore the effects of water requirement of normal consistency, setting time, mortar fluidity, and mechanical properties of mortar specimens. The results show that with the increase of sediment replacement rate, the water requirement of normal consistency of composite cementitious material paste increases, the setting time increases, the fluidity of mortar decreases, and the mechanical properties of mortar specimens decrease significantly. With the increase of mechanical activation time, the water requirement of normal consistency decreases, the setting time decreases, the fluidity of mortar increases, and the mechanical properties of the mortar specimen increase. The microcosmic results show that mechanical activation can promote the formation of C-S-H gel in reservoir sediment, and fill each other with unreacted reservoir sediment particles, resulting in a denser microstructure. The purpose of this paper is to provide theoretical support for the preparation of reservoir mud-cement composite cementitious material.

Mechanical properties and hydration behavior of dehydrated cement paste-slag based composite cementitious material

Mechanical properties are critical in engineering applications involving cementitious materials. While dehydrated cement paste-based materials are considered environmentally friendly, they suffer from poor mechanical properties, limiting their engineering applications. Therefore, enhancing the mechanical properties of these materials is crucial to overcoming their limitations in engineering and supporting the development of sustainable building engineering. Based on the pozzolanic reaction mechanism and alkali activation theory, a composite cementitious material was created by combining dehydrated cement paste with slag. Na2SiO3 and NaAlO2 were used as alkali activators. The study investigated the effects of the ratio of dehydrated cement paste to slag, alkali activator dosage, water-binder ratio, and calcination temperature on the material’s mechanical properties through an orthogonal test. The optimized ratio, determined through the orthogonal test and mathematical model, resulted in a compressive strength of 37.33 MPa (7d) and 46.89 MPa (28d), surpassing the compressive strength of the original Portland cement paste. Hydration products of the composite cementitious material primarily comprised C-(A)-S-H, C-A-H, and CaCO3, with no observed presence of AFt, AFm, and Ca(OH)2. Notably, the hydration products of the composite material exhibited clear distinctions from those of Portland cement.

Study on multi-objective matching ratio optimization and strength development law of basalt stone powder composite cementitious materials

In order to better apply basalt powder to supplement cementitious materials ( SCMs), this paper chooses the content of basalt powder, fly ash and slag as the influencing factors, and the early strength ( 3d, 7d) and long-term strength ( 28d, 60d) of mortar as the evaluation index of SCMs. Based on the response surface and multi-objective optimization method, the Box-Benken experimental design method was used to study the influence of physical and chemical superposition between various factors on the strength development and optimize the ratio.The low-field nuclear magnetic resonance technique was used to detect the water change and porosity difference between the reference group and the optimization group at each age. The results show that the content of stone powder and fly ash has a significant effect on the early and long-term strength, the slag only has a significant effect on the early strength, the interaction between the three has no significant effect on the early and 60 d strength, and the interaction between stone powder and slag has a significant effect on the 28 d strength. The optimal compatibility of basalt stone powder-assisted cementitious materials is stone powder ( 10%), fly ash ( 5%), and slag ( 25.11%). Under this mix ratio, the strength activity index of mortar at each age meets the requirements. In the early hydration process, the physical filling effect of supplement cementitious materials such as stone powder in the optimization group can better optimize the pore structure inside the slurry, and the strength improvement is slower than that of the reference group. In the later stage of hydration, the number of hydration products in the optimized group increased, and the strength increased significantly, which was closer to the strength of the benchmark group.

Experimental Study on the Properties and Hydration Mechanism of Gypsum-Based Composite Cementitious Materials

In order to achieve the resourceful, large-scale and high-value utilization of bulk industrial solid wastes such as flue gas desulfurization gypsum (FGDG), fly ash (FA) and ground blast furnace slag (GGBS), and to reduce the dosage of cementitious materials, orthogonal experimental methods were used to prepare composite cementitious materials based on the principle of synergistic coupling and reconstruction of multi-solid wastes. Through the method of extreme difference and ANOVA, the influence law of different factor levels on the performance of the cementitious materials was studied, and the maximum compressive strength of cementitious materials was reached when the ordinary Portland cement (OPC) dosage was 20%, the FGDG dosage was 56%, the FA dosage was 19.2% and the slag dosage was 4.8%, and the W/B was 0.55. The hydration products and microscopic morphology of the cementitious materials were analyzed by means of XRD, SEM and MIP techniques, so as to elucidate the complex synergistic hydration mechanism, and then to determine the more optimal group distribution ratio. The results show that the hydration reaction between FGDG and OPC can be synergistic with each other, and C-A-H further generates AFt under the action of SO42−, and at the same time, it plays the role of alkali-salt joint excitation for FA–GGBS, generates a large amount of cementitious materials, fills up the pores of the gypsum crystal structure, and forms a dense microstructure.

Ultra-High-Performance Concrete (UHPC) Piles and Splicing Options

Conventional driven piles are made from steel, concrete, timber, or composite materials. These piling options have limitations with respect to corrosion, durability, driveablity, and performance. Ultra-High-Performance Concrete (UHPC) pile is a new alternative that has already been adopted by various state Departments of Transportation in the United States for addressing the limitations that exist with conventional piles. UHPC piles are made of a cementitious composite material mixture that possesses exceptional properties such as higher strength, low capillary porosity, and high resistance to corrosion, making them a suitable option for use as a deep foundation. For several reasons, it is necessary to cast piles with a shorter length and splice them at the site to reach the desired lengths. These reasons include shipping limitations, unpredictable soil condition, reducing transportation costs, construction time, and damage during installation. This study aims to explore and summarize the currently available options for connecting UHPC pile segments. Accordingly, after a brief introduction on driven piles, this paper investigates various splicing systems that can be used for UHPC piles through reviewing previous research studies and field applications. The applicable splices are then compared based on several criteria such as capacity, durability, cost, and ease of application.

Improvement on engineering properties and volumetric stability of alkali-activated slag-based composite cementitious material with rice husk ash and magnesium oxide

This study used rice husk ash (RHA) and magnesium oxide (MgO) to improve the engineering properties and volumetric stability of alkali-activated slag-based cementitious material. Experimental results found that when the amount of slag substituted by RHA increased from 0% to 20%, the initial setting time was extended from 15 to 20 min, and the final setting time was shortened from 35 to 30 min for the paste without MgO. The optimal substitution amount of RHA was 10%, which had the maximum compressive strengths at ages of 28 and 56 days to achieve 104.44 and 102.43 MPa for the paste without MgO, respectively. The drying shrinkage of paste without MgO increased with both the increase of RHA from 0% to 20% and ages from 3 to 56 days. With RHA, the drying shrinkage substantially decreased with the substitutions of MgO from 0% to 2.0% and almost remained steady after ages of 7 or 14 days. The optimal amount of RHA to replace slag was less than 10% and the optimal content of MgO was less than 1%, which can effectively improve the volumetric stability and reduce the heat of hydration of paste.

Mechanical properties and microstructure of waterborne polyurethane-modified cement composites as concrete repair mortar

With the increase in service life, the durability of a large number of concrete structures deteriorates year by year under the influence of the external environment, requiring urgent repair and reinforcement. Ordinary cement concrete materials have disadvantages such as high brittleness, low tensile strength, poor bonding ability, and insufficient durability, making it difficult to meet the repair needs of high-quality building structures. Therefore, self-synthesized anion WPU was used to study its effect on the mechanical properties of cement-based repair mortar. The research found that waterborne polyurethane (WPU)-modified cementitious composite repair materials exhibit better bonding strength with old concrete substrates compared to traditional cement-based materials. Additionally, the incorporation of WPU significantly reduces the brittleness of the repair material, ensuring enhanced efficiency in the repair process. However, according to the hydration heat test results, the addition of WPU delays the hydration process of the composite repair materials. In addition, the addition of WPU enhances the flexibility and bonding ability of the materials, with flexural strength and tensile strength at the age of 28 days reaching 13 MPa and 6 MPa, respectively. Through stress‒strain curve analysis, it was found that the maximum strain amplification of WPU cement mortar can reach 134 %. Combined with SEM testing, it was observed that cured WPU films were present in specimens aged 28 days with P/C ratios of 10 % and 15 %. WPU and cement hydration products permeate each other, forming a three-dimensional polymer network structure, improving the pore structure, and fully exerting the flexibility of WPU. This also validates the experimental results of increased ultimate tensile strain. WPU cement-based composite repair materials have higher bonding strength at a high P/C ratio (15 %), and WPU alone, used as an interface agent for pretreating the interface of old concrete, can improve the interface bonding strength, up to twice that of the group without the surface agent.