Ordinary Portland Cement Counterparts Research Articles

How does carbonation of alkali-activated slag and Portland cement systems impact steel corrosion differently?

Carbonation of cementitious materials, associated with neutralization of pore solution, is one of the prevalent reasons for inducing corrosion of steel in concrete. Compared with ordinary Portland cement (OPC) concrete, the poorer carbonation resistance of alkali-activated slag (AAS) concrete leads to serious durability concerns of a higher risk of steel corrosion. This work investigates the corrosion behaviors of steel embedded in intact and carbonated AAS and OPC mortars exposed to various relative humidity conditions, with the aim of fostering a better understanding of how carbonation impacts steel corrosion differently in these two cementitious systems. The results show that uncarbonated AAS mortars have better protective ability against corrosion than OPC counterparts; however, poorer performance after complete carbonation. It is mainly attributed to the combined effect of severe deterioration and coarsening of pore structure and increased ratio of chloride ion concentration to hydroxide ion concentration ([Cl−]/[OH−]) in pore solution, resulted from the consumption of hydroxide ions and release of bound chloride after carbonation. Besides, the different shifts of R-C (material resistivity versus corrosion rate) relationship lines in AAS and OPC mortars after carbonation are observed, which is ascribed to the different compositional and microstructural evolutions induced by carbonation in these two cementitious systems.

Basic Properties of fly Ash/Slag -Concrete Slurry Waste Geopolymer Activated by Sodium Carbonate and Different Silicon Sources

As a kind of granular waste with complex composition and alkali corrosiveness, concrete slurry waste (CSW) has severe recycling limitations in the ordinary Portland cement (OPC). Considering this, a new type of geopolymer, prepared by granulated blast furnace slag/fly ash, concrete slurry waste, and powdered activators (sodium carbonate and different silicon sources including sodium metasilicate pentahydrate and silica fume), was adopted to conduct a comparative study with the OPC counterpart. In this study, the homogenized CSW was mixed in the OPC and geopolymer with a constant ratio of 50 wt%, respectively. Then the properties were studied in terms of the flowability, setting times, mechanical strengths, and microstructures. The results showed that better flowability (200 mm) could be achieved in the obtained geopolymer than in the OPC reference group (95 mm) by increasing the powdered activators. The setting time of the OPC was significantly shortened due to the addition of CSW. The strengths of geopolymer were supported by the produced C-A-S-H and carbonates, with less chemically bonded water than the hydration products in the reference group. The dominant size of pores in the hardened geopolymer was much smaller than that in the OPC group which was 80 nm. Silica fume could be the alternate of the sodium metasilicate pentahydrate and had an insignificant negative impact on the fresh and hardened properties and microstructures of the geopolymer when the incorporation rate was within 5%.

Formulating eco-friendly geopolymer foam concrete by alkali-activation of ground brick waste

This study investigates ground brick waste powder as a primary precursor to develop eco-friendly geopolymer foam concrete (BGFCs) using a chemical foaming method. The effect of alkali content on the reaction kinetics of brick powder is studied and correlated with the properties of BGFCs. The reaction rates and the formation of crystalline phases of BGFCs were identified by Fourier Transform Infrared Spectroscopy and X-Ray Diffraction test method, respectively. It is found that increasing the alkali content (Na2O) in the mix has significantly enhanced the reaction kinetics of brick powder with the rapid formation of geopolymer gel in the matrix. This has resulted in the enhancement of mechanical properties and rapid setting in BGFCs. For instance, the increase in Na2O concentration from 1:40 to 1:12 has increased the compressive strength of BGFC by 245%. The corresponding initial setting times were reduced from 315 min to 140 min. The rapid setting of geopolymer mixture has resulted in a fine, regular, and homogenous distribution of air voids in the BGFCs. Finally, carbon emission calculations were conducted to assess the environmental impacts of developed BGFCs and compared with Ordinary Portland cement (OPC) foamed concrete. It is found that the developed BGFCs could significantly reduce the carbon emission up to 60% compared to their OPC counterparts.

Mechanical and durability properties of blended OPC mortar modified by low-carbon belite (C2S) nanoparticles

In this study, manually synthesised low-energy belite nanoparticles via a one-step method were employed into normal ordinary Portland cement (OPC) binding systems to improve properties and to reduce OPC carbon footprint. The mechanical and durability performance of the modified OPC mortar specimens were quantified and corresponding microstructural analysis was also conducted. Results showed that 5% dosage of the belite nanoparticles led to a significantly higher compressive strength of the modified OPC mortar, compared to the normal OPC counterpart without any belite additions. Besides, the carbonation depth and chloride penetration depth was reduced by up to around 43.5% and 28.1% respectively, compared to the corresponding values of the mortar without belite. However, when the content of belite increased to 15% and 20%, significant loss in mechanical strengths was observed. Worse still, the durability also exhibited no improvements or even worse behaviours evidenced by similar or greater carbonation depth and chloride penetration depth. These weaknesses are probably associated with the low hydration rate and possible agglomerations of belite nanoparticles in the OPC binding systems. Microstructural analysis reveals that belite nanoparticles refined the porous structure by reducing the average pore sizes but also led to higher porosity when higher dosage levels (10%, 15% and 20%) were applied. Finally, it was also found that belite nanoparticles could to some extent densify the microstructure of C–S–H gels due probably to their nanoscale particle sizes. This study thus provides guidelines for further improving the mechanical and durability properties of OPC-based binders through the use of belite nanoparticles synthesised via a simple method at a relatively low temperature.

Mechanical and durability-related performance of graphene/epoxy resin and epoxy resin enhanced OPC mortar

Abstract In this research, normal epoxy resin was reinforced by graphene nanosheets which was then used for improving mechanical properties and durability of ordinary Portland cement (OPC) mortars in terms of water sorptivity, resistance towards chloride penetration and freeze–thaw actions. Results showed that epoxy resin modified OPC mortar had a similar or lower compressive strength in the early curing ages whereas a higher one was obtained after 56 days of curing, especially for the mortar with the graphene addition. Flexural strength was all improved after the addition of epoxy resin, particularly for the graphene incorporated one. Besides, epoxy resin modified mortars were endowed with higher durability evidenced by lower capillary sorptivity and negligible mass loss and changes in relative dynamic modulus during freeze–thaw cycles, as compared to the normal OPC counterpart. With the addition of graphene, epoxy resin modified OPC mortar displayed an obvious lower chloride contents (especially at deeper depth) and capillary sorptivity. Microstructure analysis revealed that epoxy resin could refine the porous structure of the OPC mortar and graphene is able to enhance this benefit even further by densifying the mortar binding matrix. Considering the extremely small dosage of graphene but significant improvements in resistance against water ingress via capillary forces and chloride penetration, this novel method of combing graphene as a nanofiller with epoxy resin could be further utilised for OPC binders in practical aggressive engineering environments.

Preparation of TiO2/epoxy resin composite and its effect on mechanical and bonding properties of OPC mortars

Bisphenol A-type epoxy resin reinforced by TiO2 with different dosages (0, 1%, 3%, 5%) was for the first time used as a polymeric admixture to prepare ordinary Portland cement (OPC) mortars in this study. Tensile strength of the TiO2/epoxy resin composite, and flowability, compressive strength and flexural strength as well as the bonding strength of OPC mortars were measured. Scanning electron microscopy (SEM) was also carried out to observe the interfacial transition zone (ITZ) between sand and OPC paste, and the bonding interfacial transition zone (BITZ) between OPC mortar (either modified by the epoxy resin or not) and a substrate material. Results showed the tensile strength of the TiO2/epoxy resin composite was enhanced to a greater extent with a higher content of TiO2 nanoparticles. A lower flowability of mortar samples when modified by the epoxy resin was observed compared to the pure OPC counterpart. The compressive, flexural strength, flexural toughness indicated by the flexural-to-compressive strength ratio and bonding strengths of TiO2/epoxy resin-modified mortars quantified by the interface tensile strength were noted to be the highest for mortars modified by the epoxy resin containing 5% TiO2. For interface tensile strength, it was found that the increment was 7.3% and 14.6% for the mortar specimen modified by the epoxy resin with 5% TiO2 compared to the mortar modified by the same amount of epoxy resin but without TiO2 and the control with no epoxy resin respectively. SEM results showed that both ITZ and BITZ were denser when epoxy resin was added into the OPC mortar systems, leading to an improved flexural toughness and bonding strength. Several enhancing mechanisms are proposed including epoxy resin bridging effect, pore-filling effect, claw-like adhesion and delayed water loss and rigid TiO2 particles with good adhesion towards epoxy resin polymer molecules.

Development of limestone calcined clay cement concrete in South China and its bond behavior with steel reinforcement

Limestone calcined clay cement (LC3), consisting of ordinary Portland cement (OPC) clinker, calcined clay, limestone powder, and gypsum, has been considered a promising solution to current challenges in the cement and concrete industry, such as high carbon emissions, high energy consumption, and resource shortages. This study carries out a series of experimental investigations of LC3-based paste, mortar, and concrete, including microstructural analyses (e.g. hydration product characterization and pore structure analysis) and macro-scale testing (e.g. workability and mechanical properties), using raw materials from south China. The results show that, in LC3 paste, the replacement of clinker by calcined clay and limestone leads to an increased volume of small pores but decreased total volume of pores. The workability of LC3 mortar and concrete can be readily tailored using conventional superplasticizers. When designed for comparable 28-d compressive strength, the LC3 mortar and concrete tend to have lower early-age compressive strength, but comparable compressive strength and higher flexural strength than those of the OPC counterparts at late ages. This study also examines the bond-slip behavior between LC3 concrete and steel bars and finds that the bond strength is comparable to that of OPC concrete with the same 28-d compressive strength, but that the LC3 concrete-rebar interface exhibits higher bond-slip stiffness. These findings on LC3 concrete provide fundamental information and guidance for furthering the application of LC3 binder in structural concrete in the near future.

Glass powder as a partial precursor in Portland cement and alkali-activated slag mortar: A comprehensive comparative study

Prior studies were conducted in an isolated manner for the use of glass powder (GP) either as ordinary Portland cement (OPC) replacement material in blended cement or as a precursor in AAM material. There was no reported literature that made a direct comparison on the properties of blended cement and AAM containing GP with an equivalent mix composition. The present study was aimed to evaluate and compare the effects of GP as a partial precursor on properties of both OPC and alkali-activated slag (AAS) mortars. For this purpose, cement and ground granulated blast furnace slag (GGBFS) was partially replaced with 10%, 20%, 30%, and 40% (by weight) GP in OPC and AAS mortars, respectively. The mortar mixes were prepared for a fixed water-to-solid (W/S) ratio to make them equivalent. The evaluation was carried out on the workability, mechanical strength, water absorption, alkali-silica reaction (ASR) response and ecological analysis of the mortar. According to the test results, the flow of the fresh mortar increased monotonically with GP content, and AAS mortars showed a higher flow value than OPC mortars. Based on the mechanical properties, the optimum GP content in OPC and AAS mortars was found to be 20% and 30% by the total weight of the binder, respectively. In general, the inclusion of GP was more effective in enhancing the strength properties of AAS mortars as compared to OPC counterparts. Furthermore, the use of GP led to a systematic increase in the water absorption of mixes due to moisture instability of the sodium silicate gel produced by GP activation. According to the results of SEM analysis, AAS mortars showed a higher degree of micro-cracking, which resulted in higher water absorption and a lower flexural strength as compared to OPC mortars. Also, the incorporation of GP densified the pore system of the paste. The potential of ASR was evaluated via the accelerated mortar bar test. The OPC mortars showed up to a 55% reduction in the ASR expansion at 40% GP content. The AAS mortars, on the other hand, showed shrinkage during the first days and then started to show expansion with increasing GP content, which was beneficial as it compensated the shrinkage. The ecological analysis of mixes revealed that the carbon footprint of AAS mortars was about 60% lower than that of OPC counterpart mixes, and the use of GP reduced the CO2 emissions and energy demand of mixes.

Mechanical Properties and Leaching Characteristics of Geopolymer‐Solidified/Stabilized Lead‐Contaminated Soil

Ordinary Portland cement (OPC) is widely used in the solidification/stabilization of Pb‐contaminated soils. However, many studies have suggested that the high content of Pb would degrade the mechanical properties of OPC‐solidified/stabilized soils. This paper presents a new binder, geopolymer fine aggregate (GFA), composed of ground granulated blast furnace slag, fly ash, CaO, and Na2SiO3. For comparison, OPC was used as a conventional binder. Mechanical properties and leaching characteristics are typically used to evaluate the effects of binders on solidified/stabilized soils. Nevertheless, limited information on the mechanical properties and leaching characteristics of the GFA‐solidified/stabilized soils is available. This study thus investigated the mechanical properties and leaching characteristics of geopolymer‐solidified/stabilized Pb‐contaminated soil. Unconfined compressive strength test, permeability test, synthetic precipitation leaching procedure, simplified bioaccessibility extraction, phytoavailability extraction (with diethylene‐triamine penta‐acetic acid), sequential extraction procedure, mercury intrusion porosimetry, and scanning electron microscopy (SEM) were performed on OPC‐ and GFA‐solidified/stabilized soil. The results showed that the GFA presented a better effect on the mechanical properties and leachability of the solidified/stabilized soils than the OPC‐solidified/stabilized soils. The GFA‐solidified/stabilized soil displayed considerably lower leachability, bioaccessibility, and phytoavailability of Pb and higher mechanical properties and chemical stability than the OPC counterpart. This study demonstrated that GFA had a better effect than OPC on the solidification/stabilization of Pb‐contaminated soils.

Fracture properties of alkali-activated slag and ordinary Portland cement concrete and mortar

Three-point bending (TPB) tests were conducted on notched beams to compare the fracture properties of alkali-activated slag (AAS) concrete and ordinary Portland cement (OPC) concrete at three different compressive strength levels (30, 50 and 70 MPa). Parallel comparisons were also conducted between AAS mortar (AASM) and OPC mortar (OPCM). Load vs. mid-span deflection (P-δ) curves, load vs. crack mouth opening displacement (P-CMOD) curves and load vs. crack tip opening displacement (P-CTOD) curves of the tested beams were obtained. The fracture energy GF and the characteristic length (lch) of the AAS and OPC concrete and mortar were calculated and analyzed. It was found that the GF of AAS concrete was always higher than that of OPC concrete given the same compressive strength, due to their denser and stronger interfacial transition zones (ITZs). At a compressive strength of 30 MPa, the GF of AASM was also larger than its OPC counterparts. However, the GF of AASM became lower than that of OPCM when the compressive strengths were 50 and 70 MPa, as more initial micro-cracks were formed in the AASM matrix with strength increase. In addition, the lch values of AAS concrete and mortar were all smaller than those of OPC, implying that the formers were more brittle given the same compressive strengths. The elastic modulus of AAS concrete and mortar were found to be always lower than those of OPC. Micro-structural observations were carried out to explain the above phenomena.

Rheology, hydration, and strength evolution of interground limestone cement containing PCE dispersant and high volume supplementary cementitious materials

The aim of this paper is to elucidate fundamental composition-reaction-property correlations in blended binder systems prepared with Portland-limestone cement (PLC) and high volume of aluminosilicate supplementary cementitious materials (SCMs). An extensive investigation involving coordinated experiments and numerical simulations was carried out to quantify synergistic interactions between PLC and SCM (i.e., fly ash and slag), and benchmark them against corresponding blended ordinary Portland cement (OPC) systems. The study was carried out on mortars provisioned with optimum dosages of polycarboxylate ether (PCE) dispersant to improve fluidity and achieve enhanced particle packing. The results indicate that in spite of incorporating PCE at high dosages, PLC systems consistently show greater hydration kinetics compared to their OPC counterparts on account of their higher specific surface area. The SCMs are more effective in terms of improving strength in PLC systems as compared to OPC systems. This superiority of the PLC systems over OPC systems is attributed to enhanced particle packing and the carbonate-rich chemistry of PLC which allows enhanced formation of space-filling carboaluminate hydrates. Further, the results are consolidated to develop reliable correlations between hydration kinetics, rheological properties, and strength evolution for OPC-SCM and PLC-SCM systems. These correlations provide insights into optimizing binder formulation and proportioning.

Damping and microstructure of fly ash-based geopolymers

As environmentally-friendly materials, geopolymers have the potential to replace ordinary Portland cement (OPC) for the construction of railway sleepers and multi-flue chimneys, where the vibration control capabilities of the material must be considered. The critical damping value (ξ) is the main parameter in relation to vibration reduction. In this study, the traditional logarithmic decrement technique was used to measure the ξ of geopolymers. Geopolymers were prepared by activating fly ash using alkali solutions with different SiO2/Na2O ratios. The results show that the ξ of the geopolymers is similar to that of the OPC counterpart. Finite element analysis (FEM) based on the Rayleigh damping model was conducted to replicate the test results, and scanning electron microscopy and mercury-intrusion porosimetry were used to study the microstructure of the geopolymers. A discussion of the possible damping mechanisms based on the microstructural investigation and the FEM analysis is presented.

Examination of Chloride-Induced Corrosion in Reinforced Geopolymer Concretes

The durability of steel reinforced-concrete specimens made from three alkali-activated fly ash (FA) stockpiles and ordinary portland cement (OPC) in cyclic wet-dry chloride environment was evaluated over a period of 12 months. Testing methods included electrochemical methods, chloride diffusion and contents analysis, chemical and mechanical analyses, and visual examination. Geopolymer concrete (GPC) specimens made from Class F FA exhibited lower diffusion coefficients, chloride contents, and porosity compared with their GPC Class C FA and OPC counterparts. Overall, GPC specimens displayed limited signs of leaching and corrosion product formation, whereas OPC specimens exhibited the formation of multiple corrosion products along with significant leaching.

Fracture properties of geopolymer paste and concrete

Geopolymers are an emerging type of cementitious material purported to provide an environmentally friendly alternative to Portland cement-based concrete. This paper reports the results of experimental research on fracture properties (fracture energy and brittleness) of fly ash based geopolymer concrete and paste with various mix parameters. The characteristic length of the geopolymer concrete was approximately three times less than that of ordinary Portland cement (OPC) concrete, due to an increase in tensile splitting strength of about 28%, a decrease in elastic modulus of about 22% and a decrease in fracture energy of about 24%. The difference in characteristic length is similar to that reported between high-strength and normal-strength OPC concretes, indicating that the geopolymer concrete exhibits higher brittleness than its OPC counterpart. This trend was found to be consistent between pastes and concretes, implying that the difference between geopolymer and OPC concrete is due to the type of matrix formation (geopolymerisation or hydration). For geopolymer concretes made with different mix parameters, fracture properties are closely correlated to their compressive strength.