Early-age Strength Development Research Articles

Structuration and yield strength characterization of hybrid alkali-activated cement composites (HACC) for ultra-rapid 3D construction printing

This study investigates the critical time-dependent rheological properties, including structuration rate and yield stress, of hybrid alkali-activated cement composites (HACC). HACC, a blend of geopolymer and cement paste, is examined to evaluate its suitability for ultra-rapid 3D construction printing (3DCP), which refers to a material’s ability to attain rapid structuration and buildability within 5–10 min after mixing, essential for additive manufacturing of 3D structures. The investigation extends to the impact of filler materials like silica sand and PVA fibers on HACC’s rapid structuration. Vicat needle, fast- and slow-penetration resistance measurements indicate that setting time increases with decreasing the Silicon/Aluminum (S/A) molar ratio and the addition of silica sand. Conversely, the addition of fibers reduces setting time while enhancing yield strength. Specifically, HACC mixtures with 0.5% and 1% PVA fiber additions, alongside combined sand (30%, 40%) and fiber (0.5%) mixtures, demonstrate ultra-rapid setting behavior within 3–5 min. Notably, mixtures with 40% silica sand and 0.5% PVA fiber exhibit highest yield strength development, i.e., 14 MPa at 12 min. Remarkably, incorporating 1% fibers into the HACC mixture yields the highest 28-day compressive strength (∼80 MPa) and flexural strength (∼11 MPa). The synergistic interplay between geopolymer and cement facilitates rapid setting time and high early-age strength development in HACC mixtures. The varying effects of silica sand and PVA fiber on setting time and yield strength provide opportunities for designing suitable structuration profiles. These findings underscore the exceptional rapid setting, structuration, and mechanical properties of HACC, positioning it as a promising material for ultra-rapid 3D construction printing.

Using metakaolin to improve properties of aged Portland cement: Effectiveness and the mechanism

Aging of Portland cement, due to its reaction with moisture and atmospheric CO2 prior to regular use, has the potential to severely impair its hydration reactivity. The effectiveness of using metakaolin to improve the properties of aged Portland cement exposed to a laboratory conditions (RH=65 %, 20 °C) as well as the associated mechanism are studied. The incorporation of 5–10 % metakaolin changes the early-stage hydration kinetics of aged cement obviously by enhancing reactions of both C3S and C3A through the filler effect at the early hydration stage, thereby mitigating detrimental influences of aging on setting time and early-age strength development (up to 7 days). Metakaolin exhibits significant effectiveness on enhancing 28-day strength of aged cement, and the enhancement arises from the synergic reaction between metakaolin and CaCO3, which converts CaCO3 resulting from aging into carboaluminate phases and produces secondary C-S-H. The incorporation of 5 % metakaolin increases the 28-day compressive strength of the sample with cement aged for one month to a comparable level as that of fresh cement.

Optimization of production variables for metakaolin and rice husk ash-based geopolymer cement

Geopolymer concrete has emerged as an eco-friendly alternative to conventional Portland cement-based concrete, boasting superior mechanical performance and a significantly reduced carbon footprint. This research delves into the effects of incorporating rice husk (RH) and metakaolin (MK) as supplementary materials in geopolymer concrete, with the goal of enhancing its properties and sustainability while making productive use of industrial by-products. The research commences with a comprehensive characterization of the raw materials, including rice husk ash (RHA) and metakaolin, via physical, chemical, and mineralogical analyses. Different combinations of RH and MK are then blended with an alkali activator solution to formulate geopolymer paste mixtures. The findings of this study reveal that the incorporation of RH and MK as partial substitutes for conventional materials in geopolymer concrete yields positive effects on its performance. The utilization of rice husk ash enhances workability and reduces the alkaline activation time of the geopolymer paste, leading to improved early-age strength development. This study demonstrates that the synergistic use of rice husk and metakaolin in geopolymer concrete promotes a more sustainable construction material with a diminished environmental impact. It offers valuable potential to guide engineers, researchers, and industry professionals in adopting sustainable and cost-effective strategies for enhancing the properties of geopolymer concrete, thus fostering its wider adoption in the construction sector.

Carbon-captured sodium hydroxide solution for sustainable alkali-activated slag

The escalating impact of climate change, primarily driven by greenhouse gas emissions, has prompted global efforts to achieve carbon neutrality. Ordinary Portland cement production contributes approximately 7% to global carbon dioxide (CO2) emissions, necessitating innovative approaches for its sustainable production. A novel method is introduced that utilizes a sodium hydroxide (NaOH) solution captured with CO2 as an alkali activator for ground granulated blast furnace slag (GGBFS); it can substitute for the cement in producing construction materials. The method integrates CO2-dissolved alkaline solutions into the activation process of GGBFS, which results in the production of sustainable alkali-activated slag materials. Various concentrations of dissolved CO2 in the NaOH-based activator were used to evaluate their impact on compressive strength, setting time, and changes in phase composition. The results demonstrate that higher CO2 concentrations lead to increased compressive strength, but an excess hinders early-age strength development. X-ray diffraction and thermogravimetry analyses explain the phase changes during carbonation, emphasizing the role of CO2 in forming calcite and enhancing C-S-H production. Additionally, the study quantifies CO2 uptake, revealing a linear relationship with dissolved CO2 concentrations.

Performances of concrete with binder and/or aggregates replacement by all-solid waste materials

Aiming for an all-solid waste concrete, a quaternary solid waste binder (SWBs) system comprising blast furnace slag, fly ash, desulfurization gypsum, and Bayer red mud was developed and utilized for both binders and preparation of cold bonded artificial aggregates. The first section investigated and discussed the hydration properties and strength gain mechanism of SWBs, revealing that ettringite formation and growth contribute to early age strength development of SWBs, while C-A-S-H formation contributes significantly to the later strength gain. The second section systematically studied the concrete performances, including mechanical properties, shrinkage, chlorides, and seawater resistance. An all-solid waste-based concrete strategy was presented by gradually substituting cement and natural aggregates. It was observed that SWBs-based concrete exhibited significantly lower shrinkage and excellent chloride ion resistance, although slightly lower mechanical properties were observed. The solid waste-based artificial aggregates proved viable alternatives to natural rock aggregates in low-strength concrete classes. Incorporating artificial aggregates endowed the concrete with self-curing properties and reduced its shrinkage and chloride migration, with only a minor effect on its mechanical strength. Lastly, an all-solid waste concrete combining the characteristics of SWBs and cold-bonded artificial aggregates was achieved. The resulting concrete exhibited lower shrinkage and excellent chloride and seawater resistance. This study provides a reference for designing and developing solid waste-based binders and concrete.

Feasibility of Preparing Steel Slag–Ground Granulated Blast Furnace Slag Cementitious Materials: Synergistic Hydration, Fresh, and Hardened Properties

Steel slag and GBFS are wastes generated during the steel and iron smelting process, characterized by their considerable production rates and extensive storage capacities. After grinding, they are often used as supplementary cementitious materials. However, the intrinsic slow hydration kinetics of steel slag–GBFS cementitious material (SGM) when exposed to a pure water environment result in prolonged setting times and diminished early-age strength development. The incorporation of modifiers such as gypsum, clinker, or alkaline activators can effectively improve the various properties of SGM. This comprehensive review delves into existing research on the utilization of SGM, examining their hydration mechanisms, workability, setting time, mechanical strengths, durability, and shrinkage. Critical parameters including the performance of base materials (water-to-cement ratio, fineness, and composition) and modifiers (type, alkali content, and dosage) are scrutinized to understand their effects on the final properties of the cementitious materials. The improvement mechanisms of various modifiers on properties are discussed. This promotes resource utilization of industrial solid wastes and provides theoretical support for the engineering application of SGM.

Synthesis of nano-SiO2@PTPEG-VPA copolymer and its effects on early-age cement hydration.

Incorporating nano-SiO2 particles into cement paste has garnered significant attention for enhancing the performance of hardened cement paste. However, the agglomeration of nanoparticles in the pore solution of cement-water system poses a challenge for cost-effective and efficient applications. Meanwhile, superplasticizers containing phosphate groups exhibit strong complexation with calcium ions and show promise in improving the dispersion performance. This study introduces a surface chemical modification technique to enhance the dispersibility of nano-SiO2. Firstly, poly(isoprenyl oxy poly(ethylene glycol) ether-random-vinylphosphonic acid) (PTPEG-VPA), a silanized superplasticizer containing phosphate moieties, is copolymerized and chemically grafted onto pristine nano-SiO2 surfaces through condensation and silanization processes. The resulting core-shell SiO2@PTPEG-VPA nanoparticles are comprehensively characterized using FT-IR spectroscopy, TGA, DLS, TEM, BET surface area analysis, and zeta potential measurements. The results indicate that introducing phosphate moieties improves the dispersion capacity of grafted copolymers, thereby reducing the severe agglomeration of nano-SiO2 in solution. Subsequently, the impact of SiO2@PTPEG-VPA on cement hydration and early-age strength development is investigated using microcalorimetry and TGA characterization. Finally, a mechanism is proposed to explain the observed retarding effects of grafted PTPEG-VPA on pristine SiO2. Overall, this study provides novel insights into the chemical design of nanoparticles, aimed at manipulating cement paste properties.

Effect of elevated temperature on concrete incorporating zeolite, silica fume and fly ash as replacement for cement

In this study, the effect of utilizing zeolite and industrial by-products, silica fume, and Class C fly ash, as partial replacement for cement is investigated at room and elevated temperatures of 200°C, 400°C, and 600°C. Four different high-strength concrete mixes of M50 concrete grade with varying proportions of zeolite, silica fume, and Class C fly ash are tested and observed for workability, compressive strength, and microstructural characteristics using Scanning Electron Microscope analysis, and the optimum mix has arrived. The results indicate that the addition of zeolite, silica fume, and fly ash resulted in lesser early-age strength development but the strength after 28 days of curing is found satisfactory and the desired target strength is achieved. Temperature rise resulted in degradation of concrete properties for both conventional and optimum mix but the concrete cubes with 10% of zeolite, 7.5% of silica fume, and 10% of fly ash have higher retainment of strength at elevated temperatures compared with conventional mix. The results from the microstructural study are found to correlate with strength test results. Incorporation of 10% zeolite, 7.5% silica fume, and 10% fly ash improved mechanical and microstructural characteristics at room and elevated temperature, however, the replacement is effective for 28 days of curing.

New biopolymers as viscosity-modifying admixtures to improve the rheological properties of cement-based materials

The performance of various new biopolymers as viscosity-modifying admixtures (VMAs), including tragacanth, almond, and locust bean gums and giant kelp extract, in addition to two commercial biopolymer-VMAs, welan and guar gums in cement-based materials was evaluated. The effect of the preparation/addition method, type, and dosage of the investigated biopolymers on rheology, hydration kinetics, stability, and compressive strength of various cement paste mixtures was evaluated. The pre-dispersion and pre-dispersion/pre-hydration techniques showed greater influence on the rheology of the investigated mixtures compared to dry addition. Moreover, the investigated biopolymers led to a shear-thinning response with higher viscoplastic properties than the reference. The structural build-up of the mixtures was mainly influenced by the type and dosage of the biopolymers, observing higher performances in mixtures containing guar gum, giant kelp extract, and welan gum. The use of the investigated biopolymers enhanced the stability of the paste mixtures, while negatively influenced the early-age strength development.

Thermal Cracking in High Volume of Fly Ash and GGBFS Concrete

Supplementary cementitious materials (SCMs) such as fly ash and ground granulated blast furnace slag (GGBFS) are found to control the maximum temperature and the accompanying thermal gradients effectively. However, SCMs also lead to low early age strength development. Thus, it is crucial to understand the cracking behaviour of SCMs-based concrete affected by the mix design parameters. In this paper, the thermal cracking resistance was evaluated using a rigid cracking frame (RCF) with a computer-controlled temperature profile. The temperature profile was determined using the software ConcreteWorks by assuming the centre point of the mass concrete. The free shrinkage frame (FSF) and match-curing oven follow the same temperature profile as RCF to measure the free total deformation and time-dependent mechanical properties of concrete, respectively. An analytical model was proposed to calculate the autogenous shrinkage and the thermal stress separately. A time-dependent cracking risk coefficient allowing to estimate the risk of early age cracking of concrete was also proposed.

Evaluation of neutron shielding performance of cementless binder manufactured with desalination reject brine

This study investigated the feasibility of desalination reject brine as a new water resource for the nuclear construction industry by evaluating the thermal neutron shielding properties of Ca(OH)2–Na2CO3-activated fly ash (FA-NC) binder manufactured with reject brine since the high chloride concentration is believed to promote neutron shielding performance. High Cl− concentration encouraged rapid and greater early-age strength development of FA-NC binder mixed with reject brine, which is the main mechanism of the accelerator. A series of chemical and microstructural analyzes were conducted, namely, XRD, TG, and MIP tests, to investigate the influence of the reject brine. The use of reject brine remarkably improved the thermal neutron attenuation performance of the FA-NC binder and achieved 98.76 % of thermal neutron shielding efficiency. Furthermore, the reject brine provided an increased efficacy in fast neutron attenuation. To further analyze the practical application potential, a 190 × 90 × 57 mm sized brick sample was made using rejected brine mixed FA-NC binder and tested with the followings: water absorption and TCLP.

Aging and Temperature Effects on the Performance of Sustainable One-Part Geopolymers Developed for Well-Cementing Applications

Summary This study elucidates the effects of aging and temperature over the performance of one-part “just add water” (JAW) granite-based geopolymers for application in well cementing and well abandonment. Additionally, the investigation delves into the fluid-state and early-age solid-state properties of these geopolymers, with a particular emphasis on their performance after aging. The aging process extended up to 56 days for assessing mechanical properties and up to 28 days for evaluating hydraulic sealability through dedicated tests. The obtained results unveil a nonlinear correlation between the designated temperature and pumping duration. Notably, the issue of fluid loss emerged as a significant concern for these geopolymers. The early-age strength development of the mix design containing zinc demonstrates adherence to industry norms by achieving minimal strength requirements within 24 hours of curing. Zinc plays a pivotal role as a strength enhancer during the initial curing stages of geopolymers, both under ambient conditions and at elevated temperatures (70℃). However, upon extended curing at elevated temperatures, zinc’s impact slightly diminishes compared with the unmodified mix design. After around 30 days of curing, a consecutive reaction occurs in both the unmodified and zinc-modified mix designs. Aging leads to a decline in the material’s hydraulic sealability that was initially established during the early stages of curing.

Innovative geopolymer-based cold asphalt emulsion mixture as eco-friendly material

In recent years, there has been a growing interest in cold asphalt emulsion mixture (CAEM) due to its numerous advantages, including reduced CO2 emissions, energy savings, and improved safety during construction and application. However, CAEM has often been considered inferior to hot mix asphalt (HMA) in terms of performance. To address this issue and achieve desirable performance characteristics, researchers have been exploring the modification of CAEM using high-cost additives like ordinary Portland cement. In this study, the focus was on investigating the effects of utilizing waste alkaline Ca(OH)2 solution, ground granulated blast-furnace slag (GGBFS), and calcium carbide residue (CCR) as modifiers to enhance the properties of CAEM. The aim was to develop an innovative geopolymer geopolymer-based cold asphalt emulsion mixture (GCAE). The results of the study revealed that the use of waste alkaline Ca(OH)2 solution led to an increase in early hydration, which was confirmed through scanning electron microscopy. Furthermore, the experimental findings demonstrated that waste alkaline Ca(OH)2 solution significantly contributed to the rapid development of early-age strength in GCAE. As a result, GCAE showed great potential for utilization in pavement applications, particularly for roads subjected to harsh service conditions involving moisture and temperature. By exploring these alternative modifiers, the study highlights a promising avenue for enhancing the performance of CAEM and potentially reducing the reliance on expensive additives like ordinary Portland cement. The development of GCAE has the potential to offer improved performance and durability in pavement applications, thus contributing to sustainable and efficient road infrastructure.

Rapid early age strength development of in-line activated geopolymer for concrete 3D printing

The in-line activation using print head mixing technology involves two parts pumping and mixing at the print head. This technology can be applied in geopolymer concrete, where the base mix (precursors, sand and water) and alkaline activators can be pumped as two separate parts and mixed at the print head for activation. While such methods provide prolonged open time, the early age strength development is low since water from the base mix dilutes the activator solution during print head mixing. Besides, choosing an activator solution with high alkalinity to combat the dilution effect reduces the immediate static yield strength development after print head mixing, which is essential for rapid building. Therefore, this study investigates the hydrated lime as an additive in the base mix to overcome the challenge to attain high 1-day compressive strength along with immediate static yield strength development. The effect of hydrated lime dosage on the pumpability (i.e. evolution of viscosity with time), static yield strength development after print head mixing (buildability) and hardened properties were assessed. The optimum hydrated lime dosage of 1 wt% of the precursors exhibited 1-day compressive strength of 20 MPa and static yield strength of 38.8 kPa after 5 min from print head mixing with minimal changes in pumpability of the base mix for up to 6 h. Moreover, the effect of rapid static yield strength development of the printed layers on the interlayer bond strength was assessed with varying cycle times between 5 s and 40 min. It was found that the interlayer bond strength reduced by 63% when the cycle time was increased from 5 s to 40 min, however, the reduction can be decreased to 15% by surface wetting of the previous layers.

Enhancing Stabilization and Early Age Hydration of Dicalcium Silicate Using TiB2 Derived Nanosheets

Enhancing the proportion of dicalcium silicate (C2S) in Portland cement is an effective method to mitigate the environmental impact of cement production. However, the early age strength development in C2S is insignificant when compared to that of tricalcium silicate (C3S). Mixing of additives during C2S synthesis has been known to stabilize C2S and enhance its hydration behavior. There has been a growing interest in incorporating graphene, the pioneering 2D nanomaterial, as a sustainable additive in concrete. The superior mechanical and catalytic properties exhibited by graphene not only reinforce the composite but also influence the stabilization and hydration behavior of C2S. The present work investigates the candidacy of a relatively new class of nanosheets derived from titanium diboride (TiB2), a class of 2D materials recently discovered by us, as cement additives. We find that addition of 3% chemically modified TiB2 nanosheets increases the formation of β-C2S polymorph by ∼4 times. Isothermal calorimetry of the stabilized C2S reveals an increased peak heat and cumulative heat evolution during the early age hydration (24 h), indicating that the TiB2 based nanosheets also enhance the rate of C2S hydration. Long-term hydraulic characterization (28 days) of C2S also indicates a higher degree of hydration in the nanomodified samples. This unconventional, and yet rich, prospect offered by TiB2 derived nanosheets exemplifies the potential 2D materials have to offer in developing substantial cement additives.

The Influence of Alkali-Free Shotcrete Accelerators on Early Age Hydration and Property Development within Cement Systems.

Fluoride-containing alkali-free setting accelerators are a common type of admixture used in tunnel shotcrete but few studies in the literature focus on the effect of their fluoride compounds on the setting and hardening properties of accelerated cement paste under low environment temperatures. Tunnel shotcrete in cold regions or winter construction periods would be obviously influenced by low environment temperatures, especially for its fast setting and quick support applications. The objective of this work is to evaluate the early age hydration behavior of different accelerated cement pastes under 20 °C and 5 °C environment temperatures. In this study, setting time measurement, early age strength development, hydration ion leaching concentration, isothermal calorimetry, X-ray diffraction, and ESEM were performed on cement systems prepared with a non-fluoride alkali-free accelerator (aluminum sulfate solution with over 60% solid content) and a designed fluoride-containing alkali-free setting accelerator (aluminum sulfate and fluoride compound). The results showed that the fluorides obtained in alkali-free accelerators promote C3S dissolution and massive ettringite needles together with monosulfoaluminate (AFm) hydrate formation, thus leading to a quicker setting effect and low sensitivity to low environment temperatures than in non- fluoride groups. However, the rate of mechanical strength development of cement pastes hydrated within 24 h was decreased obviously when fluorine-containing alkali-free accelerator was used. This phenomenon is mainly related to the crystallization of thin-plate shape calcium fluoride (CaF2) formations and promoted conversion of ettringite to monosulfoaluminate hydrate in the accelerating period, thus weakening the denseness of C-S-H gel and inhibiting alite further hydration.

Tuning the Hydration Acceleration Efficiency of Calcium Carbonate by Pre-Seeding with Calcium Silicate Hydrate.

Nanomaterials are promising candidates for refined performance optimization of cementitious materials. In recent years, numerous studies about the performance improvement of nanomaterials using polymers have been conducted, but the modification of cement-oriented nanomaterials with inorganic modifiers is seldom assessed. In this study, we explored the performance tuning and optimization of nanomaterials by inorganic modification. In this work, hydration acceleration efficiency of calcium carbonate (CaCO3, CC) was tuned via surface deposition with calcium silicate hydrate (C–S–H) nanogel through seeding. Multiple calcium carbonate–calcium silicate hydrate (CC–CSH) samples with varying degrees of surface modification were prepared via dosage control. According to characterizations, the degree of C–S–H modification on the CaCO3 surface has a maximum that is controlled by available surface space. Once the available space is depleted, excessive C–S–H turns into free form and causes adhesion between CC–CSH particles. The resultant CC–CSH samples in this work showed enhanced hydration acceleration efficiency that is tuned by the actual degree of C–S–H modification. Elevated C–S–H modification causes CC–CSH’s acceleration behavior to shift to enhanced early-age acceleration. According to mortar strength tests, CC–CSH with 5% C–S–H modification showed the most balanced performance, while CC–CSH with higher C–S–H modification showed faster early-age strength development at the cost of lower later-age strength. The inferior later-age strength of highly C–S–H-modified CC–CSH samples may be due to the coarsening of hydration products and stiffening of their network, as well as agglomeration caused by C–S–H adhesion. This study may offer a novel route for performance tuning of cement-oriented nanomaterials.

The Effect of Cement Mortar Recipe on Early Age Strength Development

The paper summarises the results of experimental analysis focused on the maturity method. The method is used to determine the early-age compressive strength of concrete on a construction site to optimise the construction process. All presented calibration curves of different recipes were determined experimentally. Comparison of calibration curves shows effects of specific changes in the recipe on early-age compressive strength development of cement mortar. Investigated changes in the recipe are the following: use of different cement types from different sources, amount of cement, water-cement ratio, used fine aggregates, admixtures and additions. Besides mentioned comparison, the topic of accuracy and deviations is included. Deviations were investigated by repeated determination of calibration curves at a specific time after the creation of the first batch of the calibration curves. The experience acquired during cement mortar experimental analysis will be later used in upcoming experiments at a concrete supplier with different concrete recipes.

Enhancement of early age cementitious properties of yellow phosphorus slag via CO2 aqueous carbonation

Yellow phosphorus slag (YPS) is an industrial waste by-product and has pozzolanic characteristics. Due to its poor early-age reactivity, this study aims to overcome this limitation via CO2 activation of raw YPS (RYPS) and use as a cement substitute. The experimental results showed that carbonation of RYPS under optimal conditions (liquid to solid ratio of 8 ml/g, temperature of 80 °C, pressure of 0.1 MPa, and for 9 h) can form rich amorphous silica gel and hydro calcite-type CaCO3. As a result, carbonated YPS (CYPS) was utilized as a supplementary cementitious material to improve the early age of pozzolanic characteristics in cement paste. This is because the formation of hemi- and mono-carboaluminates and amorphous silica gel played the important role in early-age strength development in CYPS incorporated cement pastes. Moreover, the presence of CYPS can accelerate the hydration rate of cement clinker, together with the intensification of the alite reaction. The microstructure of composite cement paste containing CYPS was denser compared to that of corresponding samples with RYPS.

Accelerating the Reaction Kinetics of Na2CO3-Activated Slag Mortars by Calcined Recycled Concrete Fines.

Sodium carbonate (Na2CO3), an environmentally friendly activator, has been shown to have vast potential for the development of sustainable alkali-activated slag mortars. However, Na2CO3-activated slag mortars exhibit a delayed reaction process and limited early-age strength development, restricting their wider application. In this work, the recycled concrete fines were calcined at a temperature of 800 °C for 1 h and then used as an auxiliary activator to improve the reaction kinetics of Na2CO3-activated slag mortars. The impact of the calcined recycled concrete fines (CRCF) dosage and Na2CO3 concentration on the compressive strength, hydration kinetics, and phase assemblage of mortars was evaluated. The results show that CRCF can react directly with Na2CO3 in the early stages, swiftly removing the CO32− in aqueous solution and providing an alkaline environment suitable for the dissolution of slag. This promotes the development of C-(A)-S-H, hydrotalcite, hemicarbonate, and monocarbonate. The hydration process and strength-giving phase of mortars can be improved further, as an increase in Na2CO3 concentration increases the initial alkaline content. Additionally, the most remarkable compressive strength value of 39.2 MPa was observed at 28 days in the mortar with 6% sodium oxide equivalent (Na2O-E) of Na2CO3 and 15% CRCF because of the synergistic effect of CRCF and Na2CO3.