News Column

Colloidal Silica Admixture

July 1, 2014

Fisher, Frank T



A novel approach for increasing early strength and durability

Colloidal silica (CS)-nanoscale particles of pure silicon dioxide (silica) dispersed in a liquid medium- has been available to the concrete industry for over 20 years. While it's known to be a pozzolan, CS is generally used only as a finishing aid and densifier for concrete floors. Recently, a series of tests were undertaken to evaluate CS as an admixture. With the specific goal of determining whether CS could mitigate alkali-silica reactivity (ASR) in dosages that would not retard early concrete strength development, mixtures were evaluated in the laboratory and on a commercial job site.

When concrete contains aggregates with a significant amount of reactive silica and cement with a moderate-tohigh alkali content, there is potential for ASR. Silica is liberated from the aggregates by [OH-] ions in solution. The silica then combines with alkalis from the portland cement, and a gel is formed around and inside the aggregate. As this gel forms, it exerts expansive forces, breaking the bond between hydrated paste and aggregate particles, pushing the concrete apart from the inside and causing extensive cracking.1-4 The cracking increases the absorption of water, thus increasing the propensity for further expansion and making the concrete more vulnerable to carbonation, sulfate attack, and intrusion of corrosive agents that can compromise steel reinforcement.

Mitigative measures include the use of a lithium admixture or cement replacement with Class F fly ash, slag cement, or silica fume.2,3,5 In general, these materials suppress ASR by binding calcium hydroxide and limiting its availability for reaction with the siliceous minerals in the aggregate.1,4 However, these measures also tend to reduce the early-age compressive strengths of concrete.

Lafarge North America's concrete plant in Gypsum, CO, recently received the contract to supply concrete for a taxiway pavement at the Eagle County Airport AircraftRescue and Fire Fighting facility. Because the aggregates available to this plant are known to contain high amounts of reactive silica, mitigative methods were required. But given the critical function of the facility, we were tasked with finding measures that would provide resistance to ASR without degrading the fresh concrete properties or compressive strengths.

Materials

CS particles are about 1/1000 the size of fly ash particles. The small CS particles can accelerate cement dissolution and nucleation as well as provide a much larger surface area of free silica for pozzolanic reaction. CS admixtures can thus provide rapid early-strength development and binding of the calcium hydroxide so that it does not participate in ASR.5-11 The CS admixture used in our study and in the taxiway pavement has a pH of 10 and comprises silica particles suspended in a solution with 85% water and 15% solids content. The particles have a surface area of 500,000 m2/kg (of dry particles) with particle diameters ranging from 3 to 6 nm. The material properties for the CS and other cementitious material constituents used in the concrete mixtures are listed in Table 1.

The coarse aggregate used in the mixtures was size 57/67 per ASTM C33/C33M, "Standard Specification for Concrete Aggregates," and it had a specific gravity of 2.63. The fine aggregate met the requirements for concrete sand per ASTM C33/C33M, and it had a specific gravity of 2.62. Both were sourced from a river bed in Gypsum, CO, and were classified as having potentially deleterious expansion per ASTM C289-07, "Standard Test Method for Potential Alkali-Silica Reactivity of Aggregates (Chemical Method)."

The CS admixture can be dosed to the fresh concrete at the plant or in the concrete mixer truck. Prior to the construction of the pavement, we evaluated the fresh and hardened properties of concrete produced with the CS admixture. The testing was a collaborative effort among the Stevens Institute of Technology; Lafarge North America; Kumar and Associates; and Intelligent Concrete, LLC.

Proportions for the control and experimental mixtures are listed in Table 2. The control mixtures (Control 1 and Control 2) had been used by the ready mixed concrete producer in the past for pavement applications. Both mixtures include Class F fly ash to mitigate ASR gel expansion. Both also include a nonchloride accelerator to counter the retarding effect of the fly ash. It has been shown that such mixtures tend to have a relatively coarse microstructure that reduces the durability of concrete.12

The water from the CS solution was accounted for when determining the batch water to maintain the design watercementitious material ratio (w/cm). Preliminary trials using Control 1 as the basis for a CS mixture resulted in uncharacteristically low compressive strengths, so Control 2 was used as the basis for laboratory, plant, and commercial project trials (Table 2). Plant trial data were used for scaling the mixtures for manually placed or slip-formed pavement projects.

Lafarge North America evaluated the variation of fresh properties (slump per ASTM C143/C143M, "Standard Test Method for Slump of Hydraulic-Cement Concrete," and air content per ASTM C231/C231M, "Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method") over the initial 2 hours after mixing and compres- sive strength (per ASTM C39/C39M, "Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens") at ages of 1, 4, 7, and 28 days. Stevens Institute of Technology; Intelligent Concrete, LLC; and Kumar and Associates partnered to determine the effects of CS on cement hydration, focusing primarily on the temperature development over the first 24 hours and ASR resistance per ASTM C1260, "Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method)," and ASTM C1567, "Standard Test Method for Determining the Potential Alkali- Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method)."

Results and Discussion

Fresh concrete properties

Two in-plant trials and two commercial-project trials were coordinated to determine the dosages of high-range water-reducing admixture (HRWRA) and air-entraining admixture (AEA) needed to make the CS mixtures meet the specified fresh properties. Tables 2 and 3 list the results.

In stark contrast to the control mixture, the CS mixtures maintained slump and air content for 2 hours, with minimal losses. Furthermore, the CS mixtures required lower dosages of HRWRA and AEA to generate the specified fresh properties. In contrast, the AEA dosage for the control mixture was further increased to compensate for the air content losses during transportation to the job site.

Temperature data were collected by Stevens Institute of Technology to understand how the CS impacted concrete temperature development during placement and initial curing. Adiabatic temperature changes were monitored in samples representing Control 2, CS-P, CS-1, and CS-2 mixtures, using proportional weights from the mixtures listed in Table 2 but excluding the coarse aggregate. Figure 1 illustrates the temperature as a function of time behaviors for the CS and control mixtures. Concrete temperature evolution can be delineated into four regions. In Region 1, Slump Retention, the temperature will typically plateau after a brief but rapid increase. With other factors remaining constant, a lower temperature plateau will tend to correlate with a longer slump retention period. The CS mixtures had a lower plateau than the control mixture, and as seen in Table 3, slump retention increased from 60 minutes for the control mixture (Control 2) to over 2 hours for the CS mixtures. Furthermore, the CS mixtures required less HRWRA to retain slump on the job site. It should be noted that the ambient conditions were identical during testing of the CS and control mixtures.

Region 2, Setting Time, denotes the period when the fluid mortar starts to gel and reaches initial set. Vicat needle tests (ATSM C807, "Standard Test Method for Time of Setting of Hydraulic Cement Mortar by Modified Vicat Needle") on mortar samples showed that setting time of the CS mixture was delayed by 23 minutes relative to the control mixture. However, contractors reported no significant delays in finishing operations.

Region 3, Early Strength I, is the period following initial setting up to 12 hours after mixing. In this region, concrete strengths typically range from 500 to 2500 psi (4 to 17 MPa), so forms can be stripped or a pavement may be opened to traffic. Earlier and higher maximum temperatures in this region normally indicate greater early strength. For the CS mixture, the maximum temperature and the time that the maximum temperature was reached both indicate that the CS mixture reached early compressive strength values before the control mixture.

Region 4, Early Strength II, is the period from 12 to 24 hours after mixing. The CS mixture had a higher temperature in both the Early Strength I and II regions, indicating greater early compressive strengths than the control mixture. This was verified by tests of the hardened concrete properties.

Hardened concrete properties

It has become a standard practice to use concrete mixtures with high early strength. While nonchloride accelerators are normally used to increase the early strength of concrete with Class F fly ash, similar acceleration can be generated with the use of CS. The partial replacement (by weight) of portland cement with CS increases the total pozzolanic surface available for chemical reaction. It has also been proposed that CS causes the dissolution of single cement particles.10,11 As indicated by the data in Table 4, the CS mixtures had greater early strength gain than the Control 2 mixture.

Cement efficiency-the ratio of compressive strength per unit weight of cement for a unit volume of concrete- provides an indication of the potential for reductions in cement used in a concrete mixture. Table 4 and Fig. 2 show the increase of cement efficiency of the CS and control mixtures. Compared with the Control 2 mixture, the CS mixtures had greater cement efficiency at every tested age.

Figure 3 illustrates the change in expansion in mixtures that included cement, CS, and fly ash tested per ASTM C1260 and C1567. While ASTM C1260 evaluates a potential of ASR reaction of aggregate in mortar bars (cement only), ASTM C1567 tests the ability of a pozzolanic or secondary cementitious material in a cement-composite sample (mortar bar) to reduce ASR expansion of aggregate.

Proportional amounts of the coarse and fine aggregates listed in Table 2 were used for the ASTM C1260 and C1567 mortar bar tests. The aggregate to cement ratio was 2.25 to 1.00 for both tests. Mixtures with cement plus fly ash or cement plus fly ash plus CS exhibited expansions below the 0.10% indicative of low risk of deleterious expansion due to ASR. The CS also reduced the rate of expansion, as shown by the slope of the linear trend line and the ultimate expansion at 14 days.

Great potentials

Two different placement methods were used to analyze the changes induced by the CS on the fresh concrete. Figures 4 and 5 show the slipform paver and the manual placement of concrete pavements for the Eagle County Airport facility. Each paving application required 10 yd3 (8 m3) of concrete. During the delivery and placement of the concrete, the fresh properties (slump and air) were maintained without additions of water or admixtures on the job site. Furthermore, the CS did not alter the workability of the mixture. (Normally, accelerating admixtures tend to reduce workability.) Field and laboratory tests showed that CS provided increased early strength of Class F fly ash mixtures while enhancing the strength development and durability.

Our study showed that CS used as an admixture in concrete with 20% Class F fly ash has the potential to:

* Enhance fresh properties and allow a reduction in HRWRA and AEA dosages;

* Increase early and 28-day compressive strengths;

* Increase cement efficiency and thus allow a reduction in cement content while still maintaining specified hardened properties; and

* Complement fly ash as an ASR mitigator.

Based on this initial work, we believe that the concrete community could benefit from the use of the CS admixture. Industry peers agree, as the application was awarded Mix Innovation of the Year (2012) by the Rocky Mountain Chapter - American Concrete Institute.

Acknowledgments

The authors thank Bill Arrasmith, Dan Diaz, Rick Seymour, and Rick Juedemann from Lafarge North America (Gypsum Plant) for their contribution in equipment and experimentation time. The authors also thank Matt Reed from AkzoNobel for graciously supplying the colloidal silica solutions; Katie Bartojay of the Bureau of Reclamation for reviewing the data, results, and discussion of this research; and Steven H. Miller for editing drafts for final publication. The partial support of the National Science Foundation under Grant No. NSF DGE-0742462 and CMMI-0846937 is gratefully acknowledged.

Aditivo de sÍlice coloidal

La sÍlice coloidal (CS) se emplea generalmente como aditivo para el acabado y como densificador para los suelos de hormigÓn. Recientemente se realizaron pruebas para evaluar el CS como aditivo para mitigar la reactividad alcalino-silÍcea (ASR) en dosis que no retardaran la fragua de resistencia del hormigÓn. El artÍculo presenta el desarrollo y la evaluaciÓn de mezclas de hormigÓn con CS como aditivo en el laboratorio y en un sitio de trabajo comercial. Las pruebas de campo y de laboratorio mostraron que la CS proporciona un aumento de resistencia en la fragua de las mezclas de cenizas sueltas Clase F, a la par que reduce la expansiÓn ASR.

References

1. Thomas, M., "The Effect of Supplementary Cement Materials on Alkali-Silica Reaction: A Review," Cement and Concrete Research, V. 41, No. 12, Dec. 2011, pp. 1224-1231.

2. Taylor, H., Cement Chemistry, second edition, Thomas Telford Services, Ltd., London, UK, 1997.

3. Bonakdar, A.; Mobasher, B.; Dey, S.; and Roy, D., "Correlation of Reaction Products and Expansion Potential of Alkali-Silica Reaction for Blended Cement Materials," ACI Materials Journal, V. 107, No. 4, July-Aug. 2010, pp. 380-386.

4. Ichikawa, T., "Alkali-Silica Reaction, Pessimum, and Pozzolanic Effect," Cement and Concrete Research, V. 39, No. 8, Aug. 2009, pp. 716-726.

5. Hou, P.; Wang, K.; Qian, J.; Kawashima, S.; Kong, D.; and Shah, S.P., "Effects of Colloidal NanoSiO2 on Fly Ash Hydration," Cement and Concrete Composites, V. 34, No. 10, Nov. 2012, pp. 1095-1103.

6. Belkowitz, J.S., and Armentrout, D., "An Investigation of Nano Silica in the Cement Hydration Process," National Ready Mixed Concrete Association, 2010 Concrete Sustainability Conference, Apr. 2009, pp. 1-15.

7. Nazari, A., and Riahi, S., "The Effects of SiO2 Nanoparticles on Physical and Mechanical Properties of High Strength Compacting Concrete," Composites: Part B: Engineering, V. 42, No. 3, Apr. 2011, pp. 570-578.

8. Constantinides, G., and Ulm, F.-J., "The Nanogranular Nature of C-S-H," Journal of the Mechanical and Physics of Solids, V. 55, No. 1, June 2006, pp. 64-90.

9. Sanchez, F., and Sobolev, K., "Nanotechnology in Concrete - A Review," Construction and Building Materials, V. 24, No. 11, Nov. 2010, pp. 2060-2071.

10. BjÖrnstrÖm, J.; Martinelli A.; Matic, A.; BÖrjesson, L.; and Panas, I., "Accelerating Effects of Colloidal Nano-Silica for Beneficial Calcium-Silicate-Hydrate Formation in Cement," Chemical Physics Letters, V. 392, No. 1-3, July 2004, pp. 242-248.

11. Jayapalan, A.R.; Lee, B.Y.; and Kurtis, K.E., "Effect of Nano-sized Titanium Dioxide on Early Age Hydration of Portland Cement," Nanotechnology in Construction 3: Proceedings of NICOM3, 2009, pp. 267-273.

12. Van Dam, T.J.; Peterson, K.R.; Sutter, L.L.; Panguluri, A.; Sytsma, J.; Buch, N.; Kowli, R.; and Desaraju, P., "Guidelines for Early-Opening-to-Traffic Portland Cement Concrete for Pavement Rehabilitation," NCHRP Report 540, Transportation Research Board, Washington, DC, 2005, 38 pp.

Note: Additional information on the ASTM standards discussed in this article can be found at www.astm.org.

Selected for reader interest by the editors.

ACI member Jon S. Belkowitz is a Research and Development Engineer in the Materials Division at Intelligent Concrete, LLC, Freehold, NJ. He received his BS in civil engineering from the Colorado School of Mines and his MS from the University of Denver. He is currently a PhD student at Stevens Institute of Technology. He is Secretary of ACI Subcommittee 236-D, Material Science - Nanotechnology of Concrete. His research interests include cementitious and geopolymer composites and the use of nanotechnology in concrete.

Whitney B. Belkowitz, an ACI, ASME, and ASCE member, is the President and part of the Research and Development Division of Intelligent Concrete, LLC. Her technical research interests include characterization of the concrete composite, manipulation and optimization of cementitious and pozzolanic materials, technology transfer, and concrete and cementitious education.

ACI member Matthew A. Best is a Construction Services and Lab Manager with Kumar and Associates in Frisco, CO. He received his BS in biology from Colorado Christian University and is currently enrolled in the civil engineering technology program at Metro State College of Denver. He is a Board member of the Rocky Mountain Chapter - ACI. His research interests include the internal structure and reactions of cementitious and pozzolanic products with silica in aggregates and concrete.

Frank T. Fisher, PhD, is an Associate Professor in the Department of Mechanical Engineering and Co- Director of the Nanotechnology Graduate Program at Stevens Institute of Technology. His research interests involve advanced material systems at the nanoscale, including polymer and cement-based nanocomposites. He received the National Science Foundation's CAREER award, the American Society of Engineering Education Mechanics Division Beer and Johnston Outstanding New Educator Award, and the 2009 Outstanding Teacher Award from the Stevens Alumni Association.


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Source: Concrete International


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