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.
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
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
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.
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
Region 2, Setting Time, denotes the period when the fluid mortar starts to gel and reaches initial set.
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.
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
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
The authors thank
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.
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5. Hou, P.; Wang, K.; Qian, J.;
6. Belkowitz, J.S., and Armentrout, D., "An Investigation of Nano Silica in the Cement Hydration Process,"
7. Nazari, A., and Riahi, S., "The Effects of SiO2 Nanoparticles on
8. Constantinides, G., and Ulm, F.-J., "The Nanogranular Nature of C-S-H,"
9. Sanchez, F., and Sobolev, K., "Nanotechnology in Concrete - A Review," Construction and
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,
11. Jayapalan, A.R.;
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.
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