- Open Access
Microstructural, Mechanical, and Durability Related Similarities in Concretes Based on OPC and Alkali-Activated Slag Binders
© The Author(s) 2014
- Received: 20 February 2014
- Accepted: 26 May 2014
- Published: 2 September 2014
Alkali-activated slag concretes are being extensively researched because of its potential sustainability-related benefits. For such concretes to be implemented in large scale concrete applications such as infrastructural and building elements, it is essential to understand its early and long-term performance characteristics vis-à-vis conventional ordinary portland cement (OPC) based concretes. This paper presents a comprehensive study of the property and performance features including early-age isothermal calorimetric response, compressive strength development with time, microstructural features such as the pore volume and representative pore size, and accelerated chloride transport resistance of OPC and alkali-activated binder systems. Slag mixtures activated using sodium silicate solution (SiO2-to-Na2O ratio or Ms of 1–2) to provide a total alkalinity of 0.05 (Na2O-to-binder ratio) are compared with OPC mixtures with and without partial cement replacement with Class F fly ash (20 % by mass) or silica fume (6 % by mass). Major similarities are noted between these binder systems for: (1) calorimetric response with respect to the presence of features even though the locations and peaks vary based on Ms, (2) compressive strength and its development, (3) total porosity and pore size, and (4) rapid chloride permeability and non-steady state migration coefficients. Moreover, electrical impedance based circuit models are used to bring out the microstructural features (resistance of the connected pores, and capacitances of the solid phase and pore-solid interface) that are similar in conventional OPC and alkali-activated slag concretes. This study thus demonstrates that performance-equivalent alkali-activated slag systems that are more sustainable from energy and environmental standpoints can be proportioned.
- alkali-activated slag
- isothermal calorimetry
- chloride transport
- electrical impedance
The demand for constructed facilities has been increasing in both the developed and the developing world, and ordinary portland cement (OPC) concrete is among the most used materials in infrastructure. The greenhouse gas emissions associated with Portland cement production (Mehta 2007)1 necessitate the development of alternate cementitious materials to replace a part of OPC concretes, and alkali-activated aluminosilicates are among the novel materials that are expected to fit this bill. Aluminosilicate materials such as fly ash and blast furnace slag, which are by-product materials from other industries, when used for concrete production, makes concrete more sustainable from a resource and energy viewpoint.
Alkaline activation of waste or by-product aluminosilicates such as fly ash or slag has been reported (Duxson et al. 2007; Palomo et al. 1999; Provis and Van Deventer 2009; Pacheco-Torgal et al. 2008; Shi et al. 2006), with adequate mechanical and durability properties (Fernández-Jiménez et al. 1999; Bernal et al. 2012; Pacheco-Torgal et al. 2012; Ravikumar et al. 2010). The use of low calcium fly ashes as precursors generally require heat curing to obtain desirable properties, even though slow strength gain at ambient temperatures has also been reported. Hence ground granulated blast furnace slag or a combination of fly ash and slag (Chithiraputhiran and Neithalath 2013) is employed for ambient temperature cured alkali-activated systems. In addition, a fairly rigorous understanding of slag as a cementitious material exists, and alkali activation of slag also produces C–S–H gel as the reaction product, similar to OPC systems but typically with a lower Ca/Si ratio (Yip et al. 2005; Song et al. 2000). This has led to extensive studies on the alkali activation of slag (Fernández-Jiménez et al. 1999; Bernal et al. 2012; Ravikumar et al. 2010; Chithiraputhiran and Neithalath 2013; Ravikumar and Neithalath 2012a). The depolymerized silica in slag results in higher dissolution rates under alkaline conditions. Caustic alkalis or alkali compounds whose anions can react with Ca2+ to form compounds less soluble than calcium hydroxide can act as activators for slag. The use of sodium silicate based activators (Na2SiO3·xH2O + NaOH) has been found to be ideal to produce desired mechanical properties (Fernández-Jiménez et al. 1999; Chithiraputhiran and Neithalath 2013; Chang 2003; Altan and Erdoğan 2012) even though alkali hydroxides, carbonates, and sulfates also have been reported to be used as activating agents (Shi et al. 2006). The nature and type of the activating agent determines the reaction rate and extent, and the reaction products formed, thereby exerting a significant influence on the early- and later-age properties of alkali-activated binder systems (Chithiraputhiran and Neithalath 2013; Ravikumar and Neithalath 2012a, b; Puertas et al. 2004; Hajimohammadi et al. 2011). The energy requirement towards the production of activators and the associated environmental impact also plays an important role in determining the sustainability of alkali-activated systems, along with their potential to replace large amounts of OPC.
In order for novel cementitious systems such as alkali-activated slags to gain acceptance in the concrete industry, it is imperative to have a thorough understanding of the properties and performance of these concretes vis-à-vis conventional OPC concretes. While several studies have reported comparisons between any one property of conventional and alkali-activated concretes, there have been no comprehensive studies that look at the early-age and later-age mechanical, microstructural, and durability (including alkali leaching and efflorescence) characteristics of these different materials. This study intends to bridge that knowledge gap by performing detailed studies on early-age reaction kinetics, strength development with time as a function of activator characteristics or cement replacement materials, microstructure and reaction products, and chloride transport resistance of conventional and activated concretes. Only waterglass-activated slag concretes are evaluated so as to keep the discussions succinct. The comparisons are made between the activated concretes and OPC concretes with and without partial cement replacement materials. Since most of the modern concretes contain high performance cement replacement materials such as fly ash or silica fume, such concretes are also proportioned in an attempt to position the alkali-activated concretes based on its relative performance.
Chemical compositions of the binder materials.
Composition (% by mass)
Iron oxide (Fe2O3)
Calcium oxide (CaO)
Magnesium oxide (MgO)
Sodium oxide (Na2O)
Potassium oxide (K2O)
Sulfur trioxide (SO3)
Loss on ignition
Specific surface area (m2/kg)
Waterglass (sodium silicate) solution was used as the activating agent for slag. The as-obtained waterglass solution had a silica modulus (ratio of SiO2-to-Na2O in the activator, termed Ms) of 3.26. Since it was previously observed that lower Ms values are needed for desired levels of activation, NaOH solution was added to bring the Ms to desired levels (1.0, 1.5, and 2.0 in this study). The ratio of Na2O-to-the total powder (n) was maintained as 0.05.
2.2 Paste, Mortar, and Concrete Mixtures
OPC and alkali-activated slag paste mixtures were used for isothermal calorimetry studies to evaluate the heat release response of these pastes. OPC mixtures were proportioned by replacing 0–20 % of OPC by fly ash and 0–12 % of OPC by silica fume, by mass. However the results for 20 % cement replacement and 6 % silica fume replacement alone are reported in this study to keep the comparisons focused. The water-to-cementing materials ratio (w/cm) used for the paste mixtures was 0.40. For the alkali-activated paste mixtures, the liquid-to-powder ratio (l/p) used was 0.50 because the use of an l/p of 0.40 resulted in a mixture of lower workability. The liquid consists of water that is added, along with the liquid portion of the waterglass solution, which was 60 %. It should be noted that no workability enhancing aids were used. The l/p used for the alkali-activated mortar and concrete mixtures were 0.40. The binder content used for the concrete mixtures was 400 kg/m3 while the mortar mixtures had a 50 % aggregate volume. Limestone aggregates and river sand was used. The concrete specimens were cast in cylinders (100 mm diameter and 200 mm height) and moist cured at 23 ± 2 °C and 98 % RH until the testing time.
2.3 Test Methods
2.3.1 Calorimetry and Porosimetry on Pastes
Isothermal calorimetry experiments were carried out in accordance with ASTM C 1679. The pastes were mixed externally and loaded into the isothermal calorimeter. The time elapsed between the instant water or the activating solution was added to the powder and the paste loaded into the calorimeter was around 2–3 min. The tests were run for 48 or 72 h with the calorimeter set at 25 °C. Mercury intrusion porosimetry (MIP) was performed on small samples of pastes (both conventional and activated) oven-dried at 60 °C for 2 h. This treatment procedure was found to yield similar total pore volumes and critical pore sizes as a more involved vacuum drying method and hence was used in this study. A mercury intrusion porosimeter that can generate a maximum pressure of 414 MPa and evaluate a minimum pore diameter of 0.003 µm was used. The test, after 28 days of hydration, was performed in two steps—the low pressure step evacuates the gases, fills the sample holder with mercury, and carries out the test up to 345 kPa, and the high pressure step reaches pressures of up to 414 MPa. The contact angle and surface tension values used were 130° and 0.485 N/m respectively for both the OPC and alkali-activated pastes. Accurate determination of these values could result in changes in the extracted pore structure features but the variations are expected to be minor in nature that does not influence the behavioral interpretations.
2.3.2 Mechanical and Durability Tests on Concretes
2.3.3 Electrical Impedance Spectroscopy
This section reports the results of early age hydration response, compressive strength, pore structure, and chloride transport resistance of conventional and alkali-activated slag concretes and facilitates a comparison of their properties.
3.1 Early Age Reaction Response and Compressive Strength
It is expected by further decreasing the M s , the dormant periods can be reduced even more and the total reaction rate at early ages, as evidenced by the main peak heights, can be increased. However, there is little benefit to improving the early age reaction rates in activated slag pastes by decreasing the M s (or increasing the Na2O content in the mixture) because the later-age strengths are found to be higher at higher M s values (or higher silica contents) as will be shown in a forthcoming section.
When the total heat released at the end of 48 h for OPC and modified pastes and 72 h for the activated slag pastes (note that the steady state is not reached until about 60 h for the high M s value pastes) are compared, it can be noticed that a distinct plateau is observed for the activated slag pastes, corresponding to the very long dormant period. It has been shown that increasing the reaction temperature also results in reducing the extent of this plateau, as is the use of higher alkalinity (either through increase in the Na2O-to-powder ratio (n) or reducing the M s ) (Chithiraputhiran and Neithalath 2013; Ravikumar and Neithalath 2012a). Figure 2b also shows that the cumulative heat of the mixture with a higher M s tends to approach that of the lower M s mixture after 72 h, and the trends indicate a higher cumulative heat for the higher M s mixture at a much longer duration. This is substantiated by the compressive strength results shown below.
3.2 Pore Structure and Product Constitution in Conventional and Alkali-Activated Pastes
As shown in the previous section, the conventional OPC mortars and alkali-activated slag mortars show similar 28-day compressive strengths. It is therefore instructive to examine their pore structure, which is the focus of this section. The pore structure of these systems is also influential in their transport (moisture and ionic) performance which is a key determinant of their durability. Since the highest compressive strength was obtained for the activated mixture with an M s of 2.0 in this study, this paste is used for the pore structure and reaction product studies along with a plain OPC paste. 28-day old pastes are used for the analysis. Mercury intrusion porosimetry (MIP) is used to investigate the pore structure of these pastes. The use of MIP as an indicator of the total pore volume is well-accepted, but its use for pore size determination has several drawbacks (Diamond 2000; Moro and Böhni 2002), primarily because of the presence of “pore-throats” in cement pastes and similar systems. Mercury cannot intrude into some of the larger pores until the applied pressure is large enough to saturate the pore throats. However, an indication of the threshold pore diameter, ideally not as an absolute value, but as a parameter to facilitate comparison between specimens, can be obtained from the differential pore volume-pore diameter relationships (Atahan et al. 2009). While the resistance to high pressure could vary depending on the type of the matrix, its effects are not considered in this paper based on the fact that these matrices are shown to demonstrate similar compressive strengths.
The critical pore sizes or the threshold pore sizes are taken as the sizes corresponding to the maximum in the pore diameter (D)—dV/dlogD relationships. It is the size below which the pore system is depercolated, and hence plays an important role in the moisture and ionic transport properties of the material. Figure 4b shows the D-dV/dlogD relationships for both the plain OPC and alkali-activated pastes. The critical pore sizes for the OPC and alkali-activated paste respectively are 0.058 and 0.068 µm respectively, which are very comparable, once again pointing to the similarity between these two different materials. These similarities in total pore volume and pore size can be considered to be responsible for the similarities in mechanical properties of OPC and alkali-activated mortars as shown previously in Fig. 3.
3.3 Accelerated Chloride Transport Test Parameters
This section discusses the accelerated chloride transport resistance of conventional and alkali-activated slag concretes and compares their performance. Rapid chloride permeability test (RCPT) and non-steady state migration (NSSM) test are used as the transport tests. RCPT, conforming to ASTM C 1202 is one of the common durability tests used as a quality control tool and acceptance criteria for OPC concretes. RCPT measures the electrical conductivity of concretes. Conductivity of porous materials saturated with a conducting fluid is influenced by the conductivity of the saturating medium in addition to the pore structure parameters like the overall pore volume and its geometry. Thus this test does not adequately represent the pore structure characteristics of the material that determines ionic transport. This aspect has been reported elsewhere. However, its ease of use and documented relationships with other performance characteristics of concrete has made this test widely accepted in practice. The NSSM test avoids some of the drawbacks of the RCP test by using a lower applied potential and a longer test duration. The catholyte solution used is 10 % NaCl, thereby ensuring that the catholyte concentration does not undergo large changes during the duration of the test.
When the RCP values of the alkali-activated concretes at 28 days are compared to those of OPC concretes, it can be seen that the waterglass-activated slag concretes demonstrate RCP values similar to those of 56-day cured conventional concretes or 20 % fly ash modified concretes of similar w/cm. The 28 day NSSM values of the alkali-activated slag concretes of all the three M s values examined here are lower than those of the plain and modified OPC concretes at the same age. In fact, the 28-day NSSM coefficient of the activated concrete with an M s of 2.0 is similar to the 90-day NSSM coefficient of 6 % silica fume modified OPC concrete. This points to the effectiveness of alkali activation of slag in reducing chloride ion transport through its microstructure, and the capability of this material to be performance-comparable to those of OPC systems. The determination of chloride diffusion coefficients of alkali-activated slag concretes using other test methods are likely to provide widely different values (differences in 2 orders of magnitude between the NSSM coefficients reported here and the effective diffusion coefficients reported in Bernal et al. (2012), but the trends remain the same.
3.4 Models to Establish Similarities in Material Microstructure of OPC and Activated Slag Concretes
The foregoing sections have provided detailed accounts on the comparison between conventional cementitious binders with and without cement replacement and alkali-activated slag binders with respect to early age reactions, strength, microstructure, and chloride transport performance. In this section, electrical impedance spectroscopy (EIS) and associated electrical circuit models are used to explore the microstructures of these systems further with an aim of understanding the underlying similarities that are responsible for similar mechanical and transport properties.
3.4.1 An Electrical Model for Material Microstructure and Its Parameters
Alkali-activated slag pastes showed similar isothermal calorimetric response as those of OPC pastes with respect to the characteristic features of the thermal signature. However the major reaction peak was much more delayed in the alkali-activated pastes, and exhibited lower intensities than those of OPC pastes. The overall heat released due to the alkali activation reaction was much lower than that from OPC hydration at the end of 48 or 72 h, even though the cumulative heat release curve had not plateaued for the alkali-activated paste, especially the one with a higher M s .
The compressive strengths of the alkali-activated slag mortars as proportioned in this study, and the plain and modified OPC systems were similar. For the alkali-activated slag mortars, the 28-day compressive strength was found to be higher at higher activator M s , which facilitates a lower Ca/Si ratio in the C-(A)–S–H gel. From considerations of early age reaction kinetics and strength development, an activator M s of 2.0 was found to be optimal. Mercury intrusion porosimetry studies revealed that the plain OPC paste and the alkali-activated slag paste with an M s of 2.0 have comparable porosities after 28 days. The critical pore size from MIP was found to be slightly lower for the OPC paste.
The chloride transport parameters of the alkali-activated slag concretes also showed similarities with those of OPC concretes. The 28-day cured waterglass activated slag concretes demonstrated RCP values similar to those of 56-day cured conventional concretes or 20 % fly ash modified concretes of similar w/cm, while the NSSM values were similar to those of 90-day cured 6 % silica fume modified OPC concrete. This establishes the improved ionic transport resistance of alkali-activated slag concretes. Equivalent electric circuit models and associated parameters were also used to identify the similarities in the microstructure of both these types of concretes—particularly the resistance of connected pores or the percolating pore network, and the capacitances of the pore-solid interfaces.
Estimated OPC production of 3.2 billion tons in 2012 results in about 5 % of the global CO2 emissions being attributed to cement production.
The authors sincerely acknowledge partial financial support from National Science Foundation (CMMI: 1068985) and the New York State Energy Research and Development Authority (NYSERDA) towards the conduct of this study. The materials were provided by US Concrete, Headwaters Resources, Holcim U.S, and PQ Corporation, and are acknowledged. The work was carried out in the Laboratory for the Science and Sustainable Infrastructural Materials (LS-SIM) at Arizona State University, and the support that has made the establishment and operation of this laboratory is also acknowledged.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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