Effect of Autoclave Curing on the Microstructure of Blended Cement Mixture Incorporating Ground Dune Sand and Ground Granulated Blast Furnace Slag
© The Author(s) 2015
Received: 7 June 2014
Accepted: 19 July 2015
Published: 12 August 2015
Investigating the microstructure of hardened cement mixtures with the aid of advanced technology will help the concrete industry to develop appropriate binders for durable building materials. In this paper, morphological, mineralogical and thermogravimetric analyses of autoclave-cured mixtures incorporating ground dune sand and ground granulated blast furnace slag as partial cementing materials were investigated. The microstructure analyses of hydrated products were conducted using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), differential thermal analysis (DTA), thermo-graphic analysis (TGA) and X-ray diffraction (XRD). The SEM and EDX results demonstrated the formation of thin plate-like calcium silicate hydrate plates and a compacted microstructure. The DTA and TGA analyses revealed that the calcium hydroxide generated from the hydration binder materials was consumed during the secondary pozzolanic reaction. Residual crystalline silica was observed from the XRD analysis of all of the blended mixtures, indicating the presence of excess silica. A good correlation was observed between the compressive strength of the blended mixtures and the CaO/SiO2 ratio of the binder materials.
Concrete is a composite material consisting of aggregate, water and Portland cement (PC). When PC is placed in contact with water, several chemical reactions occur (Hewlett 2003; Taylor 1997). These reactions produce many phases, such as calcium silicate hydrates (C–S–H gel), calcium hydroxide (CH or portlandite), ettringite (AFt), and monasulfomonite (AFm) (Englehardt and Peng 1995; Hewlett 2003). To ensure the continuity of cement hydration, the cast mixture should be cured under appropriate conditions (Mehta and Monteiro 2006; Topçu and Uygunoğlu 2007). The typical curing conditions used in concrete technology include normal curing, low pressure steam curing, high pressure steam curing and membrane curing. The so-called normal curing is conducted under a moist and ambient temperature. Under these conditions, the hydration process and strength development rate of cement paste are slow (Liu et al. 2005). Consequently, concrete takes several days, or sometimes several months, to reach its ultimate strength (Gutteridge and Dalziel 1990; Topçu and Uygunoğlu 2007). As a result, in precast concrete plants and other applications where higher early strength is of great concern, normal curing conditions are not preferred (Erdoğdu and Kurbetci 2005; Hanson 1963; Kołakowski et al. 1994).
In precast concrete technology, accelerated curing conditions, such as high-pressure steam curing (autoclave curing), are adopted (Mindess et al. 1981). The advantages of autoclave curing include that compressive strength equivalent to that of 28 days of normal curing can be achieved within 24 h under autoclave curing, less drying shrinkage, elimination of efflorescence, and better resistance to sulfate attacks (Kołakowski et al. 1994; Menzel 1934; Mindess et al. 1981; Neville 1973). Another advantage of autoclave curing is that many types of siliceous materials can be used as supplementary cementing materials even though some siliceous materials may not be feasible to react under normal curing conditions (Kalousek 1954).
The microstructure of cement paste cured under autoclave conditions is different than that produced at ambient temperature (Menzel 1934). The primary formed C–S–H gel converts to crystalline alpha dicalcium silicate hydrate (α-C2SH) or C3SH1.5 (Jupe et al. 2008; Mindess et al. 1981; Taylor 1997). These phases are porous and weak, which leads to a deterioration in the compressive strength and concrete durability (Eilers et al. 1983; Grabowski and Gillott 1989). To avoid the formation of undesired phases (α–C2SH and C3SH1.5), fine siliceous material should be added at 30–40 % by weight of the cement. The added siliceous materials react with the CH generated from the PC hydration to produce new C–S–H phases, such as tobermorite and xonotlite (Berardi et al. 1975; Eilers et al. 1983; Mindess et al. 1981; Taylor 1997). The hydration product features of autoclaved mixtures depend on many parameters, including the curing conditions, calcium to silica ratio (Ca/Si), and type of added silica (Bresson et al. 2002; Eilers et al. 1983; Hope 1981; Kołakowski et al. 1994). It has been stated that, under autoclave conditions, the use of crystalline silica produces high strength compared to the use of amorphous silica (Assarsson and Rydberg1956; Grabowski and Gillott 1989; Jupe et al. 2008; Luke 2004; Sanders and Smothers 1957; Yazici 2007). Additionally, it has been demonstrated that, at each autoclaving temperature, there is an optimum period of curing that results in good mechanical and physical properties (Hanson 1963; Menzel 1934; Neville 1973; Shi and Hu 2003; Yazıcı et al. 2008).
Recently, many tools have been used to examine the concrete structure not only at microscale but also at the nano-level (Chae et al. 2013; Kar et al. 2014). Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and thermo-graphic analysis (DTA and TGA) are being used to study the microscale changes that occur in the cement paste and concrete structure (Murmu and Singh 2014; Singh et al. 2015). Understanding the hydration mechanism and microstructure properties with the aid of advanced technology will help the concrete industry develop appropriate binders for durable building materials (Lange et al. 1997).
Previous studies by the authors have reported that ground dune sand and ground granulated blast furnace can be used as high volume cement replacement materials in autoclave concrete production (Alawad et al. 2015). However, the effect of autoclave conditions on microstructure of blended cement mixtures containing ground dune sand and slag have not been studied in depth. This study aimed to investigate the microstructures of autoclave-cured products of blended cement paste mixtures that incorporate ground dune sand and ground granulated blast furnace slag as cement replacement materials. Microstructure analyses were performed using SEM, EDX, XRD, DTA, and TGA.
2 Experimental Work
2.1 Raw Material
Chemical and physical properties of PC, GDS and Slag.
Chemical composition (%)
Specific surface area, BET cm2/g
2.2 Sample Preparation
Binder proportions for the ternary blended cement mixtures.
The paste and mortar mixtures were prepared according to ASTM C 305 and cast in 50 mm cubic steel molds. The cast samples were covered with plastic sheets and kept under laboratory conditions (23 ± 3 °C and 50 ± 5 relative humidity) for 24 h. After being demolded at the age of 24 h, the samples were immersed in water at 23 ± 3 °C (normal curing conditions) for 16 h to develop initial strength and then placed in the autoclave chamber. The chamber temperature was increased from room temperature to 182 ± 3 °C within 1 h. Consequently, the pressure was increased from atmospheric pressure to 1.0 MPa. The adopted autoclave conditions (temperature and pressure) are similar to those used by Yang et al. (2000). The temperature and pressure were kept constant at 182 ± 3 °C and 1.0 MPa for 5 h, then the autoclave heater was turned off and the chamber was allowed to cool naturally. Room temperature was reached in approximately 1.5 h.
2.3 Sample Characterization
Microstructural and microanalytical characterizations of the hydrated pastes were conducted using a Jeol JSM 6610LV coupled with EDX. The samples for SEM analysis were prepared by taking fractured surface specimens of the cured pastes. The specimens were glued to carbon stubs with carbon paint prior to the SEM analysis. The SEM analysis was carried out using accelerating voltages of 10 and 15 kV and magnifications of ×3500 and ×8000. The chemical composition analysis of selected spots (fields of view) was carried out using EDX.
Thermogravimetric analysis (DTA and TGA) was carried out using a TA instrument (model SDT Q600). DTA and TGA analyses were conducted to monitor the phase changes and to evaluate the amount of CH consumed in the cured samples. A predefined amount of the selected sample in powder form was weighed, placed in a platinum sample pan and then heated from room temperature to 1000 °C at a heating rate of 10 °C min−1 in a nitrogen gas flow. XRD analysis was conducted using a Shimadzu XRD-6000 diffractometer with a scanning rate of 2°/min from 10° to 60° (2θ) to obtain the mineralogical information for each sample. The samples for XRD analysis were prepared by grinding pieces of hydrated pastes into a powder form that could pass a 75-μm sieve. The compressive strength test of the cast mortars was carried out in accordance with ASTM C109.
3 Results and Discussion
3.1 Morphological Study
Figure 5 presents the EDX analysis of the M2-AC and M4-AC mixtures. The image and EDX analysis of the M2-AC mixture are shown in Fig. 5a and b, respectively. High peak intensities of Ca and Si with a Ca/Si ratio approximately equal to unity were observed from the EDX analysis (Fig. 5b). This analysis indicated that the crystalline structure phases are newly formed CSH (i.e., tobermorite). The formation of CSH phases with a Ca/Si ratio close to unity is a favored result for the concrete strength and physical properties (Eilers et al. 1983; Yazıcı et al. 2008). The image and EDX analysis of the M4-AC mixture are shown in Fig. 5c and d, respectively. The presence of slag leads to the formation of thin crystalline structure phases of newly formed aluminum bearing CSAH (i.e., C4SAH4) (Klimesch and Ray 1998; Kyritsis et al. 2009; Mostafa et al. 2009). The reason for the formation of C4SAH4 phases is attributed to the presence of the element Al in the system as the slag contains a significant amount of Al2O3 (Table 1). The EDX analysis indicated that the presence of GDS and slag not only prevented the formation of weak and permeable phases (α-C2SH) but also introduced new CSH phases, such as tobermorite (C5S6H5) and hydrogarnet (C4SAH4), which are associated with high strength and low permeability (Yazıcı et al. 2008).
3.2 Thermogravimetric Analysis
3.3 Mineralogical Analysis
3.4 Compressive Strength
The SEM and EDX results indicated that the incorporation of GDS and slag produces denser and more homogeneous microstructures. The SEM showed the clear formation of crystalline plate-like tobermorite and hydrogarnet in the blended mixture cured under autoclave conditions. In addition, the EDX indicated that the CSH products formed had C/S approximately equal to unity.
The DTA and TGA confirmed the consumption of CH and clear formation of an exothermic peak at 850 °C. The intensity of exothermic peaks at 850 °C of the PC-GDS mixture (M2) was higher than those of the PC-GDS-slag mixtures (M3, M4, M5).
The XRD indicated the presence of residual crystalline silica of the PC-GDS mixture (M2), whereas the CH peaks disappeared. The incorporation of slag did not affect the peak intensities of residual crystalline silica.
The incorporation of GDS significantly enhanced the compressive strength of the autoclave-cured mixture, whereas the inclusion of slag maintained the compressive strength to be higher or comparable to that of the control mixture.
A good correlation was found between the compressive strength and the CaO/SiO2 ratio. The compressive strength decreased linearly as the CaO/SiO2 ratio decreased. The authors recommend using materials rich in CaO to increase the CaO/SiO2 ratio and to utilize the excess SiO2 of GDS in the pozzolanic reaction.
This study is part of a joint research project between King Saud University, Saudi Arabia and Universiti Putra Malaysia titled “Development of local sand as a cementitious material for high-performance concrete”. The funding of this work by King Saud University is gratefully acknowledged.
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