Experimental Procedures

1998), as well as to adjust the PCE dosage to achieve a target fluidity of 130 ± 10 mm. A truncated mini-cone with a top diameter of 70 mm, a bottom diameter of 100 mm, and a height of 60 mm was used. The test procedure consists of placing the mini-slump cone at the center of the plate, followed by filling with the fresh mortar sample. Subsequently,

the mini-cone is smoothly lifted and two perpendicular spread diameters (d1 and d2, in

mm) are recorded once the flow has stopped. The mini-slump flow spread is calculated as the average of the two measured diameters.

Determination of solid concentration in fresh state: In this study, the solid concentrations of the investigated mortar systems at various PCE dosages were measured using wet density approach (Wong and Kwan 2008b) in order to determine the optimum PCE dosage corresponding to the maximum possible solid concentration for a given system. The addition of PCE improves the dispersion of particles, and hence the agglomeration of solid particles is mitigated and solid concentration is enhanced in system. Consequently, these features results in an increase in the particle packing of the

system. The test involves measuring the wet density of mixtures at various (w/b)v and

PCE dosages. Towards this, a cylindrical mold of 400 ml in volume is filled with mortar

and consolidated using a vibrating table for 30 sec. As indicated in the representative

example shown in Figure 4-3, there is an optimum w/b (point A in Figure 4-3(a)) at

which maximum solid concentration in system is achieved. The maximum solid concentration occurs when the particles have achieved their maximum packing density. However, beyond the optimum w/b, further addition of water drops the solid concentration (i.e., reduction in wet density) in the system. It is clarified that no signs of segregation, such as bleeding and sedimentation of cementitious grains, for the investigated mixtures were observed throughout the solid concentration test. For the descending part of solid concentration curve, the w/b of the investigated mixtures was increased to the extent that the mixture maintained its homogeneity and stability. The volume of the solid phase in system can be quantified as follows:

max ( / ) n s si n i w V si si i M V V w b R = = +

∑

∑

where, Vs (m3) is the total volume of solid phase in the mold,

n si i

V

∑

volumes of all solid phases, and Mmax (kg) is the mass of sample corresponding to the

maximum wet density. ρw and ρsi (kg/m3) are the density of water and solids

volumetric ratio of solids (i.e., ratio of the volume of cementitious materials to the volume of sand).

(a) (b)

(c)

Figure 4-3 Determination of optimum PCE dosage to achieve maximum possible solid

concentration in system. (a) Variations in wet density with water addition for the reference OPC mortar provisioned with 0.10% PCE dosage, (b) variations in wet density with water addition for the reference OPC mortar provisioned with various PCE dosages,

and (c) variations in solid concentration with respect to PCE dosage for the reference OPC mortar. In all cases, PCE dosage is expressed as vol.% of binder.

In order to determine the optimum PCE dosage required to achieve the maximum possible solid concentration, the procedure described above was replicated at various

PCE dosages (see Figure 4-3(b)). As expected, PCE addition improved the wet density

markedly due to the mitigation of agglomeration of particles. From Figure 4-3(b), it can

be seen that at PCE dosages between 0% and 0.10%, there is a significant enhancement in the wet density. However, further PCE addition (e.g., beyond 0.10%) leads to no

distinct changes in wet density. This is better shown in Figure 4-3(c), which plots the variations in solid concentration of system in proportion to the PCE dosage. In this study, the optimum PCE dosage was estimated from the intersection between two linear

segments (i.e., lines A and B in Figure 4-3(c)), which represent linear fits to the two

regimes. This intersection corresponds to threshold PCE dosage, beyond which there is less than 5% deviation in solid concentration compared to that obtained for the highest PCE dosage.

Rheological properties: The time-dependent rheological properties of mortars were evaluated over 70 min following initial contact of cementitious materials with water. The rheometer used in this study consists of coaxial cylinders (ConTec Viscometer 6) with inner and outer radii of 50 and 62 mm, respectively. This rheometer, therefore, has a gap of 12 mm between inner and outer cylinders, which is three times larger than the nominal maximum size of sand used in this study. The testing procedure consists of pre-shearing the fresh sample at the maximum rotational velocity of 18 rpm for 60 sec, followed by a step-wise (10 steps of 5 seconds each) decrease of the rotational velocity from 18 rpm to 1.5 rpm. Attention was paid to identify and eliminate artifacts, such as thixotropy (i.e., structural recovery of suspension when maintained at rest), plug flow (i.e., un-sheared part of suspension due to relatively high yield stress), and segregation (i.e., inhomogeneity of suspension due to shear-induced particle migration). The Reiner– Riwlin transformation (Reiner 1949) was applied to calculate fundamental rheological properties, namely dynamic yield stress (i.e., stress required to initiate flow) and plastic viscosity (i.e., internal resistance of suspension to flow once applied stress is more than yield stress) of suspension, as follows:

0 2 2 0 0 1 1 1 4 ln i i G h R R R R   τ = _ − _ p _{ }       4-2 2 2 2 0 1 1 8 i H h R R   µ = _ − _ p   4-3

where, τ0 and µ are dynamic yield stress (Pa) and plastic viscosity (Pa.s), respectively, G

(Nm) and H (Nm.s) are intercept and slope of the linear relationship between torque and rotational velocity using the Bingham model (Bingham 1922). h (m) is height of inner

cylinder submerged in the mortar, and Ri (m) and Ro (m) refer to the cylinder’s inner and

outer radii, respectively. In addition to the rheological properties (i.e., dynamic yield stress and plastic viscosity), the thixotropy (structural recovery) of mortars were evaluated using the static yield stress at rest approach (i.e., stress growth method) at 10 and 70 min after initial contact with water. Towards this, a constant rotational velocity of 1.5 rpm was applied and the resulting torque was measured as a function of time. The torque-time profile typically shows a linear elastic region followed by a yielding moment where torque reaches a maximum value, indicating that the majority of the bonds are broken (Assaad et al. 2003). The presence of such maximum torque response is an indication of thixotropy that can be explained by the concept of structural recovery and breakdown of the bond in the system when maintained at rest (Assaad et al. 2003; Roussel et al. 2012; Saric-Coric et al. 2002).

Hydration kinetics: The rate of hydration heat evolution and extent of heat release for the investigated systems were monitored up to 7 days using isothermal induction calorimetry (Calmetrix I-CAL 8000) programmed to maintain the sample at a constant temperature of 20 ± 0.1 °C. The thermal power and energy measured to maintain the temperature of the samples at 20 °C were used to evaluate the influence of SCM and PLC replacements on hydration kinetics of the binders.

Thermogravimetric and X-ray diffraction analyses: A Netzsch STA 409 PC thermal analyzer was used to identify and measure the quantities of phases present in mortars. The mass loss (TG) and the differential mass loss (DTG) traces at 7 days were used to calculate residual CH content present in the system. The amounts of phases derived from such analyses were consistently normalized by the OPC fraction in the mortar to account for the dilution effect. In addition, for the plain OPC and PLC systems, the non-evaporable water content derived from TGA/DTG traces was used to estimate the degree of hydration of the cement (Stoian et al. 2015).

X-ray diffraction (XRD) analysis was conducted on powdered cementitious

mixtures after 7 days of hydration using a Philips X'pert diffractometer in a θ–θ

configuration using CuKa (k = 1.54 Å) radiation.Samples were scanned between 5° and

detector with a time per step of 150 sec. A rotating stage was used to suitably sample the powder during acquisition.

Compressive strength: The 1, 3, 7, 28, 56, and 91 days compressive strengths of mortars were determined using 50 mm cubes as outlined in ASTM C109 (Annual Book of ASTM Standards n.d.). After demolding at 24 h, all mortar samples were stored in lime-saturated solution at 21 ± 2 °C until testing. The results of compressive strength represent the average of three replicate specimens. Among triplicate specimens, the coefficient of variation in the compressive strength for the investigated mixtures was found to be less than 6%.