Page 6 - November-2022

TECHNICAL PAPER

a plastic state. Finally, the plastic stated mixtures were mixed 3. RESULTS AND DISCUSSIONS
at a speed of 125 rpm and lower speed as known for 5, and 2
minutes, respectively. After the end of mixing operation, a batch 3.1 Mechanical performances
of test piece including three prismatic specimens with sizes of 40
× 40 × 160 mm were poured as well as vibrated. The prepared Figure 2 presents the development tendency of the compressive
3
specimens with molds were firstly preserved in a curing room and flexural strengths for UHPC mortars with long term water
at the temperature of 20°C and relative humidity of 96 % for curing from 90 to 365 days. According to Figure 2 (a), the
1 day. Then the specimens were demolded and immersed into compressive strength for mortars generally present an increasing
saturated calcium hydroxide solution at 20°C till testing age. tendency from 90 to 270 days while a decreasing tendency from
270 to 365 days. It may be attributed to part dissolution of gel
2.2.3 Test methods products in matrix induced by excess intruded water at late
ages, which stimulates a reduced compressive strength [27] . At
The compressive and flexural strengths for a set of samples 150 days, the compressive strength of MK contained mortars
(mortar specimens) were measured at 90, 150, 210, 270, and
365 days with the required loading rate of 2.4 kN/s conforming including binary 15 % MK contained mortar (C15) and ternary
to GB/T 17671 (1999) [26] . The three specimens from same 15% MK contained mortar (CLS15) have slight growth compared
batch were broken for testing flexural strength. It is noted that to that at 90 days. On the other hand, it is worth noting that
six broken samples can be obtained from the previous test. the compressive strength of the references without MK such as
Immediately, these broken samples were used to measure the C00 and CLS00 have significant increase from 90 to 150 days.
compressive strength. The final result was obtained by the The compressive strength of every specimen has similar
average value of six specimens for compressive strengths and development rate after 150 days. For instance, the compressive
three specimens for flexural strengths. strength for binary mortar incorporated with MK (C15) has an

Thermogravimetry and differential thermal analysis (TG/DTA) increase of 6 MPa from 150 to 270 days and a decrease of 6 MPa
were conducted by a differential thermal analyzer from 20°C to after 270 days similar to that of the cement mortar (C00) with an
1000°C. It is noted that a 10°C/min heating rate was employed. increase of 7 MPa from 150-270 days and a decrease of 10 MPa
The mass of powder sample used for TG/DTA test was kept after 270 days. At 365 days, the increasing rate of compressive
at 10 mg with an accuracy of 0.1 mg. The nitrogen flow during strength of mortars are 17, 25, 6, and -7 % for cement,
the experiment was 40 ml/min to prevent oxidation of samples. LS cement, MK cement and MK-LS cement mortar, respectively
Chemically bound water of paste cured in saturated calcium relative to that at 90 days. It is obviously that UHPC mortars
hydroxide solution at ages of 90, 150, 210, 270 and 365 days incorporated with MK have slighter gradient of compressive
were determined with TG analysis results. The chemically bound
water of paste sample was calculated with formula Equation (1), strength than that of the references not containing MK from 90
and (2). to 365 days with water curing.
m − m L It is noted that the compressive strengths of UHPC mortars
w = 1 2 − B (1)
ne m 1 L
−
2 B in this study have more obvious development after 90 days
−
L = B L Cement × (1 w MK − w LS ) L MK × w MK + wL LS (2) compared to that of mixtures with a w/b of 0.4 [28] , which
+
LS
substantiates a measurable re-hydration of UHPC. While the
where w ne was chemically bound water of paste sample. m 1 was re-hydration can not apparently decrease the compressive
the residual mass of sample at 60°C and m 2 was the residual strengths of MK contained UHPC mortars in one year, which is
mass of sample at 950°C. As well as L B, L Cement, L MK and L LS were analogous to the conclusion investigated by Wang [29] .
the mass wastage for the above mentioned binary or ternary
binders, PC, MK and LS, respectively when they were heat to In terms of Figure 2 (b), the flexural strengths tendency is similar
950°C. w MK and w LS were required weight fraction of MK and LS to that of compressive strength with age. In the age range of
in the sample, respectively. 90-365 days, the flexural strengths development degree for
MK contained mortars are obviously smaller than that of blank
X-ray diffraction (XRD) test on the paste sample was carried
out with a XRD apparatus. The selected parameters were Cukα mortars without MK. For instance, cement, LS-cement, MK
radiation with 40 kV and 35 mA as well as 2θ values from 5-65°. cement, MK-LS cement mortars have an increase of 3.3, 4.9, 1.1
It is worth noting that the pore structures for paste samples were and 1.2 MPa, respectively from 90 to 270 days while a decrease
measured with a Mercury porosimeter apparatus owning the of -1.5, -1.6, -0.4 and -0.3 MPa, respectively from 270 to 365 days.
maximum measuring pressure of 400 MPa as well as the micro- In general, MK can significantly stabilize the mechanical property
morphology of pastes were tested with a scanning electron of UHPC mortars after 90 days refraining from the influence of
microscope equipment at the accelerating voltage of 30 kV. re-hydration under water curing in one year.

20 THE INDIAN CONCRETE JOURNAL | NOVEMBER 2022