Experimental Investigation to improve the early strength and durability properties of very high volume fly ash mortar with addition of lime and silica fume

Concrete is the most used material on earth after water and least energy consuming material after wood. But the production of cement is responsible for world’s 7% greenhouse gas emissions due to the large application of concrete worldwide. 1 ton of cement produces around 0.82 tons of CO₂ worldwide. There is a scope to reduce the emissions produced by cement by using alternative cementitious materials such as fly ash. A high volume fly ash concrete (HVFAC) can address the two environmental problems caused by cement production and environmental waste caused by disposing of fly ash in the landfills.

This study presents the investigation on combination of various factors on improving the early strength of very high volume fly ash concrete with 80% class F fly ash. Factors considered were using two different grades of ultra-fine fly ash, addition of lime and SF in quantities 0%, 5% and 10%. Compressive strengths and water absorption tests were carried out and the results confirm the developing high strength VHVFA mortar with 80% replacement of cement with class F type ultra-fine fly ash.

Keywords: High Volume Fly Ash Concrete; Lime; Ca(OH)₂; Silica Fume; Compressive Strength; Water absorption.

Introduction

The cement industry has experienced explosive growth due to world’s urbanisation and in 2014 alone, around 4.2 billion tons of cement was produced (Xing). But, the process of cement production is causing environmental problems as 1 ton of cement produces around 0.82 tons of CO₂ (Wilson and Tagaza 2006).

Fly ash is a by-product of coal combustion from thermal power plants. Australia is a country where around 84% of the total electricity generation is produced from thermal power plants. Based on the current demand for electricity, the production of fly ash is expected to increase 3-5% annually. As per Ash Development of Australia (ADAA), 2012 (ADAA 2012), about 12.5 Mt of fly ash was produced in 2002, in which only 32.8% was utilized effectively of which only 10% in concrete and rest being used in other applications (Heidrich 2002). Therefore, replacing cement with higher amounts of pozzolana for the structural purpose should be widely accepted and should be accelerated to achieve sustainability in the construction industry (Malhotra and Mehta 2005).

HVFA system has a drawback of low early strength development. Well-cured super-plasticized HVFA concrete shows better strength and durability properties than OPC concrete (Malhotra and Mehta 2005). This study aims to improve early strength and properties of VHVFA mortar with 80%fly ash by using different strength improving mechanisms from the literature. Literature suggests the following methods which can improve the early strength of HVFAC when designed properly.

Addition of Lime

Addition of small amounts of limestone powder in the mix design which improved the early strength and mechanical properties of concrete which were investigated by many studies (De Weerdt et al. 2011, Bentz and Ferraris 2010, Barbhuiya et al. 2009, Antiohos et al. 2008). However, in VHVFA system, where replacement levels are 65% and above, smaller quantities of lime may not be adequate for full pozzolanic reaction.

A research by (Myadaraboina and Patnaikuni 2017),  concluded that addition of lime between 20-32% is optimum to react with 80% raw fly ash based on its calculated pozzolanic index.  And from another study on pozzolanic index of fly ashes by Myadaraboina et al., 2016 conveyed that Gladstone fly ash (UFFA) requires around 31% with 80% replacement of cement with fly ash. From both the studies, lime addition of 25-30% is considered for 80% replacement of cement has been taken for the present study with mortar with type E and UFFA.

Type of cement and w/b ratio

A study by Halse et al., 1984 suggests that cement with higher C₃A (Type III cement) and fly ash with smaller size and higher surface area should be used to achieve high early strength (Halse et al. 1984). Malhotra and Mehta, 2005 also suggested to use type III cement (HES) cement with a w/c ratio 0.3, if the compressive strength of 15 MPa or more is required for 1 day.

According to Aïtcin, 2004, the use of w/b ratio of less than 0.4 becomes the requirement for producing high performance concrete (Aïtcin 2004).  It was argued in a study by Yasar, Erdogan & KilIç, 2004 that “the use of w/c ratio of 0.3 with the consequence of the decreasing workability, it is possible to produce the highest compressive strength result of concrete in comparison to higher w/c ratio”. A research by Poon, Lam & Wong, 2000 has shown that HVFAC will have better strength performance when they are prepared at lower w/b ratio. 0.24 to 0.19 (Poon, Lam, and Wong 2000). A, higher dosage of superplasticizer was required for mixes having HVFAC to achieve good workability with low w/b ratios for high strengths.

Using finer grade fly ash

Many researchers confirmed that using UFFA in fly ash concrete enhances the compressive strength and durability properties over using raw fly ash due to its better reacting nature (Obla et al. 2003, Chindaprasirt, Homwuttiwong, and Jaturapitakkul 2007, Kiattikomol et al. 2001, Jo et al. 2007, Alvarez, Salas, and Veras 1988, Shaikh and Supit 2015). The addition of lime water as mixing water and using ultra-fine fly ash (UFFA) with 50% replacement of cement has shown improving the early strength of HVFAC (Solikin 2012).

Addition of silica fume (SF)

Addition or replacement of SF up to minor quantities such as 5-11% in OPC concrete will improve the early and later strengths and durability properties significantly. Research by Ting and Patnaikuni, 2011 shows that addition 10% SF gives optimum results (Ting et al. 1992). Hence, in this project addition of SF 5 and 10% were considered for optimum results.

This paper investigates combination of all the above-mentioned factors to improve the early strength of VHVFA mortar with cement replaced by 80% fly ash with addition of 30% lime and varying percentages of SF from 0, 5 and 10%. The study focused on analysing the effect of different quantities of SF additions on compressive strength up to 56 days of curing age of VHVFAC with 80% replacement of cement with class F fly ash.

Materials & Methods

Materials

For this study, a cement of HES Type III which conforms to the Australian Standard AS 3972, 2010, for Type HE cement was used. Class F fly ash from Gladstone (G) power plant and Microash (M) from Fly ash Australia Pvt Ltd, which conforms to the requirements of Australian Standard AS 3582.1 were used. Locally available uncrushed river sand conformed to  the Australian Standard AS 1141.5 2000 (AS 1141.5 2000) was used as a fine aggregate. The fine aggregate has a grading of size between 150 µm to 4.75 mm as specified. Tap water was used as mixing water. Table 1 presents the chemical compositions of Type I and Type III (HE) cements.

 Table 1 Chemical composition of ASTM type III cement.

ASTM	C3S (%)	C2S (%)	C3A (%)	C4AF (%)	Fineness (m2/kg)
Type I	55	19	10	7	370
Type III (HES)	56	19	10	7	540

Table 2 presents the chemical compostion of fly ashes, lime and SF. Gladstone (Type G) fly ash and microash (Type M) comes under category of UFFA according to their particle size (Table 2). Microash is a special grade fly ash which conforms to the requirements of (AS 3582.1 2016) AS 3582.1 2016, Part 1. It is composed of very fine particles of spherical shape.

Table 1 Chemical composition of Gladstone and Microash fly ashes, lime, and SF

	Gladstone (G)	Microash (M)	Lime	SF
SiO2	50.82	70.70	1.8	85-97
Al2O3	29.89	20.70	0.5	–
Fe2O3	10.26	3.90	0.6	–
CaO	3.24	1.13	72.0	<1
MnO	–	0.05	–	–
MgO	0.08	0.77	1.0	–
TiO2	2.05	0.92	–	–
Na2O	0.00	0.26	–	–
K2O	0.58	1.09	–	–
P2O5	1.61	0.15	–	–
SO3	0.28	0.2	–	–
LOI	0.43	0.7

 

Table 2 Physical properties of UFFA fly ash

Property	Gladstone (G)	Microash (M)
Mean particle diameter(m)	7	3.5
Specific Gravity	2.15	2.35
Fineness (particles passing 45 m)	88%	99%

The specific gravity of HES used in the mix design was of 3.15, fine aggregate of 2.60, and the hydrated lime has specific gravity of 1.9. A HRWRR polycarboxylate polymer superplasticizer used with a density of 1.05 kg /litre was used. The average particle diameter of SF is 0.10 m and specific gravity of 2.2.

 Method

An experimental analysis was carried out of VHVFA mortars with 80% fly ash and 20% cement and 30% lime to check the effect of the addition of SF in amounts of 0%, 5%, and 10% on the compressive strengths and water absorption. The summary of mix proportion is shown in Table 3. Four mix proportions for testing high volume ultra-fine fly ash mortar for its compressive strength development and two optimum mixes for water absorption test were prepared.

Mix Design

The mix proportion of mortar in the experiment shows that the lower use of w/b ratio leads to the decrease of water volume in fresh mortar and results in higher amount of binder.  To achieve sufficient workability with low w/b ratio, 2% of superplasticizer is added to all mixes. The total binder in the mix proportion is around 550 kg/m3 and the superplasticizer content is between 26 – 33 litres/m³. The difference of the total binder in each mix proportion is caused by the specific gravity of ultra-fine fly ash and SF which are less than the specific gravity of Portland cement. The mix design and proportions of VHVFA mortar with SF are presented in Table 3 and Table 4, respectively.

Table 3 Mix Design of VHVFA mortar with SF

Mix	w/b ratio	Cement%	FA%	SF %	Lime%	FA Type
G0	0.23	20	80	0	30	G
G5	0.23	20	80	5	30	G
G10	0.30	20	80	10	30	G
M10	0.27	20	80	10	30	M

Table 4  Mix proportions of VHVFA mortar with SF

Mix	Water (kg/m3)	Cement (kg/m3)	Fly Ash (kg/m3)	SF (kg/m3)	Lime (kg/m3)	HRWRRe (l/m3)	Sand (kg/m3)
G0	92	110	440	0	165	26	1470
G5	90	110	440	28	165	30	1481
G10	111	110	440	55	165	33	1447
M10	130	110	440	55	165	33	1310

The result of compressive strength development for all four mix proportions and strength development are presented in Table 5 & Figure 4.

Compressive Strength

The mortar compressive strength for all the mix proportion combinations shows the compressive strength of mortar increases along with the curing age. The compressive strength of mortars with SF with 5-10 % observed to increase rapidly from the testing age of 29 days to 59 days.

Table 5 Compressive strength result of VHVFA mortar

Mix	SF	Compressive strength (MPa)
		7 days	29 days	59 days
G0	0%	18.0	37.0	–
G5	5%	19.5	38.0*	76.0
G10	10%	23.0	45.5*	79.0
M10	10%	22.0	58.5*	84.0**

(Note: * indicates compressive strength measured at 30 days and * *indicates compressive strength measured at 90 days)

Figure 4  Compressive strength development of VHVFA mortar with Gladstone fly ash

VHVFA mortar with 10% SF can reach the high compressive strength of 41 MPa which starts at the age of 28 days. Moreover, VHVFA mortar without SF can reach high compressive strength mortar after 59 days of curing. It is observed that the VHVA mortar without SF didnot reach the high strength up to the testing age of 59 day.

With the addition of 5% SF, there is an increase in compressive strength at all ages compared to VHVFA with 0% SF, but the increase is not significant at early ages. There is a very rapid increase in strength from 29 days to 59 days, which reached from 38 MPa to 76 MPa, where the increase is 100%. With the addition of 10% of SF, though the increase is less compared to 5% addition of SF at 7 days, the increase in strength is significant at 29 days and 59 days.

Water absorption of mortar

Compressive strength analysis results show that the VHVFA mortar with UFFA fly ash is giving best results with the addition of SF. Hence to optimise the experiments, the water absorption test was conducted on only mixes with the addition of SF with 5% and 10% to compare the effect of the amount of SF on water absorption.

Water absorption test of mortar was conducted based on Australian Standard AS 1012.21 1999 (AS 1012.21. 1999) to study moisture transport in cement mortar as stated in Kim, Jeon & Lee 2012 (Kim, Jeon, and Lee 2012). The test was carried out after the curing age of mortar at 56 days. The type of absorption tested was immersed water absorption. In addition, the apparent volume of permeable voids (AVPV) was also determined. The results are given in Table 6.

Table 6 Water absorption values at 56 days of curing age

	Ai (%)	AVPV(%)
M2	2.22	4.98
M3	1.62	3.72

Analysis and Discussion

Compressive strength of VHVFA mortar

It can be noted that the addition of SF in high microash which gives a significant contribution to mortar compressive strength development. At early ages, the compressive strength of mortar with 5% SF is similar to that of mortar with 10% microash. At the ages of 29 days and 59 days the compressive strength of VHVFA mortar with microash with 10% SF is higher than that of VHVFA mortar with 0% and 5% SF. The increase of compressive strength in a VHVFA mortar using microash and 10% SF is 57.5%, 55% and 29.2% at the age of 56 days respectively compared to that of VHVFA mortar with ultrafine fly ash (Gladstone) with 0%, 5% & 10% SF.

It is remarkable to note that the contribution of adding 10% SF is quite significant in increasing the compressive strength. The result of compressive strength provides evidence that the presence of lime provides the amount of Ca(OH)₂ needed to react with silica (SiO₂) in high volume ultra-fine fly ash mortar to give a contribution to compressive strength starting at early ages as stated by Fraay, Bijen & Haan 1989 (Fraay, Bijen, and Haan 1989).

To understand the influence of each factor on the compressive strength of mortar, the results were analysed in Minitab, a statistical analysis software. The results of the analysis using Minitab software consist of main effect plot, interaction plot and contour plot. The contour plot in Figure 5 shows that the mortar compressive strength shows maximum (>70 MPa) with SF percentages of 5 & 10 after a curing period of 50 days. However, maximum shows for 10% SF after 50 days.

 Figure 5 Contour plot for compressive strength Vs Curing period and % of SF

Figure 6 Main effects plot between % of SF and fly ash type with compressive strength

Both graphs of main effect & interaction plot for compressive strength (Figure 6 & Figure 7) confirm that the highest mortar compressive strength was obtained when the microash is used combined with 10% SF in mix proportion.

Figure 7 Interaction plot for compressive strength

Water absorption of VHVFA mortar

The results of water absorption indicate that the use of VHVFA mortar with 10% SF significantly reduces water absorption and apparent volume of permeable voids in mortar in comparison to a mortar with VHVFA mortar with 5% SF (Figure 8). The decrease of immersed absorption and AVPV in VHVFA mortar with addition of 10% SF are 60% and 58.6% respectively.

Figure 8 Water absorption of VHVFA mortar

The results of immersed absorption indicates that the mortar produced using VHVFA mortar with 10% SF has higher compactness than mortar with 5% SF. Based on the AVPV values of vibrated cylinders (CCAA 2009), the high volume fly ash mortars with SF have an excellent level of durability.

Conclusion

This investigation on mortar compressive strength and water absorption convincingly support the hypothesis that VHVFA mortar with 80% replacement can produce high strength mortar using very large quantities of fly ash as high as 80% of volume of binder. This study also conforms with the other researchers stating that:

• Reducing the w/b ratio and increasing the superplasticiser as well as using high early strength cement improved early strength
• Addition of 5% SF over 0% SF has shown very little improvement in strength of VHVFA mortar
• Addition of 10% SF has exhibited a significant difference in compressive strength at all ages as well as in decreasing permeability
• Microash showed better results than Gladstone fly ash in achieving later strengths because of its finer nature.
• Addition of 10% SF has shown improved early strength and water absorption properties for VHVFA mortar.

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