Paper • open access

Keywords
: Portland cement, pozzolans, rice husk ash, calcium carbide waste, metakaolin, 
consistency, setting times 
1
 
Introduction 
The use of concrete is over centuries and has retained its popularity because of its properties.
Setting time as one of the fresh properties of concrete is vital in concrete construction [1]. In 
concreting, this property (setting time) find application in its development in terms of 
transportation, placing, compacting and finishing. The placement of concrete in formwork 
depends on the setting time for it to harden [2, 3]. With the advent of new types of concrete such 
as high strength concrete (HSC), self-compacting concrete (SCC) and high performance concrete 
(HPC), application of Pozzolans is imperative so as improve their performance, achieve better 
engineering properties and most especially reduce cost and lower the carbon foot print. Mineral 
admixtures such as rice husk ash (RHA), calcium carbide waste (CCW), metakaolin (MK), fly-
ash (FA) and others are applied to replace cement partially for more efficacy of blended 
concrete. 
Since these mineral admixtures differ in chemical, mineralogical composition and particle 
characteristics, their effects on concrete properties including setting time characteristics is also 
likely to differ. It is not out of place to say that knowledge of the setting characteristics of an 
admixture is imperative as this will enhance the scheduling of stages in concrete construction 
operations vis-a-vis transporting, placing, compacting and finishing of concrete work. Such vital 
knowledge cannot be over emphasized in the choice for a retarder or an accelerator admixture.
The mixing of water with cement triggers-off hydration products formation. Through the rigid 
behavior of the matrix, its setting times is determined. The beginning of the hardening of mortar 
or concrete is termed the initial setting time, while its sufficient hardness is termed final setting 
time [3]. Previous work [4] on cementing material from calcium carbide residue-rice husk ash 
showed that the product of the mixture is pozzolanic. Also, similar work on activated CCW 
indicated that it has prolonged setting times compared with control values [5]. 
When calcium carbide (CaC
2
) reacts with water, CCW and acetylene gas is obtained (equation 
1). 

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Nanotechnology Applications in Africa: Opportunities and Constraints
IOP Conf. Series: Materials Science and Engineering 805 (2020) 012031
IOP Publishing
doi:10.1088/1757-899X/805/1/012031
3
CaC
2
+ 2H
2
O C
2
H
2
+ Ca(OH)
2
(1) 
It is a by-product from acetylene gas (C
2
H
2
) production [6]. Acetylene gas find application in 
ripening of fruits, a veritable choice for heating processes such as flame heating, flame gouging, 
welding, flame hardening, thermal spraying and other heating applications [7]. However, in 
Nigeria, acetylene gas finds application in oxy-acetylene gas welding. The carbide residue 
(CCW) is disposed as waste which is becoming a health hazard [8]. Also, CCW contains 
Ca(OH)
2
and is alkaline with pH greater than twelve [9]. To mitigate this negative effect on the 
environment, attempts were made to use CCW positively in both building and construction 
industry. When CCW is mixed with rice husk ash (RHA), pozzolanic reaction occurs and mortar 
produced from this mix has a compressive strength of 15 MPa at 28 days [10]. 
It has been shown that cement paste and concrete containing CCW at 1 % addition of CCW had 
initial and final setting times of 78 % and 57 % of control values, but increased in consistency by 
14 % [11]. Also 5 % addition of CCW to PC mix, increased consistency but decreased the setting 
times [12]. From these results, CCW can influence properties of PC blended cement such as 
consistency and setting times. Consistency can be said to be the extent of wetness or dryness; an 
indication that a concrete sample is workable or not through the entire process of concreting 
operations (transportation, placement, finishing) without segregation. RHA mortar paste has 
higher consistency compared with control paste values. The consistency increases with increase 
in replacement level of PC with RHA [13-15]. This is not unconnected with the natural structure 
of RHA with large surface area [16]. The adsorption capacity, high fineness and specific surface 
area require more water [17, 18] which could be as high as 100 % [19]. The use of RHA of high 
fineness also leads to increased water demand as opined in earlier studies [20-22. From the effect 
of RHA and RHA-CCW in concrete, the setting time of RHA mortar decreased compared with 
that for RHA-CCW mortar [23].
Rice husk ash (RHA) is a pozzolanic material which is produced by incineration of (RH) under 
controlled temperature (550- 800 
o
C), time and rate of burning. When it is incinerated under 
these conditions, highly reactive amorphous silica (SiO
2
) is obtained. At incineration temperature 
above 700 
o
C, the amorphous SiO
2
changes to a crystalline SiO
2
(Cristoballite, Quartz and 
Tridymite) and below 500 
o
C, the product has high LOI which affects rate of reaction. In the 
aforementioned cases, the reactivity of RHA is greatly decreased. It has been shown to have low 
specific gravity [24], hence finds application in the making of light weight building materials. It 

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Nanotechnology Applications in Africa: Opportunities and Constraints
IOP Conf. Series: Materials Science and Engineering 805 (2020) 012031
IOP Publishing
doi:10.1088/1757-899X/805/1/012031
4
has been reported that when PC is replaced at 10 % level, optimum workability and strength is 
obtained [25]. Its usage improves concrete properties; mitigate global warming and waste 
disposal problems [26, 27]. In a study [28], the physico-chemical and hardened properties of 
blended cement binders containing RHA and (MK), there was an increase in water demand and 
setting times. MK is a highly reactive alumino silicate and alumina. These oxides combine with 
slake lime (Ca(OH)
2 
at ambient temperature with moisture to form compounds that were 
virtually identical to the compounds in hydrated Portland cement. MK is widely used for its 
highly pozzolanic properties and considered to have twice the reactivity of most other pozzolans 
and considered a very viable admixture. It is not like other SCMs like fly ash, silica fume which 
are by-products of an industrial process. When kaolin clay is calcined within a temperature range 
of 650
-
800 
o
C, metakaolin is obtained; a relatively new material in the concrete industry, 
effective in strength development and mitigates sulphate attack. The pozzolanic reactions 
changed the microstructure of concrete and chemistry of hydration products by consuming the 
released CH to form more calcium silicate hydrate (C-S-H), thus enhancing properties of mortar 
and concrete [29]. When PC is replaced with MK, up to 20 % increase in strength and good 
results in other variables were obtained when compared with control values. The use of MK as a 
supplement in concrete up to 20 % can reduce cost of cement and also mitigates negative 
environmental pollution due to cement production. 
Results so far analyzed indicated prolonged consistency and setting times. Since pozzolans 
enhance both fresh and hardened properties and ensure the production of concrete at minimum 
cost, there has been increasing use of pozzolans both in the building and construction industry. 
Although CCW as a pozzolan improves strength of concrete, it prolongs setting due to its low 
pozzolanicity. However, (MK) though improves early strength development, lowers workability 
due to high heat of hydration which is detrimental to hardened properties of mortar and concrete. 
It is also weak in magnesium sulphate environment as well as at high temperatures. But when 
RHA replaced PC, it accelerates the early hydration of Tri-calcium silicate (C
3
S) and increase in 
the early hydration rate of C
3
S which is attributed to its high fineness [30]; but due to its porous 
nature, absorbs more water. Thus, extent of scope of application of binary blends of CCW, MK, 
RHA and the ternary blends of CCW-MK, and MK-RHA in building and construction industry 
may be limited. However, the multiple applications of these admixtures in the form of blended 
binders have the potential to compensate for the differences due to their synergistic interactions. 

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Nanotechnology Applications in Africa: Opportunities and Constraints
IOP Conf. Series: Materials Science and Engineering 805 (2020) 012031
IOP Publishing
doi:10.1088/1757-899X/805/1/012031
5
Hence, this study was set out to evaluate the fresh properties of blended binders containing RHA, 
CCW and MK with a view to determine its suitability as a retarder or accelerator in concrete 
production.