Chemically Activated Blended Cements as Sustainable Cements

Introduction

Sustainable cements with clinker factor of 0.5 or lower can be made using high volume of supplementary cementitious materials like fly ash, slag, metakaolin, silica fume and natural pozzolanas [1-2]. Since these cements are more suitable for producing highly durable concrete products and thus can be called sustainable cements. If these cements are judiciously blended with proper selection of admixtures, mixture proportioning and curing can noticeably improve the durability of concrete [3-4].

These cements are slower in setting and hardening compared to ordinary portland cement. Further these mineral additives reduce the early strength of concrete [5-10]. To compensate the low early strength chemical activation of cement has been employed. Though thermal and mechanical activation is also possible but are not usually employed due to certain limitations. Calcium chloride is most popular and cheapest activator available for accelerating setting behaviour of cement. However, considering corrosion effect on reinforcement it is not used in the concrete. Other available admixtures such as nitrites, nitrates, aluminates, calcium formate and triethanolamine have been reported [10-13].

In the present study, sodium sulfate and superplasticizer has been used to compensate loss of early strength. This paper presents the optimization of cement by replacing it with slag and fly ash of first field by activating with sodium sulfate and superplasticizer. The study will further help in reducing replacement of ordinary portland cement by these cementitious materials and in turn reducing clinker factor.

Experimental Investigation

Materials
Ordinary portland cement of 43 grade conforming to BIS 8112-2005 [14], Fly ash of first field collected from thermal power plant near Delhi and slag were used for preparation of cement cubes. The chemical composition of these cementitious materials used in the present study is given in table 1. Technical grade sodium sulfate was used as chemical activator. Superplasticizer (SP) based on sulphonated naphthalene formaldehyde condensate (SNF) conforming to BIS 9103 (2004) was used in the present study.

Table1- Chemical/Physical composition of Cementitious Materials
Composition,%	Cement	Fly ash	Slag
SiO2	20.5	60.24	33.95
Al2O3	3.8	25.14	10.53
Fe2O3	2.6	4.75	1.25
CaO	60.5	3.10	40.40
MgO	3.2		8.65
Chloride content			0.03
SO3	2.5	1.35	0.10
CaO+MgO+ SiO2	__	__	83.0
SiO2 + Al2O3 + Fe2O3	__	90.13	__
LOI	1.0	1.32	0.54
Fineness cm2/gm	3100	2850	4020

The x-ray of the materials used in the present study is shown in figure 2-4.