Quality control and assessment of geopolymer cements based on reacted and free alkalis Ali Allahverdi a,b,⇑ , Mostafa Vafaei a , Hojjatollah Maghsoodloorad a a Research Laboratory of Inorganic Chemical Process Technologies, School of Chemical Engineering, Iran University of Science and Technology, Narmak 1684613114, Tehran, Iran b Cement Research Center, Iran University of Science and Technology, Narmak 1684613114, Tehran, Iran highlights  Reacted and free alkalis were proposed as geopolymer quality control parameters.  Reacted alkali is correlated to compressive strength.  Free-alkali is correlated to efflorescence formation intensity. article info Article history: Received 29 July 2016 Received in revised form 15 June 2017 Accepted 4 July 2017 Keywords: Geopolymer Reacted alkali Free alkali Quality control Soundness abstract In this paper, new parameters based on reacted and free alkalis are introduced to assess and control the qualitative properties of geopolymer cements. For this, Fly ash-based geopolymer paste were prepared to study the correlation between compressive strength and reacted alkali at different curing time and tem- peratures. Since the properties of geopolymers strongly depend on the reactive oxides content (Al 2 O 3 + SiO 2 ) reactive and composition [Al 2 O 3 /(Al 2 O 3 + SiO 2 )] reactive of the precursors, variation of this ratio on the relationship between compressive strength and reacted alkali and between efflorescence intensity and free alkali were also investigated using by blended natural pozzolan/calcium aluminate cement and fly ash/calcium aluminate cement geopolymer pastes. The results confirm the existence of strong corre- lations between compressive strength and reacted alkali and between efflorescence intensity and free alkali. The findings, therefore, prove the suitability of the reacted and free alkali contents as proper qual- ity control parameters in geopolymer cements. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Quality control is one of the most important activities of a pro- duction unit, because it plays a critical role in the continuity, per- manence and stability of the products. Cement as a commodity is produced during a complex process accompanied by persistent variations of the raw materials quality. Since cement is used in almost all construction applications, the issue of quality and spe- cial attention to it is important. Various factors including physical and chemical properties of cement (cement fineness, expansion, initial and final setting times, compressive strength and heat of hydration, chemical and mineral composition and free lime con- tent) have direct influence on the performance and soundness of Portland cements. Measurement and control of these parameters will result in the quality control and assessment of Portland cement [1,2]. As an example, the relatively high free lime content in Portland cement clinker which indicates the insufficient pro- gress of alite formation reaction in rotary kiln can cause unsound- ness of Portland cement (destructive expansion). Therefore, the amount of free lime in the kiln output clinker is constantly mea- sured to obtain the information about progress of alite formation reaction, providing the control of the production process and ensuring the product quality and soundness [3]. Concerns related to energy consumption and environmental issues in Portland cement manufacturing have resulted in signifi- cant interest in finding binders alternative to Portland cement [4]. Geopolymer cements are a group of clinker-free binders that are synthesized by combining aluminosilicate materials such as metakaolin and fly ash with strong alkali solutions such as sodium hydroxide and/or sodium silicate (in most cases). Based on Davi- dovits (1994), the molecular structure of geopolymer include a three-dimensional aluminosilicate network of AlO 4 and SiO 4 tetra- hedral groups linked by oxygen bridges and the negative charge of http://dx.doi.org/10.1016/j.conbuildmat.2017.07.073 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Research Laboratory of Inorganic Chemical Process Technologies, School of Chemical Engineering, Iran University of Science and Technology, Narmak 1684613114, Tehran, Iran. E-mail address: ali.allahverdi@iust.ac.ir (A. Allahverdi). Construction and Building Materials 153 (2017) 274–283 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat