Influence of calcium additions on the compressive strength and microstructure of alkali‐activated ceramic sanitary‐ware

Abstract

The ceramic sanitary‐ware market generates large amounts of waste, both during the production process and due to construction and demolition practices. In this paper, the effect of different amounts and calcium sources (calcium hydroxide Ca(OH)2, calcium aluminate cement CAC, Portland cement PC) on the alkaline activation of ceramic sanitary‐ware waste (CSW) was assessed. Blended samples were activated with NaOH and sodium silicate solutions and cured for 3 and 7 days at 65°C. The maximum amount of calcium source‐type added to the system varied according to its influence on the compactability of the mortars.CSW was physico‐chemically characterized and the compressive strength development of activated samples was assessed on the mortars. The nature of the reaction products was analyzed in pastes, by X‐ray diffraction, thermogravimetric analysis, infrared spectroscopy and microscopic studies. The results show a great positive influence with the addition of moderate amounts of Ca(OH)2, PC and CAC on the mechanical properties. Among the typical hydrates usually observed in plain water‐hydrated PC or CAC, only AH3 and a small amount of C3AH6 were identified in the alkali‐activated CSW/CAC blended pastes, which indicates that Al and Ca from PC, CAC and Ca(OH)2 are taken up in the newly formed (N,C)‐A‐S‐H or C‐A‐S‐H gels.

The Compressive Strength and Microstructure of Alkali-Activated Binary Cements Developed by Combining Ceramic Sanitaryware with Fly Ash or Blast Furnace Slag

“…Calcium silicate rankinite (R, Ca 3 Si 2 O 7 , PDF#220539) emerged in the spectra of the 100 wt % BFS pastes (with and without calcium addition), and small amounts of Natron (N, Na 2 CO 3 ·10H 2 O, PDF#150800) formed in the 100 wt % CSW activated paste. This sodium carbonate has also been noted in previous works conducted into the alkali-activation of CSW [24,26], and has been attributed to the existence of non reacted reagents or to the carbonation of pastes. Although, as described by Marjanovic et al [2], hydrotalcite gel usually forms in alkali-activated 100 wt % BFS when the original BFS contains more than 5% MgO (the BFS used in our study had 7.47 wt % MgO; see Table 4) and is activated with sodium silicate solutions, no signals associated with this compound were clearly distinguished in any activated paste.…”

“…In this sense, the CSW mortars containing 20-30 wt % BFS or FA exhibited compressive strength values within the 22.1-37.6 MPa range after 7 curing days at 65 • C, and 6.9-21.1 MPa after 28 days at 20 • C. These binders can be used mainly in low-strength applications (e.g., nonstructural concrete or blocks) or prefabricated systems (where temperature can be easily applied to cure samples). For high-strength applications, the C-BFS40 and C-BFS50 systems can be selected with strength values that come close to 30 and 40 MPa after 28 curing days at 20 • C. However, as these mortars use large amounts of BFS, CSW can alternatively be blended with CAC, PC [24] or fluid catalytic cracking waste (FCC, an industrial by-product) [26] when high strength binders or shorter curing periods are required.…”

“…Compressive strength values of 66.3 MPa and 78.6 MPa have been reported in alkali-activated CSW mortars blended with 15 wt % PC or 20 wt % CAC, respectively (cured at 65 • C for 7 days). However, as highlighted by the authors [24], CAC or PC production implies the use of natural resources and requires vast amounts of energy. Thus, an alternative with a higher added value would be to blend CSW with FCC [26], which gave compressive strength results that came close to 40 MPa in the mortars cured at 20 • C for 28 days (made with 30 wt % FCC and the same mix proportions as those used herein).…”

“…observed that the heat of reaction in an alkali-activated natural pozzolan/BFS binary cement increased with BFS addition, which accelerated the kinetics of the process and, consequently, favored the activation of the pozzolan at room temperature, as well as strength development. A very positive influence of PC, CAC, and Ca(OH) 2 on the microstructure and mechanical properties of alkali-activated CSW is reported in [24], where compressive strength values of 40.06 MPa, 64.41 MPa, and 56.62 MPa are reported in mortars containing 6 wt % Ca(OH) 2 , 10 wt % PC and 10 wt % CAC, respectively (all cured at 65 • C for 7 days). However, the natural resources used, and the large amounts of CO 2 emitted to the atmosphere during CAC or PC production, motivated a later study, which explored the influence of spent fluid catalytic cracking waste (an industrial by-product) on the microstructure and compressive strength of alkali-activated CSW [26].…”

“…However, addition of Ca(OH) 2 and applying temperature to cure samples proved essential to successfully activate this ceramic waste material, which limited its use mainly to prefabricated applications. In order to improve the properties of alkali-activated binders developed using a single material as a precursor, several studies have lately combined low-calcium precursors, like CSW, with different sources of calcium and reactive Al 2 O 3 , such as BFS [1,2,[19][20][21][22][23], FA [6], calcium aluminate cement (CAC) or Portland cement (PC) [24,25]. observed that the heat of reaction in an alkali-activated natural pozzolan/BFS binary cement increased with BFS addition, which accelerated the kinetics of the process and, consequently, favored the activation of the pozzolan at room temperature, as well as strength development.…”